Gold Core–Semiconductor Shell Nanowires Prepared by

ARTICLE pubs.acs.org/JPCC

Gold Core Semiconductor Shell Nanowires Prepared by Lithographically Patterned Nanowire Electrodeposition Somnath Ghosh,† Justin E. Hujdic,‡ Alfredo Villicana-Bedolla,‡ and Erik J. Menke*,‡ † ‡

Department of Physics, University of California, Merced, California 95343, United States Department of Chemistry, University of California, Merced, California 95343, United States ABSTRACT: Here, we report the synthesis of Au/PbSe and Au/copper indium diselenide (CIS) core/shell nanowires. The nanowires were synthesized by the aqueous electrodeposition of either PbSe or CIS onto gold nanowires prepared by lithographically patterned nanowire electrodeposition. The resulting nanowires are nanocrystalline, exhibit a sharp boundary between the gold core and the semiconducting coating, and are photoconductive, with enhanced light absorption in smaller nanowires.

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ore shell nanowires have been synthesized from a variety of materials to take advantage of the enhanced properties over their single-material counterparts. For instance, core shell nanowires have been used to prepare a variety of devices, including lasers and light-emitting diodes,1 6 field-effect transistors,7 11 and solar cells,12 18 that either would not work or would be inefficient if prepared from single-material nanowires. However, the majority of the work on core shell nanowires has focused on semiconductor/semiconductor nanowires, with very few reports on metal/semiconductor core/shell nanowires19 23 despite the unique properties demonstrated by metal/semiconductor heterostructures. For instance, metallic nanoparticles have been shown to enhance light absorption in solar cells,24 29 while metal/dielectric/metal heterostructures operate as broad-band waveguides, emitters, and detectors.30 34 Here, we report a general, scalable method for preparing gold-core, semiconductor-shell nanowires to address the lack of synthetic methods for preparing metal/semiconductor core/shell nanowires. The core shell nanowires were fabricated via a multistep process shown schematically in Scheme 1. This process essentially consists of two phases. In the first phase (steps 1 7 of Scheme 1), gold nanowires are grown on a templated glass substrate by the lithographically patterned nanowire electrodeposition (LPNE) method.35 In the second phase, the gold nanowire arrays produced by the LPNE method are then used as electrodes for the electrodeposition of a semiconductor. For this work, we have focused on the electrodeposition of PbSe and copper indium diselenide (CIS) as the semiconductor coatings, as PbSe and CIS are both useful materials for photovoltaic applications. To prepare the gold nanowires, we start with 1 in.  1 in. cleaning of float glass slides that have been immersed for ∼24 h in a standard Nochromix solution. These were then rinsed with NANOPure water (resistivity = 18.0 MΩ 3 cm) and dried with compressed air. Nickel evaporation, ∼5 100 nm thick (typical r 2011 American Chemical Society

values ∼ 40 nm), is done by the physical vapor deposition (PVD) technique using a Denton BTT-IV evaporator at a rate of 0.7 nm/s. In situ thickness monitoring is used to determine the film thickness using an SQM-160 film thickness monitor (INFICON). After the nickel deposition, a positive photoresist (ROHM and HAAS) is spin-coated on top of the Ni-coated slides, photopatterned using a 350 W UV (365 nm) light source (OAI), and then developed in an MF-24A solution (ROHM and HAAS). This is followed by rinsing with NANOPure water and drying with compressed air. These slides are then etched in 0.8 M HNO3 solution for 5 min to strip exposed nickel, creating a trench with a height equal to the thickness of the Ni layer and a depth of approximately 300 nm. After the trench has been created, gold is electrodeposited from a commercial gold-plating solution (Clean Earth) at room temperature. Finally, the photoresist and Ni film are chemically stripped away. Once the gold nanowires have been prepared by LPNE, the second phase of the core shell nanowire synthesis begins. In the second phase, the gold nanowires are used as electrodes for the electrodeposition of either PbSe or CIS. PbSe electrodeposition was achieved by a constant potential method following the procedure used by Hujdic et al.36 In this method, PbSe electrodeposition was achieved from a deposition solution containing 0.1 M Na2EDTA (Sigma Aldrich, purity = 99%), 10 mM Pb(NO3)2 (Sigma Aldrich, purity = 99.999%), and 1 mM SeO2 (Sigma Aldrich, purity = 99.999%) in NANOPure water (resistivity = 18.0 MΩ 3 cm), with the final solution pH adjusted to 4 by the dropwise addition of concd HNO3 (Sigma Aldrich, reagent grade). Figure 1 summarizes the results of PbSe electrodeposition on gold nanowires. Figure 1a shows a cyclic voltammagram (CV) of the PbSe electrodeposition solution with a gold nanowire array working Received: May 3, 2011 Revised: August 2, 2011 Published: August 10, 2011 17670

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Scheme 1. The Core Shell Nanowire Synthesis Process

Figure 2. (a) CV of the CIS electrodeposition solution. (b) SEM image showing a 100 nm gold nanowire with a 250 nm CIS shell. (c) Calibration curves relating the CIS shell thickness to electrodeposition time and gold nanowire width. (d) EDX spectrum of a CIS-coated gold nanowire showing the presence of copper, indium, and selenium.

Figure 1. (a) CV of the PbSe electrodeposition solution. (b) SEM image showing a 100 nm gold nanowire with a 200 nm PbSe shell. (c) Calibration curves relating the PbSe shell thickness to electrodeposition time and gold nanowire width. (d) EDX spectra of a PbSe-coated gold nanowire showing the presence of lead and selenium.

electrode. The broad peak between 0.5 and 1.0 V vs Ag/AgCl corresponds to the codeposition of lead and selenium, while the reduction that occurs below 1.0 V vs Ag/AgCl is due to hydrogen formation. The scanning electron microscopy (SEM) image in Figure 1b shows a single 100 nm wide gold nanowire that has been held at a potential of 0.6 V vs Ag/AgCl for 300 s in the PbSe deposition solution. The gold nanowire, which can be seen in the top portion of the image, has been coated with approximately 200 nm of lead selenide. The width of this coating depends on both the electrodeposition time and the width of the gold nanowire, as the calibration curve in Figure 1c shows. Energy-dispersive X-ray (EDX) reveals the presence of both lead and selenium in the

coating, as shown in Figure 1d, with a lead-to-selenium ratio of 45:55, close to the 50:50 expected in PbSe. CIS electrodeposition was achieved by a constant potential method following the procedure used by Phok et al.37 In this method, CIS electrodeposition is achieved from a deposition solution containing 1.5 mM CuSO4 (Sigma Aldrich, purity = 99.99%, metal basis), 2 mM InSO4 (Sigma Aldrich, purity = 99.99%, trace metal basis), and 1 mM SeO2 (Sigma Aldrich, purity = 99.999%) in NANOPure water (resistivity = 18.0 MΩ 3 cm), with the final solution pH adjusted to 2.8 by the dropwise addition of 1 M H2SO4 (Sigma Aldrich, reagent grade). Figure 2 summarizes the results of the CIS electrodeposition on gold nanowires. Figure 2a shows a CV of the CIS electrodeposition solution with a gold nanowire array working electrode. The broad peak between 0.3 and 0.6 V vs Ag/AgCl corresponds to the codeposition of copper, indium, and selenium, while the oxidation that occurs near 0.0 V vs Ag/AgCl is oxidation of indium. The SEM image in Figure 2b shows a single 100 nm wide gold nanowire that has been held at a potential of 0.6 V vs Ag/AgCl for 500 s in the CIS deposition solution. The gold nanowire, which can be seen in the top portion of the image, has been coated with approximately 250 nm of copper, indium, and selenium. As with the PbSe deposition above, the width of this coating depends on both the electrodeposition time and the width of the gold nanowire, as the calibration curve in Figure 2c shows. EDX, shown in Figure 2d, reveals the presence of copper, indium, and selenium in atomic percents of 27, 28, and 45%, respectively, close to the expected values of 25, 25, and 50%. To further characterize the semiconductor coating on the gold nanowires, we carried out transmission electron microscopy (TEM), selected area electron diffraction (SAED), and powder 17671

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Figure 4. (a) UV vis extinction spectra of a glass slide (black), an array of 40  100 nm gold nanowires (green), an array of 40  100 nm gold nanowires coated with 250 nm of PbSe (blue), and an array of 40  100 nm gold nanowires coated with 250 nm of CIS (red). (b) Light (green) and dark (black) IV curves of an array of 40  100 nm gold nanowires coated with 250 nm of PbSe. (c) Light (green) and dark (black) IV curves of an array of 40  100 nm gold nanowires coated with 250 nm of CIS.

Figure 3. (a) TEM image of a PbSe-coated gold nanowire. (b) TEM image of a CIS-coated gold nanowire. (c) SAED image of a PbSe-coated gold nanowire with the 200 and 222 peaks of gold and the 222, 113, and 220 peaks of PbSe labeled. (d) SAED image of a CIS-coated gold nanowire with the 111, 200, and 113 peaks of gold and the 204, 312, and 400 peaks of CIS labeled. (e) XRD spectrum of a PbSe-coated gold nanowire array with the 111 and 200 peaks of gold (red labels) and the 111, 200, and 220 peaks of PbSe (black labels) labeled. (f) XRD spectrum of a CIS-coated gold nanowire array with the 111 and 200 peaks of gold (red labels) and the 112 and 220 peaks of CIS (black labels) labeled.

X-ray diffraction (XRD) studies on the PbSe- and CIS-coated gold nanowires. The results of these characterization studies are summarized in Figure 3. Figure 3a,b, for instance, shows typical TEM images of gold nanowires coated with either PbSe (Figure 3a) or CIS (Figure 3b). As can be seen in these TEM images, the center of the nanowire is much denser than the edges, corresponding to a gold core with a semiconductor shell, with a strong delineation between the core and the shell indicating that the gold semiconductor boundary is sharp. In addition, it is apparent that the grains that make up the shell are much larger than the core material, with the grain boundaries primarily along the axial direction. This is further confirmed in the SAED images for the PbSe-coated gold nanowires (Figure 3c) and for the CIScoated gold nanowires (Figure 3d). In both images, the electron diffraction from the gold core shows as rings, corresponding to the nanocrystalline nature of the gold nanowire, while the electron diffraction from the shell shows up as individual spots,

indicating fewer, larger grains. Finally, XRD spectra on arrays of PbSe-coated gold nanowires (Figure 3e) and CIS-coated gold nanowires (Figure 3f) show peaks that correspond to only gold (JCPDS 19-629), cubic PbSe (JCPDS 06-0354), and tetragonal CIS (JCPDS 40-1487), indicating the purity of the materials and the lack of diffusion between the core and shell. In addition to the materials characterization described above, we have also measured the UV vis extinction spectra and electronic properties of the nanowire arrays. Figure 4a compares the extinction spectra of the nanowire arrays over a wavelength range of 400 800 nm. The curve labeled “Glass” corresponds to a clean glass slide and serves as a reference. The curve labeled “Gold” corresponds to an array of gold nanowires with widths of 100 nm and heights of 40 nm running the length of the sample and separated by 12 μm. We attribute the increased absorption over the glass to both absorption by the gold nanowires as well as light scattering by the gold nanowires and residual nickel on the glass surface. We see an increase in absorbance over the bare gold nanowires after electrodepositing approximately 250 nm of either PbSe (PbSe/Au curve) or CIS (CIS/Au curve) onto 100  40 nm gold nanowires, which we attribute to light absorption by the semiconductor layer. Light absorption by the semiconductor material can also be measured as an increase in current through the nanowire when exposed to light. Figure 4b shows current voltage (IV) curves over a potential range of 2 to +2 V of an array of PbSe-coated gold nanowires in the dark (black curve) and exposed to white light from a Hg arc lamp (green curve) with an AM 1.5 filter, whereas Figure 4c shows IV curves over a potential range of 2 to +2 V of an array of CIS-coated gold nanowires in the dark (black curve) and exposed to white light from a Hg arc 17672

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Table 1. Average Photon-to-Electron Conversion Efficiency for Different CIS Thicknesses

Figure 5. (a) The current through an array of 40  100 nm gold nanowires coated with 250 nm of CIS before (t < 20 s) and after (t > 20 s) light exposure. (b) IV curves on an array of 40  100 nm gold nanowires coated with 250 nm of CIS at temperatures of 15 (black), 28 (green), 40 (blue), and 50 °C (red). (c) The photocurrent through an array of 40  100 nm gold nanowires coated with 250 nm of CIS under white light chopped at 50 Hz. (d) The photocurrent through an array of 40  100 nm gold nanowires coated with 250 nm of CIS under white light chopped at 400 Hz. (e) The percentage of incident photons absorbed by two separate arrays of 40  100 nm gold nanowires coated with 250 nm of CIS, as a function of incident light wavelength. (f) A comparison of the percentage of incident photons absorbed by an array of 40  100 nm gold nanowires coated with 100 nm of CIS (black), an array of 40  100 nm gold nanowires coated with 250 nm of CIS (green), and an array of 40  100 nm gold nanowires coated with 450 nm of CIS (blue).

lamp (green curve) with an AM 1.5 filter. Although the overall current scale is different for the two samples, both samples show an increase in conductivity when illuminated and a fairly Ohmic behavior, indicating good electrical contact with the gold core. The majority of this increase in conductivity, however, comes from heating the sample, rather than the direct creation of charge carriers by photon absorption. This can be seen in Figure 5a, which shows the current through a CIS-coated gold nanowire array held at +1 V as a function of time. After turning on the light at approximately 20 s, we see a slow increase in the current over approximately 10 s, which is far too slow to be an optical effect and is instead attributed to an increase in sample temperature due

CIS shell

light absorbed

active area

light absorbed by

thickness

by sample (%)

of sample

nanowires (%)

100 nm 250 nm

0.10 0.29

1.7% 2.9%

6.0 9.9

450 nm

0.45

4.6%

9.5

primarily to plasmon-induced heating of the gold core, but also to other factors, including Joule heating and light absorption by the substrate. The current dependence on temperature of the CIScoated gold nanowires is further quantified in Figure 5b, where IV curves on an array of CIS-coated gold nanowires were taken at 15 (black curve), 28 (green curve), 40 (blue curve), and 50 °C (red curve). These curves show an increase in conductivity with increasing temperature, consistent with electronic conduction through a semiconductor, and from these curves, we can estimate a temperature increase of approximately 10 °C when the samples are illuminated with the Hg arc lamp, consistent with other reports on plasmon heating.38,39 Previous measurements on gold nanowires prepared via LPNE have shown a decrease in conductivity with increasing temperature, indicating that the semiconductor shell controls the electrical conductivity of the nanowires.40 To measure the generation of light-induced charge carriers in the CIS-coated gold nanowires, we performed photoconductivity measurements on the CIS-coated gold nanowire arrays using a white light source chopped at 50 Hz, shown in Figure 5c, and at 400 Hz, shown in Figure 5d, and a constant potential bias of 1 V. The samples still exhibit an increase in current when illuminated, but here the current increase is about 20 times smaller than the current increase under constant illumination. As the light is chopped much faster than the rise time shown in Figure 5a, we assume that this current increase of ∼70 nA at 50 Hz and ∼42 nA at 400 Hz is dominated by photogenerated charge carriers. Assuming that each photon absorbed generates one electron hole pair, a reasonable assumption at a 1 V bias, we calculate that only 1 2 of every 1000 photons incident on the sample are absorbed by the nanowires. However, as only 3% of the sample area is CIS, this correlates to approximately 7% of light incident on a nanowire being absorbed. In addition to broad-band photoconductivity, we also measured the photoconductivity of the CIS-coated gold nanowire arrays as a function of incident wavelength. By comparing the current through the nanowire array with the incident photon flux, and again assuming unit conversion efficiency, we find the percentage of photons absorbed by the sample. Figure 5e shows the absorption efficiency of two different 250 nm CIS-coated gold nanowire arrays. In both samples, we find narrow regions of enhanced absorption, which red shift as the nanowires increase in size, as can be seen in Figure 5f. As a result of the regions of enhanced absorption, we find that, while the 450 nm wide nanowires absorb more light, the 250 nm wide nanowires are more efficient at absorbing light than either the 100 or the 450 nm wide nanowires, as summarized in Table 1. The most reasonable explanation for the increase in absorption efficiency is due to the presence of leaky-mode resonances, first observed by Cao et al. in single-crystal nanowires,41,42 but until now not observed in nanocrystalline nanowires. 17673

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The Journal of Physical Chemistry C In summary, we have demonstrated the synthesis of nanocrystalline Au/PbSe and Au/CIS core/shell nanowires by LPNE. The synthetic process, which combines photolithography and electrodeposition, is scalable, facile, and provides control over the size, shape, and placement of the nanowires on a dielectric substrate and provides a general method for realizing Au/semiconductor core/shell nanowires for any semiconductor that can be electrodeposited. The nanowires are electrically conductive and demonstrate classical semiconductor electronic properties, such as photoconductivity and increasing conductivity with temperature. Finally, the ease of synthesis coupled with the enhanced light absorption due to leaky-mode resonances make these nanowires potentially useful for nanowire-based solar cells or optical detectors.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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