Treatment of graphene films in the early and late afterglows of N₂ plasmas: comparison of the defect generation and N-incorporation dynamics

Abstract

Graphene films grown on copper substrate by chemical vapor deposition were exposed to the flowing afterglow of a reduced-pressure N 2 plasma sustained by microwave electromagnetic fields (surface-wave plasma). Two set of conditions were examined by controlling the gas flow rate: the late afterglow (LA) characterized by a high number densities of reactive N atoms and the early afterglow (EA) in which significant populations of metastable N 2 (A) states and positive ions (N 2 + and N 4 +) coexist with plasma-generated N atoms. LA treatments of graphene films show monotonous and steady incorporation of nitrogen atoms along with very low damage. However, given the very mild LA treatment conditions, a large part of the N atoms remains weakly bonded to the graphene surface; a feature ascribed to the plasma-induced functionalization of airborne hydrocarbon contaminants. In such conditions, graphitic inclusion of plasma-generated N atoms is limited to native defect sites. On the other hand, the presence of highly energetic species in the EA induces significant damage combined with much higher N-incorporation. Detailed Raman analysis of EA-treated samples further reveals a transition from vacancy-type defects to much larger multi-vacancies with increasing treatment time. This complete set of data indicates that through a judicious control of the populations of reactive N atoms, metastable N 2 (A) states, and positive ions (N 2 + and N 4 + ), the flowing afterglow of microwave N 2 plasmas represents a highly promising tool for precise, post-growth tuning of the defect generation and N-incorporation dynamics in graphene films.

Plasma-assisted nitrogen fixation in nanomaterials: fabrication, characterization, and application

“…It was shown that a large proportion of the N atoms remained weakly bonded by the late afterglow treatment, while significant damage together with much higher N incorporation in graphene films were observed when exposed to the early afterglow plasma treatment. Therefore, the judicious control of nitrogen species is a promising tool for the precise tuning of the defect generation and for N incorporation in graphene films [77]. Aizawa et al designed a low-temperature plasma nitriding system to improve the wear resistance and corrosion toughness of micro-nozzles and micro-springs, as shown in figure 7(a) [78].…”

Plasma–graphene interactions: combined effects of positive ions, vacuum-ultraviolet photons, and metastable species

“…nanotubes [2,3], graphene [3][4][5], cosmic dust analogues [6], vertically-aligned nanostructures [7][8][9][10][11], etc. The intrinsic properties of the plasma state further allow for numerous applications in post-growth processing of nanomaterials, for example of monolayer graphene films [12][13][14][15]. Such a vast area of applications in materials processing is made possible by the versatility of plasma discharges and the numerous energetic species found within the gas phase, such as positive ions, electrons, photons, radicals and long-lived metastable states.…”