Tuning the electronic structure of graphene by molecular charge transfer: a computational study

Insulating semiconducting conductors? The electronic properties of graphene can be tuned through molecular charge transfer, induced by organic electron donor (acceptor), TTF (TCNE, TCNQ), molecules. Such molecular-charge-transfer process has significant effects on optical and transport properties, in particular the characteristic Raman spectra of graphene. We have studied the modification in the electronic structure, as well as optical and transport properties of graphene induced by molecular charge transfer using ab initio density functional theory. Our results from first-principles spin-polarized calculations are compared with those of the available data from Raman spectroscopic studies of modified graphene systems. We find that electron donor and acceptor molecules adsorbed onto the graphene surface exhibit effective molecular charge transfer, giving rise to mid-gap molecular levels with tuning of the band gap region near the Dirac point. The molecular charge transfer causes the stiffening or softening of the Raman G-band frequency in graphene, and we find that it also has a significant impact on the intensity ratio of the D- to G-band, corroborating experimental findings. We suggest that these charge transfer mechanisms can be probed through the low-frequency profile of the optical conductivity