Effect of Electron–Hole Separation on the Spectral Diffusion of Small-Sized CdSe Quantum Dots under an External Electric Field

Spectral diffusion occurs in semiconductor quantum dots (QDs) under an external electric field, resulting from the transition energy shift of one single dot during optical absorption and emission processes. The classical electric polarization model fails to interpret the spectral diffusion in some cases when strong quantum confinement Stark effects and large Stokes shifts exist in the small-sized QDs. Based on the two-process (optical absorption and emission) four-state (the ground- and excited-states involved in each process) model, the field-induced spectral shifts of the (CdSe)33 and (CdSe)34 structures, which are computed with the first-principles calculations, are well fitted by the dipole moment and static polarizability changes in the separate processes. Furthermore, the linear and the quadratic variations of the field-induced spectral shifts are interpreted with the compressible and the incompressible electron–hole separation distances, which are computed from the Kohn–Sham levels involved in the excited states. Our study provides the direct pieces of evidence on the hypothesis summarized experimentally that the electron transition energy increases with the decreasing electron–hole separation. As a result, the spectral shifts of one single QD, as well as the spectral diffusion of QDs, are then adjustable by tuning the direction and strength of the external field.