  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 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.
