Ligand-Surface Interactions and Surface Oxidation of Colloidal PbSe Quantum Dots Revealed by Thin-film Positron Annihilation Methods

TL;DR

This study uses positron annihilation spectroscopy to investigate how different ligands and oxidation affect the surface electronic structure of colloidal PbSe quantum dots in thin films.

Contribution

It demonstrates the application of 2D-ACAR measurements to reveal ligand-specific surface modifications and oxidation effects in PbSe quantum dot thin films.

Findings

Ligand type influences electron momentum density at the surface.

Small ethylediamine ligands promote electronic coupling between QDs.

Positron annihilation detects surface oxidation upon air exposure.

Abstract

Positron Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) measurements reveal modifications of the electronic structure and composition at the surfaces of PbSe quantum dots (QDs), deposited as thin films, produced by various ligands containing either oxygen or nitrogen atoms. In particular, the 2D-ACAR measurements on thin films of colloidal PbSe QDs capped with oleic acid ligands yield an increased intensity in the electron momentum density (EMD) at high momenta compared to PbSe quantum dots capped with oleylamine. Moreover, the EMD of PbSe QDs is strongly affected by the small ethylediamine ligands, since these molecules lead to small distances between QDs and favor neck formation between near neighbor QDs, inducing electronic coupling between neighboring QDs. The high sensitivity to the presence of oxygen atoms at the surface can be also exploited to monitor the surface oxidation of PbSe QDs upon exposure to air. Our study clearly demonstrates that positron annihilation spectroscopy applied to thin films can probe surface transformations of colloidal semiconductor QDs embedded in functional layers.