Evidence for increased metallicity arising from carbon-sulfur bonding and amorphization effects in sulfur-doped pyrolytic graphite

We report a novel investigation of the electronic and magnetic properties of exfoliated lamellae of highly-oriented-pyrolytic-graphite (HOPG) doped with great excess of sulfur. The sulfur-doping process was achieved through annealing at T ~ 250 °C, ~500 °C and ~990 °C. The obtained lamellae were investigated by employing transmission electron microscopy (TEM), Raman point/map spectroscopy and temperature dependent electron paramagnetic resonance (T-EPR). HRTEM analyses of samples doped at T ~ 990 °C revealed an unusual amorphization-effect arising within hexagonal moiré superlattices as a consequence of the sulfur-doping. Complete amorphization was found at T ~ 500 °C. Raman-spectroscopy point/mapping analyses highlighted a dominant carbon-sulfur-band, appearing at Raman-Shifts of 1450 cm⁻¹. T-EPR of heavily doped samples revealed presence of shifts in the position of the π-electron differential absorption feature together with an anomalous signal broadening at T_doping ~ 500 °C (from g ~ 1.99 to g ~ 2.10), while zero field cooled (ZFC) and field cooled FC superconducting quantum interference device (SQUID) magnetometry from 2 K to 300 K revealed a critical percolative ferromagnetic temperature at T ~ 25 K and the absence of superconductive transitions. Density functional theory (DFT) calculations applied to a Bernal (AB stacked) bilayer-graphene-system in presence of sulfur-bonding revealed a significant variation of the system-metallicity. We highlight the appearance of insulating states (E-gap of 0.325 eV) at sulfur concentrations of ~1%, which then vanish at ~5.5% of sulfur-doping, due to an enhanced degree of system-metallicity and a modification of the π-bonding- and antibonding-signals.