  It was recently shown that nitrogen-doped graphene (NG) can exhibit both p-
and n-type characters depending on the C-N bonding nature, which represents a
significant bottleneck for the development of graphene-based electronics. Based
on first-principles calculations, we herein scrutinise the correlations between
atomic and electronic structures of NG, and particularly explore the
feasibility of converting p-type NG with pyridinic, pyrrolic, and nitrilic N
into n- or bipolar type by introducing an additional dopant atom. Out of the
nine candidates, B, C, O, F, Al, Si, P, S, and Cl, we find that the B-, Al-,
and P-codoping can anneal even relatively large vacancy defects in p-type NG.
It will be also shown that, while the NG with pyridinic N can be converted into
n-type via codoping, only the bipolar type conversion can be achieved for the
NG with nitrilic or pyrrolic N. The amount of work function reduction was up to
0.64 eV for the pyridinic N next to a monovacancy. The atomistic origin of such
diverse type changes is analysed based on Mulliken and crystal orbital
Hamiltonian populations, which provide us with a framework to connect the local
bonding chemistry with macroscopic electronic structure in doped and/or
defective graphene. Moreover, we demonstrate that the proposed codoping scheme
can recover the excellent charge transport properties of pristine graphene.
Both the type conversion and conductance recovery in codoped NG should have
significant implications for the electronic and energy device applications.
