Optical and X-ray Photo-emission Spectroscopies of Core/Shell Colloidal CdSe/CdS Quantum Dots: Modeling and Experimental Determination of Band Alignment

TL;DR

This paper develops a theoretical model to accurately determine the conduction band offset in CdSe/CdS core/shell quantum dots, validated by optical and X-ray photoemission experiments, enabling better prediction of their electronic properties.

Contribution

It introduces a comprehensive model that accounts for interface effects and ligands to predict band alignment, providing a reliable method to determine band offsets for various quantum dot geometries.

Findings

The model accurately predicts the conduction band offset for CdSe/CdS quantum dots.

Experimental validation confirms the model's predictions.

The determined band offset improves understanding of quantum dot electronic behavior.

Abstract

Optical properties of multilayer semi-conductor nano-emitters are crucially dependent on the relative energy levelsof their different components. For core/shell quantum dots, the relative energy difference between conduction bandedge of core and shell materials induces, depending on its value, either a confinement of the electron within the coreor a delocalization of its wave function within the whole quantum dot. This results in drastic consequences on theenergy and the oscillator strength of the fundamental transition. Surprisingly, the literature currently lacks a definitivevalue for the energy difference between CdSe and CdS conduction band edges as most of the experimental studiesprovide values corresponding to specific geometries of quantum dots. Here, we develop a full theoretical modelexpressing energy levels considering core/shell interface pressure, ligands and enabling the accurate prediction ofthe bandgap value with the nanocrystal size. It allows to reliably determine the energy difference between theconduction band edge of CdSe and CdS materials, known as the conduction band offset, in such a way that this valuecan later be used to model quantum dots of any geometry. This value is determined using our model and two differentexperimental methods: optical spectroscopy and X-ray photoemission (XPS) experiments.