Formation and photoluminescence properties of colloidal ZnCuIn(Se x S 1 − x ) 2 /ZnS nanocrystals with gradient composition

Photoluminescent CuIn(SexS1 − x)2 (CISeS) nanocrystals (NCs) have been synthesized utilizing copper iodide (CuI) and indium acetate (In(Ac)3) as metal cation precursors, Se powder and n-dodecanethiol (DDT) as selenide and sulfur source, combined with oleylamine and n-dodecanethiol as coordinate ligands at a moderate reaction temperature of 200 °C. The obtained CISeS NCs consisted of a gradient composition that the content of selenide increased from the nuclei to the surface. With the growth of NCs, they exhibited a tunable photoluminescent emission ranging from 722 to 830 nm which was size- and composition-dependent. The photoluminescent quantum yield of the NCs could be further enhanced from 1.2 to 40.7% after ZnS coating due to the formation of ZnCuIn(SexS1 − x)2/ZnS (ZnCISeS/ZnS) core–shell structures with the alloyed core. The photoluminescence emission mechanisms of CISeS NCs indicated that the gradient composition structure of CISeS led to a gradient energy level of its conduction band (CB)/valence band (VB), resulting in the charge transfer from the nuclei to the surface through the quantized CB/VB. This transfer process has quenched fluorescence emission arising from the quantized CB/VB and defect states located in the nuclei, such as irradiation relaxation between quantized CB and \( V_{\text{Cu}}^{ \prime } \). This mechanism could illuminate why the indium-rich CISeS NCs did not exhibit better photoluminescence properties.