PbSe nanocrystals represent the paradigm nanoscale system exhibiting carrier multiplication upon light absorption, yet their absorption spectrum is poorly understood. Two very different interpretations of the absorption peaks have been proposed: is the second absorption peak a dipole-forbidden $S_h\text{−}{P}_e$ or $P_h\text{−}{S}_e$ transition or a dipole-allowed $P_h\text{−}{P}_e$ transition? A recent two-photon photoluminescence-excitation experiment favored the first interpretation, raising the question of why a dipole-forbidden transition would be strongly absorptive. Here we report atomistic pseudopotential calculations of the one-photon and two-photon absorption spectra of PbSe nanocrystals, showing unequivocally that, contrary to previous interpretations by other authors, the second one-photon absorption peak originates from dipole-allowed $P_h\text{−}{P}_e$ transitions.

• Received 7 May 2009

DOI:https://doi.org/10.1103/PhysRevB.79.241311