  We employ electrostatic force microscopy to study the electrostatic
environment of graphene sheets prepared with the micro-mechanical 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 10^9 V/m. We expect that water molecules
can significantly modify the electrical properties of graphene devices.
