Source-drain current ( I
) dependence. (a) ISD measured at VBG from VBG = 0 to 85 V at 1.32 K with a fixed source and drain voltage, VSD = 0.1 mV, before hydrogen plasma treatment. The neutrality point voltage VNP is near 74 V. Inset: the optical image of a single-layer graphene flake in contrast (b) ISD measured from VBG = −50 to 110 V at T = 1.41 K with a fixed source and drain voltage VSD = 20 mV after hydrogen plasma treatment. The Coulomb blockade oscillations occur between 30 and 50 V. Inset: the Coulomb peaks at T = 1.32 K with a fixed source and drain voltage VSD = 1 mV.