Effect of shell thickness on optical properties of ZnSe/ZnS quantum dots under solar and laser excitation

Monodispersed and highly luminescent quantum dots (QDs) of ZnSe/ZnS with different shell thicknesses between 0 and 4 monolayer (ML) were fabricated via an aqueous route. The photoluminescence (PL) of ZnSe/ZnS QDs showed red-shifting of the emission peaks from 390 to 425 nm when the shell thicknesses were 0 and 4 ML, respectively. A defective state–dependent PL peak appeared when the shell thickness exceeded 2 ML. The electronic structure of ZnSe/ZnS core–shell QDs was also investigated through photoluminescence excitation (PLE) spectroscopy and four excited state transitions, \({1\mathrm{S}}^{\mathrm{e}}-{1\mathrm{S}}_{3/2}^{\mathrm{h}}\), \({1\mathrm{S}}^{\mathrm{e}}-{2\mathrm{S}}_{3/2}^{\mathrm{h}}\), \({1\mathrm{P}}^{\mathrm{e}}-{1\mathrm{P}}_{3/2}^{\mathrm{h}}\), and \({1\mathrm{S}}^{\mathrm{e}}-{1\mathrm{S}}^{\mathrm{so}}\), were observed. The correlation between solar absorption and shell thickness of ZnSe/ZnS QDs was simulated using the finite-difference time-domain (FDTD) methods for the first time, and the results showed that the average solar absorption efficiency of ZnSe/ZnS QDs was improved with the increase of the shell thickness. This research expands ZnSe/ZnS QD application for solar thermal utilization including photovoltaic and photothermal systems.