  Combining experiment and theory, we investigate how the naturally created
heterojunction at a graphene and metallic contact is modulated via interaction
with molecular hydrogen (H2). Due to electrostatic interaction, a Cr/Au
electrode induces a pn junction in graphene, leading to an asymmetrical
resistance between the charge carriers (electron and hole). This asymmetry is
well modeled by considering the preferential charge scattering at the pn
junction, and we show that it can be modulated in a reversible, selective and
asymmetrical manner by exposing H2 to the metal-graphene interface. Our results
are valuable for understanding the nature of the metal-graphene interfaces and
demonstrate a novel route towards hydrogen sensor application. KEYWORDS:
graphene, contact resistance,

Mesoscale and Nanoscale Physics