Impact of Structural Changes to the Electronic Properties of PbS Colloidal Quantum Dots via DFT Study*

Correlating morphologic stoichiometry, atomic coordination, and bonding character in PbX (X=S,Se,Te) colloidal quantum dots to their allowable low energy optical transitions (<0.5eV) could pave the way for new infrared photodetector architechtures. This work specifically employed three toy models to isolate, within the PbS core, the impacts of atomic coordination, cation-rich structures, and lattice distortions. Analysis of the electron localication function, density of states, and charge density suggests metavalent bonding occurs in PbS nanocrystals. Metavalent bonding is a recently recognized novel bonding type with characteristics which lie between covalent and metallic; in PbS it is primarily studied in the bulk crystalline structure. Density functional theory results support the persistence of this bonding type in confined PbS nanocrystals. It is observed that low-coordinated Pb and S atoms on the surface impact the overall electronic environment of the nanocrystal and is exacerbated by the presence of Peirels distortions. The Peirels distortions lower the overall energy, reducing the number of states near the Fermi energy which reduces the number of states available for optical transitions. These results correlate with recent experimental observations in which a Pb_xCl_y shell was shown to result in longer bond lengths and enhanced optical absorption.