Degenerately n-Doped Colloidal PbSe Quantum Dots: Band Assignments and Electrostatic Effects.

We present a spectroscopic study of colloidal PbSe quantum dots (QDs) that have been photodoped to introduce excess delocalized conduction-band (CB) electrons. High-quality absorption spectra are obtained for these degenerately doped QDs with excess electron concentrations up to ∼10 ²⁰ cm ^-3 . At the highest doping levels, electrons have completely filled the 1S _e orbitals of the CB and partially populated the higher-energy 1P _e orbitals. Spectroscopic changes observed as a function of carrier concentration permit an unambiguous assignment of the second excitonic absorption maximum to 1P _h -1P _e transitions. At intermediate doping levels, a clear absorption feature appears between the first two excitonic maxima that is attributable to parity-forbidden 1S _h,e -1P _e,h excitations, which become observable because of electrostatic symmetry breaking. Redshifts of the main excitonic absorption features with increased carrier concentration are also analyzed. The Coulomb stabilization energies of both the 1S _h -1S _e and 1P _h -1P _e excitons in n-doped PbSe QDs are remarkably similar to those observed for multiexcitons with the same electron count. The origins of these redshifts are discussed.