Size-dependent radiative recombination characteristics of isolated CuInS2 nanocrystals

Abstract

Ternary chalcopyrite semiconductor nanocrystals of the I–III–VI2 type are potential nontoxic alternatives to Cd- and Pb-based chalcogenide nanocrystals, which can be adopted for light emission and photovoltaic applications. Most I–III–VI2 nanocrystals exhibit luminescence originated from defect-related recombination, which is characterized by a broad spectrum with a large Stokes shift regardless of the size-dependent spectral shift due to the quantum size effect. However, the origin of the luminescence in I–III–VI2 nanocrystals is still under debate, and deeper investigations of the carrier recombination processes are necessary to gain further understanding of the luminescence mechanisms. For this purpose, we investigated the temperature dependence of the luminescence properties of CuInS2 nanocrystals down to cryogenic temperatures to suppress the nonradiative recombination. The intrinsic luminescence properties of the isolated CuInS2 nanocrystals were successfully evaluated by eliminating the influence of the interactions among the nanocrystals, which is different from previous studies on nanocrystal aggregates. Both the thermal quenching behavior and the temperature dependence of the carrier recombination rate could be satisfactorily described using a simple model that assumes a thermally activated nonradiative recombination. The radiative recombination rates of the CuInS2 nanocrystals were found to be 106–107 s−1, and a characteristic cubic dependence of the radiative recombination rate on the nanocrystal diameter was found.