Band And Bonding Characteristics Of N-2(+) Ion-Doped Graphene

We report that the doping of energetic nitrogen cations (N-2(+)) on graphene effectively controls the local N-C bonding structures and the pi-band of graphene critically depending on ion energy E-k (100 eV <= E-k <= 500 eV) by using a combined study of photoemission spectroscopy and density functional theory (DFT) calculations. With increasing E-k, we find a phase transformation of the N-C bonding structures from a graphitic phase where nitrogen substitutes carbon to a pyridinic phase where nitrogen loses one of its bonding arms, with a critical energy E-k(c) = 100 eV that separates the two phases. The N-2(+)-induced changes in the pi-band with varying E-k indicate an n-doping effect in the graphitic phase for E-k < E-k(c) but a p-doping effect for the pyridinic graphene for E-k > E-k(c). We further show that one may control the electron charge density of graphene by two orders of magnitude by varying E-k of N-2(+) ions within the energy range adopted. Our DFT-based band calculations reproduce the distinct doping effects observed in the pi-band of the N-2(+)-doped graphene and provide an orbital origin of the different doping types. We thus demonstrate that the doping type and electron number density in the N-2(+) ion-doped SLG can be artificially fine-controlled by adjusting the kinetic energy of incoming N-2(+) ions.