Extraordinary low sheet resistance of CVD graphene by thionyl chloride chemical doping

Chemical doping is an effective way to increase the conductivity of graphene. Despite the efforts made by many to achieve p-doping, mainly on CVD graphene, by using a variety of oxidant agents, there is still space to evaluate and optimize methodologies to further reduce the sheet resistance while maintaining high optical transparency. In this study, we developed “new” routes to dope graphene heavily using thionyl chloride that is known to act as chlorinating agent. It was found that, in addition to the nucleophilic reactions replacing oxygen with chlorine, the thermal (T ≥ 120 °C) and pyridine-catalytic activations of the SOCl2 chemistry result in more p-doping functionalities in the graphene basal plane. A sheet resistance value of 18 Ω/□ was obtained for a 6-layer stacked graphene films with an optical transmittance of 85% at 550 nm. Furthermore, the possibility offered by the plasma oxidation of graphene in a controlled way, allows to introduce in the graphene basal plane the right amount of epoxy and hydroxyl defects so as to maximize the dopant functionalization and, hence, the carrier density, without affecting significantly the carrier mobility. This route allowed to reach a minimum sheet resistance value of 120 Ω/□ for a single layer graphene with 97.5% optical transparency.