▎ 摘　　要

Graphene is an ultrathin material, which allows us to control surface phenomena by means of field-effect gating. Among various surface phenomena, photo-oxidation is known to be a facile method to largely control the electronic structure of graphene. In this study, gate controllability of photo-oxidation of graphene is thoroughly examined using a field-effect-transistor configuration. The presence of water molecules enhances gate controllability, which can be explained using the water-oxygen co-adsorption picture. In addition, the photo-oxidation reaction evolves from the edge and proceeds towards the center of the graphene channel, which can be understood by the fringing field effect. Semiconducting characteristics are successfully obtained by narrowing of the graphene channel, suggesting possible formation of a graphene nanoribbon under mild conditions, i.e., in air at room temperature.