Investigation of the surface chemical and electronic states of pyridine-capped CdSe nanocrystal films after plasma treatments using H-2, O-2, and Ar gases

Abstract

Surface chemical bonding and the electronic states of pyridine-capped CdSe nanocrystal films were evaluated using x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy before and after plasma treatments using H-2, O-2, and Ar gases from the viewpoint of studying the effects of surface capping organic molecules and surface oxidation. Surface capping organic molecules could be removed during the plasma treatment due to the chemical reactivity, ion energy transfer, and vacuum UV (VUV) of the plasma gases. With O-2 plasma treatment, surface capping organic molecules were effectively removed but substantial oxidation of CdSe occurred during the plasma treatment. The valence band maximum energy (E-VBM) of CdSe nanocrystal films mainly depends on the apparent size of pyridine-capped CdSe nanocrystals, which controls the interparticle distance, and also on the oxidation of CdSe nanocrystals. Cd-rich surface in O-2 and H-2 plasma treatments partially would compensate for the decrease in E-VBM. After Ar plasma treatment, the smallest value of E-VBM resulted from high VUV photon flux, short wavelength, and ion energy transfer. The surface bonding states of CdSe had a strong influence on the electronic structure with the efficient strip of capping molecules as well as different surface oxidations and surface capping molecule contents.