Quantitative Analysis of Graphene Doping by Organic Molecular Charge Transfer

In this paper, both n-type and p-type doped exfoliated graphene sheets are presented by virtue of adsorbing organic molecules. Flat organic layers are uniformly grown on graphene sheets by the technique. Meanwhile, the high-mobility attribute of graphene is largely preserved, as adsorption on the graphene surface does not significantly modify the pi-bonding networks of graphene. By employing Kelvin probe force microscopy, we show that the 2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane (F4-TCNQ) molecules obtain electrons from graphene whereas vandyl-phthalocyanine (VOPc) molecules donate electrons to it. The amount of charge transfer by F4-TCNQ and VOPc to bilayer graphene on silicon dioxide substrate is estimated to be approximately 0.4 and 0.1 electron/molecule by a tight-binding self-consistent model, respectively. The consistent theoretical and experimental results demonstrate that doping of graphene by organic molecular charge transfer has great implications for future large-scale applications of graphene-based nanoelectronics.