Stability of pyridine-like and pyridinium-like nitrogen in graphene

Identification of N configurations in N-doped graphene is important to improve its functionalities. In most experiments, graphite-like, pyridine-like, and pyrrole-like N was usually chosen to assign the spectral features of N-doped graphene. Based on the first-principles simulations, we find that since one of nitrogen's electrons occupies the LUMO state, stability of pyridinium-like N (pyridine-like N with hydrogen termination) in multi-benzenes systems increases with the HOMO-LUMO gap decreasing. Therefore, unlike benzene with a large HOMO-LUMO gap, pyridinium-like N is a stable configuration in graphene with zero band gap, and the pyridinium-like N should be considered during the assignments of N configurations even under the environments with higher temperatures and lower hydrogen partial pressure than ambient conditions.