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.