Water desorption and re-adsorption on epitaxial graphene studied by SPM

We demonstrate the temperature-mediated and completely reversible process of desorption-readsorption of water on a few layers of epitaxial graphene on a 4H-SiC(0001) substrate. We show that under ambient conditions water forms solid structures on top of the second and third layers of graphene. In the case of strained or highly defective graphene domains, these features produce strongly correlated and reproducible patterns, implying importance of the underlying defects for the initial stages of water adsorption. Hydrophobicity increases with number of graphene layers. Evolution of the water layer as a function of temperature is accompanied by a significant (two-fold) change of the absolute surface potential difference between one and two layers of graphene. In situ observation of water evolution during heating also potentially provides a direct method for measurement of the heat adsorption on the nanoscale.