Pauli blocking versus electrostatic attenuation of optical transition intensities in charged PbSe quantum dots

Quantum dots can be charged selectively by electrons or holes. This leads to changes in the intensity of interband and intraband optical transitions. Using atomistic pseudopotential calculations, we show that (i) when carriers are injected into dot-interior quantum-confined states, the intensity of interband transitions that have those states as their initial or final states is attenuated ("Pauli blocking") and (ii) when carriers are injected into localized states near the surface of the dots, the electrostatic field set up by these charges attenuates all optically allowed interband transitions. We describe and explain these two mechanisms of intensity attenuation in the case of charged PbSe quantum dots. In addition, this study reveals a new assignment of the peaks in the absorption spectrum. The absorption spectrum of charged PbSe dots was previously interpreted assuming that all injected electrons reside in dot-interior states. This assumption has led to the suggestion that the second absorption peak originates from S-h-P-e and P-h-S-e optical transitions, despite the fact that such transitions are expected to be dipole forbidden. Our results show that the observed bleaching of absorption peaks upon electron or hole charging does not imply that the S-h-P-e or P-h-S-e transitions are allowed. Instead, the observed bleaching sequence is consistent with charging of both dot-interior and surface-localized states and with the assignment of the second absorption peak to the allowed P-h-P-e transition.