Electronic Properties of Nonstoichiometric PbSe Quantum Dots from First Principles

The electronic properties of PbSe quantum dots containing a nonstoichiometric Pb:Se ratio are investigated by A initio density functional theory. We take five nearly spherical PbSe nanocrystals with effective diameters ranging from 11.22 to 31.86 angstrom into account and compare their electronic properties before and after passivations. We find that despite the strong ionic character of the Pb-Se bond, their dangling bonds at the surface of nonstoichiometric PbSe nanocrystals introduce in-gap states, which are quite different from those of the stoichiometric PbSe nanocrystals. The same phenomenon is also observed for bare PbSe (011) and (111) surfaces. Compared with that of the self-passivated (001) surface, there is large surface relaxation and rumpling at the unsaturated (011) and (111) surfaces. We expect this might also be the origin of surface states in nonstoichiometric PbSe nanocrystals. We observed the almost recovery of structures and at the same time the elimination of in-gap states by passivating the dangling bonds with pseudo-hydrogen atoms. Meanwhile, the size dependencies of the QDs' gaps are obtained, which are in accordance with experimental measurement and other theoretical calculations.