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Solution-Phase Epitaxial Growth of Quasi-Monocrystalline Cuprous Oxide on Metal Nanowires
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† Center for Nanophotonics, FOM Institute AMOLF, Science Park Amsterdam 104, 1098 XG Amsterdam, The Netherlands
‡ National Center for HREM, Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628CJ, Delft, The Netherlands
§ Debye Institute for Nanomaterials Science and Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584CC Utrecht, The Netherlands
*E-mail [email protected] (E.C.G.).
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Nano Letters

Cite this: Nano Lett. 2014, 14, 10, 5891–5898
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https://doi.org/10.1021/nl502831t
Published September 18, 2014

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Abstract

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The epitaxial growth of monocrystalline semiconductors on metal nanostructures is interesting from both fundamental and applied perspectives. The realization of nanostructures with excellent interfaces and material properties that also have controlled optical resonances can be very challenging. Here we report the synthesis and characterization of metal–semiconductor core–shell nanowires. We demonstrate a solution-phase route to obtain stable core–shell metal–Cu2O nanowires with outstanding control over the resulting structure, in which the noble metal nanowire is used as the nucleation site for epitaxial growth of quasi-monocrystalline Cu2O shells at room temperature in aqueous solution. We use X-ray and electron diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, and absorption spectroscopy, as well as density functional theory calculations, to characterize the core–shell nanowires and verify their structure. Metal–semiconductor core–shell nanowires offer several potential advantages over thin film and traditional nanowire architectures as building blocks for photovoltaics, including efficient carrier collection in radial nanowire junctions and strong optical resonances that can be tuned to maximize absorption.

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The development of inexpensive and efficient solar cells has been a major research focus over the past 10 years to face the projected increase in global energy consumption. (1) Ideally, the ultimate solar cell would convert light into electricity in the smallest possible volume of material. This desire to minimize volume is not only motivated by the reduced costs associated with using less semiconductor material but also fundamentally linked to a higher solar conversion efficiency. This higher theoretical efficiency has been well documented and arises from two considerations: (1) using less material reduces bulk recombination and thus can boost the open-circuit voltage (Voc), (2) and (2) reaching full absorption in less material could lead to higher photogenerated carrier densities and thus higher Voc. (3, 4) The light concentration effect has been historically applied using macroscale concentrating optics, and the efficiency enhancement comes via a <60 mV increase of Voc per decade of concentration. For example, this Voc enhancement is largely responsible for the increase in efficiency from 31.3% at 1 sun to 40.7% at 240 sun (5) in triple-junction solar cells. More recently, researchers have shown that single semiconducting nanowires and nanowire arrays can act as antennas, providing a similar concentration effect without external optics and employing a reduced amount of material. (6, 7) Furthermore, the possibility to combine materials with high lattice mismatch in heteroepitaxial junctions, (8-10) and the opportunity to decrease the material volume without compromising light absorption, (11-13) make the development of nanowire-based solar cells intriguing. (14-16)

These results motivated us to investigate a novel core–shell nanowire geometry consisting of a metal nanowire coated by an ultrathin semiconductor shell, which theoretically shows superior absorption compared to solid semiconductor nanowires (Figure 1). (17) In this hybrid core–shell geometry there are several resonances with high field intensity in the shell, leading to efficient light absorption in the semiconductor. Furthermore, this geometry is particularly appealing because the metal core can also function as an electrode embedded within the semiconductor that locally collects photogenerated charge carriers; (18, 19) this indeed simplifies the realization of a working device and might reduce fabrication costs. For this scheme to work, the quality of the semiconductor and the nature of the interface are extremely important to provide sufficient carrier mobility and to reduce recombination. (9, 14, 20) Fabrication of related metal–semiconductor heterostructures has recently attracted a lot of attention, (21, 22) and the synthesis of core–shell nanoparticles with monocrystalline shells has allowed for the exploration of new avenues in fundamental nanomaterial research (23-25) as well as the demonstration of new technological applications; (26, 27) however, solution-phase synthesis of this class of heterostructures has so far been limited mainly to nanoparticles. (24, 28-31)

Here we report the synthesis and characterization of metal–Cu2O core–shell nanowires. Cu2O was chosen as a first model system to demonstrate this concept because it is an earth abundant material with a high absorption coefficient and a band gap close to ideal for the top layer in a tandem solar cell with silicon. (32) Additionally, it provides a relatively low lattice mismatch with both Ag and Au (∼4%), which have been used for high-performance nanowire transparent electrodes. (18, 19)

We begin by describing the synthetic procedure and resulting morphology and then use numerical simulations and analytical calculations to demonstrate the high electric field intensities in the thin semiconducting shell and the large absorption efficiency that can be reached with these structures. We confirm these theoretical predictions with quantitative single-nanowire absorption measurements. The good agreement with theory gives us confidence to calculate absorption in periodic arrays of these nanowires to predict how they would perform in a macroscopic solar cell. The results show that in our core–shell configuration a 40 nm Cu2O shell can absorb approximately the same amount of light as a semi-infinite Cu2O slab without an antireflection coating. Photoluminescence measurements on single nanowires also confirm that carriers are not completely quenched by the local metal contact, and the band gap value is similar to what is observed in bulk films. In addition to the optical properties we provide electron microscopy and X-ray spectroscopy to show that the cuprous oxide shell is spatially uniform, quasi-monocrystalline, pure-phase Cu2O. Selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HR-TEM) confirm the epitaxial relationship between the core and the shell. Density functional theory (DFT) calculations provide further insight into the binding configuration at the interface. Finally, the cuprous oxide shell shows no evidence of further oxidation to cupric oxide (CuO) even after extended storage in air. Combined, these results suggest that our core–shell nanowires could be an excellent platform for fundamental studies of metal–semiconductor interfaces, which are critical in many optoelectronic devices. Furthermore, the efficient absorption and local contacting features of such a geometry could have an impact in applications beyond photovoltaics such as sunlight-to-fuel conversion, photodetectors, and light-emitting diodes.

Figure 1

Figure 1. Ag–Cu2O core–shell nanowires. Artist’s impression of a core–shell nanowire illuminated from the top. The light absorption profile is overlaid on the schematic image, and the inset shows the lattice matching at the interface.

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The synthesis of metal–Cu2O core–shell nanowires (Figure 2) is performed entirely in solution and involves two steps (see Supporting Information for further details): (i) synthesis of metal nanowires via the polyol process in ethylene glycol; (ii) employing metal nanowires as the nucleation site for the growth of a Cu2O shell at room temperature in water.

Core–shell nanowires with a silver core were chosen to illustrate the structural, chemical, and optical properties of such heterostructures (see Supporting Information for core–shell nanowires with a gold core). The advantage of using Ag versus Au is the lower cost and the better conductivity. Compared to other metals, such as Cu, Ag is more stable to chemical reactions, but other metals such as Al could be interesting from the optical and economical point of view.

Figure 2c shows a representative cross section of a Ag–Cu2O nanowire after focused ion beam milling. In the cross-sectional image there is clear contrast between the Ag core and the Cu2O shell. Interestingly, it is also possible to resolve the five twin planes of the Ag nanowire and appreciate a different contrast for different single-crystalline subunits. Note that Ag nanowires grow from 5-fold twinned decahedral seeds along the [110] direction (33) and therefore feature a pentagonal cross section. The bright features visible on the shell are due to adsorption of sputtered material during preparation of the cross section. Elemental maps recorded using energy dispersive spectroscopy (EDS) in an SEM verify the elemental distribution in our core–shell nanowires (Figure 2e). The emission intensity of characteristic X-rays is plotted as a function of the electron beam position, for three different X-ray energies, characteristic of Ag (L shell), Cu (K shell), and O (K shell). The three plots in Figure 2e confirm the localization of Ag only in the core of the nanowire and show that X-rays from Cu and O are emitted from a larger region in the radial direction. Note that the intensity in the Cu and O chemical maps is higher at the edge, where the projected shell thickness is higher as expected for the proposed Ag–Cu2O core–shell nanowire structure. It should be emphasized that different core diameters and shell thicknesses can be achieved by adjusting the synthetic conditions and that other metal nanowires can be employed for the nucleation of the Cu2O shell with the same synthetic procedure (see Supporting Information Figure S1 for an example of a Au–Cu2O nanowire).

Figure 2

Figure 2. Ag–Cu2O core–shell nanowire: (a) in-column-detector (ICD) and (b) secondary electrons (SE) image of a Ag–Cu2O nanowire lying on a substrate; (c) focused ion beam (FIB) cross-sectional view; (d) distribution profile of the total absorbed power averaged over AM 1.5 (unpolarized), calculated from FDTD simulations for a Ag–Cu2O core–shell nanowire in vacuum. The color scale is in 104 W/m2 for a 1 V/m plane wave incident from the top. (e) EDS elemental maps of a Ag–Cu2O nanowire, for Ag, Cu, and O showing the presence of a core–shell geometry.

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Within the same synthetic batch, some difference in shell thicknesses can be observed for nanowires with very different core sizes. As the shell growth is typically very fast (1–2 min for first nucleation and growth stage), adjacent nanowires in solution compete for Cu precursor. This means that nanowires with larger cores, which require a larger volume of Cu2O for the same shell thickness, end up with thinner shells. This often results in higher surface roughness or, in extreme cases, even incomplete shell coverage (see Supporting Information Figure S3).

We used finite-difference time domain (FDTD) to model light absorption in pentagonal Ag–Cu2O nanowires. The wavelength-dependent absorbed power density in the nanowire was weighted over the AM1.5 solar spectrum and integrated for photon energies above the band gap (290–650 nm). Figure 2d shows a 2D spatial map of the integrated absorbed power. The power profile is averaged over TE and TM polarizations for the best comparison to unpolarized sunlight. From Figure 2d it is clear that most of the absorption occurs in the Cu2O shell, but there is some parasitic absorption in the metal core (<16%). In order to provide a comparison to a thin-film geometry, in Supporting Information Figure S2 we show the absorbed power distribution for three control systems: a 100 nm thick Ag film (Figure S2a), a 40 nm thick Cu2O membrane (Figure S2b), and the combination of the previous two (Figure S2c) upon illumination by a plane wave. Note that as for the Ag–Cu2O nanowire the absorbed power is weighted over the AM1.5 spectrum and integrated from 290 to 650 nm. The maximum of the absorbed power density in the core–shell geometry is 3 times larger than that absorbed in a Cu2O membrane supported on a Ag film. This corroborates the large optical cross section of this new core–shell nanowire architecture. Note that the dimensions of the Ag–Cu2O nanowire shown in Figure 2d correspond to the optimum dimensions for the largest absorbed power density (core radius: rc = 50 nm; shell thickness: ts = 20 nm).

To verify the absorption properties of such core–shell nanowires experimentally, we measured the quantitative absorption in both polarizations (Figure 3a). In the TE polarization (electric field polarized perpendicular to the nanowire’s axis) the core–shell nanowire shows two resonant absorption peaks in the experimental spectrum, while in TM (electric field polarized along nanowire’s axis) only one is visible. Figure 3a compares the measured absorption cross section to Mie theory calculations for a cylindrical core–shell nanowire with roughly the same dimensions (see Supporting Information). There is good agreement between theory and experiment. Since we have verified these quantitative absorption measurements in a simpler silicon nanowire system (which will be discussed in a future publication), we attribute the differences between measurement and theory (in particular, the emergence of a second resonance in TE) to significant surface roughness (see Figure S4 for SEM images). In the smooth cylindrical geometry TE31 is strongly overdamped, but the surface roughness increases the radiative loss rate, which alters the absorption cross section and thus leads to the apparent emergence of resonances. (17) Note that the ratio between the beam diameter, measured by the knife-edge technique, and the core–shell nanowire diameter was taken into account to quantitatively calculate the absorption efficiency reported in Figure 3a.

Figure 3

Figure 3. Optical characterization of a single Ag–Cu2O core–shell nanowire with a core radius of 55 nm and a shell thickness of 65 nm. (a) Experimental (solid lines) and calculated (dashed lines) quantitative absorption spectrum of a single core–shell nanowire in TE (red) and TM (blue) polarizations. (b) FDTD simulation of the photons absorbed in the shell of a Ag–Cu2O nanowire array weighted over the AM 1.5 (red line); photon flux in the solar spectrum (black line); absorption in the Cu2O shell before weighting for AM 1.5 (blue line). (c) Photoluminescence emission of a single core–shell nanowire.

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Interestingly, the absolute values of absorption do not differ substantially from the values calculated for a cylindrical core–shell nanowire, and the measured spectral dependence is similar to calculations for both TE and TM polarizations.

To provide a comparison of the absorption properties of Ag–Cu2O nanowires with bulk absorbing materials, we carried out FDTD simulations of Ag–Cu2O periodic arrays lying on a perfect electric conductor, with the dimensions used in Mie theory to calculate the absorption spectrum of Figure 3a (rc = 55 nm, ts = 65 nm). The spectral dependence of the absorbed photon flux in the shell material (weighted for the AM1.5 spectrum) in a Ag–Cu2O nanowire array is presented in Figure 3b (red line), along with the total photon flux in the AM1.5 spectrum (black line) and the absolute absorption (not weighted) fraction in the Cu2O shell (blue line). The total absorbed flux integrated in the range of 290–650 nm is 66% (absorbed in the Cu2O shell). With further optimization of the core radius and shell thickness it is possible to achieve a total integrated absorption as large as 72% in the shell material for such a Ag–Cu2O nanowire array (rc = 100 nm, ts = 40 nm, nanowires touching). For comparison, a semi-infinite Cu2O film without an antireflection coating absorbs roughly 75% of the AM1.5 spectrum. In the case of a Cu2O thin film on a perfect electric conductor, an absorption as large as 71% of the AM1.5 spectrum could be achieved for the optimized case (50 nm Cu2O film thickness). In such a geometry, however, there are no electrical contacts, while in the core–shell geometry, both contacts are already present, and thus shading is taken into account.

In addition to optical absorption measurements, photoluminescence (PL) experiments were performed on individual Ag–Cu2O nanowires by exciting at a wavelength of 532 nm with a laser (Figure 3c). The band gap luminescence at 670 nm from individual nanowires suggests that the metal–semiconductor interface does not fully quench radiative processes. The emission wavelength is slightly different from what has been observed in bulk Cu2O which could be related to the lattice mismatch or optical resonances in the nanostructure. (34-37)

Below we use electron microscopy and X-ray spectroscopy to investigate the quality of the metal–semiconductor interface and to analyze the characteristics of the Cu2O shell. Figure 4a shows a representative bright field transmission electron microscopy (BF-TEM) image of a Ag–Cu2O nanowire. The Ag core is clearly visible in the center. The apparent double layer contrast in the Cu2O shell is the result of a 2D projection of the 3D pentagonal morphology, whereby Cu2O shell domains from different pentagonal facets can be overlapping, depending on the orientation of the nanowire on the substrate. The scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) image (inset of Figure 4a) displays so-called Z-contrast and highlights the substantial difference in atomic number between the Ag core and the Cu2O shell. Figure 4b shows a HR-TEM image of the area indicated in Figure 4a. The yellow axes represent the crystallographic directions of core and shell.

Figure 4

Figure 4. Structural characterization of Ag–Cu2O core–shell nanowire. (a) Bright field TEM of a representative Ag–Cu2O core–shell nanowire, showing a neat contrast between Ag core and Cu2O shell; inset: the HAADF image emphasizes Z contrast in the Ag–Cu2O nanowire. (b) HRTEM image of the interface between core and shell in the region highlighted in (a). The two lattices match at the interface along the [110] direction and show epitaxial growth. (c, d) SAED pattern for an individual Ag nanowire and Ag–Cu2O nanowire, respectively. The indexing of the SAED pattern has been performed along [001̅] and [1̅12] zone axes and shows a cube-on-cube orientation relationship of the Cu2O shell on the Ag core. The spot size included the entire core–shell nanowire shown in (a).

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Ag and Cu2O both have a cubic crystal lattice, and Ag has space group Fm3̅m with a lattice parameter of 4.090 Å, (38) while Cu2O has space group Pn3̅m with a lattice parameter of 4.269 Å. (39) Interestingly, in the core–shell nanowire, the primary axes of both crystals are mutually aligned, resulting in a cube-on-cube orientation relationship, with a lattice mismatch of 4.4% (see inset in Figure 5a). The crystals are both oriented with their [110] axes along the length of the nanowire and have the [11̅1] and [001] axes pointing in lateral directions. From this analysis it is evident that the Cu2O shell grows epitaxially on the Ag nanowire core, and therefore polyvinylpyrrolidone (PVP), which is known to passivate Ag nanowire facets, (33) must be displaced during the nucleation and growth of Cu2O, as no interlayer is observed. HR-TEM measurements were performed on multiple Ag–Cu2O nanowires along various zone axes to confirm the epitaxial growth and showed similar orientations between the Ag core and the Cu2O shell. In addition, epitaxial growth was observed for core–shell nanowires with larger core diameters (above 200 nm, Figure S3).

The long-range order of the Cu2O shell, its crystallographic structure and relationship to the underlying Ag lattice were studied by selected area electron diffraction (SAED), by collecting signal from the entire core–shell nanowire shown in Figure 4a. As a reference, a SAED pattern for an individual Ag nanowire is shown in Figure 4c. The diffraction pattern cannot be assigned to a simple FCC crystal because of the presence of five twinned subcrystals, leading to two individual diffraction patterns superimposed: one along the [001̅] zone axis (solid line) and one along the [1̅12] zone axis (dashed line). (33)

A series of new spots appear in the diffraction pattern of the Ag–Cu2O core–shell nanowire (Figure 4d), as denoted by the orange circles. Some key features emerge from this pattern: (i) individual spots are observed, as opposed to a continuous ring, demonstrating that the Cu2O shell on every Ag facet is quasi-monocrystalline; (ii) two sets of superimposed quasi-single-crystal diffraction patterns are observed for Cu2O, one with square symmetry along the [001̅] zone axis and one with rhomboidal symmetry along the [1̅12] zone axis, confirming the cube-on-cube crystallographic alignment of the Cu2O shell with the underlying Ag nanowire crystal, as depicted in the inset of Figure 5a; (iii) epitaxial relationship (02̅0)Ag||(02̅0)Cu2O, (200)Ag||(200)Cu2O and [001̅]Ag||[001̅]Cu2O; (iv) epitaxial relationship (2̅2̅0)Ag||(2̅2̅0)Cu2O, (11̅1)Ag||(22̅2)Cu2O, and [1̅12]Ag||[1̅12]Cu2O; (v) the presence of two zone axes aligned with those of the Ag core (diffraction spots from the Cu2O in positions contiguous to those of Ag nanowire pattern) suggests that the crystal orientation for Cu2O is the same everywhere for a specific Ag nanowire subcrystal, therefore confirming the quasi-monocrystallinity of the shell; (vi) it is interesting to note that the (110) diffraction is forbidden for a Ag nanowire by the FCC structure factor rule (see Figure 4c); however, in the core–shell nanowire a (110) spot is present because of diffraction in Cu2O, which belongs to the Pn3̅m group. Note that the high intensity of spot (2̅2̅0) is due to the overlap of diffraction along this direction for both core and shell. Also note that diffraction along [110] for Cu2O occurs along both zone axes and therefore is more intense.

Figure 5

Figure 5. Structural characterization of Ag–Cu2O core–shell nanowire. (a) XRD of Ag–Cu2O nanowire ensemble in the range 20°–90°, showing diffraction peaks of Ag and Cu2O. Top: reference values for Ag (blue) and Cu2O (red) diffraction peaks. (23) Inset: artistic impression of the cube-on-cube orientation between the core and the shell material. (b, c) Structural atomic models of the Ag–Cu2O interface obtained after structural relaxation using DFT calculations for mixed and unmixed Ag/Cu atomic layers, respectively. Brown spheres represent copper atoms, red spheres oxygen atoms, and gray spheres silver atoms.

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Figure 4d demonstrates that the matching between Ag and Cu2O lattices occurs for every twinned subcrystal along the whole interface, and it is consistent with HRTEM measurements. The SAED pattern of Figure 4d included signal from the entire core–shell nanowire shown in Figure 4a and is therefore representative of the crystallinity of the Cu2O shell on a large scale. On the individual nanowire analyzed in Figure 4, there are no signs of either Cu or CuO phases present in the shell material.

The growth process of the Cu2O shell occurs in three steps: (23, 25) (1) epitaxial nucleation of Cu2O nanoparticles on the metal substrate, (2) Cu2O nanoparticle growth until the reagents are consumed, and (3) crystal reconstruction to release stress created during the growth. The shell consists of multiple grains that are aligned in rows along each of the five Ag{100} facets, as borne out by SAED measurements on the entire core–shell nanowire. These grains might crystallographically be slightly misaligned and not have exactly the same height because of local differences in the growth rate, resulting in surface roughness, which causes the contrast visible in the SEM images (Figure 2b and Figures S3 and S4). However, they all follow the same orientation relationship with the five subcrystals in the pentagonal Ag core, and therefore their mutual crystallographic misalignment is less than what is detectable in SAED. The morphological and structural configuration of the Cu2O shell is a result of the growth process, whereby Cu2O nucleates simultaneously at many points along the Ag nanowire. These Cu2O nuclei grow until their edges touch, leading to rows of almost perfectly aligned Cu2O grains along each of the pentagonal Ag facets. The five elongated Cu2O domains covering the Ag nanowire are therefore nearly single crystalline but may contain planar defects such as low-angle tilt boundaries and low-angle twist boundaries or dislocations. From this point of view, it is more appropriate to call it a quasi-monocrystal, using a terminology employed in similar materials for silicon photovoltaics. (40) These low-angle planar defects are indeed not visible in the SAED pattern, showing that the angle misalignment between the grains has to be very low not to be resolved. Although these low-angle planar defects are not visible in the SAED pattern, in bright-field TEM images it is sometimes possible to observe the existence of both low-angle grain boundary regions as well as fully monocrystalline regions (Figure S5).

In order to demonstrate that the growth of pure Cu2O is achievable in large ensembles, we performed XRD analysis on a thick film of Ag–Cu2O nanowires drop-cast from solution in a 2θ range of 20°–90° (see Figure 5a). Intense diffraction peaks matching crystalline Ag were observed, along with peaks matching Cu2O, as labeled in the spectrum of Figure 5a. For comparison, reference values for both Ag (blue) and Cu2O (red) are reported on the top of the figure. The low intensity of the Cu2O reflection peak is most likely due to the low Cu2O ratio in the core–shell nanowire sample employed for the measurements. Importantly, no undesirable phases such as copper(II) oxide (CuO), Cu, mixed metal oxides, or intermetallics were detected even after storage for 6 months in air, revealing the stability of the heterostructure interface and uniformity on a large scale.

While the HR-TEM, SAED, and XRD results confirm epitaxial growth of Cu2O from the Ag surface, they do not provide information about the atomic binding configurations at the Ag–Cu2O interface. Therefore, plane-wave DFT calculations (41, 42) using the generalized gradient approximation (GGA) were performed. A plausible atomic model was constructed in which the FCC metal (sub)lattice of Ag/Cu atoms is continuous across the Ag{001}/Cu2O{001} interface. Two models were considered: one in which the interface contains Ag/Cu mixed atomic layers (Figure 5b) and one model without mixed layers (Figure 5c). The difference in interfacial energy between the two models is very small, indicating that both types of interfaces may be formed. More details are given in the Supporting Information. Free energy calculations and Auger spectroscopy results reported on another noble metal–Cu2O interface, namely Au–Cu2O, (43) are consistent with the DFT result of Figure 5b, which shows that there is no Ag–O bonding at the interface.

We have shown that under the appropriate experimental conditions silver nanowires can be used as a nucleation site for the epitaxial growth of quasi-monocrystalline, pure phase cuprous oxide shells at room temperature in a water environment. SAED, HRTEM, and XRD analyses prove that the shell consists of pure Cu2O, which is unusual in bulk Cu2O samples, whose oxidation to CuO has been reported to occur in ambient conditions. (44) By tuning the synthetic parameters, various core diameters and shell thicknesses can be obtained, leading to fine control over optical resonances and ultimately light absorption. We showed that the optical response of Ag–Cu2O is in good agreement with theory/simulations, and most of the power absorption takes place in the semiconductor shell due to the nature of the resonances. FDTD simulations show a 3-fold increase of the maximum absorbed power density within the semiconductor shell, compared to a thin Cu2O membrane with the same dimensions supported on a Ag film.

Other oxides with similar band gaps and lattice constants, such as CoO, can potentially be interesting within this application as well. Metal sulfides such as Cu2S or CdS could also be intriguing absorbing layers, but they require a core material that does not react with sulfur (such as Au). Indeed, heterostructures with a Au core and a CdS shell have been synthesized by a nonepitaxial method using an amorphous intermediate, (21) and this approach might be extendable to nanowire core–shell systems with large lattice mismatches.

By combining high quality quasi-monocrystalline materials made at room temperature and efficient light absorption in extraordinarily thin absorbing layers, we expect substantial improvements in the performance of solar devices based on Ag–Cu2O core–shell nanowires. On the other hand, the lower material consumption and the employment of a simple and inexpensive fabrication process—the solution-phase synthesis—could have a large impact on reducing the module cost. Finally, the opportunity to achieve high quality quasi-monocrystalline semiconductor grown on a metal contact with an excellent interface is indeed compelling to pursue fundamental studies on semiconductor properties at the nanoscale.

Supporting Information

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Details on the synthesis of Ag nanowires and the Cu2O shell growth, experimental details on the structural and optical characterization, details on the density functional theory (DFT) calculations, and additional SEM and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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Author Information

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  • Corresponding Author
    • Erik C. Garnett - Center for Nanophotonics, FOM Institute AMOLF, Science Park Amsterdam 104, 1098 XG Amsterdam, The Netherlands;  Email: [email protected]
  • Authors
    • Beniamino Sciacca - Center for Nanophotonics, FOM Institute AMOLF, Science Park Amsterdam 104, 1098 XG Amsterdam, The Netherlands
    • Sander A. Mann - Center for Nanophotonics, FOM Institute AMOLF, Science Park Amsterdam 104, 1098 XG Amsterdam, The Netherlands
    • Frans D. Tichelaar - National Center for HREM, Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628CJ, Delft, The Netherlands
    • Henny W. Zandbergen - National Center for HREM, Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628CJ, Delft, The Netherlands
    • Marijn A. van Huis - National Center for HREM, Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628CJ, Delft, The Netherlands;  Debye Institute for Nanomaterials Science and Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584CC Utrecht, The Netherlands
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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The authors acknowledge AMOLF technical support, Henk-Jan Boluijt for the realization of the schematic drawing in Figure 1a, and Dr. Toon Coenen, Dr. Sarah Brittman, and Sebastian Oener for helpful discussions. We thank Hans Meeldijk (Utrecht Univ.) for TEM assistance. Furthermore, we acknowledge support from the Light Management in New Photovoltaic Materials (LMPV) center at AMOLF. This work is part of the research program of the Foundation for Fundamental Research on Matter (FOM), which is part of The Netherlands Organization for Scientific Research (NWO). The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 337328, “NanoEnabledPV”.

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    The use of quantum and photon confinement has enabled a true revolution in the development of high-performance semiconductor materials and devices. Harnessing these powerful phys. effects relies on an ability to design and fashion structures at length scales comparable to the wavelength of electrons (∼1 nm) or photons (∼1 μm). Unfortunately, many practical optoelectronic devices exhibit intermediate sizes where resonant enhancement effects seem to be insignificant. Here, leaky-mode resonances, which can gently confine light within subwavelength, high-refractive-index semiconductor nanostructures, are ideally suited to enhance and spectrally engineer light absorption in this important size regime. This is illustrated with individual Ge nanowire photodetectors. This notion, together with the ever-increasing control over nanostructure synthesis opens up tremendous opportunities for the realization of a wide range of high-performance, nanowire-based optoelectronic devices, including solar cells, photodetectors, optical modulators and light sources.
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  8. 8
    Tang, J. Y.; Huo, Z. Y.; Brittman, S.; Gao, H. W.; Yang, P. D. Solution-processed core-shell nanowires for efficient photovoltaic cells Nat. Nanotechnol. 2011, 6 (9) 568– 572
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    8
    Solution-processed core-shell nanowires for efficient photovoltaic cells
    Tang, Jinyao; Huo, Ziyang; Brittman, Sarah; Gao, Hanwei; Yang, Peidong
    Nature Nanotechnology (2011), 6 (9), 568-572CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Semiconductor nanowires are promising for photovoltaic applications, but, so far, nanowire-based solar cells have had lower efficiencies than planar cells made from the same materials, even allowing for the generally lower light absorption of nanowires. It is not clear, therefore, if the benefits of the nanowire structure, including better charge collection and transport and the possibility of enhanced absorption through light trapping, can outweigh the redns. in performance caused by recombination at the surface of the nanowires and at p-n junctions. Here, the authors fabricate core-shell nanowire solar cells with open-circuit voltage and fill factor values superior to those reported for equiv. planar cells, and an energy conversion efficiency of ∼5.4%, which is comparable to that of equiv. planar cells despite low light absorption levels. The device is made using a low-temp. soln.-based cation exchange reaction that creates a heteroepitaxial junction between a single-cryst. CdS core and single-cryst. Cu2S shell. The authors integrate multiple cells on single nanowires in both series and parallel configurations for high output voltages and currents, resp. The ability to produce efficient nanowire-based solar cells with a soln.-based process and Earth-abundant elements could significantly reduce fabrication costs relative to existing high-temp. bulk material approaches.
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  9. 9
    Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Single-crystal metallic nanowires and metal/semiconductor nanowire heterostructures Nature 2004, 430 (7000) 704– 704
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    Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Epitaxial core-shell and core-multishell nanowire heterostructures Nature 2002, 420 (6911) 57– 61
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    10
    Epitaxial core-shell and core-multishell nanowire heterostructures
    Lauhon, Lincoln J.; Gudiksen, Mark S.; Wang, Deli; Lieber, Charles M.
    Nature (London, United Kingdom) (2002), 420 (6911), 57-61CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Semiconductor heterostructures with modulated compn. and/or doping enable passivation of interfaces and the generation of devices with diverse functions. In this regard, the control of interfaces in nanoscale building blocks with high surface area will be increasingly important in the assembly of electronic and photonic devices. Core-shell heterostructures formed by the growth of cryst. overlayers on nanocrystals offer enhanced emission efficiency, important for various applications. Axial heterostructures also were formed by a 1-dimensional modulation of nanowire compn. and doping. However, modulation of the radial compn. and doping in nanowire structures has received much less attention than planar and nanocrystal systems. Here the authors synthesize Si and Ge core-shell and multishell nanowire heterostructures using a CVD method applicable to a variety of nanoscale materials. The authors' studies of the growth of B-doped Si shells on intrinsic Si and Si-Si oxide core-shell nanowires indicate that homoepitaxy can be achieved at relatively low temps. on clean Si. The authors also demonstrate the possibility of heteroepitaxial growth of cryst. Ge-Si and Si-Ge core-shell structures, in which band-offsets drive hole injection into either Ge core or shell regions. The authors' synthesis of core-multishell structures, including a high-performance coaxially gated field-effect transistor, indicates the general potential of radial heterostructure growth for the development of nanowire-based devices.
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  11. 11
    Garnett, E.; Yang, P. D. Light trapping in silicon nanowire solar cells Nano Lett. 2010, 10 (3) 1082– 1087
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    Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Aberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgstrom, M. T. InP nanowire array solar cells achieving 13.8% efficiency by exceeding the ray optics limit Science 2013, 339 (6123) 1057– 1060
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    12
    InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit
    Wallentin, Jesper; Anttu, Nicklas; Asoli, Damir; Huffman, Maria; Aaberg, Ingvar; Magnusson, Martin H.; Siefer, Gerald; Fuss-Kailuweit, Peter; Dimroth, Frank; Witzigmann, Bernd; Xu, H. Q.; Samuelson, Lars; Deppert, Knut; Borgstroem, Magnus T.
    Science (Washington, DC, United States) (2013), 339 (6123), 1057-1060CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diam. and the length of the top n-segment were crit. for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nm-diam. nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 V exceeds that of its planar counterpart, despite about 30 times higher surface-to-vol. ratio of the nanowire cell.
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  13. 13
    Pala, R. A.; Liu, J. S. Q.; Barnard, E. S.; Askarov, D.; Garnett, E. C.; Fan, S. H.; Brongersma, M. L. Optimization of non-periodic plasmonic light-trapping layers for thin-film solar cells. Nat. Commun. 2013, 4.
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  14. 14
    Tian, B.; Kempa, T. J.; Lieber, C. M. Single nanowire photovoltaics Chem. Soc. Rev. 2009, 38 (1) 16– 24
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    Krogstrup, P.; Jorgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Morral, A. F. I. Single-nanowire solar cells beyond the Shockley-Queisser limit Nat. Photonics 2013, 7 (4) 306– 310
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    15
    Single-nanowire solar cells beyond the Shockley-Queisser limit
    Krogstrup, Peter; Jorgensen, Henrik Ingerslev; Heiss, Martin; Demichel, Olivier; Holm, Jeppe V.; Aagesen, Martin; Nygard, Jesper; Fontcuberta i Morral, Anna
    Nature Photonics (2013), 7 (4), 306-310CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    Light management is of great importance in photovoltaic cells, as it dets. the fraction of incident light entering the device. An optimal p-n junction combined with optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At 1 sun illumination, a short-circuit current of 180 mA cm-2 is obtained, which is more than one order of magnitude higher than that predicted from the Lambert-Beer law. The enhanced light absorption is shown to be due to a light-concg. property of the standing nanowire, as shown by photocurrent maps of the device. The results imply new limits for the max. efficiency obtainable with III-V based nanowire solar cells under 1 sun illumination.
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    Garnett, E. C.; Brongersma, M. L.; Cui, Y.; McGehee, M. D. Nanowire solar cells Annu. Rev. Mater. Res. 2011, 41, 269– 295
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    Nanowire solar cells
    Garnett, Erik C.; Brongersma, Mark L.; Cui, Yi; McGehee, Michael D.
    Annual Review of Materials Research (2011), 41 (), 269-295CODEN: ARMRCU; ISSN:1531-7331. (Annual Reviews Inc.)
    A review. The nanowire geometry provides potential advantages over planar wafer-based or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the max. efficiency above std. limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost redns. Addnl., nanowires provide opportunities to fabricate complex single-cryst. semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technol. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.
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  17. 17
    Mann, S. A.; Garnett, E. C. Extreme light absorption in thin semiconductor films wrapped around metal nanowires Nano Lett. 2013, 13 (7) 3173– 3178
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  18. 18
    Garnett, E. C.; Cai, W. S.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Self-limited plasmonic welding of silver nanowire junctions Nat. Mater. 2012, 11 (3) 241– 249
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    18
    Self-limited plasmonic welding of silver nanowire junctions
    Garnett, Erik C.; Cai, Wenshan; Cha, Judy J.; Mahmood, Fakhruddin; Connor, Stephen T.; Greyson Christoforo, M.; Cui, Yi; McGehee, Michael D.; Brongersma, Mark L.
    Nature Materials (2012), 11 (3), 241-249CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Nanoscience provides many strategies to construct high-performance materials and devices, including solar cells, thermoelecs., sensors, transistors, and transparent electrodes. Bottom-up fabrication facilitates large-scale(coating process) chem. synthesis without the need for patterning and etching processes that waste material and create surface defects. However, assembly and contacting procedures still require further development. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into large interconnected networks. The small gaps that form naturally at nanowire junctions enable effective light concn. and heating at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a phys. connection between the wires is made. The localized nature of the heating prevents damage to low-thermal-budget substrates such as plastics and polymer solar cells. This work opens new avenues to control light, heat and mass transport at the nanoscale.
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  19. 19
    Wu, H.; Kong, D. S.; Ruan, Z. C.; Hsu, P. C.; Wang, S.; Yu, Z. F.; Carney, T. J.; Hu, L. B.; Fan, S. H.; Cui, Y. A transparent electrode based on a metal nanotrough network Nat. Nanotechnol. 2013, 8 (6) 421– 425
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    19
    A transparent electrode based on a metal nanotrough network
    Wu, Hui; Kong, Desheng; Ruan, Zhichao; Hsu, Po-Chun; Wang, Shuang; Yu, Zongfu; Carney, Thomas J.; Hu, Liangbing; Fan, Shanhui; Cui, Yi
    Nature Nanotechnology (2013), 8 (6), 421-425CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide-the prototypical transparent electrode material-demonstrate excellent electronic performances, but film brittleness, low IR transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low cond. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10Ω .box.-1 at 90% transmission because of the high cond. of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the no. of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ∼2Ω .box.-1 at 90% transmission) and remarkable mech. flexibility under both stretching and bending stresses. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.
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  20. 20
    Fan, Z. Y.; Razavi, H.; Do, J. W.; Moriwaki, A.; Ergen, O.; Chueh, Y. L.; Leu, P. W.; Ho, J. C.; Takahashi, T.; Reichertz, L. A.; Neale, S.; Yu, K.; Wu, M.; Ager, J. W.; Javey, A. Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates Nat. Mater. 2009, 8 (8) 648– 653
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    Lambright, S.; Butaeva, E.; Razgoniaeva, N.; Hopkins, T.; Smith, B.; Perera, D.; Corbin, J.; Khon, E.; Thomas, R.; Moroz, P.; Mereshchenko, A.; Tarnovsky, A.; Zamkov, M. Enhanced lifetime of excitons in nonepitaxial Au/CdS core/shell nanocrystals ACS Nano 2014, 8 (1) 352– 361
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    Ha, E.; Lee, L. Y.; Wang, J.; Li, F.; Wong, K. Y.; Tsang, S. C. Significant enhancement in photocatalytic reduction of water to hydrogen by Au/Cu2 ZnSnS4 nanostructure Adv. Mater. 2014, 26 (21) 3496– 500
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    Kuo, C. H.; Hua, T. E.; Huang, M. H. Au nanocrystal-directed growth of Au-Cu2O core-shell heterostructures with precise morphological control J. Am. Chem. Soc. 2009, 131 (49) 17871– 17878
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    23
    Au Nanocrystal-Directed Growth of Au-Cu2O Core-Shell Heterostructures with Precise Morphological Control
    Kuo, Chun-Hong; Hua, Tzu-En; Huang, Michael H.
    Journal of the American Chemical Society (2009), 131 (49), 17871-17878CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Formation of metal-semiconductor core-shell heterostructures with precise morphol. control of both components remains challenging. Heterojunctions, rather than core-shell structures, were typically produced for metal-semiconductor composites. Also, growth of semiconductor shells with systematic shape evolution using the same metal particle cores can also present a significant challenge. Here, the authors synthesized Au-Cu2O core-shell heterostructures using gold nanoplates, nanorods, octahedra, and highly faceted nanoparticles as the structure-directing cores for the overgrowth of Cu2O shells by a facile aq. soln. approach. The gold nanoparticle cores guide the growth of Cu2O shells with morphol. and orientation control. Systematic shape evolution of the shells can be easily achieved by simply adjusting the vol. of reductant added. For example, truncated cubic to octahedral Cu2O shells were produced from octahedral gold nanocrystal cores. Unusual truncated stellated icosahedral and star column structures also were synthesized. The heterostructures are formed via an unusual hollow-shell-refilled growth mechanism not reported before. The approach has potential toward the prepn. of other complex Cu2O structures with well-defined facets.
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    Jiang, R.; Li, B.; Fang, C.; Wang, J. Metal/semiconductor hybrid nanostructures for plasmon-enhanced applications Adv. Mater. 2014,  DOI: 10.1002/adma.201400203
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    Meir, N.; Plante, I. J. L.; Flomin, K.; Chockler, E.; Moshofsky, B.; Diab, M.; Volokh, M.; Mokari, T. Studying the chemical, optical and catalytic properties of noble metal (Pt, Pd, Ag, Au)-Cu2O core-shell nanostructures grown via a general approach J. Mater. Chem. A 2013, 1 (5) 1763– 1769
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    25
    Studying the chemical, optical and catalytic properties of noble metal (Pt, Pd, Ag, Au)-Cu2O core-shell nanostructures grown via a general approach
    Meir, Noga; Jen-La Plante, Ilan; Flomin, Kobi; Chockler, Elina; Moshofsky, Brian; Diab, Mahmud; Volokh, Michael; Mokari, Taleb
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (5), 1763-1769CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The authors studied the chem., optical and catalytic properties of metal (Pt, Pd, Ag, Au)-Cu2O core-shell nanoparticles grown via a simple and reproducible approach which uses aq.-phase reactions at room temp. The authors were able to control the thickness of the Cu2O shell and examine the effect of the core's shape and size on the structure and properties of the hybrid nanocrystals. The authors also studied the optical properties of the hybrid nanocrystals, in particular the effect of the Cu2O shell thickness on the frequency of the plasmon of gold nanorods. In addn., the catalytic activity of the hybrid nanostructures was examd. by testing the redn. reaction of 4-nitrophenol with NaBH4. Finally, the hybrid metal-Cu2O nanostructures were used as templates to form the yolk-shell of metal-Cu2S materials. The interface and the cryst. structures of the four hybrid nanostructures were extensively characterized by high resoln. TEM (HRTEM), energy-filtered TEM (EFTRM) and XRD.
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  26. 26
    Li, J. T.; Cushing, S. K.; Bright, J.; Meng, F. K.; Senty, T. R.; Zheng, P.; Bristow, A. D.; Wu, N. Q. Ag@Cu2O core-shell nanoparticles as visible-light plasmonic photocatalysts ACS Catal. 2013, 3 (1) 47– 51
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    26
    Ag@Cu2O Core-Shell Nanoparticles as Visible-Light Plasmonic Photocatalysts
    Li, Jiangtian; Cushing, Scott K.; Bright, Joeseph; Meng, Fanke; Senty, Tess R.; Zheng, Peng; Bristow, Alan D.; Wu, Nianqiang
    ACS Catalysis (2013), 3 (1), 47-51CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Compared to pristine Cu2O nanoparticles (NPs), Ag@Cu2O core-shell NPs exhibit photocatalytic activity over an extended wavelength range because of the presence of localized surface plasmon resonance (LSPR) in the Ag core. The photocatalysis action spectra and transient absorption measurements show that the plasmonic energy is transferred from the metal to the semiconductor via plasmon-induced resonant energy transfer (PIRET) and direct electron transfer (DET) simultaneously, generating electron-hole pairs in the semiconductor. The LSPR band of Ag@Cu2O core-shell NPs shows a red-shift with an increase in the Cu2O shell thickness, extending the light absorption of Ag@Cu2O heterostructures to longer wavelengths. As a result, the photocatalytic activity of the Ag@Cu2O core-shell NPs is varied by modulation of the shell thickness on the nanometer scale. This work has demonstrated that the Ag@Cu2O core-shell heterostructure is an efficient visible-light plasmonic photocatalyst, which allows for tunable light absorption over the entire visible-light region by tailoring the shell thickness.
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  27. 27
    Cushing, S. K.; Li, J. T.; Meng, F. K.; Senty, T. R.; Suri, S.; Zhi, M. J.; Li, M.; Bristow, A. D.; Wu, N. Q. Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor J. Am. Chem. Soc. 2012, 134 (36) 15033– 15041
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    27
    Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor
    Cushing, Scott K.; Li, Jiangtian; Meng, Fanke; Senty, Tess R.; Suri, Savan; Zhi, Mingjia; Li, Ming; Bristow, Alan D.; Wu, Nianqiang
    Journal of the American Chemical Society (2012), 134 (36), 15033-15041CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously detd. in Au@SiO2@Cu2O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu2O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.
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  28. 28
    Sun, H.; He, J. T.; Wang, J. Y.; Zhang, S. Y.; Liu, C. C.; Sritharan, T.; Mhaisalkar, S.; Han, M. Y.; Wang, D.; Chen, H. Y. Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation J. Am. Chem. Soc. 2013, 135 (24) 9099– 9110
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    Zhang, J. T.; Tang, Y.; Weng, L.; Ouyang, M. Versatile strategy for precisely tailored core@shell nanostructures with single shell layer accuracy: The case of metallic shell Nano Lett. 2009, 9 (12) 4061– 4065
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    Jin, M. S.; Zhang, H.; Wang, J. G.; Zhong, X. L.; Lu, N.; Li, Z. Y.; Xie, Z. X.; Kim, M. J.; Xia, Y. N. Copper can still be epitaxially deposited on palladium nanocrystals to generate core-shell nanocubes despite their large lattice mismatch ACS Nano 2012, 6 (3) 2566– 2573
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    30
    Copper Can Still Be Epitaxially Deposited on Palladium Nanocrystals To Generate Core-Shell Nanocubes Despite Their Large Lattice Mismatch
    Jin, Mingshang; Zhang, Hui; Wang, Jinguo; Zhong, Xiaolan; Lu, Ning; Li, Zhiyuan; Xie, Zhaoxiong; Kim, Moon J.; Xia, Younan
    ACS Nano (2012), 6 (3), 2566-2573CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Here the authors report the synthesis of Pd@Cu core-shell nanocubes via epitaxial growth, where the lattice mismatch is 7.1%. The synthesis involved the use of Pd seeds with different shapes (including cubes, cuboctahedra, and octahedra) for the epitaxial growth of Cu shells. Different from the conventional growth mode, Cu atoms initially nucleated only on a few of the many faces of a Pd seed, onto which more Cu atoms were continuously added to generate Cu blocks. Later, the Cu atoms also started to nucleate and grow on other faces of the Pd seed until the entire surface of the seed was covered by a Cu shell. As a result, the Pd seed was rarely located in the center of each core-shell structure. The final product took a cubic shape enclosed by {100} facets regardless of Pd seeds used because of the selective capping of Cu(100) surface by hexadecylamine. The edge lengths of the Pd@Cu nanocubes could be tuned from 50 to 100 nm by varying the amt. of Pd seeds while keeping the amt. of CuCl2 precursor.
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  31. 31
    Zhang, L.; Jing, H.; Boisvert, G.; He, J. Z.; Wang, H. Geometry control and optical tunability of metal-cuprous oxide core-shell nanoparticles ACS Nano 2012, 6 (4) 3514– 3527
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    Beiley, Z. M.; McGehee, M. D. Modeling low cost hybrid tandem photovoltaics with the potential for efficiencies exceeding 20% Energy Environ. Sci. 2012, 5 (11) 9173– 9179
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    Sun, Y. G.; Mayers, B.; Herricks, T.; Xia, Y. N. Polyol synthesis of uniform silver nanowires: A plausible growth mechanism and the supporting evidence Nano Lett. 2003, 3 (7) 955– 960
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    Polyol Synthesis of Uniform Silver Nanowires: A Plausible Growth Mechanism and the Supporting Evidence
    Sun, Yugang; Mayers, Brian; Herricks, Thurston; Xia, Younan
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  • Abstract

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    Figure 1

    Figure 1. Ag–Cu2O core–shell nanowires. Artist’s impression of a core–shell nanowire illuminated from the top. The light absorption profile is overlaid on the schematic image, and the inset shows the lattice matching at the interface.

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    Figure 2

    Figure 2. Ag–Cu2O core–shell nanowire: (a) in-column-detector (ICD) and (b) secondary electrons (SE) image of a Ag–Cu2O nanowire lying on a substrate; (c) focused ion beam (FIB) cross-sectional view; (d) distribution profile of the total absorbed power averaged over AM 1.5 (unpolarized), calculated from FDTD simulations for a Ag–Cu2O core–shell nanowire in vacuum. The color scale is in 104 W/m2 for a 1 V/m plane wave incident from the top. (e) EDS elemental maps of a Ag–Cu2O nanowire, for Ag, Cu, and O showing the presence of a core–shell geometry.

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    Figure 3

    Figure 3. Optical characterization of a single Ag–Cu2O core–shell nanowire with a core radius of 55 nm and a shell thickness of 65 nm. (a) Experimental (solid lines) and calculated (dashed lines) quantitative absorption spectrum of a single core–shell nanowire in TE (red) and TM (blue) polarizations. (b) FDTD simulation of the photons absorbed in the shell of a Ag–Cu2O nanowire array weighted over the AM 1.5 (red line); photon flux in the solar spectrum (black line); absorption in the Cu2O shell before weighting for AM 1.5 (blue line). (c) Photoluminescence emission of a single core–shell nanowire.

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    Figure 4

    Figure 4. Structural characterization of Ag–Cu2O core–shell nanowire. (a) Bright field TEM of a representative Ag–Cu2O core–shell nanowire, showing a neat contrast between Ag core and Cu2O shell; inset: the HAADF image emphasizes Z contrast in the Ag–Cu2O nanowire. (b) HRTEM image of the interface between core and shell in the region highlighted in (a). The two lattices match at the interface along the [110] direction and show epitaxial growth. (c, d) SAED pattern for an individual Ag nanowire and Ag–Cu2O nanowire, respectively. The indexing of the SAED pattern has been performed along [001̅] and [1̅12] zone axes and shows a cube-on-cube orientation relationship of the Cu2O shell on the Ag core. The spot size included the entire core–shell nanowire shown in (a).

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    Figure 5

    Figure 5. Structural characterization of Ag–Cu2O core–shell nanowire. (a) XRD of Ag–Cu2O nanowire ensemble in the range 20°–90°, showing diffraction peaks of Ag and Cu2O. Top: reference values for Ag (blue) and Cu2O (red) diffraction peaks. (23) Inset: artistic impression of the cube-on-cube orientation between the core and the shell material. (b, c) Structural atomic models of the Ag–Cu2O interface obtained after structural relaxation using DFT calculations for mixed and unmixed Ag/Cu atomic layers, respectively. Brown spheres represent copper atoms, red spheres oxygen atoms, and gray spheres silver atoms.

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      Tang, Jinyao; Huo, Ziyang; Brittman, Sarah; Gao, Hanwei; Yang, Peidong
      Nature Nanotechnology (2011), 6 (9), 568-572CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Semiconductor nanowires are promising for photovoltaic applications, but, so far, nanowire-based solar cells have had lower efficiencies than planar cells made from the same materials, even allowing for the generally lower light absorption of nanowires. It is not clear, therefore, if the benefits of the nanowire structure, including better charge collection and transport and the possibility of enhanced absorption through light trapping, can outweigh the redns. in performance caused by recombination at the surface of the nanowires and at p-n junctions. Here, the authors fabricate core-shell nanowire solar cells with open-circuit voltage and fill factor values superior to those reported for equiv. planar cells, and an energy conversion efficiency of ∼5.4%, which is comparable to that of equiv. planar cells despite low light absorption levels. The device is made using a low-temp. soln.-based cation exchange reaction that creates a heteroepitaxial junction between a single-cryst. CdS core and single-cryst. Cu2S shell. The authors integrate multiple cells on single nanowires in both series and parallel configurations for high output voltages and currents, resp. The ability to produce efficient nanowire-based solar cells with a soln.-based process and Earth-abundant elements could significantly reduce fabrication costs relative to existing high-temp. bulk material approaches.
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    9. 9
      Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Single-crystal metallic nanowires and metal/semiconductor nanowire heterostructures Nature 2004, 430 (7000) 704– 704
      There is no corresponding record for this reference.
    10. 10
      Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Epitaxial core-shell and core-multishell nanowire heterostructures Nature 2002, 420 (6911) 57– 61
      10
      Epitaxial core-shell and core-multishell nanowire heterostructures
      Lauhon, Lincoln J.; Gudiksen, Mark S.; Wang, Deli; Lieber, Charles M.
      Nature (London, United Kingdom) (2002), 420 (6911), 57-61CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Semiconductor heterostructures with modulated compn. and/or doping enable passivation of interfaces and the generation of devices with diverse functions. In this regard, the control of interfaces in nanoscale building blocks with high surface area will be increasingly important in the assembly of electronic and photonic devices. Core-shell heterostructures formed by the growth of cryst. overlayers on nanocrystals offer enhanced emission efficiency, important for various applications. Axial heterostructures also were formed by a 1-dimensional modulation of nanowire compn. and doping. However, modulation of the radial compn. and doping in nanowire structures has received much less attention than planar and nanocrystal systems. Here the authors synthesize Si and Ge core-shell and multishell nanowire heterostructures using a CVD method applicable to a variety of nanoscale materials. The authors' studies of the growth of B-doped Si shells on intrinsic Si and Si-Si oxide core-shell nanowires indicate that homoepitaxy can be achieved at relatively low temps. on clean Si. The authors also demonstrate the possibility of heteroepitaxial growth of cryst. Ge-Si and Si-Ge core-shell structures, in which band-offsets drive hole injection into either Ge core or shell regions. The authors' synthesis of core-multishell structures, including a high-performance coaxially gated field-effect transistor, indicates the general potential of radial heterostructure growth for the development of nanowire-based devices.
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    11. 11
      Garnett, E.; Yang, P. D. Light trapping in silicon nanowire solar cells Nano Lett. 2010, 10 (3) 1082– 1087
      There is no corresponding record for this reference.
    12. 12
      Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Aberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgstrom, M. T. InP nanowire array solar cells achieving 13.8% efficiency by exceeding the ray optics limit Science 2013, 339 (6123) 1057– 1060
      12
      InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit
      Wallentin, Jesper; Anttu, Nicklas; Asoli, Damir; Huffman, Maria; Aaberg, Ingvar; Magnusson, Martin H.; Siefer, Gerald; Fuss-Kailuweit, Peter; Dimroth, Frank; Witzigmann, Bernd; Xu, H. Q.; Samuelson, Lars; Deppert, Knut; Borgstroem, Magnus T.
      Science (Washington, DC, United States) (2013), 339 (6123), 1057-1060CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diam. and the length of the top n-segment were crit. for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nm-diam. nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 V exceeds that of its planar counterpart, despite about 30 times higher surface-to-vol. ratio of the nanowire cell.
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    13. 13
      Pala, R. A.; Liu, J. S. Q.; Barnard, E. S.; Askarov, D.; Garnett, E. C.; Fan, S. H.; Brongersma, M. L. Optimization of non-periodic plasmonic light-trapping layers for thin-film solar cells. Nat. Commun. 2013, 4.
      There is no corresponding record for this reference.
    14. 14
      Tian, B.; Kempa, T. J.; Lieber, C. M. Single nanowire photovoltaics Chem. Soc. Rev. 2009, 38 (1) 16– 24
      There is no corresponding record for this reference.
    15. 15
      Krogstrup, P.; Jorgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Morral, A. F. I. Single-nanowire solar cells beyond the Shockley-Queisser limit Nat. Photonics 2013, 7 (4) 306– 310
      15
      Single-nanowire solar cells beyond the Shockley-Queisser limit
      Krogstrup, Peter; Jorgensen, Henrik Ingerslev; Heiss, Martin; Demichel, Olivier; Holm, Jeppe V.; Aagesen, Martin; Nygard, Jesper; Fontcuberta i Morral, Anna
      Nature Photonics (2013), 7 (4), 306-310CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      Light management is of great importance in photovoltaic cells, as it dets. the fraction of incident light entering the device. An optimal p-n junction combined with optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At 1 sun illumination, a short-circuit current of 180 mA cm-2 is obtained, which is more than one order of magnitude higher than that predicted from the Lambert-Beer law. The enhanced light absorption is shown to be due to a light-concg. property of the standing nanowire, as shown by photocurrent maps of the device. The results imply new limits for the max. efficiency obtainable with III-V based nanowire solar cells under 1 sun illumination.
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    16. 16
      Garnett, E. C.; Brongersma, M. L.; Cui, Y.; McGehee, M. D. Nanowire solar cells Annu. Rev. Mater. Res. 2011, 41, 269– 295
      16
      Nanowire solar cells
      Garnett, Erik C.; Brongersma, Mark L.; Cui, Yi; McGehee, Michael D.
      Annual Review of Materials Research (2011), 41 (), 269-295CODEN: ARMRCU; ISSN:1531-7331. (Annual Reviews Inc.)
      A review. The nanowire geometry provides potential advantages over planar wafer-based or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the max. efficiency above std. limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost redns. Addnl., nanowires provide opportunities to fabricate complex single-cryst. semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technol. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.
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    17. 17
      Mann, S. A.; Garnett, E. C. Extreme light absorption in thin semiconductor films wrapped around metal nanowires Nano Lett. 2013, 13 (7) 3173– 3178
      There is no corresponding record for this reference.
    18. 18
      Garnett, E. C.; Cai, W. S.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Self-limited plasmonic welding of silver nanowire junctions Nat. Mater. 2012, 11 (3) 241– 249
      18
      Self-limited plasmonic welding of silver nanowire junctions
      Garnett, Erik C.; Cai, Wenshan; Cha, Judy J.; Mahmood, Fakhruddin; Connor, Stephen T.; Greyson Christoforo, M.; Cui, Yi; McGehee, Michael D.; Brongersma, Mark L.
      Nature Materials (2012), 11 (3), 241-249CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Nanoscience provides many strategies to construct high-performance materials and devices, including solar cells, thermoelecs., sensors, transistors, and transparent electrodes. Bottom-up fabrication facilitates large-scale(coating process) chem. synthesis without the need for patterning and etching processes that waste material and create surface defects. However, assembly and contacting procedures still require further development. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into large interconnected networks. The small gaps that form naturally at nanowire junctions enable effective light concn. and heating at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a phys. connection between the wires is made. The localized nature of the heating prevents damage to low-thermal-budget substrates such as plastics and polymer solar cells. This work opens new avenues to control light, heat and mass transport at the nanoscale.
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    19. 19
      Wu, H.; Kong, D. S.; Ruan, Z. C.; Hsu, P. C.; Wang, S.; Yu, Z. F.; Carney, T. J.; Hu, L. B.; Fan, S. H.; Cui, Y. A transparent electrode based on a metal nanotrough network Nat. Nanotechnol. 2013, 8 (6) 421– 425
      19
      A transparent electrode based on a metal nanotrough network
      Wu, Hui; Kong, Desheng; Ruan, Zhichao; Hsu, Po-Chun; Wang, Shuang; Yu, Zongfu; Carney, Thomas J.; Hu, Liangbing; Fan, Shanhui; Cui, Yi
      Nature Nanotechnology (2013), 8 (6), 421-425CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide-the prototypical transparent electrode material-demonstrate excellent electronic performances, but film brittleness, low IR transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low cond. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10Ω .box.-1 at 90% transmission because of the high cond. of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the no. of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ∼2Ω .box.-1 at 90% transmission) and remarkable mech. flexibility under both stretching and bending stresses. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.
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    20. 20
      Fan, Z. Y.; Razavi, H.; Do, J. W.; Moriwaki, A.; Ergen, O.; Chueh, Y. L.; Leu, P. W.; Ho, J. C.; Takahashi, T.; Reichertz, L. A.; Neale, S.; Yu, K.; Wu, M.; Ager, J. W.; Javey, A. Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates Nat. Mater. 2009, 8 (8) 648– 653
      There is no corresponding record for this reference.
    21. 21
      Lambright, S.; Butaeva, E.; Razgoniaeva, N.; Hopkins, T.; Smith, B.; Perera, D.; Corbin, J.; Khon, E.; Thomas, R.; Moroz, P.; Mereshchenko, A.; Tarnovsky, A.; Zamkov, M. Enhanced lifetime of excitons in nonepitaxial Au/CdS core/shell nanocrystals ACS Nano 2014, 8 (1) 352– 361
      There is no corresponding record for this reference.
    22. 22
      Ha, E.; Lee, L. Y.; Wang, J.; Li, F.; Wong, K. Y.; Tsang, S. C. Significant enhancement in photocatalytic reduction of water to hydrogen by Au/Cu2 ZnSnS4 nanostructure Adv. Mater. 2014, 26 (21) 3496– 500
      There is no corresponding record for this reference.
    23. 23
      Kuo, C. H.; Hua, T. E.; Huang, M. H. Au nanocrystal-directed growth of Au-Cu2O core-shell heterostructures with precise morphological control J. Am. Chem. Soc. 2009, 131 (49) 17871– 17878
      23
      Au Nanocrystal-Directed Growth of Au-Cu2O Core-Shell Heterostructures with Precise Morphological Control
      Kuo, Chun-Hong; Hua, Tzu-En; Huang, Michael H.
      Journal of the American Chemical Society (2009), 131 (49), 17871-17878CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Formation of metal-semiconductor core-shell heterostructures with precise morphol. control of both components remains challenging. Heterojunctions, rather than core-shell structures, were typically produced for metal-semiconductor composites. Also, growth of semiconductor shells with systematic shape evolution using the same metal particle cores can also present a significant challenge. Here, the authors synthesized Au-Cu2O core-shell heterostructures using gold nanoplates, nanorods, octahedra, and highly faceted nanoparticles as the structure-directing cores for the overgrowth of Cu2O shells by a facile aq. soln. approach. The gold nanoparticle cores guide the growth of Cu2O shells with morphol. and orientation control. Systematic shape evolution of the shells can be easily achieved by simply adjusting the vol. of reductant added. For example, truncated cubic to octahedral Cu2O shells were produced from octahedral gold nanocrystal cores. Unusual truncated stellated icosahedral and star column structures also were synthesized. The heterostructures are formed via an unusual hollow-shell-refilled growth mechanism not reported before. The approach has potential toward the prepn. of other complex Cu2O structures with well-defined facets.
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    24. 24
      Jiang, R.; Li, B.; Fang, C.; Wang, J. Metal/semiconductor hybrid nanostructures for plasmon-enhanced applications Adv. Mater. 2014,  DOI: 10.1002/adma.201400203
      There is no corresponding record for this reference.
    25. 25
      Meir, N.; Plante, I. J. L.; Flomin, K.; Chockler, E.; Moshofsky, B.; Diab, M.; Volokh, M.; Mokari, T. Studying the chemical, optical and catalytic properties of noble metal (Pt, Pd, Ag, Au)-Cu2O core-shell nanostructures grown via a general approach J. Mater. Chem. A 2013, 1 (5) 1763– 1769
      25
      Studying the chemical, optical and catalytic properties of noble metal (Pt, Pd, Ag, Au)-Cu2O core-shell nanostructures grown via a general approach
      Meir, Noga; Jen-La Plante, Ilan; Flomin, Kobi; Chockler, Elina; Moshofsky, Brian; Diab, Mahmud; Volokh, Michael; Mokari, Taleb
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (5), 1763-1769CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The authors studied the chem., optical and catalytic properties of metal (Pt, Pd, Ag, Au)-Cu2O core-shell nanoparticles grown via a simple and reproducible approach which uses aq.-phase reactions at room temp. The authors were able to control the thickness of the Cu2O shell and examine the effect of the core's shape and size on the structure and properties of the hybrid nanocrystals. The authors also studied the optical properties of the hybrid nanocrystals, in particular the effect of the Cu2O shell thickness on the frequency of the plasmon of gold nanorods. In addn., the catalytic activity of the hybrid nanostructures was examd. by testing the redn. reaction of 4-nitrophenol with NaBH4. Finally, the hybrid metal-Cu2O nanostructures were used as templates to form the yolk-shell of metal-Cu2S materials. The interface and the cryst. structures of the four hybrid nanostructures were extensively characterized by high resoln. TEM (HRTEM), energy-filtered TEM (EFTRM) and XRD.
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    26. 26
      Li, J. T.; Cushing, S. K.; Bright, J.; Meng, F. K.; Senty, T. R.; Zheng, P.; Bristow, A. D.; Wu, N. Q. Ag@Cu2O core-shell nanoparticles as visible-light plasmonic photocatalysts ACS Catal. 2013, 3 (1) 47– 51
      26
      Ag@Cu2O Core-Shell Nanoparticles as Visible-Light Plasmonic Photocatalysts
      Li, Jiangtian; Cushing, Scott K.; Bright, Joeseph; Meng, Fanke; Senty, Tess R.; Zheng, Peng; Bristow, Alan D.; Wu, Nianqiang
      ACS Catalysis (2013), 3 (1), 47-51CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Compared to pristine Cu2O nanoparticles (NPs), Ag@Cu2O core-shell NPs exhibit photocatalytic activity over an extended wavelength range because of the presence of localized surface plasmon resonance (LSPR) in the Ag core. The photocatalysis action spectra and transient absorption measurements show that the plasmonic energy is transferred from the metal to the semiconductor via plasmon-induced resonant energy transfer (PIRET) and direct electron transfer (DET) simultaneously, generating electron-hole pairs in the semiconductor. The LSPR band of Ag@Cu2O core-shell NPs shows a red-shift with an increase in the Cu2O shell thickness, extending the light absorption of Ag@Cu2O heterostructures to longer wavelengths. As a result, the photocatalytic activity of the Ag@Cu2O core-shell NPs is varied by modulation of the shell thickness on the nanometer scale. This work has demonstrated that the Ag@Cu2O core-shell heterostructure is an efficient visible-light plasmonic photocatalyst, which allows for tunable light absorption over the entire visible-light region by tailoring the shell thickness.
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    27. 27
      Cushing, S. K.; Li, J. T.; Meng, F. K.; Senty, T. R.; Suri, S.; Zhi, M. J.; Li, M.; Bristow, A. D.; Wu, N. Q. Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor J. Am. Chem. Soc. 2012, 134 (36) 15033– 15041
      27
      Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor
      Cushing, Scott K.; Li, Jiangtian; Meng, Fanke; Senty, Tess R.; Suri, Savan; Zhi, Mingjia; Li, Ming; Bristow, Alan D.; Wu, Nianqiang
      Journal of the American Chemical Society (2012), 134 (36), 15033-15041CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously detd. in Au@SiO2@Cu2O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu2O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.
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    28. 28
      Sun, H.; He, J. T.; Wang, J. Y.; Zhang, S. Y.; Liu, C. C.; Sritharan, T.; Mhaisalkar, S.; Han, M. Y.; Wang, D.; Chen, H. Y. Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation J. Am. Chem. Soc. 2013, 135 (24) 9099– 9110
      There is no corresponding record for this reference.
    29. 29
      Zhang, J. T.; Tang, Y.; Weng, L.; Ouyang, M. Versatile strategy for precisely tailored core@shell nanostructures with single shell layer accuracy: The case of metallic shell Nano Lett. 2009, 9 (12) 4061– 4065
      There is no corresponding record for this reference.
    30. 30
      Jin, M. S.; Zhang, H.; Wang, J. G.; Zhong, X. L.; Lu, N.; Li, Z. Y.; Xie, Z. X.; Kim, M. J.; Xia, Y. N. Copper can still be epitaxially deposited on palladium nanocrystals to generate core-shell nanocubes despite their large lattice mismatch ACS Nano 2012, 6 (3) 2566– 2573
      30
      Copper Can Still Be Epitaxially Deposited on Palladium Nanocrystals To Generate Core-Shell Nanocubes Despite Their Large Lattice Mismatch
      Jin, Mingshang; Zhang, Hui; Wang, Jinguo; Zhong, Xiaolan; Lu, Ning; Li, Zhiyuan; Xie, Zhaoxiong; Kim, Moon J.; Xia, Younan
      ACS Nano (2012), 6 (3), 2566-2573CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Here the authors report the synthesis of Pd@Cu core-shell nanocubes via epitaxial growth, where the lattice mismatch is 7.1%. The synthesis involved the use of Pd seeds with different shapes (including cubes, cuboctahedra, and octahedra) for the epitaxial growth of Cu shells. Different from the conventional growth mode, Cu atoms initially nucleated only on a few of the many faces of a Pd seed, onto which more Cu atoms were continuously added to generate Cu blocks. Later, the Cu atoms also started to nucleate and grow on other faces of the Pd seed until the entire surface of the seed was covered by a Cu shell. As a result, the Pd seed was rarely located in the center of each core-shell structure. The final product took a cubic shape enclosed by {100} facets regardless of Pd seeds used because of the selective capping of Cu(100) surface by hexadecylamine. The edge lengths of the Pd@Cu nanocubes could be tuned from 50 to 100 nm by varying the amt. of Pd seeds while keeping the amt. of CuCl2 precursor.
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    31. 31
      Zhang, L.; Jing, H.; Boisvert, G.; He, J. Z.; Wang, H. Geometry control and optical tunability of metal-cuprous oxide core-shell nanoparticles ACS Nano 2012, 6 (4) 3514– 3527
      There is no corresponding record for this reference.
    32. 32
      Beiley, Z. M.; McGehee, M. D. Modeling low cost hybrid tandem photovoltaics with the potential for efficiencies exceeding 20% Energy Environ. Sci. 2012, 5 (11) 9173– 9179
      There is no corresponding record for this reference.
    33. 33
      Sun, Y. G.; Mayers, B.; Herricks, T.; Xia, Y. N. Polyol synthesis of uniform silver nanowires: A plausible growth mechanism and the supporting evidence Nano Lett. 2003, 3 (7) 955– 960
      33
      Polyol Synthesis of Uniform Silver Nanowires: A Plausible Growth Mechanism and the Supporting Evidence
      Sun, Yugang; Mayers, Brian; Herricks, Thurston; Xia, Younan
      Nano Letters (2003), 3 (7), 955-960CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors have recently demonstrated an approach based on the polyol process for the large-scale synthesis of silver nanowires with uniform diams. (see Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833. Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736). Although the capability and feasibility of this method were successfully illustrated with the prodn. of silver nanowires 30-60 nm in diam. and 1-50 μm in length, the growth mechanism of this process is yet to be elucidated. Some of the progress are reported. Electron microscopy studies on microtomed samples indicated that the cross sections of such silver nanowires had a pentagonal shape, together with a 5-fold twinned crystal structure. The side surfaces (bounded by {100} facets) and the ends (bounded by {111} facets) of each nanowire have significant difference in reactivity toward dithoil mols., with the side surfaces being completely passivated by poly(vinyl pyrrolidone) (PVP) and the ends being partially passivated (or essentially uncovered) by PVP. This result implied that the PVP macromols. interacted more strongly with the {100} planes than with the {111} planes of silver. From these new results, probably each silver nanowire evolved from a multiply twinned nanoparticle (MTP) of silver with the assistance of PVP at the initial stage of the Ostwald ripening process. The anisotropic growth was maintained by selectively covering the {100} facets with PVP while leaving the {111} facets largely uncovered by PVP and thus highly reactive.
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    34. 34
      Jang, J. I. A unique system hosting various excitonic matter and exhibiting large third-order nonlinear optical responses. Optoelectronics - Materials and Techniques, 2011; DOI  DOI: 10.5772/18416 .
      There is no corresponding record for this reference.
    35. 35
      Li, J. Q.; Mei, Z. X.; Ye, D. Q.; Liang, H. L.; Liu, L. S.; Liu, Y. P.; Galeckas, A.; Kuznetsov, A. Y.; Du, X. L. Engineering of optically defect free Cu2O enabling exciton luminescence at room temperature Opt. Mater. Express 2013, 3 (12) 2072– 2077
      35
      Engineering of optically defect free Cu2O enabling exciton luminescence at room temperature
      Li, Junqiang; Mei, Zenxia; Ye, Daqian; Liang, Huili; Liu, Lishu; Liu, Yaoping; Galeckas, Augustinas; Kuznetsov, Andrej Yu; Du, Xiaolong
      Optical Materials Express (2013), 3 (12), 2072-2077, 6 pp.CODEN: OMEPAX; ISSN:2159-3930. (Optical Society of America)
      Cu2O is an interesting semiconductor with extraordinary high exciton binding energy, however exhibiting weak room temp. excitonic luminescence. The issue was addressed in literature emphasizing a detrimental role of native point defects responsible for optical quenching. Resolving the problem, we propose a method to manipulate the Cu and O vacancies contents opening a gateway for optoelectronic applications of Cu2O. Specifically, applying oxygen lean conditions, we observe a remarkable suppression of VCu enabling strong room temp. exciton luminescence, while manipulating with VO reveals no impact on the signal. As a result, the excitonic signature was interpreted in terms of phonon assisted transitions.
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    36. 36
      Biccari, F. Defects and Doping in Cu2O; University of Rome: Rome, 2012.
      There is no corresponding record for this reference.
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      Gu, Q.; Wang, B. Correlation between structural defects and optical properties of Cu2O nanowires grown by thermal oxidation. arXiv:1012.5338, 2011.
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      Srnova-Sloufova, I.; Lednicky, F.; Gemperle, A.; Gemperlova, J. Core-shell (Ag)Au bimetallic nanoparticles: Analysis of transmission electron microscopy images Langmuir 2000, 16 (25) 9928– 9935
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      Oba, F.; Ernst, F.; Yu, Y. S.; Liu, R.; Kothari, M.; Switzer, J. A. Epitaxial growth of cuprous oxide electrodeposited onto semiconductor and metal substrates J. Am. Ceram. Soc. 2005, 88 (2) 253– 270
      There is no corresponding record for this reference.
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      Ervik, T.; Stokkan, G.; Buonassisi, T.; Mjos, O.; Lohne, O. Dislocation formation in seeds for quasi-monocrystalline silicon for solar cells Acta Mater. 2014, 67, 199– 206
      40
      Dislocation formation in seeds for quasi-monocrystalline silicon for solar cells
      Ervik, Torunn; Stokkan, Gaute; Buonassisi, Tonio; Mjoes, Oeyvind; Lohne, Otto
      Acta Materialia (2014), 67 (), 199-206CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
      An investigation of two industrially cast quasi-monocryst. silicon blocks revealed a high dislocation d. originating at intersections between the seed crystals. This may be ascribed to three different generation mechanisms. Firstly, a dislocation cell structure was obsd. in the seed crystals, probably as an effect of poor surface prepn. of the seeds. Furthermore, clusters of dislocations form around contact points in the interface between two neighboring seeds. At contact points, the two monocryst. silicon seeds plastically deform and sinter together. Dislocation rosettes form as a result of an indentation mechanism at high temps. A third mechanism acts at the bottom surface, where dislocation clusters also form by indentation of contact points between the seed and the crucible. Since dislocations forming in the seeds will continue into the growing ingot, it is crucial to depress the dislocation formation in the seeds.
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      Kresse, G.; Hafner, J. Ab initio molecular-dynamics for liquid-metals Phys. Rev. B 1993, 47 (1) 558– 561
      There is no corresponding record for this reference.
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      Kresse, G.; Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set Comput. Mater. Sci. 1996, 6 (1) 15– 50
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      Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
      Kresse, G.; Furthmuller, J.
      Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
      The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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      Olsen, L. C.; Addis, F. W.; Miller, W. Experimental and theoretical studies of Cu2O solar cells Sol. Cells 1982–1983, 7, 247– 279
      43
      Experimental and theoretical studies of copper(I) oxide solar cells
      Olsen, L. C.; Addis, F. W.; Miller, W.
      Solar Cells (1982), 7 (3), 247-79CODEN: SOCLD4; ISSN:0379-6787.
      Schottky-barrier devices based on metals of widely different work functions were investigated. Cu-Cu2O cells were developed that exhibited active-area air-mass-1 values of photocurrent and efficiency of 8.5 mA/cm2 and 1.8%, resp. A detailed photon and carrier loss anal. conducted for Cu-Cu2O cells was used to project the ultimate values of the photocurrent for Cu2O cells to be 12-14 mA/cm2. From the thermodn. considerations, Tl is the only metal which can be combined with Cu2O to yield an adequate efficiency. However, Tl-Cu2O Schottky-barrier cells exhibit properties similar to Cu-Cu2O devices. Depth-concn. profiles show that, although no Tl-O bonding exists in the interfacial region, the region is Cu rich. The O deficiency occurs because of preferential sputtering of O during the Tl deposition process. As a result of these Cu2O Schottky-barrier studies, significant improvements in the efficiency of Cu2O solar cells can be achieved only with a homojunction structure. Thus, an approach to doping Cu2O n-type must be developed to realize the potential of this material for low-cost photovoltaics.
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      Ram, S.; Mitra, C. Formation of stable Cu2O nanocrystals in a new orthorhombic crystal structure Mater. Sci. Eng., A 2001, 304, 805– 809
      44
      Formation of stable Cu2O nanocrystals in a new orthorhombic crystal structure
      Ram, S.; Mitra, C.
      Materials Science & Engineering, A: Structural Materials: Properties, Microstructure and Processing (2001), A304-306 (), 805-809CODEN: MSAPE3; ISSN:0921-5093. (Elsevier Science S.A.)
      Substantially stable Cu2O nanocrystals of 10-30 nm in size were synthesized by an ion exchange Cu2+ → Cu → Cu+ reaction in an aq. soln. (using a reducing agent NaBH4) at 80-100°. In such small particles, a large fraction of surface atoms, i.e. 20% or even more, which suffer with a reduced coordination no. of the core atoms, support the formation of the low oxidn. state Cu2O oxide of copper in a stable structure. Otherwise, CuO is the most stable form of its oxides. The 10-30 nm Cu2O nanocrystals have a modified x-ray diffractogram of FCC bulk Cu2O structure. The diffractogram is very simple with a total of only seven peaks, over 0.25-0.12 nm dhkl interplanar spacing, which are successfully indexed assuming an orthorhombic crystal structure with lattice parameters a 0.421, b 0.324 and c 0.361 nm (at 25 nm size of the sample).
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  • Supporting Information

    Supporting Information

    Details on the synthesis of Ag nanowires and the Cu2O shell growth, experimental details on the structural and optical characterization, details on the density functional theory (DFT) calculations, and additional SEM and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.


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