Airborne contamination of graphite as analyzed by ultra-violet photoelectron spectroscopy

Highlights: • Surface contamination by airborne molecules influences the interfacial properties of graphitic materials. • UPS and orthogonal techniques have been applied to analyze the composition of airborne contaminants on HOPG. • Immediate surface contamination of HOPG upon exposure to air and the evolution of its composition over time can be resolved by UPS. - Abstract: This paper reports the surface chemistry of highly ordered pyrolytic graphite (HOPG) upon exposure to ambient air. Ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements were collected from freshly-cleaved HOPG in ultra-high vacuum and after exposure to ambient air. We observed an immediate surface contamination during exfoliation of HOPG in ambient air, which we attribute to functionalization of surface defects and step edges. Additional air exposures result in further growth of the contamination layer, as well as dynamic composition changes from more weakly bound to more strongly bound species over time. This work is the first to show the immediate contamination of graphite upon exfoliation in air and the dynamic exchange of surface contaminants over long exposure times.