Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

TL;DR

This study uses positron annihilation spectroscopy to investigate the surface composition and electronic structure of PbSe nanocrystals, revealing how positrons interact with surface electrons and ligands, and correlating surface changes with particle morphology.

Contribution

It demonstrates that positron annihilation techniques can effectively probe the surface chemistry and electronic structure of PbSe nanocrystals within multilayer assemblies, supported by theoretical calculations.

Findings

Positrons are strongly trapped at nanocrystal surfaces.

Surface composition includes Se electrons and ligand-bound O electrons.

Surface changes correlate with particle morphology modifications.

Abstract

Positron annihilation lifetime spectroscopy (PALS) and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy (TEM), show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab-initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.