Lifetime and polarization of the radiative decay of excitons, biexcitons, and trions in CdSe nanocrystal quantum dots

Abstract

Using the pseudopotential configuration-interaction method, we calculate the intrinsic lifetime and polarization of the radiative decay of single excitons $(X)$, positive and negative trions (${X}^{+}$ and ${X}^{\ensuremath{-}}$), and biexcitons $(XX)$ in CdSe nanocrystal quantum dots. We investigate the effects of the inclusion of increasingly more complex many-body treatments, starting from the single-particle approach and culminating with the configuration-interaction scheme. Our configuration-interaction results for the size dependence of the single-exciton radiative lifetime at room temperature are in excellent agreement with recent experimental data. We also find the following. (i) Whereas the polarization of the bright exciton emission is always perpendicular to the hexagonal $c$ axis, the polarization of the dark exciton switches from perpendicular to parallel to the hexagonal $c$ axis in large dots, in agreement with experiment. (ii) The ratio of the radiative lifetimes of mono- and biexcitons $\ensuremath{\tau}(X):\ensuremath{\tau}(XX)$ is $\ensuremath{\sim}1:1$ in large dots $(R=19.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. This ratio increases with decreasing nanocrystal size, approaching 2 in small dots $(R=10.3\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. (iii) The calculated ratio $\ensuremath{\tau}({X}^{+}):\ensuremath{\tau}({X}^{\ensuremath{-}})$ between positive and negative trion lifetimes is close to 2 for all dot sizes considered.