Characteristics and Effects of Diffused Water Between Graphene and a SiO₂ Substrate

The graphene/SiO₂ system is a promising building block for next-generation electronic devices, integrating the high electromagnetic performance of graphene with the mature technology of Si-based electronic devices. It is well known that the electromagnetic performance of graphene/SiO₂ is dramatically reduced by structural defects, such as wrinkles and folding, which are suspected to result from water droplets. Therefore, understanding water diffusion between graphene and SiO₂ is required for controlling structural defects and thus improving the electromagnetic performance of this system. Although the behavior of water between graphene and atomically flat mica has been investigated, the characteristics and effects of diffused water between graphene and SiO₂ remain unidentified. We have investigated water diffusion between monolayer graphene and SiO₂ under high humidity conditions using atomic force microscopy. For a relative humidity of over 90%, water diffuses into graphene/SiO₂ and forms an ice-like structure up to two layers thick. Liquid-like water can further diffuse in, stacking over the ice-like layer and evaporating relatively easily in the air causing graphene to wrinkle and fold. By similarly investigating water diffusion between graphene and mica, we argue that water-induced wrinkle formation depends on the hydrophilicity and roughness of the substrate.