Synthesis and Phase Transformation of In₂Se₃ and CuInSe₂ Nanowires

Abstract

Bulk and thin films of III-VI and I-III-VI semiconductors such as In 2 Se 3 (IS), 1 CuInSe 2 (CIS) 2 and CuGaSe 2 3 have been actively studied for photovoltaic applications. Among them, polycrystalline thin films of CuIn x Ga 1-x Se 2 (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 cells 10 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 AgInSe 2 nanorods and nanoparticles with small aspect ratios less than 5. 12 We exploit a VLS growth [13][14][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 studies 16 (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

Topological Insulator Nanowires and Nanoribbons

“…As a result, nanoribbons have a rectangular cross section. TEM studies on a wide nanoribbon ( Figure 2B) reveal similar crystalline characteristics to the narrow nanoribbon ( Figure 2A); it grows along [11][12][13][14][15][16][17][18][19][20] direction as well. The TEM data of a nanowire with rough surface are shown in Figure 2C.…”

A new crystal: layer-structured rhombohedral In3Se4

“…After stirring for ~2 min, the solution was transferred into a Teflon- 15 lined autoclave which was sealed and annealed at 200-230 º C for 24 h. The reacted product was collected by centrifuge, washed with deionised water and absolute ethanol for several times, and then dried for 12 h at 60 º C.…”

“…These anisotropic bonding characteristics, including strong intralayer covalent bonding and weak interlayer van der Waals interactions, give rise to highly 25 anisotropic optical, electrical, and thermal properties. 10,11,15,16 In particular, the layered indium selenide nanomaterials are anticipated to exhibit superior physical properties, owing to their anisotropic photon geometries and large surface-to-volume ratios, together with photon and carrier confinement effects. 10 [28][29][30][31][32][33][34] have been successfully synthesized recently through this approach by controlling the growth kinetics.…”

“…It is of interest to note that this synchrotron XRD pattern cannot be indexed by any other existing phase, but can be well indexed by a rhombohedral structure with lattice parameters of a = 4.0 ± 0.2 Å and c = 39.7 ± 0.3 Å. Using 15 this set of lattice parameters and crystal system determined from TEM characterization, together with the atomic coordinates and symmetry of the analogous r-In 3 Te 4 , the crystal structure of rIn 3 Se 4 crystal can be optimized and simulated using the spinpolarized density functional theory, 40 and the theoretical XRD 20 pattern can be simulated. The green plot shown in Figure 3a is such a simulated XRD pattern, which shows an excellent agreement with the synchrotron XRD pattern (refer to the positions of the diffraction peaks).…”

Fabrication and Photoelectrochemical Behavior of Ordered CIGS Nanowire Arrays for Application in Solar Cells

“…[17][18][19][20] Aligned NWs with a controllable composition modulation can give ordered p-n junctions and continuous charge carrier transport pathways without recombination. 21 Therefore, NW solar cells might provide an even higher efficiency, as shown by Law et al 22 To increase electron diffusion length in a dye-sensitized solar cell, these authors replaced the nanoparticle film with an array of oriented single-crystalline NWs. Because of the very large surface area of this device, a better absorption of red light and a very efficient carrier collection were achieved.…”