  Catalyst-free vertically aligned graphene nanoflakes possessing a large
amount of high density edge planes were functionalized using nitrogen species
in a low energy N+ ion bombardment process to achieve pyridinic, cyanide and
nitrogen substitution in hexagonal graphitic coordinated units. The evolution
of the electronic structure of the functionalized graphene nanoflakes over the
temperature range 20-800^{\circ}C was investigated in situ, using high
resolution x-ray photoemission spectroscopy. We demonstrate that low energy
irradiation is a useful tool for achieving nitrogen doping levels up to 9.6
at.%. Pyridinic configurations are found to be predominant at room temperature,
while at 800^{\circ}C graphitic nitrogen configurations become the dominant
ones. The findings have helped to provide an understanding of the thermal
stability of nitrogen functionalities in graphene, and offer prospects for
controllable tuning of nitrogen doping in device applications.
