  A significant advance toward achieving practical applications of graphene as
a two-dimensional material in nanoelectronics would be provided by successful
synthesis of both n-type and p-type doped graphene. However reliable doping and
a thorough understanding of carrier transport in the presence of charged
impurities governed by ionized donors or acceptors in the graphene lattice are
still lacking. Here we report experimental realization of few-layer
nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of
organic molecule 1, 3, 5-triazine on Cu metal catalyst. By reducing the growth
temperature, the atomic percentage of nitrogen doping is raised from 2.1 % to
5.6 %. With increasing doping concentration, N-doped graphene sheet exhibits a
crossover from p-type to n-type behavior accompanied by a strong enhancement of
electron-hole transport asymmetry, manifesting the influence of incorporated
nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron
spectroscopy, Raman spectroscopy, and electrical measurements, we show that
pyridinic and pyrrolic N impurities play an important role in determining the
transport behavior of carriers in N-doped graphene sheets.
