2, North Dakota State University, Fargo, North Dakota, United States
Significance of the surface passivation of II-VI colloidal quantum dots (QDs) on their photophysical properties is discussed. Optically forbidden nature of surface-associated states makes their direct measurements challenging. Our DFT-based simulations of CdSe QDs ligated by anionic ligands, such as carboxylates, thiolates, and hydrides, provide insights into the role the acidic ligands play during synthesis and ligand exchange, as well as in manipulating QD’s optical response. Thus, our calculations reveal much more complicated exchange mechanism of the native surface ligands of CdSe QDs with phenyl-dithiocarbamates (PTCs) as it was thought before. PTCs decompose during exchange with native ligands, while only a small portion of deprotonated PTCs covalently bounds to the Cd-enriched surface. Our calculations also reveal that attachment of the hydride to Se sites results in strong distortions of Cd-Se bounds leading to ‘cleaning’ out of extra Se ions from the QD surface (in a form of SeH2 gas) and eliminating Se-associated trap states. On the other hand, adsorption of H- on Cd, when the surface is enriched by metal ions, results in blue-shifted lower-energy transitions with very high oscillator strength, which likely responsible for experimentally observed emission enhancement of CdSe QDs treated by hydrides. The calculated results allow for explanations of experimental trends and observables sensitive to surface defects and ligand passivation and offering guidance for controlling the optical response of II-VI nanostructures by means of surface ligand engineering.