Bonding behavior and passivation mechanism of organic ligands (-SH, -NH2, -COOH) on ZnS ([formula omitted]) surface from first-principles calculations

Abstract

Ligand design is an important aspect of the surface engineering on the quantum dots. Organic ligands with three types of widely used function groups (-SH, -NH2, -COOH) are chosen to investigate the bonding behavior and electronic properties modification on ZnS (101¯0) based on first-principles calculations. The thermodynamic studies of (101¯0) ligands suggest that -SH and -COOH are prone to undergo a proton dissociation when binding on the surface, while -NH2 remains molecular adsorption on the 2-coordinated Zn atom. The binding strengths of these ligands exhibits the order of -SH > -COOH > -NH2, which is consistent with their ligand exchange ability in experiments. The gap states of ZnS (101¯0) mainly come from 2-coordinated S/Zn, and the passivation effect of -NH2 ligand is inferior to the other two due to the absence of dissociated H which covers the 2-coordinated S. The change of ligand structures shows that ligand length has slight influence on the binding energies, while branched ligands bind more strongly on the surface than those linear ones with the same number of carbon atoms. As for the surface vacancies that cause severe trap states, the ligands tend to be located at bridge sites between two 2-coordinated Zn rather than filling the vacancies. Our study provides an insight into the bonding behavior and passivation mechanism of typical ligands on II-VI QDs at the atomic level and gives helpful guidance for surface engineering aimed at improving QD performance.