Exciton Fine Structure in Lead Salt Quantum Dots

Lead chalcogenide quantum dots (QDs) have attracted significant scientific and technological interest as their photoluminescence (PL) is tunable by the size change over a wide infrared wavelength range. Electron and hole states in bulk lead chalcogenides are highly degenerate leading to the 64-fold degenerate exciton states. In quantum dots (QDs), the 64-fold degeneracy of the excitons is lifted by the valley mixing and by the electron-hole exchange interaction. We thoroughly investigate the relative importance of the inter-valley, spin-orbit, and electron-hole exchange couplings for the splittings and calculate the resulting exciton fine structure in the framework of the empirical tight-binding method. The dependence of the exciton fine structure on QD size and shape is analyzed. The calculated exciton fine structure allows one to explain the temperature dependence of the PL lineshapes observed in recent single QD experiments on PbS/CdS core/shell colloidal nanocrystals.