Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods

TL;DR

This study investigates how annealing affects the electronic states in CdSe nanocrystal thin films, revealing increased inter-nanocrystal electronic interaction and structural changes at elevated temperatures.

Contribution

It introduces a novel application of 2D-ACAR to monitor temperature-induced electronic and structural changes in colloidal CdSe nanocrystal arrays.

Findings

Significant reduction in positron annihilation spectrum feature after annealing above 200°C.

Increased electronic interaction due to inorganic interface formation.

Structural transformation into polycrystalline film at around 486°C.

Abstract

The effect of temperature controlled annealing on the confined valence electron states in CdSe nanocrystal arrays, deposited as thin films, was studied using two-dimensional angular correlation of annihilation radiation (2D-ACAR). A reduction in the intensity by ~35% was observed in a feature of the positron annihilation spectrum upon removal of the pyridine capping molecules above 200 degrees Celsius in a vacuum. This reduction is explained by an increased electronic interaction of the valence orbitals of neighboring nanocrystals, induced by the formation of inorganic interfaces. Partial evaporation of the nanoporous CdSe layer and additional sintering into a polycrystalline thin film was observed at a relatively low temperature of ~486 degrees Celsius.