Effect of Surface Adsorbates on Carrier Transport in Graphene

Interaction of graphene with adsorbates on its surface was investigated by analyzing two important electrical parameters - graphenes carrier density and mobility, recorded simultaneously. NH
₃
and NO
₂
molecules were exposed to the graphene separately as ionized donor and acceptor, respectively. With the exposure of NO
₂
on the graphene, graphenes carrier density was observed to be increasing monotonically, and NH
₃
exposure showed an opposite behavior as expected. Even though gas molecules attached on the graphene supposed to be working as ionized impurity scattering centers, graphenes mobility did not drop for both cases - the mobility dropped with NO
₂
exposure while it increased with NH
₃
exposure. To understand the mechanism, we proposed a theoretical model here which includes ionized impurities both from substrate and external adsorbates influencing the graphenes mobility change as one system. A strong donor C
₉
H
₂₂
N
₂
molecules effect on the graphene was also examined. Its high electron donating effect transformed graphene from p-type to n-type, while the mobility change during the whole transformation still followed the proposed model which further verified the model's reliability.