Density functional theory (DFT) calculations were performed to study doping of two nitrogen atoms at different positions on a
finite-sized graphene model of C82H24.We examined 21 structures of double nitrogen doped graphene to calculate their relative
stabilities. The structure with two nitrogen atoms located apart is the most stable among the positional isomers considered in this
study. For double nitrogen doping within a six-membered ring, the 1,4-position is more preferred than 1,3- or 1,2-positions for the
finite-sized single layer graphene sheet. Our computational study supports the experimental observation of two nitrogen atoms at
the 1,3- and 1,4-positions in a single six-membered ring of graphene. Furthermore, the structures with N-N bond are the least
stable among two nitrogen doped graphene structures. The effects of nitrogen doping and the positions of two nitrogen atoms on
the HOMO-LUMO energy gap of pristine graphene were analyzed.

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