We provide a direct demonstration of water desorption and re-adsorption on epitaxial graphene as studied by functional SPM.

For numerous industrial applications it is essential to know and control the state of the graphene surface. The presence of water is unavoidable when graphene is exposed to air for a prolonged period of time and crucial both for fundamental physics and optimal functionality of devices (i.e. wetting, catalysis, batteries, supercapacitors, chemical and biological sensors and electronic devices). To fully exploit possibilities of graphene devices, it is essential to understand the interaction between the graphene surface and atmospheric water.

We study 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 ^[1]. 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 (Fig. 1).

Fig1:Tapping phase images of heart and stripe domains obtained at different temperatures, top to bottom: RT1, 50 °C, 80 °C and RT2. The image demonstrates the reversible temperature-dependent process of desorption and re-adsorption of water on 2LG and 3LG. A clearly seen pattern on the right side of the image reveals defects and strain in the graphene layer.

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 (Fig. 2).

Fig2 - Left: SKPM potential image of a 5 μm size area containing 2LG (medium bright) and 3LG (bright) islands on the top of 1LG (dark)at room temperature. Right: Linear profiles of the surface potential difference at 20 and 120 °C corresponding to the left image. Vertical dashed lines show the approximate boundaries of 2LG domains.

In situ observation of water evolution during heating also potentially provides a direct method for measurement of the heat adsorption on the nanoscale.

[1] T. L. Burnett, J. Patten and O. Kazakova.
Water desorption and re-adsorption on epitaxial graphene studied by SPM.