Chemical Understanding of the Mechanisms Involved in Mitigation of Charged Impurity Effects by Polar Molecules on Graphene

It is well-known that the transport properties of monolayer graphene are degraded by charged impurities present between graphene and either a given substrate or air. Such impurities cause charge scattering of holes and electrons in graphene. In previous work, our group has used both fluoropolymer thin films and polar vapor molecules to dramatically improve graphene field-effect transistor (FET) device characteristics, including Dirac voltage and mobility. We attributed the graphene device improvements to mitigation of charged impurities and defects due to electrostatic interaction with the dipoles of the applied fluoropolymers and polar molecules. In this work, we present theoretical support to this hypothesis, in the form of computational chemical simulations involving the interaction of polar molecules and impurities on a graphene sheet. We examine two types of impurities which may occur at graphene interfaces: ionic impurities and molecular dipole impurities. Upon introduction of polar vapor molecules to an impurity/graphene system, we observed a dramatic reduction in the electrostatic potential in the plane of the graphene from the impurity. The magnitude of potential reduction scales with the average dipole moment of each polar molecule. We were able to determine two separate mechanisms which contribute to the total potential reduction, impurity displacement, and electrostatic screening of the impurity. The respective impacts of the mechanisms vary with distance from the impurity. Additionally, in the case of the molecular dipole impurity, the orientation of the impurity atop graphene is a key factor that determines the potential impact.