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Competitive interfacial charge transfer to graphene from the electrode contacts and surface adsorbates

Charge transfer (CT) at metal-graphene contacts induces a potential variation from the contact edges that extends to ~1 micrometer. Potential variations with a similar length should be observed around charge-transferring surface adsorbates. Thus, it is expected that a competition exists between these two CT sources when one source is within ~1 micrometer from the other. In this letter, weakly-coupled Ni contacts and 7,7,8,8-tetracyanoquinodimethan molecules are employed as the CT sources to investigate their possible competition. The CT from the molecules adsorbed only in the channel region change the charge density of the graphene in the under-contact regions. The extent of the CT effect in the under-contact region is as long as ~4 micrometers. The considerably long CT is ascribed to the high effective dielectric constant of the graphene under the contacts, resulting from a thin interfacial NiOx layer containing carbon impurities acquired from the graphene.