Quantitative model for the surface-related electron transfer in CdS quantum dots

The influence of surface S^2- dangling bonds and surface doped ions(Se^2-, Cu²⁺, and Hg²⁺) on the photoluminescence of Cd²⁺-rich CdS QDs was investigated. A quantitative model was proposed to understand the complex transfer processes of excited electrons in CdS QDs. The transfer of excited electrons from either the conduction band or the Cd²⁺-related trap-state to the surface S^2--related shallow hole trap-state is effective. However, the trap of excited electrons by surface doped ion trap-states from the Cd²⁺-related trap-state is more effective than that from the conduction band. The efficiency of trapping electrons from both the conduction band and the Cd²⁺-related trap-state can be quantitatively understood with the help of the proposed model. The results show that the transfer efficiency of excited electrons is dependent on the location of the energy-level of the relevant surface-related trap-state. The trap of excited electrons by the surface trap-state with energy-level closer to that of the conduction band is more effective, especially for the trap of excited electrons from Cd²⁺-related trap-state.