Determination of the Energy Band Gap depending on the Oxidized Structures of Quantum Dots

Abstract: Theoretical and experimental studies on the changes of the optical properties of CdSe/CdS/ZnS (core/double shell) quantum dots (QDs) during the oxidation process were first performed. Effective medium approach using modified Khon-Sham equation presents a new method to predict the effects of the oxidation and to determine the oxidized ratio of nano scale materials by a quantitative comparison with the experimental photoluminescence (PL) changes. As the oxidation progressed from the CdSe/CdS/ZnS nanocrystal surface, the PL peak shifted to longer wavelength and the quantum efficiency (QE) continuously decreased. It was also found that such changes were accelerated when the thickness of the outermost ZnS shell became thinner than a monolayer. The radial wave functions showed that the electron carriers rapidly extended into the shell region while the hole carriers spread very little into the core region. This indicates that the electrons are the key carriers to induce the changes in the energy band gap and the QE.