Synthesis and phase transformation of In(2)Se(3) and CuInSe(2) nanowires.

Bulk and thin films of III-VI and I-III-VI semiconductors such as In2Se3 (IS),1 CuInSe2 (CIS)2 and CuGaSe2 have been actively studied for photovoltaic applications. Among them, polycrystalline thin films of CuInxGa1-xSe2 (CIGS) have been demonstrated to have a high-power efficiency of 19.2%,4 which even outperforms the best single crystalline devices.5 This extraordinary performance was proposed to be caused by a hole energy barrier at grain boundaries for preventing electron-hole recombination,6,7 although this hypothesis is still under question.8 In addition, the high efficiency is also attributed to the formation of random p-n junctions distributed in compositionally inhomogeneous polycrystalline thin films.9 Nanowire (NW) morphology of I-III-VI chalcopyrite materials can provide a well-defined nanoscale domain with clearly identifiable “grain boundaries” for studying these effects. Aligned NWs with a controllable composition modulation can afford ordered p-n junctions and continuous charge carrier transport pathways without deadends, which is an advantage over the random p-n junctions. Therefore, NW solar cells10 might provide an even higher efficiency. The promise will not be fulfilled without a method for fabricating the required NW structures. Herein, we report the synthesis of IS and CIS single crystalline NWs via a Au-catalyzed vapor-liquid-solid (VLS) growth. We demonstrate the temperature-induced reversible superlattice transformation in IS NWs. We also show that the crystal structure of CIS NWs has dependence on Cu concentration. A solvothermal method was used previously for producing CIS nanowiskers and nanoparticles although their morphology and crystallinity are ill-defined.11 Solution colloidal synthesis was used to produce AgInSe2 nanorods and nanoparticles with small aspect ratios less than 5.12 We exploit a VLS growth13-15 because this method has been shown to be among the most powerful ones for predictably synthesizing single-crystalline NW structures with a size, position, and orientation control. The synthesis of IS and CIS NWs has been carried out in a similar way as that in our previous studies16 (Supporting Information). In a tube furnace, a carrier gas transports the vapor of R-phase IS or chalcopyrite-type CIS downstream. Gold colloids dispersed on Si substrates were used as VLS catalysts. Typical synthesis conditions are pressure ) 50 Torr, temperature ) 700 °C, time ) 5 h, and gas flow ) 120 sccm. To controllably adjust the Cu concentration in CIS NWs, additional CuI powder was placed upstream. The NWs were characterized in an FEI Sirion scanning electron microscope (SEM), a Hitachi 300 kV H-9500 and a Philips CM20 transmission electron microscope (TEM), and by energy dispersive X-ray spectrometry (EDX). Figure 1a shows a typical SEM image of as-grown IS NWs. The NW diameters range from 20 to 150 nm with lengths up to over 100 μm. The TEM image (Figure 1b) demonstrates that most NWs have a uniform diameter along their length and have a particle at the tip, suggesting a VLS growth mechanism. EDX spectra (Figure S1 in Supporting Information) reveal the presence of Au in the tip, while the NW consists of In and Se with an atomic ratio of 2:3 confirming the chemical composition of In2Se3. Highresolution TEM (HRTEM) (Figure 1c) and selected-area electron diffraction (SAED) (Figure 1e) of the typical IS NWs show the clearly identified hexagonal lattice planes with lattice spacing of 0.350 nm (Figure 1c), corresponding to the (1-100) plane of the hexagonal R-In2Se3 (JCPDS-00-034-1279) looking along the [0001] zone axis. The NW growth direction is [11-20], confirmed by the corresponding SAED pattern (Figure 1e). Most interestingly, a onedimensional superlattice perpendicular to the NW growth direction is observed in most of the IS NWs at room temperature (Figure 1c). The superlattice has a periodicity of 1.40 nm and one period consists of four (1-100) planes. The presence of the superlattice is confirmed by the SAED pattern (Figure 1e), in which the distance between two basic spots is subdivided into four equal parts along the [1-100] direction. We also noted the eight-plane periodicity (twice of four-planes) in our TEM studies. Superlattices were previously observed in thin film IS with a periodicity of eight or nine (1-100) planes at temperatures between † Stanford University. ‡ Hitachi High Technologies America, Inc. Figure 1. (a) SEM image of IS NWs on a Si 〈111〉 substrate. (b) TEM image of IS NWs. (c) HRTEM image of the IS NW (white spot in panel b). (d) TEM image of a curly IS NW. The insets show a HRTEM image recorded from the area indicated by the white rectangle and the corresponding SAED pattern. (e-g) SAED patterns of the IS NW recorded at 22, 267, and 23 °C, sequentially (white spot in panel b). Published on Web 12/15/2006