Excitonic characteristics of blue-emitting quantum dot materials in group II-VI using hybrid time-dependent density functional theory

Abstract

Colloidal quantum dots (QDs) of group II-VI are key ingredients of next-generation QD light-emitting diodes technology for display and lighting, yet the understanding of their luminescent characteristics are far from being mature. Using a hybrid time-dependent density functional theory, we have studied the electronic and excitonic properties of blue-emitting colloidal QDs within group II-VI containing a thousand atoms or more, including CdSe, CdS, ZnSe, and ZnS QDs, considering both quantum confinement and surface ligand effects. It is found that the calculated optical gaps are in excellent quantitative agreement with experiment, irrespective of the QD nature. Scaling laws of size-dependent energy gaps governed solely by quantum confinement effects have further been explored at both single-particle level and correlated excitonic level for all QDs. With concurrently stoichiometric control and enhancing quantum confinement effects, we have predicted an unusual switching of symmetry character of the highest occupied molecular orbital state from a ${\mathrm{\ensuremath{\Gamma}}}_{3}$ to a ${\mathrm{\ensuremath{\Gamma}}}_{1}$ symmetry at ultrasmall size ($\ensuremath{\sim}1$ nm) for all QDs. After the switching, pronounced linearly polarized band-edge excitonic emission is activated. The radiative exciton decay lifetime is found to increase monotonically with increasing the QD size and tends to saturate at larger sizes. Finally, we have explored the surface passivation mechanism of inorganic chloride ligand, and identified various favorable Cd-Cl bonding configurations which enable an effective surface passivation resembling the commonly applied pseudohydrogen passivation scheme. We find that chloride ligand serves as a hole delocalization ligand and tends to redshift the absorption spectra, reduce the absorption intensity, and significantly enhance the exciton decay lifetime. Our results provide a guideline for spectroscopic studies of excitonic characteristics of colloidal QDs within group II-VI.