▎ 摘　　要

Nitrogen content in graphene influences application performance. Although many studies have been conducted on single-process nitrogen (N)-doping, dopant content is still quite low. Therefore, the objective of this study was to develop a novel two-step economical doping technique for improved nitrogen content and device performance. First, graphene oxide (GO) was hydrothermally nitrogen-doped. This nitrogen-doped reduced graphene oxide (NrGO) was then subjected to secondary plasma treatment. Two nitrogen(N)-doping mechanisms were observed depending on graphene's oxygen functionalized group (OFG) content. OFG assisted N-doping mechanism was dominant if graphene had a high OFG content. Despite the presence of oxygen in plasma, significant OFG reduction and a higher degree of N -doping were observed. On the other hand, if graphene had a low OFG content, activated radicals in graphene structure by plasma ion bombardment mainly influenced secondary N-doping. Here, oxygen contaminants in the plasma stream showed dual effects. Low-power conditions showed no significant effect, whereas high-power plasma influenced re-oxidation and suppressed further N-doping. The maximum N/C ratio of 0.168 was observed in 5 W plasma-treated NrGO material, which had nitrogen content over 40%. Compared to a single-step N-doping technique, this new technique can facilitate dominant n-type transistors with strong oxygen reduction characteristics. (C) 2022 Vietnam National University, Hanoi. Published by Elsevier B.V.