Modification of the electronic structures of graphene by viologen

Electronic band structures of viologen-doped graphene have been investigated. Pristine graphene film, synthesized by chemical vapor deposition (CVD), showed numerous oxygen-related functional groups, and was annealed to remove oxygen content. Two species of viologen, di-cationic and mono-cationic, were observed in the viologen-doped graphene. The work function of the CVD graphene was dramatically decreased by 1.1 eV after viologen doping. Viologen, therefore, is a strong n-type dopant for graphene, and this report shows the first direct evidence of changing the work function of viologen-doped graphene by cutting-off the kinetic energy of secondary electrons.