Tuning of Charge Densities in Graphene by Molecule Doping

The tuning of carrier concentrations in graphene is at the heart of graphene-based nanoelectronic and optoelectronic applications. Molecular doping, that is, taking charges from the adsorbed molecules, shows promise as a means by which to change carrier density in graphene while retaining relative high mobility. However, poor control over doping concentrations is a major obstacle to practical applications. Here, we show that lattice disorders induced by plasma exposure can be used as anchor groups. These groups serve as centers of molecule adsorption and facilitate orbital overlap between graphene and adsorbates (melamine), thus allowing for selective and tunable doping. The carrier concentration revealed by Raman shift can be progressively adjusted up to 1.4 x 10(13) cm(-2), depending on the coverage of melamine molecules and doping temperature. The electronic band structures of the graphene melamine complex were calculated using density functional theory for adsorption over ideal graphene and over non-ideal graphene with Stone-Wales (5-7-7-5) defects. It is shown that charge transfer for adsorption on ideal graphene is negligible, while adsorption on graphene with Stone-Wales defects results in weak hole doping, which is consistent with the progressive increase of carrier density with increasing melamine coverage.