Electronic states of graphite edge surfaces fabricated by ultrashort-pulse laser processing: Synchrotron micro X-ray photoelectron spectroscopy, micro X-ray absorption spectroscopy and density functional theory studies

Graphite edges are important sites for understanding the electronic properties of carbon materials. However, their electronic states remain poorly understood because spatially selective observation of edge surfaces separately from basal surfaces has been experimentally challenging. Here, we compared a laser-fabricated edge surface and a basal surface of highly oriented pyrolytic graphite (HOPG) by in-situ synchrotron-radiation micro X-ray photoelectron spectroscopy (XPS) and micro X-ray absorption spectroscopy (XAS), together with density functional theory (DFT) calculations. Ex-situ Raman spectroscopy indicates inhomogeneous structures in the laser-fabricated edge surface, including reconstructed, defective, amorphous/sp 3 -like, residual oxygen-containing, and mixed-termination species. After annealing at 700 °C in vacuum, the laser-fabricated edge surface exhibited an enhanced density of states (DOS) near the Fermi level, whereas the basal surface did not. In the C 1s XPS spectrum, an edge-specific low-binding-energy component was observed at 283.5 eV. Carbon K-edge XAS also showed an edge-specific low-energy feature at 284.5 eV. DFT calculations using ideal hydrogen-terminated edge models indicate that zigzag-edge reference structures can produce enhanced near-Fermi-level DOS, whereas armchair-edge reference structures show much weaker near-Fermi-level contributions. Therefore, although zigzag-like local structures may represent only a part of the structurally inhomogeneous laser-fabricated edge surface, they may contribute to the observed metallic-like near-Fermi-level spectral feature. A comparison with the present limited DFT models suggests that the C 1s XPS, valence-band photoelectron spectra and XAS features could be explained by contributions from hydrogen-containing zigzag-edge structures; however, this is not a unique interpretation of the present spectroscopic results.