Understanding Ligand Exchange and Surface Passivation in CdS Quantum Dots from First Principles

The capping ligands on quantum dots (QDs) play a critical role in determining their stability and properties, making the ligand exchange process pivotal. However, rational design rules for selecting ligands to effectively passivate QD surfaces are lacking. In this study, we employ first-principles density functional theory and many-body perturbation theory to systematically examine how various ligands affect surface passivation and modulate the electronic and optical properties of QDs. We focus on the (111) and (100) facets of zinc-blende CdS QDs and survey several common ligands: chlorine (-Cl), amine (-NH2), oleic acid [CH3(CH2)7CH = CH(CH2)7COOH], stearic acid [CH3(CH2)16COOH], and thioglycolic acid (HS-CH2-COOH). We elucidate the trends in surface passivation based on an electron counting argument. We discuss different effects associated with the ligand exchange, including the interface dipole, ligand binding mode, and many-body interactions. Our work provides structure-property relationships to pave the way for the rational design of effective QD ligands.