Epitaxially Oriented Sn:In2O3 Nanowires Grown by the Vapor–Liquid

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Cite This: ACS Appl. Energy Mater. 2019, 2, 4274−4283

Epitaxially Oriented Sn:In2O3 Nanowires Grown by the Vapor− Liquid−Solid Mechanism on m‑, r‑, a‑Al2O3 as Scaffolds for Nanostructured Solar Cells Andreas Charalampous,† Matthew Zervos,*,† Joseph Kioseoglou,‡ Katerina Tsagaraki,§ Maria Androulidaki,§ George Konstantinidis,§ Eugenia Tanasă,∥ and Eugeniu Vasile∥

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Nanostructured Materials and Devices Laboratory, School of Engineering, University of Cyprus, P.O. Box 20537, Nicosia 1678, Cyprus ‡ Department of Physics, Aristotle University of Thessaloniki, Nanostructured Materials Microscopy Group, Thessaloniki 54124, Greece § Foundation of Research and Technology Hellas, Institute of Electronic Structure and LASER, Microelectronics Group, P.O. Box 527, Heraklion, Crete GR 711 10, Greece ∥ Department of Science and Engineering of Oxide Materials and Nanomaterials, Politehnica University of Bucharest, 313 Splaiul Independentei, Bucharest 060042, Romania ABSTRACT: We have grown highly directional, epitaxial Sn:In2O3 nanowires via the vapor−liquid−solid mechanism on m-, r- and a-Al2O3 between 800 and 900 °C at 1 mbar. The Sn:In2O3 nanowires have the cubic bixbyite crystal structure and are tapered with lengths of up to 80 μm, but they are inclined at ϕ ≈ 60° along one direction on m-Al2O3 while those on r-Al2O3 are inclined at ϕ ≈ 45° and oriented along two mutually orthogonal directions. In contrast, vertical Sn:In2O3 nanowires were obtained on a-Al2O3. We obtain excellent uniformity and reproducible growth of Sn:In2O3 nanowires up to 15 mm × 15 mm on m- and r-Al2O3, which is important for the fabrication of nanowire solar cells. All of the Sn:In2O3 nanowires had a resistivity of 10−4 Ω cm and carrier densities on the order of 1021 cm−3, in which case the charge distribution has a maximum at the surface of the Sn:In2O3 nanowires as a result of the occupancy of sub-bands residing well below the Fermi level, as shown via the self-consistent solution of the Poisson−Schrödinger equations in the effective mass approximation. We also show that the Sn:In2O3 nanowires are capable of light emission and exhibited room-temperature photoluminescence at 3.1 eV as a result of band-to-band radiative transitions but also at 2.25 eV as a result of donor-like states residing energetically in the upper half of the energy band gap. We discuss the advantages of using ordered networks of Sn:In2O3 nanowires in solar cell devices and issues pertaining to their fabrication. KEYWORDS: epitaxial, ordered, nanowires, indium tin oxide, solar cells structure have been obtained by Nguyen et al.16 on a-Al2O3, which enabled their integration into a vertical field-effect transistor. Furthermore, Wan et al.17,18 obtained epitaxial, ordered Sn:In2O3 NWs with square sections and the cubic crystal structure on a Sn:In2O3 buffer layer that was deposited on Y:ZrO2 (i.e., yttrium-stabilized zirconia (YSZ)) at 900 °C. The electrical properties of individual Sn:In2O3 NWs were measured by Wan et al.17,18 in a two- and four-point probe terminal configuration, and the average resistivity was found to be on the order of 10−4 Ω cm. However, one of the drawbacks of YSZ is that it is not transparent like m-, r-, and a-Al2O3. Consequently their Sn:In2O3 NWs were dispersed on glass, forming a 500-nm-thick layer, and the optical transmission was measured to be 80% all the way from 300 to 800 nm.

1. INTRODUCTION Indium tin oxide (ITO) is a transparent conducting oxide (TCO) with high transparency and conductivity that is used extensively in solar cells and displays. The epitaxial growth and structural, electrical, and optical properties of ITO epitaxial layers on c-, m-, r-, and a- Al2O3 have been investigated extensively in the past1−5 and remain active topics of interest.6 In addition to epitaxial layers, ITO (i.e., Sn:In2O3 nanowires (NWs)) have also been investigated for the fabrication of nanowire solar cells, light-emitting diodes, sensors, photocatalysis, and so forth.7−14 In the past, we have grown Sn:In2O3 NWs at 800 °C and 10−1 mbar via the vapor−liquid−solid (VLS) mechanism on Si, fused SiO2, and C fibers using metallic Sn and In.15 However, they were not ordered in any way, which prohibits the realization of nanoscale devices such as nanowire solar cells (NWSCs). Only a few have obtained epitaxial, ordered Sn:In2O3 NWs. More specifically vertical Sn:In2O3 NWs with a cubic crystal © 2019 American Chemical Society

Received: March 9, 2019 Accepted: May 16, 2019 Published: May 16, 2019 4274

DOI: 10.1021/acsaem.9b00519 ACS Appl. Energy Mater. 2019, 2, 4274−4283

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ACS Applied Energy Materials

temperature was ramped up to 800 °C at 30 °C/min using the same flow of Ar. Upon reaching 800 °C, a flow of 10 sccm O2 was added to the flow of Ar in order to grow the Sn:In2O3 NWs over 10 min at 1 mbar, followed by a cool down without O2. We have also grown the Sn:In2O3 NWs on m-, r-, and a-Al2O3 at 700, 750, 800, 900, and 1000 °C, keeping all else equal, in order to determine if this has an effect on the ordering. 2.2. Structural, Electrical, and Optical Characterization of the Sn:In2O3 NWs. The morphology and crystal structure of the Sn:In2O3 NWs were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). High-resolution transmission electron microscopy (HRTEM) was carried out using a TECNAI F30 G2 S-TWIN operated at 300 kV and equipped for energy-dispersive X-ray (EDX) analysis. In addition, the electrical properties and carrier density were measured by the Hall effect in the Van der Pauw geometry, at B = 0.2 T using a Keithley 2182 Nano Voltmeter and a Keithley 2635A constant current source, after the formation of ∼1mm-diameter Ag paint contacts over the Sn:In2O3 NWs at the four corners of the 10 mm × 10 mm Al2O3.29 The room-temperature photoluminescence (PL) spectra were measured with a He−Cd laser operating at 325 nm and 20 mW.

In addition to these efforts, Gao et al.19 obtained highly ordered, vertical Sn:In2O3 NWs via the VLS mechanism on aAl2O3 at 840 °C by using In2O3, SnO2, and C. These also had the cubic crystal structure, a diameter of 180 nm, and lengths of a few micrometers and were highly transparent with transmission greater than 80% above 600 nm but also exhibited blue emission under ultraviolet (UV) excitation, suggesting that they are attractive, not just for their conductivity and transparency but also for light emission. More recently, Shen et al.20 obtained Sn:In2O3 NWs via the VLS mechanism on YSZ at 800 °C using a mixture of In2O3, SnO2, and C (0.6:0.4:1) under a small flow of Ar at 1 mbar. The Sn:In2O3 NWs had cubic and orthorhombic crystal structures, square sections with sides of 140 ±18 nm, and lengths of up to 1 μm and were oriented at ϕ = 90, 45, and 35°. Besides, Chen et al.21,22 have grown vertical, undoped In2O3 NWs with a cubic crystal structure but hexagonal cross sections via the VLS mechanism on a-Al2O3 at 620 °C using C and In2O3 under a flow of Ar and O2. These In2O3 NWs exhibited photoluminescence at 378 nm (= 3.28 eV) which was attributed to radiative transitions from oxygen donor levels to the valence band. Finally, it is important to mention that Hsin et al.23 obtained lateral self-aligned In2O3 NWs on Si (111) while Shen et al.24 has grown laterally aligned Sn:In2O3 NWs on YSZ. All other TCO NWs that have been obtained in the past (e.g., Sb:SnO2, Al:ZnO, and In:ZnO NWs25−27) were not ordered along any particular direction. Only Sn:In2O3 NW ordered networks have been obtained on a-Al2O3 and YSZ via the reduction of In2O3, SnO2, or SnO by C, which is not best for achieving a high uniformity over areas greater than 10 mm2. To the best of our knowledge, no one has investigated the growth and properties of Sn:In2O3 NWs on m- and r-Al2O3, although these have oxygen-terminated surfaces that are suitable for the growth of cubic, tetragonal, and orthorhombic crystal structures while being optically transparent. Consequently, here we have carried out a detailed investigation into the structural, electrical, and optical properties of Sn:In2O3 NWs grown on m-, r-, and a-Al2O3 by the VLS mechanism. We obtain highly directional, epitaxial Sn:In2O3 NWs with excellent uniformity up to 15 mm × 15 mm on m- and r-Al2O3 in a reproducible fashion but to a lesser extent on a-Al2O3. We describe their structural, electrical, and optical properties in conjunction with theoretical calculations and discuss the advantages of these Sn:In2O3 NWs for the fabrication of NWSCs.28

3. RESULTS AND DISCUSSION In the past, we have shown that the reaction of Sn and In with O2 at 800 °C and 10−1 mbar results in the growth of Sn:In2O3 NWs on 1 nm Au/Si(001) with an average diameter of ∼50 nm and lengths of up to ∼100 μm.15 These Sn:In2O3 NWs had square sections and a cubic crystal structure, but they were not oriented in any particular direction because of the fact that they do not obey any epitaxial relation with the underlying Si, which is also not transparent. This is a constraint in exploiting the full potential of Sn:In2O3 NWs as a TCO for the realization of devices such as NWSCs. Highly directional, epitaxial Sn:In2O3 NWs have been obtained only on a-Al2O3 and YSZ, although m- and r-Al2O3 have oxygen-terminated surfaces that are suitable for the growth of cubic, tetragonal, and orthorhombic crystal structures and are also optically transparent. Consequently, we have grown Sn:In2O3 NWs on m-, r-, and a-Al2O3 and investigated their structural, electrical, and optical properties. 3.1. Structural Properties of Sn:In2O3 NWs on m-, r-, and a-Al2O3. We readily obtained highly ordered Sn:In2O3 NWs on m-, r-, and a-Al2O3 at 800 °C and 1 mbar as shown by the SEM images in Figures 1, 2, and 3, respectively. No Sn:In2O3 NWs were grown without supplying O2 during the growth step, suggesting that the residual O2 after purging and the temperature ramp was negligibly small. In other words, the growth of the Sn:In2O3 NWs occurred only during the growth step at 800 °C, upon supplying a small flow of O2 at 1 mbar, which also prevented the oxidation of the metallic Sn and In that melt at 232 and 157 °C, respectively, during the temperature ramp. As a result, we obtained epitaxially ordered Sn:In2O3 NWs on m- and r-Al2O3 in a highly reproducible fashion and with excellent uniformity over areas of up to 15 mm × 15 mm but not to the same extent on a-Al2O3. We did not obtain ordered networks of Sn:In2O3 NWs on m-, r-, or aAl2O3 at 800 °C by reducing the growth pressure down to 10−1 mbar as a result of the fact that this results in a larger amount or flux of Sn and In in arriving at the Au particles, which seems to be incompatible with epitaxial growth. Furthermore, temperatures below 800 °C or above 900 °C resulted in the growth of Sn:In2O3 NWs that were not oriented in any particular direction. A similar dependence on growth temperature and pressure has been observed by Chern et al.5 Consequently, the optimum temperature range for the epitaxial

2. EXPERIMENTAL SECTION 2.1. Epitaxial Growth of Sn:In2O3 NWs on m-, r-, and aAl2O3. The Sn:In2O3 NWs were grown using a 1 in. hot wall, lowpressure chemical vapor deposition (LPCVD) reactor capable of reaching 1100 °C, which was fed via a microflow leak valve positioned on the upstream side just after the gas manifold which consists of four mass flow controllers. A chemically resistant rotary pump that can reach 10−4 mbar was connected downstream. For the growth of the Sn:In2O3 NWs, 5 mg of Sn and 95 mg of In (Aldrich, 100 mesh, 99.9%) were weighed to an accuracy of ±1 mg. Square samples of 10 mm × 10 mm m-, r-, and a-Al2O3 were cleaned sequentially in trichloroethylene, methanol, acetone, and isopropanol, rinsed with deionized water, dried with nitrogen, and then coated with ∼1 nm Au. The elemental Sn and In and the clean m-, r-, and a-Al2O3 substrates were loaded into the same quartz boat that was positioned at the center of the 1 in. LPCVD reactor, pumped down to 10−4 mbar, and purged with 1000 sccm of Ar for 10 min at 1 mbar. Subsequently, the 4275

DOI: 10.1021/acsaem.9b00519 ACS Appl. Energy Mater. 2019, 2, 4274−4283

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Figure 3. (a, b) SEM images of the vertical Sn:In2O3 NWs obtained at 900 °C on a-Al2O3 with In:Sn = 20, (c) side view showing that the Sn:In2O3 NWs grow at ϕ ≈ 90°, (d) side view after rotation of ω = 90°, (e) square section of the vertical Sn:In2O3 NWs after removing them from a-Al2O3, (f) longer, tilted Sn:In2O3 NWs, (g) side view of longer tilted Sn:In2O3 NWs, and (h) representative image of a textured layer of Sn:In2O3 grown on a-Al2O3 with no Au. Similar morphology was obtained on m- and r-Al2O3.

Figure 1. (a−c) SEM images of the Sn:In2O3 NWs obtained at 900 °C on m-Al2O3 with In:Sn = 20, (d) side view of the Sn:In2O3 NWs which appear to be vertical, (e) side view of the same Sn:In2O3 NWs showing that they actually grow at ϕ = 60° after a rotation by ω = 90°, (f, g) high-magnification images of side views, (h, i) SEM images showing the square and hexagonal sections of Sn:In2O3 NWs after removing them from m-Al2O3, and (j) SEM image of the Sn:In2O3 NWs showing that their surface is not smooth.

magnification image of the side views of the Sn:In2O3 NWs are shown in Figure 1f,g, from which it is clear that the minority Sn:In2O3 NWs are also inclined at ϕ = 60°. Most of the Sn:In2O3 NWs have square sections, as shown in Figure 1h, with lengths of up to 80 μm, but we have also observed the existence of a few with hexagonal-like sections and flat tops as shown in Figure 1i. In addition, one may observe a periodic variation in the diameter of the Sn:In2O3 NWs, as shown in Figure 1j, which gives a sawtooth-like profile along their length. In the past, Nguyen et al.30 has tried to grow Sn:In2O3 NWs on m-Al2O3 via the reduction of In2O3 and SnO2 by C but obtained SnO2 NWs despite the fact that they used more In2O3 than SnO2. In essence, they obtained oriented SnO2 NWs with diameters of 86 ±12 nm and average lengths of 2.5 μm on m- and a-Al2O3 substrates at 840 °C. The In:SnO2 NWs on m-Al 2 O 3 were oriented along two mutual perpendicular directions similar to those for the undoped SnO2 NWs obtained by Mathur et al.31 and Leonardy et al.32 To the best of our knowledge, no one else has previously obtained epitaxially ordered Sn:In2O3 NWs on m-Al2O3. However, it is worthwhile to note that Chern et al.5 has grown Sn:In2O3 epitaxial layers by pulsed laser deposition on m-Al2O3 which consisted of elongated grains oriented along one direction, consistent with the alignment of the Sn:In2O3 NWs on m-Al2O3 shown in Figure 1. In contrast, we find that the Sn:In2O3 NWs on r-Al2O3 are oriented along two mutually perpendicular directions, as shown in Figure 2a,b. The Sn:In2O3 NWs are inclined at ϕ = 45° with respect to the surface of r-Al2O3 as shown from the side views in Figure 2c,d and have lengths of up to 80 μm. Higher-magnification images of the side views are shown in Figure 2e,f. Most of the Sn:In2O3 NWs have square sections as shown in Figure 2g, but again we observed a few that have hexagonal sections with flat tops as shown in Figure 2h. The Sn:In2O3 NWs are tapered, as shown in Figure 2i, but one may also observe the formation of pyramidal heads on their ends as opposed to spherical Au particles as shown in Figure 2g. In the past, Sn:In2O3 epitaxial layers with a cubic crystal structure

Figure 2. (a, b) SEM images of the Sn:In2O3 NWs obtained at 900 °C on r-Al2O3 with In:Sn = 20, (c) side view of the Sn:In2O3 NWs showing that they grow at an angle of ϕ = 45°, (d) side view after rotation by ω = 90° (e, f) high-magnification images of side views, (g, h) SEM images showing the square and hexagonal sections of the Sn:In2O3 NWs after removing them from the r-Al2O3, and (i) SEM image showing tapering and steps on the surface of the Sn:In2O3 NWs.

growth of highly directional Sn:In2O3 NWs on m-, r-, and aAl2O3 falls between 800 and 900 °C at 1 mbar. We find that a majority of Sn:In2O3 NWs on m-Al2O3 are aligned along one direction as shown in Figure 1a,b, but one may also observe the existence of a minority of Sn:In2O3 NWs in two other directions which form a herringbone-like pattern. The Sn:In2O3 NWs are tapered as shown in Figure 1c, and we do not observe any Au particles on their ends. Upon closer inspection, we find that a majority of the Sn:In2O3 NWs appear to be vertical upon viewing from one side, as shown in Figure 1d, but in fact they are inclined at ϕ = 60° with respect to the surface of m-Al2O3 when viewed from the side after an in-plane rotation of ω = 90° as shown in Figure 1e. A higher4276

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ACS Applied Energy Materials have been deposited on r-Al2O3,4,5 but to the best of our knowledge, no one has tried to grow Sn:In2O3 NWs on rAl2O3. Nevertheless, Chern et al.5 observed that Sn:In2O3 epitaxial layers on r-Al2O3 consisted of elongated grains aligned along two mutually perpendicular directions consistent with the ordering of the Sn:In2O3 NWs on r-Al2O3 as shown in Figure 2. Finally, we obtained vertical Sn:In2O3 NWs on a-Al2O3 as shown in Figure 3a,b. Side-view images of these are shown in Figure 3c,d, from which we find that they have lengths of up to 50 μm and sides of up to 300 nm as shown in Figure 3e. The vertical Sn:In2O3 NWs on a-Al2O3 are similar to those obtained by Gao et al.19 on a-Al2O3 at 840 °C by using In2O3, SnO2, and C. However, we also observed on occasion the formation of larger nanowires as shown in Figure 3f, which are inclined with respect to the surface of a-Al2O3 as shown in Figure 3g. Consequently, the growth was not very reproducible in the sense that we did not always obtain vertical Sn:In2O3 NWs all over the a-Al2O3. In all of the above cases, a majority of the Sn:In2O3 NWs had square sections, but a few appeared to be hexagonal with flat tops. One would be inclined then to suggest that the Sn:In2O3 NWs with hexagonal and square sections have different crystal structures. Before considering the structural properties of the Sn:In2O3 NWs in further detail, it is useful to note that In2O3 is polymorphic and has the cubic bixbyite crystal structure (i.e., body-centered cubic bcc-In2O3 with a unit cell consisting of 32 In and 48 O atoms that has a lattice constant of a = 10.11 Å under ambient conditions). The rhombohedral or corundum structure of In2O3 was first obtained by Shannon et al.33 at 1250 °C and 6.5 GPa and is a metastable form that belongs to the hexagonal family and consists of hexagonal close-packed oxygen ions. As such, the rhombohedral cell can also be described in terms of a hexagonal cell with a = 5.487 Å, b = 5.487 Å, and c = 15.51 Å. Corundum-type rh-In2O3 has been used to make sensors34 but also for energy storage35 and photocatalysis.36 It has been shown that bcc-In2O3 may be transformed to rh-In2O3 upon compression,37 but Wang et al.38 obtained bcc-In2O3 as well as rh-In2O3 layers on c-Al2O3 by metal organic chemical vapor deposition between 400 and 600 °C. Similarly, Nishinaka et al.6 obtained epitaxial layers of rh-In2O3 on m-, r-, and a-Al2O3 by mist chemical vapor deposition at 475 °C and 1 bar using SnCl4·5H2O and In (OCCH3CHOCCH3)3, suggesting that rhIn2O3 may be obtained under moderate growth conditions via a suitable choice of the underlying substrate. Consequently, the formation of Sn:In2O3 NWs with bcc-In2O3 and rh-In2O3 crystal structures that have square and hexagonal sections would not be surprising. However, it is important to emphasize that Chen et al.21 obtained In2O3 NWs on a-Al2O3 at 620 °C with a cubic crystal structure but hexagonal sections consisting of a horizontal triangular top surface surrounded by tilted side surfaces, which is possible by a consideration of the crystallographic symmetry of the cubic crystal of In2O3. Therefore, the Sn:In2O3 NWs with square- and hexagonallike sections shown in Figures 1, 2, and 3 may both correspond to bcc-In2O3. All of the Sn:In2O3 NWs on m-, r-, and a-Al2O3 exhibited one or two major peaks in the XRD as shown in Figures 4, 5, and 6, respectively, resulting from the fact that they grow in an epitaxial fashion along specific crystallographic directions. In contrast, the Sn:In2O3 NWs that we obtained previously on Si displayed a multitude of peaks because they do not obey any

Figure 4. XRD of Sn:In2O3 NWs obtained on m-Al2O3. Also shown is the trace from plain m-Al2O3.

Figure 5. XRD of Sn:In2O3 NWs obtained on r-Al2O3. The inset shows a rocking curve around the (440) peak of cubic bixbyite Sn:In2O3.

Figure 6. XRD of Sn:In2O3 NWs obtained on a-Al2O3. The inset shows the trace from a-Al2O3.

epitaxial relation with the underlying Si. Note that the XRD traces of the m-, r-, and a-Al2O3 without any Sn:In2O3 NWs are also shown for comparison in Figures 4, 5, and 6 respectively. The Sn:In2O3 NWs on m-Al2O3 exhibit one major peak at θ = 21° and a weaker but nevertheless clearly resolved peak at θ = 43.5°, as shown in Figure 4. These correspond to the (422) and (211) crystallographic planes of bcc-In2O3 but cannot be indexed to rh-In2O3, in agreement with the findings of Nishinaka et al.,6 who obtained bcc Sn:In2O3 layers on mAl2O3. Similarly, the Sn:In2O3 NWs obtained on r-Al2O3 exhibit one major peak at θ = 51° and a weaker one at θ = 30.5° as shown in Figure 5, corresponding to the (440) and (222) crystallographic planes of bcc-In2O3, similar to the XRD obtained by Vogt et al.3 from a Sn:In2O3 epitaxial layer that was grown on r-Al2O3. The same peaks and the strong side peak belonging to Al2O3 (024) were also observed by Chern et al.,5 who obtained (110)-oriented Sn:In2O3 layers on r-Al2O3 4277

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ACS Applied Energy Materials by pulsed laser deposition. It is also consistent with the fact that the Sn:In2O3 epitaxial layers obtained by Nishinaka et al.6 directly on r-Al2O3 also had a cubic crystal structure. A rocking curve around the (440) peak is shown as an inset in Figure 5, giving a full width at half-maximum of 1.6° because not all of the Sn:In2O3 NWs are perfectly parallel. We did not observe any peaks belonging to the tetragonal rutile crystal structure of SnO2 in Figures 4 and 5. This is due to the fact that we used a mass ratio of In:Sn ≈ 20, which results in the formation of Sn:In2O3 NWs, where Sn is incorporated into In2O3 as a substitutional donor impurity.15 Finally, the Sn:In2O3 NWs obtained on a-Al2O3 exhibit two major peaks at θ = 30.5 and 63.5° corresponding to the (222) and (444) crystallographic planes of bcc-In2O3 as shown in Figure 6. These cannot be indexed to rh-In2O3 or o-In2O3 and are identical to those observed by Chen et al.,21 who obtained undoped In2O3 NWs with hexagonal sections on a-Al2O3. The formation of bcc Sn:In2O3 NWs on a-Al2O3 is also consistent with the fact that Nishinaka et al.6 and Chern et al.4 obtained bcc Sn:In2O3 epitaxial layers directly on a-Al2O3. The minor peak at θ = 27.4° cannot be indexed to bcc-, rh-, or o-In2O3 but surprisingly appears to belong to the tetragonal rutile crystal structure of SnO2. However, this peak was not always observed, consistent with the fact that the growth on a-Al2O3 was not highly reproducible compared to that on m- and rAl2O3. From the above discussion, we conclude that the highly ordered Sn:In2O3 NWs obtained on m-, r-, and a-Al2O3 have a cubic crystal structure similar to that of all epitaxially ordered Sn:In2O3 NWs obtained previously on a-Al2O3 and Y:ZrO2. For completeness, a high-magnification TEM image of an individual Sn:In2O3 NW after being removed from the mAl2O3 is shown in Figure 7a while an HRTEM image of the

Figure 8. (a) TEM image of a Sn:In2O3 NW after removing it from rAl2O3. (b) HRTEM showing the (440) crystallographic planes of Sn:In2O3. (c) HRTEM image showing that the lattice planes are not vertical with respect to the sides of the Sn:In2O3 NW. Note that the sides are not straight, consistent with the SEM images. (d) Twodimensional arrangement of Al and O on the oxygen-terminated surface of r-Al2O3.

crystallographic planes of In2O3 consistent with the XRD in Figure 5 where the lattice spacing between the (440) crystallographic planes is 1.79 Å. Finally, a typical TEM image of a Sn:In2O3 NW on a-Al2O3 is shown in Figure 9a.

Figure 9. (a) TEM image of a Sn:In2O3 NW after removing it from aAl2O3. (b) HRTEM image of Sn:In2O3 NW showing the (222) crystallographic planes that grow on top of a-Al2O3. (c) EDX spectrum showing that the Sn is incorporated into the In2O3 NWs. (d) Two-dimensional arrangement of Al and O on the oxygenterminated surface of a-Al2O3.

Figure 7. (a) TEM and (b) HRTEM images of a Sn:In2O3 NW after being removed from m-Al2O3. (c) EDX spectrum showing peaks belonging to In and Sn. (d) Two-dimensional arrangement of Al and O on the oxygen-terminated surface of m-Al2O3.

The Sn:In2O3 NWs grow along the [111] direction as shown by the HRTEM in Figure 9b where the lattice spacing between the (222) crystallographic planes is 2.9 Å. An EDX spectrum is also shown for completeness in Figure 9c, confirming the existence of In, O, and Sn. 3.2. Epitaxial Relation and Growth Mechanism of Sn:In2O3 NWs on m-, r-, and a-Al2O3. At first sight, the epitaxial growth of bcc Sn:In2O3 on m-, r-, and a-Al2O3 appears to be impossible as a result of the large lattice constant of cubic Sn:In2O3 (i.e., 10.067 Å), but it is possible to maintain epitaxial growth by matching integral multiples of lattice planes of In2O3

same Sn:In2O3 NW is shown in Figure 7b, from which we find that the lattice spacing between the (211) crystallographic planes is 4.13 Å. A typical EDX spectrum is shown in Figure 7c, from which we observed peaks corresponding to In, Sn, and O. Similarly, a TEM image of an individual Sn:In2O3 NW that was grown on r-Al2O3 is shown in Figure 8a. HRTEM images are shown in Figure 8b,c, from which we observe the (440) 4278

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ACS Applied Energy Materials with the underlying lattice of m-, r-, or a-Al2O3, shown as insets in Figure 7, 8, and 9. To investigate the epitaxial relationships between bcc-In2O3 (space group Ia3̅ (206)) and Al2O3 (space group R3̅c (167)), all of the possible atomistic models were constructed and are shown in Figure 10. From these, we

Figure 11. Schematic representation of the growth of Sn:In2O3 NWs via the VLS mechanism.

via the VLS mechanism, but a sideways deposition must also occur via the VS mechanism, which explains why we do not clearly observe the Au particles on the ends of the Sn:In2O3 NWs. 3.3. Electrical and Optical Properties of Sn:In2O3 NWs on m-, r-, and a-Al2O3. The carrier density of the Sn:In2O3 NWs obtained at 800 °C and 1 mbar on m-, r-, and a-Al2O3 with In:Sn = 20 was measured by the Hall effect in the Van der Pauw geometry by forming ∼1-mm-diameter Ag contacts at the corners of the 10 mm × 10 mm sample. The contacts showed excellent linear and ohmic behavior due to the metallic-like conductivity of the Sn:In2O3 NWs, so no annealing was carried out. In this case, the current is expected to flow through the Sn:In2O3 layer that exists between the freestanding Sn:In2O3 NWs and the underlying Al2O3. However, we expect that the current will also flow through the freestanding ordered network of Sn:In2O3 NWs because many of these are in contact with each other and the Sn:In2O3 NWs, which have lengths of many tens of micrometers, are essentially buried in the Ag paint contact at the corners. For comparison, we removed the Sn:In2O3 NWs from their parent substrate by dry transfer onto 10 mm × 10 mm fused SiO2. This resulted in an interconnected, planar network of Sn:In2O3 NWs which also exhibited metallic-like conductivity, and we obtained carrier densities of between 1020 and 1021 cm−3, in very good agreement with Gao et al.,19 who also measured the carrier density of Sn:In2O3 NWs in the Van der Pauw geometry. We obtained a maximum carrier density of ∼1021 cm−3 but did not observe a significant variation in the carrier density of the Sn:In2O3 NWs on the different m-, r-, and a- orientations of Al2O3. The Sn:In2O3 NWs constitute in essence a highly degenerate n-type semiconductor with a large carrier density between 1020 and 1021 cm−3 that is related to the substitutional Sn donor impurities. The metallic-like conductivity is on the order of 106 S/m, similar to that obtained by Wan et al.,17,18 who measured the resistivity of individual Sn:In2O3 NWs in a two- and four-point probe terminal configuration and found that the average resistivity was on the order of 10−4 Ω cm. The Fermi level as such resides in the conduction band as discussed in more detail below. Consequently, a reduction in temperature will not result in a significant change in carrier density, similar to the case of metals. This has been observed in temperature-dependent measurements of the carrier density and conductivity of both Sn:In2O3 NWs and ITO layers.39−42 As such, the native defects in the Sn:In2O3 NWs are not expected to contribute strongly to the overall conductivity. To gain a better understanding of the charge distribution in the Sn:In2O3 NWs, we have calculated the conduction band (CB)

Figure 10. Atomistic models of In2O3 and Al2O3 constructed by VESTA: (a) m-Al2O3, (b) r-Al2O3, (c) a-Al2O3, (d) (110) In2O3, (e) (100) In2O3, and (f) (111) In2O3. The small, red spheres represent O, while large mauve and blue spheres represent In and Al atoms, respectively. The black outline denotes the unit cell.

conclude that the majority of Sn:In2O3 NWs grow on m-Al2O3 by a stacking of (211) crystallographic planes at an angle of ϕ ≈ 60°. We also deduce that the in-plane epitaxial relationships are Sn:In 2 O 3 [001] || [101̅ 0] Al 2 O 3 and Sn:In 2 O 3 [11̅0] || [0001] Al2O3 with lattice mismatches of 5.24 and 10.1%, respectively. Consequently, the Sn:In2O3 NWs are preferentially oriented along the direction of the smaller lattice mismatch. The Sn:In2O3 NWs on r-Al2O3 grow by a stacking of (110) planes at an angle of ϕ ≈ 45°. In this case, the inplane epitaxial relationships are Sn:In2O3 [001] || 1/2[426̅1̅] Al2O3 and Sn:In2O3 [11̅0] || 2/3[22̅01] Al2O3 with lattice mismatches of 4.09 and 5.49%, respectively, which are comparable. Consequently, the Sn:In2O3 NWs are oriented along two mutually perpendicular directions. The epitaxial relationships for the Sn:In2O3 NWs on m- and r-Al2O3 are very similar to those observed by Chern et al.,5 who deposited Sn:In2O3 epitaxial layers on m- and r-Al2O3. In contrast, the Sn:In2O3 NWs on [112̅0] Al2O3 grow vertically along the [111] direction, so they obey the epitaxial relation Sn:In2O3 (111) || (112̅0) Al2O3. All of the Sn:In2O3 NWs grow by the VLS mechanism, whereby Sn and In enter the Au catalyst particles and form liquid Au:Sn:In particles at elevated temperatures. Upon saturation, solid Sn:In2O3 NWs forms beneath the liquid Au particles via the reaction with O2 at the triple-phase junction as shown in Figure 11, leading to one-dimensional growth. We did not obtain Sn:In2O3 NWs on m-, r-, and a-Al2O3 with no Au. Instead, we observed the deposition of a textured layer of Sn:In2O3 as shown in Figure 3h which grows by a vapor−solid (VS) mechanism. This textured layer that grows by the VS mechanism directly on m-, r-, and a-Al2O3 between the Sn:In2O3 NWs is not an impediment to the realization of NWSCs as discussed in more detail later. Considering the above, it is reasonable to suggest that the Sn:In2O3 NWs grow 4279

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is 2 nm. When the radius is greater than 100 nm, we find that most of the charge resides near the surface, similar to the charge distribution in bulk Sn:In2O3. The large carrier density and small resistivity which was found to be ∼10−4 Ω cm gives a mobility of 10 cm2/(V s) at 300 K due to ionized impurity scattering. However, before describing in detail the advantages of using these highly conductive and ordered networks of Sn:In2O3 NWs in NWSCs, it is worthwhile to point out that the Sn:In2O3 NWs exhibited room-temperature PL as shown in Figure 13. It has been shown by Boer et al.45 that In2O3 has

potential profile along the radial direction but also the energetic position of the one-dimensional sub-bands and one-dimensional electron gas (1DEG) charge distribution via the self-consistent solution of the Poisson−Schrö dinger (SCPS) equations in the effective mass approximation. In such a calculation, one begins with the solution of Schrödinger’s equation taking a trial potential, subject to specific boundary conditions, which gives the sub-band energies and wave functions that are normalized in order to obtain the one-dimensional electron gas (1DEG) using the one-dimensional density of states (1D DOS) in conjunction with Fermi−Dirac statistics. Finally, Poisson’s equation is solved to get a correction potential that is added to the initial trial potential, and the process is repeated until convergence is reached, after which charge neutrality is evaluated for completeness. The SCPS calculations were carried out by taking into account the effective mass and dielectric constants of In2O3 (i.e., me* = 0.35 mo and εr = 9.3).43,44 Note that the energy band gap and effective masses of cubic and rhombohedral In2O3 are very similar to each other.45 In addition, we have taken into account work function ϕ = 4.5 eV and electron affinity χ = 3.3 eV of In2O3.46 Finally, the Fermi level was taken to reside ∼0.4 eV above the CB edge in accordance with the theoretical and experimental investigation of King et al.47 on bulk In2O3. The band profile and charge distribution of a 40-nmdiameter Sn:In2O3 NW taking a uniform distributon of donors (i.e., ND = 1 × 1019 cm−3) throughout is shown in Figure 12.

Figure 13. Room-temperature PL of Sn:In2O3 NWs grown on mAl2O3.

energy band gaps of 3.2 ± 0.3, 3.1 ± 0.3, and 2.9 ± 0.3 eV corresponding to the Ia3, R3c, and Pbcn In2O3 polymorphs, respectively, in excellent agreement with experiment. We suggest then that the PL emission at 400 nm (= 3.1 eV) is attributed to band-to-band radiative transitions while the PL at 550 nm (= 2.25 eV) is attributed to (a) radiative transitions via donorlike states that are energetically located in the upper half of the energy band gap and which are related to crystal imperfections and/or (b) the radiative recombination of photoexcited core holes and conduction band electrons in the metal Au particles.48 The PL at 3.1 and 2.25 eV were observed at different points in the ordered network of Sn:In2O3 NWs, but not that at 800 nm (= 1.55 eV). It should also be noted that we have not observed PL at 1.55 eV in the past from randomly oriented networks of Sn:In2O3 NWs grown using an identical thickness of Au catalyst.15 Similarly, others like Gao et al.19 have not observed near-infrared PL from Sn:In2O3 NWs. It is highly unlikely that the PL at 1.55 eV is related to Au particles because these will emit at around 2.0 eV.48 In addition, the near-infrared PL at 800 nm is not related to the laser excitation or surface plasmon resonance (SPR) effects related to the ordered network of Sn:In2O3 NWs; otherwise, we would have observed it consistently at different positions.49 Instead, it is more likely that the near-IR emission at 800 nm (= 1.55 eV) is related to some kind of defect. Indium vacancies (VIn) give rise to acceptor-like levels residing in the lower half of the energy band gap as shown by Wang et al.50 and Reunchan et al.51 More specifically, In vacancies in ntype In2O3 act as triple acceptors with a low formation energy and in fact may reduce the n-type carrier density. In contrast, oxygen vacancies VO give rise to deep-donor midgap states starting at around 1.5 eV above the valence band edge.52 From the above discussion, it appears reasonable to suggest that the near-infrared emission at 800 nm (= 1.55 eV) is related to radiative transitions occurring between the conduction band and VO midgap states or alternatively between the VO midgap states and the valence band. Despite this, it is evident that the

Figure 12. SCPS CB edge potential profile relative to the Fermi level (i.e., EC − EF (eV)) and 1DEG charge distribution (× 1019 cm−3) versus distance along the radial direction r for a 40-nm-diameter Sn:In2O3 NW taking a doping level of ND = 1 × 1019 cm−3 at 300 K. The Fermi level resides above the CB edge at the surface (i.e., EC − EF = −0.4 eV).

The potential is inverted, and one may observe a downward band bending toward the surface of the Sn:In2O3. We find a total of eight sub-bands residing below the Fermi level, the lowest one at −0.26 eV and the shallowest at −0.015 eV or 15 meV below the Fermi level. The 1DEG charge distribution resulting from the occupancy of the lowest-energy sub-bands is confined in the vicinity of the surface as shown in Figure 12. In contrast, the shallower-energy sub-bands are occupied by electrons with wave functions that have a maximum near the core which gives rise to a peak in the 1DEG charge distribution near r = 0. The number of sub-bands occupied by electrons in equilibrium is strongly dependent on the diameter of the Sn:In2O3 NW and is reduced to just two only when the radius 4280

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ACS Applied Energy Materials Sn:In2O3 NWs are capable of light emission and are promising for the realization of light-emitting devices in addition to acting as TCOs in NWSCs. 3.4. Advantages of Ordered Sn:In2O3 NW Networks in NWSCs. All of the epitaxially ordered Sn:In2O3 NWs that were grown on m-, r-, and a-Al2O3 are attractive for the fabrication of high-performance NWSCs as a result of their large surface area. We estimate that the ordered network of Sn:In2O3 TCO NWs grown on 15 mm × 15 mm (i.e., 225 mm2) r-Al2O3 has an effective area of 195 mm × 195 mm, which is ∼170 times larger than the area of a 15 mm × 15 mm film of ITO on glass. Consequently, the junction area formed by the deposition of a barrier, absorber, and p-type layer over the ordered network of Sn:In2O3 TCO NWs is expected to be considerably larger compared to that of a planar device, which in turn is expected to lead to a significant increase in the short circuit current. In the past, Sn:In2O3 NWs have been used mainly for the fabrication of dye-sensitized solar cells, but they were not grown in an epitaxial fashion and as such were not ordered.53,54 To the best of our knowledge, no one has previously used ordered networks of Sn:In2O3 NWs in NWSCs despite the fact that periodic photonic nanostructures have been shown to outperform their random counterparts in trapping light in solar cells.55 Although other metal oxide (MO) NWs such as ZnO NWs have been used for the fabrication of all-solid-state NWSCs, they are not as highly conductive as the Sn:In2O3 NWs described here, which in turn limits the short circuit current and performance of NWSCs. Short ZnO NWs with a length of 300 nm have been grown by Jean et al.56 on top of a 50 nm ZnO layer that was deposited on an ITO film on soda lime glass (SLG). The ZnO NWs were subsequently coated with PbS quantum dots (QDs) that filled the space in between, leading to a planar structure that was completed by the deposition of MoO3 and a 100-nm-thick metal contact of Au.56 A similar structure employing graphene on quartz as opposed to ITO on SLG was fabricated by Park et al.57 More recently, ZnO NWs were grown on a compact layer of TiO2 on ITO/ SLG, after which a perovskite hole transport layer (HTL) and a metal contact for Au were deposited, also resulting in a planar structure.58 In all of the above cases, the ZnO NWs did not act as the TCO per se but were grown on a film of ITO on SLG in order to increase the surface area of the junction and promote light trapping. It is desirable then to use highly conductive ordered networks of Sn:In2O3 NWs such as those described here for the fabrication of all-solid-state NWSCs. We propose the architecture shown in Figure 14 which could consist of (i) TiO2 as a barrier, (ii) CH3NH3PbI3 as an absorber, and (iii) CuxZn1−xS as a p-type layer, but one could envisage other materials too. The device must include separate metal contacts on the n-type Sn:In2O3 NWs and p-type layer. To fabricate the device shown in Figure 14, it would be necessary to deposit a layer of photoresist (PR) all over the ordered network of n-type Sn:In2O3 NWs and then use a photomask (PM) to expose the PR to ultraviolet light over the right part of the network as shown in Figure 14a, followed by the development as depicted in Figure 14b. Alternatively, one could apply acrylic polymer adhesive tape over the left part of the n-type Sn:In2O3 NW network instead of using PR, which would allow the deposition of the barrier and absorber shells as shown in Figure 14c. Subsequently, a p-type layer would be deposited to fill in the voids between the Sn:In2O3 NWs and create a planar device that would also allow the deposition of a metal contact on top of the p-type layer as shown in Figure

Figure 14. Schematic outline of the process for the fabrication of an NWSC using an ordered network of Sn:In2O3 NWs: (a) deposition of photoresist (PR) and exposure through a photomask (PM), (b) the development of PR, (c) sequential deposition of barrier and absorber shells, (d) planarization via the deposition of a p-type layer, (e) formation of a contact with the p-type layer, (f) lift-off PR, and (g) formation of contact to the underlying n-type Sn:In2O3. The final device is depicted in the center where light enters from the back through the Al2O3 and is absorbed. The top contact to the p-type layer may also serve as a mirror to reflect back light not absorbed during its first passage through the device.

14d,e. Finally, the PR would be lifted off in order to deposit a metal contact such as Au over the n-type Sn:In2O3. We have found that it is possible to apply tape and remove Sn:In2O3 NWs from certain areas of the ordered network, leaving a conductive layer on Al2O3 that is connected to the freestanding ordered network of Sn:In2O3 NWs. In other words, it is possible to make a planar contact to the side of the threedimensional, ordered network of Sn:In2O3 NWs. In general, the overall device structure has to be carefully prepared to avoid direct contact between the n-type Sn:In2O3 NWs and the metal contact deposited on top of the p-type layer through voids. This would inadvertently lead to shorting, and the device would not be able to sustain a voltage upon exposure to light.

4. CONCLUSIONS We have grown highly ordered Sn:In2O3 NWs by the VLS mechanism on m-, r-, and a-Al2O3 between 800 and 900 °C using metallic Sn and In with In:Sn ≈ 20 under flows of Ar and O2 at 1 mbar. All of the Sn:In2O3 NWs have a cubic crystal structure, they are tapered with lengths of up to 80 μm, and they are inclined at ϕ = 60° on m-Al2O3. The Sn:In2O3 NWs on r-Al2O3 are inclined at ϕ = 45° along two mutually perpendicular directions, while those on a-Al2O3 are vertical. The Sn:In2O3 NWs have a high carrier density on the order of 1021 cm−3 and a resistivity of 10−4 Ω cm but a low mobility of 10 cm2/(V s) at 300 K due to the large density of donor impurities and ionized impurity scattering. We show that the charge distribution is confined to the vicinity of the surface as a result of the occupation of sub-bands that fall well below the Fermi level via the self-consistent solution of the Poisson− Schrödinger equations in the effective mass approximation. In addition, the Sn:In2O3 NWs exhibited room-temperature photoluminescence at 3.1 eV due to band-to-band radiative 4281

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transitions and at 2.25 eV due to donor like states residing energetically in the upper half of the energy band gap. These epitaxially ordered Sn:In2O3 NWs can be grown with excellent uniformity up to 15 mm × 15 mm on m- and r-Al2O3, which is important for the realization of high-performance NWSCs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Matthew Zervos: 0000-0002-6321-233X Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsaem.9b00519 ACS Appl. Energy Mater. 2019, 2, 4274−4283

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DOI: 10.1021/acsaem.9b00519 ACS Appl. Energy Mater. 2019, 2, 4274−4283