Shape control of $CuInS_2$ /ZnS core/shell quantum dots and their degradation characteristics with reactive oxygen species

Abstract

Shape control of $CuInS_2$ /ZnS quantum dots (CIS/ZnS QDs) was examined during the growth of ZnS shell by utilizing difference in ligand binding energy and surface energy of crystal facets. At reaction tempera-ture below $250^\circ C$, tetrahedral QDs enveloped with {111} facets were obtained through ZnS growth on the {111} facets with higher ligand binding energy than that of {200}. In contrast, at higher temperature above $250^\circ C$, ZnS was preferentially grown on {111} facets with high surface energy, remaining {200}-enveloped cubic QDs. With respect to the time evolution, a large blue shift in emission by inter-diffusion of zinc ions into CIS core was observed at the beginning of reaction. As the reaction progressed, selective growth of ZnS was allowed by large concentration of sulfur ions decomposed from dodecanethiol without large shift in emission. Furthermore, the emission of CIS/ZnS nanocubes was tuned from green (540 nm) to red (630 nm) with the variation in Zn amount. The shape-dependent degradation characteristics of QDs in luminescence were investigated in a hydrogen peroxide ($H_2O_2$) medium of different concentration. With a supposed chain reac-tion of sulfur and zinc oxidation at the surface by $H_2O_2$ and hydroxyl radical, cubic QDs were shown to have lower degradation rate than tetrahedrons at low $H_2O_2$ concentration (0.1-1 mM) up to 3.4 times in half-life due to a rate-limiting step of sulfur oxidation. On the contrary, the trend was reversed at high $H_2O_2$ concentration (10-100 mM) by a rate-limiting step of zinc oxidation. These studies on the shape-dependent degradation characteristics will provide a fundamental understanding for further applications, especially those with strong oxidative environment such as light-emitting diodes and bio-imaging.