  We develop a simple chemical method to obtain bulk quantities of N-doped,
reduced graphene oxide (GO) sheets through thermal annealing of GO in ammonia.
X-ray photoelectron spectroscopy (XPS) study of GO sheets annealed at various
reaction temperatures reveals that N-doping occurs at a temperature as low as
300C, while the highest doping level of ~5% N is achieved at 500C. N-doping is
accompanied by the reduction of GO with decreases in oxygen levels from ~28% in
as-made GO down to ~2% in 1100C NH3 reacted GO. XPS analysis of the N binding
configurations of doped GO finds pyridinic N in the doped samples, with
increased quaternary N (N that replaced the carbon atoms in the graphene plane)
in GO annealed at higher temperatures (>900C). Oxygen groups in GO were found
responsible for reactions with NH3 and C-N bond formation. Pre-reduced GO with
fewer oxygen groups by thermal annealing in H2 exhibits greatly reduced
reactivity with NH3 and lower N-doping level. Electrical measurements of
individual GO sheet devices demonstrate that GO annealed in NH3 exhibits higher
conductivity than those annealed in H2, suggesting more effective reduction of
GO by annealing in NH3 than in H2, consistent with XPS data. The N-doped
reduced GO shows clearly n-type electron doping behavior with Dirac point (DP)
at negative gate voltages in three terminal devices. Our method could lead to
the synthesis of bulk amounts of N-doped, reduced GO sheets useful for various
practical applications.
