Effects of adsorbates on the electronic properties of graphene
Fuhrer, Michael S.
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Graphene, an atom-thick sheet of carbon, is a novel two-dimensional material in which the low-energy electrons behave as massless Dirac fermions. This thesis explores the effects of adsorbates on the electronic properties graphene by adsorption in controlled environment in ultra-high vacuum (UHV), coupled with in situ measurement of transport properties. Two types of adsorbates on graphene are investigated. First, the effects of charged impurity scattering are studied by controlled adsorption of potassium on bilayer graphene at low temperature in UHV. The results indicate that the magnitude of charged-impurity scattering in bilayer graphene is similar to that in single layer graphene, and in good agreement with theory. The widely observed lower mobility in bilayer graphene on SiO2 is likely due to another source of disorder. Second, the dielectric screening of bilayer graphene is modified by deposition of ice overlayers at low temperature in UHV. No screening effect is observed in pristine bilayer graphene. However, ice overlayers significantly increase the mobility of potassium-doped bilayer graphene through screening of potassium ions. Together, the ice deposition experiments demonstrate the existence of screening effect in bilayer graphene and support that charge impurities are not the dominant scatters in pristine bilayer graphene on SiO2. The screening of adsorbed potassium ions on single-layer graphene is also investigated both experimentally and theoretically. The increase in mobility upon ice deposition is much larger than expected assuming ice's bulk relative dielectric constant of 3.2. A simple model assuming stronger local screening near potassium ions is proposed which can explain the experimental observations. Temperature-dependent studies of electronic transport in the system of coadsorbed potassium and ice show that graphene's resistivity is sensitive to phase transitions in overlayers as well as desorption, opening new opportunities to study surface phases with electronic measurements.