Influence of oxide substrates on monolayer graphene doping process by thermal treatments in oxygen

The structural and the electronic properties of monolayer graphene made by chemical vapor deposition and transferred on various oxide substrates (SiO2, Al2O3, and HfO2)are investigated by Raman Spectroscopy and Atomic Force Microscopy in order to highlight the influence of the substrate on the features of p-doping obtained by O2 thermal treatments. By varing the treatment temperature up to 400 °C, the distribution of the reaction sites of the substrates is evaluated. Their total concentration and the consequent highest doping available is determined and it is shown that this latter is linked to the water affinity of the substrate. Finally, by varing the exposure time to the gas up to 2 h, the kinetics of doping is investigated. The doping process is found to be better described by a diffusion limited kinetic model, ascribable to the diffusion of O2 in the interstitial space between graphene and the substrate. After this step, the doping process is completed by a faster redox reaction between O2 adsorbed to graphene and interstitial H2O.