A First-Principles Study of CdSe Nanoclusters Capped by Thiol Ligands

TL;DR

This study uses first-principles calculations to analyze how thiol and thiol-radical ligands affect the structure and electronic properties of small CdSe nanoclusters, relevant for solar cell applications.

Contribution

It provides detailed insights into the effects of different thiol ligands and radicals on CdSe quantum dots' properties, highlighting ligand-specific structural and electronic modifications.

Findings

Thiol-radical ligands significantly alter the electronic structure of CdSe QDs.

Ligand position and length influence binding strength and electronic effects.

New absorption peaks are induced by thiol-radical ligands.

Abstract

A first-principles study of small CdnSen quantum dots (QDs) (n = 6, 13, and 33) has been performed for the study of QD-sensitized solar cells. We assessed the effects of the passivating thiol-radical ligands on the optimized structure, the energy gap, and on the absorption spectrum. The simplest thiol, methanethiol, and four other thiol type ligands, namely - cysteine (Cys), mercaptopropionic acid (MPA), and their reduced-chain analogues, were investigated. We have come to the following conclusions. (a) Thiol-radical ligands possessed greater effects on the structure and electronic properties of the CdSe QDs than thiol ligands alone. (b) The sulfur 3p orbitals were localized as the midgap states for the thiol-radical-ligated complex, which altered the absorption spectrum of bare Cd6Se6 by inducing a new lower energy absorption peak at 2.37 eV. (c) The thiol-radical-ligated complex was also found to be sensitive to the position and number of ligands. (d) Both the amine group on Cys and the carboxyl group on Cys and MPA showed a strong tendency to bond with the neighboring Cd atom, especially when the length of the ligand was reduced. This formation of Cd-N and Cd-O bonds resulted in smaller HOMO-LUMO gaps and a stronger binding between the ligands and the surface atoms of CdSe nanoclusters.