A nanoscopic approach to studying evolution in graphene wettability

The equilibrium state of graphene surfaces exposed to ambient conditions is of significant importance for applications from electronics to anti-corrosion layers and from a fundamental perspective. The environmental exposure influences macroscopic properties of the graphene surface, as recent studies on wettability reported. However, these studies are controversial for two reasons: firstly, time dependency of graphene wettability complicates comparison of results from different groups. Secondly it's inherently difficult to understand the underlying physical phenomena by means of macroscopic measurements alone. Here we study the evolution of the wettability of graphene monolayers on copper by exposing samples to controlled ambient conditions. Static contact angle measurements reveal that the graphene undergoes a transition in its wettability from slightly hydrophilic to hydrophobic over timescales of the order of hours. To gain further insight, we apply Fourier transform infrared spectroscopy and a recently developed dynamic atomic force microscopy-based force spectroscopy to probe the sample surface at the nanoscale. This novel nanoscopic approach allows us to demonstrate that the transition in wettability of graphene is induced by the adsorption of water molecules from the environment in combination with hydrocarbon contamination. Furthermore, our nanoscale observations challenge the wetting transparency theory of graphene after exposure to the environment.