Defect healing and doping of CVD graphene by thermal sulfurization

CVD graphene layers are intrinsically polycrystalline; depending on grain size, their structure at the atomic level is scarcely free of defects, which affects the properties of graphene. On the one hand, atomic-scale defects act as scattering centers and lead to a loss of carrier mobility. On the other hand, structural disorder at grain boundaries provides additional resistance in series that affects material conductivity. Graphene chemical functionalization has been demonstrated to be an effective way to improve its conductivity mainly by increasing carrier concentration. The present study reports the healing effects of sulfur doping on the electrical transport properties of single-layer CVD graphene. A post-growth thermal sulfurization process operating at 250 °C is applied on single layers of graphene on Corning-glass and Si/SiO₂ substrates. XPS and Raman analyses reveal the covalent attachment of sulfur atoms in graphene carbon lattice without creating new C-sp³ defects. Measurements of transport properties show a significant improvement in hole mobility as revealed by Hall measurements and related material conductivity. Typically, Hall mobility values as high as 2500 cm² V^-1 s^-1 and sheet resistance as low as 400 Ohm per square are measured on single-layer sulfurized graphene.