Electrical tunability of molecular doping of graphene has been investigated using back-gated field effect transistors. Variation of the gate voltage from positive to negative values resulted in reduced p-type doping by NO2, which decreased below detection limit at −45 V. A reverse trend was observed for NH3, where its n-type doping increased with more negative gate voltage, becoming undetectable at 5 V. Our results indicate that adsorption induced molecular doping of graphene could not be detected when the Fermi level coincides with the adsorption induced defect states, which yields NO2 acceptor energy level of ∼320 meV below the Dirac point.

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