Bonding and thermal stability in very low-energy nitrogen ion implantation into graphite studied by synchrotron-based spectroscopy and DFT modeling

This work investigates the nitrogen bonding configurations and thermal stability of highly oriented pyrolytic graphite (HOPG) subjected to N 2 ⁺ implantation at a dose of 5×10 14 ions/cm 2 and ion energies of 60 and 100 eV, using synchrotron–based XPS, NEXAFS, and DFT simulations. The 60- and 100-eV implantations incorporate nitrogen into the first and second graphitic layers with minimal damage. Three distinct nitrogen species—pyridine–N, pyrrolic–N, and graphitic–N form at surface and subsurface regions depending on ion energy. For 60 eV implantation, pyridine-N is concentrated at the surface, pyrrolic-N is distributed across the surface and subsurface, whereas graphitic-N is located mainly in the subsurface. In contrast, for 100 eV, pyridine–N predominantly forms subsurface, pyrrolic–N resides at the surface, and graphitic–N is distributed uniformly throughout both regions. Upon annealing to 700 °C, pyrrolic–N exhibits the lowest thermal stability, whereas graphitic–N shows the highest stability and forms in the subsurface, while pyridine–N forms on the surface. These findings highlight the importance of depth-resolved characterization for understanding nitrogen incorporation via low-energy implantation and provide a potential route to control the spatial distribution and bonding configurations, which enables precision doping of HOPG networks to tailor local structural and electronic properties in designing catalysts and tunable electronic devices.