  Adatom-decorated graphene offers a promising new path towards spintronics in
the ultrathin limit. We combine experiment and theory to investigate the
electronic properties of dilutely fluorinated bilayer graphene, where the
fluorine adatoms covalently bond to the top graphene layer. We show that
fluorine adatoms give rise to resonant impurity states near the charge
neutrality point of the bilayer, leading to strong scattering of charge
carriers and hopping conduction inside a field-induced band gap. Remarkably,
the application of an electric field across the layers is shown to tune the
resonant scattering amplitude from fluorine adatoms by nearly twofold. The
experimental observations are well explained by a theoretical analysis
combining Boltzmann transport equations and fully quantum-mechanical methods.
This paradigm can be generalized to many bilayer graphene-adatom materials, and
we envision that the realization of electrically tunable resonance may be a key
advantage in graphene-based spintronic devices.
