  This article addresses the much debated question whether the degree of
hydrophobicity of single-layer graphene (1LG) is different from the one of
double-layer graphene (2LG). Knowledge of the water affinity of graphene and
its spatial variations is critically important as it can affect the graphene
properties as well as the performance of graphene devices exposed to humidity.
By employing chemical force microscopy (CFM) with a probe rendered hydrophobic
by functionalization with octadecyltrichlorosilane (OTS), the adhesion force
between the probe and epitaxial graphene on SiC has been measured in deionized
water. Owing to the hydrophobic attraction, a larger adhesion force was
measured on 2LG domains of graphene surfaces, thus showing that 2LG is more
hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved
through Kelvin probe force microscopy and Raman spectral mapping. Approximate
values of the adhesion force per OTS molecule have been calculated through
contact area analysis. Furthermore, the contrast of friction force images
measured in contact mode was reversed to the 1LG/2LG adhesion contrast and its
origin was discussed in terms of the likely water depletion over hydrophobic
domains as well as deformation in the contact area between AFM tip and 1LG.
