Layered Structures of Interfacial Water and Their Effects on Raman Spectra in Graphene-on-Sapphire Systems

We investigated the structure of the interfacial water layers between graphene sheets and a sapphire substrate by observing them through graphene sheets at room temperature using atomic force microscopy. When graphene sheets were deposited at low relative humidity, the interfacial water layers appeared as small islands. They grew in layer-by-layer stacking with an increase in the relative humidity. We also investigated effects of the interfacial water layers on Raman spectra from the graphene sheets that cover the water layers. The correlation between G-peak position (intensity) and 2D-peak position (intensity) shows that the interfacial water induces hole-doping in graphene sheets. The doping density increases with increasing the amount of interfacial water. This study shows that the electrical properties of graphene sheets are tunable by controlling the hydrophilicity of substrate surfaces.