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Degenerately <i>n</i>‑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<sup>20</sup> cm<sup>–3</sup>. At the highest doping levels, electrons
have completely filled the 1S<sub>e</sub> orbitals of the CB and partially
populated the higher-energy 1P<sub>e</sub> orbitals. Spectroscopic
changes observed as a function of carrier concentration permit an
unambiguous assignment of the second excitonic absorption maximum
to 1P<sub>h</sub>-1P<sub>e</sub> transitions. At intermediate doping
levels, a clear absorption feature appears between the first two excitonic
maxima that is attributable to parity-forbidden 1S<sub>h,e</sub>-1P<sub>e,h</sub> 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<sub>h</sub>-1S<sub>e</sub> and
1P<sub>h</sub>-1P<sub>e</sub> excitons in <i>n</i>-doped
PbSe QDs are remarkably similar to those observed for multiexcitons
with the same electron count. The origins of these redshifts are discussed