Synthesis of Ternary CuInS2 Nanocrystals; Phase Determination by Complex Ligand Species

Colloidal CuInS2 nanocrystals were synthesized in a hot organic solvent containing surfactant molecules. The CuInS2 phase was controlled by the ligand species of the metallic monomers. When the metallic monomers were coordinated with trioctylphosphite, the resulting CuInS2 had a chalcopyrite or zincblende phase. When the metallic monomers were coordinated with hexadecylamine or oleylamine, the thermodynamically metastable wurtzite phase appeared. The experimental results indicated that the obtained phase was predominantly determined by the growth rate of the nanocrystals. The bond strength between the metallic monomers and ligand molecules and steric size of the ligand molecules influenced the growth rate. The CuInS2 nanocrystals showed photoluminescence in the near-infrared region. Its energy was far from the optical energy band gap; the luminescence was attributable to the electron−hole recombination via deep defect levels. In the photoluminescence spectrum of the CuInS2/ZnS core/shell nanocrystals, a band near the optical energy band gap, whose Stokes’ shift was ∼50 meV, appeared. The band was suggested to be attributable to the defect-related emission from CuInS2−ZnS alloy formed at the interfaces between the core CuInS2 and shell ZnS.