Time-dependent evolution of the nitrogen configurations in N-doped graphene films

Large-area time-controlled N-doped graphene films were grown on a Cu foil using an ammonia-assisted atmospheric pressure chemical vapour deposition (APCVD) technique. The films were characterized using optical microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy was used to verify the doping level and lattice distortion in the graphene films, while the degree of N-doping (N/C at%) and nitrogen configuration were studied by XPS. The results showed that both total nitrogen content and configurations were strongly dependent on the growth time. Notably, at short growth time (2 min) pyridinic-rich films with high oxygen content (similar to 47.02%) were produced, and the total N-content reached a maximum of 4.68%. Interestingly longer growth times (20 min) also resulted in pyridinic-rich films, however both the nitrogen and oxygen content were at lower values of 2.84% and similar to 26.07%, respectively. With increasing growth time, Raman spectra showed a decreasing doping level as seen by the decreasing I-D/I-G ratio values (1.2 to 0.9). Additionally, Raman peaks exhibited a systematic blue shift due to the compressive strain on the C-C bonds during the incorporation of N atoms into the graphene lattice. The study presents an in-depth understanding of how exposure time of N-dopants influences the bonding states of nitrogen atoms to carbon atoms, thereby dictating the resulting type of N-configurations as well as the overall nitrogen content.