We employ electrostatic force microscopy to study the electrostatic environment of graphene sheets prepared with the micromechanical exfoliation technique. We detect the electric dipole of residues left from the adhesive tape during graphene preparation, as well as the dipole of water molecules adsorbed on top of graphene. Water molecules form a dipole layer that can generate an electric field as large as . We expect that water molecules can significantly modify the electrical properties of graphene devices.
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The slope of is different on the oxide and on the graphene sheet. This is attributed to the graphene sheet that changes the capacitance between the tip and the Si substrate. In principle, it should be possible to extract the dielectric constant of the graphene sheet. We numerically solved the Poisson equation and found that has a very weak dependence on when . As a result, estimating the dielectric constant of graphene is difficult.
Note that varies slightly as humidity is changed. The size and the sign of the variation are consistent with previous work on surfaces contaminated with a submonolayer of carbon (Ref. 10).
Note that the wafer was exposed to air during manipulation before these measurements, at which point water and other molecules were likely to adsorb on the graphene sheet.
