Nitrogen implantation of suspended graphene flakes: Annealing effects and selectivity of sp2 nitrogen species

Nitrogen inclusion in both chemical vapour deposition and exfoliated few-layer graphene flakes was performed by nitrogen ion implantation in ultra-high vacuum. Inclusion of up to ∼20 at.% nitrogen can be reached through this clean technique with absence of oxygen species in the final product, while maintaining a largely sp2-carbon network. The nitrogen inclusion was observed by scanning X-ray photoelectron microscopy (SPEM) with energy resolution of 0.2 eV and spatial resolution of 10 nm. SPEM can be used to follow the evolution of nitrogen species: pyridinic, graphitic, and pyrrolic, at different doping stages and annealing temperatures. Variations in the ratio between sp2 nitrogen species was observed for increasing treatment time; annealing results in quenching of the sp3 component, suggesting the graphitic nitrogen as the most thermal stable species. The occurrence of graphitic species together with the absence of pyrrolic is indicative of N-incorporation into a hexagonal graphene-based lattice. Ion irradiation followed by annealing performed in a controlled way is a promising strategy to fine control the nature of the nitrogen species grafted to the graphene while focusing on selected applications.