  Chemical functionalization is a promising route to band gap engineering of
graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer
graphene sheets in the form of substrate-supported or free-standing films. Our
transport measurements demonstrate that non-suspended functionalized graphene
behaves as a granular metal, with variable range hopping transport and a
mobility gap ~ 0.1 eV at low temperature. For suspended graphene that allows
functionalization on both surfaces, we demonstrate tuning of its electronic
properties from a granular metal to a gapped semiconductor, in which charge
transport occurs via thermal activation over a gap ~ 80 meV. This non-invasive
and scalable functionalization technique paves the way for CMOS-compatible band
gap engineering of graphene electronic devices.
