Twisted graphene in graphite: Impact on surface potential andchemical stability

Highly-oriented pyrolytic graphite (HOPG),i.e., the 3D stack of sp2-hybridized carbon sheets, is anattractive material thanks to its high electrical conductivity, chemical inertness, thermal stability, atomic-scaleflatness, and ease of exfoliation. Despite an apparently ideal and uniform material, freshly cleavedHOPG shows domains in Kelvin probe force microscopy (KPFM) with surface potential contrast over30 mV. We systematically investigated these domains using an integrated approach, including time-dependent KPFM and hyperspectral Raman imaging. The observed time-evolving domains are attrib-uted to locally different hydrocarbon adsorption from the environment, driven by structural defectslikely related to rotational mismatch,i.e., twisted layers. These defects affect the interlayer couplingbetween topmost graphene and the underlying layers. Our hypothesis was supported by Raman spec-troscopy results, showing domains with G peak shifts and 2D line shape compatible with bilayer gra-phene. We attribute the selective sensitivity of our Raman spectroscopy results to the top graphenelayers as resonances due to van Hove singularities. Our results show that the chemical and electricalproperties of HOPG are far more complex than what is generally believed due to the broken symmetry atthe top surface, giving rise to graphene bilayer-like behavior.