Interactions of bound excitons in doped core/shell quantum dot heterostructures

Abstract

Spatial localization to a defect or a dopant of one of the charge carriers comprising an exciton, has a significant effect on the optical properties of bulk semiconductors. It is not clear, however, how these effects would change when considering semiconductor nanocrystals in the strong confinement regime. As we show here, under strong confinement, doping has a dramatic effect on the energetics of multiply excited states, which exhibit a strong size dependence. This is experimentally shown by performing multiexciton spectroscopy of CdSe/CdS and ZnSe/CdS colloidal quantum dot (QD) heterostructures, whose cores are nucleation-doped with few atoms of tellurium, leading to localization of the holes. The biexciton (BX) is shown to be strongly blueshifted relative to the bound exciton, in stark contrast with the corresponding undoped nanocrystals exhibiting a BX redshift. The energetics of the BX is shown to be determined mostly by the energy difference between the dopant state and the valence-band edge while the emission color is mostly determined by quantum confinement in the conduction band. By tailoring the nanocrystal's structure we can thus independently control the emission color, the radiative decay rate and the BX repulsion. QD heterostructures harboring bound excitons are therefore excellent candidates for colloidal-based gain devices required to operate in the single exciton gain regime.