Analysis of Graphite Contamination in Air Using Ultraviolet Photoelectron Spectroscopy (UPS)

UPS has been used to characterize organic thin films and molecular adsorption on surfaces. Extensive studies on sputtering and thin-film growth in various amorphous systems, graphite systems, HOPG, and polymers have shown that changes in the convolution of the density of states (DOS) are related to surface composition.

At UPS photon energies, C 2p states are emphasized over C 2s-derived states. This characteristic makes UPS a more sensitive tool than XPS for probing the valence states, molecular bonding, and chemical composition of surface-adsorbed species, allowing UPS to distinguish different chemical states of carbon more clearly.

In the work of Muhammad Salim et al., the surface chemistry of highly ordered pyrolytic graphite (HOPG) after exposure to ambient air was studied. Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS) measurements were performed on freshly cleaved HOPG under ultrahigh vacuum (UHV) and after exposure to ambient air. The experimental setup is shown in Figure 1.

Figure 1: Schematic of the experimental setup

In addition, HOPG samples were cleaved in air, and their UPS spectra were measured after different durations of air exposure. Any adsorption of contaminants on the surface was expected to alter the total peak position/area of the signals compared to the spectra of HOPG cleaved under UHV conditions.

As shown in Figure 2, a rapid decrease in the intensity of the primary graphite σ peak at 7.6 eV was observed in the total UPS spectral area of these samples. During prolonged air exposure, this main peak also exhibited gradual broadening. This broadening is attributed to an increase in the quantity and/or variety of adsorbed volatile organic compounds (VOCs).

Figure 2: UPS spectra of HOPG after cleavage under ultrahigh vacuum (UHV Cleave), cleavage in air (Air Cleave), and exposure to air for different durations (15 minutes, 2 hours, and 20 hours)

After increased air exposure, a slight downward shift and an increase in intensity were observed for the graphite π peak at 2.9 eV. A similar shift was observed for the graphite peak at 13.7 eV, which gradually shifted to 12.8 eV as air exposure increased, accompanied by an increase in intensity. Notably, UPS measurements were conducted under UHV conditions, meaning UPS underestimates molecular adsorption under atmospheric pressure. Weakly bound species, potentially low-molecular-weight and/or nonpolar molecules, may adsorb onto the surface under ambient conditions but desorb before UPS measurements are performed.

Temperature-programmed desorption (TPD) studies under UHV conditions have reported the desorption of organic molecules from graphene and graphite surfaces, providing insight into the adsorbates that may be present during UPS measurements.

By comparing the UPS spectra of HOPG cleaved in air with those cleaved under UHV, the experiment revealed an instantaneous change in surface composition for the former. Surface contamination was observed immediately upon HOPG cleavage in ambient air, which was attributed to the functionalization of surface defects and step edges.

Further air exposure led to continued growth of the contamination layer and a dynamic shift in the composition of adsorbed species over time, from weakly bound to more strongly bound species. This study provides the first direct demonstration of immediate contamination of graphite upon cleavage in air, as well as the dynamic exchange of surface contaminants during prolonged exposure.

[1] Muhammad Salim, Justin Hurst, Michelle Montgomery, Nathan Tolman, Haitao Liu, Airborne contamination of graphite as analyzed by ultra-violet photoelectron spectroscopy, Journal of Electron Spectroscopy and Related Phenomena, Volume 235, 2019, Pages 8-15.