Crystallinity, Surface Morphology, and Photoelectrochemical Effects in

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Crystallinity, surface morphology, and photoelectrochemical effects in conical InP and InN nanowires grown on silicon Vijay Parameshwaran, Xiaoqing Xu, and Bruce M. Clemens ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05749 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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ACS Applied Materials & Interfaces

Crystallinity, surface morphology, and photoelectrochemical effects in conical InP and InN nanowires grown on silicon Vijay Parameshwaran,†,‡ Xiaoqing Xu,¶ and Bruce Clemens∗,‡ †Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States ‡Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ¶Stanford Nanofabrication Facility, Stanford University, Stanford, California 94305, United States E-mail: [email protected] Abstract In this work, the growth conditions of two types of indium-based III-V nanowires, InP and InN, are tailored such that instead of yielding conventional wire-type morphologies, single-crystal conical structures are formed with an enlarged diameter either near the base or near the tip. By using indium droplets as a growth catalyst, combined with an excess indium supply during growth, “ice-cream cone” type structures are formed with a nanowire “cone” and an indium-based “ice cream” droplet on top for both InP and InN. Surface polycrystallinity and annihilation of the catalyst tip of the conical InP nanowires are observed when the indium supply is turned off during the growth process. This growth design technique is extended to create single-crystal InN nanowires with the same morphology. Conical InN nanowires with an enlarged base

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are obtained through the use of an excess combined Au-In growth catalyst. Electrochemical studies of the InP nanowires on silicon demonstrate a reduction photocurrent as a proof of photovolatic behavior, and provide insight as to how the observed surface polycrystallinity and the resulting interface affects these device-level properties. Additionally, a photovoltage is induced in both types of conical InN nanowires on silicon, which is not replicated in epitaxial InN thin films.

Keywords Nanowires, chemical vapor deposition, photovoltaics, electrochemistry, semiconductors

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Introduction

The ability to engineer crystal growth processes for high quality III-V nanowires on silicon opens up several possibilities of electronic and photonic devices without relying on homoepitaxy with relatively expensive substrates. This has particular significance within solar energy conversion technologies. It eliminates the need for expensive growth stacks and substrates in traditional multijunction photovoltaics architectures, yet it still preserves single crystal domains within the photoactive III-V material for efficient light absorption and charge transport. Recent engineering work has demonstrated high efficiencies for single-nanowire gallium arsenide 1 (GaAs) and gallium arsenide phosphide 2 (GaAsP) solar cells grown on silicon, with designs shown as envisioning a spectrum-splitting array of III-V nanowires that achieves a multijunction effect. 3 These advances show that III-V nanowire growth on silicon is a basis for future designs in nanoscale-based photovoltaics. Nanowire-based semiconductor materials are also of interest for driving solar-driven electrochemial reactions such as the hydrogen evolution reaction and the oxygen evolution reaction for photoelectrochemical (PEC) water splitting. The nanowire morphology provides a larger overall surface area for catalyst loading to improve overpotential losses, and im2 ACS Paragon Plus Environment

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proves charge carrier collection efficiency as compared to films and bulk crystals. 4 Moreover, nanowire growth uses less material than a thin film or bulk crystal growth, allowing for scalable architectures or dispersed arrays of nanoscale devices to work as high-efficiency photocatalysts. 5 The materials chosen for study as nanowires in this work are indium phosphide (InP) and indium nitride (InN). InP is a material of choice due to its direct bandgap energy of 1.3 eV being close to the ideal value for the Shockley-Queisser limit in photovoltaics. This bandgap also matches the requirement for one of the absorber materials in a dual electrode PEC device. 6 Furthermore its low surface recombination velocity, as compared to GaAs, 7 makes it ideal for use in a nanowire morphology which has a high surface area-to-volume ratio. InP nanowires have been demonstrated within both high-efficiency solar cells 8 and photocathodes for the hydrogen evolution reaction, 9 but these designs rely on epitaxial growth using InP substrates. Extending these innovations to a less costly substrate such as silicon can help in proliferation for lower-cost renewable energy applications. InN is a material of choice due to the bandgap energy of the InGaN system being nominally tunable between the visible and infrared light spectrum by varying the In/Ga ratio in the ternary alloy. However, high-quality InGaN with more than 30-40% indium incorporation in GaN is difficult to achieve due to spinodal decomposition and subsequent microstructure and threading dislocations within a film. 10 This limits its use in solar energy conversion as its bandgap cannot be lowered to a value that allows for absorption of a large amount of incident photons. One advantage of growing InGaN as nanowires is the accommodation of strain-relaxed growth, which improves alloy solubility and minimizes phase separation. 11 This has led to demonstrations in materials synthesis showing full compositional tunability between InN and GaN. 12 GaN and gallium-rich InGaN nanowires have been explored in both solar cells 13,14 and photoelectrodes for water splitting, 15–17 but little work has been done with indium-rich InGaN or InN nanowires. 18 Performing electrochemical studies on InN nanowires can give insight on how indium-rich InGaN nanowires, which are more favor-



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able for solar energy conversion as they absorb more of the solar spectrum as compared to gallium-rich InGaN nanowires, operate under device-level conditions. Herein, this work describes the growth of conical InP and InN nanowires on silicon through chemical vapor deposition (CVD) processes, and how the subsequent development in morphology and crystallinity affects device-level and interface properties through analysis of electrolyte junctions formed with methyl viologen (MV+/++ ) and cobaltocene/cobaltocenium (CoCp2 0/+ ) in non-aqueous solvents. By viewing photocurrent and photovoltage under these conditions, important insights correlated to the photovoltaic behavior can be obtained in an electrode junction created at the silicon/nanowire/electrolyte interface. These insights provide a template for engineering III-V nanowire morphologies during the growth process to enable advanced photonic design for light absorption in solar energy conversion devices.

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Experimental methods

Materials growth InP nanowires The InP nanowires are grown on single crystal (100) boron-doped silicon wafers with a resistivity of 0.001 - 0.005 Ω-cm (MTI Corp.). After native oxide removal, growth is performed at a temperature of 375 ◦ C with trimethylindium (TMIn) and tertiarybutylphosphine (TBP) precursors for 16 minutes at a V/III ratio of 28.65. Indium droplets are deposited by decomposing TMIn for 15 seconds. Once growth is finished, the system is cooled down under a TBP overpressure. No active dopant source is flowed. A variation of the nanowire synthesis has the growth step proceed for 8 minutes, after which the trimethylindium source is turned off and the process continues at a temperature of 375 ◦ C under a tertiarybutylphosphine overpressure for another 8 minutes before cooldown. A variation of this growth process is conducted on single crystal (100) zinc-doped GaAs wafers with a nominal carrier density of


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1 × 1019 cm−3 (Wafer Technology Ltd.). All growth experiments are done in an Aixtron AIX 200/4 MOCVD reactor. A more detailed description of the growth conditions is given in the Supporting Information.

InN nanowires The InN nanowires are grown on single crystal (100) boron-doped silicon wafers with a resistivity of 0.001 - 0.005 Ω-cm (MTI Corp.). For the gold-catalyzed wires, after native oxide removal, poly-L-lysine solution (0.1% w/v, Ted Pella) is pipetted onto the surface of the substrates for 2 minutes to form a positively charged surface, and then blown off with a nitrogen stream. A colloidal solution of gold nanoparticles (50 nm diameter, BBI Solutions) is then drop cast onto the substrates. After 5 minutes, the colloidal solution is blown off with a nitrogen stream. The substrates with gold nanoparticles are loaded into a single-zone tube furnace with a metal indium source upstream. After heatup to 550 ◦ C under an argon overpressure, the growth is carried out under a flow of NH3 at a pressure of 6 Torr for 240 minutes. For the indium-catalyzed wires, the bare silicon substrates are loaded into a single-zone tube furnace with a metal indium source upstream. The temperature gradient of the furnace is used to decouple the indium metal volatilization heating and the growth temperature on the substrate. After heatup to 700 ◦ C under an argon overpressure, with measured temperature of the indium source at 710 ◦ C and the silicon substrate at 493 ◦ C, these conditions are held for 20 minutes to promote indium transport to the substrate as surface droplets. The gas is switched to a 80%/20% mixture of N2 /NH3 at a dynamic pressure of 5 Torr, and the nominal furnace temperature is decreased to 618 ◦ C with the indium source at a measured temperature of 620 ◦ C and the silicon substrate at a measured temperature of 410 ◦ C. The growth is carried out for 150 minutes. A more detailed description of the growth conditions is given in the Supporting Information.



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Materials characterization X-ray diffraction (XRD) is performed using a PANAlytical X’Pert PRO system with a CuKα1 wavelength of 1.5405 Å. The hybrid monochromator is removed in order to increase the amount of x-ray fluence on the sample. Grazing incidence XRD (GIXD) scans, with ω fixed at 2◦ , are used to analyze crystallinity. Specifically, the (100), (002), and (101) peaks are analyzed as characteristic diffraction peaks for wurtzite hexagonal materials. Scanning electron microscopy (SEM) is performed using a FEI Magellan 400 XHR system with the electron beam operating at a voltage of 5 kV and a current of 25 pA. Transmission electron microscopy (TEM) is performed using a FEI Tecnai G2 F20 X-TWIN system with the electron beam operating at a voltage of 200 kV, and is used in imaging mode to visualize the crystallinity and surface properties, fast Fourier transform (FFT) mode to view the lattice spacing and crystallinity in reciprocal space, and energy dispersive x-ray spectroscopy (EDX) mode for elemental analysis. Auger electron spectroscopy (AES) two-dimensional mapping is performed using a PHI 700 Scanning Auger Nanoprobe system with the electron beam operating at a voltage of 25 kV and a current of 10 nA.

Electrode fabrication and chemicals preparation An indium-gallium eutectic is applied on the back side of the silicon, and a copper wire is soldered onto the liquid contact with pure indium. The wire, rear contact, and sample edges are protected by applying Apiezon Black Wax W. For measuring the photoreduction current in the InP nanowires, the reference electrode is a silver wire, anodized in HCl to create an AgCl coating, immersed in a 0.3 M NH4 Cl filling solution, encased in a glass tube with a fritted tip (CH Instruments), and calibrated to the Fc/Fc+ potential. The electrolyte is 0.1 M NH4 Cl (99.5%, Sigma-Aldrich), and is prepared using anhydrous methanol. The redox salt is methyl viologen dichloride (98%, Sigma-Aldrich), added at a concentration of 40 mM to form the MV+/++ redox couple. The counter electrode is a glassy carbon disk integrated within a Kel-F rod (CH Instruments). 6 ACS Paragon Plus Environment

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Additional details are provided in the Supporting Information. For measuring the photovoltage in the InN nanowires, the electrolyte is prepared using anhydrous acetonitrile (99.8%, Sigma-Aldrich). The redox salt is bis(cyclopentadienyl)cobalt(II) (Sigma-Aldrich), added at a concentration of 2 mM. The supporting electrolyte salt is is 0.1 M tetramethylammonium hexafluorophosphate (TMAHFP, ≥ 98%, Sigma-Aldrich). The reference electrode is an Ag wire immersed in a 0.1 M TMAHFP/10 mM AgNO3 filling solution, encased in a glass tube with a fritted tip (CH Instruments), and calibrated to the Fc/Fc+ potential. A series of cyclic voltammetry scans are taken around the open-circuit voltage for the solution with the dissolved salt to ensure the presence of both CoCp2 0 and CoCp2 + species during photovoltage measurements. Additional details are provided in the Supporting Information.

Photoelectrochemistry Lock-in photocurrent measurements are performed with illumination from a high-power Thorlabs white LED operating at a power density of 6.33 mW/cm2 . The output light is focused with a condenser lens. The LED is biased to apply a square wave signal at a frequency of 7 Hz such that chemical transients can follow the incident light signal. The photocurrent is measured by a Pine Research AFRDE5 potentiostat, with the current output connected into a Stanford Research Systems 530 lock-in amplifier. Photovoltage measurements are performed with illumination from a Newport Oriel Sol3A solar simulator lamp at 1 sun (100 mW/cm2 ) intensity. The shutter is chopped over the duration of the measurement to measure the open-circuit voltage Voc both in the dark and in 1 sun illumination conditions. Electrochemical impedance spectroscopy (EIS) measurements are performed with a Bio-Logic SP-200 potentiostat using a 10 mV peak-to-peak AC voltage and a frequency range between 1 Hz and 500 kHz. The data is fit with EC-Lab ZFit software to extract the junction capacitance C using Randle’s circuit model. The area used to normalize the photocurrent and the capacitance is the physical area of the samples exposed in electrolyte. The area was 7 ACS Paragon Plus Environment


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obtained from the as-fabricated electrodes by using ImageJ software with a ruler for scale. Although the effective area can change based on a three-dimensional morphology introduced from the nanowires, the density and length are small enough such that the only apparent change is in a roughness factor γ. Since a goal is not in showing competitive performance of these materials, but instead viewing the physical and interface effects, the physical electrode area is used for normalization without factoring in changes with surface roughness. All cells are stirred during measurements. An argon (99.998% purity, Praxair) overpressure is applied to minimize O2 and water vapor solvation.

3

Results and discussion

InP nanowires SEM images of the as-grown nanowires are shown in Figure 1. These nanowires grow as inverted cones with an enlarged catalyst tip, much like ice cream cones with InP cones and indium metal as the “ice cream” in the cones. The reasoning behind this cone formation is the presence of excess indium during the VLS growth process from having a small V/III ratio. It should be noted that this precursor ratio of phosphorus to indium is less than the minimum value used in previous studies under similar growth conditions, 19–21 which leads the nanowire shape in Figure 1 as opposed to the traditional diameter-confined VLS semiconductor nanowires. The solvation of TBP within the indium tip provides the excess concentration required to nucleate InP. However, unlike a growth process in which a single-element semiconductor grows out of a metal nanoparticle, like silicon from gold, the growth catalyst metal itself is a reactant in the formation of the nanowire compound, so this element needs to be sourced in order to prevent consumption of the catalyst tip and limit the nanowire growth process. It appears that due to the low V/III ratio in these growth experiments, the excess indium provides a source for the growth catalyst particle to enlarge as the growth proceeds. Previous 8 ACS Paragon Plus Environment

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Figure 1: (a) SEM image of indium-catalyzed InP nanowires on silicon, showing an “ice cream cone” type morphology. (b) SEM image of an indium-catalyzed InP nanowire on silicon under higher magnification. work on self-catalyzed InPSb nanowires 22 also observed the “ice cream cone” morphology, with a similar explanation as to the enlargement of the growth catalyst. In the case of the InPSb nanowires, they did not grow from molten indium, but instead from a molten InSb alloy, which makes sense since antimony has a high solubility within liquid indium. The V/III ratio, taking only the indium and phosphorus into account since the antimony is dissolved within the liquid phase of the growth catalyst tip, is a similar value (35.66) to the conditions in this work, providing support for the theory of excess amount of indium flow increasing the size of the nanoparticle growth catalyst tip. InP nanowires of similar morphology 23 have also been reported using a low V/III ratio with PH3 as a phosphorus source, but were limited in scope to InP substrates due to the reliance on an indium-rich InP surface assisting catalyst droplet formation. TEM is used to further analyze the crystallinity of these nanowires, with the results shown in Figure 2. The nanowires are predominantly single crystal with the (002) planes viewable in high-resolution mode, as shown in Figure 2(b). However, these indium-catalyzed InP nanowires have a degraded and rough surface with several grains, as shown in Figure 2(c). The penetration of these grains is only to a depth of about 8 nm, which does not significantly affect the crystallinity of the nanowire, but does affect the integrity of the surface within



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any photovoltaic-type junction created, either solid-state or electrochemically. The AES two-dimensional scan of indium and phosphorus signals in the nanowires, shown in Figure 2(d) and 2(e), reveals that the nanowire growth catalyst tip is pure indium.

Figure 2: (a) Bright-field TEM image of a InP nanowire grown on p+ silicon. (b) Highresolution TEM image of the InP nanowire at a segment where single crystallinity is maintained. (c) High-resolution TEM image of the InP nanowire showing grain formation at the surface. For the high-resolution TEM images, the measured lattice spacing is 2.893 Å, corresponding to the (002) InP planes. (d) AES two-dimensional scan of the indium MNN transition energy signal. (e) AES two-dimensional scan of the phosphorus LMM transition energy signal, showing that the growth catalyst tip is pure indium. Analysis of how the growth process and conditions lead to an “ice cream cone” morphology uses thermodynamic and kinetic considerations. The VLS mechanism can be modeled by visualizing a chemical potential for the elements in the vapor phase, liquid phase, and solid phase. The difference between the chemical potentials in these phases provide the thermodynamic driving force for the growth process, and under steady-state conditions, the kinetic fluxes from vapor to liquid phases, and from liquid to solid phases, are all equal. For a multi-element nanowire such as InP, one has to consider the chemical potential of both elements: indium and phosphorus. The net flux of indium into the nanoparticle is the 10 ACS Paragon Plus Environment

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difference between the vapor-to-liquid flux and the liquid-to-solid flux. The difference in the fluxes causes the size of the indium droplet to increase, and is dependent on the amount of indium being in excess of the amount of phosphorus. The overall result is an increase in the nanowire diameter as the growth progresses from the base to the tip. The absence of phosphorus in the tip from the EDS scan is an indicator of its limited solubility in indium. A visualization of indium incorporation in the nanowire is presented in Figure 3. Under the growth conditions, a simplified nanoparticle morphology in the form of a disk will increase its diameter as a result of the excess indium.

Figure 3: A visualization of the nanowire growth process with excess indium in the growth catalyst tip, causing an increase in the nanowire diameter during the growth process. The analysis can be simplified into two dimensions by viewing the spherical growth catalyst tip as a cylinder with uniform diameter.

Within the context of using these nanowires within photovoltaic applications, the observed surface polycrystallinity in Figure 2(c) is an important feature for investigation as it can potentially affect junction electrostatics and induce non-radiative recombination processes. As a further extension, the same growth is performed with the TMIn precursor source turned off midway through the growth process. The temperature, pressure, and TBP flow conditions are maintained for the remainder of the time. Under these conditions, the 11 ACS Paragon Plus Environment


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Figure 4: (a) SEM image of InP nanowires with the TMIn source cut off after the growth is halfway complete. (b) SEM image of InP nanowire with TMIn source cut off after the growth is halfway complete, under higher magnification. (c) High-resolution TEM image of a central segment of the InP nanowire, with the FFT (inset) showing single crystallinity with single points in reciprocal space. (d) High-resolution TEM image of a segment of the InP nanowire closer to the surface, with the FFT (inset) showing polycrystallinity with a distributed circle in reciprocal space. surface polycrystallinity and roughening is more pronounced, as evidenced by the SEM and TEM images in Figure 4. The indium metal as a catalyst tip eventually becomes totally phosphidized and becomes a part of the crystal. The SEM images in Figure 4(a) and 4(b) show the original imprint of the indium catalyst tip that is now a part of the InP crystal. The TEM images and corresponding FFT patterns in Figure 4(c) and 4(d) confirm that the surface is polycrystalline with grains, while the core still maintains single crystallinity. Overall, the self-catalyzed InP nanowires exhibit a surface polycrystallinity and roughening under growth conditions with a relatively low V/III ratio, and this effect is more pronounced if the TMIn precursor is turned off during the nanowire growth process. Some clues are provided due to the growth mode being VLS instead of vapor phase epitaxy. A 12 ACS Paragon Plus Environment

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general rule of MOCVD thin-film growth processes is having a V/III ratio of at least 10 such that the growth rate is limited by the Group III molar flow rate. When the Group III precursor is reduced, the growth rate correspondingly goes down, and when it is turned off, the growth stops and the III-V film is passivated by the Group V overpressure at the growth temperature. However, turning off the TMIn precursor does not passivate the asgrown nanowire surface. This could be attributed to the limited flow rate and low pyrolysis conditions of the TBP precursor under the conditions of low flow rate and low temperature, thereby not providing enough of an overpressure to induce surface passivation. An additional mechanism is possible adatom diffusion of indium atoms between the liquid phase of the catalyst tip and the solid InP phase within the nanowire. Surface diffusion of the Group III metal is an effect that has been proposed with gallium adatoms on GaN nanowires, 24 and with indium adatoms on both InAs 25 and InP 26,27 nanowires. Therefore, it is proposed that in the InP nanowires grown in this study, the surface grains and polycrystallinity arises from an insufficient phosphorus overpressure and indium adatom kinetics within the growth catalyst tip and on the surface. Additionally, the phosphidization of the indium growth catalyst tip is a result that has further implications on how these conical nanowires are grown. The VLS process has been refined to a degree such that the diameter 28 and kinking 29–31 of semiconductor nanowires is controlled through modulation of the sidewalls and contact angle of the growth catalyst tip, respectively. The growth model presented earlier establishes that the difference between the vapor-to-liquid flux and the liquid-to-solid flux causes the catalyst tip to grow over the process. However, these experiments show that when the phosphorus source is turned off, the catalyst tip does not slowly annihilate while continuing the nanowire growth, but it instead totally becomes indium phosphide in a quenching effect. This shows that the phosphorus flux through the catalyst tip, under these conditions, has a low diffusion length and instead preferentially reacts with the resident indium. Within this context, InP formation is thermodynamically limited rather than kinetically limited.



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Figure 5: (a) MV+/++ reduction photocurrent of InP nanowires on p+ silicon, as compared to a p+ silicon electrode. (b) Mott-Schottky curve of InP nanowires on p+ silicon, as compared to a p+ silicon electrode. To correlate how these crystal growth effects translate into photovolatic effects, the photoelectrochemical properties of the conical InP nanowires on p+ silicon are shown in Figure 5 as compared to bare p+ silicon, with the photocurrent in Figure 5(a) and the Mott-Schottky curves in Figure 5(b). The photocurrent for the InP nanowires is higher than the bare silicon substrate over the biasing range, confirming that the InP nanowires exhibit photovoltaic behavior within the electrochemical junction with the p+ silicon. The Mott-Schottky plot helps to explain the electrochemical junction with the InP nanowires. Between reduction biases of 0.05 V and -0.45 V vs. Fc/Fc+ , 1/C 2 is constant, which is indicative of the charge being set by the surface defects of the InP nanowires. The independence of 1/C 2 in this applied bias range shows that there is no depletion within the electrode, and the charge is instead supplied from energetic trap states at the interface. The trap states are tied to the polycrystalline grains at the surface of the nanowires, and show that the limitations of the photocurrent can be attributed to recombination near the surface of these wires. A shift of the Mott-Schottky plot across voltage is typically indicative of a p-n junction formation between the nanowire and the substrate. This is likely the case in this interface, due to InP grown by MOCVD exhibiting n-type behavior either from background impurities in the MOCVD process 32 or from silicon diffusion as a donor dopant. 33 However, it would be expected that 1/C 2 would increase with applied bias to reflect n-type doping. 14 ACS Paragon Plus Environment

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The static nature of 1/C 2 is more indicative of the equilibrium potential being set by the defect states on the InP nanowire surface. Beyond -0.45 V vs. Fc/Fc+ , 1/C 2 shows a trend consistent with depletion in the silicon substrate, indicating the presence of photocurrent generated from the silicon as well as the InP nanowires. At this point, even with the surface defects present, the junction has fully depleted the nanowires and the space charge region has entered the p+ silicon.

Figure 6: (a) MV+/++ reduction photocurrent of InP nanowires with TMIn turned off midway through the growth on p+ silicon, as compared to a p+ silicon electrode. (b) MottSchottky curve of InP nanowires with TMIn turned off midway through the growth on p+ silicon, as compared to a p+ silicon electrode. The photoelectrochemical measurements for the more roughened InP nanowires (with a midway cut off TMIn flow) are shown in Figure 6 as compared to bare p+ silicon, with the photocurrent in Figure 6(a) and the Mott-Schottky curves in Figure 6(b). Although the photocurrent is slightly higher with the presence of the degraded InP nanowires, it is smaller in magnitude than the less roughened nanowires, indicating that more photogenerated carriers recombine within the surface region of the nanowire. The Mott-Schottky curve confirms the further degradation and roughening of the nanowires, since as a function of the applied bias, 1/C 2 does not significantly change, even at high reduction bias. The energetic trap states arising from the grain boundaries within the degraded surface prevent depletion within the wire and the substrate to supply the charge. These measurements provide evidence of a relation between the material properties and the photovoltaic properties of InP nanowires 15 ACS Paragon Plus Environment


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grown under different conditions, especially at the surface where the electrochemical junction is made. To put these results in perspective of the junction created at the interface, the p+ silicon/electrolyte junction has a small amount of depletion within the silicon. This means that there is little photoreduction current from the incident light excitation. When the n-type InP nanowires are placed at the interface, the overall nature of the junction is still operating in a photoreduction mode. This scheme, with a p-type substrate and an n-type emitter on top, is similar to several thin-film designs for photocathodes 34–36 with silicon p-n junctions. The p-n junction created would provide additional space charge within the nanowire to source a higher photoreduction current under illumination. A similar result, in the form of a photovoltage, was observed with InP nanowires on p-type silicon. 19 However, the other junction to consider at this interface is the conformal liquid contact between the nanowires and the solution that performs carrier extraction to complete the photoreduction process. From the results in Figure 5(b), the defect states at the nanowire-liquid interface are the likely cause of the bias-independent capacitance measurement between the reduction biases of 0.05 V and -0.45 V vs. Fc/Fc+ ; trapped charges in these defect states would be supplied to compensate the small-signal voltage excitation in the impedance measurement. For this nanowire morphology, the conformal liquid contact has a higher surface area at the interface than the substrate contact, therefore defect charges likely reside at that location. This assertion is additionally confirmed with the results in Figure 6(b). The capacitance of the rougher nanowires is bias-independent over the entire scan range, therefore a higher defect density would supply the charge to prevent depletion in both the InP and the silicon. The experiments show that the surface crystallinity of the InP nanowires, when utilized within an electrochemical interface, is an important design consideration when designing heterojunction-based photovoltaic or PEC devices for solar energy conversion. The experiments also reveal the electrostatics of the interface as the surface crystallinity is changed from materials synthesis conditions. Recent work has also addressed the importance of surface


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crystallinity in Fe2 O3 near the electrochemical interface within the context of its open-circuit voltage and overall performance as a photoelectrode, 37 indicating that research in this specific area is important to pursue in the future. One possible extension of this InP nanowire array is in sensitizers of other III-V materials such as GaAs, but since indium and gallium form a low-temperature eutectic, the result of growing InP nanowires on GaAs to form a p-n heterojunction (Figure S1) leads to an indium-rich InGaAs interlayer (shown in the XRD on Figure S2) that serves as a low-bandgap recombination center near the interface. Looking forward, the versatility of using CVD of InP nanowires on various substrates, as was recently demonstrated on GaN and molybdenum foil 38 opens up several opportunities to investigate the nature of of these electrochemical interfaces as well.

InN nanowires SEM images of the gold-catalyzed InN nanowires are shown in Figure 7 along with the GIXD scan confirming the wurtzite hexagonal phase. The nanowires exhibit a faceted pyramidal structure with the gold growth catalyst tips still present at the end of the wires, as indicated in Figure 7(b). The growth mode of these wires is likely from the formation of a liquid gold-indium alloy that initially increases in diameter as the gold nanoparticle is initially exposed to the indium during the furnace heatup. The formation of this liquid alloy is thermodynamically possible; the gold-indium binary phase diagram shows that at 550 ◦ C, a liquid alloy is achievable. 39 When exposed to NH3 , the indium part of the droplet reacts to form InN and is consumed, while the gold part of the droplet remains unreacted. The result is an inverted cone that, through the growth process, decreases its diameter as the growth proceeds. The distinctive pyramidal shape of the nanowire is different than previous reports of diameter-limited InN nanowires. 40–42 TEM images of the gold-catalyzed InN nanowires are shown in Figure 8. Although the nanowires exhibit some stacking fault defects as shown by the contrast within the TEM images, they are single crystal under high-resolution with the (101) planes visible. More 17 ACS Paragon Plus Environment


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Figure 7: (a) SEM image of gold-catalyzed InN nanowires grown on silicon. (b) SEM image of gold-catalyzed InN nanowires grown on silicon under high magnification, showing the gold catalyst tips at the end of the wires. (c) GIXD scan, with ω fixed at 2◦ , showing the (100), (002), and (101) peaks of wurtzite hexagonal InN nanowires. importantly, they do not exhibit any surface degradation, with the facets visible on the SEM images. Wurtzite nanowire faceting has been previously attributed to adatom diffusion and coalescensce on the c-plane directions, 24 so the initial nucleation and growth of these wires appears to follow a similar trend. The CoCp2 0/+ redox couple, with a standard redox potential of approximately -1.3 V vs. Fc/Fc+ , 43 is a reducing agent that facilitates a one-electron transfer. This makes it ideal for analyzing n-type photoanodic materials. One example has been in investigating photovoltaic effects in iron pyrite grown by chemical vapor transport. 44 Therefore, the CoCp2 0/+ redox couple is used to probe anodic photoeffects within InN nanowires without inducing any oxidation and corrosion of the material.


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Figure 8: (a) Bright-field TEM image of the gold-catalyzed InN nanowire, showing some stacking fault type defects from the contrast in the image. (b) High-resolution TEM image of the InN nanowire near the surface, showing that single crystallinity is maintained in this region. (c) High-resolution TEM image of the InN nanowire at another region, showing that single crystallinity is maintained. For the high-resolution TEM images, the measured lattice spacing is 2.714 Å, corresponding to the (101) InN planes. The photoelectrochemical properties of the gold-catalyzed InN nanowires in contact with the CoCp2 0/+ redox couple are shown in Figure 9, with the open-circuit voltage in response to 1 sun illumination in Figure 9(a) and the junction capacitance C in Figure 9(b). When the InN nanowire electrode is illuminated, a photovoltage of 4 mV is developed in the nanowires. The p+ silicon does not itself develop a measurable photovoltage, as confirmed by performing this same experiment on a bare substrate. Although this result demonstrates a photovoltaic effect in the InN nanowires, is still a small value based on the incident power of 100 mW/cm2 . An effect that can contribute to the low photovoltage of the InN nanowires is the presence of the surface accumulation layer observed in thin films. 45 The presence of this layer is indicated with the measurement of



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C in Figure 9(b). The capacitance is constant and at a higher value than the substrate, meaning that the charge supplied at the interface can come from the surface accumulation layer instead from depletion in the nanowire interior. The surface accumulation layer arises due to the high electron affinity of InN, lowering it below the Fermi stabilization energy EF S . It is likely that there is a screening effect such that photogenerated holes would be electrostatically be attracted to the surface electron layer and not transfer into the CoCp2 0 species easily. This effect is important in the future for analyzing high-indium InGaN alloys for both photovoltaic and PEC devices.

Figure 9: (a) Photovoltage measurement of gold-catalyzed InN nanowires in contact with the CoCp2 0/+ redox couple with 1 sun illumination shuttered in 4-second intervals. (b) Junction capacitance C of the junction made between the InN nanowires in contact with the CoCp2 0/+ redox couple as compared to the bare p+ silicon substrate. SEM images and two-dimensional AES scans of the indium-catalyzed InN nanowires are shown in Figure 10. The “ice cream cone” morphology of these wires is the same as the indium-catalyzed InP nanowires discussed previously, signifying a similar growth mechanism from indium droplets with the associated materials considerations. Similarly to the gold-catalyzed wires, the morphology obtained has not been previously observed for InN nanowires. One important difference between the indium-catalyzed InN nanowires and the InP nanowires is the tip material. The InP nanowire tip is pure indium, whereas the InN nanowire tip exhibits an InN surface as evidenced by the two-dimensional AES scan.


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Figure 10: (a) SEM image of indium-catalyzed InN nanowires showing the same “ice cream cone” morphology as the InP nanowires shown previously. (b) SEM image, under higher magnification, of a indium-catalyzed InN nanowire, showing facets and a InN growth catalyst tip. (c) AES two-dimensional scan of the indium MNN transition energy signal. (d) AES two-dimensional scan of the nitrogen KLL transition energy signal, showing that the growth catalyst tip surface is indium nitride. The nucleation of InN crystallites as a result of dissolved nitrogen within the indium growth catalyst tip has been previously proposed as a growth mechanism of InN nanostructures. 40,46 The wetting of additional indium onto the crystallite forms the basis for growing a InN nanowire in the case of a small amount of initial indium, whereas multiple crystallites within the growth catalyst will nucleate several nanowires in the case of a large amount of initial indium. However, previous work has shown that excess indium at the nanowire tip solidifies with an InN shell formed. 40 This mechanism is also possible for these structures, especially given that in the historical context of liquid phase epitaxy of As/P materials, indium and gallium melts often develop a phosphide and arsenide crust once a growth experiment is complete. In either case, there is a greater driving force to form InN within the growth catalyst tip; comparison of the indium-phosphorus and indium-nitrogen phase



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diagrams shows a higher degree of nitrogen solubility, as compared to phosphorus solubility, under similar growth temperatures. It is proposed that for the indium-catalyzed InN nanowires in this work, crystallites form within the growth catalyst tip in addition to the dominant VLS process that occurs to form the “ice cream cone” nanowire. The tip is the same material and phase as the nanowire, although it would likely be polycrystalline. However, it cannot be proved in this study that this mechanism is occurring. The two-dimensional AES scan does not probe below the surface. The tip thickness in the interior is too thick to perform HRTEM analysis as the material would absorb all electrons. The EDX analysis generally produces a weak signal for light elements such as nitrogen, since it is increasingly difficult to ionize the atom and produce an x-ray signal. The nitrogen Kα signal (at 0.392 eV) is very close to the carbon Kα signal (at 0.277 eV), and there is concern that the EDX signal from the carbon support film will wash out the nitrogen signal from the nanowires. Therefore, it remains an open question as to whether the growth catalyst tip is fully InN, or indium with a InN shell. TEM images of the indium-catalyzed InN nanowires are shown in Figure 11. Although the nanowires are single crystal InN with the (101) planes viewable under high-resolution, they undergo surface degradation similar to the InP nanowires. Some regions have a single crystal surface that does not undergo degradation, as shown in Figure 11(b), but other regions of the nanowires have a degraded surface with grains that exhibit different crystallographic directions as shown in Figure 11(c). These grains are not totally polycrystalline in a random order, but instead exhibit texturing as evidenced by the FFT showing broader spots in reciprocal space instead of rings. Therefore, a direct comparison between the two is indicative of a less degraded surface for InN, which is further supported by faceting in the SEM image. The photoelectrochemical properties of the indium-catalyzed InN nanowires are shown in Figure 12, with the open-circuit voltage in response to 1 sun illumination in Figure 12(a) and the junction capacitance C in Figure 12(b). When the InN nanowire electrode is illuminated, a photovoltage of approximately 6 mV is developed in the nanowires, but unlike the


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Figure 11: (a) Bright-field TEM image of the indium-catalyzed InN nanowire. (b) Highresolution TEM image of the InN nanowire of a segment exhibiting a non-degraded surface, with the FFT (inset) showing single crystallinity with single points in reciprocal space. (c) High-resolution TEM image of the InN nanowire of a segment exhibiting a degraded surface, with the FFT (inset) showing textured polycrystallinity with broader points in reciprocal space. For the high-resolution TEM images, the measured lattice spacing is 2.697 Å, corresponding to the (101) InN planes. photovoltage in the gold-catalyzed InN nanowires, it linearly increases over time instead of stabilizing at a steady-state value. This linear progression can be tied to a gradual buildup and decrease of the open-circuit voltage as electron-hole pairs are generated under illumination and recombine in the dark. There is a possibility of the redox potential of CoCp2 0/+ not being completely stable due to the relative lack of both species in the electrolyte, but since the gold-catalyzed InN nanowires did not show this drift, it is unlikely as the experimental conditions were same for both sets of materials. The measurement of C in Figure 12(b) also qualitatively confirms the existence of the surface accumulation layer on the InN nanowires due to the capacitance being at a high value and relatively constant as a function of applied voltage. The high value of C is indicative 23 ACS Paragon Plus Environment


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of charge being supplied close to the semiconductor-electrolyte interface instead of through depletion-based charge within the nanowire. Unlike the gold-catalyzed InN nanowires, the indium-catalyzed nanowires show a slight decrease in capacitance as the potential is increased. This suggests that the InN is also being depleted in the bulk under oxidizing biases. Since the acetonitrile environment with the redox couple is inert and does not have dissolved oxygen or water vapor, oxidation of either the InN or the silicon does not occur and subsequently does not contribute to the variation of the photovoltaic behavior or change in the capacitance. The presence of depletion within the nanowire gives a possible explanation for this photovoltage and why it can achieve a larger value than the gold-catalyzed InN nanowires. More quasi-Fermi level splitting is present, and larger voltage can be built up.

Figure 12: (a) Photovoltage measurement of indium-catalyzed InN nanowires in contact with the CoCp2 0/+ redox couple with 1 sun illumination shuttered in 8-second intervals. (b) Junction capacitance C of the junction made between the InN nanowires in contact with the CoCp2 0/+ redox couple as compared to the bare p+ silicon substrate. Another observation is that no photovoltage is observed for MBE-grown InN films on c-plane GaN/sapphire templates or for highly dense indium-catalyzed InN nanowires on p+ silicon that agglomerate into a polycrystalline film. This means that the observed photovoltaic effect in this work is only within nanowires of InN. Quantum confinement effects are ruled out since they only influence Eg and the photovoltage at dimensions of less than 20 nm. A more likely reason is that the volumetric confinement of photogenerated charge carriers


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gives rise to a larger quasi-Fermi level splitting between the nanowires and the CoCp2 0/+ redox couple, and therefore, a higher photovoltage. For any semiconductor exposed to a light flux, an electron-hole pair generation rate is established, but in lower-volume structures such as nanowires, the total amount of photogenerated carriers in comparison with the equilibrium carriers would be higher, causing high-level injection conditions that can generate a photovoltage. The transient buildup and decay of the indium-catalyzed InN nanowires is likely a result of the combined depletion, surface accumulation, and high-level photogeneration effects when the electrochemical junction is irradiated with solar illumination. To put these results in perspective of the junction created at the interface, InN behaves very different than InP. Due to its high electron affinity 47 that puts its conduction band minimum energy below the silicon’s valence band maximum energy, as well as its high electron concentration due to nitrogen vacancy point defects and the surface electron accumulation layer, InN makes a “quasi-ohmic” contact with p-type silicon 18 that operates through carrier annihilation between electrons in the InN conduction band and holes in the silicon valence band. Therefore, the only rectifying junction at this interface is between the InN nanowires and the electrolyte. The surface accumulation layer is viewed through the capacitance measurement in Figure 6(b), and its relatively large value as compared to the depletion capacitance in silicon closely resembles the Helmholtz layer capacitance. 48 The electrochemical junction is created by a conformal liquid contact around the nanowires. Since this provides a larger area junction as compared to the nanowire-substrate interface, and since the silicon-InN interface is “quasi-ohmic”, it is likely that impedance measurement data comes from the electrochemical interface. When the junction is illuminated, a quasi-Fermi level split within the nanowire demonstrates an anodic photovoltage under open-circuit conditions. The photoeffects at this specific interface with CoCp2 0/+ is additionally supported with results showing an anodic photovoltage with iron pyrite. 44 The emphasis in this work was to obtain a better understanding of the InP and InN nanowires, both in terms of materials properties and junction properties at the electrochem-



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ical interface. Although the performance parameters are poor, future steps can be taken in optimizing synthesis conditions and post-growth processing of the nanowire electrodes to improve the photocurrent and photovoltage in InP and InN, respectively. This has an additional advantage in analyzing the materials properties of the nanowires. Similar work has been done with the development of silicon microwires. Optimizing materials growth and processing conditions, 49 especially within the context of integrating heterogeneous light absorber materials, 50,51 has been important in obtaining high-performance photoelectrodes for solar energy conversion. Using this methodology with the InP and InN nanowires will help to improve both their performance and understanding of the physical properties. Another extension of the findings in this work is in isolating single nanowires and probing the electrical, photonic, and chemical properties. This can be done with solid-state transport measurements using metal contacts, 52,53 incorporating scanning light excitation sources, 54 and developing new electrochemical measurement methods. 55,56

Conclusions In summary, the first set of experiments investigated the synthesis of indium-catalyzed InP nanowires exhibiting an “ice cream cone” morphology with an enlarged catalyst tip. Electrochemical photocurrent and impedance spectroscopy studies, combined with high-resolution TEM measurements, provide insight into the material and photovoltaic properties of these electrodes. Insight from thermodynamic and kinetic processes in crystal growth helps to provide an explanation for how nanowires of these morphologies and crystal quality are formed. The effects of nanostructuring and surface states on photovoltaic performance are investigated through electrochemical analysis. These experiments provide a basis for future work on using indium-catalyzed InP nanowires for materials studies, photoelectrodes, or integration within solid-state photovoltaic systems. These experiments help to build a picture of the electrostatics of the junction formed between the semiconductor substrate and a redox


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couple in electrolyte with nanowire integration. The second set of experiments investigated the synthesis of InN nanowires by CVD, using both gold and indium as a growth catalyst. In both cases, single-crystal nanowires are grown in the form of conical and pyramidal structures, with the gold-catalyzed InN wires showing a larger diameter near the base and the indium-catalyzed InN wires showing a larger diameter near the tip. The indium-catalyzed nanowires show the same “ice cream cone” morphology as the InP nanowires, indicating that a similar growth process is occurring in these structures. The nanowires demonstrate an photovoltage in contact with the CoCp2 0/+ redox couple, which represents the first time a photovoltage is observed within pure InN, either in film or nanowire form. EIS measurements show evidence of the surface accumulation layer that is typically present in single crystal InN films. These experiments provide a basis in using high-indium InGaN alloys for photovoltaic energy conversion applications, both for solar cells and photoelectrodes for water splitting. By isolating the electrochemical experiments for the electrodes by using non-aqueous environments, material oxidation is removed, and so anodic behavior can be isolated to light conversion into photocurrent. The conical morphology developed in these materials can be potentially utilized in nanophotonic-inspired designs for improved light trapping, and provides a basis for nanoscale engineering of light-absorbing sensitizers on top of earth-abundant materials such as silicon.

Acknowledgement This work is primarily supported as a part of the Center for Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001060. Additional support comes from the Precourt Institute of Energy and the Bay Area Photovoltaic Consortium. The authors would like to thank Ryan Enck, Chad Gallinat, and Eric Readinger at the U.S. Army Research Laboratory for providing the



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MBE-grown InN films on sapphire.

Supporting Information Available Detailed procedures for materials growth and electrochemical characterization of electrodes, SEM and XRD reciprocal space map of InP nanowires grown on GaAs. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Krogstrup, P.; Jørgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Fontcuberta i Morral, A. Single-Nanowire Solar Cells Beyond the ShockleyQueisser Limit. Nat. Photonics 2013, 7, 306–310. (2) Holm, J. V.; Jørgensen, H. I.; Krogstrup, P.; Nygård, J.; Liu, H.; Aagesen, M. SurfacePassivated GaAsP Single-Nanowire Solar Cells Exceeding 10% Efficiency Grown on Silicon. Nat. Commun. 2013, 4, 1498. (3) Dorodnyy, A.; Alarcon-Lladó, E.; Shklover, V.; Hafner, C.; Fontcuberta i Morral, A.; Leuthold, J. Efficient Multiterminal Spectrum Splitting via a Nanowire Array Solar Cell. ACS Photonics 2015, 2, 1284–1288. (4) Liu, C.; Dasgupta, N. P.; Yang, P. Semiconductor Nanowires for Artificial Photosynthesis. Chem. Mater. 2013, 26, 415–422. (5) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983–2002. 28 ACS Paragon Plus Environment

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(6) Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F. Modeling Practical Performance Limits of Photoelectrochemical Water Splitting Based on the Current State of Materials Research. ChemSusChem 2014, 7, 1372–1385. (7) Hoffman, C. A.; Jaraši¯ unas, K.; Gerritsen, H. J.; Nurmikko, A. V. Measurement of Surface Recombination Velocity in Semiconductors by Diffraction from Picosecond Transient Free-Carrier Gratings. Appl. Phys. Lett. 1978, 33, 536–539. (8) Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Åberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgström, M. InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit. Science 2013, 339, 1057–1060. (9) Gao, L.; Cui, Y.; Wang, J.; Cavalli, A.; Standing, A.; Vu, T. T. T.; Verheijen, M. A.; Haverkort, J. E.; Bakkers, E. P. A. M.; Notten, P. H. L. Photoelectrochemical Hydrogen Production on InP Nanowire Arrays With Molybdenum Sulfide Electrocatalysts. Nano Lett. 2014, 14, 3715–3719. (10) Stringfellow, G. B. Microstructures Produced During the Epitaxial Growth of InGaN Alloys. J. Cryst. Growth 2010, 312, 735–749. (11) Xiang, H. J.; Wei, S.-H.; Da Silva, J. L. F.; Li, J. Strain Relaxation and Band-Gap Tunability in Ternary Inx Ga1-x N Nanowires. Phys. Rev. B 2008, 78, 193301. (12) Kuykendall, T.; Ulrich, P.; Aloni, S.; Yang, P. Complete Composition Tunability of InGaN Nanowires Using a Combinatorial Approach. Nat. Mater. 2007, 6, 951–956. (13) Dong, Y.; Tian, B.; Kempa, T. J.; Lieber, C. M. Coaxial Group III-Nitride Nanowire Photovoltaics. Nano Lett. 2009, 9, 2183–2187. (14) Wierer, J. J.; Li, Q.; Koleske, D. D.; Lee, S. R.; Wang, G. T. III-Nitride Core–Shell Nanowire Arrayed Solar Cells. Nanotechnology 2012, 23, 194007. 29 ACS Paragon Plus Environment


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(15) Hwang, Y. J.; Wu, C. H.; Hahn, C.; Jeong, H. E.; Yang, P. Si/InGaN Core/Shell Hierarchical Nanowire Arrays and Their Photoelectrochemical Properties. Nano Lett. 2012, 12, 1678–1682. (16) Wang, D.; Pierre, A.; Kibria, M. G.; Cui, K.; Han, X.; Bevan, K. H.; Guo, H.; Paradis, S.; Hakima, A.-R.; Mi, Z. Wafer-Level Photocatalytic Water Splitting on GaN Nanowire Arrays Grown by Molecular Beam Epitaxy. Nano Lett. 2011, 11, 2353–2357. (17) Kamimura, J.; Bogdanoff, P.; Lähnemann, J.; Hauswald, C.; Geelhaar, L.; Fiechter, S.; Riechert, H. Photoelectrochemical Properties of (In,Ga)N Nanowires for Water splitting Investigated by In-Situ Electrochemical Mass Spectroscopy. J. Am. Chem. Soc. 2013, 135, 10242–10245. (18) Nguyen, H. P. T.; Chang, Y.-L.; Shih, I.; Mi, Z. InN p-i-n Nanowire Solar Cells on Si. IEEE J. Sel. Top. Quantum Electron. 2011, 17, 1062–1069. (19) Woo, R. L.; Xiao, R.; Kobayashi, Y.; Gao, L.; Goel, N.; Hudait, M. K.; Mallouk, T. E.; Hicks, R. F. Effect of Twinning on the Photoluminescence and Photoelectrochemical Properties of Indium Phosphide Nanowires Grown on Silicon (111). Nano Lett. 2008, 8, 4664–4669. (20) Woo, R. L.; Gao, L.; Goel, N.; Hudait, M. K.; Wang, K. L.; Kodambaka, S.; Hicks, R. F. Kinetic Control of Self-Catalyzed Indium Phosphide Nanowires, Nanocones, and Nanopillars. Nano Lett. 2009, 9, 2207–2211. (21) Gao, L.; Woo, R. L.; Liang, B.; Pozuelo, M.; Prikhodko, S.; Jackson, M.; Goel, N.; Hudait, M. K.; Huffaker, D. L.; Goorsky, M. S.; Kodambaka, S.; Hicks, R. F. SelfCatalyzed Epitaxial Growth of Vertical Indium Phosphide Nanowires on Silicon. Nano Lett. 2009, 9, 2223–2228. (22) Ngo, C.; Zhou, H.; Mecklenburg, M.; Pozuelo, M.; Regan, B. C.; Xiao, Q. F.; Shenoy, V. B.; Hicks, R. F.; Kodambaka, S. Effect of Precursor Flux on Composi30 ACS Paragon Plus Environment

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tional Evolution in InP1-x Sbx Nanowires Grown via Self-Catalyzed Vapor–Liquid–Solid Process. J. Cryst. Growth 2011, 336, 14–19. (23) Novotny, C. J.; Yu, P. K. L. Vertically Aligned, Catalyst-Free InP Nanowires Grown by Metalorganic Chemical Vapor Deposition. Appl. Phys. Lett. 2005, 87, 203111. (24) Li, H.; Chin, A. H.; Sunkara, M. K. Direction-Dependent Homoepitaxial Growth of GaN Nanowires. Adv. Mater. 2006, 18, 216–220. (25) Jensen, L. E.; Björk, M. T.; Jeppesen, S.; Persson, A. I.; Ohlsson, B. J.; Samuelson, L. Role of Surface Diffusion in Chemical Beam Epitaxy of InAs Nanowires. Nano Lett. 2004, 4, 1961–1964. (26) Lu, X.-L.; Zhang, X.; Yan, X.; Liu, X.-L.; Cui, J.-G.; Li, J.-S.; Huang, Y.-Q.; Ren, X.-M. Growth of Self-Catalyzed InP Nanowires by Metalorganic Chemical Vapour Deposition. Chin. Phys. Lett. 2012, 29, 126102. (27) Paiman, S.; Gao, Q.; Joyce, H. J.; Kim, Y.; Tan, H. H.; Jagadish, C.; Zhang, X.; Guo, Y.; Zou, J. Growth Temperature and V/III Ratio Effects on the Morphology and Crystal Structure of InP Nanowires. J. Phys. D: Appl. Phys. 2010, 43, 445402. (28) Musin, I. R.; Boyuk, D. S.; Filler, M. A. Surface Chemistry Controlled DiameterModulated Semiconductor Nanowire Superstructures. J. Vac. Sci. Technol. B 2013, 31, 020603. (29) Tian, B.; Xie, P.; Kempa, T. J.; Bell, D. C.; Lieber, C. M. Single-Crystalline Kinked Semiconductor Nanowire Superstructures. Nat. Nanotechnol. 2009, 4, 824–829. (30) Plissard, S. R.; van Weperen, I.; Car, D.; Verheijen, M. A.; Immink, G. W.; Kammhuber, J.; Cornelissen, L. J.; Szombati, D. B.; Geresdi, A.; Frolov, S. M.; Kouwenhoven, L. P.; Bakkers, E. P. A. M. Formation and Electronic Properties of InSb Nanocrosses. Nat. Nanotechnol. 2013, 8, 859–864. 31 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(31) Musin, I. R.; Filler, M. A. Chemical Control of Semiconductor Nanowire Kinking and Superstructure. Nano Lett. 2012, 12, 3363–3368. (32) Bass, S. J.; Pickering, C.; Young, M. L. Metal Organic Vapour Phase Epitaxy of Indium Phosphide. J. Cryst. Growth 1983, 64, 68–75. (33) Astles, M. G.; Smith, F. G. H.; Williams, E. W. Indium Phosphide II. Liquid Epitaxial Growth. J. Electrochem. Soc. 1973, 120, 1750–1757. (34) Warren, E. L.; Boettcher, S. W.; Walter, M. G.; Atwater, H. A.; Lewis, N. S. pH-Independent, 520 mV Open-Circuit Voltages of Si/Methyl Viologen2+/+ Contacts Through use of Radial n+ p-Si Junction Microwire Array Photoelectrodes. J. Phys. Chem. C 2011, 115, 594–598. (35) Boettcher, S. W.; Warren, E. L.; Putnam, M. C.; Santori, E. A.; Turner-Evans, D.; Kelzenberg, M. D.; Walter, M. G.; McKone, J. R.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays. J. Am. Chem. Soc. 2011, 133, 1216–1219. (36) Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F. Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Adv. Energy Mater. 2014, 4, 1400739. (37) Jang, J.-W.; Du, C.; Ye, Y.; Lin, Y.; Yao, X.; Thorne, J.; Liu, E.; McMahon, G.; Zhu, J.; Javey, A.; Guo, J.; Wang, D. Enabling Unassisted Solar Water Splitting by Iron Oxide and Silicon. Nat. Commun. 2015, 6 . (38) Kornienko, N.; Gibson, N. A.; Zhang, H.; Eaton, S. W.; Yu, Y.; Aloni, S.; Leone, S. R.; Yang, P. Growth and Photoelectrochemical Energy Conversion of Wurtzite Indium Phosphide Nanowire Arrays. ACS Nano 2016, 10, 5525–5535.


Page 32 of 35

Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Massalski, T. B.; Okamoto, H.; Subramanian, P.; Kacprzak, L.; Scott, W. W. Binary Alloy Phase Diagrams; American Society for Metals, 1986. (40) Vaddiraju, S.;

Mohite, A.;

Chin, A.;

Meyyappan, M.;

Sumanasekera, G.;

Alphenaar, B. W.; Sunkara, M. K. Mechanisms of 1D Crystal Growth in Reactive Vapor Transport: Indium Nitride Nanowires. Nano Lett. 2005, 5, 1625–1631. (41) Chang, Y.-L.; Li, F.; Fatehi, A.; Mi, Z. Molecular Beam Epitaxial Growth and Characterization of Non-Tapered InN Nanowires on Si (111). Nanotechnology 2009, 20, 345203. (42) Liang, C. H.; Chen, L. C.; Hwang, J. S.; Chen, K. H.; Hung, Y. T.; Chen, Y. F. SelectiveArea Growth of Indium Nitride Nanowires on Gold-Patterned Si (100) Substrates. Appl. Phys. Lett. 2002, 81, 22–24. (43) Connelly, N. G.; Geiger, W. E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996, 96, 877–910. (44) Cabán-Acevedo, M.; Kaiser, N. S.; English, C. R.; Liang, D.; Thompson, B. J.; Chen, H.-E.; Czech, K. J.; Wright, J. C.; Hamers, R. J.; Jin, S. Ionization of HighDensity Deep Donor Defect States Explains the Low Photovoltage of Iron Pyrite Single Crystals. J. Am. Chem. Soc. 2014, 136, 17163–17179. (45) King, P. D. C.; Veal, T. D.; Lu, H.; Jefferson, P. H.; Hatfield, S. A.; Schaff, W. J.; McConville, C. F. Surface Electronic Properties of n-and p-Type InGaN Alloys. Phys. Status Solidi B 2008, 245, 881–883. (46) Xu, G.; Li, Z.; Baca, J.; Wu, J. Probing Nucleation Mechanism of Self-Catalyzed InN Nanostructures. Nanoscale Res. Lett. 2010, 5, 7–13. (47) Moses, P. G.; Van de Walle, C. G. Band Bowing and Band Alignment in InGaN Alloys. Appl. Phys. Lett. 2010, 96, 021908. 33 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(48) De Gryse, R.; Gomes, W. P.; Cardon, F.; Vennik, J. On the Interpretation of MottSchottky Plots Determined at Semiconductor/Electrolyte Systems. J. Electrochem. Soc. 1975, 122, 711–712. (49) Warren, E. L.; Atwater, H. A.; Lewis, N. S. Silicon Microwire Arrays for Solar EnergyConversion Applications. J. Phys. Chem. C 2013, 118, 747–759. (50) Strandwitz, N. C.; Turner-Evans, D. B.; Tamboli, A. C.; Chen, C. T.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Behavior of Planar and Microwire-Array Si|GaP Electrodes. Adv. Energy Mater. 2012, 2, 1109–1116. (51) Shaner, M. R.; Fountaine, K. T.; Ardo, S.; Coridan, R. H.; Atwater, H. A.; Lewis, N. S. Photoelectrochemistry of Core–Shell Tandem Junction n–p+ -Si/n-WO3 Microwire Array Photoelectrodes. Energy Environ. Sci. 2014, 7, 779–790. (52) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Indium Phosphide Nanowires as Building Blocks for Nanoscale Electronic and Optoelectronic Devices. Nature 2001, 409, 66–69. (53) Huang, Y.; Duan, X.; Cui, Y.; Lieber, C. M. Gallium Nitride Nanowire Nanodevices. Nano Lett. 2002, 2, 101–104. (54) Graham, R.; Miller, C.; Oh, E.; Yu, D. Electric Field Dependent Photocurrent Decay Length in Single Lead Sulfide Nanowire Field Effect Transistors. Nano Lett. 2011, 11, 717–722. (55) Liu, C.; Hwang, Y. J.; Jeong, H. E.; Yang, P. Light-Induced Charge Transport Within a Single Asymmetric Nanowire. Nano Lett. 2011, 11, 3755–3758. (56) Su, Y.; Liu, C.; Brittman, S.; Tang, J.; Fu, A.; Kornienko, N.; Kong, Q.; Yang, P. Single-Nanowire Photoelectrochemistry. Nat. Nanotechnol. 2016, 11, 609–612.


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