Abstract
Advances in lightweight, flexible and conformal electronic devices depend on materials that exhibit high electrical conductivity coupled with high mechanical strength. Defect-free graphene is one such material that satisfies both these requirements and which offers a range of attractive and tunable electrical, optoelectronic and plasmonic characteristics for devices that operate at microwave, THz, infra-red, or optical frequencies. Essential to the future success of such devices is therefore the ability to control the frequency dependent conductivity of graphene. Looking to accelerate the development of high frequency applications of graphene, here we demonstrate how readily accessible and processable organic and organometallic molecules can efficiently dope graphene to carrier densities in excess of 10^13 cm^-2 with conductivities at GHz frequencies in excess of 60 mS. In using the molecule F2-HCNQ, a high charge transfer (CT) of 0.5 electrons per adsorbed molecule is calculated resulting in p-type doping of graphene. N-type doping is achieved using cobaltocene and the sulphur containing molecule TTF with a CT of 0.41 and 0.24 electrons donated per adsorbed molecule, respectively. Efficient CT is associated with the interaction between the electrons present in the molecule and in graphene. Calculation of the high frequency conductivity shows a dispersion-less behaviour of the real component of the conductivity over a wide range of GHz frequencies. Potential high frequency applications in graphene antennas and communications that can exploit these properties and the broader impacts of using molecular doping to modify functional materials which possess a low energy Dirac cone are also discussed.