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Recent developments in monolayer graphene production allow its use as the active layer in field-effect transistor technology. Favorable electrical characteristics of monolayer graphene include high mobility, operating frequency, and good stability. These characteristics are governed by such key transport physical phenomena as electron-hole transport symmetry, Dirac point voltage, and charged impurity effects. Doping of graphene occurs during device fabrication, and is largely due to charged impurities located at or near the graphene/substrate interface. These impurities cause scattering of charge carriers, which lowers mobility. Such scattering is detrimental to graphene transistor performance, but our group has shown that coating with fluoropolymer thin films or exposure to polar organic vapors can restore favorable electrical characteristics to monolayer graphene. By partially neutralizing charged impurities and defects, we can improve the mobility by approximately a factor of 2, change the Dirac voltage by fairly large amounts, and reduce the residual carrier density significantly. We hypothesize that this phenomena results from screening of charged impurities by the polar molecules. To better understand such screening interactions, we performed computational chemistry experiments to observe interactions between polar organic molecules and monolayer graphene. The molecules interacted more strongly with defective graphene than with pristine graphene, and the electronic environment of graphene was altered. These computational observations correlate well with our experimental results to support our hypothesis that polar molecules can act to screen charged impurities on or near monolayer graphene. Such screening favorably mitigates charge scattering, improving graphene transistor performance.
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