Synthesis of CuInS₂ colloidal nanocrystals and their optical properties

Colloidal CuInS₂ nanocrystals (NCs) were synthesized in a hot organic solvent containing a surfactant. The crystal structure of the obtained CuInS₂ NCs phases was controllable by the ligand species coordinating with the metallic monomers. The metallic monomers weakly coordinated by ligands resulted in zincblende-type (ZB-type) CuInS₂ NCs. The metallic monomers strongly coordinated by ligands resulted in wurtzite-type (WZ-type) CuInS₂ NCs. The optical band gap of ZB-type CuInS₂ NCs synthesized varied in the range from 1.70 eV to 2.25 eV as corresponding to the crystal size from 5.5 nm to 2.1 nm due to the quantum size effect. This experimental result agreed well with the calculation of the finite depth well effective mass approximation. Broad photoluminescence (PL) emissions were observed for the ZB-type CuInS₂ NCs; however, the observed large Stokes' shift between the optical band gap and PL emission indicated that the origin of emission was not an excitonic recombination but an electron-hole recombination via defect levels. In the PL spectrum of the CuInS₂/ZnS core/shell NCs, the emission shoulder with small Stokes' shift of ∼ 50 meV was observed together with the emission relating to the defect levels. The emission shoulder of the core/shell NCs was suggested to be due to direct excitonic recombination.