Ab initio calculation of halide ligand passivation on PbSe quantum dot facets

The passivation and charge compensation provided by inorganic halide ligands on low index facets of lead selenide (PbSe) nanocrystals has been studied using density functional theory to produce projected densities of states (PDOS), bond lengths and to perform Bader analysis. The calculations were made using a grid-based planar augmented wave code with a localized double zeta potential basis and the generalized gradient approximation. Surface energies of halide ligands bonded onto surface Pb atoms show trends that are consistent with the increased electronegativity of the species, with iodine having the lowest binding energy of the halides investigated. Different densities of iodine ligands lead to different levels of passivation with a continuous widening of the bandgap on particular facets for increasing levels of coverage. In particular, the (111) plane shows a clear recovery of surface layer back to bulk property and widening of bandgap when the ligands cover most Pb atoms on the surfaces. Additionally, a possible increase of carrier conductance along with the increase of ligand density has been found using Bader analysis. Relative increases in the conductance for large halide atoms stem from the measurable increases to electronic states near the top of the valence band in these p-type semiconductors. The passivation is observed to increase along with the s-type character of the electron density at the surface, suggesting that a higher degree of symmetry in the electron density accompanies the reduction in defect levels.