Effect of heterojunction on exciton binding energy and electron-hole recombination probability in CdSe/ZnS quantum dots.

Abstract

Presence of heterojunctions is important for generation of free charge carriers and the dissociation of bound electron-hole pairs in semiconductor nanoparticles. This work presents a theoretical investigation of the effect of core/shell heterojunction on electron-hole interaction in CdSe/ZnS quantum dots. The excitonic wave function in the CdSe/ZnS dots was calculated using the electron-hole explicitly correlated Hartree-Fock (eh-XCHF) method and the effect of successive addition of the ZnS shell on exciton binding energy, electron-hole recombination probability, and the electron-hole separation distance was investigated. It was found that the scaling of all the three quantities as a function of dot diameter did not follow conventional volume scaling laws of core-only dots, and the scaling laws were significantly altered due to the presence of the heterojunction. The spatial localization of the quasiparticles in the core/shell quantum dot was analyzed by calculating the 1-particle reduced density from the eh-XCHF wave function and partitioning the density spatially into core and shell regions. It was found that in the 15 nm CdSe/ZnS dot, the relative probability of the electron localization in the shell region was higher than the hole by a factor of 3. The degree of spatial localization of the quasiparticles was found to depend strongly on the initial size of the CdSe core in the core/shell quantum dot. It was found that a reduction in the CdSe core diameter by a factor of 1.7 resulted in an enhancement of the preferential localization of the electron in the shell region by a factor of 11.3. The results demonstrate that large CdSe/ZnS quantum dots with a small CdSe core have the necessary characteristics for efficient exciton dissociation and generation of free charge carriers.