Nature of the positron state in CdSe quantum dots

TL;DR

This study uses positron-annihilation spectroscopy and ab-initio calculations to determine that positrons in CdSe quantum dots are primarily localized at the surface, providing insights into surface composition and ligand interactions.

Contribution

It resolves the longstanding question about the positron state in semiconductor quantum dots by demonstrating the presence of a surface state through combined experimental and theoretical approaches.

Findings

Positron lifetime component indicates surface trapping in CdSe QDs.

Ab-initio calculations confirm surface positron state and match experimental lifetimes.

Study enables quantitative analysis of surface composition and ligand interactions.

Abstract

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor Quantum Dots (QDs) embedded in thin films. Nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy (PALS) studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab-initio calculations of the positron wave function and lifetime employing a recent formulation of the Weighted Density Approximation (WDA) demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the longstanding question regarding the nature of the positron state in semiconductor QDs, and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.