  We study the electronic structure of diluted F atoms chemisorbed on graphene
using density functional theory calculations. We show that the nature of the
chemical bonding of a F atom adsorbed on top of a C atom in graphene strongly
depends on carrier doping. In neutral samples the F impurities induce a
sp^3-like bonding of the C atom below, generating a local distortion of the
hexagonal lattice. As the graphene is electron-doped, the C atom retracts back
to the graphene plane and for high doping (10^14 cm^-2) its electronic
structure corresponds to a nearly pure sp^2 configuration. We interpret this
sp^3-sp^2 doping-induced crossover in terms of a simple tight binding model and
discuss the physical consequences of this change.
