Strong hole-doping and robust resistance-decrease in proton-irradiated graphene

Great effort has been devoted in recent years to improve the electrical conductivity of graphene for use in practical applications. Here, we demonstrate the hole carrier density of CVD graphene on a SiO2/Si substrate increases by more than one order of magnitude to n = 3 x 10(13) cm(-2) after irradiation with a high energy 5 MeV proton beam. As a result, the dc-resistance (R) of graphene is reduced significantly by 60%. Only a negligible amount of defect is created by the irradiation. Also the hole-doped low resistance state of graphene remains robust against external perturbations. This carrier doping is achieved without requiring the bias-gate voltage as is the case for other field effect devices. We make two important observations, (i) occurrence of the doping after the irradiation is turned off (ii) indispensability of the SiO2-layer in the substrate, which leads to a purely electronic mechanism for the doping where electron-hole pair creation and interlayer Coulomb attraction play a major role. A flux-dependent study predicts that an ultrahigh doping may be obtained by longer irradiation. We expect the irradiation doping method could be applied to other atomically thin solids, facilitating the fundamental study and application of the 2d materials.