Synthesis of CuInS 2 -ZnS Alloyed Nanorods and Hybrid Nanostructures
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xture model implemented into the MAUD program. The anisotropy of the shape of the particles was analyzed using a size-strain model developed by Popa.15 The integral stoichiometry was obtained by the EDAX detector integrated into a FEI Quanta 200 3D scanning electron microscope. Stoichiometry of individual nanoparticles was analyzed with the EDX detector of the JEOL JEM-2100F microscope. DISCUSSION The synthesis described here is based on a procedure, we developed previously for CuInS2 particles.16 Briefly, copper indium zinc sulfide particles were synthesized with zinc, copper and indium acetate as the sources of the cations, using oleic acid and 1-dodecanethiol as ligands and oleylamin as solvent. Tert-dodecanethiol served as the source of sulfur, because this compound can be relatively easy decomposed thermally.16,12,17,18 Figure 1 shows the different growth stages observed in a typical synthesis. The first nanocrystals, which appear in the reaction solution, are composed of copper sulfide (Fig. 1 a). Copper sulfide is a superionic conductor at temperatures above 105°C,19 with cations moving like a liquid across the lattice of the anions. Because of the high mobility of the cations copper sulfide can play the role of a catalyst for further growth of other semiconductor materials. This has already been demonstrated for semiconductors, such as, CuInS2,12,13,20 CuInxGa1-xS2,21 and Cu2ZnSnS4.4,22 In this kind of synthesis, usually, an intermediate hybrid compound can be found, composed of copper sulfide and the other semiconductor. Such hybrid nanocrystals are shown in Fig. 1 b. Within these nanorods we can distinguish two regions with different contrast and crystallographic structure. The smaller particles within the hybrid nanostructure are copper sulfide nanocrystals; the rods attached to them consist of copper indium zinc sulfide. This could be confirmed by spatially resolved EDX measurements. The copper sulfide part of the hybrid nanostructure disappears during the synthesis and pure and uniform copper indium zinc sulfide nanorods are formed.
Figure 1 HRTEM images of copper sulfide seeds (a), copper sulfide- copper indium zinc sulfide hybrid nanostructures (b), and copper indium zinc sulfide nanorods (c) obtained at different reaction times (2 min, 3 min, and 4 min at 230°C) in a reaction with a 2:1.5:1 ratio between zinc, copper and indium in the reaction solution.
The formation of copper indium zinc sulfide nanorods was tested with different amounts of zinc in the reaction solution. The x-ray diffraction patterns of the resulting particles are shown in Figure 2. The obtained patterns can be assigned to the hexagonal wurtzite structure. The positions of the reflections shift toward higher 2 θ values for larger amounts of zinc in the reaction solution. This contraction of the lattice is expected for the formation of a ZnS-CuInS2 alloy with increasing ZnS content, because the lattice parameters of ZnS have slightly smaller values, compared with those of CuInS2. The reflections are broadened in all the diffractio
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