Band structures of blue luminescent nitrogen-doped graphene quantum dots by synchrotron-based XPS

Abstract This paper is dedicated to Professor P.R. Norton on the occasion of his 75th birthday, in honor of his profound contributions to Surface Science. Graphene quantum dots (GQDs) attracted great interests attributed to their outstanding photoluminescence properties. Nitrogen doping is an effective strategy to engineer the optical and electron transfer properties of GQDs. Nitrogen doped GQDs (N-GQDs) were obtained by using cow milk as the precursor via a facile hydrothermal process with a Teflon-lined autoclave heated at 200 °C for 4.5 h. The purified N-GQDs were discovered to have high crystallinity with a lattice spacing of 0.21 nm corresponding to that of in-plane graphene (100 facet). The optical band gap of the N-GQDs was estimated from the Tauc plot derived from their UV–vis absorption spectrum. The full valence band spectrum of N-GQDs was collected and analyzed by means of synchrotron-based X-ray photoelectron spectroscopy for the first time. Corresponding full band structures were elucidated.