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Kinking in Semiconductor Nanowires: A Review

Cite this: Cryst. Growth Des. 2022, 22, 1, 871–892
Publication Date (Web):December 3, 2021
https://doi.org/10.1021/acs.cgd.1c00802

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Abstract

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The growth direction of nanowires (NWs) can change during synthesis as a result of stochastic processes or modulation of certain growth conditions. This phenomenon is known as kinking. Although deviations from a uniform vertical growth are typically considered to be undesirable, kinking opens a route for additional tweaking of the characteristics and functionalities of NWs in a controllable manner, thus extending the range of potential applications. In the present Review, we give an insight into the kinking mechanisms and summarize the most crucial factors that can lead to kinking of NWs during synthesis. Additionally, the properties and applications of kinked NWs are discussed.

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Synopsis

In the present Review, we give an insight into the kinking mechanisms and summarize the most crucial factors that can lead to kinking of NWs during synthesis. Additionally, the properties and applications of kinked NWs are discussed.

Introduction

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Nanowires (NWs) are solid one-dimensional (high-aspect-ratio) nanostructures that are among the most studied objects in modern material science. (1) Having a diameter in nanometers, NWs exhibit a wide range of unique characteristics, typically attributed to nanoscale objects in general. For instance, they have greatly enhanced mechanical strength, (2,3) increased chemical activity, (4) and other features generally related to quantum confinement and increased surface-to-volume ratio. At the same time, the length of NWs often lies in the micrometric range and can reach tens or even hundreds of μm, allowing for a relatively simple manipulation of individual NWs that facilitates their characterization (e.g., mechanical, (5,6) tribological, (2,7) or electrical (8,9) testing) and the assembly of single-NW-based prototype devices. (10,11) Attractive properties of NWs have inspired a great number of applied studies, in which NWs are tested as key elements of bio/chemical sensors, (12,13) energy-efficient light-emitting devices, (8,14) solar cells, (15,16) photodetectors, (10,17) material for batteries, (18) logic gates, (19,20) field-effect transistors, (8,21) and other nanoelectronic devices. (8)
In order to meet the specific requirements of various applications, it is important to have the ability to tune the properties of NWs by controlling their composition, (3) diameter, (22,23) crystallographic orientation, (24) as well as their defect structure. (25) Usually, defects are intuitively associated with negative effects on the properties of a material. Indeed, defects often cause reduced efficiency, malfunctions, or even failures in various systems. (26,27) However, in spite of a negative flavor in the term, defects can be highly desirable and are widely used to tune the properties of NWs and supplement them with essential functionalities required in particular applications. (28,29) Defects can be created either at the synthesis stage (30,31) or introduced later by post-treatment with plasma, (32,33) chemicals, (34,35) mechanical stresses, (36) etc.
In this work, we focus on a particular type of defect─so-called kinks, i.e., sudden changes in the axial growth orientation of an NW. (37) Similar to other defects, kinks can significantly worsen essential properties of NWs. (38−40) Therefore, in most applications, straight and defect-free NWs are desirable. On the other hand, kinking opens a route for additional control of the characteristics of NWs. Moreover, kinking may give a NW totally new functionalities. In recent years, there is an increasingly growing interest toward kinked NWs because of their potential use in integrated electronic devices, nanoscale springs, nanoelectronic bioprobes, etc. (41−46) Kinking is often reported for semiconductor NWs made of materials having practical significance such as Si, (47,48) Ge, (49,50) GaAs, (51) GaSe, (52) and numbers of other materials. Kinked Si NWs are the most studied ones, having the highest potential to serve as building blocks in electronic (41,53) or nanomechanical devices. (54,55) Moreover, kinks in heterostructures formed by at least two different materials are also reported. (56−58)
Changes in the growth direction are often accompanied by changes in the properties of an NW. (59) Therefore, it is important to understand the mechanisms responsible for these changes. A better control over the kink location and growth direction will allow for advanced spatial NW structures. (42) In the present work, we analyze the available literature dedicated to the kinking of NWs in order to summarize the main methods that allow the controlled introduction of kinks to NWs during synthesis and discuss the impact of the kinks upon properties and behavior of NWs.

Kinking in Nanowires: General Information and Kinking Mechanisms

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Most often, kinking is observed during the synthesis of NWs by any method that involves one-dimensional growth using metal catalyst nanoparticles (chemical vapor deposition, laser ablation, molecular beam epitaxy, etc.). In these types of methods, gaseous precursors are introduced into the reactor either in molecular or atomic form and are gathered and decomposed (for a molecular precursor) on metal nanoparticles. Upon supersaturation of the catalyst particles, the precursor components start to form a crystal seed in the substrate–nanoparticle interface. Continuous supply of precursor components from the gas phase to the solid crystal that grow beneath the particles leads to the formation of NWs with diameters defined by the size of the catalyst. Depending on whether the catalyst is in the liquid or solid state, the growth will follow either the so-called vapor–liquid–solid (VLS) (37,48,60,61) or vapor–solid–solid (VSS) (60−62) mechanisms. The growth direction of NWs is closely related to the surface free energy, (51) since NWs preferentially grow in the direction that minimizes it. The growth direction can change during the synthesis due to changes in some of the key parameters in the system, such as pressure, (42,43,45,47,48,63−65) temperature, (49,50,52,56,64) process time or rate, (41,45,52,63) chemical composition of the atmosphere in the reactor, (41) etc. Although even the simple interruption of the growth process can lead to kinking, (41) typically a change in several parameters is needed. Kinks may appear also as a result of stochastic processes related to the interplay between the surface energies of the NW facets and the interface energy between the catalyst and NW. (37,58,61,62) Often, a mixture of straight and kinked NWs appears in the same sample. (51)
Depending on conditions, the whole kinked NW may have the same crystallographic orientation, (41,66) or kinking can involve a change in crystallographic orientation. (67) In the latter case, kinking occurs mostly between growth directions that are common for the 1D forms of a given material. (60) For instance, the main growth directions for Si NWs are ⟨100⟩, ⟨110⟩, ⟨111⟩, ⟨112⟩, and ⟨113⟩, depending on various factors. (68) For the most widespread Au-catalyzed VLS growth, ⟨111⟩ growth is observed for diameters >40 nm, ⟨110⟩ for diameters below ∼10 nm, and a mixture of ⟨112⟩ and ⟨111⟩ for diameters ranging from ∼10 to 40 nm. This is valid for both epitaxially (68) or nonepitaxially (69) grown Si NWs. VSS-grown Cu-catalyzed Si NWs favor the ⟨110⟩ direction, with occasional growth along ⟨111⟩. (70) Al-catalyzed Si NWs grow epitaxially along ⟨111⟩, normal to the {111} Si substrate surface; (71,72) ⟨110⟩ Al-catalyzed Si NWs can grow epitaxially on {110}-oriented Si substrates. (73) For Ga-catalyzed VLS growth, (74) all ⟨110⟩, ⟨112⟩, ⟨111⟩, and ⟨100⟩ growth directions have been reported. (75) For In-catalyzed growth, the variety of Si NWs growth directions include ⟨100⟩, ⟨111⟩, ⟨112⟩, and ⟨113⟩, depending on the NW diameter. (76) When there are multiple growth directions that are nearly equally favorable, small changes in the growth conditions can shift the balance between them. (77,78)
NWs can have single (56,79) or multiple kinks (multikinked NWs). (79) In the latter case, periodically modulated structures sometimes referred to as zigzag (52,78,80) (Figure 1a–c), sawtooth, (81,82) or even wormy (64) (Figure 1d) NWs may appear. The mechanisms responsible for periodic kinking may vary for different materials and conditions. For instance, anisotropic sawtooth structures observed for wurtzite ZnO, ZnS, and CdSe were explained by the equilibrium crystal surface structure. (81) The formation of sawtooth-type Si NWs in the VLS process was linked to a periodic shrinking and expansion of the liquid catalyst, induced by the interplay between surface energy and geometry of NWs and the catalyst droplet. (52) Sawtooth and zigzag morphologies in GaSe NWs supposedly related to the participation of Ga in the catalyst. (52) A liquid Ga catalyst droplet has a lower surface-interfacial energy compared to Au, and its shape is more sensitive to perturbations during growth, which results in a constant change in growth direction that leads to formation of zigzag and sawtooth structures. The authors of ref (52) conclude that liquid droplets are perturbed by the combination of surface-interfacial energy, thermal energy, gas flow, or other mechanical motions.

Figure 1

Figure 1. (a,b) Bright-field TEM images and corresponding electron diffraction patterns for zigzag ZnO nanostructures. (c) Dark-field image showing periodic strain inside a zigzag ZnO nanobelt. Adapted from ref (78). Copyright 2004 American Chemical Society. (d) TEM image of a Si NW grown using 2.5 mTorr disilane acquired along the [110] azimuth. Arrows indicate segments grown at different temperatures. The scale bar is 250 nm. Adapted from ref (64). Copyright 2009 American Chemical Society.

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Kinking often involves dropping of or displacing the metal catalyst. (59,61,83) As observed by Dick et al. (57) for heterostructured NWs, once the nucleus of a new material is formed on the already grown part decorated with the catalyst, further growth can displace the catalyst asymmetrically. Thus, the NW gets kinked or even folds over to grow backward (Figure 2). Furthermore, Sivaram et al. (84) attributed the appearance of kinked Ge NWs to the rapid expulsion of Ge atoms from the solid AuGe catalyst, as has been also observed previously. (85)

Figure 2

Figure 2. Bright-field TEM images recorded during the growth of Ge on GaP NWs by UHV–CVD. Images represent different wires. (a) After the Au particle becomes saturated with Ge, nucleation is observed as a small compact Ge crystal at the boundary between GaP, Au, and the vacuum. This brighter nucleus is indicated by an arrow. The horizontal lines are stacking faults. (b) As the nucleus enlarges, the Au particle is pushed horizontally across the GaP surface. Ge does not form a layer on the GaP. (c) As the growth proceeds, a new growth front is visible between the Ge and Au, apparently a different (111) interface. (d) When the particle reaches the edge of the GaP, the growth may continue outward from the wire or may wrap around the top. Adapted from ref (57). Copyright 2007 American Chemical Society.

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Kinks are often accompanied by crystallographic defects such as twin boundaries. Characteristic examples are the kinking of non-centrosymmetric crystalline NWs during their growth along the polar axis (66) and the kinking of Si NWs that involves segments of ⟨112⟩ axial orientation. (83,86) He et al. (60) demonstrated that most turning angles in kinked Si NWs can be considered as various combinations of different types of {111} coherent twins that coexist at the transition regions between different segments. Thicker NWs have a higher concentration of defects, leading to situations where a mixture of thin straight and thick kinked NWs is obtained from the same material during a synthesis. (62) For instance, inclined defects may affect the geometry of NW, (58) making the change in growth direction easier. There is also a so-called coherent kinking that does not have defects around the kink. (37,58,64) For example, coherent kinking is observed in Ge NWs for diameters at which ⟨111⟩ and ⟨110⟩ VLS growth orientations are almost equally favored, governed by the balance of surface/interface energies. (66,87,88) Coherent kinking from the vertical [111] orientation to an inclined ⟨111⟩ axis can be caused by an error in the sidewall facet development as the cross section area of the NWs is rapidly reduced, while the catalyst droplet “lifts off” the substrate surface. (88,89)
Surface adsorbates can alter the NW sidewall energies and promote kinking, as has been observed for the Au sidewall wetting during VLS growth of Si NWs. (64) According to Filler et al., (67) kinking of Si NWs and subeutectic VLS growth of Ge NWs (84) (using a Au catalyst in both cases) is strongly influenced by hydrogen adsorption on the sidewalls of NWs.
Generally, kinking in the VLS process is closely related to the stability of the triple boundary between the liquid catalyst droplet, the solid NW, and the gas phase. (57) In the dynamic model of the VLS process developed by Schwarz and Tersoff, (90,91) the growth is controlled by the balance of capillary forces acting on the triple boundary line, combined with the difference in chemical potential between the liquid and solid phases. They propose that many of the complex morphological phenomena observed during NW growth can be explained by the interplay of only three elementary processes: facet growth, droplet statics, and the introduction of new facets. Their model agrees well with most of the experimental data related to NWs and kinking.
Although kinking is observed mostly in VLS and VSS processes, kinks can occur also during catalyst-free NW synthesis. For instance, Geaney et al. (49) observed kinking in Ge NWs synthesized via the catalyst-free metal–organic vapor-phase route. The authors attributed these kinks to the interplay between the NW’s tendency to grow along a preferred crystallographic axis and defects perpendicular to the growth direction. These catalyst-free (seedless) synthesis processes are less understood and have been utilized less frequently than seeded methods so far. Moreover, sometimes fundamentally different approaches are used, such as anisotropic etching of the substrate. (92) A more detailed description will be given in the next section.

Synthesis of Kinked NWs: Critical Parameters

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There are several different strategies used to obtain kinks in NWs. The most common approaches rely on changing certain parameters during the synthesis─mainly temperature and pressure. Modulations in precursor composition or molar flow rate are also used rather often. Other methods are based on tweaking some characteristics prior to synthesis (e.g., catalyst particle size, choice of substrate). It should be also noted that the growth parameters are not necessarily independent of each other and modification of one may influence the other. In this section, we briefly overview chosen works that involve synthesis of kinked NWs in order to highlight the most common factors and quantitative parameters responsible for kinking during growth.

Temperature

The growth direction of NWs is known to depend on the temperature in the reactor. (42) If the temperature is varied during the synthesis, it may result in a change of the growth direction of an already growing NW. Therefore, it is fair to say that the temperature is probably the simplest and most straightforward parameter that can be tweaked in order to achieve or avoid kinking. However, the mechanisms responsible for kinking at certain temperatures may be not that obvious. Two distinct scenarios have been reported in the literature: depending on the system and method, kinking may happen either at “lower-than-optimal” or “higher-than-optimal” growth temperatures. To understand the role of temperature in the formation of kinks, it is important to briefly discuss a few thermal aspects of the VLS growth of NWs in general. According to the logic that is reflected in the name of the VLS mechanism, NWs are expected to grow by the incorporation of material from the vapor into the growing NW via a liquid catalyst, commonly a low-melting-point eutectic alloy. However, the growth of NWs at temperatures below the eutectic temperature with either liquid or solid catalysts at the same temperature has often been observed. (85) The catalyst state was shown to be dependent on the growth pressure and thermal history, which has been explained by the kinetic enrichment of the eutectic alloy composition. (85) At temperatures “lower-than-optimal”, NW growth becomes unstable and prone to kinking, as the process is not perfectly epitaxial. (93,94)
A good example of subeutectic growth is given in the study performed by Kim et al., (50) where the authors exploited the growth of Ge NWs on Si (111) substrates by CVD with Au nanoparticle catalysts at different temperatures (Figure 3). They found that for temperatures above 270 °C, the grown NWs are straight and vertically aligned (Figure 3a–d). When the temperature is decreased to 260 °C, the NWs start to kink (Figure 3e), while the growth is terminated at temperatures below 250 °C (Figure 3f). The authors suggest that such an abrupt change in the NW morphology indicates that a rapid solidification of the catalyst occurs at 260–250 °C, because the liquid phase cannot be maintained at such a low temperature. This indicates that kinking is achieved only within a very narrow temperature range, in which the transition between the solid and liquid state of the catalyst is expected. Multiple kinks and tortuous morphology were reported (62) for ⟨111⟩-oriented Ge NWs, synthesized by low-temperature (in the range of 300 to 375 °C, below the eutectic reaction in the Ni–Ge system) VSS growth, using a NiGe catalyst. In another work by Kim et al., (86) kinked Ge NWs have been successfully formed on a Si substrate at subeutectic temperatures by a purging/nucleation process. The nucleation was performed at 350 °C for 2 min followed by NW growth for 20 min at a subeutectic temperature of 300 °C. Multiply kinked NWs were grown, and the segment length was controlled by growth time. According to the authors, the Au catalysts were in the liquid state at the subeutectic temperature, and thereby, the growth of Ge NWs still proceeds under the VLS mechanism. Their finding is in a good agreement with Hillerich et al. (56) who showed that increasing the temperature increases the number of straight NWs.

Figure 3

Figure 3. Field emission SEM images of Ge nanowires on GeBSi substrates grown at (a) 300 °C, (b) 290 °C, (c) 280 °C, (d) 270 °C, (e) 260 °C, and (f) 250 °C for 20 min. The arrow points at the kink in the NW. Scale bar = 500 nm. Adapted from ref (50). Copyright 2011 American Chemical Society.

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On the other hand, “higher-than-optimal” temperatures may induce a number of deviations from regular growth, including significant radial overgrowth manifested as NW tapering, (95,96) undesirable shell structures, (97) and compositional nonuniformity along the length of ternary NWs. (98) The density of crystallographic defects may also increase with growth temperature. (99) Already back in 1968, Wagner et al. (100) demonstrated that if temperature is abruptly increased, it may induce the displacement of the droplet, leading to the formation of kinks or branches. Since then, kinking as a result of increased temperature has been demonstrated in numerous studies. Li et al. (37) studied the morphologies of kinked Ge NWs synthesized in a cold-wall CVD chamber using Au catalysts and found up to a 6-fold (depending on diameter) increase in the fraction of kinked NWs as the growth temperature increases from 390 to 420 °C. Peng et al. (52) synthesized GaSe NWs using Au-catalyst-assisted VLS at relatively high temperatures (800–900°). For both synthesis temperatures, the authors found multiply kinked zigzag and sawtooth NWs, with kinking angles that were uniformly 120°. However, the relative yield of kinked NWs was significantly higher at higher temperatures. Song et al. (101) observed kinking near the base of epitaxial Ge NWs grown by CVD at any temperature in the range required for synthesis (from 300 to 380 °C). However, the kinking was more prominent for NWs grown at higher temperatures. Geaney et al. (49) also observed a significant increase in the relative proportion of kinked Ge NWs to straight ones, when the temperature of the synthesis was increased. However, it is important to note that they used metal–organic chemical vapor deposition (MOCVD) without a catalyst; therefore, there are no eutectics involved. Moreover, the NWs were periodically twinned, which may be another important factor in relation to kinking. A link between kinks and stacking fault content in the NWs was found. The interplay between the preferable NW growth direction and the defect orientation was found to dictate the morphology of these NWs. Growth-direction-driven large-angle kinks prevailed, while the occurrence of defect-driven small-angle kinks was small.
Finally, we would like to note that the measurement of the growth temperature is often performed indirectly, and the approach may vary across different growth systems. These aspect should be taken into account when interpreting and comparing results from the research community.

Pressure

Pressure (both total pressure (47,48) and precursor partial pressure (73)) is another common factor that influences the growth direction of NWs and is often varied during the synthesis in order to introduce kinks.
Already in relatively early studies dedicated to the synthesis of Si NWs by the Au-catalyzed VLS process using silane (SiH4) as the Si source gas, it was shown that decreasing the partial pressure of silane leads to an increase in NW diameter and reduces the amount of growth defects. (93) Later, Lugstein et al. (47) studied the influence of silane pressure on the epitaxial growth of Au-catalyzed Si NWs on Si(111) via a VLS process at 500 °C. They found that at 3 mbar, the NWs grow preferentially in the ⟨111⟩ direction, while at 15 mbar, the ⟨112⟩ direction is favored. When the pressure was abruptly switched during the synthesis, NWs that were already growing kinked and changed their growth direction without changing their diameter. The same year, Schmid et al. (102) also reported a growth direction change for epitaxial Si NWs grown on Si (111) substrates by CVD when they kept total reactor pressure constant but varied the partial pressure of silane. Shortly afterward, their results were confirmed with the same material and conditions by Hyun et al. (48) Pressure-controlled kinking between the [111] and [211] growth directions was obtained with an angle between straight segments of 62° (Figure 4). These results clearly demonstrate that kinking in Si NWs can be introduced in a well-controllable manner.

Figure 4

Figure 4. (a) TEM image of a kinked Si NW grown in the two-step process. The (111) lattice planes of a [111] growth direction wire have grown continuously within the [211̅] wire. (b) Schematics of the crystallographic relationship between growth directions of a kinked Si NW. Reprinted with permission from ref (48). Copyright 2009 IOP Publishing.

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Schmid et al. (102) also studied kinking in silicon NWs grown by VLS on Si(111), using an Au catalyst and silane as a precursor. They performed two sets of experiments. In the first set, the SiH4 partial pressure was varied between 50 and 400 mTorr while keeping the temperature constant. In the second set, the temperature was varied from 470 to 600 °C at constant pressure of 80 mTorr. For the first set, it was found that at lower pressure the preferred growth direction is vertical. (111) At higher partial pressures, increasingly more NWs kinked to one of the other three possible directions of the ⟨111⟩ family. For the second set, kinking increased significantly with temperature.
Another study dedicated to kinking of Si NWs grown from Au seeds on a Si(111) substrate was performed in the same period by Madras et al. (64) However, as compared to the above-mentioned studies, here the authors used disilane instead of silane. In addition to varying the pressure, the influence of the temperature on the growth process was also exploited. They found that Si NWs growing perpendicularly to the substrate can be induced to kink away from [111] by either reducing the substrate temperature (from 500 to 400 °C) or increasing the disilane pressure (from 0.2 to 10 mTorr). An important finding was also that for growth at 400 and 500 °C, the NWs kinked mostly toward another ⟨111⟩ direction at lower disilane pressures and preferred the ⟨112⟩ direction at higher disilane pressures. The authors explained this observation to the inability of Au to wet the NW sidewalls at the higher silane pressure, causing the initially [111]-oriented NWs to switch to a lower-energy ⟨112⟩ growth axis, which agrees with the results reported in ref (47).
Later, Shin and Filler (103) studied kinking of Si NWs in a silane atmosphere further at different pressures and temperatures. They found that the hydrogen originating from the Si2H6 precursor is responsible for kinking. The authors continued their study, (67) revealing the interplay between growth orientation, defect propagation, and surface chemistry. In the latter work, kinking was obtained by switching between two growth conditions that involve changes both in pressure and temperature:
(1)

Condition I: 5 × 10–4 Torr Si2H6 and 410 °C.

(2)

Condition II: 2 × 10–4 Torr Si2H6 and 490 °C.

The authors observed multiple degenerate kink directions as shown in Figure 5.

Figure 5

Figure 5. (a) Substrate temperature and disilane pressure as a function of time for ⟨211⟩ → ⟨111⟩ kinking for conditions I and II. SEM images along the [011̅] direction of Si NWs showing a ⟨211⟩ → ⟨111⟩ kinking: (b) [211] → [111̅], (c) [121] → [111̅], and (d) [112] → [1̅11]. (e) Substrate temperature and disilane pressure as a function of time for ⟨211⟩ → ⟨211⟩ kinking. SEM images along the [01̅1] direction of Si NWs showing ⟨211⟩ → ⟨211⟩ kinking: (f) [211] → [121̅], (g) [121] → [211̅], and (h) [112] → [1̅12]. Both types of kinked NWs have [111] segments at the base and are grown for 10 min under condition II. The axial positions where condition I is initially applied are indicated with dotted lines at the lower part of SEM images. Scale bars are 100 nm. Adapted from ref (67). Copyright 2014 American Chemical Society.

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A more recent study on the kinking of Si NWs grown in silane atmosphere was conducted by Hainey et al. (73) This time, Al catalysis was used instead of the more traditional Au, and the NWs grew in the ⟨110⟩ and ⟨111⟩ directions. The authors tested the effect of hydrogen partial pressure by varying the reactor pressure from 100 to 500 Torr in 100 Torr increments. At 100 Torr, NW growth was nonepitaxial, resulting in all or at least most of the NWs being extensively kinked. This may be due to the vapor–solid deposition of amorphous silicon on the tip and sidewalls of the NWs during synthesis. The amorphous coating may destabilize the Al catalyst/NW interface during the growth, promoting the kinking. Kinking was observed for the majority of NWs also at 200 Torr, especially at the tips of the wires. This was explained by catalyst destabilization occurring at lower pressures. With increasing the pressure to 300 Torr, significant kinking was still observed, but the yield of vertical wires increased further. However, starting from 300 Torr and above (up to 500 Torr), the amount of kinking did not significantly change further, suggesting that this pressure is sufficient for suppressing the rate of vapor–solid deposition of silicon.
Multiple-pressure modulation was recently described by Sun et al., (65) for Au-catalyzed Si NWs grown in a hot-wall CVD reactor at 450 °C and a total pressure of 40 Torr using a silane precursor, H2 as a carrier gas, and B2H6 (100 ppm in He) as the doping precursor. Kinking was introduced by temporally decreasing the pressure to 3.5 Torr for 10 s followed by restoration of initial value. Pressure modulations was repeated every 2–4 min for a total of 10 cycles. The resulting kinking probability was approximately 50% for a 100 nm diameter ⟨211⟩ NW.
Xu et al. (45) demonstrated an exceptional degree of control in the formation of U-, V-, and W-shaped kinked Si NWs in Au-catalyzed VLS growth. NWs were grown at 40 Torr and 460 °C. To introduce one 120° kink, the growth was paused for 15 s to introduce kinks by rapidly evacuating the chamber to the lowest pressure and shutting down the gas lines. The procedure was repeated for a sufficient amount of times to obtain the required geometries.
It should be noted that although a modulation of global process parameters such as precursor partial pressure or substrate temperature is a simple way to achieve kinking, multiple aspects of NW morphology (surface defects, tapering, etc.) can be undesirably impacted by this approach. (104−108) Therefore, alternative methods are constantly being developed and are actively studied as will be shown further in this Review.

Vapor/Gas Composition

The composition of the atmosphere in the reactor during a VLS growth process can be modulated in order to achieve certain morphologies and more complex compositions such as heterostructured NWs. This can also lead to kinking as a result of changes in vapor or gas composition, especially when the precursor is modulated. Moreover, the location of the kink introduced by this method can be controlled with high accuracy.
Schwarz et al. (77) reversibly switched the orientation of Au-catalyzed Si NWs during in situ TEM growth by introducing or ceasing a small flow of O2, adapting a method published earlier. (109) According to the authors, oxygen changes the relative energies of {111} and {113} facets so that either one or the other growth direction is favored. Joyce et al. (110) reported a transition from vertical to kinked GaAs NWs by increasing the ratio of arsenic (group V) to trimethylgallium (group III precursors) in Au-catalyzed metal–organic CVD synthesis.
Tian et al. (41) utilized the so-called “nanotectonic” approach for synthesis of two-dimensional kinked Si NW chains using the Au-catalyzed VLS process. The idea behind nanotectonics is the spontaneous or directed assembly of nanoscale building blocks into organized structures. (111) Similarly to metal–organic framework materials, the authors created a secondary building unit (SBU) consisting of two straight single-crystalline arms connected by one fixed 120° angle joint (Figure 6a). SBU formation procedure includes three main stages:
(1)

Axial growth of a straight NW arm segment.

(2)

Purging of gaseous reactants to suspend NW growth and induce perturbation.

(3)

Supersaturation and initiation of NW growth following reintroduction of reactants.

Figure 6

Figure 6. (a) Schematic of a coherently kinked NW and SBU containing two arms (blue) and one joint (green). The multiply kinked NWs (middle panel) are derived from the corresponding 1D NW by introducing the joints at the locations indicated by the dashed lines in the upper panel. Subscripts c and h denote cubic and hexagonal structures, respectively. (b) Schematic illustrating the key stages of kink formation. Arrows 1–4 denote purge, reintroduction of reactant, joint growth, and subsequent arm growth, respectively. (c) SEM images of the kinked Si NW (upper) grown with periodic 15 s purges. Inset: SEM image of a multiply kinked 2D silicon NW with equal arm segment lengths. The arrow indicates the location of the catalyst. (d) TEM image of a single kink with crystallographic directions and facets indicated by arrows and dashed lines, respectively. The green (I) and blue (II) squares correspond to high-resolution images in the two last panels. Dashed lines and solid arrows indicate crystallographic planes and growth directions, respectively. Reprinted with permission from ref (41). Copyright 2009 Springer.

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The concentration of the silicon reactant dissolved in the nanocatalyst drops during purging and then reaches a maximum upon supersaturation.
The kink formation is shown schematically in Figure 6b. Arrows 1–4 indicate purge, reintroduction of reactant, joint growth, and subsequent arm growth, respectively.
SEM images of kinked silicon NW with a 150 nm diameter, (upper) grown with periodic 15 s purges, are shown in Figure 6b. The inset shows an SEM image of a multiply kinked 2D silicon NW with equal arm segment lengths. The arrow indicates the position of the nanocatalyst. TEM images of a single kink (Figure 6d) revealed no twin defects or stacking faults, confirming that the entire arm–joint–arm junction has a single-crystal structure. Steps (1) to (3) can be repeated to bond a number of SBUs, resulting in a two-dimensional chain structure.
Musin and Filler (104) demonstrated that the introduction of a methylgermane ((GeH3CH3), MG) to a growth environment of Ge NWs (GeH4/H2) can change the growth direction and lead to kinked superstructures with controllable segment lengths and angles. Ge NWs grown using only the traditional GeH4/H2 chemistry always yield a ⟨111⟩ orientation. The growth, however, can be temporarily switched to ⟨110⟩ and then back to ⟨111⟩ if MG is introduced and removed from the growth environment at temperatures below 425 °C. Authors repeated the procedure multiple times obtaining complex multikinked structures. Examples of ⟨111⟩/⟨110⟩, ⟨110⟩/⟨110⟩, and ⟨111⟩/⟨111⟩ superstructures grown at 325 °C with a corresponding schematic of each kink type is shown in Figure 7. Kinked NWs remained single-crystalline, and no twin defects or stacking faults were observed near the kink. MG also created a passivating layer on the sidewalls of NWs, preventing tapering and significantly expanding the process window for Ge NW growth from hydride-based precursors. At temperatures above 475 °C, no deviation from ⟨111⟩ growth was observed even in the presence of MG.

Figure 7

Figure 7. SEM images of (a) ⟨111⟩/⟨110⟩, (b) ⟨110⟩/⟨110⟩, and (c) ⟨111⟩/⟨111⟩ Ge NW superstructures grown at 325 °C by introducing and removing MG during synthesis. An asterisk indicates the locations where transition did not occur as planned. Schematics for each growth direction change are shown below each corresponding superstructure with the smallest deviation angle labeled. Bolded sidewalls in the schematic for (b) denote that the diamond cubic lattice dictates that neighboring ⟨110⟩ segments of the ⟨110⟩/⟨110⟩ superstructure cannot lie in the same plane. Adapted from ref (104). Copyright 2012 American Chemical Society.

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Wang et al. (112) demonstrated that by changing the catalyst composition and shape in situ, it is possible to switch the growth direction of InP NWs between ⟨100⟩ and ⟨111⟩ in the VLS process, in a controllable manner. The process started with the production of vertical ⟨100⟩ InP NW arrays on a Au–In catalyst, under trimethylindium (TMI) and phosphine (PH3) flow. TMI is important for maintaining the In content in the catalyst droplet and avoiding the destabilization of the droplet’s shape. Then, TMI was switched off, while PH3 flow was kept on. The wires continued to grow while the In fraction in the droplet gradually decreased below the supersaturation level required for NW growth. For In depletion times exceeding 1.5 min, the growth direction of NWs switched from ⟨100⟩ to ⟨111⟩. Switching occurred uniformly across the entire substrate. Next, the authors demonstrated the ability to switch the growth direction back from ⟨111⟩ to ⟨100⟩. After growing in the ⟨100⟩ and ⟨111⟩ directions consecutively, the PH3 was switched off while TMI was kept on for different periods. As a result, the growth was interrupted, and the In was absorbed by the catalyst droplet from TMI decomposition. Then, the PH3 was switched back on before cooling down.
Zhang et al. (113) reported a somewhat similar approach to control the growth direction of defect-free zinc blende InAs NWs in molecular beam epitaxy (MBE) by carefully controlling the In content in the Au catalyst. When an In source is introduced to the reactor, In atoms diffuse into the Au catalyst to form Au–In alloy nanoparticles. Au–In particles with lower In content tend to initiate vertical ⟨0001̅⟩ NW growth, while a higher indium concentration induces the epitaxial growth of ⟨001̅⟩ NWs.
Shin and Filler (103) controlled the growth direction of VLS-grown Si NWs via surface chemistry. Kinking was achieved upon the addition of hydrogen on the NW’s sidewall. The authors demonstrated that covalently bonded hydrogen atoms are responsible for the commonly observed growth transition in Si NWs from ⟨111⟩ to ⟨112⟩.
Kinking can be achieved also when different segments of an NW have different chemical composition. For instance, Dick et al. (57) studied the growth of various heterostructured NWs (combinations of the III–V materials GaAs, GaP, InAs, InP, AlAs, as well as Si) by changing the precursor composition during synthesis by Au-catalyzed low-pressure metal–organic vapor-phase epitaxy (LP-MOVPE). No annealing was performed between the synthesis of segments with different composition. The hydrogen was used to flush the reactor between growth of the two different compositions to remove precursor residuals. The authors highlighted two typical growth categories. In some systems (GaP on InP or GaP on Si), the NW growth continued in the same crystal direction after introduction of the second material (Figure 8a). In other cases (InP on GaP, Si on GaP, InAs on GaAs, InAs on AlAs), NWs kinked (Figure 8b) or even wrapped around themselves and grew backward (Figure 8c). Note that the same pair of materials may or may not produce kinked nanostructures, depending on which material was grown first (e.g., GaP/Si). In general, the authors did not observe a mixture of straight and kinked morphologies─mostly either only straight or only kinked morphologies (often approaching 100%)─were observed for a given material combination for all growth conditions. Dick et al., as well as Ross et al., (43) speculated that for heterostructured NWs that do not kink, the second material grows in a layer-by-layer mode, while for the ones that do kink, the second material grows from a compact nucleus.

Figure 8

Figure 8. TEM images of heterostructured kinked NWs. (a) Si → GaP NW. The Si part is single-crystalline, while the GaP part exhibits extensive stacking faults perpendicular to the growth direction. (b) GaAs → InAs NW. The interface between materials is indicated by a line. (c) GaP → InP NW. The InP wraps along the top of the GaP and grows backward. Adapted from ref (57). Copyright 2007 American Chemical Society.

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A drastic change in the growth direction of heterostructured NWs was observed also by Paladugu et al., (59) for InAs segments grown on GaAs NWs. InAs/GaAs heterostructures were grown using Au nanoparticles of ∼30 nm diameter as catalysts, in a horizontal-flow metal–organic chemical vapor deposition (MOCVD) reactor. The growth pressure and temperature were 100 mbar and 450 °C, respectively. GaAs NWs were grown on a {111}B GaAs substrate by flowing trimethylgallium (TMG) and AsH3 into the chamber. Then, InAs NW sections were grown on the GaAs NWs by replacing the TMG flow with a TMI while maintaining the AsH3 flow. It was found that thinner NWs tend to bend more than thicker ones. SEM and TEM studies revealed that in the case of thinner NWs, the Au particle has moved away from the tip region down the NW, while for the thicker ones, a sideward movement of the Au particle was observed (similar to the process of Figure 2). Since during GaAs growth the Au particles remained on top of the NWs, (98) these Au movements must take place during the InAs growth period. In their work, the authors provide an in-depth analysis of the mechanisms responsible of the observed behavior and results. Shortly, they conclude (59) that the axial growth failure of InAs on GaAs NWs is explained by the lower Au/GaAs interfacial energy as compared to Au/InAs. In general, when an axial NW heterostructure is grown by the VLS mechanism with two materials A and B, through a catalyst particle C, if the interfacial energy between A and C is higher than that between B and C, the axial growth of A on B fails, whereas the axial growth of B on A is feasible.
Svensson et al. (63) studied the kink formation in InAs–InP heterostructured NWs grown by chemical beam epitaxy (CBE) on InAs (111)B substrates, using TMI, tertiarybutylarsine (TBAs), and tertiarybutylphosphine (TBP) as precursors. The authors studied the impact of different parameters (pressure, NW density and diameter) on the probability of kinking and found it to be always dependent on the amount of available In in the system. Kinking always occurred at the InP → InAs interface. By tweaking the growth parameters, the authors were able to increase the yield of kinked NWs up to 68%.
Kinking in hybrid NWs was observed also by Dayeh et al. (58) for VLS-grown Au-catalyzed Ge–Si NWs. The authors utilized precise sequencing of the gas precursor partial pressure and temperature upon transitioning from Ge to Si during growth. This resulted in pure Ge and pure Si NW segments with dimensions suitable for a detailed structural study and potential applications. A 19.5° change in growth direction was observed between segments at certain growth rates, which was attributed to the formation of twin boundaries. Ref (58) contains a detailed high-resolution structural analysis and atomistic modeling of the nucleation and propagation of stacking defects throughout single NWs, greatly contributing to the understanding of the NW kinking mechanisms.

Size Factor (Catalyst Manipulation)

In numerous studies, a clear connection between the diameters of the NWs and the probability of kinking was observed. Considering that the NW diameters in both the VLS and VSS processes are directly related to the size of the catalyst particles, the proper choice of the catalyst particle size is another factor that can be used to manipulate the amount of kinked NWs in a sample.
Thombare et al. (62) reported a strong dependence of the morphology on diameter of Ge NWs synthesized by low-temperature VSS growth using a NiGe catalyst. NWs with diameter below 25 nm were straight and grew preferentially in the ⟨110⟩ direction. NWs with diameter exceeding 25 nm were ⟨111⟩-oriented, had a high density of defects, and exhibited multiple-kinking, resulting in a tortuous shape. The authors demonstrated that kinking in thicker NWs involves multiple twinning events facilitated by the slow growth and anisotropic catalyst/wire interfaces, typical for VSS growth. Such effects are expected also for other VSS systems where the NWs exhibit different morphologies. According to TEM observations, NWs with a tortuous morphology had catalyst particles larger than 25 nm, while particles on straight NWs were smaller than 25 nm. This clearly indicates that the diameter of catalyst particles is the deciding factor that can be chosen correspondingly to either induce or avoid kinking.
Hillerich et al. (56) introduced conditions that decreased the Au droplet volume during the growth of a pure Si NW in the UHV TEM at 645 °C in 1.1 × 10–6 Torr disilane. Gold is highly mobile on the silicon surface, and its migration leads to Ostwald ripening: Au atoms detach from a droplet, diffuse across the surface, and rejoin the same or another droplet. As the droplets shrink, the corresponding NW diameters also decrease, and inclined sidewall facets are formed. This sequence of events results in a single kink without other defects. The authors speculated that if a droplet is “too small” for the initial area, it grows a segment that decreases in diameter resulting in inclined (111) sidewall facets. The droplet then moves to an inclined facet, and growth continues in a kinked ⟨111⟩ direction. Once kinked, the new diameter is correctly matched to the droplet volume and contact angle, and no further kinking is required. This scenario is summarized in Figure 9.

Figure 9

Figure 9. Chosen frames of Si NWs growth inside UHV TEM under conditions of decreasing droplet volume. The electron beam is parallel to ⟨110⟩. (a) Migration of Au from the droplet (Ostwald ripening) reduces the droplet volume. The NW diameter decreases via formation of inclined (111) facets. (b) A different NW with a larger inclined facet. (c) Schematic diagram showing the outcomes for NW morphology in the case of a mismatch between the droplet volume/contact angle and the NW diameter. Adapted from ref (56). Copyright 2013 American Chemical Society.

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Madras et al. (64) studied Au-catalyzed Si NWs, grown in a disilane atmosphere. They found that the probability of kinking depends on the NW diameter for most experimental conditions (temperature and disilane pressure) except one (500 °C, 0.7 mTorr) as summarized in Figure 10. Moreover, the growth direction after kinking was also found to be dependent on the NW diameter. The smallest (D < 20 nm) Si NWs kinked toward ⟨110⟩, while larger (D > 40 nm) NWs favor either ⟨111⟩ or ⟨112⟩, depending on whether or not growth conditions favor Au wetting the NW sidewalls.

Figure 10

Figure 10. Fraction of NWs that kink away from [111] vs average diameter, D, at the indicated P in mtorr (mt) and T. Adapted from ref (64). Copyright 2009 American Chemical Society.

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A dependence of the kinking probability on the catalyst size and corresponding NW diameter was reported also by Li et al. (37) for CVD-grown Ge NWs using Au catalyst particles with diameters of 20, 40, and 80 nm. The authors observed two distinct, defect-free kinking modes (Figure 11a,b). For diameters of ∼20 nm, the NW growth axis changed from vertical [111] to ⟨110⟩. According to the authors, this observation agrees with the phase field simulations that show a spontaneous transition from ⟨111⟩ to ⟨110⟩ at this diameters. Thicker NWs kinked from [111] to an inclined ⟨111⟩ axis. The authors link this to an error in sidewall facet development that leads to a contraction of the (111) growth facet area as NW grows, resulting in an instability of the Au–Ge droplet at the tip of the NW. (37) Moreover, the probability of kinking at different tested temperatures strongly depended on the catalyst size (Figure 11c).

Figure 11

Figure 11. (a,b) Two types of kinking observed in experiments. (c) Dependence of the kinking probability at different tested temperatures on the catalyst size. Adapted from ref (37). Copyright 2016 American Chemical Society.

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Sometimes the catalyst is applied on the substrate in the form of a thin film. Wacaser et al. (72) examined the supercooled VLS growth of silicon NWs on Si substrates with thin aluminum films. The authors varied the thickness of the Al layer, which clearly influenced the NW diameters, the density of vertically aligned epitaxial NWs, and the density of other growth irregularities, including kinks. It was found that optimal growth conditions for vertically aligned NWs were achieved at a 5 nm Al layer thickness. For thinner Al layers, the density of aligned NWs decreased, and many NWs deviated from vertical growth and kinked.
Wen et al. (70) studied the CVD growth of copper-catalyzed silicon NWs on a copper-coated Si substrate at around 500–600 °C. They used TEM to observe the NW growth mechanism directly and quantify the growth kinetics of individual wires. The authors found that the growth direction of individual NWs is related to the orientation and shape of the catalyst, leading to a rich morphology and extensive kinking (about 50% of the observed NWs).
Potts et al. (83) further advanced the techniques of catalyst manipulation. They demonstrated an unusual kinking method for achieving L-shapes in self-catalyzed InAs and InAsSb NWs, by in situ formation and manipulation of indium droplets obtained either by annealing NWs in vacuum or by direct deposition on the NWs. The authors demonstrated that the size and position of indium droplets can be tuned by changing the annealing time. The indium droplets were then used as seed particles to initiate growth in different crystallographic directions. Depending on the position of the seed, linear or L-shaped nanostructures were obtained. The authors then further investigated the formation of droplets by annealing L-shaped structures. They observed different positions of the indium droplets and different shapes of nanostructures including thicker L-shaped structures, longer branches, and nanobridges. The latter is particularly interesting as the second section of the NW that is formed after the second annealing step grows toward ⟨111⟩, which is a quite untypical direction for self-catalyzed arsenide NWs.
The probability of kinking strongly correlated with the NW diameters for catalyst-free synthesis methods as well. For instance, Ge NWs grown in the unseeded organic vapor-phase-based process at 420 °C were mostly straight with diameters in the range of 7–20 nm and large aspect ratios. (49) A small percentage of the NWs had a larger average diameter (up to 50 nm) and higher diameter variance, with a considerable degree of kinking.

Growth Rate

The degree of kinetic forcing required to induce kinking is different for different materials and therefore can depend on the growth rate, which is often regulated by the precursor flow rate in the chamber. For example, Ge NWs kink at much larger kinetic force than Si, and therefore, Ge NWs grown at T = 300 °C do not kink at growth rates below 5 nm/sec, while Si NWs grown at 400 °C kink already at a growth rate of 1 nm/sec. (64) For III–V materials, the ratio between the molar flow rates of the group V and group III precursors (V/III ratio) can considerably influence the morphology of NWs. (42,110,114,115) For instance, kinking in GaAs NWs was observed in Au-catalyzed MOCVD when a V/III ratio exceeded 90. (110) Dayeh et al. (58) studied the VLS growth of Au-catalyzed hybrid Ge–Si NWs with a precise control over the growth conditions. The authors observed a 19.5° kinking at higher Si growth rates resulting from twin boundary formation. As the twin boundary propagates into the NW, it locally increases its diameter and disturbs the liquid growth seed, increasing the line tension that is restored by switching the growth direction from a ⟨111⟩ to ⟨211⟩. The occurrence of such stacking faults was shown to decrease by reducing the growth rate.

Etching

In a few works, kinked NWs were prepared by etching−a method that is fundamentally different form the VLS and VSS processes. For instance, in a recent study, Chen et al. (92,116) prepared kinked Si NWs by alternating metal-assisted chemical etching. First, patterns were formed from polystyrene microspheres on a (100)-oriented single-crystalline Si substrate through a self-assembly process. Then, Ti and Au films were deposited on Si as catalysts. Next, straight NWs were formed by immersing the Si substrate into etchant A, which consisted of deionized water, H2O2, and hydrofluoric acid. Then, the substrate was removed from etchant A and immersed into etchant B, which contained also glycerol. The etching directions were different in the two etchants, which led to kink formation at the joints of different crystallographic directions. The straight and slanted etching directions were [100] and [211̅], respectively. By repeating the etching procedures A–B–A–B–A, multikinked Si NWs were fabricated, consisting mainly of three straight segments and two segments slanted at an angle of 54.8°.
A similar technique was used by Sandu et al., (117) who synthesized kinked Si superstructures with aspect ratios above 200. They used metal-assisted chemical etching of a Si substrate via a patterned Au mask, as shown in Figure 12. To form the kinks at the desired location, Si etching was interrupted at specific times by submersing the sample in methanol. The angles of the kinks were found to be dependent on the size of the openings in the Au catalyst.

Figure 12

Figure 12. Schematics of the synthesis of kinked Si NWs 200 by metal-assisted chemical etching of Si substrate. First, the Si substrate is patterned with a holey Au mask by polystyrene nanosphere lithography. Second, the nanostructuring of Si follows repetitive etch–quench sequences. The SEM pictures show the k-kinked Si NWs that resulted after five successive etch–quench sequences. Adapted from ref (117). Copyright 2019 American Chemical Society.

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The advantage of the described etching protocols is that they enable to control the number, angle, and location of the kinks via sequential etch–quench sequences.

Summary of modulation parameters

Based on the examined literature, we can draw some conclusions regarding the efficiency of the different methods in obtaining kinked NWs of a desired geometry. Although modulating the temperature during the synthesis is the simplest way to achieve kinks, it is not the most efficient method for the production of kinked NWs if high quality and good control over the location of the kink and the geometry of the kinked NWs is needed. Kinking that occurs when the growth temperature is outside of the optimal range required for straight vertical growth is unpredictable and can be considered more like a random defect. Moreover, changing the temperature of the whole reactor is rather time-consuming. The situation is even more complicated by the fact that the precise control and measurement of the temperature at the sample site with sufficient precision and reliability is not trivial.
Pressure modulation is significantly faster, gives better control, and can lead to a higher quality of the kinked NWs, compared to temperature modulation. However, based on the available literature, it seems that the applicability of precursor pressure modulation is very limited in terms of material choice and so far is used mainly for production of kinked Si NWs. This is also the case for kinking achieved by anisotropic etching that has good control over the process but is limited mainly to silicon.
Regulating the growth rate by controlling the precursor flow is another simple method that, however, can suffer from a compromised quality, as increased flow rates lead to defects like stacking faults. The highest flexibility is obtained when the composition of the gas in the chamber is modulated during the synthesis in a controllable manner, following the well-elaborated recipes available in dedicated literature. Such an approach allows synthesis of high-quality long multikinked NWs, with various geometries and compositions, including heterostructured ones. Gas composition modulation can be combined with other methods when needed, for even more advanced control over the process. Therefore, we can conclude that in most cases, composition modulation is the preferable method for achieving kinked NWs in a well-controllable manner. However, some materials may benefit from the simplicity of other methods, like pressure modulation for Si NWs.
In order to have a better overview of the materials and methods analyzed in the present work, we summarized some relevant data in Table 1. The synthesis method alongside with the parameter that should be modulated to achieve kinking are shown for each material and its corresponding crystal structure. We also added data on the growth directions and kinking angles observed in the studies that were analyzed.
Table 1. Most Frequent Kink Directions in Nanowires
materialstructuremethodmodulation parametergrowth and kink directionsangle deg
Sidiamond cubicVLS (most works)pressure (45,47,48,65,73,102)110 ↔ 11171, 90, 109, 125, 130, 141, 160
chemical etching (92,116,117)pressure and temperature (64,67,103)110 ↔ 112
vapor/gas composition (41,77,103)112 ↔ 111
size factor (56,64,70,72)112 ↔ 110 ↔ 112
111 ↔ 331
Gediamond cubicVLS (most works)temperature, (49,101,104)110 ↔ 11135, 70, 60, 90, 120
seedless MOCVD (49)vapor/gas composition (104)110 ↔ 112
size factor (37,49,62)
GaAszinc blende (ZB)VLS (51,110)vapor/gas composition (110)101 ↔ 110 ↔ 01145, 60, 90
growth rate (110)[111]B ↔ non-[111]B
GaSewurtzite (WZ)VLS (52)temperature (52)11–20 ↔ 2–1–10120
InPZB and ZB/WZ mixVLS (112)vapor/gas composition (112)100 ↔ 11116, 55
InAsWZ ↔ ZBMBE (113)vapor/gas composition (113)000–1 ↔ 00–1no data
size factor (83)
CdSWZVLS (41)vapor/gas composition (41)11–20 ↔ 11–20120
SnO2rutileVLS (123)spontaneous (123)001 ↔ 211123
heterostructurematerial-dependentVLSvapor/gas composition (47,53,58,59,63)material-dependent 

Preventing Kinks

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Considering that for most situations kinking is an undesirable effect, we would like to briefly summarize some of the aspects of the synthesis process that are essential for achieving kink-free NWs.
First of all, growth conditions should be chosen carefully, as often straight kink-free NWs can be achieved only in a narrow range of temperatures and pressures. (93) Two distinct scenarios have been reported in the literature─depending on the system and method, kinking may happen either at “lower-than-optimal” or “higher-than-optimal” growth temperatures. In some studies, (118) a two-temperature growth technique with separate nucleation and growth temperatures was applied to achieve kink-free vertical NWs. Choosing the proper temperature for synthesis does not yet guarantee kink prevention. For instance, Song et al. (101) observed kinking near the base in epitaxial Ge NWs at any temperature in the range required for synthesis (from 300 to 380 °C). However, kinking was more prominent for NWs grown at higher temperatures, indicating that at least minimization of kinking is still possible. Fortuna et al. (42) emphasized the importance of maintaining the stability of the catalyst and the NW–catalyst interface through isothermal growth techniques to avoid kinking and other unwanted defects. Precursor flow rates, as well as composition, should also be kept in the optimal range. For instance, it was demonstrated (110) that a high ratio of group V to group III precursors dramatically improves the quality and purity of GaAs NWs. However, a further increase of the V/III ratio leads to kinking and increases NW tapering.
Since the appearance of kinks was shown to be dependent on the size of the catalyst particle, (62) the diameter of the catalyst should be chosen correspondingly to avoid kinking. Composition of the catalyst is another factor that can play an important role in the growth direction of NWs. Wang et al. (112) demonstrated the high (97%) yield of vertical ⟨100⟩-oriented InP NWs when the Au catalyst droplet was filled with In prior to NW nucleation to the equilibrium composition during VSL growth. Otherwise, the shape of the droplet changes during the nucleation process due to the uptake of In, which disturbs the growth. Prefilling the catalyst with In brought it into equilibrium conditions, and ⟨100⟩ growth was stabilized. If the In content will be too high, the NW will first kink to one side to release excess material and then start to grow. Therefore, optimization is important.
For heterostructured NWs (GaP/Si, GaAs/Ge, and GaAs/S), the interlayer between segments can enable the growth of unkinked hybrid NWs. (56) Dayeh et al. (58) found for the VLS synthesis of Au-catalyzed Ge–Si NWs that maintaining a liquid Au particle with high supersaturation is required to initiate the epitaxial growth of a Si segment on the previously grown Ge NW, to stabilize the Au seed on top of the NW, and prevent kinking.
Finally, considering that kinking is often attributed to interactions at NW/catalyst interface, choosing a catalyst-free method like, for example, MBE over CVD when possible can help achieving straight nanowires. (119)

Properties and Applications of Kinked NWs

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Kinks are expected to influence the properties and behavior of NWs. However, the extent of the impact strongly depends on the type of kink, i.e., whether it involves defects and/or change of crystallographic direction. It is evident that these factors are crucial for the mechanical strength and propagation of phonons or charge carriers inside a NW. Although the number of studies related to kinked NWs is rather large, most of them are dedicated only to synthesis, with far less including measurements of their properties and/or discussion on the applications of kinked NWs. Here, we overview some of the works that go beyond synthesis and include measurements or simulations of some of the properties of kinked NWs.

Mechanical

The mechanical properties of kinked NWs can play an important role in the operation of nano-electromechanical devices. For now, most studies in this direction are done using computer simulations, as experimental measurements are challenging. For instance, Jiang and Rabczuk (55) performed classical molecular dynamics (MD) simulations to demonstrate the application of kinked silicon NWs as nanoscale springs. Spring-like oscillations in a GHz frequency range were actuated in kinked NWs of various aspect ratios, using a procedure similar to the actuation of a classical mass spring oscillator. Using energy spectrum analysis, the authors detected spring-like mechanical oscillations as well as some other low-frequency oscillations. They obtained a dimensional crossover (from one-dimensional to three-dimensional) of the transverse oscillation mode as the aspect ratio of the kinked NWs decreased.
Jiang et al., (54) performed both MD simulations and finite element method (FEM) beam model calculations to investigate the elastic properties of kinked (zigzag) silicon NWs. The Young’s modulus was found to be highly sensitive on the arm length, with a dependence of the functional form Y = c1/(b + c2), where b is the arm length and c1,2 are coefficients that depend on the material and the geometry of the kinks. This dependence was derived via a simplified model that represents the kinked NWs as a series of springs, which replace both the elastic arms and angles. Still, they gave excellent agreement with MD simulation results, demonstrating the spring-like elastic properties of kinked Si NWs.
Furthermore, MD simulations performed by Jing et al. (120) demonstrated that kinked silicon NWs have a much larger fracture strain compared to straight ones, as shown in Figure 13. For the symmetric structure (left diagram), it was found that the fracture stress decreased with the periodic length, while the fracture strain abruptly increased to large values and then stabilized. Note here that L = 0 corresponds to a straight NW. For the asymmetric case (right diagram), the fracture stress did not significantly depend on the arm length Larm in contrast to the fracture strain that increased monotonically with Larm. The authors concluded that kinked Si NWs with a larger fracture strain can be a promising anode material in high-performance Li-ion batteries, since the Si anode can swell by a large factor during initial lithiation.

Figure 13

Figure 13. Fracture stress (left axes, blue dot markers) and fracture strain (right axes, orange square markers) as a function of the periodic length L in symmetric zigzag kinked Si NWs (left diagram) and the arm length Larm in asymmetric ones with Lfixed = 19.5 nm (right diagram). The NW configurations for each case are shown in the insets. Data are from ref (113).

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Chen et al. (92) studied the effect of kinks on the mechanical properties of silicon NWs, using MD simulations, indirectly validated by experiments. The authors found that kinks are weak points in the NW. They also calculated the elastic modulus of kinked NWs, finding it significantly smaller (∼30 GPa) compared to a straight NW (∼106 GPa), in line with the findings of ref (54). Moreover, in the experiments by Chen et al., the kinked NWs fractured at kinks under load.
Finally, Jiang (121) performed a lattice dynamical study of intrinsic out-of-plane twisting vibrations that are self-generated in zigzag-shaped kinked Si NWs to withstand the out-of-plane mechanical instability. The authors explored the effect of kinking on the phonon spectrum of the NWs and obtained an analytical formula for the geometrical (cross-sectional area and the arm length at given temperature) dependence of the twisting amplitude of the kinked Si NWs. The result provides valuable information on the kinking-induced twisting stability of kinked NWs when used as bioprobes for intracellular recording application.

Thermal

Jiang et al. (38) performed MD simulations to investigate the reduction of the thermal conductivity by kinks in silicon NWs. According to their results, the reduction can achieve 70% at room temperature. The thermal conductivity was found to be insensitive to the temperature, since the heat transport is mainly limited by the phonon scattering at the kink interface. Two mechanisms responsible for the reduced thermal conductivity were proposed: (1) the interchange between the longitudinal and transverse phonon modes and (2) the pinching effect, which is a new type of localization for the twisting and transverse phonon modes in the kinked silicon NWs. The authors demonstrated that these kink-induced effects offer a novel and effective approach for modulating heat transfer in NWs, making the kinked silicon NWs a promising candidate for thermoelectric materials.
The first experimental demonstration of the kink thermal resistance was reported in a recent study by Zhang et al. (122) They found a 15–36% reduction of thermal conductivity in kinked boron carbide NWs compared to straight ones, which is up to ∼7 times more pronounced than the effect predicted by Juang et al. (38) in the MD simulations (if the results are normalized to the investigated size accordingly). The authors attributed this effect to the high thermal resistance of the kinks, estimated to be ∼30 times that of a straight wire segment with a length equivalent to that of the kink’s central line. According to TEM studies, the kinked regions were all single-crystalline and defect-free. Therefore, the reduced thermal conductivity cannot be attributed to low a structural quality at the kinks, but it is rather related to the kinked morphology itself. Furthermore, larger diameter NWs were generally found to have higher kink resistance. This might be due to the weaker boundary scattering from the NW surface in larger wires. In addition to defect-free kinks, some NWs had kinks with clear structural defects, which were oriented in parallel to the stacking faults in one of their legs. Surprisingly, the thermal conductivity of such NWs was higher than that of comparable NWs with defect-free kinks but still lower than that of straight wires. This contradicts the well-known fact that defects can scatter phonons and obstruct their transport. In order to get a deeper understanding of the observed phenomena, the authors performed a Monte Carlo (MC) simulation of phonon transport through an “L”-shaped kink and concluded that the highly focused phonons have to undergo scattering to make the turn and enter the opposite leg of a kink. More scattering events in the kink assist the change of phonon direction and thus reduce the kink resistance. These findings provide valuable data for the accurate thermal management of nanoelectronic devices that involve kink-like wire morphologies.
Further insights on the phonon transport in kinked NWs can be found in a recent study of Zhao et al., (40) who performed nonequilibrium MD studies of thermal transport through kinked and straight Si NWs. They confirmed that kinks lead to lower (up to 1.6 times) thermal conductivity for kinked NWs compared to straight ones, by reflecting phonons back into their incoming arms. The decrease in thermal conductivity becomes more pronounced as the arm length increases. However, when heavier isotope atoms were introduced in the kink region, simulations replicated the experimental observation that defects in the kink region can in fact facilitate thermal transport through deflecting phonons into the adjacent arm.

Electronic

Similar to the situation with phonons, kinks are expected to cause resistance on the propagation of charge carriers in semiconducting NWs. However, as was shown by Cook and Varga (39) in their first-principles transport calculations, the conductance strongly depends on the kink geometry. For certain geometries, the conductance of the kinked NWs is close to that of the ideal wire, due to the introduction of resonance states. Moreover, kinked NWs with larger diameters are expected to be better conductors, as there are more conduction channels available and the perturbation induced by the kink has a lesser effect.
There are also experimental studies available. For instance, Tian et al. (41) demonstrated and characterized topologically defined nanoelectronic devices based on kinked semiconductor NWs by combining precisely controlled 120° kinking with additional modulation of the dopant, to modify the electronic properties of NWs in a well-defined manner. Figure 14 represents the characterization of a kinked Si NW with integrated n- and p-type arms that was synthesized by switching phosphine and diborane dopants during the Au-catalyzed VLS growth sequence. The current–voltage (I–V) data (Figure 14a) reveal a current rectification in reverse bias with an onset at a forward bias voltage of ∼0.6 V, consistent with the synthesis of a well-defined p–n diode within the kinked structure. According to the electrostatic force microscopy (EFM) characterization of the kinked p–n NW in reverse bias (Figure 14b), the voltage drop occurs primarily at the p–n junction, localized by the kink during growth.

Figure 14

Figure 14. (a) I–V data recorded from a kinked p–n silicon NW-based device. Inset: SEM image of the device structure. Scale bar is 2 μm. (b) Electrostatic force microscopy image of a p–n diode reverse-biased at 5 V. The atomic force microscope (AFM) tip voltage was modulated by 3 V at the cantilever-tip resonance frequency. The signal brightness is proportional to the NW device surface potential and has an abrupt drop around the kink position. The dashed lines indicate the NW position. Scale bar is 2 μm. (c,d) AFM and scanned gate microscopy (SGM) images of one n+–kink–n+–kink–(n–n+) dopant modulated double-kinked silicon NW structure. The scale bar in (c) is 2 μm. The SGM images were recorded with a Vtip of 10 V (I) and −10 V (II), respectively, and a Vsd of 1 V. The dark and bright regions correspond to reduced and enhanced conductance, respectively. The black dashed lines denote the NW position. Reprinted with permission from ref (41). Copyright 2009 Springer.

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This concept can be extended to the design of NWs with a distinct functionality at sequential kinks. Scanned gate microscopy (SGM) data (Figure 14d) acquired from the area denoted on the atomic force microscopy image (Figure 14c) reveal increased (decreased) NW conductance as the SGM probe with positive (negative) gate potential is scanned across the n-type segment adjacent to the upper-left kink junction. This confirms the integration of an n-type field-effect transistor (FET) at a well-defined and recognizable location of the structure. The absence of a gate response from the lower-right kink junction suggests that the single-crystalline kink structure itself will not alter the electronic transport properties of the NW. The authors stated that their results significantly contribute to the realization of ab initio designed and “self-labeled” 2D NW structures and may initiate applications in the bottom-up integration of active devices in nanoelectronics, photodetector arrays, and multiplexed biological sensors and the development of multiterminal 3D nanodevices.
Utilization of kinked structures in fabrication of FETs was reported also by Shen et al. (79) for In2O3 NWs grown by Au-catalyzed CVD. The authors report n-type conductivity with mobilities exceeding 200 cm2/(V·s), opening a route for fabricating high-performance electronic and optoelectronic devices. The synthesized nanostructures demonstrated ultrafast photoinduced reversible wettability, switching from hydrophobic to superhydrophilic in 14 min.
Potts et al. (83) studied the electrical properties of L-shaped kinked nanostructures grown by the annealing method (see details of Potts et al. above) using two-point electrical measurements. The conductivity of the nanostructures (81 ± 23 S cm–1) was found to be comparable to standard ⟨111⟩B plane self-catalyzed InAs NWs. All parts of the investigated device exhibited an n-type gate response, although the intensity of the response varied significantly.
Gan et al. (123) compared the efficiency of straight and kinked SnO2 NWs in photodetector applications and found a significant enhancement of the local electric field at the kink, which improves the electron–hole separation efficiency and the transmission time of the carrier, thus improving both the photocurrent and response time of the SnO2 NW.

Biological

Nanoelectronic devices made of various nanostructures including NWs offer substantial potential for studying biological systems. FETs based on semiconductor NWs and other semiconducting nanostructures have been extensively studied as biological/chemical (bio/chem) sensors for many years. (53) High sensitivity was demonstrated for one- and two-dimensional sensors. However, most works were focused on planar device designs, while the realization of ultimate point-like and 3D detectors remains challenging. Kinked NWs may help to overcome this limitation.
Superstructures made of kinked NWs have been utilized to fabricate 3D bend-up nanoelectronic probes for recording extra- and intracellular action potentials from single cells and tissues. (44,45,53) For instance, Tian et al., (44) using the synthesis method described in ref (41), presented the concept of synthetic integration of a nanoscale FET (nanoFET), localized by modulating the doping at the tip of an acute-angle kinked Si NW. At this location, nanoscale connections are made by the arms of the kinked NW, and remote multilayer interconnects allow a 3D probe operation (Figure 15). 3D nanoFET probes exhibited electrical conductivity and high pH sensitivity in aqueous solution, independent of severe mechanical deflections. According to the authors, 3D nanoprobes, if modified with phospholipid bilayers, can enter single cells allowing recording of intracellular potentials.

Figure 15

Figure 15. Surface modification and cellular entry. (a) Fluorescence image of a lipid-coated NW probe. (b) Schematics of NW probe entrance into a cell. Dark purple, light purple, pink, and blue colors denote the phospholipid bilayers, heavily doped NW segments, active sensor segment, and cytosol, respectively. (c) Differential interference contrast microscopy images and electrical recording of an HL-1 cell and 60° kinked NW probe as the cell approaches (I), contacts and internalizes (II), and is retracted from (III) the nanoprobe. A pulled-glass micropipette (inner tip diameter ≈ 5 mm) was used to manipulate and voltage clamp the HL-1 cell. The dashed green line corresponds to the micropipette potential. Scale bars are 5 mm. Reprinted with permission from ref (44). Copyright 2010 American Association for the Advancement of Science.

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In another study, (53) the same team discussed the advantages of nanoscale p–n diodes, described in details in their earlier work, (41) in bio- and chemosensing applications. The nanoscale axial p–n junctions were synthetically introduced at the joints of kinked Si NWs. Electrical transport measurements revealed a rectifying behavior with well-defined turn-on in the forward bias, in accordance with expectations for a p–n diode. Scanning gate microscopy measurements demonstrated that the most sensitive region was localized near the kink at the p–n junction. High-spatial-resolution sensing of fluorescent charged polystyrene nanobeads was carried out using p–n diode probes in aqueous solution. Multiplexed electrical measurements with simultaneous confocal imaging demonstrated detection of single nanoparticles. In addition, kinked p–n junction NWs configured as 3D probes demonstrated the capability of intracellular recording of action potentials from electrogenic cells. The probes were functionalized with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Embryonic chicken cardiomyocyte cells cultured on a polydimethylsiloxane (PDMS) substrate were then positioned over an NW probe. Next, the probe was inserted into the cell, and the evolutions of its conductance were recorded successfully, registering the signal from a spontaneously beating cardiomyocyte cell (Figure 16).

Figure 16

Figure 16. Intracellular electrical recording from spontaneously beating chicken cardiomyocytes. (Top) Steady state intracellular recording using 3D kinked p–n nanoprobes from a spontaneously beating cardiomyocyte cell. (Bottom) Zoom of the single-action potential peak from the green-dashed region. Inset: Schematic of intracellular recording from spontaneously beating embryonic chicken cardiomyocytes cultured on PDMS substrate using 3D kinked p–n nanoprobes. Adapted from ref (53). Copyright 2012 American Chemical Society.

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Further development in this field was done by Xu et al., (45) who substantially expanded the scope of functional building blocks made of kinked NWs, through the ab initio design and synthesis of several new structures. They demonstrated spatially isolated point-like nanoelectronic probes made of kinked Au-catalyzed VLS-grown Si NWs. Their structures are especially useful for monitoring biological systems where a precisely engineered size and structure are highly desired. The authors presented three types of novel functional kinked NWs:
(1)

Zero-degree kinked nanostructures with two parallel heavily doped arms of U-shaped structures, where three 120° kinks can define the “U” shape. A nanoFET detector is formed at the tip of the “U” by modulating the doping between two of these kinks. The heavily doped arms before and after the three kinks function as a source and drain (S–D), respectively. The width of this U-shaped element is determined by the interkink segment lengths and is independent of the arm length. This design should allow a deep penetration of the nanoFET detector into cells and tissue, without increasing its cross-sectional area.

(2)

Series multiplexed functional kinked (at 120°) NWs, integrating multiple nanoFETs along the arm and at the tips of V-shaped structures.

(3)

Parallel multiplexed kinked NWs integrating nanoFETs at the two tips of W-shaped nanostructures with 120° kinking angles.

The authors succeeded to synthesize U-shaped kinked NWs with a distance between parallel arms of just 650 nm. One of the arms/tip of V-shaped and distinct arms/tips of W-shaped NWs were encoded with multiple nanoFETs during synthesis. NWs were then assembled into 3D nanoFET probes. (Figure 17)

Figure 17

Figure 17. (a,b) Overview of designs and potential applications of kinked NWs. (a) U-shaped NW with integrated nanoFET (red) shown as a bioprobe for intracellular recording. (b) W-shaped NW with multiple nanoFETs (red) illustrated as a bioprobe for simultaneous intracellular/extracellular recording. Green color indicates heavily doped (n++) nanoelectrode arms, red indicates the point-like active nanoFET elements, and gold indicates the fabricated metal interconnects. A schematic of a cell to scale is drawn with the different device designs to show the potential for achieving minimally invasive deep penetration (a) and multiplexed intracellular and extracellular recording (b). (c) U- and W-shaped NWs synthesized in the study. Upper right image is the dark-field optical microscopy image of a KOH-etched kinked NW with four nanoFETs. The dark segments correspond to the four lightly doped nanoFET elements. Adapted from ref (45). Copyright 2012 American Chemical Society.

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Synthesis and design of p–n junction devices based on the kinked NWs described above greatly contribute to a growing array of methods and building elements for fabrication of highly compact and multiplexed 3D nanoprobes with highly localized sensing regions for applications in life sciences. They hence open substantial opportunities in areas ranging from bio- and chemosensing, nanoscale photon detection, to 3D imaging from within living cells and tissue. (53)

Structural

Kim et al. (86) have formed nanobridges by using single-kinked Ge NWs on uncommon V-grooved (100) Si substrates to take advantage of their vertical (111) sidewalls, (124) without resorting to the noncompatible (110) substrate. This is another example that reveals the potential of kinked NWs as promising building blocks for future devices. However, according to the authors, (86) the control of the crystallographic direction of segments in multiply kinked NWs still remains a challenging issue to be solved.

Summary and Outlook

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Kinking in NWs (i.e., abrupt changes in NW growth directions) is a phenomenon that has been known to scientists for decades. However, in recent years, kinking has attracted increased attention and is now considered a powerful tool for additional tweaking of the properties and geometry of one-dimensional nanostructures. To gain comprehensive control over the kinking process and utilize kinked NWs for practical applications, a thorough understanding of the kinking mechanisms is critical. As was shown in the present Review, there is a wide range of factors that may affect the NW growth direction, including the growth temperature, pressure, precursor composition, substrate orientation and treatment, NW diameter, catalyst size, etc. Moreover, these factors are often interconnected. The resulting growth direction is determined by the energies of the NW surface facets, NW–catalyst interface, and the availability of nucleation sites. (42) Some of the parameters can be tailored during the synthesis, leading to a change of the growth direction of already growing NWs, thus resulting in kinked structures. There are also several different mechanisms that lead to the NW kinking. Depending on the conditions and materials, one or another mechanism may prevail.
The properties of kinked NWs strongly depend on the type of kinking. Depending on various factors, the whole kinked NW may have the same crystallographic orientation, or kinking can involve a change in the crystallographic orientation. Kinks are often accompanied by crystallographic defects; however, defect-free kinking is also observed. NWs can have single or multiple kinks.
Despite a considerable amount of studies dedicated to kinking in NWs, there is still a lot to contribute. The majority of studies are dedicated to kinking of Si NWs in the VLS growth processes, while other materials and methods are significantly less exploited. Moreover, the number of studies that involve characterization of kinked NWs that extends beyond simulations or routine microscopy and spectroscopy is very limited. Even less studied are potential applications of kinked NWs. The most exploited and promising field seems to be the utilization of kinked NWs as nanoscale sensors in biological applications: various operational prototypes are already demonstrated in several recent studies. Extension of research to other materials and potential applications in various areas with corresponding characterization of mechanical, electrical, thermal, and other properties is a real challenge and has to be further explored.

Author Information

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  • Corresponding Author
  • Authors
    • Sven Oras - Tallinn University of Technology, Tartu College, Puiestee 78, Tartu 51008, Estonia;  Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia
    • Boris Polyakov - Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia;  Orcidhttps://orcid.org/0000-0002-6626-1065
    • Edgars Butanovs - Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia;  Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia;  Orcidhttps://orcid.org/0000-0003-3796-1190
    • Andreas Kyritsakis - Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia
    • Veronika Zadin - Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia
  • Notes
    The authors declare no competing financial interest.

Biographies

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Sergei Vlassov

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Sergei Vlassov (born in 1980) is an associate professor at the Institute of Physics, University of Tartu (Estonia). He received an MSc in applied physics (2007) and PhD in material science (2011) in the University of Tartu (Estonia) and then spent 2 years as a postdoc in the Institute of Solid State Physics, University of Latvia. His research interests are mainly related to experimental studies of mechanical and structural properties of individual nanostructures. By the spring of 2021, Sergei has 85 publications in the Google Scholar database, with an h-index of 19.

Sven Oras

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Sven Oras (born in 1989) has a PhD in Materials Chemistry (2018, University of Upper Alsace) and a Master’s Degree in Physics (2014, University of Tartu). His research activities are focused on the experimental characterization of nanostructures focusing mainly on mechanical and tribological properties. In the MATTER project, Sven is a postdoc responsible for performing nanomanipulation experiments inside a scanning electron microscope (SEM) affecting single nanostructures and performing characterization of nanostructures with an atomic force microscope (AFM). By the end of 2020, Sven has 15 publications in the Google Scholar database, with an h-index of 8.

Boris Polyakov

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Boris Polyakov is a senior researcher in the Institute of Solid State Physics, University of Latvia. He received his BSc and MSc degrees in mechanical engineering in Riga Technical University. He completed his Ph.D. in chemical physics in the University of Latvia in 2007. Later, he won a postdoctoral position in a postdoc project competition in the field of nanotribology in the Institute of Physics, University of Tartu, Estonia. After returning to Latvia, he continued his work in the Institute of Solid State Physics, University of Latvia. His research interests include synthesis and physical properties of nanomaterials and 2D layered materials. By the spring of 2021, Boris has 79 publications in the Google Scholar database with an h-index of 19.

Edgars Butanovs

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Edgars Butanovs is a senior researcher in the Thin Films Laboratory in the Institute of Solid State Physics, University of Latvia, a Research Fellow in the Institute of Technology, University of Tartu, and a lecturer in the University of Latvia. He received his BSc and MSc degrees in physics in 2014 and 2016, respectively, and completed his Ph.D. in materials physics in the University of Latvia in 2020. His research interests are related to the growth and studies of 1D and 2D nanomaterials (semiconducting nanowires and layered 2D materials), thin film deposition technologies, and optoelectronics applications. By the spring of 2021, Edgars has 16 publications in the Google Scholar database with an h-index of 5.

Andreas Kyritsakis

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Andreas Kyritsakis (born in 1987) has a PhD in Electrical & Computer Engineering (2014, National Technical University of Athens) and a Master Degree (Diploma) in Electrical & Computer Engineering (2010, National Technical University of Athens). His research activities are focused mainly on the theoretical/computational modelling of electron emission, vacuum arcs, and nanomaterial behavior under high electric fields. At the moment, Andreas is the ERA Chair Holder of the MATTER project, leading its scientific activities. As a simulation specialist, he focuses mainly on the subgroup of multiscale nanomaterial modelling. By the spring of 2021 Andreas has 45 publications in the Google Scholar database, with an h-index of 11.

Veronika Zadin

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Veronika Zadin is a professor of materials technology in the Institute of Technology, University of Tartu. She received her PhD in physical engineering in 2012 (University of Tartu) followed by postdoctoral research in the University of Helsinki (2012–2014). Next to the scientific work, she is involved in managing the collaboration between University of Tartu and CTF3 activities in CERN, holding there the position of Estonian group leader. Her main expertise and research interest are multiphysics finite element simulations in the nano- and microscale. Her specialization includes elastic and plastic deformations, heat and mass transport, charge transport, and interactions between the electric field and metals. By the spring of 2021, Veronika has 65 publications in the Google Scholar database, with an h-index of 15.

Acknowledgments

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The work was funded by ERA Chair MATTER from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 856705.

References

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This article references 124 other publications.

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    Antsov, M.; Polyakov, B.; Zadin, V.; Mets, M.; Oras, S.; Vahtrus, M.; Lõhmus, R.; Dorogin, L.; Vlassov, S. Mechanical characterisation of pentagonal gold nanowires in three different test configurations: A comparative study. Micron 2019, 124, 102686,  DOI: 10.1016/j.micron.2019.102686
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    6
    Mechanical characterisation of pentagonal gold nanowires in three different test configurations: A comparative study
    Antsov, Mikk; Polyakov, Boris; Zadin, Vahur; Mets, Magnus; Oras, Sven; Vahtrus, Mikk; Lohmus, Runno; Dorogin, Leonid; Vlassov, Sergei
    Micron (2019), 124 (), 102686CODEN: MCONEN; ISSN:0968-4328. (Elsevier Ltd.)
    Mech. characterization of individual nanostructures is a challenging task and can greatly benefit from the utilization of several alternative approaches to increase the reliability of results. In the present work, we have measured and compared the elastic modulus of five-fold twinned gold nanowires (NWs) with at. force microscopy (AFM) indentation in three different test configurations: three-point bending with fixed ends, three-point bending with free ends and cantilevered-beam bending. The free-ends condition was realized by introducing a novel approach where the NW is placed diagonally inside an inverted pyramid chem. etched in a silicon wafer. In addn., all three configurations were simulated with a finite element method to obtain better insight into stress distribution inside NWs during bending depending on test conditions. The free-ends configuration yielded elastic modulus similar to a classical fixed-ends approach (88 ± 20 GPa vs 87 ± 16 GPa), indicating the reliability of the proposed method. At the same time, the free-ends configuration benefits from a more favorable NW position relative to the probe with facet facing upwards in contrast to the sharp edge in the case of fixed ends. From the other hand, the free-ends configuration was less suitable for strength measurements, as NW can run into the bottom of the inverted pyramid because of a higher degree of deformation before fracture. The cantilevered-beam configuration was less suitable for mech. testing with indentation because of the instabilities of the free end under the AFM probe.
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  7. 7
    Polyakov, B.; Vlassov, S.; Dorogin, L. M.; Kulis, P.; Kink, I.; Lohmus, R. The effect of substrate roughness on the static friction of CuO nanowires. Surf. Sci. 2012, 606, 1393– 9,  DOI: 10.1016/j.susc.2012.04.029
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    7
    The effect of substrate roughness on the static friction of CuO nanowires
    Polyakov, Boris; Vlassov, Sergei; Dorogin, Leonid M.; Kulis, Peteris; Kink, Ilmar; Lohmus, Rynno
    Surface Science (2012), 606 (17-18), 1393-1399CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    The dependence of static friction on surface roughness was measured for CuO nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chem. etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an AFM tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calc. the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estd. using a multiple elastic asperity model, which is based on the Derjaguin-Muller-Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.
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  8. 8
    Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 2001, 409, 66– 9,  DOI: 10.1038/35051047
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    8
    Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices
    Duan, Xiangfeng; Huang, Yu; Cui, Yl; Wang, Jianfang; Lieber, Charles M.
    Nature (London) (2001), 409 (6816), 66-69CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Nanowires and nanotubes carry charge and excitons efficiently, and are therefore potentially ideal building blocks for nanoscale electronics and optoelectronics. Carbon nanotubes have already been exploited in devices such as field-effect and single-electron transistors, but the practical utility of nanotube components for building electronic circuits is limited, as it is not yet possible to selectively grow semiconducting or metallic nanotubes. Here we report the assembly of functional nanoscale devices from indium phosphide nanowires, the elec. properties of which are controlled by selective doping. Gate-voltage-dependent transport measurements demonstrate that the nano- wires can be predictably synthesized as either n- or p-type. These doped nanowires function as nanoscale field-effect transistors, and can be assembled into crossed-wire p-n junctions that exhibit rectifying behavior. Significantly, the p-n junctions emit light strongly and are perhaps the smallest light-emitting diodes that have yet been made. Finally, we show that elec.-field-directed assembly can be used to create highly integrated device arrays from nanowire building blocks.
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  9. 9
    Stern, A.; Aharon, S.; Binyamin, T.; Karmi, A.; Rotem, D.; Etgar, L.; Porath, D. Electrical Characterization of Individual Cesium Lead Halide Perovskite Nanowires Using Conductive AFM. Adv. Mater. 2020, 32, 1907812,  DOI: 10.1002/adma.201907812
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    9
    Electrical Characterization of Individual Cesium Lead Halide Perovskite Nanowires Using Conductive AFM
    Stern, Avigail; Aharon, Sigalit; Binyamin, Tal; Karmi, Abeer; Rotem, Dvir; Etgar, Lioz; Porath, Danny
    Advanced Materials (Weinheim, Germany) (2020), 32 (12), 1907812CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Perovskite nanostructures have attracted much attention in recent years due to their suitability for a variety of applications such as photovoltaics, light-emitting diodes (LEDs), nanometer-size lasing, and more. These uses rely on the conductive properties of these nanostructures. However, elec. characterization of individual, thin perovskite nanowires has not yet been reported. Here, conductive at. force microscopy characterization of individual cesium lead halide nanowires is presented. Clear differences are obsd. in the cond. of nanowires contg. only bromide and nanowires contg. a mixt. of bromide and iodide. The differences are attributed to a higher d. of cryst. defects, deeper trap states, and higher inherent cond. for nanowires with mixed bromide-iodide content.
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  10. 10
    Butanovs, E.; Vlassov, S.; Kuzmin, A.; Piskunov, S.; Butikova, J.; Polyakov, B. Fast-Response Single-Nanowire Photodetector Based on ZnO/WS 2 Core/Shell Heterostructures. ACS Appl. Mater. Interfaces 2018, 10, 13869– 76,  DOI: 10.1021/acsami.8b02241
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    10
    Fast-Response Single-Nanowire Photodetector Based on ZnO/WS2 Core/Shell Heterostructures
    Butanovs, Edgars; Vlassov, Sergei; Kuzmin, Alexei; Piskunov, Sergei; Butikova, Jelena; Polyakov, Boris
    ACS Applied Materials & Interfaces (2018), 10 (16), 13869-13876CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The surface plays an exceptionally important role in nanoscale materials, exerting a strong influence on their properties. Consequently, even a very thin coating can greatly improve the optoelectronic properties of nanostructures by modifying the light absorption and spatial distribution of charge carriers. To use these advantages, 1D/1D heterostructures of ZnO/WS2 core/shell nanowires with a-few-layers-thick WS2 shell were fabricated. These heterostructures were thoroughly characterized by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Then, a single-nanowire photoresistive device was assembled by mech. positioning ZnO/WS2 core/shell nanowires onto gold electrodes inside a scanning electron microscope. The results show that a few layers of WS2 significantly enhance the photosensitivity in the short wavelength range and drastically (almost 2 orders of magnitude) improve the photoresponse time of pure ZnO nanowires. The fast response time of ZnO/WS2 core/shell nanowire was explained by electrons and holes sinking from ZnO nanowire into WS2 shell, which serves as a charge carrier channel in the ZnO/WS2 heterostructure. First-principles calcns. suggest that the interface layer i-WS2, bridging ZnO nanowire surface and WS2 shell, might play a role of energy barrier, preventing the backward diffusion of charge carriers into ZnO nanowire.
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  11. 11
    Vlassov, S.; Oras, S.; Antsov, M.; Butikova, J.; Lõhmus, R.; Polyakov, B. Low-friction nanojoint prototype. Nanotechnology 2018, 29, 195707,  DOI: 10.1088/1361-6528/aab163
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    11
    Low-friction nanojoint prototype
    Vlassov, Sergei; Oras, Sven; Antsov, Mikk; Butikova, Jelena; Lohmus, Runno; Polyakov, Boris
    Nanotechnology (2018), 29 (19), 195707/1-195707/6CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    High surface energy of individual nanostructures leads to high adhesion and static friction that can completely hinder the operation of nanoscale systems with movable parts. For instance, silver or gold nanowires cannot be moved on silicon substrate without plastic deformation. In this paper, we exptl. demonstrate an operational prototype of a low-friction nanojoint. The movable part of the prototype is made either from a gold or silver nano-pin produced by laser-induced partial melting of silver and gold nanowires resulting in the formation of rounded bulbs on their ends. The nano-pin is then manipulated into the inverted pyramid (i-pyramids) specially etched in a Si wafer. Due to the small contact area, the nano-pin can be repeatedly tilted inside an i-pyramid as a rigid object without noticeable deformation. At the same time in the absence of external force the nanojoint is stable and preserves its position and tilt angle. Expts. are performed inside a scanning electron microscope and are supported by finite element method simulations.
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  12. 12
    Rahong, S.; Yasui, T.; Kaji, N.; Baba, Y. Recent developments in nanowires for bio-applications from molecular to cellular levels. Lab Chip 2016, 16, 1126– 38,  DOI: 10.1039/C5LC01306B
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    12
    Recent developments in nanowires for bio-applications from molecular to cellular levels
    Rahong, Sakon; Yasui, Takao; Kaji, Noritada; Baba, Yoshinobu
    Lab on a Chip (2016), 16 (7), 1126-1138CODEN: LCAHAM; ISSN:1473-0189. (Royal Society of Chemistry)
    This review highlights the most promising applications of nanowires for bioanal. chem. and medical diagnostics. The materials discussed here are metal oxide and Si semiconductors, which are integrated with various microfluidic systems. Nanowire structures offer desirable advantages such as a very small diam. size with a high aspect ratio and a high surface-to-vol. ratio without grain boundaries; consequently, nanowires are promising tools to study biol. systems. This review starts with the integration of nanowire structures into microfluidic systems, followed by the discussion of the advantages of nanowire structures in the sepn., manipulation and purifn. of biomols. (DNA, RNA and proteins). Next, some representative nanowire devices are introduced for biosensors from mol. to cellular levels based on elec. and optical approaches. Finally, we conclude the review by highlighting some bio-applications for nanowires and presenting the next challenges that must be overcome to improve the capabilities of nanowire structures for biol. and medical systems.
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  13. 13
    Verardo, D.; Lindberg, F. W.; Anttu, N.; Niman, C. S.; Lard, M.; Dabkowska, A. P.; Nylander, T.; Månsson, A.; Prinz, C. N.; Linke, H. Nanowires for Biosensing: Lightguiding of Fluorescence as a Function of Diameter and Wavelength. Nano Lett. 2018, 18, 4796– 802,  DOI: 10.1021/acs.nanolett.8b01360
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    13
    Nanowires for Biosensing: Lightguiding of Fluorescence as a Function of Diameter and Wavelength
    Verardo, Damiano; Lindberg, Frida W.; Anttu, Nicklas; Niman, Cassandra S.; Lard, Mercy; Dabkowska, Aleksandra P.; Nylander, Tommy; Maansson, Alf; Prinz, Christelle N.; Linke, Heiner
    Nano Letters (2018), 18 (8), 4796-4802CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Semiconductor nanowires can act as nanoscaled optical fibers, enabling them to guide and conc. light emitted by surface-bound fluorophores, potentially enhancing the sensitivity of optical biosensing. While parameters such as the nanowire geometry and the fluorophore wavelength can be expected to strongly influence this lightguiding effect, no detailed description of their effect on in-coupling of fluorescent emission is available to date. Here, the authors use confocal imaging to quantify the lightguiding effect in GaP nanowires as a function of nanowire geometry and light wavelength. Using a combination of finite-difference time-domain simulations and anal. approaches, the authors identify the role of multiple waveguide modes for the obsd. lightguiding. The normalized frequency parameter, based on the step-index approxn., predicts the lightguiding ability of the nanowires as a function of diam. and fluorophore wavelength, providing a useful guide for the design of optical biosensors based on nanowires.
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  14. 14
    Johar, M. A.; Song, H.-G.; Waseem, A.; Hassan, M. A.; Bagal, I. V.; Cho, Y.-H.; Ryu, S.-W. Universal and scalable route to fabricate GaN nanowire-based LED on amorphous substrate by MOCVD. Appl. Mater. Today 2020, 19, 100541,  DOI: 10.1016/j.apmt.2019.100541
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    There is no corresponding record for this reference.
  15. 15
    Thiyagu, S; Fukata, N Chapter 9 - Silicon nanowire-based solar cells. Nanomaterials for Solar Cell Applications 2019, 325– 348,  DOI: 10.1016/B978-0-12-813337-8.00009-6
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    15
    Silicon nanowire-based solar cells
    Thiyagu, Subramani; Fukata, Naoki
    Nanomaterials for Solar Cell Applications (2019), (), 325-348CODEN: 69YKZ8 ISSN:. (Elsevier B.V.)
    In summary, the use of a silicon nanostructure is an effective way of fabricating high-performance and cost-effective solar cells because of their superior properties for carrier transport, charge sepn., and light absorption. Radial p-n junction SiNW arrays offers a more viable approach to cost-effective Si-based solar cells than do conventional planar Si solar cells. Reflectance can be dramatically suppressed relative to that of planar Si substrates due to the formation of SiNW array structures over a broad range of spectral wavelengths of 300-1000 nm. By constructing p-n junctions radially in SiNWs, the light trapping effect effectively increases and the carrier recombination rate is also effectively reduced due to the shorter carrier collection path. SiNW-based solar cells appear to be promising candidates; however, many challenges need to be tackled; in particular, the carrier recombination problem at the interface caused by increased surface-to-vol. ratio. We showed that three kinds of passivation techniques improve the performance of SiNW solar cells. As a result of these enhancements, cell properties such as Jsc, Voc, and FF increase, leading to higher power conversion efficiency. The NRET effect using nc-Si is a new way of improving solar cell properties. This technique can be simply and easily applied to any kind of Si solar cell. These approaches broaden the scope for developing cost-effective, large-area solar energy devices in industries at large scales.
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  16. 16
    Garnett, E. C.; Brongersma, M. L.; Cui, Y.; McGehee, M. D. Nanowire Solar Cells Annu. Annu. Rev. Mater. Res. 2011, 41, 269– 95,  DOI: 10.1146/annurev-matsci-062910-100434
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    16
    Nanowire solar cells
    Garnett, Erik C.; Brongersma, Mark L.; Cui, Yi; McGehee, Michael D.
    Annual Review of Materials Research (2011), 41 (), 269-295CODEN: ARMRCU; ISSN:1531-7331. (Annual Reviews Inc.)
    A review. The nanowire geometry provides potential advantages over planar wafer-based or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the max. efficiency above std. limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost redns. Addnl., nanowires provide opportunities to fabricate complex single-cryst. semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technol. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.
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  17. 17
    Lan, C.; Li, C.; Wang, S.; Yin, Y.; Guo, H.; Liu, N.; Liu, Y. ZnO–WS2 heterostructures for enhanced ultra-violet photodetectors. RSC Adv. 2016, 6, 67520– 4,  DOI: 10.1039/C6RA12643J
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    17
    ZnO-WS2 heterostructures for enhanced ultra-violet photodetectors
    Lan, Changyong; Li, Chun; Wang, Shuai; Yin, Yi; Guo, Huayang; Liu, Nishuang; Liu, Yong
    RSC Advances (2016), 6 (72), 67520-67524CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Two-dimensional (2D) materials have attracted wide attention due to their exotic properties. In particular, the lack of dangling bonds makes it possible to build highly lattice mismatched heterostructures composed of 2D materials and conventional semiconductors. Here, we report that by simply stacking a chem. vapor deposition grown monolayer WS2 film onto the surface of a room temp. sputtered ZnO film, significant enhanced ultra-violet (UV) photoresponse can be achieved. In this heterostructure of ZnO-WS2, the ZnO film acts as a light harvesting layer while the WS2 monolayer functions as a carrier transport layer which facilitates the photocarrier transport and reduces its recombination. Such a mechanism was confirmed by the observation of further photoresponsivity improvement of the ZnO-WS2 heterostructure under vacuum which removes the surface absorbates and thereby increases the carrier mobility of WS2. The strategy presented here can be applied to other wide band-gap semiconductors, shedding light on high sensitivity and flexible UV photodetectors based on van der Waals heterostructures.
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  18. 18
    Stokes, K.; Geaney, H.; Sheehan, M.; Borsa, D.; Ryan, K. M. Copper Silicide Nanowires as Hosts for Amorphous Si Deposition as a Route to Produce High Capacity Lithium-Ion Battery Anodes. Nano Lett. 2019, 19, 8829– 35,  DOI: 10.1021/acs.nanolett.9b03664
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    18
    Copper Silicide Nanowires as Hosts for Amorphous Si Deposition as a Route to Produce High Capacity Lithium-Ion Battery Anodes
    Stokes, Killian; Geaney, Hugh; Sheehan, Martin; Borsa, Dana; Ryan, Kevin M.
    Nano Letters (2019), 19 (12), 8829-8835CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Herein, copper silicide (Cu15Si4) nanowires (NWs) grown in high densities from a metallic Cu substrate are utilized as nanostructured hosts for amorphous silicon (aSi) deposition. The conductive Cu15Si4 NW scaffolds offer an increased surface area, vs. planar substrates, and enable the prepn. of high capacity Li-ion anodes consisting of a nanostructured active material. The formation method involves a two-step process, where Cu15Si4 nanowires are synthesized from a Cu substrate via a solvent vapor growth (SVG) approach followed by the plasma-enhanced chem. vapor deposition (PECVD) of aSi. These binder-free anodes are investigated in half-cell (vs. Li-foil) and full-cell (vs. LCO) configurations with discharge capacities greater than 2000 mAh/g retained after 200 cycles (half-cell) and reversible capacities of 1870 mAh/g exhibited after 100 cycles (full-cell). A noteworthy rate capability is also attained where capacities of up to 1367 mAh/g and 1520 mAh/g are exhibited at 5C in half-cell and full-cell configurations, resp., highlighting the active material's promise for fast charging and high power applications. The anode material is characterized prior to cycling and after 1, 25, and 100 charge/discharge cycles, by SEM (SEM) and transmission electron microscopy (TEM), to track the effects of cycling on the material.
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  19. 19
    Wei, H.; Wang, Z.; Tian, X.; Käll, M.; Xu, H. Cascaded logic gates in nanophotonic plasmon networks. Nat. Commun. 2011, 2, 387,  DOI: 10.1038/ncomms1388
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    Cascaded logic gates in nanophotonic plasmon networks
    Wei Hong; Wang Zhuoxian; Tian Xiaorui; Kall Mikael; Xu Hongxing
    Nature communications (2011), 2 (), 387 ISSN:.
    Optical computing has been pursued for decades as a potential strategy for advancing beyond the fundamental performance limitations of semiconductor-based electronic devices, but feasible on-chip integrated logic units and cascade devices have not been reported. Here we demonstrate that a plasmonic binary NOR gate, a 'universal logic gate', can be realized through cascaded OR and NOT gates in four-terminal plasmonic nanowire networks. This finding provides a path for the development of novel nanophotonic on-chip processor architectures for future optical computing technologies.
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  20. 20
    Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K.-H.; Lieber, C. M. Logic Gates and Computation from Assembled Nanowire Building Blocks. Science 2001, 294, 1313– 7,  DOI: 10.1126/science.1066192
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    20
    Logic gates and computation from assembled nanowire building blocks
    Huang, Yu; Duan, Xiangfeng; Cui, Yi; Lauhon, Lincoln J.; Kim, Kyoung-Ha; Lieber, Charles M.
    Science (Washington, DC, United States) (2001), 294 (5545), 1313-1317CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Miniaturization in electronics through improvements in established "top-down" fabrication techniques is approaching the point where fundamental issues are expected to limit the dramatic increases in computing seen over the past several decades. Here we report a "bottom-up" approach in which functional device elements and element arrays have been assembled from soln. through the use of electronically well-defined semiconductor nanowire building blocks. We show that crossed nanowire p-n junctions and junction arrays can be assembled in over 95% yield with controllable elec. characteristics, and in addn., that these junctions can be used to create integrated nanoscale field-effect transistor arrays with nanowires as both the conducting channel and gate electrode. Nanowire junction arrays have been configured as key OR, AND, and NOR logic-gate structures with substantial gain and have been used to implement basic computation.
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  21. 21
    Huo, N.; Yang, S.; Wei, Z.; Li, S.-S.; Xia, J.-B.; Li, J. Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS2 Nanoflakes. Sci. Rep. 2015, 4, 5209,  DOI: 10.1038/srep05209
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    Hu, X. F.; Li, S. J.; Wang, J.; Jiang, Z. M.; Yang, X. J. Investigating Size-Dependent Conductive Properties on Individual Si Nanowires. Nanoscale Res. Lett. 2020, 15, 52,  DOI: 10.1186/s11671-020-3277-3
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    Investigating Size-Dependent Conductive Properties on Individual Si Nanowires
    Hu, X. F.; Li, S. J.; Wang, J.; Jiang, Z. M.; Yang, X. J.
    Nanoscale Research Letters (2020), 15 (1), 52CODEN: NRLAAD; ISSN:1556-276X. (Springer)
    Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated by nanosphere lithog. combined with metal-assisted chem. etching. By adjusting the etching time, both the nanowires diam. and length can be well controlled. The conductive properties of such Si NWs and particularly their size dependence are investigated by conductive at. force microscopy (CAFM) on individual nanowires. The results indicate that the conductance of Si NWs is greatly relevant to their diam. and length. Si NWs with smaller diams. and shorter lengths exhibit better conductive properties. Together with the I-V curve characterization, a possible mechanism is supposed with the viewpoint of size-dependent Schottky barrier height, which is further verified by the electrostatic force microscopy (EFM) measurements. This study also suggests that CAFM can act as an effective means to explore the size (or other parameters) dependence of conductive properties on individual nanostructures, which should be essential for both fabrication optimization and potential applications of nanostructures.
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  23. 23
    Nam, C.-Y.; Jaroenapibal, P.; Tham, D.; Luzzi, D. E.; Evoy, S.; Fischer, J. E. Diameter-Dependent Electromechanical Properties of GaN Nanowires. Nano Lett. 2006, 6, 153– 8,  DOI: 10.1021/nl051860m
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    23
    Diameter-dependent electromechanical properties of GaN nanowires
    Nam, Chang-Yong; Jaroenapibal, Papot; Tham, Douglas; Luzzi, David E.; Evoy, Stephane; Fischer, John E.
    Nano Letters (2006), 6 (2), 153-158CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The diam.-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromech. resonance anal. in a transmission electron microscope. E is close to the theor. bulk value (∼300 GPa) for a large diam. nanowire (d = 84 nm) but is significantly smaller for smaller diams. At room temp., Q is as high as 2800 for d = 84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-vol. ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromech. system (NEMS) applications. Two closely spaced resonances are obsd. and attributed to the low-symmetry triangular cross section of the nanowires.
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    Yorikawa, H.; Uchida, H.; Muramatsu, S. Energy gap of nanoscale Si rods. J. Appl. Phys. 1996, 79, 3619– 21,  DOI: 10.1063/1.361416
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    Energy gap of nanoscale Si rods
    Yorikawa, H.; Uchida, H.; Muramatsu, S.
    Journal of Applied Physics (1996), 79 (7), 3619-21CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The electronic structure of silicon rods has been studied by means of the tight-binding recursion method to investigate the dependence of the energy gap (Eg) on rod length and the direction of the rod axis. An empirical expression for Eg is derived from numerical results for the rods in 〈100〉, 〈110〉, and 〈111〉 directions. This expression is applicable to the energy gaps of wires and crystallites, which can be regarded as limiting cases of rods.
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    Roy, A.; Mead, J.; Wang, S.; Huang, H. Effects of surface defects on the mechanical properties of ZnO nanowires. Sci. Rep. 2017, 7, 9547,  DOI: 10.1038/s41598-017-09843-5
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    Effects of surface defects on the mechanical properties of ZnO nanowires
    Roy Aditi; Mead James; Wang Shiliang; Huang Han; Wang Shiliang
    Scientific reports (2017), 7 (1), 9547 ISSN:.
    The elastic modulus of ZnO nanowires was measured using a resonance method based on laser Doppler effect and their fracture strains were determined via two-point bending with the aid of optical nanomanipulation. The elastic moduli of ZnO nanowires with diameters of 78 to 310 nm vary from 123 to 154 GPa, which are close to the bulk value of 140 GPa and independent of the diameters and surface defects. However, the fracture strains of the ZnO nanowires depend significantly on their diameters, increasing from 2.1% to 6.0% with the decrease in diameter from 316 to 114 nm. Post-mortem TEM analysis of the ends of the fractured nanowires revealed that fracture initiated at surface defects. The Weibull statistical analysis demonstrated that a greater defect depth led to a smaller fracture strain. The surface-defect dominated fracture should be an important consideration for the design and application of nanowire-based nanoelectromechanical systems.
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    Shin, N.; Chi, M.; Filler, M. A. Sidewall Morphology-Dependent Formation of Multiple Twins in Si Nanowires. ACS Nano 2013, 7, 8206– 13,  DOI: 10.1021/nn4036798
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    Sidewall Morphology-Dependent Formation of Multiple Twins in Si Nanowires
    Shin, Naechul; Chi, Miaofang; Filler, Michael A.
    ACS Nano (2013), 7 (9), 8206-8213CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Precise placement of twin boundaries and stacking faults promises new opportunities to fundamentally manipulate the optical, elec., and thermal properties of semiconductor nanowires. Here the authors report on the appearance of consecutive twin boundaries in Si nanowires and show that sidewall morphol. governs their spacing. Detailed electron microscopy anal. reveals that thin {111} sidewall facets, which elongate following the 1st twin boundary (TB1), are responsible for deforming the triple-phase line and favoring the formation of the 2nd twin boundary (TB2). While multiple, geometrically correlated defect planes are known in Group III-V nanowires, the authors' findings show that this behavior is also possible in Group IV materials.
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    Arbiol, J.; Fontcuberta i Morral, A.; Estradé, S.; Peiró, F.; Kalache, B.; Roca i Cabarrocas, P.; Morante, J. R. Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires. J. Appl. Phys. 2008, 104, 064312  DOI: 10.1063/1.2976338
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    Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires
    Arbiol, Jordi; Fontcuberta i Morral, Anna; Estrade, Sonia; Peiro, Francesca; Kalache, Billel; Roca i Cabarrocas, Pere; Morante, Joan Ramon
    Journal of Applied Physics (2008), 104 (6), 064312/1-064312/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The occurrence of heterostructures of cubic Si/hexagonal Si as disks defined along the nanowire 〈111〉 growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. During growth, lamellar twinning on the cubic phase along the 〈111〉 direction is generated. Consecutive presence of twins along the 〈111〉 growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram. (c) 2008 American Institute of Physics.
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    Sosnin, I. M.; Vlassov, S.; Akimov, E. G.; Agenkov, V. I.; Dorogin, L. M. Transparent ZnO-coated polydimethylsiloxane-based material for photocatalytic purification applications. J. Coat. Technol. Res. 2020, 17, 573– 9,  DOI: 10.1007/s11998-019-00314-2
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    Transparent ZnO-coated polydimethylsiloxane-based material for photocatalytic purification applications
    Sosnin, I. M.; Vlassov, S.; Akimov, E. G.; Agenkov, V. I.; Dorogin, L. M.
    Journal of Coatings Technology and Research (2020), 17 (2), 573-579CODEN: JCTRCP; ISSN:1935-3804. (Springer)
    Abstr: We describe prodn. and photocatalytic properties of a material based on polydimethylsiloxane (PDMS) as a carrier substrate coated with microparticles of zinc oxide (ZnO). The ZnO microparticles are fabricated by our original hydrothermal method and intentionally have a defect structure. According to our understanding, this peculiar defect structure contributes to the greatly enhanced photocatalytic properties of the ZnO material. The resulting photocatalyst demonstrates high activity under visible light (410 nm) in the process of phenol degrdn. in water soln., while generally ZnO is inactive below the UV range. In addn., we compare the photocatalytic activity of our ZnO/PDMS composite to that of the same ZnO powder suspension in a similar setup. We find that the same activity is achieved by three orders of magnitude smaller amt. of ZnO in our composite compared to the powder suspension. The ZnO/PDMS interface exhibits sufficiently strong bonding for stable operation that is ensured during material prodn. The obtained photocatalytic material preserves the transparency of PDMS due to the low amt. of attached ZnO (about 0.1% by mass). The transparency of the photocatalytic ZnO/PDMS material enables easy performance upgrades by constructing multilayer or manifold fluid treatment devices.
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    Murphy, K. F.; Piccione, B.; Zanjani, M. B.; Lukes, J. R.; Gianola, D. S. Strain- and Defect-Mediated Thermal Conductivity in Silicon Nanowires. Nano Lett. 2014, 14, 3785– 92,  DOI: 10.1021/nl500840d
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    Strain- and Defect-Mediated Thermal Conductivity in Silicon Nanowires
    Murphy, Kathryn F.; Piccione, Brian; Zanjani, Mehdi B.; Lukes, Jennifer R.; Gianola, Daniel S.
    Nano Letters (2014), 14 (7), 3785-3792CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The unique thermal transport of insulating nanostructures is attributed to the convergence of material length scales with the mean free paths of quantized lattice vibrations known as phonons, enabling promising next-generation thermal transistors, thermal barriers, and thermoelecs. Apart from size, strain and defects are also known to drastically affect heat transport when introduced in an otherwise undisturbed cryst. lattice. Here we report the first exptl. measurements of the effect of both spatially uniform strain and point defects on thermal cond. of an individual suspended nanowire using in situ Raman piezothermog. Our results show that whereas phononic transport in undoped Si nanowires with diams. in the range of 170-180 nm is largely unaffected by uniform elastic tensile strain, another means of disturbing a pristine lattice, namely, point defects introduced via ion bombardment, can reduce the thermal cond. by over 70%. In addn. to discerning surface- and core-governed pathways for controlling thermal transport in phonon-dominated insulators and semiconductors, we expect our novel approach to have broad applicability to a wide class of functional one- and two-dimensional nanomaterials.
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    Barth, S.; Boland, J. J.; Holmes, J. D. Defect Transfer from Nanoparticles to Nanowires. Nano Lett. 2011, 11, 1550– 5,  DOI: 10.1021/nl104339w
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    Defect Transfer from Nanoparticles to Nanowires
    Barth, Sven; Boland, John J.; Holmes, Justin D.
    Nano Letters (2011), 11 (4), 1550-1555CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Metal-seeded growth of 1-dimensional (1D) semiconductor nanostructures is still a very active field of research, despite the huge progress which was made in understanding this fundamental phenomenon. Liq. growth promoters allow control of the aspect ratio, diam., and structure of 1-dimensional crystals via external parameters, such as precursor feedstock, temp., and operating pressure. However the transfer of crystallog. information from a catalytic nanoparticle seed to a growing nanowire was not described in the literature. Here the authors define the theor. requirements for transferring defects from nanoparticle seeds to growing semiconductor nanowires and describe why Ag nanoparticles are ideal candidates for this purpose. The authors detail the influence of solid Ag growth seeds on the crystal quality of Ge nanowires, synthesized using a supercrit. fluid growth process. Significantly, under certain reaction conditions {111} stacking faults in the Ag seeds can be directly transferred to a high percentage of 〈112〉-oriented Ge nanowires, as radial twins in the semiconductor crystals. Defect transfer from nanoparticles to nanowires could open up the possibility of engineering 1-dimensional nanostructures with new and tunable phys. properties and morphologies.
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    Shin, N.; Chi, M.; Howe, J. Y.; Filler, M. A. Rational Defect Introduction in Silicon Nanowires. Nano Lett. 2013, 13, 1928– 33,  DOI: 10.1021/nl3042728
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    Rational Defect Introduction in Silicon Nanowires
    Shin, Naechul; Chi, Miaofang; Howe, Jane Y.; Filler, Michael A.
    Nano Letters (2013), 13 (5), 1928-1933CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The controlled introduction of planar defects, particularly twin boundaries and stacking faults, in Group IV nanowires remains challenging despite the prevalence of these structural features in other nanowire systems (e.g., II-VI and III-V). User-programmable changes to precursor pressure and growth temp. can rationally generate both transverse twin boundaries and angled stacking faults during the growth of 〈111〉 oriented Si nanowires. The authors leverage this new capability to demonstrate prototype defect superstructures. These findings yield important insight into the mechanism of defect generation in semiconductor nanowires and suggest new routes to engineer the properties of this ubiquitous semiconductor.
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    Ra, H.-W.; Khan, R.; Kim, J. T.; Kang, B. R.; Bai, K. H.; Im, Y. H. Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment. Mater. Lett. 2009, 63, 2516– 9,  DOI: 10.1016/j.matlet.2009.08.054
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    Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment
    Ra, H.-W.; Khan, R.; Kim, J. T.; Kang, B. R.; Bai, K. H.; Im, Y. H.
    Materials Letters (2009), 63 (28), 2516-2519CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
    This study examd. the effects of an oxygen plasma treatment on the properties of ZnO nanowires with diams. of 80 nm using a single nanowire field effect transistor. After the oxygen plasma treatment, the carrier concn. and mobility of individual ZnO nanowires decreased with a substantial pos. shift in the threshold voltage. The shifting was accounted to the surface modification, resulted to the improved gas sensitivity under hydrogen gas exposure and an enhanced photocurrent response time in UV illumination. The plausible surface mechanisms responsible for these significant changes after the surface modification were suggested by considering the surface anal. and elec. transport mechanism.
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    Muhammad, B. L.; Cummings, F. Nitrogen plasma treatment of ZnO and TiO2 nanowire arrays for polymer photovoltaic applications. Surf. Interfaces 2019, 17, 100382,  DOI: 10.1016/j.surfin.2019.100382
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    Nitrogen plasma treatment of ZnO and TiO2 nanowire arrays for polymer photovoltaic applications
    Muhammad, Bello Ladan; Cummings, Franscious
    Surfaces and Interfaces (2019), 17 (), 100382CODEN: SIUNCN; ISSN:2468-0230. (Elsevier B.V.)
    This work reports on a simple, yet unique approach to improving the opto-electronic properties of vertically-aligned arrays of rutile TiO2 and Wurzite ZnO nanowires by means of controlled nitrogen doping during exposure to highly kinetic radio-frequency generated N2 plasma radicals. Morphol., the plasma treatment causes a distortion of the vertical alignment of the nanowires due to a dissocn. of the weak Van der Waals force clustering the nanowires. Optical spectroscopy show that plasma treatment increases the light transmission of TiO2 arrays from 48% to 90%, with the ZnO arrays exhibiting an increase from 70% to 90% in the visible to UV range. The as-synthesized TiO2 array has an indirect band gap of 3.13 eV, which reduces to 3.03 eV after N2 treatment, with the ZnO equiv. decreasing from 3.20 to 3.17 eV post plasma exposure. A study of the 3d transition metal near edge fine structure of both Ti and Zn show that the N2 plasma treatment of the nanowires results in nitrogen doping of both TiO2 and ZnO lattices; this is confirmed by scanning transmission electron microscopy coupled with energy dispersive spectroscopy x-ray maps collected of single nanowires, which show a clear distribution of nitrogen throughout the metal-oxide. Application of these structures in P3HT:PCBM polymer blends shows progressive improvement in the photoluminescence quenching of the photoactive layer when incorporating both undoped and nitrogen-doped nanowires.
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    Yang, B.; Liu, B.; Wang, Y.; Zhuang, H.; Liu, Q.; Yuan, F.; Jiang, X. Zn-dopant dependent defect evolution in GaN nanowires. Nanoscale 2015, 7, 16237– 45,  DOI: 10.1039/C5NR04771D
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    Zn-dopant dependent defect evolution in GaN nanowires
    Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin
    Nanoscale (2015), 7 (39), 16237-16245CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chem. vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calcns. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (10‾13), (10‾11) and (20‾21), as well as Type I stacking faults (...ABABCBCB...), are obsd. in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABACBA...) can be identified as Type II stacking faults. The d. of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphol. in the GaN nanowires. First-principles calcns. reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the elec. properties of p-type GaN nanowires.
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    Ghosh, S.; Gopal Khan, G.; Varma, S.; Mandal, K. Influence of Li-N and Li-F co-doping on defect-induced intrinsic ferromagnetic and photoluminescence properties of arrays of ZnO nanowires. J. Appl. Phys. 2012, 112, 043910  DOI: 10.1063/1.4747929
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    Influence of Li-N and Li-F co-doping on defect-induced intrinsic ferromagnetic and photoluminescence properties of arrays of ZnO nanowires
    Ghosh, Shyamsundar; Khan, Gobinda Gopal; Varma, Shikha; Mandal, Kalyan
    Journal of Applied Physics (2012), 112 (4), 043910/1-043910/9CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The role of N/F co-doping on the defect-driven room-temp. d0 ferromagnetism in group-I element Li doped ZnO nanowire arrays has been investigated. The ferromagnetic signature of pristine ZnO nanowires has enhanced significantly after Li doping but the Li-N co-doping has found to be more effective in the stabilization and enhancement in room-temp. ferromagnetism in ZnO nanowires. Satn. magnetization in Li-doped ZnO nanowires found to increase from 0.63 to 2.52 emu/g and the Curie temp. rises up to 648 K when 10 at. % N is co-doped with 6 at. % Li. On the other hand, Li-F co-doping leads to exhibit much poor room-temp. ferromagnetic as well as visible luminescence properties. The valance state of the different dopants is estd. by XPS while the photoluminescence spectra indicate the gradual stabilization of Zn vacancy defects or defect complexes in presence of No acceptor states, which is found to be responsible for the enhancement of intrinsic ferromagnetism in ZnO:Li matrix. Therefore, the Li-N co-doping can be an effective parameter to stabilize, enhance, and tune zinc vacancy-induced room-temp. d0 ferromagnetism in ZnO nanowires, which can be an exciting approach to prep. new class of spintronic materials. (c) 2012 American Institute of Physics.
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    Narayanan, S.; Cheng, G.; Zeng, Z.; Zhu, Y.; Zhu, T. Strain Hardening and Size Effect in Five-fold Twinned Ag Nanowires. Nano Lett. 2015, 15, 4037– 44,  DOI: 10.1021/acs.nanolett.5b01015
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    Strain Hardening and Size Effect in Five-fold Twinned Ag Nanowires
    Narayanan, Sankar; Cheng, Guangming; Zeng, Zhi; Zhu, Yong; Zhu, Ting
    Nano Letters (2015), 15 (6), 4037-4044CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Metallic nanowires usually exhibit ultrahigh strength but low tensile ductility owing to their limited strain hardening capability. Here we study the unique strain hardening behavior of the five-fold twinned Ag nanowires by nanomech. testing and atomistic modeling. In situ tensile tests within a scanning electron microscope revealed strong strain hardening behavior of the five-fold twinned Ag nanowires. Mol. dynamics simulations showed that such strain hardening was critically controlled by twin boundaries and pre-existing defects. Strain hardening was size dependent; thinner nanowires achieved more hardening and higher ductility. The size-dependent strain hardening was found to be caused by the obstruction of surface-nucleated dislocations by twin boundaries. Our work provides mechanistic insights into enhancing the tensile ductility of metallic nanostructures by engineering the internal interfaces and defects.
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    Li, Y.; Wang, Y.; Ryu, S.; Marshall, A. F.; Cai, W.; McIntyre, P. C. Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor–Liquid–Solid Nanowire Growth. Nano Lett. 2016, 16, 1713– 8,  DOI: 10.1021/acs.nanolett.5b04633
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    Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor-Liquid-Solid Nanowire Growth
    Li, Yanying; Wang, Yanming; Ryu, Seunghwa; Marshall, Ann F.; Cai, Wei; McIntyre, Paul C.
    Nano Letters (2016), 16 (3), 1713-1718CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Kinking, a common anomaly in nanowire (NW) vapor-liq.-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liq. droplets of defined size. Statistical anal. of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diam. and the growth temp. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to 〈110〉, was obsd. in Ge NWs with a nominal diam. of ∼20 nm. This size coincides with a crit. diam. at which a spontaneous transition from 〈111〉 to 〈110〉 growth occurs in the phase field simulations. Larger diam. NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined 〈111〉 axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liq. droplet at the tip of the NW.
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    Jiang, J.-W.; Yang, N.; Wang, B.-S.; Rabczuk, T. Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects. Nano Lett. 2013, 13, 1670– 4,  DOI: 10.1021/nl400127q
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    Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects
    Jiang, Jin-Wu; Yang, Nuo; Wang, Bing-Shen; Rabczuk, Timon
    Nano Letters (2013), 13 (4), 1670-1674CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors performed mol. dynamics simulations to investigate the redn. of the thermal cond. by kinks in silicon nanowires. The redn. percentage can be as high as 70% at room temp. The temp. dependence of the redn. was also calcd. By calcg. phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal cond.: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, i.e., a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. This work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelec. materials.
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    Cook, B. G.; Varga, K. Conductance of kinked nanowires. Appl. Phys. Lett. 2011, 98, 052104,  DOI: 10.1063/1.3551711
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    Conductance of kinked nanowires
    Cook, B. G.; Varga, K.
    Applied Physics Letters (2011), 98 (5), 052104/1-052104/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The conductance properties of kinked nanowires are studied by first-principles transport calcns. within a recently developed complex potential framework. Using prototypical examples of monoat. Au chains as well as small diam. single-cryst. silicon nanowires we show that transmission strongly depends on the kink geometry and one can tune the conductance properties by the kink angle and other geometrical factors. In the case of a silicon nanowire the presence of a kink drastically reduces the conductance. (c) 2011 American Institute of Physics.
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    Zhao, Y.; Yang, L.; Liu, C.; Zhang, Q.; Chen, Y.; Yang, J.; Li, D. Kink effects on thermal transport in silicon nanowires. Int. J. Heat Mass Transfer 2019, 137, 573– 8,  DOI: 10.1016/j.ijheatmasstransfer.2019.03.104
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    Kink effects on thermal transport in silicon nanowires
    Zhao, Yang; Yang, Lin; Liu, Chenhan; Zhang, Qian; Chen, Yunfei; Yang, Juekuan; Li, Deyu
    International Journal of Heat and Mass Transfer (2019), 137 (), 573-578CODEN: IJHMAK; ISSN:0017-9310. (Elsevier Ltd.)
    Kinks in nanowires have recently been shown to be able to effectively tune the nanowire thermal conductivities; however, the underlying mechanisms have not been fully understood yet. To further disclose the details of phonon transport in kinked nanowires, here we report on non-equil. mol. dynamics studies of thermal transport through kinked and straight silicon nanowires. Results show that kinks can induce addnl. resistance and lead to lower thermal cond. for kinked nanowires than that of corresponding straight wires. Detailed anal. indicates that kinks produce addnl. resistance through reflecting phonons back into their incoming arms. Moreover, through introducing heavier isotope atoms in the kink region, the simulation replicates the exptl. observation that defects in the kink regime, instead of posing addnl. resistance, can actually facilitate thermal transport through deflecting phonons into the opposite arm.
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    Tian, B.; Xie, P.; Kempa, T. J.; Bell, D. C.; Lieber, C. M. Single crystalline kinked semiconductor nanowire superstructures. Nat. Nanotechnol. 2009, 4, 824– 9,  DOI: 10.1038/nnano.2009.304
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    Single-crystalline kinked semiconductor nanowire superstructures
    Tian, Bozhi; Xie, Ping; Kempa, Thomas J.; Bell, David C.; Lieber, Charles M.
    Nature Nanotechnology (2009), 4 (12), 824-829CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    The ability to control and modulate the compn., doping, crystal structure and morphol. of semiconductor nanowires during the synthesis process has allowed researchers to explore various applications of nanowires. However, despite advances in nanowire synthesis, progress towards the ab initio design and growth of hierarchical nanostructures was limited. Here, we demonstrate a 'nanotectonic' approach that provides iterative control over the nucleation and growth of nanowires, and use it to grow kinked or zigzag nanowires in which the straight sections are sepd. by triangular joints. Moreover, the lengths of the straight sections can be controlled and the growth direction remains coherent along the nanowire. We also grow dopant-modulated structures in which specific device functions, including p-n diodes and field-effect transistors, can be precisely localized at the kinked junctions in the nanowires.
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    Fortuna, S. A.; Li, X. Metal-catalyzed semiconductor nanowires: a review on the control of growth directions. Semicond. Sci. Technol. 2010, 25, 024005  DOI: 10.1088/0268-1242/25/2/024005
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    Metal-catalyzed semiconductor nanowires: a review on the control of growth directions
    Fortuna, Seth A.; Li, Xiuling
    Semiconductor Science and Technology (2010), 25 (2), 024005/1-024005/16CODEN: SSTEET; ISSN:0268-1242. (Institute of Physics Publishing)
    A review. Semiconductor nanowires have become an important building block for nanotechnol. The growth of semiconductor nanowires using a metal catalyst via the vapor-liq.-solid (VLS) or vapor-solid-solid (VSS) mechanism has yielded growth directions in 〈1 1 1〉, 〈1 0 0〉 and 〈1 1 0〉 etc. In this paper, we summarize and discuss a broad range of factors that affect the growth direction of VLS or VSS grown epitaxial semiconductor nanowires, providing an indexed glimpse of the control of nanowire growth directions and thus the mech., elec. and optical properties assocd. with the crystal orientation. The prospect of using planar nanowires for large area planar processing toward future nanowire array-based nanoelectronics and photonic applications is discussed.
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    Ross, F. M. Controlling nanowire structures through real time growth studies. Rep. Prog. Phys. 2010, 73, 114501,  DOI: 10.1088/0034-4885/73/11/114501
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    Controlling nanowire structures through real time growth studies
    Ross, Frances M.
    Reports on Progress in Physics (2010), 73 (11), 114501/1-114501/21CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
    A review. In situ electron microscopy can be used to visualize the phys. processes that control the growth of Si and Ge nanowires through the vapor-liq.-solid mechanism. Images and movies are recorded in a transmission electron microscope that has capabilities for depositing catalysts onto a sample and for introducing CVD precursor gases while the sample remains under observation. This technique allows us to measure nucleation, catalyst stability, surface structure and growth kinetics, in some cases confirming existing models and in other cases producing unexpected results and suggesting approaches toward growing novel structures. Nanowire formation provides a unique window into the fundamentals of crystal growth as well as an opportunity to fabricate precisely controlled structures for novel applications.
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    Tian, B.; Cohen-Karni, T.; Qing, Q.; Duan, X.; Xie, P.; Lieber, C. M. Three-Dimensional, Flexible Nanoscale Field-Effect Transistors as Localized Bioprobes. Science 2010, 329, 830– 4,  DOI: 10.1126/science.1192033
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    Three-Dimensional, Flexible Nanoscale Field-Effect Transistors as Localized Bioprobes
    Tian, Bozhi; Cohen-Karni, Tzahi; Qing, Quan; Duan, Xiaojie; Xie, Ping; Lieber, Charles M.
    Science (Washington, DC, United States) (2010), 329 (5993), 830-834CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Nanoelectronic devices offer substantial potential for interrogating biol. systems, although nearly all work has focused on planar device designs. The authors have overcome this limitation through synthetic integration of a nanoscale field-effect transistor (nanoFET) device at the tip of an acute-angle kinked silicon nanowire, where nanoscale connections are made by the arms of the kinked nanostructure, and remote multilayer interconnects allow three-dimensional (3D) probe presentation. The acute-angle probe geometry was designed and synthesized by controlling cis vs. trans crystal conformations between adjacent kinks, and the nanoFET was localized through modulation doping. 3D nanoFET probes exhibited conductance and sensitivity in aq. soln., independent of large mech. deflections, and demonstrated high pH sensitivity. Addnl., 3D nanoprobes modified with phospholipid bilayers can enter single cells to allow robust recording of intracellular potentials.
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    Xu, L.; Jiang, Z.; Qing, Q.; Mai, L.; Zhang, Q.; Lieber, C. M. Design and Synthesis of Diverse Functional Kinked Nanowire Structures for Nanoelectronic Bioprobes. Nano Lett. 2013, 13, 746– 51,  DOI: 10.1021/nl304435z
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    Design and Synthesis of Diverse Functional Kinked Nanowire Structures for Nanoelectronic Bioprobes
    Xu, Lin; Jiang, Zhe; Qing, Quan; Mai, Liqiang; Zhang, Qingjie; Lieber, Charles M.
    Nano Letters (2013), 13 (2), 746-751CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Functional kinked nanowires (KNWs) represent a new class of nanowire building blocks, in which functional devices, for example, nanoscale field-effect transistors (nanoFETs), are encoded in geometrically controlled nanowire superstructures during synthesis. The bottom-up control of both structure and function of KNWs enables construction of spatially isolated point-like nanoelectronic probes that are esp. useful for monitoring biol. systems where finely tuned feature size and structure are highly desired. Here we present three new types of functional KNWs including (1) the zero-degree KNW structures with two parallel heavily doped arms of U-shaped structures with a nanoFET at the tip of the "U", (2) series multiplexed functional KNW integrating multi-nanoFETs along the arm and at the tips of V-shaped structures, and (3) parallel multiplexed KNWs integrating nanoFETs at the two tips of W-shaped structures. First, U-shaped KNWs were synthesized with sepns. as small as 650 nm between the parallel arms and used to fabricate three-dimensional nanoFET probes at least 3 times smaller than previous V-shaped designs. In addn., multiple nanoFETs were encoded during synthesis in one of the arms/tip of V-shaped and distinct arms/tips of W-shaped KNWs. These new multiplexed KNW structures were structurally verified by optical and electron microscopy of dopant-selective etched samples and elec. characterized using scanning gate microscopy and transport measurements. The facile design and bottom-up synthesis of these diverse functional KNWs provides a growing toolbox of building blocks for fabricating highly compact and multiplexed three-dimensional nanoprobes for applications in life sciences, including intracellular and deep tissue/cell recordings.
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    Barth, S.; Hernandez-Ramirez, F.; Holmes, J. D.; Romano-Rodriguez, A. Synthesis and applications of one-dimensional semiconductors. Prog. Mater. Sci. 2010, 55, 563– 627,  DOI: 10.1016/j.pmatsci.2010.02.001
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    Synthesis and applications of one-dimensional semiconductors
    Barth, Sven; Hernandez-Ramirez, Francisco; Holmes, Justin D.; Romano-Rodriguez, Albert
    Progress in Materials Science (2010), 55 (6), 563-627CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
    A review. Nanoscale inorg. materials such as quantum dots (0-dimensional) and one-dimensional (1D) structures, such as nanowires, nanobelts and nanotubes, have gained tremendous attention within the last decade. Among the huge variety of 1D nanostructures, semiconducting nanowires have gained particular interest due to their potential applications in optoelectronic and electronic devices. Despite the huge efforts to control and understand the growth mechanisms underlying the formation of these highly anisotropic structures, some fundamental phenomena are still not well understood. For example, high aspect-ratio semiconductors exhibit unexpected growth phenomena, e.g. diam.-dependent and temp.-dependent growth directions, and unusual high doping levels or compns., which are not known for their macroscopic crystals or thin-film counterparts. This article reviews viable synthetic approaches for growing high aspect-ratio semiconductors from bottom-up techniques, such as crystal structure governed nucleation, metal-promoted vapor phase and soln. growth, formation in non-metal seeded gas-phase processes, structure directing templates and electrospinning. In particular new exptl. findings and theor. models relating to the frequently applied vapor-liq.-solid (VLS) growth are highlighted. In addn., the top-down application of controlled chem. etching, using novel masking techniques, is described as a viable approach for generating certain 1D structures. The review highlights the controlled synthesis of semiconducting nanostructures and heterostructures of silicon, germanium, gallium nitride, gallium arsenide, cadmium sulfide, zinc oxide and tin oxide. The alignment of 1D nanostructures will be reviewed briefly. While specific and reliable contact procedures are still a major challenge for the integration of 1D nanostructures as active building blocks, this issue will not be the focus of this paper. However, the promising applications of 1D semiconductors will be highlighted, particularly with ref. to surface dependent electronic transduction (gas and biol. sensors), energy generation (nanomech. and photovoltaic) devices, energy storage (lithium storage in battery anodes) as well as nanowire photonics.
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    Lugstein, A.; Steinmair, M.; Hyun, Y. J.; Hauer, G.; Pongratz, P.; Bertagnolli, E. Pressure-Induced Orientation Control of the Growth of Epitaxial Silicon Nanowires. Nano Lett. 2008, 8, 2310– 4,  DOI: 10.1021/nl8011006
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    Pressure-Induced Orientation Control of the Growth of Epitaxial Silicon Nanowires
    Lugstein, A.; Steinmair, M.; Hyun, Y. J.; Hauer, G.; Pongratz, P.; Bertagnolli, E.
    Nano Letters (2008), 8 (8), 2310-2314CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Single crystal Si nanowires (SiNWs) were synthesized with silane reactant using Au nanocluster-catalyzed 1-dimensional growth. Under the authors' exptl. conditions, SiNWs grown epitaxially on Si(111) via the vapor-liq.-solid growth mechanism change their growth direction as a function of the total pressure. Structural characterization of a large no. of samples shows that SiNWs synthesized at a total pressure of 3 mbar grow preferentially in the 〈111〉 direction, while the one at 15 mbar favors the 〈112〉 direction. Specifically by dynamically changing the system pressure during the growth process morphol. changes of the NW growth directions along their length were demonstrated.
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    Hyun, Y.-J.; Lugstein, A.; Steinmair, M.; Bertagnolli, E.; Pongratz, P. Orientation specific synthesis of kinked silicon nanowires grown by the vapour–liquid–solid mechanism. Nanotechnology 2009, 20, 125606,  DOI: 10.1088/0957-4484/20/12/125606
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    Orientation specific synthesis of kinked silicon nanowires grown by the vapor-liquid-solid mechanism
    Hyun, Youn-Joo; Lugstein, Alois; Steinmair, Mathias; Bertagnolli, Emmerich; Pongratz, Peter
    Nanotechnology (2009), 20 (12), 125606/1-125606/5CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
    Kinked silicon nanowires (Si-NWs) were prepd. in a well reproducible manner using gold nanocluster-catalyzed quasi-1D growth on Si(111) substrates with SiH4 as the precursor gas. The kinking is considered to be due to the change in the growth direction induced by the sudden change of the pressure during Si-NW prepn. Structural high resoln. transmission electron microscopy (HRTEM) characterization of the sample shows that epitaxial Si-NWs prepd. on Si(111) substrates at a total pressure of 3 mbar grow along the 〈111〉 direction, while the ones at 15 mbar favor the 〈112〉 direction. By dynamically changing the system pressure during the growth process morphol. changes of the NW growth directions along their length were shown, resulting in kinked nanowires. The crystallog. orientation relation of the kinking between the 3 and 15 mbar ranges was analyzed by TEM. No defects or grain boundaries in the intersection between the two sections of the Si-NWs are necessary to form such kinks between different wire directions.
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    Geaney, H.; Dickinson, C.; Weng, W.; Kiely, C. J.; Barrett, C. A.; Gunning, R. D.; Ryan, K. M. Role of Defects and Growth Directions in the Formation of Periodically Twinned and Kinked Unseeded Germanium Nanowires. Cryst. Growth Des. 2011, 11, 3266– 72,  DOI: 10.1021/cg200510y
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    Role of Defects and Growth Directions in the Formation of Periodically Twinned and Kinked Unseeded Germanium Nanowires
    Geaney, Hugh; Dickinson, Calum; Weng, Weihao; Kiely, Christopher J.; Barrett, Christopher A.; Gunning, Robert D.; Ryan, Kevin M.
    Crystal Growth & Design (2011), 11 (7), 3266-3272CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    The authors show the impact of preferred growth directions and defects in the formation of complex Ge nanowire (NW) structures grown by a simple org. medium based synthesis. Various types of NWs are examd. including: straight defect free NWs; periodically bent NWs with precise angles between the NW segments; NWs with mutually exclusive lateral or longitudinal faults; and more complex "wormlike" structures. The authors show that choice of solvent and reaction temp. can be used to tune the morphol. of the NWs formed. The various types of NWs were probed in depth using transmission electron microscopy (TEM), SEM, selected area electron diffraction (SAED), and dark field TEM (DFTEM).
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    Kim, J. H.; Moon, S. R.; Yoon, H. S.; Jung, J. H.; Kim, Y.; Chen, Z. G.; Zou, J.; Choi, D. Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Taper-Free and Vertically Oriented Ge Nanowires on Ge/Si Substrates Grown by a Two-Temperature Process. Cryst. Growth Des. 2012, 12, 135– 41,  DOI: 10.1021/cg2008914
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    Taper-Free and Vertically Oriented Ge Nanowires on Ge/Si Substrates Grown by a Two-Temperature Process
    Kim, Jung Hyuk; Moon, So Ra; Yoon, Hyun Sik; Jung, Jae Hun; Kim, Yong; Chen, Zhi Gang; Zou, Jin; Choi, Duk Yong; Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati
    Crystal Growth & Design (2012), 12 (1), 135-141CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    Taper-free and vertically oriented Ge nanowires were grown on Si (111) substrates by CVD with Au nanoparticle catalysts. To achieve vertical nanowire growth on the highly lattice mismatched Si substrate, a thin Ge buffer layer was 1st deposited, and to achieve taper-free nanowire growth, a two-temp. process was employed. The two-temp. process consisted of a brief initial base growth step at high temp. followed by prolonged growth at lower temp. Taper-free and defect-free Ge nanowires grew successfully even at 270°, which is 90° lower than the bulk eutectic temp. The yield of vertical and taper-free nanowires is over 90%, comparable to that of vertical but tapered nanowires grown by the conventional 1-temp. process. This method is of practical importance and can be reliably used to develop novel nanowire-based devices on relatively cheap Si substrates. Addnl., the activation energy of Ge nanowire growth by the two-temp. process is dependent on Au nanoparticle size. The low activation energy (∼5 kcal/mol) for 30 and 50 nm diam. Au nanoparticles suggests that the decompn. of gaseous species on the catalytic Au surface is a rate-limiting step. A higher activation energy (∼14 kcal/mol) was detd. for 100 nm diam. Au nanoparticles which suggests that larger Au nanoparticles are partially solidified and that growth kinetics become the rate-limiting step.
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    Wu, Z. H.; Mei, X.; Kim, D.; Blumin, M.; Ruda, H. E.; Liu, J. Q.; Kavanagh, K. L. Growth, branching, and kinking of molecular-beam epitaxial ⟨110⟩ GaAs nanowires. Appl. Phys. Lett. 2003, 83, 3368– 70,  DOI: 10.1063/1.1618018
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    Growth, branching, and kinking of molecular-beam epitaxial 〈110〉 GaAs nanowires
    Wu, Z. H.; Mei, X.; Kim, D.; Blumin, M.; Ruda, H. E.; Liu, J. Q.; Kavanagh, K. L.
    Applied Physics Letters (2003), 83 (16), 3368-3370CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    GaAs nanowires were grown on GaAs (100) substrates by vapor-liq.-solid growth. About 8% of these nanowires grew in the 〈110〉 direction with straight, Y-branched, or L-shaped morphologies. The role of strain-induced redn. in surface free energy is discussed as a possible factor contributing to the evolution of 〈110〉 nanowires. Kinking and branching is attributed to growth instabilities resulting from equiv. surface free energies for 〈110〉 growth directions. Transmission electron microscopy verified that the 〈110〉 nanowires were defect free.
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    Peng, H.; Meister, S.; Chan, C. K.; Zhang, X. F.; Cui, Y. Morphology Control of Layer-Structured Gallium Selenide Nanowires. Nano Lett. 2007, 7, 199– 203,  DOI: 10.1021/nl062047+
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    Morphology Control of Layer-Structured Gallium Selenide Nanowires
    Peng, Hailin; Meister, Stefan; Chan, Candace K.; Zhang, Xiao Feng; Cui, Yi
    Nano Letters (2007), 7 (1), 199-203CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Layer-structured Group III chalcogenides have highly anisotropic properties and are attractive materials for stable photocathodes and battery electrodes. The authors report the controlled synthesis and characterization of layer-structured GaSe nanowires via a catalyst-assisted vapor-liq.-solid (VLS) growth mechanism during GaSe powder evapn. GaSe nanowires consist of Se-Ga-Ga-Se layers stacked together via van der Waals interactions to form belt-shaped nanowires with a growth direction along the [11-20], width along the [1-100], and height along the [0001] direction. Nanobelts exhibit a variety of morphologies including straight, zigzag, and saw-tooth shapes. These morphologies are realized by controlling the growth temp. and time so that the actual catalysts have a chem. compn. of Au, Au-Ga alloy, or Ga. The participation of Ga in the VLS catalyst is important for achieving different morphologies of GaSe. GaSe nanotubes are also prepd. by a slow growth process.
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    Jiang, Z.; Qing, Q.; Xie, P.; Gao, R.; Lieber, C. M. Kinked p–n Junction Nanowire Probes for High Spatial Resolution Sensing and Intracellular Recording. Nano Lett. 2012, 12, 1711– 6,  DOI: 10.1021/nl300256r
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    Kinked p-n Junction Nanowire Probes for High Spatial Resolution Sensing and Intracellular Recording
    Jiang, Zhe; Qing, Quan; Xie, Ping; Gao, Ruixuan; Lieber, Charles M.
    Nano Letters (2012), 12 (3), 1711-1716CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Semiconductor nanowires and other semiconducting nanoscale materials configured as field-effect transistors have been studied extensively as biol./chem. sensors. These nanomaterials have demonstrated high-sensitivity from one- and two-dimensional sensors, although the realization of the ultimate pointlike detector has not been achieved. In this regard, nanoscale p-n diodes are attractive since the device element is naturally localized near the junction, and while nanowire p-n diodes have been widely studied as photovoltaic devices, their applications as bio/chem. sensors have not been explored. Here the authors demonstrate that p-n diode devices can serve as a new and powerful family of highly localized biosensor probes. Designed nanoscale axial p-n junctions were synthetically introduced at the joints of kinked silicon nanowires. SEM images showed that the kinked nanowire structures were achieved, and elec. transport measurements exhibited rectifying behavior with well-defined turn-on in forward bias as expected for a p-n diode. In addn., scanning gate microscopy demonstrated that the most sensitive region of these nanowires was localized near the kinked region at the p-n junction. High spatial resoln. sensing using these p-n diode probes was carried out in aq. soln. using fluorescent charged polystyrene nanobeads. Multiplexed elec. measurements show well-defined single-nanoparticle detection, and expts. with simultaneous confocal imaging correlate directly the motion of the nanobeads with the elec. signals recorded from the p-n devices. In addn., kinked p-n junction nanowires configured as three-dimensional probes demonstrate the capability of intracellular recording of action potentials from electrogenic cells. These p-n junction kinked nanowire devices, which represent a new way of constructing nanoscale probes with highly localized sensing regions, provide substantial opportunity in areas ranging from bio/chem. sensing and nanoscale photon detection to three-dimensional recording from within living cells and tissue.
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    Jiang, J.-W.; Zhao, J.-H.; Rabczuk, T. Size-sensitive Young’s modulus of kinked silicon nanowires. Nanotechnology 2013, 24, 185702,  DOI: 10.1088/0957-4484/24/18/185702
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    Size-sensitive Young's modulus of kinked silicon nanowires
    Jiang, Jin-Wu; Zhao, Jun-Hua; Rabczuk, Timon
    Nanotechnology (2013), 24 (18), 185702/1-185702/6, 6 pp.CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    We perform both classical mol. dynamics simulations and beam model calcns. to investigate the Young's modulus of kinked silicon nanowires (KSiNWs). The Young's modulus is found to be highly sensitive to the arm length of the kink and is essentially inversely proportional to the arm length. The mechanism underlying the size dependence is found to be the interplay between the kink angle potential and the arm length potential, where we obtain an analytic relationship between the Young's modulus and the arm length of the KSiNW. Our results provide insight into the application of this novel building block in nanomech. devices.
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    Jiang, J.-W.; Rabczuk, T. Mechanical oscillation of kinked silicon nanowires: A natural nanoscale spring. Appl. Phys. Lett. 2013, 102, 123104,  DOI: 10.1063/1.4799029
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    Mechanical oscillation of kinked silicon nanowires. A natural nanoscale spring
    Jiang, Jin-Wu; Rabczuk, Timon
    Applied Physics Letters (2013), 102 (12), 123104/1-123104/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We perform classical mol. dynamics simulations to demonstrate the application of kinked Si nanowires (KSiNWs) as nanoscale springs. The spring-like oscillation in gigahertz frequency range is successfully actuated using a similar procedure as the actuation of a classical mass spring oscillator. We detect the spring-like mech. oscillation and some other low-frequency oscillations by the energy spectrum anal., where a dimensional crossover phenomenon is obsd. for the transverse mode in KSiNWs with decreasing aspect ratio. Our findings shed light on the elastic properties of the KSiNW and open a way for its application in nanomech. devices. (c) 2013 American Institute of Physics.
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    Hillerich, K.; Dick, K. A.; Wen, C.-Y.; Reuter, M. C.; Kodambaka, S.; Ross, F. M. Strategies To Control Morphology in Hybrid Group III–V/Group IV Heterostructure Nanowires. Nano Lett. 2013, 13, 903– 8,  DOI: 10.1021/nl303660h
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    Strategies To Control Morphology in Hybrid Group III-V/Group IV Heterostructure Nanowires
    Hillerich, Karla; Dick, Kimberly A.; Wen, Cheng-Yen; Reuter, Mark C.; Kodambaka, Suneel; Ross, Frances M.
    Nano Letters (2013), 13 (3), 903-908CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    By combining in situ and ex situ TEM measurements, the authors examine the factors that control the morphol. of hybrid nanowires that include group III-V and Group IV materials. The authors focus on one materials pair, GaP/Si, for which the authors use a wide range of growth parameters. The authors show through video imaging that nanowire morphol. depends on growth conditions, but that a general pattern emerges where either single kinks or inclined defects form some distance after the heterointerface. Pure Si nanowires can be made to exhibit the same kinks and defects by changing their droplet vol. From this the authors derive a model where droplet geometry drives growth morphol. and discuss optimization strategies. The authors finally discuss morphol. control for material pairs where the 2nd material kinks immediately at the heterointerface and show that an interlayer between segments can enable the growth of unkinked hybrid nanowires.
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  57. 57
    Dick, K. A.; Kodambaka, S.; Reuter, M. C.; Deppert, K.; Samuelson, L.; Seifert, W.; Wallenberg, L. R.; Ross, F. M. The Morphology of Axial and Branched Nanowire Heterostructures. Nano Lett. 2007, 7, 1817– 22,  DOI: 10.1021/nl0705900
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    The Morphology of Axial and Branched Nanowire Heterostructures
    Dick, Kimberly A.; Kodambaka, Suneel; Reuter, Mark C.; Deppert, Knut; Samuelson, Lars; Seifert, Werner; Wallenberg, L. Reine; Ross, Frances M.
    Nano Letters (2007), 7 (6), 1817-1822CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphol. of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphol. (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.
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    Dayeh, S. A.; Wang, J.; Li, N.; Huang, J. Y.; Gin, A. V.; Picraux, S. T. Growth, Defect Formation, and Morphology Control of Germanium–Silicon Semiconductor Nanowire Heterostructures. Nano Lett. 2011, 11, 4200– 6,  DOI: 10.1021/nl202126q
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    Growth, defect formation, and morphology control of germanium-silicon semiconductor nanowire heterostructures
    Dayeh, Shadi A.; Wang, Jian; Li, Nan; Huang, Jian Yu; Gin, Aaron V.; Picraux, S. Thomas
    Nano Letters (2011), 11 (10), 4200-4206CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    By the virtue of the nature of the vapor-liq.-solid (VLS) growth process in semiconductor nanowires (NWs) and their small size, the nucleation, propagation, and termination of stacking defects in NWs are dramatically different from that in thin films. We demonstrate Ge-Si axial NW heterostructure growth by the VLS method with 100% compn. modulation and use these structures as a platform to understand how defects in stacking sequence force the ledge nucleation site to be moved along or pinned at a single point on the triple-phase circumference, which in turn dets. the NW morphol. Combining structural anal. and atomistic simulation of the nucleation and propagation of stacking defects, we explain these observations based on preferred nucleation sites during NW growth. The stacking defects are found to provide a fingerprint of the layer-by-layer growth process and reveal how the 19.5° kinking in semiconductor NWs obsd. at high Si growth rates results from a stacking-induced twin boundary formation at the NW edge. This study provides basic foundations for an at. level understanding of cryst. and defective ledge nucleation and propagation during [111] oriented NW growth and improves understanding for control of fault nucleation and kinking in NWs.
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    Paladugu, M.; Zou, J.; Guo, Y.-N.; Auchterlonie, G. J.; Joyce, H. J.; Gao, Q.; Hoe Tan, H.; Jagadish, C.; Kim, Y. Novel Growth Phenomena Observed in Axial InAs/GaAs Nanowire Heterostructures. Small 2007, 3, 1873,  DOI: 10.1002/smll.200700222
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    Novel growth phenomena observed in axial InAs/GaAs nanowire heterostructures
    Paladugu, Mohanchand; Zou, Jin; Guo, Ya-Nan; Auchterlonie, Graeme J.; Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Kim, Yong
    Small (2007), 3 (11), 1873-1877CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Gold on the move... A novel growth phenomenon of axial InAs/GaAs nanowire heterostructures catalyzed by Au particles was obsd. Transmission electron microscopy has detd. a sequence of events: (1) Displacement of the Au particle at the end of the nanowire due to InAs clustering, (2) further InAs growth leading to sideways movement of the Au particle, and (3) eventual downward nanowire growth due to the preservation of a Au/GaAs interface (see scheme).
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    He, Z.; Nguyen, H. T.; Duc Toan, L.; Pribat, D. A detailed study of kinking in indium-catalyzed silicon nanowires. CrystEngComm 2015, 17, 6286,  DOI: 10.1039/C5CE00773A
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    A detailed study of kinking in indium-catalyzed silicon nanowires
    He, Zhanbing; Nguyen, Hung Tran; Le, Duc Toan; Pribat, Didier
    CrystEngComm (2015), 17 (33), 6286-6296CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
    Kinking of semiconductor nanowires grown by the vapor-solid-liq. (VSL) mechanism has long been obsd. and studied, particularly for Si. A large variety of turning angles for kinked Si nanowires (KSiNWs) has been reported in the literature, but most authors have studied the kinking mechanism rather than the structure and corresponding geometrical features of the kinks. Here, we have investigated the relationship between the turning angles and the structure (down to at. level) of KSiNWs grown by VSL from indium nanoparticles. By using transmission electron microscopy, we have characterized the transition regions between different segments of KSiNWs of various crystallog. orientations. We have found that most turning angles can be viewed as rich combinations of different types of {111} coherent twins that coexist within the transition regions between different segments of KSiNWs.
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    McIntyre, P. C.; Fontcuberta i Morral, I. Semiconductor nanowires: to grow or not to grow?. Mater. Today Nano 2020, 9, 100058,  DOI: 10.1016/j.mtnano.2019.100058
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    Thombare, S. V.; Marshall, A. F.; McIntyre, P. C. Size effects in vapor-solid-solid Ge nanowire growth with a Ni-based catalyst. J. Appl. Phys. 2012, 112, 054325  DOI: 10.1063/1.4749797
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    Size effects in vapor-solid-solid Ge nanowire growth with a Ni-based catalyst
    Thombare, S. V.; Marshall, A. F.; McIntyre, P. C.
    Journal of Applied Physics (2012), 112 (5), 054325/1-054325/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We report a dramatic size effect on the morphol. of Ge nanowires prepd. by low-temp. vapor-solid-solid (VSS) growth using a NiGe catalyst. Nanowires with diam. greater than 25 nm are [111]-oriented, have a high d. of grown-in defects, and exhibit frequent kinking. However, nanowires with diam. below 25 nm are straight, despite also having a substantial d. of crystal defects. The latter wires grow preferentially in the [110] direction. The absence of kinking in the small nanowires coincides with the observation of a low-energy, epitaxial NiGe/Ge interface. The occurrence of (1) this solid-solid epitaxial interface and (2) the low-energy sidewall facets of the [110] wire orientation strongly bias the Ni-Ge binary system toward kink-free nanowire growth in the VSS regime. Kinking in larger nanowires occurs via multiple twinning events facilitated by the slow growth and anisotropic catalyst/wire interfaces typical of VSS growth. Such effects are expected in other VSS systems where a range of nanowire morphologies are obsd. (c) 2012 American Institute of Physics.
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    Fahlvik Svensson, S.; Jeppesen, S.; Thelander, C.; Samuelson, L.; Linke, H.; Dick, K. A. Control and understanding of kink formation in InAs–InP heterostructure nanowires. Nanotechnology 2013, 24, 345601,  DOI: 10.1088/0957-4484/24/34/345601
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    Control and understanding of kink formation in InAs-InP heterostructure nanowires
    Fahlvik Svensson, S.; Jeppesen, S.; Thelander, C.; Samuelson, L.; Linke, H.; Dick, K. A.
    Nanotechnology (2013), 24 (34), 345601, 9 pp.CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Nanowire heterostructures are of special interest for band structure engineering due to an expanded range of defect-free material combinations. However, the higher degree of freedom in nanowire heterostructure growth comes at the expense of challenges related to nanowire-seed particle interactions, such as undesired compn., grading and kink formation. To better understand the mechanisms of kink formation in nanowires, we here present a detailed study of the dependence of heterostructure nanowire morphol. on indium pressure, nanowire diam., and nanowire d. We investigate InAs-InP-InAs heterostructure nanowires grown with chem. beam epitaxy, which is a material system that allows for very abrupt heterointerfaces. Our observations indicate that the crit. parameter for kink formation is the availability of indium, and that the resulting morphol. is also highly dependent on the length of the InP segment. It is shown that kinking is assocd. with the formation of an inclined facet at the interface between InP and InAs, which destabilizes the growth and leads to a change in growth direction. By careful tuning of the growth parameters, it is possible to entirely suppress the formation of this inclined facet and thereby kinking at the heterointerface. Our results also indicate the possibility of producing controllably kinked nanowires with a high yield.
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    Madras, P.; Dailey, E.; Drucker, J. Kinetically Induced Kinking of Vapor–Liquid–Solid Grown Epitaxial Si Nanowires. Nano Lett. 2009, 9, 3826– 30,  DOI: 10.1021/nl902013g
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    Kinetically Induced Kinking of Vapor-Liquid-Solid Grown Epitaxial Si Nanowires
    Madras, Prashanth; Dailey, Eric; Drucker, Jeff
    Nano Letters (2009), 9 (11), 3826-3830CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Epitaxial Si nanowires grown from Au seeds using the vapor-liq.-solid method begin growing normal to the Si(111) substrate atop a tapered base. After a kinetically detd. length, the NWs may kink away from [111] to another crystallog. direction. The smallest NWs prefer growth along 〈110〉 while larger Si NWs choose either 〈111〉 or 〈112〉 based on whether growth conditions favor Au-free sidewalls. "Vertical" growth normal to the Si(111) substrate is obtained only for slowly growing NWs with Au-decorated sidewalls. At the fastest growth rates, single-crystal Si NWs smoothly, continuously, and randomly vary their growth directions, producing a morphol. that is qual. different than highly kinked growth.
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    Sun, Z.; Seidman, D. N.; Lauhon, L. J. Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid–Solid Growth Interface. Nano Lett. 2017, 17, 4518– 25,  DOI: 10.1021/acs.nanolett.7b02071
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    Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface
    Sun, Zhiyuan; Seidman, David N.; Lauhon, Lincoln J.
    Nano Letters (2017), 17 (7), 4518-4525CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Dopants modify the electronic properties of semiconductors, including their susceptibility to etching. In semiconductor nanowires doped during growth by the vapor-liq.-solid (VLS) process, it has been shown that nanofaceting of the liq.-solid growth interface influences strongly the radial distribution of dopants. Hence, the combination of facet-dependent doping and dopant selective etching provides a means to tune simultaneously the electronic properties and morphologies of nanowires. Using atom-probe tomog., we investigated the boron dopant distribution in Au catalyzed VLS grown silicon nanowires, which regularly kink between equiv. 〈112〉 directions. Segments alternate between radially uniform and nonuniform doping profiles, which we attribute to switching between a concave and convex faceted liq.-solid interface. Dopant selective etching was used to reveal and correlate the shape of the growth interface with the obsd. anisotropic doping.
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    Yuan, X.; Caroff, P.; Wong-Leung, J.; Fu, L.; Tan, H. H.; Jagadish, C. Tunable Polarity in a III–V Nanowire by Droplet Wetting and Surface Energy Engineering. Adv. Mater. 2015, 27, 6096– 103,  DOI: 10.1002/adma.201503540
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    Tunable Polarity in a III-V Nanowire by Droplet Wetting and Surface Energy Engineering
    Yuan, Xiaoming; Caroff, Philippe; Wong-Leung, Jennifer; Fu, Lan; Tan, Hark Hoe; Jagadish, Chennupati
    Advanced Materials (Weinheim, Germany) (2015), 27 (40), 6096-6103CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors use GaAs nanowires as a model system and demonstrate control over their polarity between <111>B and <111>A using a simple combination of nucleation and surface energy engineering. All nanowires were grown via a gold-seeded vapor-liq.-solid (VLS) growth mechanism in a std. metalorg. vapor phase epitaxy (MOVPE) reactor. Preloading the Au nanoparticles with Sb or Ga is shown to be effective in favoring nanowire nucleation in the <111>A direction. Minimizing the interface energy difference between the {111}A and {111}B facets by adjusting the growth conditions stabilizes nanowire growth in the <111>A direction.
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    Shin, N.; Chi, M.; Filler, M. A. Interplay between Defect Propagation and Surface Hydrogen in Silicon Nanowire Kinking Superstructures. ACS Nano 2014, 8, 3829– 35,  DOI: 10.1021/nn500598d
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    Interplay between Defect Propagation and Surface Hydrogen in Silicon Nanowire Kinking Superstructures
    Shin, Naechul; Chi, Miaofang; Filler, Michael A.
    ACS Nano (2014), 8 (4), 3829-3835CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Semiconductor nanowire kinking superstructures, particularly those with long-range structural coherence, remain difficult to fabricate. The authors combine high-resoln. electron microscopy with IR spectroscopy to show why this is the case for Si nanowires and, in doing so, reveal the interplay between defect propagation and surface chem. during 〈211〉 → 〈111〉 and 〈211〉 → 〈211〉 kinking. The adsorbed hydrogen atoms are responsible for selecting 〈211〉-oriented growth and indicate that a twin boundary imparts structural coherence. The twin boundary, only continuous at 〈211〉 → 〈211〉 kinks, reduces the symmetry of the trijunction and limits the no. of degenerate directions available to the nanowire. These findings constitute a general approach for rationally engineering kinking superstructures and also provide important insight into the role of surface chem. bonding during vapor-liq.-solid synthesis.
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    Schmidt, V.; Senz, S.; Gösele, U. Diameter-Dependent Growth Direction of Epitaxial Silicon Nanowires. Nano Lett. 2005, 5, 931– 5,  DOI: 10.1021/nl050462g
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    Diameter-Dependent Growth Direction of Epitaxial Silicon Nanowires
    Schmidt, Volker; Senz, Stephan; Goesele, Ulrich
    Nano Letters (2005), 5 (5), 931-935CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Si nanowires grown epitaxially on Si (100) via the vapor-liq.-solid growth mechanism change their growth direction from 〈111〉 to 〈110〉 at a crossover diam. of ∼20 nm. A model is proposed for the explanation of this phenomenon. Probably the interplay of the liq.-solid interfacial energy with the Si surface energy expressed in terms of an edge tension is responsible for the change of the growth direction. The value of the edge tension is estd. by the product of the interfacial thickness with the surface energy of Si. For large diams., the direction with the lowest interfacial energy is dominant, whereas for small diams. the surface energy of the Si nanowire dets. the preferential growth direction.
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    Wu, Y.; Cui, Y.; Huynh, L.; Barrelet, C. J.; Bell, D. C.; Lieber, C. M. Controlled Growth and Structures of Molecular-Scale Silicon Nanowires. Nano Lett. 2004, 4, 433– 6,  DOI: 10.1021/nl035162i
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    Controlled Growth and Structures of Molecular-Scale Silicon Nanowires
    Wu, Yue; Cui, Yi; Huynh, Lynn; Barrelet, Carl J.; Bell, David C.; Lieber, Charles M.
    Nano Letters (2004), 4 (3), 433-436CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Single-crystal silicon nanowires with diams. approaching mol. dimensions were synthesized using gold nanocluster-catalyzed 1-dimensional growth. High-resoln. TEM studies show that silicon nanowires grown with silane reactant in hydrogen are single crystal with little or no visible amorphous oxide down to diams. ≥3 nm. Structural characterization of a large no. of samples shows that the smallest-diam. nanowires grow primarily along the 〈110〉 direction, whereas larger nanowires grow along the 〈111〉 direction. Cross-sectional TEM was used to address the importance of surface energetics in detg. the growth direction of the smallest nanowires. The ability to prep. well-defined mol.-scale single-crystal silicon nanowires opens up new opportunities for both fundamental studies and nanodevice applications.
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    Wen, C.-Y.; Reuter, M. C.; Tersoff, J.; Stach, E. A.; Ross, F. M. Structure, Growth Kinetics, and Ledge Flow during Vapor–Solid–Solid Growth of Copper-Catalyzed Silicon Nanowires. Nano Lett. 2010, 10, 514– 9,  DOI: 10.1021/nl903362y
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    Structure, Growth Kinetics, and Ledge Flow during Vapor-Solid-Solid Growth of Copper-Catalyzed Silicon Nanowires
    Wen, C.-Y.; Reuter, M. C.; Tersoff, J.; Stach, E. A.; Ross, F. M.
    Nano Letters (2010), 10 (2), 514-519CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We use real-time observations of the growth of Cu-catalyzed silicon nanowires to det. the nanowire growth mechanism directly and to quantify the growth kinetics of individual wires. Nanowires were grown in a transmission electron microscope using chem. vapor deposition on a copper-coated Si substrate. We show that the initial reaction is the formation of a silicide, η'-Cu3Si, and that this solid silicide remains on the wire tips during growth so that growth is by the vapor-solid-solid mechanism. Individual wire directions and growth rates are related to the details of orientation relation and catalyst shape, leading to a rich morphol. compared to vapor-liq.-solid grown nanowires. Furthermore, growth occurs by ledge propagation at the silicide/silicon interface, and the ledge propagation kinetics suggest that the soly. of precursor atoms in the catalyst is small, which is relevant to the fabrication of abrupt heterojunctions in nanowires.
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    Wang, Y.; Schmidt, V.; Senz, S.; Gösele, U. Epitaxial growth of silicon nanowires using an aluminium catalyst. Nat. Nanotechnol. 2006, 1, 186– 9,  DOI: 10.1038/nnano.2006.133
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    Epitaxial growth of silicon nanowires using an aluminium catalyst
    Wang, Yewu; Schmidt, Volker; Senz, Stephan; Goesele, Ulrich
    Nature Nanotechnology (2006), 1 (3), 186-189CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Si nanowires were identified as important components for future electronic and sensor nanodevices. So far Au has dominated as the catalyst for growing Si nanowires via the vapor-liq.-solid (VLS) mechanism. Unfortunately, Au traps electrons and holes in Si and poses a serious contamination problem for Si complementary metal oxide semiconductor (CMOS) processing. Although there are some reports on the use of non-Au catalysts for Si nanowire growth, either the growth requires high temps. and/or the catalysts are not compatible with CMOS requirements. From a technol. standpoint, a much more attractive catalyst material would be Al, as it is a std. metal in Si process lines. Here we report for the 1st time the epitaxial growth of Al-catalyzed Si nanowires and suggest that growth proceeds via a vapor-solid-solid (VSS) rather than a VLS mechanism. The tapering of the nanowires can be strongly reduced by lowering the growth temp.
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    Wacaser, B. A.; Reuter, M. C.; Khayyat, M. M.; Wen, C.-Y.; Haight, R.; Guha, S.; Ross, F. M. Growth System, Structure, and Doping of Aluminum-Seeded Epitaxial Silicon Nanowires. Nano Lett. 2009, 9, 3296– 301,  DOI: 10.1021/nl9015792
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    Growth System, Structure, and Doping of Aluminum-Seeded Epitaxial Silicon Nanowires
    Wacaser, Brent A.; Reuter, Mark C.; Khayyat, Maha M.; Wen, Cheng-Yen; Haight, Richard; Guha, Supratik; Ross, Frances M.
    Nano Letters (2009), 9 (9), 3296-3301CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We have examd. the formation of Si nanowires grown by self-assembly from Si substrates with thin Al films. Post-growth and in situ studies using various Al deposition and annealing conditions suggest that nanowire growth takes place with a supercooled liq. droplet (i.e., the vapor-liq.-solid system), even though the growth temps. are below the bulk Al/Si eutectic temp. Wire morphol. as a function of processing conditions is also described. When Al environmental exposure is prevented before wire growth a wide process window for wire formation can be achieved. Under optimum growth conditions, it is possible to produce excellent crystal quality nanowires with rapid growth rates, high surface densities, low diam. dispersion, and controlled tapering. Photoelectron spectroscopy measurements indicate that the use of Al leads to active doping levels that depend on the growth temp. in as-grown nanowires and increase when annealed. We suggest that these structural and electronic properties will be relevant to photovoltaic and other applications, where the more common use of Au is believed to be detrimental to performance.
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    Hainey, M.; Eichfeld, S. M.; Shen, H.; Yim, J.; Black, M. R.; Redwing, J. M. Aluminum-Catalyzed Growth of ⟨110⟩ Silicon Nanowires. J. Electron. Mater. 2015, 44, 1332– 7,  DOI: 10.1007/s11664-014-3565-8
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    Aluminum-Catalyzed Growth of '110' Silicon Nanowires
    Hainey, Mel, Jr.; Eichfeld, Sarah M.; Shen, Haoting; Yim, Joanne; Black, Marcie R.; Redwing, Joan M.
    Journal of Electronic Materials (2015), 44 (5), 1332-1337CODEN: JECMA5; ISSN:0361-5235. (Springer)
    The growth of silicon nanowires in the '110' direction is reported using a vapor-liq.-solid mechanism with aluminum as the catalyst and SiH4 as the source gas in a low pressure chem. vapor deposition process. The effects of growth conditions on the yield of '110' vs. '111' nanowires were investigated. Increasing reactor pressure beyond 300 Torr was found to improve '110' wire yield by suppressing vapor-solid thin film deposition on the nanowire sidewalls during growth that promoted nanowire kinking. Addnl., '110' growth was found to occur only at temps. below the Al-Si eutectic temp. (577°C). At temps. approx. equal to 577°C or higher, the preferential growth direction was obsd. to shift from '110' to '111'. The growth of '110' Si nanowires at sub-eutectic temps. was attributed to a redn. in the silicon concn. in the catalyst droplet which promotes (110) surface nucleation and subsequent growth in the '110' direction.
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    Conesa-Boj, S.; Zardo, I.; Estradé, S.; Wei, L.; Jean Alet, P.; Roca i Cabarrocas, P.; Morante, J. R.; Peiró, F.; Morral, A F i; Arbiol, J. Defect Formation in Ga-Catalyzed Silicon Nanowires. Cryst. Growth Des. 2010, 10, 1534– 43,  DOI: 10.1021/cg900741y
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    Defect formation in Ga-catalyzed silicon nanowires
    Conesa-Boj, Sonia; Zardo, Ilaria; Estrade, Sonia; Wei, Li; Jean Alet, Pierre; Roca i Cabarrocas, Pere; Morante, Joan R.; Peiro, Francesca; Fontcuberta i Morral, Anna; Arbiol, Jordi
    Crystal Growth & Design (2010), 10 (4), 1534-1543CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    The synthesis of Si nanowires by Ga-assisted plasma enhanced CVD (PECVD) was recently demonstrated. We study in detail the structural characteristics of the synthesized nanowires. High resoln. TEM (HRTEM) anal. reveals the existence of various types of structural defects, which can be classified mainly according to the orientation into axial twins, lateral twins, and transverse twins. We compare our results with previous studies of Si nanowires synthesized with other catalyst metals. Understanding both the origin and the effects of the obsd. defects is important for technol. applications. The presence of twinned domains changes locally the structure of the material. As a consequence, one should find a different local d. of states and band gap, which should result in a variation of the carrier transport and optical properties of the nanowires.
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    Sharma, S.; Sunkara, M. K. Direct synthesis of single-crystalline silicon nanowires using molten gallium and silane plasma. Nanotechnology 2004, 15, 130– 4,  DOI: 10.1088/0957-4484/15/1/025
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    Direct synthesis of single-crystalline silicon nanowires using molten gallium and silane plasma
    Sharma, S.; Sunkara, M. K.
    Nanotechnology (2004), 15 (1), 130-134CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
    We report the first ever demonstration of using silane directly in the gas phase and molten gallium in a microwave plasma for bulk nucleation and growth of single-crystal quality silicon nanowires. Multiple nanowires nucleated and grew from micron- to millimetre-sized gallium droplets. The resulting nanowires were tens to hundreds of nanometers in diam. and were tens to hundreds of microns long. Transmission electron microscopy results confirmed the nanowires to be single cryst., devoid of structural defects, and grown along the {100} direction. The as-synthesized silicon nanowires were sheathed with an ultrathin (< 1 nm) non-uniform native oxide amorphous layer that could be directly used in the assembly of electronic and optoelectronic devices.
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    Wang, Z. W.; Li, Z. Y. Structures and Energetics of Indium-Catalyzed Silicon Nanowires. Nano Lett. 2009, 9, 1467– 71,  DOI: 10.1021/nl803345u
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    Structures and Energetics of Indium-Catalyzed Silicon Nanowires
    Wang, Z. W.; Li, Z. Y.
    Nano Letters (2009), 9 (4), 1467-1471CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Strong size dependent structures of Si nanowires, grown via In nanoparticles, were revealed by high-resoln. TEM studies. Below the crit. value of particle diam. of ∼100 nm, the growth changes from 〈111〉 to predominantly 〈211〉 direction and the formation of multiple {111} twins changes from perpendicular to the 〈111〉 growth direction to parallel to the 〈211〉 axial direction. The growth mechanisms are discussed in terms of relative surface/interface energy, using Au catalyzed Si nanowires as a comparative benchmark.
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    Schwarz, K. W.; Tersoff, J.; Kodambaka, S.; Chou, Y.-C.; Ross, F. M. Geometrical Frustration in Nanowire Growth. Phys. Rev. Lett. 2011, 107, 265502,  DOI: 10.1103/PhysRevLett.107.265502
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    Geometrical Frustration in Nanowire Growth
    Schwarz, K. W.; Tersoff, J.; Kodambaka, S.; Chou, Y.-C.; Ross, F. M.
    Physical Review Letters (2011), 107 (26), 265502/1-265502/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Idealized nanowire geometries assume stable sidewalls at right angles to the growth front. Here we report growth simulations that include a mix of nonorthogonal facet orientations, as for Au-catalyzed Si. We compare these with in situ microscopy observations, finding striking correspondences. In both expts. and simulations, there are distinct growth modes that accommodate the lack of right angles in different ways-one through sawtooth-textured sidewalls, the other through a growth front at an angle to the growth axis. Small changes in conditions can reversibly switch the growth between modes. The fundamental differences between these modes have important implications for control of nanowire growth.
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    Hughes, W. L.; Wang, Z. L. Formation of Piezoelectric Single-Crystal Nanorings and Nanobows. J. Am. Chem. Soc. 2004, 126, 6703– 9,  DOI: 10.1021/ja049266m
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    Formation of piezoelectric single-crystal nanorings and nanobows
    Hughes, William L.; Wang, Zhong L.
    Journal of the American Chemical Society (2004), 126 (21), 6703-6709CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Bending of polar-surface-dominated (PSD) nanobelts of ZnO can be explained by one of two processes: electrostatic neutralization of the dipole moment via deformation (called an electrostatic polar charge model) or imbalances between surface tensions via surface-termination induced stresses. This article presents exptl. data on the structural features of nanorings and nanobows formed by bending single-crystal, PSD ZnO nanobelts. The authors' data exclusively support the electrostatic polar charge model as the dominant mechanism for bending.
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    Shen, G.; Liang, B.; Wang, X.; Chen, P.-C.; Zhou, C. Indium Oxide Nanospirals Made of Kinked Nanowires. ACS Nano 2011, 5, 2155– 61,  DOI: 10.1021/nn103358y
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    Indium Oxide Nanospirals Made of Kinked Nanowires
    Shen, Guo-Zhen; Liang, Bo; Wang, Xian-Fu; Chen, Po-Chiang; Zhou, Chong-Wu
    ACS Nano (2011), 5 (3), 2155-2161CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Helical inorg. nanostructures have received great attention due to their unique structures that could be interesting for both fundamental research and nanodevice applications. Using a tube-in-tube laser ablation chem. vapor deposition (CVD) method with gold nanoparticles as the catalysts, we reported the synthesis of self-assembled kinked In2O3 nanospirals and multikinked nanowires. As-synthesized nanostructures showed ultrafast photoinduced reversible wettability switching behavior from hydrophobic (132.7°) to superhydrophilic (0°) within 14 min. Single kinked In2O3 nanostructure-based field-effect transistors were fabricated, and mobilities higher than 200 cm2/(V·s) were obtained, revealing good opportunity in fabricating high-performance electronic and optoelectronic devices.
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    Zhou, X. T.; Sham, T. K.; Shan, Y. Y.; Duan, X. F.; Lee, S. T.; Rosenberg, R. A. One-dimensional zigzag gallium nitride nanostructures. J. Appl. Phys. 2005, 97, 104315,  DOI: 10.1063/1.1897834
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    One-dimensional zigzag gallium nitride nanostructures
    Zhou, X. T.; Sham, T. K.; Shan, Y. Y.; Duan, X. F.; Lee, S. T.; Rosenberg, R. A.
    Journal of Applied Physics (2005), 97 (10, Pt. 1), 104315/1-104315/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Two 1-dimensional (1D) single-cryst. GaN nanostructures with periodic zigzag (type I) and diam.-modulated (type II) shapes were synthesized by passing through NH3 over a mixt. of Ga and Ga oxide (Ga2O3) powders held at elevated temp. The process was catalyzed by the dispersion of thio-capped Au nanoparticles on the substrate onto which GaN nanostructures were condensed. The transformation between these two nanostructure morphologies was also obsd. A possible growth model for the zigzag-shaped nanostructures is proposed, in which the formation of the zigzag nanostructures results from the construction of two different nanoscale unit cells. This work provides an avenue to a group of 1-dimensional nanostructures with a zigzag shape. The possibility to form 1-dimensional nanostructures yet to be discovered by changing the stacking direction of the (0001) plane will facilitate the fabrication of nanoscale functional devices as well as the authors' understanding of the growth behavior of nanoscale crystallites.
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    Panda, S. K.; Datta, A.; Sinha, G.; Chaudhuri, S.; Chavan, P. G.; Patil, S. S.; More, M. A.; Joag, D. S. Synthesis of Well-Crystalline GaS Nanobelts and Their Unique Field Emission Behavior. J. Phys. Chem. C 2008, 112, 6240– 4,  DOI: 10.1021/jp712083d
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    Synthesis of well-crystalline GaS nanobelts and their unique field emission behavior
    Panda, Subhendu K.; Datta, Anuja; Sinha, Godhuli; Chaudhuri, Subhadra; Chavan, Padmakar G.; Patil, Sandip S.; More, Mahendra A.; Joag, Dilip S.
    Journal of Physical Chemistry C (2008), 112 (16), 6240-6244CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Well-cryst. ultrathin GaS nanobelts have been successfully synthesized on silicon substrates by a simple thermal evapn. process. The GaS nanobelts were examd. by x-ray diffraction (XRD), scanning electron microscope (SEM), high-resoln. transmission electron microscope (HRTEM), and energy dispersive x-ray anal. (EDAX). The XRD pattern indicates formation of well-cryst. hexagonal phase GaS nanostructures. The SEM image shows uniformly distributed GaS nanostructures covering the entire substrate surface. The TEM results reveal that the GaS nanostructures are "nanobelts" of widths 20 to 50 nm and lengths up to several microns, and some of them are L-shaped. The growth mechanism and formation of GaS straight and L-shaped nanobelts has been explained. The field emission studies revealed that the threshold field required to draw an emission current of ∼1 nA is to be 2.9 V/μm, and a c.d. of ∼5.7 μA/cm2 can be drawn at an applied field of 6.0 V/μm. The Fowler-Nordheim plot, derived from the obsd. c.d.-applied field characteristics depicts nonlinear behavior over the entire range of applied field. The field enhancement factor is estd. to be ∼2.0 × 104. The emission current stability investigated over a duration of more than 2 h at the preset value ∼4.0 μA shows initial increment followed by stabilization to a higher value ∼6.0 μA. The av. emission current at the stabilized value is seen to be fairly const. with current fluctuations within ±10%. The results suggest the use of GaS nanobelts as a promising electron source for applications in field emission based devices.
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    Hong, W.-K.; Sohn, J. I.; Hwang, D.-K.; Kwon, S.-S.; Jo, G.; Song, S.; Kim, S.-M.; Ko, H.-J.; Park, S.-J.; Welland, M. E.; Lee, T. Tunable Electronic Transport Characteristics of Surface-Architecture-Controlled ZnO Nanowire Field Effect Transistors. Nano Lett. 2008, 8, 950– 6,  DOI: 10.1021/nl0731116
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    Tunable Electronic Transport Characteristics of Surface-Architecture-Controlled ZnO Nanowire Field Effect Transistors
    Hong, Woong-Ki; Sohn, Jung Inn; Hwang, Dae-Kue; Kwon, Soon-Shin; Jo, Gunho; Song, Sunghoon; Kim, Seong-Min; Ko, Hang-Ju; Park, Seong-Ju; Welland, Mark E.; Lee, Takhee
    Nano Letters (2008), 8 (3), 950-956CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diam. and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diam. and a weak deep-level emission PL peak. The surface morphol. and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diam. exhibited neg. threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diam. had pos. threshold voltages, indicating n-channel enhancement-mode behavior.
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    Potts, H.; Morgan, N. P.; Tütüncüoglu, G.; Friedl, M.; Morral, A F i Tuning growth direction of catalyst-free InAs(Sb) nanowires with indium droplets. Nanotechnology 2017, 28, 054001  DOI: 10.1088/1361-6528/28/5/054001
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    83
    Tuning growth direction of catalyst-free InAs(Sb) nanowires with indium droplets
    Potts, Heidi; Morgan, Nicholas P.; Tutuncuoglu, Gozde; Friedl, Martin; Fontcuberta i Morral, Anna
    Nanotechnology (2017), 28 (5), 054001/1-054001/9CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    The need for indium droplets to initiate self-catalyzed growth of InAs nanowires has been highly debated in the last few years. Here, we report on the use of indium droplets to tune the growth direction of self-catalyzed InAs nanowires. The indium droplets are formed in situ on InAs(Sb) stems. Their position is modified to promote growth in the 〈11-2〉 or equiv. directions. We also show that indium droplets can be used for the fabrication of InSb insertions in InAsSb nanowires. Our results demonstrate that indium droplets can initiate growth of InAs nanostructures as well as provide added flexibility to nanowire growth, enabling the formation of kinks and heterostructures, and offer a new approach in the growth of defect-free crystals.
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    Sivaram, S. V.; Hui, H. Y.; de la Mata, M.; Arbiol, J.; Filler, M. A. Surface Hydrogen Enables Subeutectic Vapor–Liquid–Solid Semiconductor Nanowire Growth. Nano Lett. 2016, 16, 6717– 23,  DOI: 10.1021/acs.nanolett.6b01640
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    Surface Hydrogen Enables Subeutectic Vapor-Liquid-Solid Semiconductor Nanowire Growth
    Sivaram, Saujan V.; Hui, Ho Yee; de la Mata, Maria; Arbiol, Jordi; Filler, Michael A.
    Nano Letters (2016), 16 (11), 6717-6723CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Vapor-liq.-solid nanowire growth below the bulk metal-semiconductor eutectic temp. is known for several systems; however, the fundamental processes that govern this behavior are poorly understood. Here, hydrogen atoms adsorbed on the Ge nanowire sidewall enable AuGe catalyst supercooling and control Au transport. The authors' approach combines in situ IR spectroscopy to directly and quant. det. hydrogen atom coverage with a regrowth step that allows catalyst phase to be detd. with ex situ electron microscopy. Maintenance of a supercooled catalyst with only hydrogen radical delivery confirms the centrality of sidewall chem. This work underscores the importance of the nanowire sidewall and its chem. on catalyst state, identifies new methods to regulate catalyst compn., and provides synthetic strategies for subeutectic growth in other nanowire systems.
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    Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M. Germanium Nanowire Growth Below the Eutectic Temperature. Science 2007, 316, 729– 32,  DOI: 10.1126/science.1139105
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    Germanium Nanowire Growth Below the Eutectic Temperature
    Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M.
    Science (Washington, DC, United States) (2007), 316 (5825), 729-732CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Nanowires are conventionally assumed to grow via the vapor-liq.-solid process, in which material from the vapor is incorporated into the growing nanowire via a liq. catalyst, commonly a low-m.p. eutectic alloy. However, nanowires have been obsd. to grow below the eutectic temp., and the state of the catalyst remains controversial. Using in situ microscopy, we showed that, for the classic Ge/Au system, nanowire growth can occur below the eutectic temp. with either liq. or solid catalysts at the same temp. We found, unexpectedly, that the catalyst state depends on the growth pressure and thermal history. We suggest that these phenomena may be due to kinetic enrichment of the eutectic alloy compn. and expect these results to be relevant for other nanowire systems.
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    Kim, J. H.; Moon, S. R.; Kim, Y.; Chen, Z. G.; Zou, J.; Choi, D. Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Taper-free and kinked germanium nanowires grown on silicon via purging and the two-temperature process. Nanotechnology 2012, 23, 115603,  DOI: 10.1088/0957-4484/23/11/115603
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    Taper-free and kinked germanium nanowires grown on silicon via purging and the two-temperature process
    Kim, Jung Hyuk; Moon, So Ra; Kim, Yong; Chen, Zhi Gang; Zou, Jin; Choi, Duk Yong; Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati
    Nanotechnology (2012), 23 (11), 115603/1-115603/6CODEN: NNOTER; ISSN:1361-6528. (Institute of Physics Publishing)
    We investigate the growth procedures for achieving taper-free and kinked germanium nanowires epitaxially grown on silicon substrates by chem. vapor deposition. Singly and multiply kinked germanium nanowires consisting of 〈111〉 segments were formed by employing a reactant gas purging process. Unlike non-epitaxial kinked nanowires, a two-temp. process is necessary to maintain the taper-free nature of segments in our kinked germanium nanowires on silicon. As an application, nanobridges formed between (111) side walls of V-grooved (100) silicon substrates have been demonstrated.
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    Li, Z.; Kurtulus, Ö; Fu, N.; Wang, Z.; Kornowski, A.; Pietsch, U.; Mews, A. Controlled Synthesis of CdSe Nanowires by Solution–Liquid–Solid Method. Adv. Funct. Mater. 2009, 19, 3650– 61,  DOI: 10.1002/adfm.200900569
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    Controlled Synthesis of CdSe Nanowires by Solution-Liquid-Solid Method
    Li, Zhen; Kurtulus, Oezguel; Fu, Nan; Wang, Zhe; Kornowski, Andreas; Pietsch, Ullrich; Mews, Alf
    Advanced Functional Materials (2009), 19 (22), 3650-3661CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Semiconductor nanowires prepd. by wet chem. methods are a relatively new field of 1-dimensional electronic systems, where the dimensions can be controlled by changing the reaction parameters using soln. chem. Here, the soln.-liq.-solid approach where the nanowire growth is governed by low-melting-point catalyst particles, such as Bi nanocrystals, is presented. In particular, the focus is on the prepn. and characterization of CdSe nanowires, a material which serves a prototype structure for many kinds of low dimensional semiconductor systems. To study the influence of different reaction parameters on the structural and optical properties of the nanowires, a comprehensive synthetic study is presented, and the results are compared with those reported in literature. How the interplay between different reaction parameters affects the diam., length, crystal structure, and the optical properties of the resultant nanowires are demonstrated. The structural properties are mainly detd. by competing reaction pathways, such as the growth of Bi nanocatalysts, the formation and catalytic growth of nanowires, and the formation and uncatalytic growth of quantum dots. Systematic variation of the reaction parameters (e.g., mol. precursors, concn. and concn. ratios, org. ligands, or reaction time, and temp.) enables control of the nanowire diam. from 6 to 33 nm, while their length can be adjusted between several tens of nanometers and tens of micrometers. The obtained CdSe nanowires exhibit an admixt. of wurtzite (W) and zinc blende (ZB) structure, which was studied by x-ray diffraction. The diam.-dependent band gaps of these nanowires can be varied between 650 and 700 nm while their fluorescence intensities are mainly governed by the Cd/Se precursor ratio and the ligands used.
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    Schmidt, V.; Senz, S.; Gösele, U. The shape of epitaxially grown silicon nanowires and the influence of line tension. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 445– 50,  DOI: 10.1007/s00339-004-3092-1
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    The shape of epitaxially grown silicon nanowires and the influence of line tension
    Schmidt, V.; Senz, S.; Goesele, U.
    Applied Physics A: Materials Science & Processing (2005), 80 (3), 445-450CODEN: APAMFC; ISSN:0947-8396. (Springer GmbH)
    Si nanowires grown epitaxially via the vapor-liq.-solid mechanism show a larger diam. at the base of the nanowire, which cannot be explained by an overgrowth of the nanowire alone. By considering the equil. condition for the contact angle of the droplet, the Neumann quadrilateral relation, a quasi-static model of epitaxial nanowire growth is derived. A change of the contact angle of the droplet is responsible for the larger diam. of the nanowire base, so that the expansion has to be considered a fundamental aspect of epitaxial vapor-liq.-solid growth. By comparison of exptl. results with theor. calcns., an est. for the line tension is obtained. In addn., the growth model predicts the existence of two different growth modes. Only within a certain range of line-tension values is the mode corresponding to ordinary nanowire growth realized, whereas nanowire growth stops at a relatively small height if the line tension exceeds an upper boundary. An approx. analytic expression for the upper boundary as a function of the surface tensions is given.
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    Ghisalberti, L.; Potts, H.; Friedl, M.; Zamani, M.; Güniat, L.; Tütüncüoglu, G.; Carter, W. C.; Morral, A F i Questioning liquid droplet stability on nanowire tips: from theory to experiment. Nanotechnology 2019, 30, 285604,  DOI: 10.1088/1361-6528/ab139c
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    89
    Questioning liquid droplet stability on nanowire tips: from theory to experiment
    Ghisalberti, Lea; Potts, Heidi; Friedl, Martin; Zamani, Mahdi; Guniat, Lucas; Tutuncuoglu, Gozde; Carter, W. Craig; Fontcuberta i Morral, Anna
    Nanotechnology (2019), 30 (28), 285604CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Liq. droplets sitting on nanowire (NW) tips constitute the starting point of the vapor-liq.-solid method of NW growth. Shape and vol. of the droplet have been linked to a variety of growth phenomena ranging from the modification of growth direction, NW orientation, crystal phase, and even polarity. In this work we focus on numerical and theor. anal. of the stability of liq. droplets on NW tips, explaining the peculiarity of this condition with respect to the wetting of planar surfaces. We highlight the role of droplet pinning at the tip in engineering the contact angle. Exptl. results on the characteristics of In droplets of variable vol. sitting on the tips or side facets of InAs NWs are also provided. This work contributes to the fundamental understanding of the nature of droplets contact angle at the tip of NWs and to the improvement of the engineering of such nanostructures.
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    Schwarz, K. W.; Tersoff, J. From Droplets to Nanowires: Dynamics of Vapor-Liquid-Solid Growth. Phys. Rev. Lett. 2009, 102, 206101,  DOI: 10.1103/PhysRevLett.102.206101
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    From Droplets to Nanowires: Dynamics of Vapor-Liquid-Solid Growth
    Schwarz, K. W.; Tersoff, J.
    Physical Review Letters (2009), 102 (20), 206101/1-206101/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Starting with a liq. eutectic droplet on a surface, we calc. its dynamical evolution into an epitaxial nanowire via the vapor-liq.-solid growth process. Our continuum approach incorporates kinetic effects and cryst. anisotropy in a natural way. Some realistic features appear automatically even for an isotropic solid, e.g., the tapered wire base. Crystal anisotropy leads to a richer variety of morphologies. For example, sixfold anisotropy leads to a wire shape having broken symmetry and an intriguing resemblance to the 〈110〉-oriented Si wires seen in Au-catalyzed growth on Si (111), while higher symmetry leads to a shape more like 〈111〉 Si wires.
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    Schwarz, K. W.; Tersoff, J. Elementary Processes in Nanowire Growth. Nano Lett. 2011, 11, 316– 20,  DOI: 10.1021/nl1027815
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    Elementary Processes in Nanowire Growth
    Schwarz, K. W.; Tersoff, J.
    Nano Letters (2011), 11 (2), 316-320CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    It is proposed that many of the complex morphol. phenomena obsd. during nanowire growth arise from the interplay of just three elementary processes: facet growth, droplet statics, and the introduction of new facets. These processes are incorporated into an explicit model for the vapor-liq.-solid growth of fully faceted nanowires. In numerical simulations with this model, different conditions can lead to either growth of a free-standing wire or lateral growth where the catalyst droplet crawls along the surface. An external perturbation can cause the wire to kink into a different direction. Different growth conditions can also change the shape of the growth tip. All of these phenomena have been obsd., and the model behavior is consistent with the exptl. observations.
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    Chen, Y.; Zhang, C.; Li, L.; Tuan, C.-C.; Chen, X.; Gao, J.; He, Y.; Wong, C.-P. Effects of Defects on the Mechanical Properties of Kinked Silicon Nanowires. Nanoscale Res. Lett. 2017, 12, 185,  DOI: 10.1186/s11671-017-1970-7
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    Effects of Defects on the Mechanical Properties of Kinked Silicon Nanowires
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    Nanoscale research letters (2017), 12 (1), 185 ISSN:1931-7573.
    Kinked silicon nanowires (KSiNWs) have many special properties that make them attractive for a number of applications. The mechanical properties of KSiNWs play important roles in the performance of sensors. In this work, the effects of defects on the mechanical properties of KSiNWs are studied using molecular dynamics simulations and indirectly validated by experiments. It is found that kinks are weak points in the nanowire (NW) because of inharmonious deformation, resulting in a smaller elastic modulus than that of straight NWs. In addition, surface defects have more significant effects on the mechanical properties of KSiNWs than internal defects. The effects of the width or the diameter of the defects are larger than those of the length of the defects. Overall, the elastic modulus of KSiNWs is not sensitive to defects; therefore, KSiNWs have a great potential as strain or stress sensors in special applications.
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    Westwater, J.; Gosain, D. P.; Tomiya, S.; Usui, S.; Ruda, H. Growth of silicon nanowires via gold/silane vapor–liquid–solid reaction. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 1997, 15, 554– 7,  DOI: 10.1116/1.589291
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    Silicon nanowires (whiskers) have been grown on Si(111) via the vapor-liq.-solid (VLS) reaction using silane as the Si source gas and Au as the mediating solvent. The silane partial pressure and temp. ranges were 0.01-1 torr and 320-600°, resp. Growth at high partial pressure and low temp. leads to the growth of Si nanowires as thin as 10 nm. These wires are single crystals but exhibit growth defects such as bending and kinking. Lowering the silane partial pressure leads to an increase in the wire width and a redn. in the tendency to form growth defects. At low pressure, 40-100 nm wide well-formed wires have been grown at 520°. The VLS reaction using silane allows the growth of Si wires, which are significantly thinner than those grown previously using SiCl4.
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    Adhikari, H.; Marshall, A. F.; Chidsey, C. E. D.; McIntyre, P. C. Germanium Nanowire Epitaxy: Shape and Orientation Control. Nano Lett. 2006, 6, 318– 23,  DOI: 10.1021/nl052231f
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    Nano Letters (2006), 6 (2), 318-323CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Epitaxial growth of nanowires along the 〈111〉 directions was obtained on Ge(111), Ge(110), Ge(001), and heteroepitaxial Ge on Si(001) substrates at temps. of 350° or less by Au-nanoparticle-catalyzed CVD. On Ge(111), the growth was mostly vertical. In addn. to 〈111〉 growth, 〈110〉 growth was obsd. on Ge(001) and Ge(110) substrates. Tapering was avoided by the use of the 2-temp. growth procedure, reported earlier by Greytak et al.
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    Hiruma, K.; Yazawa, M.; Haraguchi, K.; Ogawa, K.; Katsuyama, T.; Koguchi, M.; Kakibayashi, H. GaAs free-standing quantum-size wires. J. Appl. Phys. 1993, 74, 3162– 71,  DOI: 10.1063/1.354585
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    Gallium arsenide free-standing quantum-size wires
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    Journal of Applied Physics (1993), 74 (5), 3162-71CODEN: JAPIAU; ISSN:0021-8979.
    Ultrathin GaAs wires as thin as 15-40 nm and ∼2 μm long were grown on a GaAs substrate by OMVPE. The wires, which consist of whiskers, are grown at 380-550° using trimethylgallium and arsine (AsH3) as source materials. The wire growth direction is parallel to the [111] As dangling-bond direction and can be perfectly controlled by the crystallog. orientation of the GaAs substrate surface. The crystal structure of the wire coincides with the Zn-blende type for the growth temp. range of 460-500°, but it changes to the wurtzite type at 420° and >500°. Also the wires have a twin-type structure around the [111] growth axis for Zn blende and [0001] growth axis for wurtzite. The luminescence peak energy shifts to a higher energy as the wire width decreases from 100 to ∼35 nm. In terms of luminescence polarization the luminescence intensity parallel to the wires is 4 times greater than that perpendicular to the wires. As a preliminary application to devices, a p-n junction was formed along the GaAs wire. Light emission by current injection to the p-n junction wires was obsd. in continuous operation at room temp.
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    Borgström, M.; Deppert, K.; Samuelson, L.; Seifert, W. Size- and shape-controlled GaAs nano-whiskers grown by MOVPE: a growth study. J. Cryst. Growth 2004, 260, 18– 22,  DOI: 10.1016/j.jcrysgro.2003.08.009
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    Size- and shape-controlled GaAs nano-whiskers grown by MOVPE: a growth study
    Borgstrom, M.; Deppert, K.; Samuelson, L.; Seifert, W.
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    We have investigated the Au-catalyzed GaAs 〈1 1 1〉B whisker growth under low-pressure metal-org. vapor phase epitaxy conditions. By varying the growth temp. we found a max. in the whisker growth rate at about 450-475°C. With increasing temp. the growth rate decreases due to competing growth at the (1 1 1) substrate surface and at the {1 1 0} whisker side facets, which leads to significant tapering of the whiskers. For low temps., the growth rate R in the ln R=f(1/T)-plot results in an Arrhenius activation energy of about 67-75 kJ/mol, a value which is in agreement with activation energies reported for low-temp. planar growth of GaAs from TMG and AsH3. The Au acts as a local catalyst and as a collector of reactants, enabling a liq.-phase-epitaxy-like growth with high growth rates at the GaAs (1 1 1)B/(Au,Ga) interface.
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    Hiruma, K.; Murakoshi, H.; Yazawa, M.; Katsuyama, T. Self-organized growth of GaAsInAs heterostructure nanocylinders by organometallic vapor phase epitaxy. J. Cryst. Growth 1996, 163, 226– 31,  DOI: 10.1016/0022-0248(95)00714-8
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    Self-organized growth of GaAs/InAs heterostructure nanocylinders by organometallic vapor phase epitaxy
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    Journal of Crystal Growth (1996), 163 (3), 226-231CODEN: JCRGAE; ISSN:0022-0248. (Elsevier)
    Free-standing GaAs/InAs heterostructure wires as thin as 20 nm and as long as 1 μm were formed by vapor-liq.-solid (VLS) growth during OMVPE. The grown wires were analyzed by TEM, which revealed that the crystal structure of the GaAs portion coincides with that of zinc-blende, and the InAs portion coincides with that of wurtzite. The at. compn. along the heterojunction was also measured by energy dispersive x-ray anal. The compn. changes within a width of 5 nm at the heterojunction interface. The InAs/GaAs wires show a photoluminescence peak around 1.5 eV at 14 K, which indicates significant improvement in crystal quality over conventional GaAs/InAs layer structures.
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    Kim, Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Paladugu, M.; Zou, J.; Suvorova, A. A. Influence of Nanowire Density on the Shape and Optical Properties of Ternary InGaAs Nanowires. Nano Lett. 2006, 6, 599– 604,  DOI: 10.1021/nl052189o
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    Influence of Nanowire Density on the Shape and Optical Properties of Ternary InGaAs Nanowires
    Kim, Yong; Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Paladugu, Mohanchand; Zou, Jin; Suvorova, Alexandra A.
    Nano Letters (2006), 6 (4), 599-604CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We have synthesized ternary InGaAs nanowires on (111)B GaAs surfaces by metal-org. chem. vapor deposition. Au colloidal nanoparticles were employed to catalyze nanowire growth. We obsd. the strong influence of nanowire d. on nanowire height, tapering, and base shape specific to the nanowires with high In compn. This dependency was attributed to the large difference of diffusion length on (111)B surfaces between In and Ga reaction species, with In being the more mobile species. Energy dispersive X-ray spectroscopy anal. together with high-resoln. electron microscopy study of individual InGaAs nanowires shows large In/Ga compositional variation along the nanowire supporting the present diffusion model. Photoluminescence spectra exhibit a red shift with decreasing nanowire d. due to the higher degree of In incorporation in more sparsely distributed InGaAs nanowires.
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    Johansson, J.; Karlsson, L. S.; Svensson, C. P. T.; Martensson, T.; Wacaser, B. A.; Deppert, K.; Samuelson, L.; Seifert, W. Structural properties of ⟨111⟩B -oriented III–V nanowires. Nat. Mater. 2006, 5, 574,  DOI: 10.1038/nmat1677
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    Structural properties of <111>B-oriented III-V nanowires
    Johansson, Jonas; Karlsson, Lisa S.; Svensson, C. Patrik T.; Martensson, Thomas; Wacaser, Brent A.; Deppert, Knut; Samuelson, Lars; Seifert, Werner
    Nature Materials (2006), 5 (7), 574-580CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Controlled growth of nanowires is an important, emerging research field with many applications in, for example, electronics, photonics, and life sciences. Nanowires of zinc blende crystal structure, grown in the <111>B direction, which is the favored direction of growth, usually have a large no. of twin-plane defects. Such defects limit the performance of optoelectronic nanowire-based devices. To investigate this defect formation, we examine GaP nanowires grown by metal-org. vapor-phase epitaxy. We show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets, resulting in a microfaceting of the nanowires. We discuss these findings in a nucleation context, where we present an idea on how the twin planes form. This investigation contributes to the understanding of defect formation in nanowires. One future prospect of such knowledge is to det. strategies on how to control the crystallinity of nanowires.
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    Wagner, R. S.; Ooherty, C. J. Mechanism of Branching and Kinking during VLS Crystal Growth. J. Electrochem. Soc. 1968, 115, 93,  DOI: 10.1149/1.2411032
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    Mechanism of branching and kinking during vapor-liquid-solid crystal growth
    Wagner, Richard Siegfried; Doherty, C. J.
    Journal of the Electrochemical Society (1968), 115 (1), 93-9CODEN: JESOAN; ISSN:0013-4651.
    The morphology of the solid-liq. interface and the contact angle configuration of the liq. alloy droplet det. the direction of growth of crystals prepd. by the VLS technique. There are 4 different processes by which both growth kinks and branches can be formed. A change in solid-liq. interface shape during VLS caused by a lateral temp. gradient results in the formation of growth kinks. Branches are formed if the alloy droplet ruptures during the kinking sequence. A sudden increase in temp. can cause an unstable contact angle configuration. The alloy droplet may run down the side faces of the growing crystal, leading to the formation of growth kinks or branches. A sudden decrease in temp. may cause "pinching off" of small droplets from the main droplet, giving rise to branches. Finally, the codeposition of liq.-forming impurities may also lead to branch and kink formation. The proposed models were verified exptl. for the VLS growth of Si and Ge. Cryst. defects, such as dislocations, are not essential for the branching and kinking process. "Growth shaping" during the VLS process is possible. 16 references.
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    Song, M. S.; Jung, J. H.; Kim, Y.; Wang, Y.; Zou, J.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Vertically standing Ge nanowires on GaAs(110) substrates. Nanotechnology 2008, 19, 125602,  DOI: 10.1088/0957-4484/19/12/125602
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    Vertically standing Ge nanowires on GaAs(110) substrates
    Song, Man Suk; Jung, Jae Hun; Kim, Yong; Wang, Y.; Zou, J.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.
    Nanotechnology (2008), 19 (12), 125602/1-125602/6CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
    The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temp. range from 300 to 380 °C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the 〈110〉 direction. Using this unique property, vertical 〈110〉 Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addn., these Ge nanowires exhibit minimal tapering and uniform diams., regardless of growth temps., which is an advantageous property for device applications. Ge nanowires growing along the 〈110〉 directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.
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    Schmid, H.; Björk, M. T.; Knoch, J.; Riel, H.; Riess, W.; Rice, P.; Topuria, T. Patterned epitaxial vapor-liquid-solid growth of silicon nanowires on Si(111) using silane. J. Appl. Phys. 2008, 103, 024304  DOI: 10.1063/1.2832760
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    Patterned epitaxial vapor-liquid-solid growth of silicon nanowires on Si(111) using silane
    Schmid, H.; Bjork, M. T.; Knoch, J.; Riel, H.; Riess, W.; Rice, P.; Topuria, T.
    Journal of Applied Physics (2008), 103 (2), 024304/1-024304/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We have carried out a detailed study on the vapor-liq.-solid growth of Si nanowires (SiNWs) on (111)-oriented Si substrates using Au as catalytic seed material. Arrays of individual seeds were patterned by electron-beam lithog., followed by Au evapn. and lift-off. SiNWs were grown using dild. silane as precursor gas in a low-pressure chem. vapor deposition system. The silane partial pressure, substrate temp., and seed diam. were systematically varied to obtain the growth rate of the NWs and the rate of sidewall deposition. Activation energies of 19 kcal/mol for the axial SiNW growth and 29 kcal/mol for the radial deposition on the SiNW surface are derived from the data. SiNW growth at elevated temps. is accompanied by significant Au surface diffusion, leading to a loss of Au from the tips of the SiNWs that depends on the layout and d. of the Au seeds patterned. In contrast to NWs grown from a thin-film-nucleated substrate, the deterministic patterning of identical Au seeds of varying diams. allows accurate measurements of the nucleation yield of the SiNW, which is close to 100%, and anal. of the epitaxial relationship with the substrate. In addn. to the vertical and the 3 70.5°-inclined 〈111〉 epitaxial growth directions, we observe 3 addnl. 70.5°-inclined directions, which are rotated by 60°. The 60° rotation is explained by the occurrence of stacking faults in the SiNWs. The overall yield of vertically grown 〈111〉 NWs depends sensitively on the partial pressure of the SiH4 and, to a lesser extent, on the growth temp. At 80 mTorr partial pressure and 470°C, up to 60% of the SiNWs grow in the vertical 〈111〉 direction. In situ doping of SiNWs using arsine resulted in a significant redn. of nucleation and wire growth, whereas doping with trimethylboron and phosphine exhibited no difference in growth and epitaxy compared with undoped samples. (c) 2008 American Institute of Physics.
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    Shin, N.; Filler, M. A. Controlling Silicon Nanowire Growth Direction via Surface Chemistry. Nano Lett. 2012, 12, 2865– 70,  DOI: 10.1021/nl300461a
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    Controlling Silicon Nanowire Growth Direction via Surface Chemistry
    Shin, Naechul; Filler, Michael A.
    Nano Letters (2012), 12 (6), 2865-2870CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors report on the 1st in situ chem. study of vapor-liq.-solid semiconductor nanowire growth and reveal the important, and previously unrecognized, role of transient surface chem. near the triple-phase line. Real-time IR spectroscopy measurements coupled with postgrowth electron microscopy demonstrate that covalently bonded hydrogen atoms are responsible for the 〈111〉 to 〈112〉 growth orientation transition commonly obsd. during Si nanowire growth. The authors' findings provide insight into the root cause of this known nanowire growth phenomenon and open a new route to rationally engineer the crystal structure of these nanoscale semiconductors.
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    Musin, I. R.; Filler, M. A. Chemical Control of Semiconductor Nanowire Kinking and Superstructure. Nano Lett. 2012, 12, 3363– 8,  DOI: 10.1021/nl204065p
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    Chemical Control of Semiconductor Nanowire Kinking and Superstructure
    Musin, Ildar R.; Filler, Michael A.
    Nano Letters (2012), 12 (7), 3363-3368CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We show that methylgermane (GeH3CH3) can induce a transition from 〈111〉 to 〈110〉 oriented growth during the vapor-liq.-solid synthesis of Ge nanowires. This hydride-based chem. is subsequently leveraged to rationally fabricate kinking superstructures based on combinations of 〈111〉 and 〈110〉 segments. The addn. of GeH3CH3 also eliminates sidewall tapering and enables Ge nanowire growth at temps. exceeding 475 °C, which greatly expands the process window and opens new avenues to create Si/Ge heterostructures.
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    Dailey, E.; Drucker, J. Seedless” vapor-liquid-solid growth of Si and Ge nanowires: The origin of bimodal diameter distributions. J. Appl. Phys. 2009, 105, 064317  DOI: 10.1063/1.3088885
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    "Seedless" vapor-liquid-solid growth of Si and Ge nanowires: The origin of bimodal diameter distributions
    Dailey, Eric; Drucker, Jeff
    Journal of Applied Physics (2009), 105 (6), 064317/1-064317/5CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We identify a previously uncharacterized vapor-liq.-solid growth mode that can produce small diam., epitaxial <110> oriented Si and Ge nanowires (NWs). Disilane or digermane pyrolysis evolves H2 causing the monolayer thick Au/Si(111) layer between three dimensional Au seeds to dewet and form small Au islands. Under some conditions, these small islands facilitate "seedless" growth of small diam. NWs distinct from larger NWs that grow from the deposited seeds leading to a bimodal diam. distribution. We identify the precursor pressures and growth temp. regimes for which Si and Ge NW growth occurs in the absence of deposited seeds from the dewetted Au/Si(111) layer. (c) 2009 American Institute of Physics.
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    Xie, P.; Hu, Y.; Fang, Y.; Huang, J.; Lieber, C. M. Diameter-dependent dopant location in silicon and germanium nanowires. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 15254– 8,  DOI: 10.1073/pnas.0906943106
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    Diameter-dependent dopant location in silicon and germanium nanowires
    Xie, Ping; Hu, Yongjie; Fang, Ying; Huang, Jinlin; Lieber, Charles M.
    Proceedings of the National Academy of Sciences of the United States of America (2009), 106 (36), 15254-15258, S15254/1CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    We report studies defining the diam.-dependent location of elec. active dopants in silicon (Si) and germanium (Ge) nanowires (NWs) prepd. by nanocluster catalyzed vapor-liq.-solid (VLS) growth without measurable competing homogeneous decompn. and surface overcoating. The location of active dopants was assessed from elec. transport measurements before and after removal of controlled thicknesses of material from NW surfaces by low-temp. chem. oxidn. and etching. These measurements show a well-defined transition from bulk-like to surface doping as the diam. is decreased <22-25 nm for n- and p-type Si NWs, although the surface dopant concn. is also enriched in the larger diam. Si NWs. Similar diam.-dependent results were also obsd. for n-type Ge NWs, suggesting that surface dopant segregation may be general for small diam. NWs synthesized by the VLS approach. Natural surface doping of small diam. semiconductor NWs is distinct from many top-down fabricated NWs, explains enhanced transport properties of these NWs and could yield robust properties in ultrasmall devices often dominated by random dopant fluctuations.
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    Perea, D. E.; Hemesath, E. R.; Schwalbach, E. J.; Lensch-Falk, J. L.; Voorhees, P. W.; Lauhon, L. J. Direct measurement of dopant distribution in an individual vapour–liquid–solid nanowire. Nat. Nanotechnol. 2009, 4, 315– 9,  DOI: 10.1038/nnano.2009.51
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    Direct measurement of dopant distribution in an individual vapour-liquid-solid nanowire
    Perea, Daniel E.; Hemesath, Eric R.; Schwalbach, Edwin J.; Lensch-Falk, Jessica L.; Voorhees, Peter W.; Lauhon, Lincoln J.
    Nature Nanotechnology (2009), 4 (5), 315-319CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Semiconductor nanowires show promise for many device applications, but controlled doping with electronic and magnetic impurities remains an important challenge. Limitations on dopant incorporation were identified in nanocrystals, raising concerns about the prospects for doping nanostructures. Progress was hindered by the lack of a method to quantify the dopant distribution in single nanostructures. Recently, atom probe tomog. can be used to det. the compn. of isolated nanowires. Here, the authors report the 1st direct measurements of dopant concns. in arbitrary regions of individual nanowires. Differences in precursor decompn. rates between the liq. catalyst and solid nanowire surface give rise to a heavily doped shell surrounding an underdoped core. The authors also present a thermodn. model that relates liq. and solid compns. to dopant fluxes.
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    Jagannathan, H.; Deal, M.; Nishi, Y.; Woodruff, J.; Chidsey, C.; McIntyre, P. C. Nature of germanium nanowire heteroepitaxy on silicon substrates. J. Appl. Phys. 2006, 100, 024318  DOI: 10.1063/1.2219007
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    Nature of germanium nanowire heteroepitaxy on silicon substrates
    Jagannathan, Hemanth; Deal, Michael; Nishi, Yoshio; Woodruff, Jacob; Chidsey, Christopher; McIntyre, Paul C.
    Journal of Applied Physics (2006), 100 (2), 024318/1-024318/10CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Systematic studies of the heteroepitaxial growth of Ge nanowires on Si substrates were performed. These studies included the effect of sample prepn., substrate orientation, preanneal, growth temp., and GeH4 partial pressure on the growth of epitaxial Ge nanowires. SEM and transmission electron microscopy were used to analyze the resulting nanowire growth. Ge nanowires grew predominantly along the 〈111〉 crystallog. direction, with a minority of wires growing along the 〈110〉 direction, irresp. of the underlying silicon substrate orientation [silicon (111), (110), and (100)]. Decreasing the partial pressure of GeH4 increased the no. of 〈111〉 nanowires normal to the silicon (111) surface, compared to the other three available 〈111〉 directions. The growth rate of nanowires increased with the partial pressure of germane and to a lesser degree with temp. The nucleation d. of nanowire growth and the degree of epitaxy both increased with temp. However, increasing the growth temp. also increased the rate of sidewall deposition, thereby resulting in tapered nanowires. A 2-step temp. process was used to initiate nanowire nucleation and epitaxy at a high temp., followed by non-tapered nanowire growth at a lower temp. Preannealing Au films in H or Ar before nanowire growth reduced the yield of nanowires grown on Si samples, esp. on Si (111) substrates, but not on SiO2. Au annealing studies performed to investigate this preanneal effect showed greater gold agglomeration on the Si samples compared to SiO2. The results and conclusions obtained from these studies give a better understanding of the complex interdependencies of the parameters involved in the controlled heteroepitaxial growth of vapor-liq.-solid grown Ge nanowires.
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  109. 109
    Kodambaka, S.; Hannon, J. B.; Tromp, R. M.; Ross, F. M. Control of Si Nanowire Growth by Oxygen. Nano Lett. 2006, 6, 1292– 6,  DOI: 10.1021/nl060059p
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    Control of Si nanowire growth by oxygen
    Kodambaka, Suneel; Hannon, James B.; Tromp, Rudolf M.; Ross, Frances M.
    Nano Letters (2006), 6 (6), 1292-1296CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Semiconductor nanowires formed using the vapor-liq.-solid mechanism are routinely grown in many labs., but a comprehensive understanding of the key factors affecting wire growth is still lacking. Under conditions of low disilane pressure and higher temp., long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to O, even at low levels, reduces the diffusion of Au away from the catalyst droplets. This allows the droplet vols. to remain const. for longer times and therefore permits the growth of untapered wires. This effect is obsd. for both gas-phase and surface-bound O, so the source of O is unimportant. The control of O exposure during growth provides a new tool for the fabrication of long, uniform-diam. structures, as required for many applications of nanowires.
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  110. 110
    Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Fickenscher, M. A.; Perera, S.; Hoang, T. B.; Smith, L. M.; Jackson, H. E.; Yarrison-Rice, J. M.; Zhang, X.; Zou, J. High Purity GaAs Nanowires Free of Planar Defects: Growth and Characterization. Adv. Funct. Mater. 2008, 18, 3794– 800,  DOI: 10.1002/adfm.200800625
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    High purity GaAs nanowires free of planar defects: growth and characterization
    Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Kim, Yong; Fickenscher, Melodie A.; Perera, Saranga; Hoang, Thang Ba; Smith, Leigh M.; Jackson, Howard E.; Yarrison-Rice, Jan M.; Zhang, Xin; Zou, Jin
    Advanced Functional Materials (2008), 18 (23), 3794-3800CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We investigate how to tailor the structural, crystallog. and optical properties of GaAs nanowires. Nanowires were grown by Au nanoparticle-catalyzed metalorg. chem. vapor deposition. A high arsine flow rate, i.e., a high ratio of group V to group III precursors, imparts significant advantages. It dramatically reduces planar crystallog. defects and reduces intrinsic carbon dopant incorporation. Increasing V/III ratio further, however, instigates nanowire kinking and increases nanowire tapering. By choosing an intermediate V/III ratio we achieve uniform, vertically aligned GaAs nanowires, free of planar crystallog. defects, with excellent optical properties and high purity. These findings will greatly assist the development of future GaAs nanowire-based electronic and optoelectronic devices, and are expected to be more broadly relevant to the rational synthesis of other III-V nanowires.
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  111. 111

    Anon Nanotectonics | COMC.

    There is no corresponding record for this reference.
  112. 112
    Wang, J.; Plissard, S. R.; Verheijen, M. A.; Feiner, L.-F.; Cavalli, A.; Bakkers, E. P. A. M. Reversible Switching of InP Nanowire Growth Direction by Catalyst Engineering. Nano Lett. 2013, 13, 3802– 6,  DOI: 10.1021/nl401767b
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    Reversible Switching of InP Nanowire Growth Direction by Catalyst Engineering
    Wang, Jia; Plissard, Sebastien R.; Verheijen, Marcel A.; Feiner, Lou-Fe; Cavalli, Alessandro; Bakkers, Erik P. A. M.
    Nano Letters (2013), 13 (8), 3802-3806CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors demonstrate high yield vapor-liq.-solid (VLS) growth of 〈100〉-oriented InP nanowire arrays. The highest yield (97%) was obtained when the catalyst droplet is filled with indium prior to nanowire nucleation to the equil. compn. during nanowire growth. Using these 〈100〉 wires as a template the authors can reversibly switch between a 〈100〉 and a 〈111〉 growth direction by varying the indium content of the droplet. Modeling VLS growth by a kinetic nucleation model indicates that the growth direction is governed by the liq.-vapor interface energy that is strongly affected by the indium concn. in the catalyst droplet.
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    Zhang, Z.; Zheng, K.; Lu, Z.-Y.; Chen, P.-P.; Lu, W.; Zou, J. Catalyst Orientation-Induced Growth of Defect-Free Zinc-Blende Structured ⟨001̅⟩ InAs Nanowires. Nano Lett. 2015, 15, 876– 82,  DOI: 10.1021/nl503556a
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    Catalyst Orientation-Induced Growth of Defect-Free Zinc-Blende Structured 〈00‾1〉 InAs Nanowires
    Zhang, Zhi; Zheng, Kun; Lu, Zhen-Yu; Chen, Ping-Ping; Lu, Wei; Zou, Jin
    Nano Letters (2015), 15 (2), 876-882CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors demonstrate the epitaxial growth of 〈00‾1〉 defect-free Zn-blende structured InAs nanowires on GaAs {111}B substrate using Au catalysts in MBE. The catalysts and their underlying 〈00‾1〉 nanowires have the orientation relation of {1‾103}C//{00‾2}InAs and [‾3302]C//[1‾10]InAs due to their small in-plane lattice mismatches between their corresponding lattice spacings perpendicular to the {00‾1} at. planes of the nanowires, giving the {00‾1} catalyst/nanowire interfaces, and consequently the formation of 〈00‾1〉 nanowires. This study provides a practical approach to manipulate the crystal structure and structural quality of III-V nanowires through carefully controlling the crystal phase of the catalysts.
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    Verheijen, M. A.; Immink, G.; de Smet, T.; Borgström, M. T.; Bakkers, E. P. A. M. Growth Kinetics of Heterostructured GaP–GaAs Nanowires. J. Am. Chem. Soc. 2006, 128, 1353– 9,  DOI: 10.1021/ja057157h
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    Growth Kinetics of Heterostructured GaP-GaAs Nanowires
    Verheijen, Marcel A.; Immink, George; De Smet, Thierry; Borgstroem, Magnus T.; Bakkers, Erik P. A. M.
    Journal of the American Chemical Society (2006), 128 (4), 1353-1359CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors have studied the vapor-liq.-solid (VLS) growth dynamics of GaP and GaAs in heterostructured GaP-GaAs nanowires. The wires contg. multiple GaP-GaAs junctions were grown using metal-org. vapor phase-epitaxy (OMVPE) on SiO2, and the lengths of the individual sections were obtained from TEM. The growth kinetics was studied as a function of temp. and the partial pressures of the precursors. The growth of the GaAs sections is limited by the arsine (AsH3) as well as the trimethylgallium (GaMe3) partial pressures, whereas the growth of GaP is a temp.-activated, phosphine (PH3)-limited process with an activation energy of 115 ± 6 kJ/mol. The PH3 kinetics obeys the Hinshelwood-Langmuir mechanism, indicating that the dissocn. reaction of adsorbed PH3 into PH2 and H on the catalytic Au surface is the rate-limiting step for the growth of GaP. The authors have studied the competitive thin layer growth on the sidewalls of the nanowires. Although the rate of this process is 2 orders of magnitude lower than the growth rate of the VLS mechanism, it competes with VLS growth and results in tapered nanowires at elevated temps.
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    Dayeh, S. A.; Yu, E. T.; Wang, D. III–V Nanowire Growth Mechanism: V/III Ratio and Temperature Effects. Nano Lett. 2007, 7, 2486– 90,  DOI: 10.1021/nl0712668
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    III-V Nanowire Growth Mechanism: V/III Ratio and Temperature Effects
    Dayeh, Shadi A.; Yu, Edward T.; Wang, Deli
    Nano Letters (2007), 7 (8), 2486-2490CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors have studied the dependence of Au-assisted InAs nanowire (NW) growth on InAs(111)B substrates as a function of substrate temp. and input V/III precursor ratio using OMVPE. Temp.-dependent growth was obsd. within certain temp. windows that are highly dependent on input V/III ratios. This dependence is a direct consequence of the drop in NW nucleation and growth rate with increasing V/III ratio at a const. growth temp. due to depletion of indium at the NW growth sites. The growth rate is detd. by the local V/III ratio, which is dependent on the input precursor flow rates, growth temp., and substrate decompn. These studies advance understanding of the key processes involved in III-V NW growth, support the general validity of the vapor-liq.-solid growth mechanism for III-V NWs, and improve rational control over their growth morphol.
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    Chen, Y.; Li, L.; Zhang, C.; Tuan, C.-C.; Chen, X.; Gao, J.; Wong, C.-P. Controlling Kink Geometry in Nanowires Fabricated by Alternating Metal-Assisted Chemical Etching. Nano Lett. 2017, 17, 1014– 9,  DOI: 10.1021/acs.nanolett.6b04410
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    116
    Controlling Kink Geometry in Nanowires Fabricated by Alternating Metal-Assisted Chemical Etching
    Chen, Yun; Li, Liyi; Zhang, Cheng; Tuan, Chia-Chi; Chen, Xin; Gao, Jian; Wong, Ching-Ping
    Nano Letters (2017), 17 (2), 1014-1019CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Kinked silicon (Si) nanowires (NWs) have many special properties that make them attractive for a no. of applications, such as microfluidics devices, microelectronic devices, and biosensors. However, fabricating NWs with controlled three-dimensional (3D) geometry has been challenging. In this work, a novel method called alternating metal-assisted chem. etching is reported for the fabrication of kinked Si NWs with controlled 3D geometry. By the use of multiple etchants with carefully selected compn., one can control the no. of kinks, their locations, and their angles by controlling the no. of etchant alternations and the time in each etchant. The resulting no. of kinks equals the no. times the etchant is alternated, the length of each segment sepd. by kinks has a linear relationship with the etching time, and the kinking angle is related to the surface tension and viscosity of the etchants. This facile method may provide a feasible and economical way to fabricate novel silicon nanowires, nanostructures, and devices for broad applications.
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  117. 117
    Sandu, G.; Avila Osses, J.; Luciano, M.; Caina, D.; Stopin, A.; Bonifazi, D.; Gohy, J.-F.; Silhanek, A.; Florea, I.; Bahri, M.; Ersen, O.; Leclère, P.; Gabriele, S.; Vlad, A.; Melinte, S. Kinked Silicon Nanowires: Superstructures by Metal-Assisted Chemical Etching. Nano Lett. 2019, 19, 7681– 90,  DOI: 10.1021/acs.nanolett.9b02568
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    117
    Kinked silicon nanowires: superstructures by metal-assisted chemical etching
    Sandu, Georgiana; Avila Osses, Jonathan; Luciano, Marine; Caina, Darwin; Stopin, Antoine; Bonifazi, Davide; Gohy, Jean-Francois; Silhanek, Alejandro; Florea, Ileana; Bahri, Mounib; Ersen, Ovidiu; Leclere, Philippe; Gabriele, Sylvain; Vlad, Alexandru; Melinte, Sorin
    Nano Letters (2019), 19 (11), 7681-7690CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors report on metal-assisted chem. etching of Si for the synthesis of mech. stable, hybrid crystallog. orientation Si superstructures with high aspect ratio, above 200. This method sustains high etching rates and facilitates reproducible results. The protocol enables the control of the no., angle, and location of the kinks via successive etch-quench sequences. The authors analyzed relevant Au mask catalyst features to systematically assess their impact on a wide spectrum of etched morphologies that can be easily attained and customized by fine-tuning of the crit. etching parameters. For instance, the designed kinked Si nanowires can be incorporated in biol. cells without affecting their viability. An accessible numerical model is provided to explain the etch profiles and the physicochem. events at the Si/Au-electrolyte interface and offers guidelines for the development of finite-element modeling of metal-assisted Si chem. etching.
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    Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Zhang, X.; Guo, Y.; Zou, J. Twin-Free Uniform Epitaxial GaAs Nanowires Grown by a Two-Temperature Process. Nano Lett. 2007, 7, 921– 6,  DOI: 10.1021/nl062755v
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    Twin-Free Uniform Epitaxial GaAs Nanowires Grown by a Two-Temperature Process
    Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Kim, Yong; Zhang, Xin; Guo, Yanan; Zou, Jin
    Nano Letters (2007), 7 (4), 921-926CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallog. quality and optimal shape, grown by Au nanoparticle-catalyzed metalorg. chem. vapor deposition. This is achieved by a two-temp. growth procedure, consisting of a brief initial high-temp. growth step followed by prolonged growth at a lower temp. The initial high-temp. step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temp. employed for subsequent growth imparts superior nanowire morphol. and crystallog. quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temp. grown nanowires. Two mechanisms are proposed to explain the success of this two-temp. growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temp. hysteresis of the Au-Ga nanoparticle alloy.
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    Dick, K. A.; Caroff, P. Metal-seeded growth of III–V semiconductor nanowires: towards gold-free synthesis. Nanoscale 2014, 6, 3006– 21,  DOI: 10.1039/C3NR06692D
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    119
    Metal-seeded growth of III-V semiconductor nanowires: towards gold-free synthesis
    Dick, Kimberly A.; Caroff, Philippe
    Nanoscale (2014), 6 (6), 3006-3021CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    A review. Semiconductor nanowires composed of III-V materials have enormous potential to add new functionality to electronics and optical applications. However, integration of these promising structures into applications is severely limited by the current near-universal reliance on gold nanoparticles as seeds for nanowire fabrication. Although highly controlled fabrication is achieved, this metal is entirely incompatible with the Si-based electronics industry. In this Feature we review the progress towards developing gold-free bottom-up synthesis techniques for III-V semiconductor nanowires. Three main categories of nanowire synthesis are discussed: selective-area epitaxy, self-seeding and foreign metal seeding, with main focus on the metal-seeded techniques. For comparison, we also review the development of foreign metal seeded synthesis of silicon and germanium nanowires. Finally, directions for future development and anticipated important trends are discussed. We anticipate significant development in the use of foreign metal seeding in particular. In addn., we speculate that multiple different techniques must be developed in order to replace gold and to provide a variety of nanowire structures and properties suited to a diverse range of applications.
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    Jing, Y.; Zhang, C.; Liu, Y.; Guo, L.; Meng, Q. Mechanical properties of kinked silicon nanowires. Phys. B 2015, 462, 59– 63,  DOI: 10.1016/j.physb.2015.01.018
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    120
    Mechanical properties of kinked silicon nanowires
    Jing, Yuhang; Zhang, Chuan; Liu, Yingzhi; Guo, Licheng; Meng, Qingyuan
    Physica B: Condensed Matter (Amsterdam, Netherlands) (2015), 462 (), 59-63CODEN: PHYBE3; ISSN:0921-4526. (Elsevier B.V.)
    Mol. dynamics simulations are used to investigate the mech. properties of KSiNWs. Our results show that KSiNWs have a much larger fracture strain compared to straight SiNWs. The effects of the periodic length of KSiNWs with sym. arms and the arm length of the KSiNW with asym. arms on the mech. properties of KSiNWs are studied. The fracture stress of KSiNWs decrease as the periodic length increases. However, the fracture strain of KSiNWs is not dependent on the short periodic length and the fracture strain of KSiNWs will abruptly increase to very large value and then vary slightly as the periodic length increases. In addn., the fracture stress is not dependent on arm length while the fracture strain monotonically increases as the arm length increases. We also investigate the fracture process of KSiNWs. The results in this paper suggest that the KSiNWs with larger fracture strain can be a promising anode materials in high performance Li-ion batteries.
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    Jiang, J.-W. Intrinsic twisting instability of kinked silicon nanowires for intracellular recording. Phys. Chem. Chem. Phys. 2015, 17, 28515– 24,  DOI: 10.1039/C5CP05010C
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    Intrinsic twisting instability of kinked silicon nanowires for intracellular recording
    Jiang, Jin-Wu
    Physical Chemistry Chemical Physics (2015), 17 (43), 28515-28524CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A kinked silicon nanowire (KSiNW) is a zigzag-shaped nanowire with its growth direction changing regularly at the kinking joints, resulting in a quasi-two-dimensional structure. An intrinsic tendency for the two-dimensional system is to generate some out-of-plane vibrations to withstand the mech. instability in the third dimension. In the present work, we report a lattice dynamical study of the intrinsic out-of-plane twisting vibration of KSiNWs. We derive the dynamical matrix anal., and explore the kinking effect on the phonon spectrum of the KSiNWs. Based on lattice dynamical anal., we obtain an anal. formula for the geometrical dependence of the twisting amplitude of the KSiNWs. The anal. formula provides valuable information on the kinking induced twisting stability of KSiNWs serving as bio-probes for intracellular recording application.
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    Zhang, Q.; Cui, Z.; Wei, Z.; Chang, S. Y.; Yang, L.; Zhao, Y.; Yang, Y.; Guan, Z.; Jiang, Y.; Fowlkes, J.; Yang, J.; Xu, D.; Chen, Y.; Xu, T. T.; Li, D. Defect Facilitated Phonon Transport through Kinks in Boron Carbide Nanowires. Nano Lett. 2017, 17, 3550– 5,  DOI: 10.1021/acs.nanolett.7b00666
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    Defect Facilitated Phonon Transport through Kinks in Boron Carbide Nanowires
    Zhang, Qian; Cui, Zhiguang; Wei, Zhiyong; Chang, Siang Yee; Yang, Lin; Zhao, Yang; Yang, Yang; Guan, Zhe; Jiang, Youfei; Fowlkes, Jason; Yang, Juekuan; Xu, Dongyan; Chen, Yunfei; Xu, Terry T.; Li, Deyu
    Nano Letters (2017), 17 (6), 3550-3555CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Nanowires of complex morphologies, such as kinked wires, were recently synthesized and demonstrated for novel devices and applications. However, the effects of these morphologies on thermal transport were not well studied. Through systematic exptl. measurements, single-cryst., defect-free kinks in boron carbide nanowires can pose a thermal resistance up to ∼30 times larger than that of a straight wire segment of equiv. length. Anal. suggests that this pronounced resistance can be attributed to the combined effects of backscattering of highly focused phonons and required mode conversion at the kink. Also instead of posing resistance, structural defects in the kink can actually assist phonon transport through the kink and reduce its resistance. Given the common kink-like wire morphol. in nanoelectronic devices and required low thermal cond. for thermoelec. devices, these findings have important implications in precise thermal management of electronic devices and thermoelecs.
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    Gan, L.; Liao, M.; Li, H.; Ma, Y.; Zhai, T. Geometry-induced high performance ultraviolet photodetectors in kinked SnO2 nanowires. J. Mater. Chem. C 2015, 3, 8300– 6,  DOI: 10.1039/C5TC01178G
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    Geometry-induced high performance ultraviolet photodetectors in kinked SnO2 nanowires
    Gan, Lin; Liao, Meiyong; Li, Huiqiao; Ma, Ying; Zhai, Tianyou
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2015), 3 (32), 8300-8306CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    SnO2 nanowires were widely used in photodetectors due to their large surface to vol. ratio and semiconducting characteristics. In general, SnO2 nanowire-based photodetectors exhibit high photocurrent but a slow response speed. The important role of geometry in nanowire-based photodetectors was explored, as exemplified by the study of the UV detector fabricated from a kinked single-crystal SnO2 nanowire, in which 2 kinds of photodetectors, based on the straight and kinked SnO2 nanowire, resp., were fabricated on the same SnO2 nanowire for comparison. Both the photocurrent and response time of the SnO2 nanowire were improved greatly by the kinked structure; the photoresponsivity and external quantum efficiency (EQE) are ultrahigh as 1.2 × 107 A W-1 and 6.0 × 109%, resp. The photoresponse time exhibits a hundred times improvement, compared with the straight nanowire case. Such an improvement was attributed to the enhancement effect of the kinked structure to the local elec. field, which benefits the electron-hole sepn. efficiency and the transmit time of carrier, thus improving both the photocurrent and response time of the SnO2 nanowire. As the enhancement of the photodetector results from geometry exclusively, this simple strategy is promising to be extended to the applications of other low dimensional materials.
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  124. 124
    Islam, M. S.; Sharma, S.; Kamins, T. I.; Williams, R. S. Ultrahigh-density silicon nanobridges formed between two vertical silicon surfaces. Nanotechnology 2004, 15, L5,  DOI: 10.1088/0957-4484/15/5/L01
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    Ultrahigh-density silicon nanobridges formed between two vertical silicon surfaces
    Islam, M. Saif; Sharma, S.; Kamins, T. I.; Williams, R. Stanley
    Nanotechnology (2004), 15 (5), L5-L8CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
    We report simultaneous lateral growth of a high d. of highly oriented, metal-catalyzed silicon nanowires on a patterned silicon substrate and bridging of nanowires between two vertical silicon sidewalls, which can be developed into electrodes of an electronic device. After angled deposition of catalytic metal nanoparticles on one of two opposing vertical silicon surfaces, we used a metal-catalyzed chem. vapor deposition process to grow nanowires and eventually form mech. robust 'nanobridges'. The growth and bridging of these nanowire arrays can be integrated with existing silicon processes. This method of connecting multiple nanowires between two electrodes offers the high surface-to-vol. ratio needed for nanosensor applications.
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This article is cited by 3 publications.

  1. Griffin C. Spence, Rachel C. Barbieri, David Pate, Lisa S. Graves, Ümit Özgür, Indika U. Arachchige. Sn-Induced Synthesis of Highly Crystalline and Size-Confined Si Nanorods at Moderately High Temperatures Using Hydrogen Silsesquioxane. The Journal of Physical Chemistry C 2023, 127 (24) , 11579-11590. https://doi.org/10.1021/acs.jpcc.3c01308
  2. Xin Yan, Yuqing Liu, Chaofei Zha, Xia Zhang, Yunyan Zhang, Xiaomin Ren. Non-〈111〉-oriented semiconductor nanowires: growth, properties, and applications. Nanoscale 2023, 15 (7) , 3032-3050. https://doi.org/10.1039/D2NR06421A
  3. Miguel Sinusia Lozano, Víctor J. Gómez. Epitaxial growth of crystal phase quantum dots in III–V semiconductor nanowires. Nanoscale Advances 2023, 20 https://doi.org/10.1039/D2NA00956K
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  • Abstract

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    Figure 1

    Figure 1. (a,b) Bright-field TEM images and corresponding electron diffraction patterns for zigzag ZnO nanostructures. (c) Dark-field image showing periodic strain inside a zigzag ZnO nanobelt. Adapted from ref (78). Copyright 2004 American Chemical Society. (d) TEM image of a Si NW grown using 2.5 mTorr disilane acquired along the [110] azimuth. Arrows indicate segments grown at different temperatures. The scale bar is 250 nm. Adapted from ref (64). Copyright 2009 American Chemical Society.

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    Figure 2

    Figure 2. Bright-field TEM images recorded during the growth of Ge on GaP NWs by UHV–CVD. Images represent different wires. (a) After the Au particle becomes saturated with Ge, nucleation is observed as a small compact Ge crystal at the boundary between GaP, Au, and the vacuum. This brighter nucleus is indicated by an arrow. The horizontal lines are stacking faults. (b) As the nucleus enlarges, the Au particle is pushed horizontally across the GaP surface. Ge does not form a layer on the GaP. (c) As the growth proceeds, a new growth front is visible between the Ge and Au, apparently a different (111) interface. (d) When the particle reaches the edge of the GaP, the growth may continue outward from the wire or may wrap around the top. Adapted from ref (57). Copyright 2007 American Chemical Society.

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    Figure 3

    Figure 3. Field emission SEM images of Ge nanowires on GeBSi substrates grown at (a) 300 °C, (b) 290 °C, (c) 280 °C, (d) 270 °C, (e) 260 °C, and (f) 250 °C for 20 min. The arrow points at the kink in the NW. Scale bar = 500 nm. Adapted from ref (50). Copyright 2011 American Chemical Society.

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    Figure 4

    Figure 4. (a) TEM image of a kinked Si NW grown in the two-step process. The (111) lattice planes of a [111] growth direction wire have grown continuously within the [211̅] wire. (b) Schematics of the crystallographic relationship between growth directions of a kinked Si NW. Reprinted with permission from ref (48). Copyright 2009 IOP Publishing.

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    Figure 5

    Figure 5. (a) Substrate temperature and disilane pressure as a function of time for ⟨211⟩ → ⟨111⟩ kinking for conditions I and II. SEM images along the [011̅] direction of Si NWs showing a ⟨211⟩ → ⟨111⟩ kinking: (b) [211] → [111̅], (c) [121] → [111̅], and (d) [112] → [1̅11]. (e) Substrate temperature and disilane pressure as a function of time for ⟨211⟩ → ⟨211⟩ kinking. SEM images along the [01̅1] direction of Si NWs showing ⟨211⟩ → ⟨211⟩ kinking: (f) [211] → [121̅], (g) [121] → [211̅], and (h) [112] → [1̅12]. Both types of kinked NWs have [111] segments at the base and are grown for 10 min under condition II. The axial positions where condition I is initially applied are indicated with dotted lines at the lower part of SEM images. Scale bars are 100 nm. Adapted from ref (67). Copyright 2014 American Chemical Society.

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    Figure 6

    Figure 6. (a) Schematic of a coherently kinked NW and SBU containing two arms (blue) and one joint (green). The multiply kinked NWs (middle panel) are derived from the corresponding 1D NW by introducing the joints at the locations indicated by the dashed lines in the upper panel. Subscripts c and h denote cubic and hexagonal structures, respectively. (b) Schematic illustrating the key stages of kink formation. Arrows 1–4 denote purge, reintroduction of reactant, joint growth, and subsequent arm growth, respectively. (c) SEM images of the kinked Si NW (upper) grown with periodic 15 s purges. Inset: SEM image of a multiply kinked 2D silicon NW with equal arm segment lengths. The arrow indicates the location of the catalyst. (d) TEM image of a single kink with crystallographic directions and facets indicated by arrows and dashed lines, respectively. The green (I) and blue (II) squares correspond to high-resolution images in the two last panels. Dashed lines and solid arrows indicate crystallographic planes and growth directions, respectively. Reprinted with permission from ref (41). Copyright 2009 Springer.

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    Figure 7

    Figure 7. SEM images of (a) ⟨111⟩/⟨110⟩, (b) ⟨110⟩/⟨110⟩, and (c) ⟨111⟩/⟨111⟩ Ge NW superstructures grown at 325 °C by introducing and removing MG during synthesis. An asterisk indicates the locations where transition did not occur as planned. Schematics for each growth direction change are shown below each corresponding superstructure with the smallest deviation angle labeled. Bolded sidewalls in the schematic for (b) denote that the diamond cubic lattice dictates that neighboring ⟨110⟩ segments of the ⟨110⟩/⟨110⟩ superstructure cannot lie in the same plane. Adapted from ref (104). Copyright 2012 American Chemical Society.

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    Figure 8

    Figure 8. TEM images of heterostructured kinked NWs. (a) Si → GaP NW. The Si part is single-crystalline, while the GaP part exhibits extensive stacking faults perpendicular to the growth direction. (b) GaAs → InAs NW. The interface between materials is indicated by a line. (c) GaP → InP NW. The InP wraps along the top of the GaP and grows backward. Adapted from ref (57). Copyright 2007 American Chemical Society.

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    Figure 9

    Figure 9. Chosen frames of Si NWs growth inside UHV TEM under conditions of decreasing droplet volume. The electron beam is parallel to ⟨110⟩. (a) Migration of Au from the droplet (Ostwald ripening) reduces the droplet volume. The NW diameter decreases via formation of inclined (111) facets. (b) A different NW with a larger inclined facet. (c) Schematic diagram showing the outcomes for NW morphology in the case of a mismatch between the droplet volume/contact angle and the NW diameter. Adapted from ref (56). Copyright 2013 American Chemical Society.

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    Figure 10

    Figure 10. Fraction of NWs that kink away from [111] vs average diameter, D, at the indicated P in mtorr (mt) and T. Adapted from ref (64). Copyright 2009 American Chemical Society.

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    Figure 11

    Figure 11. (a,b) Two types of kinking observed in experiments. (c) Dependence of the kinking probability at different tested temperatures on the catalyst size. Adapted from ref (37). Copyright 2016 American Chemical Society.

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    Figure 12

    Figure 12. Schematics of the synthesis of kinked Si NWs 200 by metal-assisted chemical etching of Si substrate. First, the Si substrate is patterned with a holey Au mask by polystyrene nanosphere lithography. Second, the nanostructuring of Si follows repetitive etch–quench sequences. The SEM pictures show the k-kinked Si NWs that resulted after five successive etch–quench sequences. Adapted from ref (117). Copyright 2019 American Chemical Society.

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    Figure 13

    Figure 13. Fracture stress (left axes, blue dot markers) and fracture strain (right axes, orange square markers) as a function of the periodic length L in symmetric zigzag kinked Si NWs (left diagram) and the arm length Larm in asymmetric ones with Lfixed = 19.5 nm (right diagram). The NW configurations for each case are shown in the insets. Data are from ref (113).

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    Figure 14

    Figure 14. (a) I–V data recorded from a kinked p–n silicon NW-based device. Inset: SEM image of the device structure. Scale bar is 2 μm. (b) Electrostatic force microscopy image of a p–n diode reverse-biased at 5 V. The atomic force microscope (AFM) tip voltage was modulated by 3 V at the cantilever-tip resonance frequency. The signal brightness is proportional to the NW device surface potential and has an abrupt drop around the kink position. The dashed lines indicate the NW position. Scale bar is 2 μm. (c,d) AFM and scanned gate microscopy (SGM) images of one n+–kink–n+–kink–(n–n+) dopant modulated double-kinked silicon NW structure. The scale bar in (c) is 2 μm. The SGM images were recorded with a Vtip of 10 V (I) and −10 V (II), respectively, and a Vsd of 1 V. The dark and bright regions correspond to reduced and enhanced conductance, respectively. The black dashed lines denote the NW position. Reprinted with permission from ref (41). Copyright 2009 Springer.

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    Figure 15

    Figure 15. Surface modification and cellular entry. (a) Fluorescence image of a lipid-coated NW probe. (b) Schematics of NW probe entrance into a cell. Dark purple, light purple, pink, and blue colors denote the phospholipid bilayers, heavily doped NW segments, active sensor segment, and cytosol, respectively. (c) Differential interference contrast microscopy images and electrical recording of an HL-1 cell and 60° kinked NW probe as the cell approaches (I), contacts and internalizes (II), and is retracted from (III) the nanoprobe. A pulled-glass micropipette (inner tip diameter ≈ 5 mm) was used to manipulate and voltage clamp the HL-1 cell. The dashed green line corresponds to the micropipette potential. Scale bars are 5 mm. Reprinted with permission from ref (44). Copyright 2010 American Association for the Advancement of Science.

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    Figure 16

    Figure 16. Intracellular electrical recording from spontaneously beating chicken cardiomyocytes. (Top) Steady state intracellular recording using 3D kinked p–n nanoprobes from a spontaneously beating cardiomyocyte cell. (Bottom) Zoom of the single-action potential peak from the green-dashed region. Inset: Schematic of intracellular recording from spontaneously beating embryonic chicken cardiomyocytes cultured on PDMS substrate using 3D kinked p–n nanoprobes. Adapted from ref (53). Copyright 2012 American Chemical Society.

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    Figure 17

    Figure 17. (a,b) Overview of designs and potential applications of kinked NWs. (a) U-shaped NW with integrated nanoFET (red) shown as a bioprobe for intracellular recording. (b) W-shaped NW with multiple nanoFETs (red) illustrated as a bioprobe for simultaneous intracellular/extracellular recording. Green color indicates heavily doped (n++) nanoelectrode arms, red indicates the point-like active nanoFET elements, and gold indicates the fabricated metal interconnects. A schematic of a cell to scale is drawn with the different device designs to show the potential for achieving minimally invasive deep penetration (a) and multiplexed intracellular and extracellular recording (b). (c) U- and W-shaped NWs synthesized in the study. Upper right image is the dark-field optical microscopy image of a KOH-etched kinked NW with four nanoFETs. The dark segments correspond to the four lightly doped nanoFET elements. Adapted from ref (45). Copyright 2012 American Chemical Society.

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    Sergei Vlassov

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    Sergei Vlassov (born in 1980) is an associate professor at the Institute of Physics, University of Tartu (Estonia). He received an MSc in applied physics (2007) and PhD in material science (2011) in the University of Tartu (Estonia) and then spent 2 years as a postdoc in the Institute of Solid State Physics, University of Latvia. His research interests are mainly related to experimental studies of mechanical and structural properties of individual nanostructures. By the spring of 2021, Sergei has 85 publications in the Google Scholar database, with an h-index of 19.

    Sven Oras

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    Sven Oras (born in 1989) has a PhD in Materials Chemistry (2018, University of Upper Alsace) and a Master’s Degree in Physics (2014, University of Tartu). His research activities are focused on the experimental characterization of nanostructures focusing mainly on mechanical and tribological properties. In the MATTER project, Sven is a postdoc responsible for performing nanomanipulation experiments inside a scanning electron microscope (SEM) affecting single nanostructures and performing characterization of nanostructures with an atomic force microscope (AFM). By the end of 2020, Sven has 15 publications in the Google Scholar database, with an h-index of 8.

    Boris Polyakov

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    Boris Polyakov is a senior researcher in the Institute of Solid State Physics, University of Latvia. He received his BSc and MSc degrees in mechanical engineering in Riga Technical University. He completed his Ph.D. in chemical physics in the University of Latvia in 2007. Later, he won a postdoctoral position in a postdoc project competition in the field of nanotribology in the Institute of Physics, University of Tartu, Estonia. After returning to Latvia, he continued his work in the Institute of Solid State Physics, University of Latvia. His research interests include synthesis and physical properties of nanomaterials and 2D layered materials. By the spring of 2021, Boris has 79 publications in the Google Scholar database with an h-index of 19.

    Edgars Butanovs

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    Edgars Butanovs is a senior researcher in the Thin Films Laboratory in the Institute of Solid State Physics, University of Latvia, a Research Fellow in the Institute of Technology, University of Tartu, and a lecturer in the University of Latvia. He received his BSc and MSc degrees in physics in 2014 and 2016, respectively, and completed his Ph.D. in materials physics in the University of Latvia in 2020. His research interests are related to the growth and studies of 1D and 2D nanomaterials (semiconducting nanowires and layered 2D materials), thin film deposition technologies, and optoelectronics applications. By the spring of 2021, Edgars has 16 publications in the Google Scholar database with an h-index of 5.

    Andreas Kyritsakis

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    Andreas Kyritsakis (born in 1987) has a PhD in Electrical & Computer Engineering (2014, National Technical University of Athens) and a Master Degree (Diploma) in Electrical & Computer Engineering (2010, National Technical University of Athens). His research activities are focused mainly on the theoretical/computational modelling of electron emission, vacuum arcs, and nanomaterial behavior under high electric fields. At the moment, Andreas is the ERA Chair Holder of the MATTER project, leading its scientific activities. As a simulation specialist, he focuses mainly on the subgroup of multiscale nanomaterial modelling. By the spring of 2021 Andreas has 45 publications in the Google Scholar database, with an h-index of 11.

    Veronika Zadin

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    Veronika Zadin is a professor of materials technology in the Institute of Technology, University of Tartu. She received her PhD in physical engineering in 2012 (University of Tartu) followed by postdoctoral research in the University of Helsinki (2012–2014). Next to the scientific work, she is involved in managing the collaboration between University of Tartu and CTF3 activities in CERN, holding there the position of Estonian group leader. Her main expertise and research interest are multiphysics finite element simulations in the nano- and microscale. Her specialization includes elastic and plastic deformations, heat and mass transport, charge transport, and interactions between the electric field and metals. By the spring of 2021, Veronika has 65 publications in the Google Scholar database, with an h-index of 15.

  • References

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    This article references 124 other publications.

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      In the present work, we demonstrate a novel approach to nanotribol. measurements based on the bending manipulation of hexagonal ZnO nanowires (NWs) in an adjustable halfsuspended configuration inside a scanning electron microscope. A pick-and-place manipulation technique was used to control the length of the adhered part of each suspended NW. Static and kinetic friction were found by a 'self-sensing' approach based on the strain profile of the elastically bent NW during manipulation and its Young's modulus, which was sep. measured in a three-point bending test with an at. force microscope. The calcn. of static friction from the most bent state was completely reconsidered and a novel more realistic crackbased model was proposed. It was demonstrated that, in contrast to assumptions made in previously published models, interfacial stresses in statically bent NW are highly localized and interfacial strength is comparable to the bending strength of NW measured in resp. bending tests.
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      Vlassov, S.; Polyakov, B.; Vahtrus, M.; Mets, M.; Antsov, M.; Oras, S.; Tarre, A.; Arroval, T.; Lõhmus, R.; Aarik, J. Enhanced flexibility and electron-beam-controlled shape recovery in alumina-coated Au and Ag core–shell nanowires. Nanotechnology 2017, 28, 505707,  DOI: 10.1088/1361-6528/aa973c
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      Nanotechnology (2017), 28 (50), 505707/1-505707/10CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      The proper choice of coating materials and methods in core-shell nanowire (NW) engineering is crucial to assuring improved characteristics or even new functionalities of the resulting composite structures. In this paper, we have reported electron-beam-induced reversible elastic-to-plastic transition in Ag/Al2O3 and Au/Al2O3 NWs prepd. by the coating of Ag and AuNWs with Al2O3 by low-temp. at. layer deposition. The obsd. phenomenon enabled freezing the bent core-shell NW at any arbitrary curvature below the yield strength of the materials and later restoring its initially straight profile by irradiating the NW with electrons. In addn., we demonstrated that the coating efficiently protects the core material from fracture and plastic yield, allowing it to withstand significantly higher deformations and stresses in comparison to uncoated NW.
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      Nehra, M.; Dilbaghi, N.; Marrazza, G.; Kaushik, A.; Abolhassani, R.; Mishra, Y. K.; Kim, K. H.; Kumar, S. 1D semiconductor nanowires for energy conversion, harvesting and storage applications. Nano Energy 2020, 76, 104991,  DOI: 10.1016/j.nanoen.2020.104991
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      1D semiconductor nanowires for energy conversion, harvesting and storage applications
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      Nano Energy (2020), 76 (), 104991CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      A review. The accomplishment of 1D semiconductor nanowires (SN) in the field of energy has attracted intense interest in recent years due to their advantageous properties (e.g., large surface area, unique surface chem., and tunable transport properties). Considerable efforts were devoted to explore 1D-SN building blocks as the harvesting channel/unit (e.g., in thermal, chem., mech., and solar energy applications) and as the storage media (for electrochem. energy). A wide bandgap tuning of SN in the range of 0.39 eV (in case of InAs nanowires) to 4.66 eV (in case of β-Ga2O3 nanowires) due to quantum size effect makes them a suitable candidate for optoelectronic applications. This review focuses on 1D-SN wherein the travel of electron and photon is confined in two directions but in one dimension. The SN emerged as promising nanostructures for developing electronic devices of high carrier-mobilities (e.g., >12000 cm2V-1s-1 for holes and 3000 cm2V-1s-1 for electrons in case of Ge nanowires). A list of efficient fabrication strategies (e.g., vapor-liq.-solid [VLS], hard-template approaches, and soln.-phase) are discussed along with ultrafast electron transport dynamics of SN and piezoelec. nanowires. The control on electrons, photons, and phonons transport makes 1D-SN ideal for solid-state energy conversion, harvesting, and storage applications. State-of-the-art 1D-SN energy nano-systems have been demonstrated to yield diverse outcomes of high significance including single-nanowire and array-based photovoltaic cells (InP nanowires with a max. power conversion efficiency up to 17.8%), nanogenerators (SiGe nanowires with a max. power output of 7.1μW/cm2), supercapacitors (core-shell hierarchical CoS@MoS2 nanowire array with an energy d. of 95.7 Wh kg-1 at power d. of 711.2 W kg-1), and lithium-air batteries (3D freestanding hierarchical CuCo2O4 nanowires@Ni foam with an excellent specific capacity of 13654 mAh g-1 at 0.1 mA cm-2). This review will serve as a key platform to understand 1D-SN to fabricate the next-generation novel nano-systems for developing efficient energy devices of high performance.
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      Vlassov, S.; Mets, M.; Polyakov, B.; Bian, J.; Dorogin, L.; Zadin, V. Abrupt elastic-to-plastic transition in pentagonal nanowires under bending Beilstein. Beilstein J. Nanotechnol. 2019, 10, 2468– 76,  DOI: 10.3762/bjnano.10.237
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      Vlassov, Sergei; Mets, Magnus; Polyakov, Boris; Bian, Jianjun; Dorogin, Leonid; Zadin, Vahur
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      In this study, pentagonal Ag and Au nanowires (NWs) were bent in cantilever beam configuration inside a scanning electron microscope. We demonstrated an unusual, abrupt elastic-to-plastic transition, obsd. as a sudden change of the NW profile from smooth arc-shaped to angled knee-like during the bending in the narrow range of bending angles. In contrast to the behavior of NWs in the tensile and three-point bending tests, where extensive elastic deformation was followed by brittle fracture, in our case, after the abrupt plastic event, the NW was still far from fracture and enabled further bending without breaking. A possible explanation is that the five-fold twinned structure prevents propagation of crit. defects, leading to dislocation pile up that may lead to sudden stress release, which is obsd. as an abrupt plastic event. Moreover, we found that if the NWs are coated with alumina, the abrupt plastic event is not obsd. and the NWs can withstand severe deformation in the elastic regime without fracture. The coating may possibly prevent formation of dislocations. Mech. durability under high and inhomogeneous strain fields is an important aspect of exploiting Ag and Au NWs in applications like waveguiding or conductive networks in flexible polymer composite materials.
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      Antsov, M.; Polyakov, B.; Zadin, V.; Mets, M.; Oras, S.; Vahtrus, M.; Lõhmus, R.; Dorogin, L.; Vlassov, S. Mechanical characterisation of pentagonal gold nanowires in three different test configurations: A comparative study. Micron 2019, 124, 102686,  DOI: 10.1016/j.micron.2019.102686
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      Mechanical characterisation of pentagonal gold nanowires in three different test configurations: A comparative study
      Antsov, Mikk; Polyakov, Boris; Zadin, Vahur; Mets, Magnus; Oras, Sven; Vahtrus, Mikk; Lohmus, Runno; Dorogin, Leonid; Vlassov, Sergei
      Micron (2019), 124 (), 102686CODEN: MCONEN; ISSN:0968-4328. (Elsevier Ltd.)
      Mech. characterization of individual nanostructures is a challenging task and can greatly benefit from the utilization of several alternative approaches to increase the reliability of results. In the present work, we have measured and compared the elastic modulus of five-fold twinned gold nanowires (NWs) with at. force microscopy (AFM) indentation in three different test configurations: three-point bending with fixed ends, three-point bending with free ends and cantilevered-beam bending. The free-ends condition was realized by introducing a novel approach where the NW is placed diagonally inside an inverted pyramid chem. etched in a silicon wafer. In addn., all three configurations were simulated with a finite element method to obtain better insight into stress distribution inside NWs during bending depending on test conditions. The free-ends configuration yielded elastic modulus similar to a classical fixed-ends approach (88 ± 20 GPa vs 87 ± 16 GPa), indicating the reliability of the proposed method. At the same time, the free-ends configuration benefits from a more favorable NW position relative to the probe with facet facing upwards in contrast to the sharp edge in the case of fixed ends. From the other hand, the free-ends configuration was less suitable for strength measurements, as NW can run into the bottom of the inverted pyramid because of a higher degree of deformation before fracture. The cantilevered-beam configuration was less suitable for mech. testing with indentation because of the instabilities of the free end under the AFM probe.
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    7. 7
      Polyakov, B.; Vlassov, S.; Dorogin, L. M.; Kulis, P.; Kink, I.; Lohmus, R. The effect of substrate roughness on the static friction of CuO nanowires. Surf. Sci. 2012, 606, 1393– 9,  DOI: 10.1016/j.susc.2012.04.029
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      7
      The effect of substrate roughness on the static friction of CuO nanowires
      Polyakov, Boris; Vlassov, Sergei; Dorogin, Leonid M.; Kulis, Peteris; Kink, Ilmar; Lohmus, Rynno
      Surface Science (2012), 606 (17-18), 1393-1399CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      The dependence of static friction on surface roughness was measured for CuO nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chem. etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an AFM tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calc. the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estd. using a multiple elastic asperity model, which is based on the Derjaguin-Muller-Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.
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    8. 8
      Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 2001, 409, 66– 9,  DOI: 10.1038/35051047
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      8
      Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices
      Duan, Xiangfeng; Huang, Yu; Cui, Yl; Wang, Jianfang; Lieber, Charles M.
      Nature (London) (2001), 409 (6816), 66-69CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Nanowires and nanotubes carry charge and excitons efficiently, and are therefore potentially ideal building blocks for nanoscale electronics and optoelectronics. Carbon nanotubes have already been exploited in devices such as field-effect and single-electron transistors, but the practical utility of nanotube components for building electronic circuits is limited, as it is not yet possible to selectively grow semiconducting or metallic nanotubes. Here we report the assembly of functional nanoscale devices from indium phosphide nanowires, the elec. properties of which are controlled by selective doping. Gate-voltage-dependent transport measurements demonstrate that the nano- wires can be predictably synthesized as either n- or p-type. These doped nanowires function as nanoscale field-effect transistors, and can be assembled into crossed-wire p-n junctions that exhibit rectifying behavior. Significantly, the p-n junctions emit light strongly and are perhaps the smallest light-emitting diodes that have yet been made. Finally, we show that elec.-field-directed assembly can be used to create highly integrated device arrays from nanowire building blocks.
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    9. 9
      Stern, A.; Aharon, S.; Binyamin, T.; Karmi, A.; Rotem, D.; Etgar, L.; Porath, D. Electrical Characterization of Individual Cesium Lead Halide Perovskite Nanowires Using Conductive AFM. Adv. Mater. 2020, 32, 1907812,  DOI: 10.1002/adma.201907812
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      9
      Electrical Characterization of Individual Cesium Lead Halide Perovskite Nanowires Using Conductive AFM
      Stern, Avigail; Aharon, Sigalit; Binyamin, Tal; Karmi, Abeer; Rotem, Dvir; Etgar, Lioz; Porath, Danny
      Advanced Materials (Weinheim, Germany) (2020), 32 (12), 1907812CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Perovskite nanostructures have attracted much attention in recent years due to their suitability for a variety of applications such as photovoltaics, light-emitting diodes (LEDs), nanometer-size lasing, and more. These uses rely on the conductive properties of these nanostructures. However, elec. characterization of individual, thin perovskite nanowires has not yet been reported. Here, conductive at. force microscopy characterization of individual cesium lead halide nanowires is presented. Clear differences are obsd. in the cond. of nanowires contg. only bromide and nanowires contg. a mixt. of bromide and iodide. The differences are attributed to a higher d. of cryst. defects, deeper trap states, and higher inherent cond. for nanowires with mixed bromide-iodide content.
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    10. 10
      Butanovs, E.; Vlassov, S.; Kuzmin, A.; Piskunov, S.; Butikova, J.; Polyakov, B. Fast-Response Single-Nanowire Photodetector Based on ZnO/WS 2 Core/Shell Heterostructures. ACS Appl. Mater. Interfaces 2018, 10, 13869– 76,  DOI: 10.1021/acsami.8b02241
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      10
      Fast-Response Single-Nanowire Photodetector Based on ZnO/WS2 Core/Shell Heterostructures
      Butanovs, Edgars; Vlassov, Sergei; Kuzmin, Alexei; Piskunov, Sergei; Butikova, Jelena; Polyakov, Boris
      ACS Applied Materials & Interfaces (2018), 10 (16), 13869-13876CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The surface plays an exceptionally important role in nanoscale materials, exerting a strong influence on their properties. Consequently, even a very thin coating can greatly improve the optoelectronic properties of nanostructures by modifying the light absorption and spatial distribution of charge carriers. To use these advantages, 1D/1D heterostructures of ZnO/WS2 core/shell nanowires with a-few-layers-thick WS2 shell were fabricated. These heterostructures were thoroughly characterized by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Then, a single-nanowire photoresistive device was assembled by mech. positioning ZnO/WS2 core/shell nanowires onto gold electrodes inside a scanning electron microscope. The results show that a few layers of WS2 significantly enhance the photosensitivity in the short wavelength range and drastically (almost 2 orders of magnitude) improve the photoresponse time of pure ZnO nanowires. The fast response time of ZnO/WS2 core/shell nanowire was explained by electrons and holes sinking from ZnO nanowire into WS2 shell, which serves as a charge carrier channel in the ZnO/WS2 heterostructure. First-principles calcns. suggest that the interface layer i-WS2, bridging ZnO nanowire surface and WS2 shell, might play a role of energy barrier, preventing the backward diffusion of charge carriers into ZnO nanowire.
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    11. 11
      Vlassov, S.; Oras, S.; Antsov, M.; Butikova, J.; Lõhmus, R.; Polyakov, B. Low-friction nanojoint prototype. Nanotechnology 2018, 29, 195707,  DOI: 10.1088/1361-6528/aab163
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      11
      Low-friction nanojoint prototype
      Vlassov, Sergei; Oras, Sven; Antsov, Mikk; Butikova, Jelena; Lohmus, Runno; Polyakov, Boris
      Nanotechnology (2018), 29 (19), 195707/1-195707/6CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      High surface energy of individual nanostructures leads to high adhesion and static friction that can completely hinder the operation of nanoscale systems with movable parts. For instance, silver or gold nanowires cannot be moved on silicon substrate without plastic deformation. In this paper, we exptl. demonstrate an operational prototype of a low-friction nanojoint. The movable part of the prototype is made either from a gold or silver nano-pin produced by laser-induced partial melting of silver and gold nanowires resulting in the formation of rounded bulbs on their ends. The nano-pin is then manipulated into the inverted pyramid (i-pyramids) specially etched in a Si wafer. Due to the small contact area, the nano-pin can be repeatedly tilted inside an i-pyramid as a rigid object without noticeable deformation. At the same time in the absence of external force the nanojoint is stable and preserves its position and tilt angle. Expts. are performed inside a scanning electron microscope and are supported by finite element method simulations.
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    12. 12
      Rahong, S.; Yasui, T.; Kaji, N.; Baba, Y. Recent developments in nanowires for bio-applications from molecular to cellular levels. Lab Chip 2016, 16, 1126– 38,  DOI: 10.1039/C5LC01306B
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      12
      Recent developments in nanowires for bio-applications from molecular to cellular levels
      Rahong, Sakon; Yasui, Takao; Kaji, Noritada; Baba, Yoshinobu
      Lab on a Chip (2016), 16 (7), 1126-1138CODEN: LCAHAM; ISSN:1473-0189. (Royal Society of Chemistry)
      This review highlights the most promising applications of nanowires for bioanal. chem. and medical diagnostics. The materials discussed here are metal oxide and Si semiconductors, which are integrated with various microfluidic systems. Nanowire structures offer desirable advantages such as a very small diam. size with a high aspect ratio and a high surface-to-vol. ratio without grain boundaries; consequently, nanowires are promising tools to study biol. systems. This review starts with the integration of nanowire structures into microfluidic systems, followed by the discussion of the advantages of nanowire structures in the sepn., manipulation and purifn. of biomols. (DNA, RNA and proteins). Next, some representative nanowire devices are introduced for biosensors from mol. to cellular levels based on elec. and optical approaches. Finally, we conclude the review by highlighting some bio-applications for nanowires and presenting the next challenges that must be overcome to improve the capabilities of nanowire structures for biol. and medical systems.
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    13. 13
      Verardo, D.; Lindberg, F. W.; Anttu, N.; Niman, C. S.; Lard, M.; Dabkowska, A. P.; Nylander, T.; Månsson, A.; Prinz, C. N.; Linke, H. Nanowires for Biosensing: Lightguiding of Fluorescence as a Function of Diameter and Wavelength. Nano Lett. 2018, 18, 4796– 802,  DOI: 10.1021/acs.nanolett.8b01360
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      13
      Nanowires for Biosensing: Lightguiding of Fluorescence as a Function of Diameter and Wavelength
      Verardo, Damiano; Lindberg, Frida W.; Anttu, Nicklas; Niman, Cassandra S.; Lard, Mercy; Dabkowska, Aleksandra P.; Nylander, Tommy; Maansson, Alf; Prinz, Christelle N.; Linke, Heiner
      Nano Letters (2018), 18 (8), 4796-4802CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Semiconductor nanowires can act as nanoscaled optical fibers, enabling them to guide and conc. light emitted by surface-bound fluorophores, potentially enhancing the sensitivity of optical biosensing. While parameters such as the nanowire geometry and the fluorophore wavelength can be expected to strongly influence this lightguiding effect, no detailed description of their effect on in-coupling of fluorescent emission is available to date. Here, the authors use confocal imaging to quantify the lightguiding effect in GaP nanowires as a function of nanowire geometry and light wavelength. Using a combination of finite-difference time-domain simulations and anal. approaches, the authors identify the role of multiple waveguide modes for the obsd. lightguiding. The normalized frequency parameter, based on the step-index approxn., predicts the lightguiding ability of the nanowires as a function of diam. and fluorophore wavelength, providing a useful guide for the design of optical biosensors based on nanowires.
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    14. 14
      Johar, M. A.; Song, H.-G.; Waseem, A.; Hassan, M. A.; Bagal, I. V.; Cho, Y.-H.; Ryu, S.-W. Universal and scalable route to fabricate GaN nanowire-based LED on amorphous substrate by MOCVD. Appl. Mater. Today 2020, 19, 100541,  DOI: 10.1016/j.apmt.2019.100541
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      There is no corresponding record for this reference.
    15. 15
      Thiyagu, S; Fukata, N Chapter 9 - Silicon nanowire-based solar cells. Nanomaterials for Solar Cell Applications 2019, 325– 348,  DOI: 10.1016/B978-0-12-813337-8.00009-6
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      15
      Silicon nanowire-based solar cells
      Thiyagu, Subramani; Fukata, Naoki
      Nanomaterials for Solar Cell Applications (2019), (), 325-348CODEN: 69YKZ8 ISSN:. (Elsevier B.V.)
      In summary, the use of a silicon nanostructure is an effective way of fabricating high-performance and cost-effective solar cells because of their superior properties for carrier transport, charge sepn., and light absorption. Radial p-n junction SiNW arrays offers a more viable approach to cost-effective Si-based solar cells than do conventional planar Si solar cells. Reflectance can be dramatically suppressed relative to that of planar Si substrates due to the formation of SiNW array structures over a broad range of spectral wavelengths of 300-1000 nm. By constructing p-n junctions radially in SiNWs, the light trapping effect effectively increases and the carrier recombination rate is also effectively reduced due to the shorter carrier collection path. SiNW-based solar cells appear to be promising candidates; however, many challenges need to be tackled; in particular, the carrier recombination problem at the interface caused by increased surface-to-vol. ratio. We showed that three kinds of passivation techniques improve the performance of SiNW solar cells. As a result of these enhancements, cell properties such as Jsc, Voc, and FF increase, leading to higher power conversion efficiency. The NRET effect using nc-Si is a new way of improving solar cell properties. This technique can be simply and easily applied to any kind of Si solar cell. These approaches broaden the scope for developing cost-effective, large-area solar energy devices in industries at large scales.
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    16. 16
      Garnett, E. C.; Brongersma, M. L.; Cui, Y.; McGehee, M. D. Nanowire Solar Cells Annu. Annu. Rev. Mater. Res. 2011, 41, 269– 95,  DOI: 10.1146/annurev-matsci-062910-100434
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      16
      Nanowire solar cells
      Garnett, Erik C.; Brongersma, Mark L.; Cui, Yi; McGehee, Michael D.
      Annual Review of Materials Research (2011), 41 (), 269-295CODEN: ARMRCU; ISSN:1531-7331. (Annual Reviews Inc.)
      A review. The nanowire geometry provides potential advantages over planar wafer-based or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the max. efficiency above std. limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost redns. Addnl., nanowires provide opportunities to fabricate complex single-cryst. semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technol. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.
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    17. 17
      Lan, C.; Li, C.; Wang, S.; Yin, Y.; Guo, H.; Liu, N.; Liu, Y. ZnO–WS2 heterostructures for enhanced ultra-violet photodetectors. RSC Adv. 2016, 6, 67520– 4,  DOI: 10.1039/C6RA12643J
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      17
      ZnO-WS2 heterostructures for enhanced ultra-violet photodetectors
      Lan, Changyong; Li, Chun; Wang, Shuai; Yin, Yi; Guo, Huayang; Liu, Nishuang; Liu, Yong
      RSC Advances (2016), 6 (72), 67520-67524CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Two-dimensional (2D) materials have attracted wide attention due to their exotic properties. In particular, the lack of dangling bonds makes it possible to build highly lattice mismatched heterostructures composed of 2D materials and conventional semiconductors. Here, we report that by simply stacking a chem. vapor deposition grown monolayer WS2 film onto the surface of a room temp. sputtered ZnO film, significant enhanced ultra-violet (UV) photoresponse can be achieved. In this heterostructure of ZnO-WS2, the ZnO film acts as a light harvesting layer while the WS2 monolayer functions as a carrier transport layer which facilitates the photocarrier transport and reduces its recombination. Such a mechanism was confirmed by the observation of further photoresponsivity improvement of the ZnO-WS2 heterostructure under vacuum which removes the surface absorbates and thereby increases the carrier mobility of WS2. The strategy presented here can be applied to other wide band-gap semiconductors, shedding light on high sensitivity and flexible UV photodetectors based on van der Waals heterostructures.
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    18. 18
      Stokes, K.; Geaney, H.; Sheehan, M.; Borsa, D.; Ryan, K. M. Copper Silicide Nanowires as Hosts for Amorphous Si Deposition as a Route to Produce High Capacity Lithium-Ion Battery Anodes. Nano Lett. 2019, 19, 8829– 35,  DOI: 10.1021/acs.nanolett.9b03664
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      18
      Copper Silicide Nanowires as Hosts for Amorphous Si Deposition as a Route to Produce High Capacity Lithium-Ion Battery Anodes
      Stokes, Killian; Geaney, Hugh; Sheehan, Martin; Borsa, Dana; Ryan, Kevin M.
      Nano Letters (2019), 19 (12), 8829-8835CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Herein, copper silicide (Cu15Si4) nanowires (NWs) grown in high densities from a metallic Cu substrate are utilized as nanostructured hosts for amorphous silicon (aSi) deposition. The conductive Cu15Si4 NW scaffolds offer an increased surface area, vs. planar substrates, and enable the prepn. of high capacity Li-ion anodes consisting of a nanostructured active material. The formation method involves a two-step process, where Cu15Si4 nanowires are synthesized from a Cu substrate via a solvent vapor growth (SVG) approach followed by the plasma-enhanced chem. vapor deposition (PECVD) of aSi. These binder-free anodes are investigated in half-cell (vs. Li-foil) and full-cell (vs. LCO) configurations with discharge capacities greater than 2000 mAh/g retained after 200 cycles (half-cell) and reversible capacities of 1870 mAh/g exhibited after 100 cycles (full-cell). A noteworthy rate capability is also attained where capacities of up to 1367 mAh/g and 1520 mAh/g are exhibited at 5C in half-cell and full-cell configurations, resp., highlighting the active material's promise for fast charging and high power applications. The anode material is characterized prior to cycling and after 1, 25, and 100 charge/discharge cycles, by SEM (SEM) and transmission electron microscopy (TEM), to track the effects of cycling on the material.
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    19. 19
      Wei, H.; Wang, Z.; Tian, X.; Käll, M.; Xu, H. Cascaded logic gates in nanophotonic plasmon networks. Nat. Commun. 2011, 2, 387,  DOI: 10.1038/ncomms1388
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      Cascaded logic gates in nanophotonic plasmon networks
      Wei Hong; Wang Zhuoxian; Tian Xiaorui; Kall Mikael; Xu Hongxing
      Nature communications (2011), 2 (), 387 ISSN:.
      Optical computing has been pursued for decades as a potential strategy for advancing beyond the fundamental performance limitations of semiconductor-based electronic devices, but feasible on-chip integrated logic units and cascade devices have not been reported. Here we demonstrate that a plasmonic binary NOR gate, a 'universal logic gate', can be realized through cascaded OR and NOT gates in four-terminal plasmonic nanowire networks. This finding provides a path for the development of novel nanophotonic on-chip processor architectures for future optical computing technologies.
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    20. 20
      Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K.-H.; Lieber, C. M. Logic Gates and Computation from Assembled Nanowire Building Blocks. Science 2001, 294, 1313– 7,  DOI: 10.1126/science.1066192
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      20
      Logic gates and computation from assembled nanowire building blocks
      Huang, Yu; Duan, Xiangfeng; Cui, Yi; Lauhon, Lincoln J.; Kim, Kyoung-Ha; Lieber, Charles M.
      Science (Washington, DC, United States) (2001), 294 (5545), 1313-1317CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Miniaturization in electronics through improvements in established "top-down" fabrication techniques is approaching the point where fundamental issues are expected to limit the dramatic increases in computing seen over the past several decades. Here we report a "bottom-up" approach in which functional device elements and element arrays have been assembled from soln. through the use of electronically well-defined semiconductor nanowire building blocks. We show that crossed nanowire p-n junctions and junction arrays can be assembled in over 95% yield with controllable elec. characteristics, and in addn., that these junctions can be used to create integrated nanoscale field-effect transistor arrays with nanowires as both the conducting channel and gate electrode. Nanowire junction arrays have been configured as key OR, AND, and NOR logic-gate structures with substantial gain and have been used to implement basic computation.
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    21. 21
      Huo, N.; Yang, S.; Wei, Z.; Li, S.-S.; Xia, J.-B.; Li, J. Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS2 Nanoflakes. Sci. Rep. 2015, 4, 5209,  DOI: 10.1038/srep05209
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      There is no corresponding record for this reference.
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      Hu, X. F.; Li, S. J.; Wang, J.; Jiang, Z. M.; Yang, X. J. Investigating Size-Dependent Conductive Properties on Individual Si Nanowires. Nanoscale Res. Lett. 2020, 15, 52,  DOI: 10.1186/s11671-020-3277-3
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      Investigating Size-Dependent Conductive Properties on Individual Si Nanowires
      Hu, X. F.; Li, S. J.; Wang, J.; Jiang, Z. M.; Yang, X. J.
      Nanoscale Research Letters (2020), 15 (1), 52CODEN: NRLAAD; ISSN:1556-276X. (Springer)
      Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated by nanosphere lithog. combined with metal-assisted chem. etching. By adjusting the etching time, both the nanowires diam. and length can be well controlled. The conductive properties of such Si NWs and particularly their size dependence are investigated by conductive at. force microscopy (CAFM) on individual nanowires. The results indicate that the conductance of Si NWs is greatly relevant to their diam. and length. Si NWs with smaller diams. and shorter lengths exhibit better conductive properties. Together with the I-V curve characterization, a possible mechanism is supposed with the viewpoint of size-dependent Schottky barrier height, which is further verified by the electrostatic force microscopy (EFM) measurements. This study also suggests that CAFM can act as an effective means to explore the size (or other parameters) dependence of conductive properties on individual nanostructures, which should be essential for both fabrication optimization and potential applications of nanostructures.
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    23. 23
      Nam, C.-Y.; Jaroenapibal, P.; Tham, D.; Luzzi, D. E.; Evoy, S.; Fischer, J. E. Diameter-Dependent Electromechanical Properties of GaN Nanowires. Nano Lett. 2006, 6, 153– 8,  DOI: 10.1021/nl051860m
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      Diameter-dependent electromechanical properties of GaN nanowires
      Nam, Chang-Yong; Jaroenapibal, Papot; Tham, Douglas; Luzzi, David E.; Evoy, Stephane; Fischer, John E.
      Nano Letters (2006), 6 (2), 153-158CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The diam.-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromech. resonance anal. in a transmission electron microscope. E is close to the theor. bulk value (∼300 GPa) for a large diam. nanowire (d = 84 nm) but is significantly smaller for smaller diams. At room temp., Q is as high as 2800 for d = 84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-vol. ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromech. system (NEMS) applications. Two closely spaced resonances are obsd. and attributed to the low-symmetry triangular cross section of the nanowires.
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    24. 24
      Yorikawa, H.; Uchida, H.; Muramatsu, S. Energy gap of nanoscale Si rods. J. Appl. Phys. 1996, 79, 3619– 21,  DOI: 10.1063/1.361416
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      Energy gap of nanoscale Si rods
      Yorikawa, H.; Uchida, H.; Muramatsu, S.
      Journal of Applied Physics (1996), 79 (7), 3619-21CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The electronic structure of silicon rods has been studied by means of the tight-binding recursion method to investigate the dependence of the energy gap (Eg) on rod length and the direction of the rod axis. An empirical expression for Eg is derived from numerical results for the rods in 〈100〉, 〈110〉, and 〈111〉 directions. This expression is applicable to the energy gaps of wires and crystallites, which can be regarded as limiting cases of rods.
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    25. 25
      Roy, A.; Mead, J.; Wang, S.; Huang, H. Effects of surface defects on the mechanical properties of ZnO nanowires. Sci. Rep. 2017, 7, 9547,  DOI: 10.1038/s41598-017-09843-5
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      25
      Effects of surface defects on the mechanical properties of ZnO nanowires
      Roy Aditi; Mead James; Wang Shiliang; Huang Han; Wang Shiliang
      Scientific reports (2017), 7 (1), 9547 ISSN:.
      The elastic modulus of ZnO nanowires was measured using a resonance method based on laser Doppler effect and their fracture strains were determined via two-point bending with the aid of optical nanomanipulation. The elastic moduli of ZnO nanowires with diameters of 78 to 310 nm vary from 123 to 154 GPa, which are close to the bulk value of 140 GPa and independent of the diameters and surface defects. However, the fracture strains of the ZnO nanowires depend significantly on their diameters, increasing from 2.1% to 6.0% with the decrease in diameter from 316 to 114 nm. Post-mortem TEM analysis of the ends of the fractured nanowires revealed that fracture initiated at surface defects. The Weibull statistical analysis demonstrated that a greater defect depth led to a smaller fracture strain. The surface-defect dominated fracture should be an important consideration for the design and application of nanowire-based nanoelectromechanical systems.
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    26. 26
      Shin, N.; Chi, M.; Filler, M. A. Sidewall Morphology-Dependent Formation of Multiple Twins in Si Nanowires. ACS Nano 2013, 7, 8206– 13,  DOI: 10.1021/nn4036798
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      Sidewall Morphology-Dependent Formation of Multiple Twins in Si Nanowires
      Shin, Naechul; Chi, Miaofang; Filler, Michael A.
      ACS Nano (2013), 7 (9), 8206-8213CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Precise placement of twin boundaries and stacking faults promises new opportunities to fundamentally manipulate the optical, elec., and thermal properties of semiconductor nanowires. Here the authors report on the appearance of consecutive twin boundaries in Si nanowires and show that sidewall morphol. governs their spacing. Detailed electron microscopy anal. reveals that thin {111} sidewall facets, which elongate following the 1st twin boundary (TB1), are responsible for deforming the triple-phase line and favoring the formation of the 2nd twin boundary (TB2). While multiple, geometrically correlated defect planes are known in Group III-V nanowires, the authors' findings show that this behavior is also possible in Group IV materials.
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    27. 27
      Arbiol, J.; Fontcuberta i Morral, A.; Estradé, S.; Peiró, F.; Kalache, B.; Roca i Cabarrocas, P.; Morante, J. R. Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires. J. Appl. Phys. 2008, 104, 064312  DOI: 10.1063/1.2976338
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      27
      Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires
      Arbiol, Jordi; Fontcuberta i Morral, Anna; Estrade, Sonia; Peiro, Francesca; Kalache, Billel; Roca i Cabarrocas, Pere; Morante, Joan Ramon
      Journal of Applied Physics (2008), 104 (6), 064312/1-064312/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The occurrence of heterostructures of cubic Si/hexagonal Si as disks defined along the nanowire 〈111〉 growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. During growth, lamellar twinning on the cubic phase along the 〈111〉 direction is generated. Consecutive presence of twins along the 〈111〉 growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram. (c) 2008 American Institute of Physics.
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    28. 28
      Sosnin, I. M.; Vlassov, S.; Akimov, E. G.; Agenkov, V. I.; Dorogin, L. M. Transparent ZnO-coated polydimethylsiloxane-based material for photocatalytic purification applications. J. Coat. Technol. Res. 2020, 17, 573– 9,  DOI: 10.1007/s11998-019-00314-2
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      28
      Transparent ZnO-coated polydimethylsiloxane-based material for photocatalytic purification applications
      Sosnin, I. M.; Vlassov, S.; Akimov, E. G.; Agenkov, V. I.; Dorogin, L. M.
      Journal of Coatings Technology and Research (2020), 17 (2), 573-579CODEN: JCTRCP; ISSN:1935-3804. (Springer)
      Abstr: We describe prodn. and photocatalytic properties of a material based on polydimethylsiloxane (PDMS) as a carrier substrate coated with microparticles of zinc oxide (ZnO). The ZnO microparticles are fabricated by our original hydrothermal method and intentionally have a defect structure. According to our understanding, this peculiar defect structure contributes to the greatly enhanced photocatalytic properties of the ZnO material. The resulting photocatalyst demonstrates high activity under visible light (410 nm) in the process of phenol degrdn. in water soln., while generally ZnO is inactive below the UV range. In addn., we compare the photocatalytic activity of our ZnO/PDMS composite to that of the same ZnO powder suspension in a similar setup. We find that the same activity is achieved by three orders of magnitude smaller amt. of ZnO in our composite compared to the powder suspension. The ZnO/PDMS interface exhibits sufficiently strong bonding for stable operation that is ensured during material prodn. The obtained photocatalytic material preserves the transparency of PDMS due to the low amt. of attached ZnO (about 0.1% by mass). The transparency of the photocatalytic ZnO/PDMS material enables easy performance upgrades by constructing multilayer or manifold fluid treatment devices.
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    29. 29
      Murphy, K. F.; Piccione, B.; Zanjani, M. B.; Lukes, J. R.; Gianola, D. S. Strain- and Defect-Mediated Thermal Conductivity in Silicon Nanowires. Nano Lett. 2014, 14, 3785– 92,  DOI: 10.1021/nl500840d
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      Strain- and Defect-Mediated Thermal Conductivity in Silicon Nanowires
      Murphy, Kathryn F.; Piccione, Brian; Zanjani, Mehdi B.; Lukes, Jennifer R.; Gianola, Daniel S.
      Nano Letters (2014), 14 (7), 3785-3792CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The unique thermal transport of insulating nanostructures is attributed to the convergence of material length scales with the mean free paths of quantized lattice vibrations known as phonons, enabling promising next-generation thermal transistors, thermal barriers, and thermoelecs. Apart from size, strain and defects are also known to drastically affect heat transport when introduced in an otherwise undisturbed cryst. lattice. Here we report the first exptl. measurements of the effect of both spatially uniform strain and point defects on thermal cond. of an individual suspended nanowire using in situ Raman piezothermog. Our results show that whereas phononic transport in undoped Si nanowires with diams. in the range of 170-180 nm is largely unaffected by uniform elastic tensile strain, another means of disturbing a pristine lattice, namely, point defects introduced via ion bombardment, can reduce the thermal cond. by over 70%. In addn. to discerning surface- and core-governed pathways for controlling thermal transport in phonon-dominated insulators and semiconductors, we expect our novel approach to have broad applicability to a wide class of functional one- and two-dimensional nanomaterials.
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    30. 30
      Barth, S.; Boland, J. J.; Holmes, J. D. Defect Transfer from Nanoparticles to Nanowires. Nano Lett. 2011, 11, 1550– 5,  DOI: 10.1021/nl104339w
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      Defect Transfer from Nanoparticles to Nanowires
      Barth, Sven; Boland, John J.; Holmes, Justin D.
      Nano Letters (2011), 11 (4), 1550-1555CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Metal-seeded growth of 1-dimensional (1D) semiconductor nanostructures is still a very active field of research, despite the huge progress which was made in understanding this fundamental phenomenon. Liq. growth promoters allow control of the aspect ratio, diam., and structure of 1-dimensional crystals via external parameters, such as precursor feedstock, temp., and operating pressure. However the transfer of crystallog. information from a catalytic nanoparticle seed to a growing nanowire was not described in the literature. Here the authors define the theor. requirements for transferring defects from nanoparticle seeds to growing semiconductor nanowires and describe why Ag nanoparticles are ideal candidates for this purpose. The authors detail the influence of solid Ag growth seeds on the crystal quality of Ge nanowires, synthesized using a supercrit. fluid growth process. Significantly, under certain reaction conditions {111} stacking faults in the Ag seeds can be directly transferred to a high percentage of 〈112〉-oriented Ge nanowires, as radial twins in the semiconductor crystals. Defect transfer from nanoparticles to nanowires could open up the possibility of engineering 1-dimensional nanostructures with new and tunable phys. properties and morphologies.
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      Shin, N.; Chi, M.; Howe, J. Y.; Filler, M. A. Rational Defect Introduction in Silicon Nanowires. Nano Lett. 2013, 13, 1928– 33,  DOI: 10.1021/nl3042728
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      Rational Defect Introduction in Silicon Nanowires
      Shin, Naechul; Chi, Miaofang; Howe, Jane Y.; Filler, Michael A.
      Nano Letters (2013), 13 (5), 1928-1933CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The controlled introduction of planar defects, particularly twin boundaries and stacking faults, in Group IV nanowires remains challenging despite the prevalence of these structural features in other nanowire systems (e.g., II-VI and III-V). User-programmable changes to precursor pressure and growth temp. can rationally generate both transverse twin boundaries and angled stacking faults during the growth of 〈111〉 oriented Si nanowires. The authors leverage this new capability to demonstrate prototype defect superstructures. These findings yield important insight into the mechanism of defect generation in semiconductor nanowires and suggest new routes to engineer the properties of this ubiquitous semiconductor.
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    32. 32
      Ra, H.-W.; Khan, R.; Kim, J. T.; Kang, B. R.; Bai, K. H.; Im, Y. H. Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment. Mater. Lett. 2009, 63, 2516– 9,  DOI: 10.1016/j.matlet.2009.08.054
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      Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment
      Ra, H.-W.; Khan, R.; Kim, J. T.; Kang, B. R.; Bai, K. H.; Im, Y. H.
      Materials Letters (2009), 63 (28), 2516-2519CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
      This study examd. the effects of an oxygen plasma treatment on the properties of ZnO nanowires with diams. of 80 nm using a single nanowire field effect transistor. After the oxygen plasma treatment, the carrier concn. and mobility of individual ZnO nanowires decreased with a substantial pos. shift in the threshold voltage. The shifting was accounted to the surface modification, resulted to the improved gas sensitivity under hydrogen gas exposure and an enhanced photocurrent response time in UV illumination. The plausible surface mechanisms responsible for these significant changes after the surface modification were suggested by considering the surface anal. and elec. transport mechanism.
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      Muhammad, B. L.; Cummings, F. Nitrogen plasma treatment of ZnO and TiO2 nanowire arrays for polymer photovoltaic applications. Surf. Interfaces 2019, 17, 100382,  DOI: 10.1016/j.surfin.2019.100382
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      Nitrogen plasma treatment of ZnO and TiO2 nanowire arrays for polymer photovoltaic applications
      Muhammad, Bello Ladan; Cummings, Franscious
      Surfaces and Interfaces (2019), 17 (), 100382CODEN: SIUNCN; ISSN:2468-0230. (Elsevier B.V.)
      This work reports on a simple, yet unique approach to improving the opto-electronic properties of vertically-aligned arrays of rutile TiO2 and Wurzite ZnO nanowires by means of controlled nitrogen doping during exposure to highly kinetic radio-frequency generated N2 plasma radicals. Morphol., the plasma treatment causes a distortion of the vertical alignment of the nanowires due to a dissocn. of the weak Van der Waals force clustering the nanowires. Optical spectroscopy show that plasma treatment increases the light transmission of TiO2 arrays from 48% to 90%, with the ZnO arrays exhibiting an increase from 70% to 90% in the visible to UV range. The as-synthesized TiO2 array has an indirect band gap of 3.13 eV, which reduces to 3.03 eV after N2 treatment, with the ZnO equiv. decreasing from 3.20 to 3.17 eV post plasma exposure. A study of the 3d transition metal near edge fine structure of both Ti and Zn show that the N2 plasma treatment of the nanowires results in nitrogen doping of both TiO2 and ZnO lattices; this is confirmed by scanning transmission electron microscopy coupled with energy dispersive spectroscopy x-ray maps collected of single nanowires, which show a clear distribution of nitrogen throughout the metal-oxide. Application of these structures in P3HT:PCBM polymer blends shows progressive improvement in the photoluminescence quenching of the photoactive layer when incorporating both undoped and nitrogen-doped nanowires.
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      Yang, B.; Liu, B.; Wang, Y.; Zhuang, H.; Liu, Q.; Yuan, F.; Jiang, X. Zn-dopant dependent defect evolution in GaN nanowires. Nanoscale 2015, 7, 16237– 45,  DOI: 10.1039/C5NR04771D
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      Zn-dopant dependent defect evolution in GaN nanowires
      Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin
      Nanoscale (2015), 7 (39), 16237-16245CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chem. vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calcns. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (10‾13), (10‾11) and (20‾21), as well as Type I stacking faults (...ABABCBCB...), are obsd. in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABACBA...) can be identified as Type II stacking faults. The d. of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphol. in the GaN nanowires. First-principles calcns. reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the elec. properties of p-type GaN nanowires.
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      Ghosh, S.; Gopal Khan, G.; Varma, S.; Mandal, K. Influence of Li-N and Li-F co-doping on defect-induced intrinsic ferromagnetic and photoluminescence properties of arrays of ZnO nanowires. J. Appl. Phys. 2012, 112, 043910  DOI: 10.1063/1.4747929
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      Influence of Li-N and Li-F co-doping on defect-induced intrinsic ferromagnetic and photoluminescence properties of arrays of ZnO nanowires
      Ghosh, Shyamsundar; Khan, Gobinda Gopal; Varma, Shikha; Mandal, Kalyan
      Journal of Applied Physics (2012), 112 (4), 043910/1-043910/9CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The role of N/F co-doping on the defect-driven room-temp. d0 ferromagnetism in group-I element Li doped ZnO nanowire arrays has been investigated. The ferromagnetic signature of pristine ZnO nanowires has enhanced significantly after Li doping but the Li-N co-doping has found to be more effective in the stabilization and enhancement in room-temp. ferromagnetism in ZnO nanowires. Satn. magnetization in Li-doped ZnO nanowires found to increase from 0.63 to 2.52 emu/g and the Curie temp. rises up to 648 K when 10 at. % N is co-doped with 6 at. % Li. On the other hand, Li-F co-doping leads to exhibit much poor room-temp. ferromagnetic as well as visible luminescence properties. The valance state of the different dopants is estd. by XPS while the photoluminescence spectra indicate the gradual stabilization of Zn vacancy defects or defect complexes in presence of No acceptor states, which is found to be responsible for the enhancement of intrinsic ferromagnetism in ZnO:Li matrix. Therefore, the Li-N co-doping can be an effective parameter to stabilize, enhance, and tune zinc vacancy-induced room-temp. d0 ferromagnetism in ZnO nanowires, which can be an exciting approach to prep. new class of spintronic materials. (c) 2012 American Institute of Physics.
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      Narayanan, S.; Cheng, G.; Zeng, Z.; Zhu, Y.; Zhu, T. Strain Hardening and Size Effect in Five-fold Twinned Ag Nanowires. Nano Lett. 2015, 15, 4037– 44,  DOI: 10.1021/acs.nanolett.5b01015
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      Strain Hardening and Size Effect in Five-fold Twinned Ag Nanowires
      Narayanan, Sankar; Cheng, Guangming; Zeng, Zhi; Zhu, Yong; Zhu, Ting
      Nano Letters (2015), 15 (6), 4037-4044CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Metallic nanowires usually exhibit ultrahigh strength but low tensile ductility owing to their limited strain hardening capability. Here we study the unique strain hardening behavior of the five-fold twinned Ag nanowires by nanomech. testing and atomistic modeling. In situ tensile tests within a scanning electron microscope revealed strong strain hardening behavior of the five-fold twinned Ag nanowires. Mol. dynamics simulations showed that such strain hardening was critically controlled by twin boundaries and pre-existing defects. Strain hardening was size dependent; thinner nanowires achieved more hardening and higher ductility. The size-dependent strain hardening was found to be caused by the obstruction of surface-nucleated dislocations by twin boundaries. Our work provides mechanistic insights into enhancing the tensile ductility of metallic nanostructures by engineering the internal interfaces and defects.
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    37. 37
      Li, Y.; Wang, Y.; Ryu, S.; Marshall, A. F.; Cai, W.; McIntyre, P. C. Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor–Liquid–Solid Nanowire Growth. Nano Lett. 2016, 16, 1713– 8,  DOI: 10.1021/acs.nanolett.5b04633
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      Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor-Liquid-Solid Nanowire Growth
      Li, Yanying; Wang, Yanming; Ryu, Seunghwa; Marshall, Ann F.; Cai, Wei; McIntyre, Paul C.
      Nano Letters (2016), 16 (3), 1713-1718CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Kinking, a common anomaly in nanowire (NW) vapor-liq.-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liq. droplets of defined size. Statistical anal. of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diam. and the growth temp. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to 〈110〉, was obsd. in Ge NWs with a nominal diam. of ∼20 nm. This size coincides with a crit. diam. at which a spontaneous transition from 〈111〉 to 〈110〉 growth occurs in the phase field simulations. Larger diam. NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined 〈111〉 axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liq. droplet at the tip of the NW.
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    38. 38
      Jiang, J.-W.; Yang, N.; Wang, B.-S.; Rabczuk, T. Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects. Nano Lett. 2013, 13, 1670– 4,  DOI: 10.1021/nl400127q
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      Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects
      Jiang, Jin-Wu; Yang, Nuo; Wang, Bing-Shen; Rabczuk, Timon
      Nano Letters (2013), 13 (4), 1670-1674CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors performed mol. dynamics simulations to investigate the redn. of the thermal cond. by kinks in silicon nanowires. The redn. percentage can be as high as 70% at room temp. The temp. dependence of the redn. was also calcd. By calcg. phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal cond.: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, i.e., a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. This work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelec. materials.
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      Cook, B. G.; Varga, K. Conductance of kinked nanowires. Appl. Phys. Lett. 2011, 98, 052104,  DOI: 10.1063/1.3551711
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      Conductance of kinked nanowires
      Cook, B. G.; Varga, K.
      Applied Physics Letters (2011), 98 (5), 052104/1-052104/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The conductance properties of kinked nanowires are studied by first-principles transport calcns. within a recently developed complex potential framework. Using prototypical examples of monoat. Au chains as well as small diam. single-cryst. silicon nanowires we show that transmission strongly depends on the kink geometry and one can tune the conductance properties by the kink angle and other geometrical factors. In the case of a silicon nanowire the presence of a kink drastically reduces the conductance. (c) 2011 American Institute of Physics.
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    40. 40
      Zhao, Y.; Yang, L.; Liu, C.; Zhang, Q.; Chen, Y.; Yang, J.; Li, D. Kink effects on thermal transport in silicon nanowires. Int. J. Heat Mass Transfer 2019, 137, 573– 8,  DOI: 10.1016/j.ijheatmasstransfer.2019.03.104
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      Kink effects on thermal transport in silicon nanowires
      Zhao, Yang; Yang, Lin; Liu, Chenhan; Zhang, Qian; Chen, Yunfei; Yang, Juekuan; Li, Deyu
      International Journal of Heat and Mass Transfer (2019), 137 (), 573-578CODEN: IJHMAK; ISSN:0017-9310. (Elsevier Ltd.)
      Kinks in nanowires have recently been shown to be able to effectively tune the nanowire thermal conductivities; however, the underlying mechanisms have not been fully understood yet. To further disclose the details of phonon transport in kinked nanowires, here we report on non-equil. mol. dynamics studies of thermal transport through kinked and straight silicon nanowires. Results show that kinks can induce addnl. resistance and lead to lower thermal cond. for kinked nanowires than that of corresponding straight wires. Detailed anal. indicates that kinks produce addnl. resistance through reflecting phonons back into their incoming arms. Moreover, through introducing heavier isotope atoms in the kink region, the simulation replicates the exptl. observation that defects in the kink regime, instead of posing addnl. resistance, can actually facilitate thermal transport through deflecting phonons into the opposite arm.
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      Tian, B.; Xie, P.; Kempa, T. J.; Bell, D. C.; Lieber, C. M. Single crystalline kinked semiconductor nanowire superstructures. Nat. Nanotechnol. 2009, 4, 824– 9,  DOI: 10.1038/nnano.2009.304
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      Single-crystalline kinked semiconductor nanowire superstructures
      Tian, Bozhi; Xie, Ping; Kempa, Thomas J.; Bell, David C.; Lieber, Charles M.
      Nature Nanotechnology (2009), 4 (12), 824-829CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      The ability to control and modulate the compn., doping, crystal structure and morphol. of semiconductor nanowires during the synthesis process has allowed researchers to explore various applications of nanowires. However, despite advances in nanowire synthesis, progress towards the ab initio design and growth of hierarchical nanostructures was limited. Here, we demonstrate a 'nanotectonic' approach that provides iterative control over the nucleation and growth of nanowires, and use it to grow kinked or zigzag nanowires in which the straight sections are sepd. by triangular joints. Moreover, the lengths of the straight sections can be controlled and the growth direction remains coherent along the nanowire. We also grow dopant-modulated structures in which specific device functions, including p-n diodes and field-effect transistors, can be precisely localized at the kinked junctions in the nanowires.
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      Fortuna, S. A.; Li, X. Metal-catalyzed semiconductor nanowires: a review on the control of growth directions. Semicond. Sci. Technol. 2010, 25, 024005  DOI: 10.1088/0268-1242/25/2/024005
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      Metal-catalyzed semiconductor nanowires: a review on the control of growth directions
      Fortuna, Seth A.; Li, Xiuling
      Semiconductor Science and Technology (2010), 25 (2), 024005/1-024005/16CODEN: SSTEET; ISSN:0268-1242. (Institute of Physics Publishing)
      A review. Semiconductor nanowires have become an important building block for nanotechnol. The growth of semiconductor nanowires using a metal catalyst via the vapor-liq.-solid (VLS) or vapor-solid-solid (VSS) mechanism has yielded growth directions in 〈1 1 1〉, 〈1 0 0〉 and 〈1 1 0〉 etc. In this paper, we summarize and discuss a broad range of factors that affect the growth direction of VLS or VSS grown epitaxial semiconductor nanowires, providing an indexed glimpse of the control of nanowire growth directions and thus the mech., elec. and optical properties assocd. with the crystal orientation. The prospect of using planar nanowires for large area planar processing toward future nanowire array-based nanoelectronics and photonic applications is discussed.
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      Ross, F. M. Controlling nanowire structures through real time growth studies. Rep. Prog. Phys. 2010, 73, 114501,  DOI: 10.1088/0034-4885/73/11/114501
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      Controlling nanowire structures through real time growth studies
      Ross, Frances M.
      Reports on Progress in Physics (2010), 73 (11), 114501/1-114501/21CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
      A review. In situ electron microscopy can be used to visualize the phys. processes that control the growth of Si and Ge nanowires through the vapor-liq.-solid mechanism. Images and movies are recorded in a transmission electron microscope that has capabilities for depositing catalysts onto a sample and for introducing CVD precursor gases while the sample remains under observation. This technique allows us to measure nucleation, catalyst stability, surface structure and growth kinetics, in some cases confirming existing models and in other cases producing unexpected results and suggesting approaches toward growing novel structures. Nanowire formation provides a unique window into the fundamentals of crystal growth as well as an opportunity to fabricate precisely controlled structures for novel applications.
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    44. 44
      Tian, B.; Cohen-Karni, T.; Qing, Q.; Duan, X.; Xie, P.; Lieber, C. M. Three-Dimensional, Flexible Nanoscale Field-Effect Transistors as Localized Bioprobes. Science 2010, 329, 830– 4,  DOI: 10.1126/science.1192033
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      Three-Dimensional, Flexible Nanoscale Field-Effect Transistors as Localized Bioprobes
      Tian, Bozhi; Cohen-Karni, Tzahi; Qing, Quan; Duan, Xiaojie; Xie, Ping; Lieber, Charles M.
      Science (Washington, DC, United States) (2010), 329 (5993), 830-834CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Nanoelectronic devices offer substantial potential for interrogating biol. systems, although nearly all work has focused on planar device designs. The authors have overcome this limitation through synthetic integration of a nanoscale field-effect transistor (nanoFET) device at the tip of an acute-angle kinked silicon nanowire, where nanoscale connections are made by the arms of the kinked nanostructure, and remote multilayer interconnects allow three-dimensional (3D) probe presentation. The acute-angle probe geometry was designed and synthesized by controlling cis vs. trans crystal conformations between adjacent kinks, and the nanoFET was localized through modulation doping. 3D nanoFET probes exhibited conductance and sensitivity in aq. soln., independent of large mech. deflections, and demonstrated high pH sensitivity. Addnl., 3D nanoprobes modified with phospholipid bilayers can enter single cells to allow robust recording of intracellular potentials.
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    45. 45
      Xu, L.; Jiang, Z.; Qing, Q.; Mai, L.; Zhang, Q.; Lieber, C. M. Design and Synthesis of Diverse Functional Kinked Nanowire Structures for Nanoelectronic Bioprobes. Nano Lett. 2013, 13, 746– 51,  DOI: 10.1021/nl304435z
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      45
      Design and Synthesis of Diverse Functional Kinked Nanowire Structures for Nanoelectronic Bioprobes
      Xu, Lin; Jiang, Zhe; Qing, Quan; Mai, Liqiang; Zhang, Qingjie; Lieber, Charles M.
      Nano Letters (2013), 13 (2), 746-751CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Functional kinked nanowires (KNWs) represent a new class of nanowire building blocks, in which functional devices, for example, nanoscale field-effect transistors (nanoFETs), are encoded in geometrically controlled nanowire superstructures during synthesis. The bottom-up control of both structure and function of KNWs enables construction of spatially isolated point-like nanoelectronic probes that are esp. useful for monitoring biol. systems where finely tuned feature size and structure are highly desired. Here we present three new types of functional KNWs including (1) the zero-degree KNW structures with two parallel heavily doped arms of U-shaped structures with a nanoFET at the tip of the "U", (2) series multiplexed functional KNW integrating multi-nanoFETs along the arm and at the tips of V-shaped structures, and (3) parallel multiplexed KNWs integrating nanoFETs at the two tips of W-shaped structures. First, U-shaped KNWs were synthesized with sepns. as small as 650 nm between the parallel arms and used to fabricate three-dimensional nanoFET probes at least 3 times smaller than previous V-shaped designs. In addn., multiple nanoFETs were encoded during synthesis in one of the arms/tip of V-shaped and distinct arms/tips of W-shaped KNWs. These new multiplexed KNW structures were structurally verified by optical and electron microscopy of dopant-selective etched samples and elec. characterized using scanning gate microscopy and transport measurements. The facile design and bottom-up synthesis of these diverse functional KNWs provides a growing toolbox of building blocks for fabricating highly compact and multiplexed three-dimensional nanoprobes for applications in life sciences, including intracellular and deep tissue/cell recordings.
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    46. 46
      Barth, S.; Hernandez-Ramirez, F.; Holmes, J. D.; Romano-Rodriguez, A. Synthesis and applications of one-dimensional semiconductors. Prog. Mater. Sci. 2010, 55, 563– 627,  DOI: 10.1016/j.pmatsci.2010.02.001
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      Synthesis and applications of one-dimensional semiconductors
      Barth, Sven; Hernandez-Ramirez, Francisco; Holmes, Justin D.; Romano-Rodriguez, Albert
      Progress in Materials Science (2010), 55 (6), 563-627CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
      A review. Nanoscale inorg. materials such as quantum dots (0-dimensional) and one-dimensional (1D) structures, such as nanowires, nanobelts and nanotubes, have gained tremendous attention within the last decade. Among the huge variety of 1D nanostructures, semiconducting nanowires have gained particular interest due to their potential applications in optoelectronic and electronic devices. Despite the huge efforts to control and understand the growth mechanisms underlying the formation of these highly anisotropic structures, some fundamental phenomena are still not well understood. For example, high aspect-ratio semiconductors exhibit unexpected growth phenomena, e.g. diam.-dependent and temp.-dependent growth directions, and unusual high doping levels or compns., which are not known for their macroscopic crystals or thin-film counterparts. This article reviews viable synthetic approaches for growing high aspect-ratio semiconductors from bottom-up techniques, such as crystal structure governed nucleation, metal-promoted vapor phase and soln. growth, formation in non-metal seeded gas-phase processes, structure directing templates and electrospinning. In particular new exptl. findings and theor. models relating to the frequently applied vapor-liq.-solid (VLS) growth are highlighted. In addn., the top-down application of controlled chem. etching, using novel masking techniques, is described as a viable approach for generating certain 1D structures. The review highlights the controlled synthesis of semiconducting nanostructures and heterostructures of silicon, germanium, gallium nitride, gallium arsenide, cadmium sulfide, zinc oxide and tin oxide. The alignment of 1D nanostructures will be reviewed briefly. While specific and reliable contact procedures are still a major challenge for the integration of 1D nanostructures as active building blocks, this issue will not be the focus of this paper. However, the promising applications of 1D semiconductors will be highlighted, particularly with ref. to surface dependent electronic transduction (gas and biol. sensors), energy generation (nanomech. and photovoltaic) devices, energy storage (lithium storage in battery anodes) as well as nanowire photonics.
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    47. 47
      Lugstein, A.; Steinmair, M.; Hyun, Y. J.; Hauer, G.; Pongratz, P.; Bertagnolli, E. Pressure-Induced Orientation Control of the Growth of Epitaxial Silicon Nanowires. Nano Lett. 2008, 8, 2310– 4,  DOI: 10.1021/nl8011006
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      Pressure-Induced Orientation Control of the Growth of Epitaxial Silicon Nanowires
      Lugstein, A.; Steinmair, M.; Hyun, Y. J.; Hauer, G.; Pongratz, P.; Bertagnolli, E.
      Nano Letters (2008), 8 (8), 2310-2314CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Single crystal Si nanowires (SiNWs) were synthesized with silane reactant using Au nanocluster-catalyzed 1-dimensional growth. Under the authors' exptl. conditions, SiNWs grown epitaxially on Si(111) via the vapor-liq.-solid growth mechanism change their growth direction as a function of the total pressure. Structural characterization of a large no. of samples shows that SiNWs synthesized at a total pressure of 3 mbar grow preferentially in the 〈111〉 direction, while the one at 15 mbar favors the 〈112〉 direction. Specifically by dynamically changing the system pressure during the growth process morphol. changes of the NW growth directions along their length were demonstrated.
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      Hyun, Y.-J.; Lugstein, A.; Steinmair, M.; Bertagnolli, E.; Pongratz, P. Orientation specific synthesis of kinked silicon nanowires grown by the vapour–liquid–solid mechanism. Nanotechnology 2009, 20, 125606,  DOI: 10.1088/0957-4484/20/12/125606
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      Orientation specific synthesis of kinked silicon nanowires grown by the vapor-liquid-solid mechanism
      Hyun, Youn-Joo; Lugstein, Alois; Steinmair, Mathias; Bertagnolli, Emmerich; Pongratz, Peter
      Nanotechnology (2009), 20 (12), 125606/1-125606/5CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
      Kinked silicon nanowires (Si-NWs) were prepd. in a well reproducible manner using gold nanocluster-catalyzed quasi-1D growth on Si(111) substrates with SiH4 as the precursor gas. The kinking is considered to be due to the change in the growth direction induced by the sudden change of the pressure during Si-NW prepn. Structural high resoln. transmission electron microscopy (HRTEM) characterization of the sample shows that epitaxial Si-NWs prepd. on Si(111) substrates at a total pressure of 3 mbar grow along the 〈111〉 direction, while the ones at 15 mbar favor the 〈112〉 direction. By dynamically changing the system pressure during the growth process morphol. changes of the NW growth directions along their length were shown, resulting in kinked nanowires. The crystallog. orientation relation of the kinking between the 3 and 15 mbar ranges was analyzed by TEM. No defects or grain boundaries in the intersection between the two sections of the Si-NWs are necessary to form such kinks between different wire directions.
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    49. 49
      Geaney, H.; Dickinson, C.; Weng, W.; Kiely, C. J.; Barrett, C. A.; Gunning, R. D.; Ryan, K. M. Role of Defects and Growth Directions in the Formation of Periodically Twinned and Kinked Unseeded Germanium Nanowires. Cryst. Growth Des. 2011, 11, 3266– 72,  DOI: 10.1021/cg200510y
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      Role of Defects and Growth Directions in the Formation of Periodically Twinned and Kinked Unseeded Germanium Nanowires
      Geaney, Hugh; Dickinson, Calum; Weng, Weihao; Kiely, Christopher J.; Barrett, Christopher A.; Gunning, Robert D.; Ryan, Kevin M.
      Crystal Growth & Design (2011), 11 (7), 3266-3272CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
      The authors show the impact of preferred growth directions and defects in the formation of complex Ge nanowire (NW) structures grown by a simple org. medium based synthesis. Various types of NWs are examd. including: straight defect free NWs; periodically bent NWs with precise angles between the NW segments; NWs with mutually exclusive lateral or longitudinal faults; and more complex "wormlike" structures. The authors show that choice of solvent and reaction temp. can be used to tune the morphol. of the NWs formed. The various types of NWs were probed in depth using transmission electron microscopy (TEM), SEM, selected area electron diffraction (SAED), and dark field TEM (DFTEM).
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    50. 50
      Kim, J. H.; Moon, S. R.; Yoon, H. S.; Jung, J. H.; Kim, Y.; Chen, Z. G.; Zou, J.; Choi, D. Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Taper-Free and Vertically Oriented Ge Nanowires on Ge/Si Substrates Grown by a Two-Temperature Process. Cryst. Growth Des. 2012, 12, 135– 41,  DOI: 10.1021/cg2008914
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      Taper-Free and Vertically Oriented Ge Nanowires on Ge/Si Substrates Grown by a Two-Temperature Process
      Kim, Jung Hyuk; Moon, So Ra; Yoon, Hyun Sik; Jung, Jae Hun; Kim, Yong; Chen, Zhi Gang; Zou, Jin; Choi, Duk Yong; Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati
      Crystal Growth & Design (2012), 12 (1), 135-141CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
      Taper-free and vertically oriented Ge nanowires were grown on Si (111) substrates by CVD with Au nanoparticle catalysts. To achieve vertical nanowire growth on the highly lattice mismatched Si substrate, a thin Ge buffer layer was 1st deposited, and to achieve taper-free nanowire growth, a two-temp. process was employed. The two-temp. process consisted of a brief initial base growth step at high temp. followed by prolonged growth at lower temp. Taper-free and defect-free Ge nanowires grew successfully even at 270°, which is 90° lower than the bulk eutectic temp. The yield of vertical and taper-free nanowires is over 90%, comparable to that of vertical but tapered nanowires grown by the conventional 1-temp. process. This method is of practical importance and can be reliably used to develop novel nanowire-based devices on relatively cheap Si substrates. Addnl., the activation energy of Ge nanowire growth by the two-temp. process is dependent on Au nanoparticle size. The low activation energy (∼5 kcal/mol) for 30 and 50 nm diam. Au nanoparticles suggests that the decompn. of gaseous species on the catalytic Au surface is a rate-limiting step. A higher activation energy (∼14 kcal/mol) was detd. for 100 nm diam. Au nanoparticles which suggests that larger Au nanoparticles are partially solidified and that growth kinetics become the rate-limiting step.
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      Wu, Z. H.; Mei, X.; Kim, D.; Blumin, M.; Ruda, H. E.; Liu, J. Q.; Kavanagh, K. L. Growth, branching, and kinking of molecular-beam epitaxial ⟨110⟩ GaAs nanowires. Appl. Phys. Lett. 2003, 83, 3368– 70,  DOI: 10.1063/1.1618018
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      Growth, branching, and kinking of molecular-beam epitaxial 〈110〉 GaAs nanowires
      Wu, Z. H.; Mei, X.; Kim, D.; Blumin, M.; Ruda, H. E.; Liu, J. Q.; Kavanagh, K. L.
      Applied Physics Letters (2003), 83 (16), 3368-3370CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      GaAs nanowires were grown on GaAs (100) substrates by vapor-liq.-solid growth. About 8% of these nanowires grew in the 〈110〉 direction with straight, Y-branched, or L-shaped morphologies. The role of strain-induced redn. in surface free energy is discussed as a possible factor contributing to the evolution of 〈110〉 nanowires. Kinking and branching is attributed to growth instabilities resulting from equiv. surface free energies for 〈110〉 growth directions. Transmission electron microscopy verified that the 〈110〉 nanowires were defect free.
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      Peng, H.; Meister, S.; Chan, C. K.; Zhang, X. F.; Cui, Y. Morphology Control of Layer-Structured Gallium Selenide Nanowires. Nano Lett. 2007, 7, 199– 203,  DOI: 10.1021/nl062047+
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      Morphology Control of Layer-Structured Gallium Selenide Nanowires
      Peng, Hailin; Meister, Stefan; Chan, Candace K.; Zhang, Xiao Feng; Cui, Yi
      Nano Letters (2007), 7 (1), 199-203CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Layer-structured Group III chalcogenides have highly anisotropic properties and are attractive materials for stable photocathodes and battery electrodes. The authors report the controlled synthesis and characterization of layer-structured GaSe nanowires via a catalyst-assisted vapor-liq.-solid (VLS) growth mechanism during GaSe powder evapn. GaSe nanowires consist of Se-Ga-Ga-Se layers stacked together via van der Waals interactions to form belt-shaped nanowires with a growth direction along the [11-20], width along the [1-100], and height along the [0001] direction. Nanobelts exhibit a variety of morphologies including straight, zigzag, and saw-tooth shapes. These morphologies are realized by controlling the growth temp. and time so that the actual catalysts have a chem. compn. of Au, Au-Ga alloy, or Ga. The participation of Ga in the VLS catalyst is important for achieving different morphologies of GaSe. GaSe nanotubes are also prepd. by a slow growth process.
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      Jiang, Z.; Qing, Q.; Xie, P.; Gao, R.; Lieber, C. M. Kinked p–n Junction Nanowire Probes for High Spatial Resolution Sensing and Intracellular Recording. Nano Lett. 2012, 12, 1711– 6,  DOI: 10.1021/nl300256r
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      Kinked p-n Junction Nanowire Probes for High Spatial Resolution Sensing and Intracellular Recording
      Jiang, Zhe; Qing, Quan; Xie, Ping; Gao, Ruixuan; Lieber, Charles M.
      Nano Letters (2012), 12 (3), 1711-1716CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Semiconductor nanowires and other semiconducting nanoscale materials configured as field-effect transistors have been studied extensively as biol./chem. sensors. These nanomaterials have demonstrated high-sensitivity from one- and two-dimensional sensors, although the realization of the ultimate pointlike detector has not been achieved. In this regard, nanoscale p-n diodes are attractive since the device element is naturally localized near the junction, and while nanowire p-n diodes have been widely studied as photovoltaic devices, their applications as bio/chem. sensors have not been explored. Here the authors demonstrate that p-n diode devices can serve as a new and powerful family of highly localized biosensor probes. Designed nanoscale axial p-n junctions were synthetically introduced at the joints of kinked silicon nanowires. SEM images showed that the kinked nanowire structures were achieved, and elec. transport measurements exhibited rectifying behavior with well-defined turn-on in forward bias as expected for a p-n diode. In addn., scanning gate microscopy demonstrated that the most sensitive region of these nanowires was localized near the kinked region at the p-n junction. High spatial resoln. sensing using these p-n diode probes was carried out in aq. soln. using fluorescent charged polystyrene nanobeads. Multiplexed elec. measurements show well-defined single-nanoparticle detection, and expts. with simultaneous confocal imaging correlate directly the motion of the nanobeads with the elec. signals recorded from the p-n devices. In addn., kinked p-n junction nanowires configured as three-dimensional probes demonstrate the capability of intracellular recording of action potentials from electrogenic cells. These p-n junction kinked nanowire devices, which represent a new way of constructing nanoscale probes with highly localized sensing regions, provide substantial opportunity in areas ranging from bio/chem. sensing and nanoscale photon detection to three-dimensional recording from within living cells and tissue.
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      Jiang, J.-W.; Zhao, J.-H.; Rabczuk, T. Size-sensitive Young’s modulus of kinked silicon nanowires. Nanotechnology 2013, 24, 185702,  DOI: 10.1088/0957-4484/24/18/185702
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      Size-sensitive Young's modulus of kinked silicon nanowires
      Jiang, Jin-Wu; Zhao, Jun-Hua; Rabczuk, Timon
      Nanotechnology (2013), 24 (18), 185702/1-185702/6, 6 pp.CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      We perform both classical mol. dynamics simulations and beam model calcns. to investigate the Young's modulus of kinked silicon nanowires (KSiNWs). The Young's modulus is found to be highly sensitive to the arm length of the kink and is essentially inversely proportional to the arm length. The mechanism underlying the size dependence is found to be the interplay between the kink angle potential and the arm length potential, where we obtain an analytic relationship between the Young's modulus and the arm length of the KSiNW. Our results provide insight into the application of this novel building block in nanomech. devices.
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      Jiang, J.-W.; Rabczuk, T. Mechanical oscillation of kinked silicon nanowires: A natural nanoscale spring. Appl. Phys. Lett. 2013, 102, 123104,  DOI: 10.1063/1.4799029
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      Mechanical oscillation of kinked silicon nanowires. A natural nanoscale spring
      Jiang, Jin-Wu; Rabczuk, Timon
      Applied Physics Letters (2013), 102 (12), 123104/1-123104/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We perform classical mol. dynamics simulations to demonstrate the application of kinked Si nanowires (KSiNWs) as nanoscale springs. The spring-like oscillation in gigahertz frequency range is successfully actuated using a similar procedure as the actuation of a classical mass spring oscillator. We detect the spring-like mech. oscillation and some other low-frequency oscillations by the energy spectrum anal., where a dimensional crossover phenomenon is obsd. for the transverse mode in KSiNWs with decreasing aspect ratio. Our findings shed light on the elastic properties of the KSiNW and open a way for its application in nanomech. devices. (c) 2013 American Institute of Physics.
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      Hillerich, K.; Dick, K. A.; Wen, C.-Y.; Reuter, M. C.; Kodambaka, S.; Ross, F. M. Strategies To Control Morphology in Hybrid Group III–V/Group IV Heterostructure Nanowires. Nano Lett. 2013, 13, 903– 8,  DOI: 10.1021/nl303660h
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      Strategies To Control Morphology in Hybrid Group III-V/Group IV Heterostructure Nanowires
      Hillerich, Karla; Dick, Kimberly A.; Wen, Cheng-Yen; Reuter, Mark C.; Kodambaka, Suneel; Ross, Frances M.
      Nano Letters (2013), 13 (3), 903-908CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      By combining in situ and ex situ TEM measurements, the authors examine the factors that control the morphol. of hybrid nanowires that include group III-V and Group IV materials. The authors focus on one materials pair, GaP/Si, for which the authors use a wide range of growth parameters. The authors show through video imaging that nanowire morphol. depends on growth conditions, but that a general pattern emerges where either single kinks or inclined defects form some distance after the heterointerface. Pure Si nanowires can be made to exhibit the same kinks and defects by changing their droplet vol. From this the authors derive a model where droplet geometry drives growth morphol. and discuss optimization strategies. The authors finally discuss morphol. control for material pairs where the 2nd material kinks immediately at the heterointerface and show that an interlayer between segments can enable the growth of unkinked hybrid nanowires.
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      Dick, K. A.; Kodambaka, S.; Reuter, M. C.; Deppert, K.; Samuelson, L.; Seifert, W.; Wallenberg, L. R.; Ross, F. M. The Morphology of Axial and Branched Nanowire Heterostructures. Nano Lett. 2007, 7, 1817– 22,  DOI: 10.1021/nl0705900
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      The Morphology of Axial and Branched Nanowire Heterostructures
      Dick, Kimberly A.; Kodambaka, Suneel; Reuter, Mark C.; Deppert, Knut; Samuelson, Lars; Seifert, Werner; Wallenberg, L. Reine; Ross, Frances M.
      Nano Letters (2007), 7 (6), 1817-1822CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphol. of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphol. (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.
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    58. 58
      Dayeh, S. A.; Wang, J.; Li, N.; Huang, J. Y.; Gin, A. V.; Picraux, S. T. Growth, Defect Formation, and Morphology Control of Germanium–Silicon Semiconductor Nanowire Heterostructures. Nano Lett. 2011, 11, 4200– 6,  DOI: 10.1021/nl202126q
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      58
      Growth, defect formation, and morphology control of germanium-silicon semiconductor nanowire heterostructures
      Dayeh, Shadi A.; Wang, Jian; Li, Nan; Huang, Jian Yu; Gin, Aaron V.; Picraux, S. Thomas
      Nano Letters (2011), 11 (10), 4200-4206CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      By the virtue of the nature of the vapor-liq.-solid (VLS) growth process in semiconductor nanowires (NWs) and their small size, the nucleation, propagation, and termination of stacking defects in NWs are dramatically different from that in thin films. We demonstrate Ge-Si axial NW heterostructure growth by the VLS method with 100% compn. modulation and use these structures as a platform to understand how defects in stacking sequence force the ledge nucleation site to be moved along or pinned at a single point on the triple-phase circumference, which in turn dets. the NW morphol. Combining structural anal. and atomistic simulation of the nucleation and propagation of stacking defects, we explain these observations based on preferred nucleation sites during NW growth. The stacking defects are found to provide a fingerprint of the layer-by-layer growth process and reveal how the 19.5° kinking in semiconductor NWs obsd. at high Si growth rates results from a stacking-induced twin boundary formation at the NW edge. This study provides basic foundations for an at. level understanding of cryst. and defective ledge nucleation and propagation during [111] oriented NW growth and improves understanding for control of fault nucleation and kinking in NWs.
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    59. 59
      Paladugu, M.; Zou, J.; Guo, Y.-N.; Auchterlonie, G. J.; Joyce, H. J.; Gao, Q.; Hoe Tan, H.; Jagadish, C.; Kim, Y. Novel Growth Phenomena Observed in Axial InAs/GaAs Nanowire Heterostructures. Small 2007, 3, 1873,  DOI: 10.1002/smll.200700222
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      Novel growth phenomena observed in axial InAs/GaAs nanowire heterostructures
      Paladugu, Mohanchand; Zou, Jin; Guo, Ya-Nan; Auchterlonie, Graeme J.; Joyce, Hannah J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Kim, Yong
      Small (2007), 3 (11), 1873-1877CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Gold on the move... A novel growth phenomenon of axial InAs/GaAs nanowire heterostructures catalyzed by Au particles was obsd. Transmission electron microscopy has detd. a sequence of events: (1) Displacement of the Au particle at the end of the nanowire due to InAs clustering, (2) further InAs growth leading to sideways movement of the Au particle, and (3) eventual downward nanowire growth due to the preservation of a Au/GaAs interface (see scheme).
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      He, Z.; Nguyen, H. T.; Duc Toan, L.; Pribat, D. A detailed study of kinking in indium-catalyzed silicon nanowires. CrystEngComm 2015, 17, 6286,  DOI: 10.1039/C5CE00773A
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      A detailed study of kinking in indium-catalyzed silicon nanowires
      He, Zhanbing; Nguyen, Hung Tran; Le, Duc Toan; Pribat, Didier
      CrystEngComm (2015), 17 (33), 6286-6296CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
      Kinking of semiconductor nanowires grown by the vapor-solid-liq. (VSL) mechanism has long been obsd. and studied, particularly for Si. A large variety of turning angles for kinked Si nanowires (KSiNWs) has been reported in the literature, but most authors have studied the kinking mechanism rather than the structure and corresponding geometrical features of the kinks. Here, we have investigated the relationship between the turning angles and the structure (down to at. level) of KSiNWs grown by VSL from indium nanoparticles. By using transmission electron microscopy, we have characterized the transition regions between different segments of KSiNWs of various crystallog. orientations. We have found that most turning angles can be viewed as rich combinations of different types of {111} coherent twins that coexist within the transition regions between different segments of KSiNWs.
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      McIntyre, P. C.; Fontcuberta i Morral, I. Semiconductor nanowires: to grow or not to grow?. Mater. Today Nano 2020, 9, 100058,  DOI: 10.1016/j.mtnano.2019.100058
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      Thombare, S. V.; Marshall, A. F.; McIntyre, P. C. Size effects in vapor-solid-solid Ge nanowire growth with a Ni-based catalyst. J. Appl. Phys. 2012, 112, 054325  DOI: 10.1063/1.4749797
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      Size effects in vapor-solid-solid Ge nanowire growth with a Ni-based catalyst
      Thombare, S. V.; Marshall, A. F.; McIntyre, P. C.
      Journal of Applied Physics (2012), 112 (5), 054325/1-054325/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      We report a dramatic size effect on the morphol. of Ge nanowires prepd. by low-temp. vapor-solid-solid (VSS) growth using a NiGe catalyst. Nanowires with diam. greater than 25 nm are [111]-oriented, have a high d. of grown-in defects, and exhibit frequent kinking. However, nanowires with diam. below 25 nm are straight, despite also having a substantial d. of crystal defects. The latter wires grow preferentially in the [110] direction. The absence of kinking in the small nanowires coincides with the observation of a low-energy, epitaxial NiGe/Ge interface. The occurrence of (1) this solid-solid epitaxial interface and (2) the low-energy sidewall facets of the [110] wire orientation strongly bias the Ni-Ge binary system toward kink-free nanowire growth in the VSS regime. Kinking in larger nanowires occurs via multiple twinning events facilitated by the slow growth and anisotropic catalyst/wire interfaces typical of VSS growth. Such effects are expected in other VSS systems where a range of nanowire morphologies are obsd. (c) 2012 American Institute of Physics.
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      Fahlvik Svensson, S.; Jeppesen, S.; Thelander, C.; Samuelson, L.; Linke, H.; Dick, K. A. Control and understanding of kink formation in InAs–InP heterostructure nanowires. Nanotechnology 2013, 24, 345601,  DOI: 10.1088/0957-4484/24/34/345601
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      Control and understanding of kink formation in InAs-InP heterostructure nanowires
      Fahlvik Svensson, S.; Jeppesen, S.; Thelander, C.; Samuelson, L.; Linke, H.; Dick, K. A.
      Nanotechnology (2013), 24 (34), 345601, 9 pp.CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      Nanowire heterostructures are of special interest for band structure engineering due to an expanded range of defect-free material combinations. However, the higher degree of freedom in nanowire heterostructure growth comes at the expense of challenges related to nanowire-seed particle interactions, such as undesired compn., grading and kink formation. To better understand the mechanisms of kink formation in nanowires, we here present a detailed study of the dependence of heterostructure nanowire morphol. on indium pressure, nanowire diam., and nanowire d. We investigate InAs-InP-InAs heterostructure nanowires grown with chem. beam epitaxy, which is a material system that allows for very abrupt heterointerfaces. Our observations indicate that the crit. parameter for kink formation is the availability of indium, and that the resulting morphol. is also highly dependent on the length of the InP segment. It is shown that kinking is assocd. with the formation of an inclined facet at the interface between InP and InAs, which destabilizes the growth and leads to a change in growth direction. By careful tuning of the growth parameters, it is possible to entirely suppress the formation of this inclined facet and thereby kinking at the heterointerface. Our results also indicate the possibility of producing controllably kinked nanowires with a high yield.
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      Madras, P.; Dailey, E.; Drucker, J. Kinetically Induced Kinking of Vapor–Liquid–Solid Grown Epitaxial Si Nanowires. Nano Lett. 2009, 9, 3826– 30,  DOI: 10.1021/nl902013g
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      Kinetically Induced Kinking of Vapor-Liquid-Solid Grown Epitaxial Si Nanowires
      Madras, Prashanth; Dailey, Eric; Drucker, Jeff
      Nano Letters (2009), 9 (11), 3826-3830CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Epitaxial Si nanowires grown from Au seeds using the vapor-liq.-solid method begin growing normal to the Si(111) substrate atop a tapered base. After a kinetically detd. length, the NWs may kink away from [111] to another crystallog. direction. The smallest NWs prefer growth along 〈110〉 while larger Si NWs choose either 〈111〉 or 〈112〉 based on whether growth conditions favor Au-free sidewalls. "Vertical" growth normal to the Si(111) substrate is obtained only for slowly growing NWs with Au-decorated sidewalls. At the fastest growth rates, single-crystal Si NWs smoothly, continuously, and randomly vary their growth directions, producing a morphol. that is qual. different than highly kinked growth.
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      Sun, Z.; Seidman, D. N.; Lauhon, L. J. Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid–Solid Growth Interface. Nano Lett. 2017, 17, 4518– 25,  DOI: 10.1021/acs.nanolett.7b02071
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      Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface
      Sun, Zhiyuan; Seidman, David N.; Lauhon, Lincoln J.
      Nano Letters (2017), 17 (7), 4518-4525CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Dopants modify the electronic properties of semiconductors, including their susceptibility to etching. In semiconductor nanowires doped during growth by the vapor-liq.-solid (VLS) process, it has been shown that nanofaceting of the liq.-solid growth interface influences strongly the radial distribution of dopants. Hence, the combination of facet-dependent doping and dopant selective etching provides a means to tune simultaneously the electronic properties and morphologies of nanowires. Using atom-probe tomog., we investigated the boron dopant distribution in Au catalyzed VLS grown silicon nanowires, which regularly kink between equiv. 〈112〉 directions. Segments alternate between radially uniform and nonuniform doping profiles, which we attribute to switching between a concave and convex faceted liq.-solid interface. Dopant selective etching was used to reveal and correlate the shape of the growth interface with the obsd. anisotropic doping.
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      Yuan, X.; Caroff, P.; Wong-Leung, J.; Fu, L.; Tan, H. H.; Jagadish, C. Tunable Polarity in a III–V Nanowire by Droplet Wetting and Surface Energy Engineering. Adv. Mater. 2015, 27, 6096– 103,  DOI: 10.1002/adma.201503540
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      Tunable Polarity in a III-V Nanowire by Droplet Wetting and Surface Energy Engineering
      Yuan, Xiaoming; Caroff, Philippe; Wong-Leung, Jennifer; Fu, Lan; Tan, Hark Hoe; Jagadish, Chennupati
      Advanced Materials (Weinheim, Germany) (2015), 27 (40), 6096-6103CODEN: