  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.
