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Wafer-Scale Growth of One-Dimensional Transition-Metal Telluride Nanowires

Cite this: Nano Lett. 2021, 21, 1, 243–249
Publication Date (Web):December 13, 2020
https://doi.org/10.1021/acs.nanolett.0c03456
Copyright © 2020 American Chemical Society
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Abstract

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The development of bulk synthetic processes to prepare functional nanomaterials is crucial to achieve progress in fundamental and applied science. Transition-metal chalcogenide (TMC) nanowires, which are one-dimensional (1D) structures having three-atom diameters and van der Waals surfaces, have been reported to possess a 1D metallic nature with great potential in electronics and energy devices. However, their mass production remains challenging. Here, a wafer-scale synthesis of highly crystalline transition-metal telluride nanowires is demonstrated by chemical vapor deposition. The present technique enables formation of either aligned, atomically thin two-dimensional (2D) sheets or random networks of three-dimensional (3D) bundles, both composed of individual nanowires. These nanowires exhibit an anisotropic 1D optical response and superior conducting properties. The findings not only shed light on the controlled and large-scale synthesis of conductive thin films but also provide a platform for the study on physics and device applications of nanowire-based 2D and 3D crystals.

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Introduction

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In recent decades, intensive research has focused on various functional nanomaterials, such as fullerenes,(1) carbon nanotubes (CNTs),(2) graphene,(3) and transition-metal dichalcogenides (TMDCs),(4) because of their excellent physical properties and their wide variety of potential applications such as in electronics and energy conversion devices. The ability to achieve their large-scale synthesis has been key to major breakthroughs that accelerate basic and industrial research. While significant progress has already been made, the synthesis of one-dimensional (1D) transition-metal chalcogenides (TMCs) remains challenging despite 40 years since their discovery in bulk crystals of ternary molybdenum chalcogenides(5−7) and their fascinating physical properties derived from three-atom-thick, 1D quantum confinement system.
A TMC nanowire is a 1D structure consisting of staggered triangular units of M3X3, where M = Mo or W and X = S, Se, or Te. Chalcogen atoms occupy the three vertices in each layer and metal atoms are located at the edge centers, resulting in an elongated chalcogen shell with a metal core (Figure 1a). Instead of behaving as a conventional Fermi liquid, such nanowires show 1D metallic behavior consistent with a Tomonaga–Luttinger liquid,(8−10) therefore holding great potential for study in basic physics and electronics applications. In addition, the formation of nanostructures with various geometries is possible due to van der Waals (vdW) interactions between nanowire surfaces. For instance, lateral assembly of these nanowires results in an atomically thin two-dimensional (2D) layer. This may lead to the emergence of interesting physical properties as observed in the semiconducting to metallic transition,(11) large-scale integration of hybrid heterostructures,(12) and anomalous PL properties(13) of ultrathin 2D materials. Besides this, TMC nanowires can also aggregate into three-dimensional (3D) bundles as in the case of CNTs. The 1D confinement effect and structural diversity of TMC nanowires provide an excellent opportunity for further study, as the degree of freedom available during assembly marks an important distinction from other functional nanomaterials.

Figure 1

Figure 1. 1D vdW building block. (a) Schematic diagram showing 2D and 3D assemblies of TMC nanowires having the composition MTe, where M = Mo or W. Each individual wire comprises an elongated network of staggered M3Te3 triangular units. Adjacent nanowires interact via vdW forces and either extend laterally, forming a 2D layer, or combine into 3D bundles of various size. (b, c) Cross-section HAADF-STEM images of monolayer and bilayer sheets of WTe nanowires grown on a-plane sapphire substrates. (d) Cross-section TEM image of a single WTe 3D bundle grown on a SiO2/Si substrate. (e) HAADF-STEM image showing the cross-section of a single WTe nanowire. The W core of heavier atomic mass is clearly seen as a bright hexagonal ring surrounded by Te atoms. (f) Atomic resolution HAADF-STEM image of suspended WTe nanowires, illustrating atomic alignment along the nanowire axis.

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Despite their attractive features, studies on TMC nanowires are fairly scarce due solely to limited sample availability. A major issue is the lack of a feasible strategy to produce high-quality, long-length nanowires in high yield. Currently available syntheses involve conversion of similar compounds using lithium-intercalated TMC complexes(8,9) or 2H-TMDCs.(10,14,15) A few studies have reported growth using vapor-phase techniques, such as sulfurization,(16) molecular beam epitaxy,(17) and nanotemplating reaction inside CNTs.(18,19) Although they are reliable fabrication techniques, the mass production and study of TMC nanowires are still constrained by production costs, purity, and scalability.
To solve this issue, we focus our attention on chemical vapor deposition (CVD), which is a powerful technique for the large-scale synthesis of a diverse range of nanomaterials, including CNTs,(20,21) graphene,(22−24) and TMDCs.(25−32) Here, we show wafer-scale growth of WTe and MoTe TMC nanowires, having various aggregation forms, by CVD. The growth substrate is observed to influence the structure and morphology of the TMC nanowires, with random or ordered networks forming on substrates of varying crystallinity and lattice orientation. The present synthesis method enables study of the optical and electronic properties of these TMC nanowire networks, which can be used to benefit future applications in electronics and energy-harvesting devices.

Results and Discussion

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First, 2D and 3D structures based on TMC nanowires, which act as 1D vdW building blocks, are demonstrated by changing growth substrates. Figure 1a shows schematic representations of the different configurations of the TMC nanowires synthesized in this work. Two types of geometrical structures are prepared, with 1D nanowire building blocks forming either an atomically thin 2D sheet or assembling into a 3D bundle. These two structures are observed in the cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and TEM images of the WTe nanowires, as shown in Figure 1b–d. Monolayer and bilayer sheets of WTe nanowires grown on a-plane sapphire substrates are presented in Figure 1b and c, respectively, whereas a 3D bundle containing approximately 120 nanowires grown on a SiO2/Si substrate is displayed in Figure 1d. The atomic structure of the nanowires can be clearly seen from the cross-section image of a single nanowire (Figure 1e) and from the image taken along the nanowire axis (Figure 1f), consistent with those reported for WTe(19) and other TMC nanowires.(14,15,18) The brighter spots in Figure 1e correspond to the heavier atomic mass W atoms. These atoms form a hexagonal ring surrounded by Te atoms having relatively low contrast. Elemental mapping performed by energy dispersive X-ray spectroscopy (EDS) further confirms the chemical composition of the sample (Figures S1 and S2). Quantitative analysis reveals a 1:1 ratio of W to Te, providing further evidence for the identity of the nanowires. Meanwhile, the WTe nanowires in Figure 1b–d are coated with an amorphous layer consisting of W and Te (Figure S3). This amorphous layer is likely residue from decomposed WTe nanowires, caused by damage incurred either by prolonged exposure to the ambient air or during the specimen preparation process.
In addition to the 2D and 3D aggregation forms, the CVD process results in the substrate-dependent formation of aligned and randomly oriented TMC nanowires at the wafer scale. Figure 2a displays the in-house CVD setup used in this study. A temperature range of 750–800 °C was suitable for nanowire growth on various substrates, including SiO2/Si, quartz, and sapphire. In a typical growth process, precursors such as WO3 powder and Te granular are heated separately by two furnaces. The alkali metal halide KBr, commonly used in the fabrication of TMDC atomic layers, is also added as a growth promoter near the WO3. Figure 2b (S4a) and c (S4b) show the as-grown WTe nanowires on SiO2/Si and sapphire substrates, respectively. In both cases, fiber-like structures are observed to form at high density. A wide coverage of the WTe network is found spanning over a 25 mm × 30 mm growth substrate with uniform sample quality by scanning electron microscopy (SEM) observations and Raman measurements (Figure S5). While a random network is formed on the silicon substrate in Figure 2b, the use of a-plane sapphire results in the growth of aligned fiber-like structures (Figure 2c). The individual aligned fibers have different sizes (Figure S6), with thicknesses ranging from ∼1 to 8 nm. The result agrees with those observed in Figure 1b and c. The growth method described here can also be extended to MoTe (Figure S7). These results clearly indicate that the morphology and growth orientation of TMC nanowires originate from a strong vdW interaction between the TMC nanowires and the substrate. Similar strong substrate interaction has been also observed for the MoS2 growth on Si(001) surfaces.(13)

Figure 2

Figure 2. Wafer-scale CVD growth of WTe nanowires. (a) Schematic illustration of the setup used for nanowire synthesis. Growth is conducted via vapor-phase evaporation of solid precursors with the aid of an alkali metal salt as a growth promoter. (b) Plan-view SEM image of the unoriented WTe nanowire arrays grown on a SiO2/Si substrate. (c) Atomic force microscopy (AFM) topographic image of a dense, ordered array of WTe nanowires synthesized on an a-plane sapphire substrate. The insets show the photographic images of the substrate following nanowire growth, which exhibit a slight yellowish (in b) or brownness (in c). The substrate dimensions are approximately 2.5 cm × 3 and 1 cm × 1 cm for (b) and (c), respectively.

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The atomically narrow 1D structure of TMC nanowires allows us to probe their 1D optical response. Raman spectra for WTe and MoTe nanowires grown on SiO2/Si are shown in Figure 3a. Distinct Raman peaks are observed in the range of 50 cm–1 to 300 cm–1 for both the MoTe and WTe nanowires. Prominent peaks are observed in the vicinity of 155 and 255 cm–1 for MoTe, which is similar to that reported previously(17−19) and confirms the identity of the nanowire structure. The former peak is attributed to the out-of-plane stretching of Te atoms, whereas the latter is due to the radial breathing mode of the Mo core.(18,19) This analysis hints at the origins of similar spectral features observed in the WTe sample. Further research into the detailed vibration modes for WTe is currently in progress.

Figure 3

Figure 3. 1D optical response. (a) Normalized Raman spectra for the synthesized MoTe and WTe nanowires on SiO2/Si. (b) Raman spectra for a single WTe 3D bundle recorded at four representative polarization angles of 0°, 30°, 60°, and 90°. The inset shows an optical image of the sample measured. Theta (θ) refers to the angle between the polarization of the incident laser and the nanowire axis. The spectra in (a) and (b) are shifted vertically for comparison. (c) Polarization-dependent normalized Raman intensity map for corresponding spectra in (b) taken from 0° to 360°. (d) Normalized Raman intensity polar plots for WTe nanowires in (b) at 155 cm–1 (blue) and 196 cm–1 (red). The solid line is provided as a guide to the eye. Normalization was performed with respect to the peak at 155 cm–1 in (a) and the same peak of highest intensity within the range of 0° to 360° in (c) and (d).

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TMC nanowires are expected to display optical anisotropy, as has been observed in many other 1D materials.(24,33−38) We therefore performed polarized Raman spectroscopy measurements on the WTe nanowires grown on a SiO2/Si substrate. Figure 3b shows the representative Raman spectra measured as the incident angle between the polarized laser and the nanowire axis was varied from 0° to 90°. The Raman signal reaches a maximum when the incident polarized light is parallel to the nanowire axis at 0°. As the incident angle changes, the peaks gradually diminish, reaching a minimum at 90°. Similar results are obtained when the scattered light is collected in parallel or cross configuration relative to the incident laser (Figure S8). Figure 3c presents the polarization-dependent normalized Raman intensity map in the range of 0° to 360°. A uniform optical response is observed for all the different Raman modes, where maximum intensities are observed when the incident polarized beam is parallel to the nanowire axis, at 0°, 180°, and 360°. This gives rise to the 2-fold symmetric polar curves in Figure 3d, where the angular dependences are plotted for the peak intensities at 155 and 196 cm–1.
The above phenomena are similar to those observed in CNTs,(33,34) graphene nanoribbons,(24,35) and semiconducting nanowires such as GaP and InAs(36,37) having anisotropic shapes. For these structures, photon absorption is considerably enhanced when the incident light is polarized parallel to the material’s axis. In contrast, excitation using perpendicularly polarized light could be suppressed by the depolarization effect, reducing Raman intensity.(33) The WTe 3D bundles can be conceptually visualized as a structural analogue to bundled CNTs. It is thus expected to exhibit a similar optical response, where a strong depolarization effect is observed, suggesting the 1D nature of the TMC nanowires. Furthermore, the WTe nanowires show three main absorption bands in the visible region at 1.61, 2.34, and 3.02 eV (Figure S9). These absorption bands could account for the polarization-dependent Raman scattering observed through the resonance Raman effect. It should be noted that the phonon confinement effect could lead to the broadening of Raman peaks of WTe bundles as observed for MoS2 nanoribbons.(38)
In addition to the 1D optical response of the single WTe 3D bundle, the present wafer-scale network of TMC nanowires also shows superior conducting properties. First, the electrical conductivity of a single WTe 3D bundle was monitored using a four-terminal measurement device. As shown in Figure 4a, the voltage drop increases linearly with the current, following an ohmic relationship. With a nanowire length of 1.6 μm (inset Figure 4a) and an approximate cross-sectional area of 83 nm2 (Figure 1d), the resistivity of the WTe bundle is estimated to be 1.3 μΩ m. This value is comparable to that of a single CNT bundle,(39,40) implying that the TMC nanowires exhibit excellent electrical conductivity. Additionally, the randomly oriented WTe thin film grown on a SiO2/Si substrate exhibits a similar linear I–V curve, and its sheet resistance is estimated to be 1.8 kΩ/sq (Figure 4b). Such low sheet resistance is likely due to the relatively low interbundle contact resistance, suggesting the potential application of WTe nanowires as transparent, flexible, and conductive films. The metallic behavior of the WTe network is confirmed by the temperature dependence of the electrical resistance. In Figure 4c, a monotonic decrease in resistance is observed as the temperature decreases from 300 to 10 K. The increase in resistance below 10 K is likely due to the electron localization effect in low-dimensional systems.(40) The data are reproducible, as evidenced by the overlapping heating and cooling curves. In addition, measurements taken on other samples, depicted in Figure S10, yielded similar results. Figure 4d shows the magnetoresistance (MR) of these conductive WTe networks plotted as a function of B2 at various temperatures. Here, MR is defined as [ρ(B) – ρ(0)]/ρ(0), where ρ(0) and ρ(B) refer to the resistivity in a zero and an applied magnetic field B, respectively. The MR increases with increasing magnetic field. The linear dependence of MR with B2 observed from 80 to 40 K indicates classical metallic behavior subject to Lorentz forces.(41) In contrast, a nonlinear B2 dependence is observed below 30 K. Such change in the field dependence can also be clearly seen for the MR–B curves given in Figure S11. Considering the electrical resistance curve in Figure 4c, the cusp-like positive MR that appears at low temperature (<30 K) can be attributed to the weak antilocalization effect originating from the spin–orbit interaction, which usually predominates as material thickness decreases.(41) These findings demonstrate that TMC nanowires, having uniform crystal structure with varying aggregate forms, provide an ideal system to study 1D optical, electrical, and magnetic responses.

Figure 4

Figure 4. Electron transport properties. I–V characteristic curves for (a) a single WTe 3D bundle (Figure 1d) and (b) a WTe thin film grown on a SiO2/Si substrate measured at room temperature. The insets in (a) and (b) show an SEM image of the four-point probe measurement and an optical image of a device fabricated with a channel length of 20 μm, respectively. (c) Temperature dependence of the electrical resistance of the WTe network on a SiO2/Si substrate under zero magnetic field (B = 0). Black and red curves represent the data taken on heating and cooling, as indicated by the arrows of corresponding color. The overlapping curves indicate reproducibility of the measurements. (d) Magnetoresistance as a function of B2 for the corresponding sample in (c). Solid curves correspond to linear (40 to 80 K) and power law (10 to 30 K) fits.

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To confirm the transport results obtained, energy band structures of an isolated WTe nanowire, the 2D mono/bilayer sheet, and the 3D bundle (Figure S12) were also calculated based on the density functional theory (DFT). The corresponding band structures are displayed in Figure 5a–d and Figure S13. The results indicate that the isolated WTe nanowire is a narrow-gap semiconductor with an indirect gap of 18 meV (Figure 5a and Figure S13a). Its 2D and 3D counterparts, on the contrary, become metals. These changes can be understood from the wave functions near the Fermi level of the isolated and 2D WTe structures. As illustrated in Figure 5e, the conduction band minimum (CBM) at Γ point in Figure 5a mainly comes from the 5p orbitals of Te atoms, whereas the combined attribution from W (5d) and Te (5p) orbitals accounts for its valence band maximum (VBM). In the 2D (or 3D) assemblies, these wave functions eventually overlap with those of neighboring wires (Figure 5f), which hence leads to the increases of band widths. This changes the semiconducting nature of the WTe nanowire to a metallic one. The metallic band structure of the 3D assembly is consistent to the metallic transport properties observed in Figure 4 for the network of 3D bundles.

Figure 5

Figure 5. Electronic band structures. DFT-calculated band structures of (a) an isolated single WTe nanowire, (b) its 2D monolayer, and the 3D bundle along (c) Y−Γ–B line and (d) Γ–Z line, respectively. Fermi level is set to zero as a reference for (b–d). An isolated WTe nanowire is a narrow-gap semiconductor with an indirect gap of 18 meV (an enlarged image is shown in Figure S13a), whereas its collected 2D and 3D assemblies are metals. Squared wave functions of the CBM and VBM at Γ point with energies of (e) 0.02 and 0 eV in (a) and of (f) 0.1 and 0 eV in (b), respectively.

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Finally, we discuss the effect of the substrate on the growth of TMC nanowires and corresponding mechanisms. Previous studies have reported that alkali metal halide aids the formation of highly volatile oxyhalide species and/or alkali metal complexes that promote TMDC growth.(29−32) Recently, the alkali metal was accountable to play a crucial role for initializing growth via a vapor–liquid–solid (VLS) mechanism.(31,32) The fact that the WTe nanowires are also grown by direct reaction of Te with Na2WO4·2H2O (Figure S14) in the present study has hinted that a similar growth mechanism is involved. In this scenario, solid precursors first vaporize and deposit onto the substrate surface, forming islands of molten intermediate droplets. When these droplets become supersaturated, the WTe nanowires start to emerge and elongate as the reaction proceeds. A root-growth model predominates over tip-growth as indicated by the radially extended wires with a shared nucleation center, as observed in Figure S15. With respect to the effect of the substrate, a close relationship between the mode of WTe nanowire growth and the supporting substrate is observed (Figure S16). Randomly oriented networks of TMC nanowires are usually formed on amorphous surfaces, such as the oxide (SiO2) surface of Si wafers and quartz glass. In contrast, a single-crystal alumina (sapphire) substrate offers better control over the growth direction. The orientation of the nanowires varies depending on the crystal surface plane. For example, unidirectionally aligned nanowires are observed on the a- and r-planes of sapphire, while WTe nanowires on the c-plane grow in three different directions at 120° angles to one another (Figure 2c and Figure S16). Such directed growth may take place due to the gas flow, atomic step, faceted nanogrooves, or crystallographic-induced alignment.(42−45) Nonetheless, the influence of gas flow is unlikely as it fails to explain the different projections on the c-plane sapphire substrate. Besides, the cross-section images in Figure 1b, c indicate wire formation on a flat surface with no step edge nor grooves. The alignment is therefore attributable to the lattice-oriented anisotropic wire-substrate interaction, which has also been reported in the directed growth of carbon nanotubes(42,43) and nanowires such as GaN and ZnS.(44,45) As shown earlier in Figure 1b–d, the formation of nanowire-based 2D sheets and 3D bundles is also affected by the substrate. According to these results, the tendency to attain either the sheet or bundle configuration is determined by whether the wire–substrate interaction or the wire–wire interaction is dominant, respectively. The bundle formation could be also facilitated through the aggregation of molten intermediate droplets on the SiO2 surface, which has larger surface roughness and different affinity compared to the Al2O3 surface.(32) These phenomena offer an opportunity for simultaneous control of the structural morphology, size, and geometric orientation of TMC nanowires as 1D vdW building blocks, benefiting the future design of nanowire-based materials for optimal properties.

Conclusions

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In conclusion, the ability to achieve large-scale synthesis and manipulate the nanowire growth direction is important, as it provides a possible means for scalable, direct orientation patterning of TMC nanowires via surface engineering. The present findings offer a new platform for novel studies and applications of 1D vdW nanowire systems, contributing not only to new discoveries in basic low-dimension physics but also to the design of future applications in electronics and energy storage/conversion devices.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c03456.

  • Experimental methods and supporting data for structural uniformity and crystallinity of suspended WTe bundles with the corresponding EDS measurements, decomposed WTe residue, surface morphology of the WTe network on SiO2/Si and a-plane sapphire, coverage and crystal uniformity of the WTe network, topographic analysis of WTe nanowires, SEM of MoTe nanowires, polarized Raman characterization and optical absorption of WTe nanowires, transport measurements for different WTe devices, structural models for DFT calculations, electronic band structure of bilayer 2D WTe, tellurization of Na2WO4·2H2O, nucleation of WTe nanowires, and WTe growth on various surfaces (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Yusuke Nakanishi - Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan;  Orcidhttp://orcid.org/0000-0001-8782-9556
    • Zheng Liu - Innovative Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan;  Orcidhttp://orcid.org/0000-0001-9095-7647
    • Jiang Pu - Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan;  Orcidhttp://orcid.org/0000-0002-1676-2072
    • Mina Maruyama - Department of Physics, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan;  Orcidhttp://orcid.org/0000-0002-2872-5543
    • Takahiko Endo - Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan
    • Chisato Ando - Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan
    • Hiroshi Shimizu - Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan
    • Kazuhiro Yanagi - Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan;  Orcidhttp://orcid.org/0000-0002-7609-1493
    • Susumu Okada - Department of Physics, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan
    • Taishi Takenobu - Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan
  • Author Contributions

    H.E.L., Y.N., and Y.M. conceived the experiments. H.E.L., T.E., and C.A. conducted material growth. H.E.L., C.A., and Y.M. performed Raman, SEM, and AFM characterization. Z.L. carried out the STEM observations. Y.M. and K.Y. measured the optical absorption. C.A., H.S., Y.N., Y.M., and J.P. fabricated the devices and conducted electrical measurements. T.T. provided technical support and assisted data analyses of electrical measurements. M.M. and S.O. performed DFT calculations. H.E.L. and Y.M. wrote the paper. All authors discussed and commented on the manuscript.

  • Notes

    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by JST CREST (JPMJCR16F3), Grants-in-Aid for Scientific Research (B) (JP18H01832 and JP19H02543), and a Grant-in-Aid for Young Scientists (JP19K15393) from the Japan Society for the Promotion of Science (JSPS). Y.N. acknowledges support from JSPS KAKENHI (20H02572), Murata Science Foundation 2019 (H31-068), JKA 2020 (2020M-121), and its promotion funds from KEIRIN RACE. J.P. acknowledges support from JSPS KAKENHI (JP19K15383 and 20H05189). K.Y. acknowledges support by JSPS KAKENHI through Grant Number JP20H02573 and by JST CREST through Grant Number JPMJCR17I5. T.T. acknowledges support from JSPS KAKENHI (JP26102012, JP25000003, 19K22127, and JP17H01069) and JST CREST (JPMJCR17I5). This work was partially conducted at the AIST Nano-Processing Facility supported by “Nanotechnology Platform Program” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Grant Number JPMXP09F19008709 and 20009034.

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    Two-dimensional atomic crystals
    Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K.
    Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (30), 10451-10453CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The authors report free-standing at. crystals that are strictly 2-dimensional and can be viewed as individual at. planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromech. cleavage, the authors prepd. and studied a variety of 2-dimensional crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2-dimensional mols. unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
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  5. 5
    Potel, M.; Chevrel, R.; Sergent, M.; Armici, J. C.; Decroux, M.; Fischer, Ø. New Pseudo-One-Dimensional Metals: M2Mo6Se6 (M = Na, In, K, TI), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, TI). J. Solid State Chem. 1980, 35 (2), 286– 290,  DOI: 10.1016/0022-4596(80)90505-8
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    5
    New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na, In, K, Tl), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, Tl)
    Potel, M.; Chevrel, R.; Sergent, M.; Armici, J. C.; Decroux, M.; Fischer, O.
    Journal of Solid State Chemistry (1980), 35 (2), 286-90CODEN: JSSCBI; ISSN:0022-4596.
    A new series of ternary chalcogenides, derived from divalent Mo, M2Mo6X6 is derived. These compds. crystallize in a hexagonal lattice space group P63/n, with a ≈9 and c ≈ 4.5 Å. The compds. are characterized by clusters (Mo3)∞1 in the form of linear chains, resulting from a linear condensation of Mo6 octahedral clusters. The (Mo3)∞ clusters are well sepd. from each other, with the shortest Mo-Mo intercluster distance >6 Å. The resulting pseudo-one-dimensional structures show remarkable anisotropy of phys. properties.
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  6. 6
    Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J. Facile Synthesis of Pseudo One-Monodimensional Ternary Molybdenum Chalcogenides M2Mo6X6 (X = Se, Te; M = Li, Na. Cs). Mater. Res. Bull. 1984, 19 (7), 915– 924,  DOI: 10.1016/0025-5408(84)90054-0
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    6
    A facile synthesis of pseudo one-dimensional ternary molybdenum chalcogenides M2Mo6X6 (X = Se,Te; M = Li,Na,...,Cs)
    Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J.
    Materials Research Bulletin (1984), 19 (7), 915-24CODEN: MRBUAC; ISSN:0025-5408.
    Ion-exchange reactions of In2Mo6X6 with alkali iodide and chloride salts at 550° to give M2Mo6X6 and mixed solid solns. In2-xMxMo6X6 (M = Li, Na, K, Rb, Cs; X = Se, Te) are described. Ion exchange provides a low-temp. route to prep. phases that are not stable at high temp., such as In2-xMxMo6Se6, where 0 ≤ x ≤ 2 and M = alkali. When M = Li, the Li can be partially removed by oxidn. with I2, producing nonstoichiometric ternary Mo chalcogenides In2-xMo6Se6 (0 ≤ x ≤ 1.8). Only Tl2Mo6Se6 is superconducting above 1 K.
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  7. 7
    Tarascon, J. M. Mo6Se6: A New Solid-State Electrode for Secondary Lithium Batteries. J. Electrochem. Soc. 1985, 132 (9), 2089,  DOI: 10.1149/1.2114297
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    7
    Molybdenum selenide (Mo6Se6): a new solid-state electrode for secondary lithium batteries
    Tarascon, J. M.
    Journal of the Electrochemical Society (1985), 132 (9), 2089-93CODEN: JESOAN; ISSN:0013-4651.
    Mo6Si6 can be used both as cathode and anode in secondary Li cells and can take up reversibly 9 Li atoms/Mo6Se6 without losing its linear chain structure, leading to a theor. vol. energy d. of ∼1.1 W-h/cm3, compared to 1.2 for TiS2 cathodes. Another important promising possibility is the use of LixMo6Se6 as the anode instead of Li in secondary Li cells.
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  8. 8
    Venkataraman, L.; Lieber, C. M. Molybdenum Selenide Molecular Wires as One-Dimensional Conductors. Phys. Rev. Lett. 1999, 83 (25), 5334– 5337,  DOI: 10.1103/PhysRevLett.83.5334
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    8
    Molybdenum selenide molecular wires as one-dimensional conductors
    Venkataraman, Latha; Lieber, Charles M.
    Physical Review Letters (1999), 83 (25), 5334-5337CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Scanning tunneling microscopy has been used to characterize the structure and electronic properties of molybdenum selenide (Mo6Se6) mol. wires. STM images resolve the 0.45 nm unit cell repeat in isolated mol. wires and bundles of wires. Tunneling spectroscopy measurements exhibit sharp peaks in the local d. of states that are consistent with the Van Hove singularities expected for a one-dimensional system. These data are compared with those of previous band-structure calcns. Preliminary measurements at 5 K show no evidence of a gap opening at low temps. The implications of these results to metal-insulator transitions in one-dimensional systems are discussed.
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  9. 9
    Venkataraman, L.; Hong, Y. S.; Kim, P. Electron Transport in a Multichannel One-Dimensional Conductor: Molybdenum Selenide Nanowires. Phys. Rev. Lett. 2006, 96 (7), 076601,  DOI: 10.1103/PhysRevLett.96.076601
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    9
    Electron transport in a multichannel one-dimensional conductor. Molybdenum selenide nanowires
    Venkataraman, Latha; Hong, Yeon Suk; Kim, Philip
    Physical Review Letters (2006), 96 (7), 076601/1-076601/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We have measured electron transport in small bundles of identical conducting MoSe nanowires where the no. of weakly interacting 1D chains ranges from 1 to 300. The linear conductance and current in these nanowires exhibit a power-law dependence on temp. and bias voltage, resp. The exponents governing these power laws decrease as the no. of conducting channels increase. These exponents can be related to the electron-electron interaction parameter for transport in multichannel 1D systems with a few defects.
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  10. 10
    Xia, Y.; Wang, B.; Zhang, J.; Jin, Y.; Tian, H.; Ho, W.; Xu, H.; Jin, C.; Xie, M. Quantum Confined Tomonaga–Luttinger Liquid in Mo6Se6 Nanowires Converted from an Epitaxial MoSe2 Monolayer. Nano Lett. 2020, 20 (3), 2094– 2099,  DOI: 10.1021/acs.nanolett.0c00090
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    10
    Quantum Confined Tomonaga-Luttinger Liquid in Mo6Se6 Nanowires Converted from an Epitaxial MoSe2 Monolayer
    Xia, Yipu; Wang, Bo; Zhang, Junqiu; Jin, Yuanjun; Tian, Hao; Ho, Wingkin; Xu, Hu; Jin, Chuanhong; Xie, Maohai
    Nano Letters (2020), 20 (3), 2094-2099CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Confining interacting particles in one-dimension (1D) changes the electronic behavior of the system fundamentally, which has been studied extensively in the past. Examples of 1D metallic systems include carbon nanotubes, quasi-1D org. conductors, metal chains, and domain boundary defects in monolayer thick transition-metal dichalcogenides such as MoSe2. Here single and bundles of Mo6Se6 nanowires were fabricated through annealing a MoSe2 monolayer grown by mol.-beam epitaxy on graphene. Conversion from two-dimensional (2D) MoSe2 film to 1D Mo6Se6 nanowire is reversible. Mo6Se6 nanowires form preferentially at the Se-terminated zigzag edges of MoSe2 and stitch to it via two distinct at. configurations. The Mo6Se6 wire is metallic and its length is tunable, which represents one of few 1D systems that exhibit properties pertinent to quantum confined Tomonaga-Luttinger liq., as evidenced by scanning tunneling microscopic and spectroscopic studies.
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  11. 11
    Cheng, F.; Xu, H.; Xu, W.; Zhou, P.; Martin, J.; Loh, K. P. Controlled Growth of 1D MoSe2 Nanoribbons with Spatially Modulated Edge States. Nano Lett. 2017, 17 (2), 1116– 1120,  DOI: 10.1021/acs.nanolett.6b04715
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    11
    Controlled Growth of 1D MoSe2 Nanoribbons with Spatially Modulated Edge States
    Cheng, Fang; Xu, Hai; Xu, Wentao; Zhou, Pinjia; Martin, Jens; Loh, Kian Ping
    Nano Letters (2017), 17 (2), 1116-1120CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Two-dimensional (2D) transition metal dichalcogenides (TMDCs) possess interesting one-dimensional (1D) properties at its edges and inversion domain boundaries, where properties markedly different from the 2D basal plane, such as 1D metallicity and charge d. waves, can be obsd. Although 2D TMDCs crystals are widely grown by chem. vapor deposition (CVD), the fabrication of 1D TMDCs ribbons is challenging due to the difficulty to confine growth in only one dimension. Here we report the controlled growth of MoSe2 nanoribbons with an aspect ratio >100 by using prepatterned Se reconstructions on Au(100). Using scanning tunneling microscope and spectroscopy (STM/STS), the at. and electronic structure of MoSe2 nanoribbons are studied. The ultranarrow ribbons show metallic behavior, while wider ribbons show a crossover from metallic to semiconducting behavior going from the edge to the center of the ribbon. The obsd. conductance modulations of the ultranarrow ribbons are attributed to 1D Moir´e pattern. Remarkably, it shows a different periodicity compared with the 2D Moir´e pattern in wider ribbons indicating that the 1D system is softened due to the high ratio of edge to basal plane bonds. Further, we demonstrated that the nanoribbons are stable against ambient conditions, which suggests that 1D TMDCs can be exploited for further applications.
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  12. 12
    Poh, S. M.; Tan, S. J. R.; Zhao, X.; Chen, Z.; Abdelwahab, I.; Fu, D.; Xu, H.; Bao, Y.; Zhou, W.; Loh, K. P. Large Area Synthesis of 1D-MoSe2 Using Molecular Beam Epitaxy. Adv. Mater. 2017, 29 (12), 1605641,  DOI: 10.1002/adma.201605641
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  13. 13
    Chowdhury, T.; Kim, J.; Sadler, E. C.; Li, C.; Lee, S. W.; Jo, K.; Xu, W.; Gracias, D. H.; Drichko, N. V.; Jariwala, D.; Brintlinger, T. H.; Mueller, T.; Park, H.-G.; Kempa, T. J. Substrate-Directed Synthesis of MoS2 Nanocrystals with Tunable Dimensionality and Optical Properties. Nat. Nanotechnol. 2020, 15 (1), 29– 34,  DOI: 10.1038/s41565-019-0571-2
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    13
    Substrate-directed synthesis of MoS2 nanocrystals with tunable dimensionality and optical properties
    Chowdhury, Tomojit; Kim, Jungkil; Sadler, Erick C.; Li, Chenyang; Lee, Seong Won; Jo, Kiyoung; Xu, Weinan; Gracias, David H.; Drichko, Natalia V.; Jariwala, Deep; Brintlinger, Todd H.; Mueller, Tim; Park, Hong-Gyu; Kempa, Thomas J.
    Nature Nanotechnology (2020), 15 (1), 29-34CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
    Two-dimensional transition-metal dichalcogenide (TMD) crystals are a versatile platform for optoelectronic, catalytic, and quantum device studies. However, the ability to tailor their phys. properties through explicit synthetic control of their morphol. and dimensionality is a major challenge. We demonstrate a gas-phase synthesis method that substantially transforms the structure and dimensionality of TMD crystals without lithog. Synthesis of MoS2 on Si(001) surfaces pre-treated with phosphine yields high-aspect-ratio nanoribbons of uniform width. We systematically control the width of these nanoribbons at 50-430 nm by varying the total phosphine dosage during the surface treatment step. Aberration-cor. electron microscopy reveals that the nanoribbons are predominantly 2H phase with zig-zag edges and an edge quality that is comparable to, or better than, that of graphene and TMD nanoribbons prepd. through conventional top-down processing. Owing to their restricted dimensionality, the nominally 1-dimensional MoS2 nanocrystals exhibit photoluminescence 50 meV higher in energy than that from 2-dimensional MoS2 crystals. Moreover, this emission is precisely tunable through synthetic control of crystal width. Directed crystal growth on designer substrates has the potential to enable the prepn. of low-dimensional materials with prescribed morphologies and tunable or emergent optoelectronic properties.
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  14. 14
    Lin, J.; Cretu, O.; Zhou, W.; Suenaga, K.; Prasai, D.; Bolotin, K. I.; Cuong, N. T.; Otani, M.; Okada, S.; Lupini, A. R.; Idrobo, J.-C.; Caudel, D.; Burger, A.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Pennycook, S. J.; Pantelides, S. T. Flexible Metallic Nanowires with Self-Adaptive Contacts to Semiconducting Transition-Metal Dichalcogenide Monolayers. Nat. Nanotechnol. 2014, 9 (6), 436– 442,  DOI: 10.1038/nnano.2014.81
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    14
    Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers
    Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I.; Nguyen, Thanh Cuong; Otani, Minoru; Okada, Susumu; Lupini, Andrew R.; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J.; Yan, Jiaqiang; Mandrus, David G.; Pennycook, Stephen J.; Pantelides, Sokrates T.
    Nature Nanotechnology (2014), 9 (6), 436-442CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) were predicted to be metallic. Such nanowires were chem. synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a 2-dimensional integrated circuit, has so far remained elusive. By steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometer in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ elec. measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mech. deformations, thus showing promise for mech. robust flexible electronics. D. functional theory calcns. further confirm the metallicity of the nanowires and account for their beam-induced mech. behavior. These results show that direct patterning of 1D conducting nanowires in 2D semiconducting materials with nanometer precision is possible using electron-beam-based techniques.
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  15. 15
    Zhu, H.; Wang, Q.; Zhang, C.; Addou, R.; Cho, K.; Wallace, R. M.; Kim, M. J. New Mo6Te6 Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe2. Adv. Mater. 2017, 29 (18), 1606264,  DOI: 10.1002/adma.201606264
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    There is no corresponding record for this reference.
  16. 16
    Kibsgaard, J.; Tuxen, A.; Levisen, M.; Lægsgaard, E.; Gemming, S.; Seifert, G.; Lauritsen, J. V.; Besenbacher, F. Atomic-Scale Structure of Mo6S6 Nanowires. Nano Lett. 2008, 8 (11), 3928– 3931,  DOI: 10.1021/nl802384n
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    16
    Atomic-Scale Structure of Mo6S6 Nanowires
    Kibsgaard, Jakob; Tuxen, Anders; Levisen, Martin; Laegsgaard, Erik; Gemming, Sibylle; Seifert, Gotthard; Lauritsen, Jeppe V.; Besenbacher, Flemming
    Nano Letters (2008), 8 (11), 3928-3931CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We have studied the at.-scale structure of the Mo6S6 nanowires using scanning tunneling microscopy and spectroscopy (STM and STS) and d. functional theory (DFT). A novel synthesis route based on metallic Mo precursors is presented for the selective formation of elementary pure Mo6S6 nanowires. The Mo6S6 nanowires selectively organize as trimer bundles, and each of the Mo6S6 nanowires consists of an elec. conducting Mo backbone dressed with a sulfur exterior cap. The Mo6S6 nanowires may thus be of interest as novel building blocks in nanoelectronics because the Mo6S6 nanowires exist in a robust, singular structural conformation with uniquely defined elec. (metallic) properties.
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  17. 17
    Yu, Y.; Wang, G.; Tan, Y.; Wu, N.; Zhang, X.-A.; Qin, S. Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires and Two-Dimensional MoTe2 Ultrathin Films Heterostructures. Nano Lett. 2018, 18 (2), 675– 681,  DOI: 10.1021/acs.nanolett.7b03058
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    17
    Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires and Two-Dimensional MoTe2 Ultrathin Films Heterostructures
    Yu, Yayun; Wang, Guang; Tan, Yuan; Wu, Nannan; Zhang, Xue-Ao; Qin, Shiqiao
    Nano Letters (2018), 18 (2), 675-681CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Controllable synthesizing of 1-dimensional-two-dimensional (1-dimensional-2D) heterostructures and tuning their at. and electronic structures is nowadays of particular interest due to the extraordinary properties and potential applications. Here, the authors demonstrate the temp.-induced phase-controlled growth of 1-dimensional Mo6Te6-2D MoTe2 heterostructures via MBE. In situ scanning tunneling microscopy study shows 2-dimensional ultrathin films are synthesized at low temp. range, while 1-dimensional nanowires gradually arise and dominate as temp. increasing. XPS confirms the good stoichiometry and scanning tunneling spectroscopy reveals the semimetallic property of grown Mo6Te6 nanowires. Through in situ annealing, a phase transition from 2-dimensional MoTe2 to 1-dimensional Mo6Te6 is induced, thus forming a semimetal-semiconductor junction in at. level. An upward band bending of 2H-MoTe2 is caused by lateral hole injection from Mo6Te6. The work suggests a new route to synthesize 1-dimensional semimetallic transition metal chalcogenide nanowires, which could serve as ultrasmall conducting building blocks and enable band engineering in future 1-dimensional-2D heterostructure devices.
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  18. 18
    Nagata, M.; Shukla, S.; Nakanishi, Y.; Liu, Z.; Lin, Y.-C.; Shiga, T.; Nakamura, Y.; Koyama, T.; Kishida, H.; Inoue, T.; Kanda, N.; Ohno, S.; Sakagawa, Y.; Suenaga, K.; Shinohara, H. Isolation of Single-Wired Transition-Metal Monochalcogenides by Carbon Nanotubes. Nano Lett. 2019, 19 (8), 4845– 4851,  DOI: 10.1021/acs.nanolett.8b05074
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    18
    Isolation of Single-Wired Transition-Metal Monochalcogenides by Carbon Nanotubes
    Nagata, Masataka; Shukla, Shivani; Nakanishi, Yusuke; Liu, Zheng; Lin, Yung-Chang; Shiga, Takuma; Nakamura, Yuto; Koyama, Takeshi; Kishida, Hideo; Inoue, Tsukasa; Kanda, Naoyuki; Ohno, Shun; Sakagawa, Yuki; Suenaga, Kazu; Shinohara, Hisanori
    Nano Letters (2019), 19 (8), 4845-4851CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The successful isolation of single layers from two-dimensional (2D) van der Waals (vdW)-layered materials has opened new frontiers in condensed matter physics and materials science. Their discovery and unique properties laid the foundation for exploring 1D counterparts. However, the isolation of 1D vdW-wired materials has thus far remained a challenge, and effective techniques are demanded. Here we report the facile synthesis of isolated transition-metal monochalcogenide MoTe nanowires by using carbon nanotubes (CNTs) as molds. Individual nanowires are perfectly sepd. by CNTs with a minimal interaction, enabling detailed characterization of the single wires. Transmission electron microscopy revealed unusual torsional motion of MoTe nanowires inside CNTs. Confinement of 1D vdW-wired materials to the nanotest tubes might open up possibilities for exploring unprecedented properties of the nanowires and their potential applications such as electromech. switching devices.
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  19. 19
    Kanda, N.; Nakanishi, Y.; Liu, D.; Liu, Z.; Inoue, T.; Miyata, Y.; Tománek, D.; Shinohara, H. Efficient Growth and Characterization of One-Dimensional Transition Metal Tellurides inside Carbon Nanotubes. Nanoscale 2020, 12, 17185– 17190,  DOI: 10.1039/D0NR03129A
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    19
    Efficient growth and characterization of one-dimensional transition metal tellurides inside carbon nanotubes
    Kanda, Naoyuki; Nakanishi, Yusuke; Liu, Dan; Liu, Zheng; Inoue, Tsukasa; Miyata, Yasumitsu; Tomanek, David; Shinohara, Hisanori
    Nanoscale (2020), 12 (33), 17185-17190CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Atomically thin one-dimensional (1D) van der Waals wires of transition metal monochalocogenides (TMMs) have been anticipated as promising building blocks for integrated nanoelectronics. While reliable prodn. of TMM nanowires has eluded scientists over the past few decades, we finally demonstrated a bottom-up fabrication of MoTe nanowires inside carbon nanotubes (CNTs). Still, the current synthesis method is based on vacuum annealing of reactive MoTe2, and limits access to a variety of TMMs. Here we report an expanded framework for high-yield synthesis of the 1D tellurides including WTe, an previously unknown family of TMMs. Exptl. and theor. analyses revealed that the choice of suitable metal oxides as a precursor provides a useful yield for their characterization. These TMM nanowires exhibit a significant optical absorption in the visible-light region. More important, electronic properties of CNTs can be tuned by encapsulating different TMM nanowires.
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  20. 20
    Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes. J. Phys. Chem. B 1999, 103 (31), 6484– 6492,  DOI: 10.1021/jp990957s
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    20
    Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes
    Cassell, Alan M.; Raymakers, Jeffrey A.; Kong, Jing; Dai, Hongjie
    Journal of Physical Chemistry B (1999), 103 (31), 6484-6492CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The synthesis of bulk amts. of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chem. compns. and textural properties of the catalyst material used in the chem. vapor deposition (CVD) of methane. A series of catalysts are derived by systematically varying the catalytic metal compds. and support materials. The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica-alumina multicomponent material. The high SWNT yielding catalyst exhibits high surface-area and large mesopore vol. at elevated temps. Gram quantities of SWNT materials have been synthesized in ∼0.5 h using the optimized catalyst material. The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating. This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes.
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  21. 21
    Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes. Science 2004, 306 (5700), 1362– 1364,  DOI: 10.1126/science.1104962
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    21
    Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes
    Hata, Kenji; Futaba, Don N.; Mizuno, Kohei; Namai, Tatsunori; Yumura, Motoo; Iijima, Sumio
    Science (Washington, DC, United States) (2004), 306 (5700), 1362-1364CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    We demonstrate the efficient chem. vapor deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water. Water-stimulated enhanced catalytic activity results in massive growth of superdense and vertically aligned nanotube forests with heights up to 2.5 mm that can be easily sepd. from the catalysts, providing nanotube material with carbon purity above 99.98%. Moreover, patterned, highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many crit. problems that currently plague carbon nanotube synthesis.
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  22. 22
    Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324 (5932), 1312– 1314,  DOI: 10.1126/science.1171245
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    22
    Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
    Li, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.
    Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
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  23. 23
    Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Özyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5 (8), 574– 578,  DOI: 10.1038/nnano.2010.132
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    23
    Roll-to-roll production of 30-inch graphene films for transparent electrodes
    Bae, Sukang; Kim, Hyeongkeun; Lee, Youngbin; Xu, Xiangfan; Park, Jae-Sung; Zheng, Yi; Balakrishnan, Jayakumar; Lei, Tian; Kim, Hye Ri; Song, Young Il; Kim, Young-Jin; Kim, Kwang S.; Oezyilmaz, Barbaros; Ahn, Jong-Hyun; Hong, Byung Hee; Iijima, Sumio
    Nature Nanotechnology (2010), 5 (8), 574-578CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    The outstanding elec., mech. and chem. properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll prodn. and wet-chem. doping of predominantly monolayer 30-in. graphene films grown by chem. vapor deposition onto flexible copper substrates. The films have sheet resistances as low as ∼125 Ω .box.-1 with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as ∼30 Ω .box.-1 at ∼90% transparency, which is superior to com. transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.
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  24. 24
    Suzuki, H.; Kaneko, T.; Shibuta, Y.; Ohno, M.; Maekawa, Y.; Kato, T. Wafer-Scale Fabrication and Growth Dynamics of Suspended Graphene Nanoribbon Arrays. Nat. Commun. 2016, 7 (1), 11797,  DOI: 10.1038/ncomms11797
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    24
    Wafer-scale fabrication and growth dynamics of suspended graphene nanoribbon arrays
    Suzuki, Hiroo; Kaneko, Toshiro; Shibuta, Yasushi; Ohno, Munekazu; Maekawa, Yuki; Kato, Toshiaki
    Nature Communications (2016), 7 (), 11797CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Adding a mech. degree of freedom to the elec. and optical properties of atomically thin materials can provide an excellent platform to investigate various optoelec. physics and devices with mech. motion interaction. The large scale fabrication of such atomically thin materials with suspended structures remains a challenge. Here we demonstrate the wafer-scale bottom-up synthesis of suspended graphene nanoribbon arrays (over 1,000,000 graphene nanoribbons in 2 × 2 cm2 substrate) with a very high yield (over 98%). Polarized Raman measurements reveal graphene nanoribbons in the array can have relatively uniform-edge structures with near zigzag orientation dominant. A promising growth model of suspended graphene nanoribbons is also established through a comprehensive study that combined expts., mol. dynamics simulations and theor. calcns. with a phase-diagram anal. We believe that our results can contribute to pushing the study of graphene nanoribbons into a new stage related to the optoelec. physics and industrial applications.
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  25. 25
    Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu, Y.-C.; Wang, J. T.-W.; Chang, C.-S.; Li, L.-J.; Lin, T.-W. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24 (17), 2320– 2325,  DOI: 10.1002/adma.201104798
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    25
    Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition
    Lee, Yi-Hsien; Zhang, Xin-Quan; Zhang, Wenjing; Chang, Mu-Tung; Lin, Cheng-Te; Chang, Kai-Di; Yu, Ya-Chu; Wang, Jacob Tse-Wei; Chang, Chia-Seng; Li, Lain-Jong; Lin, Tsung-Wu
    Advanced Materials (Weinheim, Germany) (2012), 24 (17), 2320-2325CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Large-area MoS2 at. layers are synthesized on SiO2 substrates by chem. vapor deposition using MoO3 and S powders as the reactants. Optical, microscopic and elec. measurements suggest that the synthetic process leads to the growth of MoS2 monolayer. The TEM images verify that the synthesized MoS2 sheets are highly cryst. To check for elec. performance bottom-gated transistors on silica/silicon using photolithog. was fabricated directly on top of the MoS2 sheets. The transfer curve (drain current vs. gate voltage) was computed and field effect mobility was detd. from anal. of the curve.
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  26. 26
    Liu, K.-K.; Zhang, W.; Lee, Y.-H.; Lin, Y.-C.; Chang, M.-T.; Su, C.-Y.; Chang, C.-S.; Li, H.; Shi, Y.; Zhang, H.; Lai, C.-S.; Li, L.-J. Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12 (3), 1538– 1544,  DOI: 10.1021/nl2043612
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    26
    Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates
    Liu, Keng-Ku; Zhang, Wenjing; Lee, Yi-Hsien; Lin, Yu-Chuan; Chang, Mu-Tung; Su, Ching-Yuan; Chang, Chia-Seng; Li, Hai; Shi, Yumeng; Zhang, Hua; Lai, Chao-Sung; Li, Lain-Jong
    Nano Letters (2012), 12 (3), 1538-1544CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS2 at. thin layers is still rare. Here we report that the high-temp. annealing of a thermally decompd. ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS2 thin layers with superior elec. performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS2 sheets are highly cryst. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS2 layer is comparable with those of the micromechanically exfoliated thin sheets from MoS2 crystals. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.
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    Zhan, Y.; Liu, Z.; Najmaei, S.; Ajayan, P. M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate. Small 2012, 8 (7), 966– 971,  DOI: 10.1002/smll.201102654
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    27
    Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate
    Zhan, Yongjie; Liu, Zheng; Najmaei, Sina; Ajayan, Pulickel M.; Lou, Jun
    Small (2012), 8 (7), 966-971CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Monolayer Mo disulfide (MoS2), a two-dimensional crystal with a direct bandgap, is a promising candidate for 2-dimensional nanoelectronic devices complementing graphene. There were recent attempts to produce MoS2 layers via chem. and mech. exfoliation of bulk material. Here the authors demonstrate the large area growth of MoS2 at. layers on SiO2 substrates by a scalable CVD method. The as-prepd. samples can either be readily used for further device fabrication or be easily released from SiO2 and transferred to arbitrary substrates. High resoln. TEM and Raman spectroscopy on the as grown films of MoS2 indicate that the no. of layers range from single layer to a few layers. The authors' results on the direct growth of MoS2 layers on dielec. leading to facile device fabrication possibilities show the expanding set of useful 2-dimensional at. layers, on the heels of graphene, which can be controllably synthesized and manipulated for many applications.
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    Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520 (7549), 656– 660,  DOI: 10.1038/nature14417
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    High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity
    Kang, Kibum; Xie, Saien; Huang, Lujie; Han, Yimo; Huang, Pinshane Y.; Mak, Kin Fai; Kim, Cheol-Joo; Muller, David; Park, Jiwoong
    Nature (London, United Kingdom) (2015), 520 (7549), 656-660CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the at., sub-nanometer length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technol. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high elec. carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technol. relevant scale without film transfer. In addn., their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high elec. performance remains an unsolved challenge. Here we report the prepn. of high-mobility 4-in. wafer-scale films of monolayer molybdenum disulfide (MoS2) and tungsten disulfide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal-org. chem. vapor deposition technique, and show high elec. performance, including an electron mobility of 30 cm2 V-1 s-1 at room temp. and 114 cm2 V-1 s-1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.
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    Li, S.; Wang, S.; Tang, D.-M.; Zhao, W.; Xu, H.; Chu, L.; Bando, Y.; Golberg, D.; Eda, G. Halide-Assisted Atmospheric Pressure Growth of Large WSe2 and WS2 Monolayer Crystals. Appl. Mater. Today 2015, 1 (1), 60– 66,  DOI: 10.1016/j.apmt.2015.09.001
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    Zhou, J.; Lin, J.; Huang, X.; Zhou, Y.; Chen, Y.; Xia, J.; Wang, H.; Xie, Y.; Yu, H.; Lei, J.; Wu, D.; Liu, F.; Fu, Q.; Zeng, Q.; Hsu, C.-H.; Yang, C.; Lu, L.; Yu, T.; Shen, Z.; Lin, H.; Yakobson, B. I.; Liu, Q.; Suenaga, K.; Liu, G.; Liu, Z. A Library of Atomically Thin Metal Chalcogenides. Nature 2018, 556 (7701), 355– 359,  DOI: 10.1038/s41586-018-0008-3
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    A library of atomically thin metal chalcogenides
    Zhou, Jiadong; Lin, Junhao; Huang, Xiangwei; Zhou, Yao; Chen, Yu; Xia, Juan; Wang, Hong; Xie, Yu; Yu, Huimei; Lei, Jincheng; Wu, Di; Liu, Fucai; Fu, Qundong; Zeng, Qingsheng; Hsu, Chuang-Han; Yang, Changli; Lu, Li; Yu, Ting; Shen, Zexiang; Lin, Hsin; Yakobson, Boris I.; Liu, Qian; Suenaga, Kazu; Liu, Guangtong; Liu, Zheng
    Nature (London, United Kingdom) (2018), 556 (7701), 355-359CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting phys. phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional supercond.5, suggesting potential applications for functional devices6-10. However, of the numerous compds. available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11-15, selenization16,17 and tellurization18 of metals and metal compds. Many TMCs are difficult to produce because of the high m.ps. of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temp.19 and this approach20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chem. vapor deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compds., including 32 binary compds. (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compds. We elaborate how the salt decreases the m.p. of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of the synthesized materials in our library are useful, as supported by evidence of supercond. in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.
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  31. 31
    Li, S.; Lin, Y.-C.; Zhao, W.; Wu, J.; Wang, Z.; Hu, Z.; Shen, Y.; Tang, D.-M.; Wang, J.; Zhang, Q.; Zhu, H.; Chu, L.; Zhao, W.; Liu, C.; Sun, Z.; Taniguchi, T.; Osada, M.; Chen, W.; Xu, Q.-H.; Wee, A. T. S.; Suenaga, K.; Ding, F.; Eda, G. Vapour–Liquid–Solid Growth of Monolayer MoS2 Nanoribbons. Nat. Mater. 2018, 17 (6), 535– 542,  DOI: 10.1038/s41563-018-0055-z
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    31
    Vapour-liquid-solid growth of monolayer MoS2 nanoribbons
    Li, Shisheng; Lin, Yung-Chang; Zhao, Wen; Wu, Jing; Wang, Zhuo; Hu, Zehua; Shen, Youde; Tang, Dai-Ming; Wang, Junyong; Zhang, Qi; Zhu, Hai; Chu, Leiqiang; Zhao, Weijie; Liu, Chang; Sun, Zhipei; Taniguchi, Takaaki; Osada, Minoru; Chen, Wei; Xu, Qing-Hua; Wee, Andrew Thye Shen; Suenaga, Kazu; Ding, Feng; Eda, Goki
    Nature Materials (2018), 17 (6), 535-542CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    Chem. vapor deposition of two-dimensional materials typically involves the conversion of vapor precursors to solid products in a vapor-solid-solid mode. Here, we report the vapor-liq.-solid growth of monolayer MoS2, yielding highly cryst. ribbons with a width of few tens to thousands of nanometers. This vapor-liq.-solid growth is triggered by the reaction between MoO3 and NaCl, which results in the formation of molten Na-Mo-O droplets. These droplets mediate the growth of MoS2 ribbons in the 'crawling mode' when satd. with sulfur. The locally well-defined orientations of the ribbons reveal the regular horizontal motion of the droplets during growth. Using at.-resoln. scanning transmission electron microscopy and second harmonic generation microscopy, we show that the ribbons are grown homoepitaxially on monolayer MoS2 with predominantly 2H- or 3R-type stacking. Our findings highlight the prospects for the controlled growth of atomically thin nanostructure arrays for nanoelectronic devices and the development of unique mixed-dimensional structures.
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  32. 32
    Li, S.; Lin, Y.-C.; Liu, X.-Y.; Hu, Z.; Wu, J.; Nakajima, H.; Liu, S.; Okazaki, T.; Chen, W.; Minari, T.; Sakuma, Y.; Tsukagoshi, K.; Suenaga, K.; Taniguchi, T.; Osada, M. Wafer-Scale and Deterministic Patterned Growth of Monolayer MoS2 via Vapor–Liquid–Solid Method. Nanoscale 2019, 11 (34), 16122– 16129,  DOI: 10.1039/C9NR04612G
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    32
    Wafer-scale and deterministic patterned growth of monolayer MoS2 via vapor-liquid-solid method
    Li, Shisheng; Lin, Yung-Chang; Liu, Xu-Ying; Hu, Zehua; Wu, Jing; Nakajima, Hideaki; Liu, Song; Okazaki, Toshiya; Chen, Wei; Minari, Takeo; Sakuma, Yoshiki; Tsukagoshi, Kazuhito; Suenaga, Kazu; Taniguchi, Takaaki; Osada, Minoru
    Nanoscale (2019), 11 (34), 16122-16129CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chem. vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of vapors. The non-stoichiometric supply of transition-metal precursors and chalcogens leads to poor control in the products' location, morphol., crystallinity, uniformity and batch to batch reproducibility. The vapor-liq.-solid (VLS) growth method often involves molten precursors (e.g., non-volatile Na2MoO4) at growth temps. higher than their m.ps. The liq. Na2MoO4 can ppt. out solid MoS2 monolayers when satd. with sulfur vapor. Taking advantage of the VLS growth, we attained three kinds of important achievements: (i) a 4-in.-wafer-scale uniform growth of MoS2 flakes on SiO2/Si substrates, (ii) a 2-in.-wafer-scale growth of continuous MoS2 film with the grain size exceeding 100μm on sapphire substrates, and (iii) a patterned (site-controlled) growth of MoS2 flakes and films. We clarified that the VLS growth thus paves a new way for the high-efficient and scalable synthesis of two-dimensional TMDC monolayers.
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    Duesberg, G. S.; Loa, I.; Burghard, M.; Syassen, K.; Roth, S. Polarized Raman Spectroscopy on Isolated Single-Wall Carbon Nanotubes. Phys. Rev. Lett. 2000, 85 (25), 5436– 5439,  DOI: 10.1103/PhysRevLett.85.5436
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    33
    Polarized Raman Spectroscopy on Isolated Single-Wall Carbon Nanotubes
    Duesberg, G. S.; Loa, I.; Burghard, M.; Syassen, K.; Roth, S.
    Physical Review Letters (2000), 85 (25), 5436-5439CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Polarized micro-Raman spectroscopy was performed on spatially sepd. single-wall C nanotubes (SWNTs) as individual nanotubes or thin ropes of only a few SWNTs. Different from bulk samples, the Raman spectra are composed of well-resolved peaks which allow a direct comparison of exptl. data with theor. calcns. Orientation-dependent measurements reveal max. intensity of all Raman modes when the nanotubes are aligned parallel to the polarization of the incident laser light. The angular dependences clearly deviate from the selection rules predicted by theor. studies. These differences are attributed to depolarization effects caused by the strongly anisotropic geometry of the nanotubes and to electronic resonance effects for excitation at 633 nm.
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  34. 34
    Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Ünlü, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R. G-Band Resonant Raman Study of 62 Isolated Single-Wall Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65 (15), 155412,  DOI: 10.1103/PhysRevB.65.155412
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    34
    G-band resonant Raman study of 62 isolated single-wall carbon nanotubes
    Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Unlu, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R.
    Physical Review B: Condensed Matter and Materials Physics (2002), 65 (15), 155412/1-155412/9CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors report G-band resonance Raman spectra of single-wall C nanotubes (SWNTs) at the single-nanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diams. dt ranging between 0.95 nm and 2.62 nm, the authors have conclusively detd. the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diam. dependent (ωG+=1591 cm-1), while the lower-frequency peak is given by ωG-=ωG+-C/dt2, with C being different for metallic and semiconducting SWNTs (CM>CS). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1<IωG-/IωG+<0.3 for most of the isolated SWNTs (∼90%). Unusually high or low IωG-/IωG+ ratios are obsd. for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E44S interband transition and scattered photons are in resonance with E33S. Since the Eii values depend sensitively on both nanotube diam. and chirality, the (n,m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theor. predictions and exptl. observations about these special G-band phenomena gives addnl. support for the (n,m) assignment from resonance Raman spectroscopy.
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  35. 35
    Xie, L.; Wang, H.; Jin, C.; Wang, X.; Jiao, L.; Suenaga, K.; Dai, H. Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties. J. Am. Chem. Soc. 2011, 133 (27), 10394– 10397,  DOI: 10.1021/ja203860a
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    Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties
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    Physical Review B: Condensed Matter and Materials Physics (2011), 84 (8), 085318/1-085318/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The authors report polarized Raman scattering and resonant Raman scattering studies on single InAs nanowires. Polarized Raman expts. show that the highest scattering intensity is obtained when both the incident and analyzed light polarizations are perpendicular to the nanowire axis. InAs wurtzite optical modes are obsd. The obtained wurtzite modes are consistent with the selection rules and also with the results of calcns. using an extended rigid-ion model. Addnl. resonant Raman scattering expts. reveal a red-shifted E1 transition for InAs nanowires compared to the bulk Zn-blende InAs transition due to the dominance of the wurtzite phase in the nanowires. Ab initio calcns. of the electronic band structure for wurtzite and Zn-blende InAs phases corroborate the obsd. values for the E1 transitions.
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    Wu, J.-B.; Zhao, H.; Li, Y.; Ohlberg, D.; Shi, W.; Wu, W.; Wang, H.; Tan, P.-H. Monolayer Molybdenum Disulfide Nanoribbons with High Optical Anisotropy. Adv. Opt. Mater. 2016, 4 (5), 756– 762,  DOI: 10.1002/adom.201500707
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    Advanced Optical Materials (2016), 4 (5), 756-762CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    2D molybdenum disulfide (MoS2) has shown promising prospects for the next generation electronics and optoelectronics devices. The monolayer MoS2 can be patterned into quasi-1D anisotropic MoS2 nanoribbons (MNRs), in which theor. calcns. have predicted novel properties. However, little work has been carried out in the exptl. exploration of MNRs with a width of less than 20 nm where the geometrical confinement can lead to interesting phenomenons. Here, MNRs are prepd. with width between 5 and 15 nm by direct helium ion beam milling. High optical anisotropy of these MNRs is revealed by the systematic study of optical contrast and Raman spectroscopy. The Raman modes in MNRs show strong polarization dependence. Besides that the E' and A'1 peaks are broadened by the phonon-confinement effect, the modes corresponding to singularities of vibrational d. of states are activated by edges. The peculiar polarization behavior of Raman modes can be explained by the anisotropy of light absorption in MNRs, which is evidenced by the polarized optical contrast. The study opens the possibility to explore quasi-1D materials with high optical anisotropy from isotropic 2D family of transition metal dichalcogenides.
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    Nature communications (2015), 6 (), 8892 ISSN:.
    Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals.
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    Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations
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    Guided Growth of Horizontal ZnS Nanowires on Flat and Faceted Sapphire Surfaces
    Rothman, Amnon; Forsht, Tamir; Danieli, Yarden; Popovitz-Biro, Ronit; Rechav, Katya; Houben, Lothar; Joselevich, Ernesto
    Journal of Physical Chemistry C (2018), 122 (23), 12413-12420CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The surface-guided growth of horizontal nanowires (NWs) allows assembly and alignment of the NWs on the substrate during the synthesis, thus eliminating the need for addnl. processes after growth. One of the major advantages of guided growth over postgrowth assembly is the control on the NWs direction, crystallog. orientation, and position. The authors use the guided growth approach to synthesize high-quality, single-crystal, aligned horizontal ZnS NWs on flat and faceted sapphire surfaces, and show how the crystal planes of the different substrates affects the crystal structure and orientation of the NWs. The authors also show initial results of the effect of Cu doping on their photoluminescence. Such high-quality aligned ZnS NWs can potentially be assembled as key components in phosphorescent displays and markers due to their unique optical properties. The ZnS NWs have either wurtzite or Zn-blende structure depending on the substrate orientations and contain intrinsic point defects such as S vacancies, which are common in this material. The crystallog. orientations are consistent with those of guided NWs from other semiconductor materials, demonstrating the generality of the guided growth phenomenon. The successfully grown ZnS NWs and the Cu doping are the 1st step toward the fabrication of optoelectronic devices based on ZnS nanostructures.
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  1. Yohei Yomogida, Mai Nagano, Hiromu Hamasaki, Kaori HIRAHARA, Yasumitsu Miyata, Kazuhiro Yanagi. Synthesis of relatively small-diameter tungsten ditelluride nanowires from solution-grown tungsten oxide nanowires. Japanese Journal of Applied Physics 2021, https://doi.org/10.35848/1347-4065/abe201

  • Abstract

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    Figure 1

    Figure 1. 1D vdW building block. (a) Schematic diagram showing 2D and 3D assemblies of TMC nanowires having the composition MTe, where M = Mo or W. Each individual wire comprises an elongated network of staggered M3Te3 triangular units. Adjacent nanowires interact via vdW forces and either extend laterally, forming a 2D layer, or combine into 3D bundles of various size. (b, c) Cross-section HAADF-STEM images of monolayer and bilayer sheets of WTe nanowires grown on a-plane sapphire substrates. (d) Cross-section TEM image of a single WTe 3D bundle grown on a SiO2/Si substrate. (e) HAADF-STEM image showing the cross-section of a single WTe nanowire. The W core of heavier atomic mass is clearly seen as a bright hexagonal ring surrounded by Te atoms. (f) Atomic resolution HAADF-STEM image of suspended WTe nanowires, illustrating atomic alignment along the nanowire axis.

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    Figure 2

    Figure 2. Wafer-scale CVD growth of WTe nanowires. (a) Schematic illustration of the setup used for nanowire synthesis. Growth is conducted via vapor-phase evaporation of solid precursors with the aid of an alkali metal salt as a growth promoter. (b) Plan-view SEM image of the unoriented WTe nanowire arrays grown on a SiO2/Si substrate. (c) Atomic force microscopy (AFM) topographic image of a dense, ordered array of WTe nanowires synthesized on an a-plane sapphire substrate. The insets show the photographic images of the substrate following nanowire growth, which exhibit a slight yellowish (in b) or brownness (in c). The substrate dimensions are approximately 2.5 cm × 3 and 1 cm × 1 cm for (b) and (c), respectively.

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    Figure 3

    Figure 3. 1D optical response. (a) Normalized Raman spectra for the synthesized MoTe and WTe nanowires on SiO2/Si. (b) Raman spectra for a single WTe 3D bundle recorded at four representative polarization angles of 0°, 30°, 60°, and 90°. The inset shows an optical image of the sample measured. Theta (θ) refers to the angle between the polarization of the incident laser and the nanowire axis. The spectra in (a) and (b) are shifted vertically for comparison. (c) Polarization-dependent normalized Raman intensity map for corresponding spectra in (b) taken from 0° to 360°. (d) Normalized Raman intensity polar plots for WTe nanowires in (b) at 155 cm–1 (blue) and 196 cm–1 (red). The solid line is provided as a guide to the eye. Normalization was performed with respect to the peak at 155 cm–1 in (a) and the same peak of highest intensity within the range of 0° to 360° in (c) and (d).

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    Figure 4

    Figure 4. Electron transport properties. I–V characteristic curves for (a) a single WTe 3D bundle (Figure 1d) and (b) a WTe thin film grown on a SiO2/Si substrate measured at room temperature. The insets in (a) and (b) show an SEM image of the four-point probe measurement and an optical image of a device fabricated with a channel length of 20 μm, respectively. (c) Temperature dependence of the electrical resistance of the WTe network on a SiO2/Si substrate under zero magnetic field (B = 0). Black and red curves represent the data taken on heating and cooling, as indicated by the arrows of corresponding color. The overlapping curves indicate reproducibility of the measurements. (d) Magnetoresistance as a function of B2 for the corresponding sample in (c). Solid curves correspond to linear (40 to 80 K) and power law (10 to 30 K) fits.

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    Figure 5

    Figure 5. Electronic band structures. DFT-calculated band structures of (a) an isolated single WTe nanowire, (b) its 2D monolayer, and the 3D bundle along (c) Y−Γ–B line and (d) Γ–Z line, respectively. Fermi level is set to zero as a reference for (b–d). An isolated WTe nanowire is a narrow-gap semiconductor with an indirect gap of 18 meV (an enlarged image is shown in Figure S13a), whereas its collected 2D and 3D assemblies are metals. Squared wave functions of the CBM and VBM at Γ point with energies of (e) 0.02 and 0 eV in (a) and of (f) 0.1 and 0 eV in (b), respectively.

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  • References

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      The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
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    4. 4
      Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102 (30), 10451– 10453,  DOI: 10.1073/pnas.0502848102
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      4
      Two-dimensional atomic crystals
      Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K.
      Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (30), 10451-10453CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The authors report free-standing at. crystals that are strictly 2-dimensional and can be viewed as individual at. planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromech. cleavage, the authors prepd. and studied a variety of 2-dimensional crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2-dimensional mols. unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
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    5. 5
      Potel, M.; Chevrel, R.; Sergent, M.; Armici, J. C.; Decroux, M.; Fischer, Ø. New Pseudo-One-Dimensional Metals: M2Mo6Se6 (M = Na, In, K, TI), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, TI). J. Solid State Chem. 1980, 35 (2), 286– 290,  DOI: 10.1016/0022-4596(80)90505-8
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      5
      New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na, In, K, Tl), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, Tl)
      Potel, M.; Chevrel, R.; Sergent, M.; Armici, J. C.; Decroux, M.; Fischer, O.
      Journal of Solid State Chemistry (1980), 35 (2), 286-90CODEN: JSSCBI; ISSN:0022-4596.
      A new series of ternary chalcogenides, derived from divalent Mo, M2Mo6X6 is derived. These compds. crystallize in a hexagonal lattice space group P63/n, with a ≈9 and c ≈ 4.5 Å. The compds. are characterized by clusters (Mo3)∞1 in the form of linear chains, resulting from a linear condensation of Mo6 octahedral clusters. The (Mo3)∞ clusters are well sepd. from each other, with the shortest Mo-Mo intercluster distance >6 Å. The resulting pseudo-one-dimensional structures show remarkable anisotropy of phys. properties.
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    6. 6
      Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J. Facile Synthesis of Pseudo One-Monodimensional Ternary Molybdenum Chalcogenides M2Mo6X6 (X = Se, Te; M = Li, Na. Cs). Mater. Res. Bull. 1984, 19 (7), 915– 924,  DOI: 10.1016/0025-5408(84)90054-0
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      6
      A facile synthesis of pseudo one-dimensional ternary molybdenum chalcogenides M2Mo6X6 (X = Se,Te; M = Li,Na,...,Cs)
      Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J.
      Materials Research Bulletin (1984), 19 (7), 915-24CODEN: MRBUAC; ISSN:0025-5408.
      Ion-exchange reactions of In2Mo6X6 with alkali iodide and chloride salts at 550° to give M2Mo6X6 and mixed solid solns. In2-xMxMo6X6 (M = Li, Na, K, Rb, Cs; X = Se, Te) are described. Ion exchange provides a low-temp. route to prep. phases that are not stable at high temp., such as In2-xMxMo6Se6, where 0 ≤ x ≤ 2 and M = alkali. When M = Li, the Li can be partially removed by oxidn. with I2, producing nonstoichiometric ternary Mo chalcogenides In2-xMo6Se6 (0 ≤ x ≤ 1.8). Only Tl2Mo6Se6 is superconducting above 1 K.
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    7. 7
      Tarascon, J. M. Mo6Se6: A New Solid-State Electrode for Secondary Lithium Batteries. J. Electrochem. Soc. 1985, 132 (9), 2089,  DOI: 10.1149/1.2114297
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      7
      Molybdenum selenide (Mo6Se6): a new solid-state electrode for secondary lithium batteries
      Tarascon, J. M.
      Journal of the Electrochemical Society (1985), 132 (9), 2089-93CODEN: JESOAN; ISSN:0013-4651.
      Mo6Si6 can be used both as cathode and anode in secondary Li cells and can take up reversibly 9 Li atoms/Mo6Se6 without losing its linear chain structure, leading to a theor. vol. energy d. of ∼1.1 W-h/cm3, compared to 1.2 for TiS2 cathodes. Another important promising possibility is the use of LixMo6Se6 as the anode instead of Li in secondary Li cells.
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    8. 8
      Venkataraman, L.; Lieber, C. M. Molybdenum Selenide Molecular Wires as One-Dimensional Conductors. Phys. Rev. Lett. 1999, 83 (25), 5334– 5337,  DOI: 10.1103/PhysRevLett.83.5334
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      8
      Molybdenum selenide molecular wires as one-dimensional conductors
      Venkataraman, Latha; Lieber, Charles M.
      Physical Review Letters (1999), 83 (25), 5334-5337CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Scanning tunneling microscopy has been used to characterize the structure and electronic properties of molybdenum selenide (Mo6Se6) mol. wires. STM images resolve the 0.45 nm unit cell repeat in isolated mol. wires and bundles of wires. Tunneling spectroscopy measurements exhibit sharp peaks in the local d. of states that are consistent with the Van Hove singularities expected for a one-dimensional system. These data are compared with those of previous band-structure calcns. Preliminary measurements at 5 K show no evidence of a gap opening at low temps. The implications of these results to metal-insulator transitions in one-dimensional systems are discussed.
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    9. 9
      Venkataraman, L.; Hong, Y. S.; Kim, P. Electron Transport in a Multichannel One-Dimensional Conductor: Molybdenum Selenide Nanowires. Phys. Rev. Lett. 2006, 96 (7), 076601,  DOI: 10.1103/PhysRevLett.96.076601
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      9
      Electron transport in a multichannel one-dimensional conductor. Molybdenum selenide nanowires
      Venkataraman, Latha; Hong, Yeon Suk; Kim, Philip
      Physical Review Letters (2006), 96 (7), 076601/1-076601/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We have measured electron transport in small bundles of identical conducting MoSe nanowires where the no. of weakly interacting 1D chains ranges from 1 to 300. The linear conductance and current in these nanowires exhibit a power-law dependence on temp. and bias voltage, resp. The exponents governing these power laws decrease as the no. of conducting channels increase. These exponents can be related to the electron-electron interaction parameter for transport in multichannel 1D systems with a few defects.
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    10. 10
      Xia, Y.; Wang, B.; Zhang, J.; Jin, Y.; Tian, H.; Ho, W.; Xu, H.; Jin, C.; Xie, M. Quantum Confined Tomonaga–Luttinger Liquid in Mo6Se6 Nanowires Converted from an Epitaxial MoSe2 Monolayer. Nano Lett. 2020, 20 (3), 2094– 2099,  DOI: 10.1021/acs.nanolett.0c00090
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      10
      Quantum Confined Tomonaga-Luttinger Liquid in Mo6Se6 Nanowires Converted from an Epitaxial MoSe2 Monolayer
      Xia, Yipu; Wang, Bo; Zhang, Junqiu; Jin, Yuanjun; Tian, Hao; Ho, Wingkin; Xu, Hu; Jin, Chuanhong; Xie, Maohai
      Nano Letters (2020), 20 (3), 2094-2099CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Confining interacting particles in one-dimension (1D) changes the electronic behavior of the system fundamentally, which has been studied extensively in the past. Examples of 1D metallic systems include carbon nanotubes, quasi-1D org. conductors, metal chains, and domain boundary defects in monolayer thick transition-metal dichalcogenides such as MoSe2. Here single and bundles of Mo6Se6 nanowires were fabricated through annealing a MoSe2 monolayer grown by mol.-beam epitaxy on graphene. Conversion from two-dimensional (2D) MoSe2 film to 1D Mo6Se6 nanowire is reversible. Mo6Se6 nanowires form preferentially at the Se-terminated zigzag edges of MoSe2 and stitch to it via two distinct at. configurations. The Mo6Se6 wire is metallic and its length is tunable, which represents one of few 1D systems that exhibit properties pertinent to quantum confined Tomonaga-Luttinger liq., as evidenced by scanning tunneling microscopic and spectroscopic studies.
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    11. 11
      Cheng, F.; Xu, H.; Xu, W.; Zhou, P.; Martin, J.; Loh, K. P. Controlled Growth of 1D MoSe2 Nanoribbons with Spatially Modulated Edge States. Nano Lett. 2017, 17 (2), 1116– 1120,  DOI: 10.1021/acs.nanolett.6b04715
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      11
      Controlled Growth of 1D MoSe2 Nanoribbons with Spatially Modulated Edge States
      Cheng, Fang; Xu, Hai; Xu, Wentao; Zhou, Pinjia; Martin, Jens; Loh, Kian Ping
      Nano Letters (2017), 17 (2), 1116-1120CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Two-dimensional (2D) transition metal dichalcogenides (TMDCs) possess interesting one-dimensional (1D) properties at its edges and inversion domain boundaries, where properties markedly different from the 2D basal plane, such as 1D metallicity and charge d. waves, can be obsd. Although 2D TMDCs crystals are widely grown by chem. vapor deposition (CVD), the fabrication of 1D TMDCs ribbons is challenging due to the difficulty to confine growth in only one dimension. Here we report the controlled growth of MoSe2 nanoribbons with an aspect ratio >100 by using prepatterned Se reconstructions on Au(100). Using scanning tunneling microscope and spectroscopy (STM/STS), the at. and electronic structure of MoSe2 nanoribbons are studied. The ultranarrow ribbons show metallic behavior, while wider ribbons show a crossover from metallic to semiconducting behavior going from the edge to the center of the ribbon. The obsd. conductance modulations of the ultranarrow ribbons are attributed to 1D Moir´e pattern. Remarkably, it shows a different periodicity compared with the 2D Moir´e pattern in wider ribbons indicating that the 1D system is softened due to the high ratio of edge to basal plane bonds. Further, we demonstrated that the nanoribbons are stable against ambient conditions, which suggests that 1D TMDCs can be exploited for further applications.
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    12. 12
      Poh, S. M.; Tan, S. J. R.; Zhao, X.; Chen, Z.; Abdelwahab, I.; Fu, D.; Xu, H.; Bao, Y.; Zhou, W.; Loh, K. P. Large Area Synthesis of 1D-MoSe2 Using Molecular Beam Epitaxy. Adv. Mater. 2017, 29 (12), 1605641,  DOI: 10.1002/adma.201605641
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      There is no corresponding record for this reference.
    13. 13
      Chowdhury, T.; Kim, J.; Sadler, E. C.; Li, C.; Lee, S. W.; Jo, K.; Xu, W.; Gracias, D. H.; Drichko, N. V.; Jariwala, D.; Brintlinger, T. H.; Mueller, T.; Park, H.-G.; Kempa, T. J. Substrate-Directed Synthesis of MoS2 Nanocrystals with Tunable Dimensionality and Optical Properties. Nat. Nanotechnol. 2020, 15 (1), 29– 34,  DOI: 10.1038/s41565-019-0571-2
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      13
      Substrate-directed synthesis of MoS2 nanocrystals with tunable dimensionality and optical properties
      Chowdhury, Tomojit; Kim, Jungkil; Sadler, Erick C.; Li, Chenyang; Lee, Seong Won; Jo, Kiyoung; Xu, Weinan; Gracias, David H.; Drichko, Natalia V.; Jariwala, Deep; Brintlinger, Todd H.; Mueller, Tim; Park, Hong-Gyu; Kempa, Thomas J.
      Nature Nanotechnology (2020), 15 (1), 29-34CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
      Two-dimensional transition-metal dichalcogenide (TMD) crystals are a versatile platform for optoelectronic, catalytic, and quantum device studies. However, the ability to tailor their phys. properties through explicit synthetic control of their morphol. and dimensionality is a major challenge. We demonstrate a gas-phase synthesis method that substantially transforms the structure and dimensionality of TMD crystals without lithog. Synthesis of MoS2 on Si(001) surfaces pre-treated with phosphine yields high-aspect-ratio nanoribbons of uniform width. We systematically control the width of these nanoribbons at 50-430 nm by varying the total phosphine dosage during the surface treatment step. Aberration-cor. electron microscopy reveals that the nanoribbons are predominantly 2H phase with zig-zag edges and an edge quality that is comparable to, or better than, that of graphene and TMD nanoribbons prepd. through conventional top-down processing. Owing to their restricted dimensionality, the nominally 1-dimensional MoS2 nanocrystals exhibit photoluminescence 50 meV higher in energy than that from 2-dimensional MoS2 crystals. Moreover, this emission is precisely tunable through synthetic control of crystal width. Directed crystal growth on designer substrates has the potential to enable the prepn. of low-dimensional materials with prescribed morphologies and tunable or emergent optoelectronic properties.
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    14. 14
      Lin, J.; Cretu, O.; Zhou, W.; Suenaga, K.; Prasai, D.; Bolotin, K. I.; Cuong, N. T.; Otani, M.; Okada, S.; Lupini, A. R.; Idrobo, J.-C.; Caudel, D.; Burger, A.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Pennycook, S. J.; Pantelides, S. T. Flexible Metallic Nanowires with Self-Adaptive Contacts to Semiconducting Transition-Metal Dichalcogenide Monolayers. Nat. Nanotechnol. 2014, 9 (6), 436– 442,  DOI: 10.1038/nnano.2014.81
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      14
      Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers
      Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I.; Nguyen, Thanh Cuong; Otani, Minoru; Okada, Susumu; Lupini, Andrew R.; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J.; Yan, Jiaqiang; Mandrus, David G.; Pennycook, Stephen J.; Pantelides, Sokrates T.
      Nature Nanotechnology (2014), 9 (6), 436-442CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) were predicted to be metallic. Such nanowires were chem. synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a 2-dimensional integrated circuit, has so far remained elusive. By steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometer in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ elec. measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mech. deformations, thus showing promise for mech. robust flexible electronics. D. functional theory calcns. further confirm the metallicity of the nanowires and account for their beam-induced mech. behavior. These results show that direct patterning of 1D conducting nanowires in 2D semiconducting materials with nanometer precision is possible using electron-beam-based techniques.
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    15. 15
      Zhu, H.; Wang, Q.; Zhang, C.; Addou, R.; Cho, K.; Wallace, R. M.; Kim, M. J. New Mo6Te6 Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe2. Adv. Mater. 2017, 29 (18), 1606264,  DOI: 10.1002/adma.201606264
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      There is no corresponding record for this reference.
    16. 16
      Kibsgaard, J.; Tuxen, A.; Levisen, M.; Lægsgaard, E.; Gemming, S.; Seifert, G.; Lauritsen, J. V.; Besenbacher, F. Atomic-Scale Structure of Mo6S6 Nanowires. Nano Lett. 2008, 8 (11), 3928– 3931,  DOI: 10.1021/nl802384n
      [ACS Full Text ACS Full Text], [CAS], Google Scholar
      16
      Atomic-Scale Structure of Mo6S6 Nanowires
      Kibsgaard, Jakob; Tuxen, Anders; Levisen, Martin; Laegsgaard, Erik; Gemming, Sibylle; Seifert, Gotthard; Lauritsen, Jeppe V.; Besenbacher, Flemming
      Nano Letters (2008), 8 (11), 3928-3931CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We have studied the at.-scale structure of the Mo6S6 nanowires using scanning tunneling microscopy and spectroscopy (STM and STS) and d. functional theory (DFT). A novel synthesis route based on metallic Mo precursors is presented for the selective formation of elementary pure Mo6S6 nanowires. The Mo6S6 nanowires selectively organize as trimer bundles, and each of the Mo6S6 nanowires consists of an elec. conducting Mo backbone dressed with a sulfur exterior cap. The Mo6S6 nanowires may thus be of interest as novel building blocks in nanoelectronics because the Mo6S6 nanowires exist in a robust, singular structural conformation with uniquely defined elec. (metallic) properties.
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    17. 17
      Yu, Y.; Wang, G.; Tan, Y.; Wu, N.; Zhang, X.-A.; Qin, S. Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires and Two-Dimensional MoTe2 Ultrathin Films Heterostructures. Nano Lett. 2018, 18 (2), 675– 681,  DOI: 10.1021/acs.nanolett.7b03058
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      17
      Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires and Two-Dimensional MoTe2 Ultrathin Films Heterostructures
      Yu, Yayun; Wang, Guang; Tan, Yuan; Wu, Nannan; Zhang, Xue-Ao; Qin, Shiqiao
      Nano Letters (2018), 18 (2), 675-681CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Controllable synthesizing of 1-dimensional-two-dimensional (1-dimensional-2D) heterostructures and tuning their at. and electronic structures is nowadays of particular interest due to the extraordinary properties and potential applications. Here, the authors demonstrate the temp.-induced phase-controlled growth of 1-dimensional Mo6Te6-2D MoTe2 heterostructures via MBE. In situ scanning tunneling microscopy study shows 2-dimensional ultrathin films are synthesized at low temp. range, while 1-dimensional nanowires gradually arise and dominate as temp. increasing. XPS confirms the good stoichiometry and scanning tunneling spectroscopy reveals the semimetallic property of grown Mo6Te6 nanowires. Through in situ annealing, a phase transition from 2-dimensional MoTe2 to 1-dimensional Mo6Te6 is induced, thus forming a semimetal-semiconductor junction in at. level. An upward band bending of 2H-MoTe2 is caused by lateral hole injection from Mo6Te6. The work suggests a new route to synthesize 1-dimensional semimetallic transition metal chalcogenide nanowires, which could serve as ultrasmall conducting building blocks and enable band engineering in future 1-dimensional-2D heterostructure devices.
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    18. 18
      Nagata, M.; Shukla, S.; Nakanishi, Y.; Liu, Z.; Lin, Y.-C.; Shiga, T.; Nakamura, Y.; Koyama, T.; Kishida, H.; Inoue, T.; Kanda, N.; Ohno, S.; Sakagawa, Y.; Suenaga, K.; Shinohara, H. Isolation of Single-Wired Transition-Metal Monochalcogenides by Carbon Nanotubes. Nano Lett. 2019, 19 (8), 4845– 4851,  DOI: 10.1021/acs.nanolett.8b05074
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      18
      Isolation of Single-Wired Transition-Metal Monochalcogenides by Carbon Nanotubes
      Nagata, Masataka; Shukla, Shivani; Nakanishi, Yusuke; Liu, Zheng; Lin, Yung-Chang; Shiga, Takuma; Nakamura, Yuto; Koyama, Takeshi; Kishida, Hideo; Inoue, Tsukasa; Kanda, Naoyuki; Ohno, Shun; Sakagawa, Yuki; Suenaga, Kazu; Shinohara, Hisanori
      Nano Letters (2019), 19 (8), 4845-4851CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The successful isolation of single layers from two-dimensional (2D) van der Waals (vdW)-layered materials has opened new frontiers in condensed matter physics and materials science. Their discovery and unique properties laid the foundation for exploring 1D counterparts. However, the isolation of 1D vdW-wired materials has thus far remained a challenge, and effective techniques are demanded. Here we report the facile synthesis of isolated transition-metal monochalcogenide MoTe nanowires by using carbon nanotubes (CNTs) as molds. Individual nanowires are perfectly sepd. by CNTs with a minimal interaction, enabling detailed characterization of the single wires. Transmission electron microscopy revealed unusual torsional motion of MoTe nanowires inside CNTs. Confinement of 1D vdW-wired materials to the nanotest tubes might open up possibilities for exploring unprecedented properties of the nanowires and their potential applications such as electromech. switching devices.
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    19. 19
      Kanda, N.; Nakanishi, Y.; Liu, D.; Liu, Z.; Inoue, T.; Miyata, Y.; Tománek, D.; Shinohara, H. Efficient Growth and Characterization of One-Dimensional Transition Metal Tellurides inside Carbon Nanotubes. Nanoscale 2020, 12, 17185– 17190,  DOI: 10.1039/D0NR03129A
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      19
      Efficient growth and characterization of one-dimensional transition metal tellurides inside carbon nanotubes
      Kanda, Naoyuki; Nakanishi, Yusuke; Liu, Dan; Liu, Zheng; Inoue, Tsukasa; Miyata, Yasumitsu; Tomanek, David; Shinohara, Hisanori
      Nanoscale (2020), 12 (33), 17185-17190CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Atomically thin one-dimensional (1D) van der Waals wires of transition metal monochalocogenides (TMMs) have been anticipated as promising building blocks for integrated nanoelectronics. While reliable prodn. of TMM nanowires has eluded scientists over the past few decades, we finally demonstrated a bottom-up fabrication of MoTe nanowires inside carbon nanotubes (CNTs). Still, the current synthesis method is based on vacuum annealing of reactive MoTe2, and limits access to a variety of TMMs. Here we report an expanded framework for high-yield synthesis of the 1D tellurides including WTe, an previously unknown family of TMMs. Exptl. and theor. analyses revealed that the choice of suitable metal oxides as a precursor provides a useful yield for their characterization. These TMM nanowires exhibit a significant optical absorption in the visible-light region. More important, electronic properties of CNTs can be tuned by encapsulating different TMM nanowires.
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    20. 20
      Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes. J. Phys. Chem. B 1999, 103 (31), 6484– 6492,  DOI: 10.1021/jp990957s
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      20
      Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes
      Cassell, Alan M.; Raymakers, Jeffrey A.; Kong, Jing; Dai, Hongjie
      Journal of Physical Chemistry B (1999), 103 (31), 6484-6492CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The synthesis of bulk amts. of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chem. compns. and textural properties of the catalyst material used in the chem. vapor deposition (CVD) of methane. A series of catalysts are derived by systematically varying the catalytic metal compds. and support materials. The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica-alumina multicomponent material. The high SWNT yielding catalyst exhibits high surface-area and large mesopore vol. at elevated temps. Gram quantities of SWNT materials have been synthesized in ∼0.5 h using the optimized catalyst material. The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating. This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes.
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    21. 21
      Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes. Science 2004, 306 (5700), 1362– 1364,  DOI: 10.1126/science.1104962
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      21
      Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes
      Hata, Kenji; Futaba, Don N.; Mizuno, Kohei; Namai, Tatsunori; Yumura, Motoo; Iijima, Sumio
      Science (Washington, DC, United States) (2004), 306 (5700), 1362-1364CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      We demonstrate the efficient chem. vapor deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water. Water-stimulated enhanced catalytic activity results in massive growth of superdense and vertically aligned nanotube forests with heights up to 2.5 mm that can be easily sepd. from the catalysts, providing nanotube material with carbon purity above 99.98%. Moreover, patterned, highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many crit. problems that currently plague carbon nanotube synthesis.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXpvVektbg%253D&md5=757c237fc8cb158d2564d9daf150a906
    22. 22
      Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324 (5932), 1312– 1314,  DOI: 10.1126/science.1171245
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      22
      Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
      Li, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.
      Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
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    23. 23
      Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Özyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5 (8), 574– 578,  DOI: 10.1038/nnano.2010.132
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      23
      Roll-to-roll production of 30-inch graphene films for transparent electrodes
      Bae, Sukang; Kim, Hyeongkeun; Lee, Youngbin; Xu, Xiangfan; Park, Jae-Sung; Zheng, Yi; Balakrishnan, Jayakumar; Lei, Tian; Kim, Hye Ri; Song, Young Il; Kim, Young-Jin; Kim, Kwang S.; Oezyilmaz, Barbaros; Ahn, Jong-Hyun; Hong, Byung Hee; Iijima, Sumio
      Nature Nanotechnology (2010), 5 (8), 574-578CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      The outstanding elec., mech. and chem. properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll prodn. and wet-chem. doping of predominantly monolayer 30-in. graphene films grown by chem. vapor deposition onto flexible copper substrates. The films have sheet resistances as low as ∼125 Ω .box.-1 with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as ∼30 Ω .box.-1 at ∼90% transparency, which is superior to com. transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.
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    24. 24
      Suzuki, H.; Kaneko, T.; Shibuta, Y.; Ohno, M.; Maekawa, Y.; Kato, T. Wafer-Scale Fabrication and Growth Dynamics of Suspended Graphene Nanoribbon Arrays. Nat. Commun. 2016, 7 (1), 11797,  DOI: 10.1038/ncomms11797
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      24
      Wafer-scale fabrication and growth dynamics of suspended graphene nanoribbon arrays
      Suzuki, Hiroo; Kaneko, Toshiro; Shibuta, Yasushi; Ohno, Munekazu; Maekawa, Yuki; Kato, Toshiaki
      Nature Communications (2016), 7 (), 11797CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Adding a mech. degree of freedom to the elec. and optical properties of atomically thin materials can provide an excellent platform to investigate various optoelec. physics and devices with mech. motion interaction. The large scale fabrication of such atomically thin materials with suspended structures remains a challenge. Here we demonstrate the wafer-scale bottom-up synthesis of suspended graphene nanoribbon arrays (over 1,000,000 graphene nanoribbons in 2 × 2 cm2 substrate) with a very high yield (over 98%). Polarized Raman measurements reveal graphene nanoribbons in the array can have relatively uniform-edge structures with near zigzag orientation dominant. A promising growth model of suspended graphene nanoribbons is also established through a comprehensive study that combined expts., mol. dynamics simulations and theor. calcns. with a phase-diagram anal. We believe that our results can contribute to pushing the study of graphene nanoribbons into a new stage related to the optoelec. physics and industrial applications.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XptV2isrk%253D&md5=0cc7b6ba3ecae0066b2e42ba346cd494
    25. 25
      Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu, Y.-C.; Wang, J. T.-W.; Chang, C.-S.; Li, L.-J.; Lin, T.-W. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24 (17), 2320– 2325,  DOI: 10.1002/adma.201104798
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      25
      Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition
      Lee, Yi-Hsien; Zhang, Xin-Quan; Zhang, Wenjing; Chang, Mu-Tung; Lin, Cheng-Te; Chang, Kai-Di; Yu, Ya-Chu; Wang, Jacob Tse-Wei; Chang, Chia-Seng; Li, Lain-Jong; Lin, Tsung-Wu
      Advanced Materials (Weinheim, Germany) (2012), 24 (17), 2320-2325CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Large-area MoS2 at. layers are synthesized on SiO2 substrates by chem. vapor deposition using MoO3 and S powders as the reactants. Optical, microscopic and elec. measurements suggest that the synthetic process leads to the growth of MoS2 monolayer. The TEM images verify that the synthesized MoS2 sheets are highly cryst. To check for elec. performance bottom-gated transistors on silica/silicon using photolithog. was fabricated directly on top of the MoS2 sheets. The transfer curve (drain current vs. gate voltage) was computed and field effect mobility was detd. from anal. of the curve.
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    26. 26
      Liu, K.-K.; Zhang, W.; Lee, Y.-H.; Lin, Y.-C.; Chang, M.-T.; Su, C.-Y.; Chang, C.-S.; Li, H.; Shi, Y.; Zhang, H.; Lai, C.-S.; Li, L.-J. Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12 (3), 1538– 1544,  DOI: 10.1021/nl2043612
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      26
      Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates
      Liu, Keng-Ku; Zhang, Wenjing; Lee, Yi-Hsien; Lin, Yu-Chuan; Chang, Mu-Tung; Su, Ching-Yuan; Chang, Chia-Seng; Li, Hai; Shi, Yumeng; Zhang, Hua; Lai, Chao-Sung; Li, Lain-Jong
      Nano Letters (2012), 12 (3), 1538-1544CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS2 at. thin layers is still rare. Here we report that the high-temp. annealing of a thermally decompd. ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS2 thin layers with superior elec. performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS2 sheets are highly cryst. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS2 layer is comparable with those of the micromechanically exfoliated thin sheets from MoS2 crystals. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.
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    27. 27
      Zhan, Y.; Liu, Z.; Najmaei, S.; Ajayan, P. M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate. Small 2012, 8 (7), 966– 971,  DOI: 10.1002/smll.201102654
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      27
      Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate
      Zhan, Yongjie; Liu, Zheng; Najmaei, Sina; Ajayan, Pulickel M.; Lou, Jun
      Small (2012), 8 (7), 966-971CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Monolayer Mo disulfide (MoS2), a two-dimensional crystal with a direct bandgap, is a promising candidate for 2-dimensional nanoelectronic devices complementing graphene. There were recent attempts to produce MoS2 layers via chem. and mech. exfoliation of bulk material. Here the authors demonstrate the large area growth of MoS2 at. layers on SiO2 substrates by a scalable CVD method. The as-prepd. samples can either be readily used for further device fabrication or be easily released from SiO2 and transferred to arbitrary substrates. High resoln. TEM and Raman spectroscopy on the as grown films of MoS2 indicate that the no. of layers range from single layer to a few layers. The authors' results on the direct growth of MoS2 layers on dielec. leading to facile device fabrication possibilities show the expanding set of useful 2-dimensional at. layers, on the heels of graphene, which can be controllably synthesized and manipulated for many applications.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xitlyht70%253D&md5=a39ddda9639d32ad598a487b777d1968
    28. 28
      Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520 (7549), 656– 660,  DOI: 10.1038/nature14417
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      28
      High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity
      Kang, Kibum; Xie, Saien; Huang, Lujie; Han, Yimo; Huang, Pinshane Y.; Mak, Kin Fai; Kim, Cheol-Joo; Muller, David; Park, Jiwoong
      Nature (London, United Kingdom) (2015), 520 (7549), 656-660CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the at., sub-nanometer length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technol. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high elec. carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technol. relevant scale without film transfer. In addn., their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high elec. performance remains an unsolved challenge. Here we report the prepn. of high-mobility 4-in. wafer-scale films of monolayer molybdenum disulfide (MoS2) and tungsten disulfide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal-org. chem. vapor deposition technique, and show high elec. performance, including an electron mobility of 30 cm2 V-1 s-1 at room temp. and 114 cm2 V-1 s-1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.
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    29. 29
      Li, S.; Wang, S.; Tang, D.-M.; Zhao, W.; Xu, H.; Chu, L.; Bando, Y.; Golberg, D.; Eda, G. Halide-Assisted Atmospheric Pressure Growth of Large WSe2 and WS2 Monolayer Crystals. Appl. Mater. Today 2015, 1 (1), 60– 66,  DOI: 10.1016/j.apmt.2015.09.001
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      Zhou, J.; Lin, J.; Huang, X.; Zhou, Y.; Chen, Y.; Xia, J.; Wang, H.; Xie, Y.; Yu, H.; Lei, J.; Wu, D.; Liu, F.; Fu, Q.; Zeng, Q.; Hsu, C.-H.; Yang, C.; Lu, L.; Yu, T.; Shen, Z.; Lin, H.; Yakobson, B. I.; Liu, Q.; Suenaga, K.; Liu, G.; Liu, Z. A Library of Atomically Thin Metal Chalcogenides. Nature 2018, 556 (7701), 355– 359,  DOI: 10.1038/s41586-018-0008-3
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      A library of atomically thin metal chalcogenides
      Zhou, Jiadong; Lin, Junhao; Huang, Xiangwei; Zhou, Yao; Chen, Yu; Xia, Juan; Wang, Hong; Xie, Yu; Yu, Huimei; Lei, Jincheng; Wu, Di; Liu, Fucai; Fu, Qundong; Zeng, Qingsheng; Hsu, Chuang-Han; Yang, Changli; Lu, Li; Yu, Ting; Shen, Zexiang; Lin, Hsin; Yakobson, Boris I.; Liu, Qian; Suenaga, Kazu; Liu, Guangtong; Liu, Zheng
      Nature (London, United Kingdom) (2018), 556 (7701), 355-359CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting phys. phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional supercond.5, suggesting potential applications for functional devices6-10. However, of the numerous compds. available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11-15, selenization16,17 and tellurization18 of metals and metal compds. Many TMCs are difficult to produce because of the high m.ps. of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temp.19 and this approach20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chem. vapor deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compds., including 32 binary compds. (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compds. We elaborate how the salt decreases the m.p. of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of the synthesized materials in our library are useful, as supported by evidence of supercond. in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.
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    31. 31
      Li, S.; Lin, Y.-C.; Zhao, W.; Wu, J.; Wang, Z.; Hu, Z.; Shen, Y.; Tang, D.-M.; Wang, J.; Zhang, Q.; Zhu, H.; Chu, L.; Zhao, W.; Liu, C.; Sun, Z.; Taniguchi, T.; Osada, M.; Chen, W.; Xu, Q.-H.; Wee, A. T. S.; Suenaga, K.; Ding, F.; Eda, G. Vapour–Liquid–Solid Growth of Monolayer MoS2 Nanoribbons. Nat. Mater. 2018, 17 (6), 535– 542,  DOI: 10.1038/s41563-018-0055-z
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      31
      Vapour-liquid-solid growth of monolayer MoS2 nanoribbons
      Li, Shisheng; Lin, Yung-Chang; Zhao, Wen; Wu, Jing; Wang, Zhuo; Hu, Zehua; Shen, Youde; Tang, Dai-Ming; Wang, Junyong; Zhang, Qi; Zhu, Hai; Chu, Leiqiang; Zhao, Weijie; Liu, Chang; Sun, Zhipei; Taniguchi, Takaaki; Osada, Minoru; Chen, Wei; Xu, Qing-Hua; Wee, Andrew Thye Shen; Suenaga, Kazu; Ding, Feng; Eda, Goki
      Nature Materials (2018), 17 (6), 535-542CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Chem. vapor deposition of two-dimensional materials typically involves the conversion of vapor precursors to solid products in a vapor-solid-solid mode. Here, we report the vapor-liq.-solid growth of monolayer MoS2, yielding highly cryst. ribbons with a width of few tens to thousands of nanometers. This vapor-liq.-solid growth is triggered by the reaction between MoO3 and NaCl, which results in the formation of molten Na-Mo-O droplets. These droplets mediate the growth of MoS2 ribbons in the 'crawling mode' when satd. with sulfur. The locally well-defined orientations of the ribbons reveal the regular horizontal motion of the droplets during growth. Using at.-resoln. scanning transmission electron microscopy and second harmonic generation microscopy, we show that the ribbons are grown homoepitaxially on monolayer MoS2 with predominantly 2H- or 3R-type stacking. Our findings highlight the prospects for the controlled growth of atomically thin nanostructure arrays for nanoelectronic devices and the development of unique mixed-dimensional structures.
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    32. 32
      Li, S.; Lin, Y.-C.; Liu, X.-Y.; Hu, Z.; Wu, J.; Nakajima, H.; Liu, S.; Okazaki, T.; Chen, W.; Minari, T.; Sakuma, Y.; Tsukagoshi, K.; Suenaga, K.; Taniguchi, T.; Osada, M. Wafer-Scale and Deterministic Patterned Growth of Monolayer MoS2 via Vapor–Liquid–Solid Method. Nanoscale 2019, 11 (34), 16122– 16129,  DOI: 10.1039/C9NR04612G
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      32
      Wafer-scale and deterministic patterned growth of monolayer MoS2 via vapor-liquid-solid method
      Li, Shisheng; Lin, Yung-Chang; Liu, Xu-Ying; Hu, Zehua; Wu, Jing; Nakajima, Hideaki; Liu, Song; Okazaki, Toshiya; Chen, Wei; Minari, Takeo; Sakuma, Yoshiki; Tsukagoshi, Kazuhito; Suenaga, Kazu; Taniguchi, Takaaki; Osada, Minoru
      Nanoscale (2019), 11 (34), 16122-16129CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chem. vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of vapors. The non-stoichiometric supply of transition-metal precursors and chalcogens leads to poor control in the products' location, morphol., crystallinity, uniformity and batch to batch reproducibility. The vapor-liq.-solid (VLS) growth method often involves molten precursors (e.g., non-volatile Na2MoO4) at growth temps. higher than their m.ps. The liq. Na2MoO4 can ppt. out solid MoS2 monolayers when satd. with sulfur vapor. Taking advantage of the VLS growth, we attained three kinds of important achievements: (i) a 4-in.-wafer-scale uniform growth of MoS2 flakes on SiO2/Si substrates, (ii) a 2-in.-wafer-scale growth of continuous MoS2 film with the grain size exceeding 100μm on sapphire substrates, and (iii) a patterned (site-controlled) growth of MoS2 flakes and films. We clarified that the VLS growth thus paves a new way for the high-efficient and scalable synthesis of two-dimensional TMDC monolayers.
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      Duesberg, G. S.; Loa, I.; Burghard, M.; Syassen, K.; Roth, S. Polarized Raman Spectroscopy on Isolated Single-Wall Carbon Nanotubes. Phys. Rev. Lett. 2000, 85 (25), 5436– 5439,  DOI: 10.1103/PhysRevLett.85.5436
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      33
      Polarized Raman Spectroscopy on Isolated Single-Wall Carbon Nanotubes
      Duesberg, G. S.; Loa, I.; Burghard, M.; Syassen, K.; Roth, S.
      Physical Review Letters (2000), 85 (25), 5436-5439CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Polarized micro-Raman spectroscopy was performed on spatially sepd. single-wall C nanotubes (SWNTs) as individual nanotubes or thin ropes of only a few SWNTs. Different from bulk samples, the Raman spectra are composed of well-resolved peaks which allow a direct comparison of exptl. data with theor. calcns. Orientation-dependent measurements reveal max. intensity of all Raman modes when the nanotubes are aligned parallel to the polarization of the incident laser light. The angular dependences clearly deviate from the selection rules predicted by theor. studies. These differences are attributed to depolarization effects caused by the strongly anisotropic geometry of the nanotubes and to electronic resonance effects for excitation at 633 nm.
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    34. 34
      Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Ünlü, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R. G-Band Resonant Raman Study of 62 Isolated Single-Wall Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65 (15), 155412,  DOI: 10.1103/PhysRevB.65.155412
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      34
      G-band resonant Raman study of 62 isolated single-wall carbon nanotubes
      Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Unlu, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R.
      Physical Review B: Condensed Matter and Materials Physics (2002), 65 (15), 155412/1-155412/9CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors report G-band resonance Raman spectra of single-wall C nanotubes (SWNTs) at the single-nanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diams. dt ranging between 0.95 nm and 2.62 nm, the authors have conclusively detd. the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diam. dependent (ωG+=1591 cm-1), while the lower-frequency peak is given by ωG-=ωG+-C/dt2, with C being different for metallic and semiconducting SWNTs (CM>CS). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1<IωG-/IωG+<0.3 for most of the isolated SWNTs (∼90%). Unusually high or low IωG-/IωG+ ratios are obsd. for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E44S interband transition and scattered photons are in resonance with E33S. Since the Eii values depend sensitively on both nanotube diam. and chirality, the (n,m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theor. predictions and exptl. observations about these special G-band phenomena gives addnl. support for the (n,m) assignment from resonance Raman spectroscopy.
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    35. 35
      Xie, L.; Wang, H.; Jin, C.; Wang, X.; Jiao, L.; Suenaga, K.; Dai, H. Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties. J. Am. Chem. Soc. 2011, 133 (27), 10394– 10397,  DOI: 10.1021/ja203860a
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      35
      Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties
      Xie, Liming; Wang, Hailiang; Jin, Chuanhong; Wang, Xinran; Jiao, Liying; Suenaga, Kazu; Dai, Hongjie
      Journal of the American Chemical Society (2011), 133 (27), 10394-10397CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors studied the at. structures, Raman spectroscopic and elec. transport properties of individual graphene nanoribbons (GNRs, widths ∼ 10-30 nm) derived from sonochem. unzipping of multiwalled carbon nanotubes (MWNTs). Aberration-cor. TEM revealed a high percentage of two-layer (2 L) GNRs and some single-layer ribbons. The layer-layer stacking angles ranged from 0° to 30° including av. chiral angles near 30° (armchair orientation) or 0° (zigzag orientation). A large fraction of GNRs with bent and smooth edges was obsd., while the rest showed flat and less smooth edges (roughness ≤ 1 nm). Polarized Raman spectroscopy probed individual GNRs to reveal D/G ratios and ratios of D band intensities at parallel and perpendicular laser excitation polarization (D‖/D.perp.). The obsd. spectroscopic trends were used to infer the av. chiral angles and edge smoothness of GNRs. Elec. transport and Raman measurements were carried out for individual ribbons to correlate spectroscopic and elec. properties of GNRs.
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    36. 36
      Xiong, Q.; Chen, G.; Gutierrez, H. R.; Eklund, P. C. Raman Scattering Studies of Individual Polar Semiconducting Nanowires: Phonon Splitting and Antenna Effects. Appl. Phys. A: Mater. Sci. Process. 2006, 85 (3), 299– 305,  DOI: 10.1007/s00339-006-3717-7
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      36
      Raman scattering studies of individual polar semiconducting nanowires: phonon splitting and antenna effects
      Xiong, Q.; Chen, G.; Gutierrez, H. R.; Eklund, P. C.
      Applied Physics A: Materials Science & Processing (2006), 85 (3), 299-305CODEN: APAMFC; ISSN:0947-8396. (Springer)
      A review. Results from Raman scattering expts. on individual cryst. GaP nanowires are presented which indicate that the shape of the nanowire, i.e., the high aspect ratio, may be responsible for two new phenomena involving optical phonons: (1) a shape-induced splitting of both the longitudinal optical (LO) and transverse optical (TO) phonons at the center of the Brillouin zone (q = 0), and (2) a Raman scattering "antenna" effect which masks the normal Raman polarization selection rules. We suggest that (1) stems from the asymmetry in the long range dipolar sums that control the electromagnetic LO-TO splitting, and we identify the Raman antenna effect (2) with the internal elec. field created by Mie resonances in the nanowire driven by the incident laser field. Although these effects are reported here for GaP, they are expected to be general effects observable in many semiconducting nanowire systems.
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    37. 37
      Möller, M.; de Lima, M. M.; Cantarero, A.; Dacal, L. C. O.; Madureira, J. R.; Iikawa, F.; Chiaramonte, T.; Cotta, M. A. Polarized and Resonant Raman Spectroscopy on Single InAs Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84 (8), 085318,  DOI: 10.1103/PhysRevB.84.085318
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      37
      Polarized and resonant Raman spectroscopy on single InAs nanowires
      Moller, M.; de Lima, M. M., Jr.; Cantarero, A.; Dacal, L. C. O.; Madureira, J. R.; Iikawa, F.; Chiaramonte, T.; Cotta, M. A.
      Physical Review B: Condensed Matter and Materials Physics (2011), 84 (8), 085318/1-085318/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The authors report polarized Raman scattering and resonant Raman scattering studies on single InAs nanowires. Polarized Raman expts. show that the highest scattering intensity is obtained when both the incident and analyzed light polarizations are perpendicular to the nanowire axis. InAs wurtzite optical modes are obsd. The obtained wurtzite modes are consistent with the selection rules and also with the results of calcns. using an extended rigid-ion model. Addnl. resonant Raman scattering expts. reveal a red-shifted E1 transition for InAs nanowires compared to the bulk Zn-blende InAs transition due to the dominance of the wurtzite phase in the nanowires. Ab initio calcns. of the electronic band structure for wurtzite and Zn-blende InAs phases corroborate the obsd. values for the E1 transitions.
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    38. 38
      Wu, J.-B.; Zhao, H.; Li, Y.; Ohlberg, D.; Shi, W.; Wu, W.; Wang, H.; Tan, P.-H. Monolayer Molybdenum Disulfide Nanoribbons with High Optical Anisotropy. Adv. Opt. Mater. 2016, 4 (5), 756– 762,  DOI: 10.1002/adom.201500707
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      38
      Monolayer Molybdenum Disulfide Nanoribbons with High Optical Anisotropy
      Wu, Jiang-Bin; Zhao, Huan; Li, Yuanrui; Ohlberg, Douglas; Shi, Wei; Wu, Wei; Wang, Han; Tan, Ping-Heng
      Advanced Optical Materials (2016), 4 (5), 756-762CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      2D molybdenum disulfide (MoS2) has shown promising prospects for the next generation electronics and optoelectronics devices. The monolayer MoS2 can be patterned into quasi-1D anisotropic MoS2 nanoribbons (MNRs), in which theor. calcns. have predicted novel properties. However, little work has been carried out in the exptl. exploration of MNRs with a width of less than 20 nm where the geometrical confinement can lead to interesting phenomenons. Here, MNRs are prepd. with width between 5 and 15 nm by direct helium ion beam milling. High optical anisotropy of these MNRs is revealed by the systematic study of optical contrast and Raman spectroscopy. The Raman modes in MNRs show strong polarization dependence. Besides that the E' and A'1 peaks are broadened by the phonon-confinement effect, the modes corresponding to singularities of vibrational d. of states are activated by edges. The peculiar polarization behavior of Raman modes can be explained by the anisotropy of light absorption in MNRs, which is evidenced by the polarized optical contrast. The study opens the possibility to explore quasi-1D materials with high optical anisotropy from isotropic 2D family of transition metal dichalcogenides.
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    39. 39
      Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tománek, D.; Fischer, J. E.; Smalley, R. E. Crystalline Ropes of Metallic Carbon Nanotubes. Science 1996, 273 (5274), 483– 487,  DOI: 10.1126/science.273.5274.483
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      Crystalline ropes of metallic carbon nanotubes
      Thess, Andreas; Lee, Roland; Nikolaev, Pavel; Dai, Hongjie; Petit, Pierre; Robert, Jerome; Xu, Chunhui; Lee, Young Hee; Kim, Seong Gon; et al.
      Science (Washington, D. C.) (1996), 273 (5274), 483-487CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Fullerene single-wall nanotubes (SWNTs) were produced in yields of >70% by condensation of a laser-vaporized C-Ni-Co mixt. at 1200°. X-ray diffraction and electron microscopy showed that these SWNTs are nearly uniform in diam. and that they self-organize into ropes, which consist of 100 to 500 SWNTs in a 2-dimensional triangular lattice with a lattice const. of 17 Å. The x-ray form factor is consistent with that of uniformly charged cylinders 13.8 ± 0.2 Å in diam. The ropes were metallic, with a single-rope resistivity of <10-4 ohm-centimeters at 300 K. The uniformity of SWNT diam. is attributed to the efficient annealing of an initial fullerene tubelet kept open by a few metal atoms; the optimum diam. is detd. by competition between the strain energy of curvature of the graphene sheet and the dangling-bond energy of the open edge, where growth occurs. These factors strongly favor the metallic (10,10) tube with C5v symmetry and an open edge stabilized by triple bonds.
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    40. 40
      Fischer, J. E.; Dai, H.; Thess, A.; Lee, R.; Hanjani, N. M.; Dehaas, D. L.; Smalley, R. E. Metallic Resistivity in Crystalline Ropes of Single-Wall Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55 (8), R4921– R4924,  DOI: 10.1103/PhysRevB.55.R4921
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      Metallic resistivity in crystalline ropes of single-wall carbon nanotubes
      Fischer, J. E.; Dai, H.; Thess, A.; Lee, R.; Hanjani, N. M.; Dehaas, D. L.; Smalley, R. E.
      Physical Review B: Condensed Matter (1997), 55 (8), R4921-R4924CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Laser ablation of (Co,Ni)-doped graphite yields ∼70% single-wall nanotubes, predominantly (10,10) armchair tubes which self-organize into cryst. "ropes" > 100 Å in a diam. and >10 μm long. The authors find ρ‖ = 0.03-0.10 mΩ-cm at 300 K, with pos. (neg.) dρ/dT above (below) T* = 35 K. Unoriented bulk samples exhibit similar behavior, with higher (directionally averaged) resistivities and T*'s. The high-T behavior is consistent with the predicted intrinsic metallic state for this structure.
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    41. 41
      Wang, L.; Gutiérrez-Lezama, I.; Barreteau, C.; Ubrig, N.; Giannini, E.; Morpurgo, A. F. Tuning Magnetotransport in a Compensated Semimetal at the Atomic Scale. Nat. Commun. 2015, 6 (1), 8892,  DOI: 10.1038/ncomms9892
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      Tuning magnetotransport in a compensated semimetal at the atomic scale
      Wang Lin; Gutierrez-Lezama Ignacio; Barreteau Celine; Ubrig Nicolas; Giannini Enrico; Morpurgo Alberto F; Wang Lin; Gutierrez-Lezama Ignacio; Ubrig Nicolas; Morpurgo Alberto F
      Nature communications (2015), 6 (), 8892 ISSN:.
      Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals.
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      Ago, H.; Nakamura, K.; Ikeda, K.-I.; Uehara, N.; Ishigami, N.; Tsuji, M. Aligned Growth of Isolated Single-Walled Carbon Nanotubes Programmed by Atomic Arrangement of Substrate Surface. Chem. Phys. Lett. 2005, 408 (4), 433– 438,  DOI: 10.1016/j.cplett.2005.04.054
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      Aligned growth of isolated single-walled carbon nanotubes programmed by atomic arrangement of substrate surface
      Ago, Hiroki; Nakamura, Kazuhiro; Ikeda, Ken-ichi; Uehara, Naoyasu; Ishigami, Naoki; Tsuji, Masaharu
      Chemical Physics Letters (2005), 408 (4-6), 433-438CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)
      Highly aligned and isolated single-walled carbon nanotubes (SWNTs) were grown on the R-face (1‾102) and A-face (11‾20) surfaces of sapphire (Al2O3) substrates by catalytic chem. vapor deposition. On the basis of the electron backscatter diffraction (EBSD) anal., it was found that the SWNTs are aligned along the specific cryst. directions corresponding to the anisotropic pseudo-1D array of Al atoms on these surfaces. This suggests that the Al array guides the SWNT growth due to the strong interaction between the Al atoms and nanotubes. On the other hand, a random orientation was obsd. for the SWNTs grown on the C-face (0001) substrate, reflecting the isotropic arrangement of Al atoms. These findings indicate that the strong interaction between the SWNTs and substrate surface is applicable for patterning or integrating SWNTs in nanoelectronics applications.
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      Han, S.; Liu, X.; Zhou, C. Template-Free Directional Growth of Single-Walled Carbon Nanotubes on a- and r-Plane Sapphire. J. Am. Chem. Soc. 2005, 127 (15), 5294– 5295,  DOI: 10.1021/ja042544x
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      43
      Template-Free Directional Growth of Single-Walled Carbon Nanotubes on a- and r-Plane Sapphire
      Han, Song; Liu, Xiaolei; Zhou, Chongwu
      Journal of the American Chemical Society (2005), 127 (15), 5294-5295CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report high-throughput growth of highly aligned single-walled carbon nanotube arrays on a-plane and r-plane sapphire substrates. This is achieved using chem. vapor deposition with ferritin as the catalyst. The nanotubes are aligned normal to the [0001] direction for growth on the a-plane sapphire. They are typically tens of micrometers long, with a narrow diam. distribution of 1.34 ± 0.30 nm. In contrast, no orientation was achieved for growth on the c-plane and m-plane sapphire, or when Fe films, instead of ferritin, were used as the catalyst. Such orientation control is likely related to the interaction between carbon nanotubes and the sapphire substrate, which is supported by the observation that when a second layer of nanotubes was grown, they followed the gas flow direction. These aligned nanotube arrays may enable the construction of integrable and scalable nanotube devices and systems.
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      Tsivion, D.; Schvartzman, M.; Popovitz-Biro, R.; von Huth, P.; Joselevich, E. Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations. Science 2011, 333 (6045), 1003– 1007,  DOI: 10.1126/science.1208455
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      44
      Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations
      Tsivion, David; Schvartzman, Mark; Popovitz-Biro, Ronit; von Huth, Palle; Joselevich, Ernesto
      Science (Washington, DC, United States) (2011), 333 (6045), 1003-1007CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. The authors report the vapor-liq.-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallog. orientations on different planes of sapphire. The growth directions, crystallog. orientation, and faceting of the nanowires vary with each surface orientation, as detd. by their epitaxial relation with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.
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    45. 45
      Rothman, A.; Forsht, T.; Danieli, Y.; Popovitz-Biro, R.; Rechav, K.; Houben, L.; Joselevich, E. Guided Growth of Horizontal ZnS Nanowires on Flat and Faceted Sapphire Surfaces. J. Phys. Chem. C 2018, 122 (23), 12413– 12420,  DOI: 10.1021/acs.jpcc.8b04063
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      45
      Guided Growth of Horizontal ZnS Nanowires on Flat and Faceted Sapphire Surfaces
      Rothman, Amnon; Forsht, Tamir; Danieli, Yarden; Popovitz-Biro, Ronit; Rechav, Katya; Houben, Lothar; Joselevich, Ernesto
      Journal of Physical Chemistry C (2018), 122 (23), 12413-12420CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The surface-guided growth of horizontal nanowires (NWs) allows assembly and alignment of the NWs on the substrate during the synthesis, thus eliminating the need for addnl. processes after growth. One of the major advantages of guided growth over postgrowth assembly is the control on the NWs direction, crystallog. orientation, and position. The authors use the guided growth approach to synthesize high-quality, single-crystal, aligned horizontal ZnS NWs on flat and faceted sapphire surfaces, and show how the crystal planes of the different substrates affects the crystal structure and orientation of the NWs. The authors also show initial results of the effect of Cu doping on their photoluminescence. Such high-quality aligned ZnS NWs can potentially be assembled as key components in phosphorescent displays and markers due to their unique optical properties. The ZnS NWs have either wurtzite or Zn-blende structure depending on the substrate orientations and contain intrinsic point defects such as S vacancies, which are common in this material. The crystallog. orientations are consistent with those of guided NWs from other semiconductor materials, demonstrating the generality of the guided growth phenomenon. The successfully grown ZnS NWs and the Cu doping are the 1st step toward the fabrication of optoelectronic devices based on ZnS nanostructures.
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    • Experimental methods and supporting data for structural uniformity and crystallinity of suspended WTe bundles with the corresponding EDS measurements, decomposed WTe residue, surface morphology of the WTe network on SiO2/Si and a-plane sapphire, coverage and crystal uniformity of the WTe network, topographic analysis of WTe nanowires, SEM of MoTe nanowires, polarized Raman characterization and optical absorption of WTe nanowires, transport measurements for different WTe devices, structural models for DFT calculations, electronic band structure of bilayer 2D WTe, tellurization of Na2WO4·2H2O, nucleation of WTe nanowires, and WTe growth on various surfaces (PDF)


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