Layer-Dependent Oxidation Spreading in Multilayer Graphene during AFM Local Anodic Oxidation.

Local anodic oxidation is a practical method for creating graphene oxide nanostructure prototypes for fundamental research, such as in nanoelectronics. It is usually performed by scanning the atomic force microscopy (AFM) tip over an area designated for oxidation. The spreading of the oxidation reaction from a stationary tip has been rarely studied, despite its potential usefulness for fabricating graphene structures. While previous work investigated oxidation spreading on a graphene monolayer, this study extends the research to multilayer graphene to examine how the number of graphene layers affects the process. Depending on the number of layers, three pattern categories were identified. For 2-8 layers, "graphene craters" (a central hole surrounded by a raised circular rim) were observed. Beginning from 8 layers to 14 layers, "isotropic etching", which removes a portion of the graphene layer in a relatively uniform manner, emerged. Finally, on 28-layer-thick graphene, irregular "anisotropic etching" was observed. Additionally, the increase of pattern radius with relative humidity was observed, whereas no clear dependence on exposure time was observed. The properties of the three categories of fabricated graphene patterns were characterized using AFM topography, Kelvin probe force microscopy, and Raman spectroscopy. The experiments were complemented by electric field calculations performed in COMSOL Multiphysics. By combining experimental results and simulations, a theoretical model was proposed to explain the formation of the observed patterns, based on oxidation initiated at defects and cleavages in the graphene.