(779d) Surface Energy of Graphene Determined By Intrinsic Friction Microscopy

Surface Energy of Graphene Determined by Intrinsic Friction Microscopy

Brad Krajina,  Lakshmi S. Kocherlakota , René M. Overney

Due to its exceptional properties, research in graphene is actively and widely pursued for applications in electronics, electro-optics, photovoltaics, and sensing. A critical and surprisingly still outstanding property is graphene’s surface energy and its transition towards bulk graphite. The reason for this shortcoming is the challenge to obtain surface energies on materials that possess surface inhomogneities, such as step edges, or thickness variations, an aspect that is of particular importance for graphene. While the graphene sheet homogeneity and size could be improved significantly by chemical routes over the initial mechanical cleaving process, oxidation effects and follow up treatments add additional complications for reaching consensus on the surface energy.

In this light, we explored the surface energy of “virgin” graphene as obtained through mechanical cleaving of highly ordered pyrolytic graphite (HOPG) on the sub-inhomogeneity scale. We utilized a scanning scattering methodology that extracts an energetic signature from the  coupling events of an atomic force microscopy (AFM) tip with the thermally active modes within the interface between AFM tip and sample. This method dubbed “intrinsic friction microscopy” (IFA) has been used in the past on organic systems to analyze the energetics of mobile groups.^1,2 It is here for the first time employed on an inorganic material, resulting in an energy value for HOPG that is in correspondence with the Hamaker constant of  1.49 × 10^-19 J known for HOPG and SiO₂ (AFM tip material). Subsequent analysis of single layer graphene reveal a decrease in the Hamaker constant by a factor of two, which leads via the Liftshift Theory of Van der Waals interaction applied to metal-insulator interfaces to an equivalent drop in the surface free energy. This paper discusses also the dependence on the surface energy on the number of graphene layers and identifies a critical graphene/HOPG thickness of around 5 nm, beyond which the HOPG bulk value for the surface energy is restored.

(1) Sills, S.E.; Gray, T.; Overney, R.M., J. Chem. Phys. (2005), 123, 134902.

(2) Knorr, D.B.; Gray, T.O.; Overney, R.M., J. Chem. Phys. (2008), 129, 074504.