Thickness dependent hydrophobicity of epitaxial graphene

TL;DR

This study demonstrates that double-layer graphene (2LG) exhibits greater hydrophobicity than single-layer graphene (1LG), using chemical force microscopy and spectral mapping to quantify water affinity differences.

Contribution

It provides the first direct measurement of hydrophobicity differences between 1LG and 2LG epitaxial graphene using advanced microscopy techniques.

Findings

2LG is more hydrophobic than 1LG based on adhesion force measurements.

Adhesion forces were quantified per OTS molecule on graphene surfaces.

Friction force contrast correlates with water depletion and deformation effects.

Abstract

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.