Determination of band alignment of core/shell colloidal CdSe/CdS quantum dots, by optical and X-ray photo-electron spectroscopies

Abstract

Optical properties of multilayer semi-conductor nano-emitters are crucially dependent on the relative energy levels of their different components. For core/shell quantum dots, the relative energy difference between conduction band edge of core and shell materials induces, depending on its value, either a confinement of the electron within the core or a delocalization of its wave function within the whole quantum dot. This results in drastic consequences on the energy and the oscillator strength of the transitions. Surprisingly, the literature currently lacks a definitive value for this energy difference, called offset, between the conduction band edge of CdSe and CdS materials. Here, we develop a theoretical model expressing energy levels and considering quantum dot dimension, core/shell interface pressure, ligands and allowing to reliably determine the conduction band offset. Its value is determined experimentally using our model and both optical and X-ray photoelectron (XPS) spectroscopies.