EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Highly oriented pyrolytic graphite (HOPG) is the ideal 3D carbon material with sp2 hybridization. Besides being a common source for single-layer graphene, obtained by exfoliation, HOPG is an attractive material thanks to its electrical conductivity, chemical inertness, and atomic-level flatness. When using HOPG in Kelvin probe force microscopy (KPFM), we observed the presence of domains with surface potential contrast as high as 50 mV for freshly-cleaved HOPG or more for HOPG after some days of air exposure. Our time-dependent studies show an evolution of the contrast and spatial extension of the surface potential domains. We attribute this result to hydrocarbon adsorption from the environment. The origin of domains, giving rise to spatial heterogeneities in hydrocarbon adsorption, is explained by rotational defects in HOPG that give rise to Moire patterns and affect the interlayer coupling with the graphene top layer. This hypothesis was supported by colocalized hyperspectral Raman imaging showing domains with G peak shifts and 2D line shape compatible with bilayer graphene. We explain the selective sensitivity or our Raman spectroscopy results to the top graphene layers as arising from resonances due to van Hove singularities. In addition, the adsorbed hydrocarbons can be effectively removed by high external tip-sample bias during scanning. Beyond providing new insights in the surface properties of graphite, our results show that chemical and optical properties of HOPG are more complex than what is widely believed due to the broken symmetry at the HOPG top surface giving rise to bilayer-like behavior [1].
[1] T.-H. Tran, R.D. Rodriguez, M. Salerno, A. Matkovic, C. Teichert, E. Sheremet, “Twisted graphene in graphite: Impact on surface potential and chemical stability”, to appear in Carbon, 2021.