Significant enhancement of blue emission and electrical conductivity of N-doped graphene

Abstract

Nitrogen (N)-doped graphene with different atomic percentages (2.3–4.7 at%) of N has been synthesized by thermal annealing of reduced graphene oxide (RGO) in ammonia gas for different times. The effects of annealing time on the structure, electrical and optical properties of N-doped graphene have been systematically investigated by using various analytical techniques. XPS, FTIR, Raman, and XRD studies show that there is a gradual structural change in N-doped graphene sheets with increasing annealing time, resulting from the increase of carbon and simultaneous decrease of oxygen and N contents. High resolution N1s spectra reveal that the pyridine-N and pyrrolic-N contents decrease with increasing annealing time, whereas the amount of quaternary-N increases. Importantly, it has been found that the annealing time caused significant changes in both the electrical and the optical properties of N-doped graphene. The electrical resistance of N-doped graphene is greatly reduced compared to that of GO and RGO, and found to further decrease with increasing annealing time, possibly due to the increase of sp² carbon networks and decrease of oxygen content as well as defects associated with the incorporation of N. The room-temperature photoluminescence (PL) properties of graphene oxide (GO), RGO and N-doped graphene were systematically studied with regard to the annealing time. The results showed that the PL spectrum of GO exhibits a peak emission maximum at around 700 nm, while that of RGO is found to be strongly blue-shifted with two distinct emission peaks: green emission at 485–500 nm and blue emission at 420–428 nm. For N-doped graphene samples, the blue emission intensity could significantly be enhanced by controlling the annealing time, which leads to a promising blue and green light-emitting material with controllable optical properties.