Structural and electronic properties of PbSe nanocrystals from first principles

This work reports density-functional calculations of the structural and electronic properties of PbSe nanocrystals ranging in radius from 8.4 to 14.8. We considered nearly spherical, rocksalt-structure nanocrystals with 1:1 Pb:Se stoichiometry and no molecular species adsorbed at the surface. We found that: (i) The Pb-Se bond lengths and bond angles are significantly distorted compared to those in bulk PbSe, in an ~8--thick shell near the surface of the nanocrystal. (ii) Even in the absence of surface passivants, there are no surface states in the band gap of the nanocrystals. (iii) The nanocrystal band gap undergoes a significant redshift (Franck-Condon shift) when an electron is promoted from the valence band to the conduction band. For nanocrystals of radius R=8.4, the calculated Franck-Condon shift is ~0.18 eV. (iv) The calculated electronic density of states shows a significant asymmetry between valence and conduction states, suggesting that the postulated ``mirror symmetry'' between valence band and conduction band does not exist.