Direct Measurement of the Surface Energy of Graphene

Graphene produced by chemical vapor deposition (CVD) is a promising candidate for implementing graphene in a range of technologies. In most device configurations, one side of the graphene is supported by a solid substrate, wheras the other side is in contact with a medium of interest, such as a liquid or other two-dimensional material within a van der Waals stack. In such devices, graphene interacts on both faces via noncovalent interactions and therefore surface energies are key parameters for device fabrication and operation. In this work, we directly measured adhesive forces and surface energies of CVD-grown graphene in dry nitrogen, water, and sodium cholate using a modified surface force balance. For this, we fabricated large (∼1 cm²) and clean graphene-coated surfaces with smooth topography at both macro- and nanoscales. By bringing two such surfaces into contact and measuring the force required to separate them, we measured the surface energy of single-layer graphene in dry nitrogen to be 115 ± 4 mJ/m², which was similar to that of few-layer graphene (119 ± 3 mJ/m²). In water and sodium cholate, we measured interfacial energies of 83 ± 7 and 29 ± 6 mJ/m², respectively. Our work provides the first direct measurement of graphene surface energy and is expected to have an impact both on the development of graphene-based devices and contribute to the fundamental understanding of surface interactions.