Influence of Stoichiometry and Surface Crystal Orientation on the Electronic Properties of PbS Quantum Dots

The electronic properties of quantum dots, such as band structure and work function, are highly sensitive to the composition and surface orientation. Based on the first-principles calculation method, the most stable configurations of PbS quantum dots with various stoichiometric ratios were established first through energy calculation in this paper. Then the variation trends and mechanisms of electronic properties of PbS quantum dots with different stoichiometry and surface crystal orientation are analyzed by the energy, density of states (DOS), and charge characteristics in this paper. The DOS and charge analysis reveal that the band structures of PbS quantum dots are significantly affected by both stoichiometry and surface crystal orientation. The quantum dot exhibits the widest bandgap (1.94 eV) with no extrinsic states within the bandgap when the Pb to S ratio is 1:1. In cases of Pb-rich or S-rich, the appearance of extrinsic states caused by surface Pb and S atoms reduces the bandgap width and leads to the quantum dot exhibiting n-type or p-type characteristics. For quantum dots with exposed {111} crystal plane, the energy of the extrinsic states and the Fermi level decreases with increasing S atom proportion. Conversely, for quantum dots with exposed {100} crystal plane, the energy of the extrinsic states and the Fermi level contributed by the rich S atoms increases with increasing S atom proportion. The calculation results also demonstrate that the work function of Pb-rich quantum dots is lower than that of S-rich quantum dots. This research provides crucial theoretical insights into the understanding and manipulation of the electronic properties of quantum dots.