Microwave plasma synthesis of freestanding sulfur-doped few-layer graphene and the impact of the dopant on the electrical transport properties: Experiment and simulation

This study investigates the formation of freestanding sulfur-doped few-layer graphene (FLG) during microwave plasma synthesis and the impact of the doping on its physicochemical and electrical properties. Varying the molar concentration of the sulfur dopant shows for low concentrations (<10 mol%) an increase in the electrical conductivity of powder pellets compared with undoped FLG. Larger dopant concentrations (≥10 mol%) lead to a reduction in conductivity, which is likely linked to the formation of undesired secondary phases. The measured conductivity values are compared to predictions from Boltzmann transport theory based on density functional theory (DFT) calculations of different sulfur dopant configurations. The results highlight that the multilayer structure of FLG significantly influences the stability of different defect types, showing that sulfur (S) doping in conjunction with carbon vacancies in FLG (−3.69 eV) is more favorable than graphitic doping (−2.27 eV). The transport calculations for the most stable configurations demonstrate a conductivity enhancement factor of 1.78–2.47 over the pristine sample matching the experimental observations. Thus, the experimentally observed conductivity increase could be associated with the formation of C–S–C defect structures at C vacancies.