In this work, we present an investigation regarding how and why molecular hydrogen changes the electronic properties of graphene field effect transistors. We demonstrate that interaction with H2 leads to local doping of graphene near of the graphene-contact heterojunction. We also show that such interaction is strongly dependent on the characteristics of the metal-graphene interface. By changing the type metal in the contact, we observe that Ohmic contacts can be strongly or weakly electrostatically coupled with graphene. For strongly coupled contacts, the signature of the charge transfer effect promoted by the contacts results on an asymmetric ambipolar conduction, and such asymmetry can be tunable under interaction with H2. On the other hand, for contacts weakly coupled with graphene, the hydrogen interaction has a more profound effect. In such situation, the devices show a second charge neutrality point in graphene transistor transfer curves (a double-peak response) upon H2 exposure. We propose that this double-peak phenomenon arises from the decoupling of the work function of graphene and that of the metallic electrodes induced by the H2 molecules. We also show that the gas-induced modifications at the metal-graphene interface can be exploited to create a controlled graphene p-n junction, with considerable electron transfer to graphene layer and significant variation in the graphene resistance. These effects can pave the way for a suitable metallic contact engineering providing great potential for the application of such devices as gas sensors.
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