Extensively encountered in radiation damage studies for its role in fission and fusion reactors, sp2-carbon has gained a surge of interest this last decade owing to the exfoliation of monolayer graphene films in 2004. Nowadays, despite the absence of a large-scale production method, the synthesis of polycrystalline monolayer graphene is well-controlled using chemical vapour deposition. It has enabled numerous studies of ion-induced damage in 2D materials. Recently, a peculiar phenomenon has been evidenced in CVD-grown sheets under low-ion energy argon plasma irradiation: the self-healing of the graphene grain boundaries (GBs) [1]. While this has been theorized [2] and experimentally demonstrated [3] in 3D materials, this is the first occurrence of such process in 2D materials.

This seminar introduces the discovery of this phenomenon in graphene using a new hyperspectral Raman spectroscopy system [4]. The plasma environment and its 12-eV argon ion irradiation lead to a high density of Frenkel pairs during the treatment. The resilience of the GBs, evidenced by the Raman mappings (Fig. 1 [1]), is explained by their role as defect sinks which results in preferential adatom-vacancy annihilation in their vicinity. The phenomenon is further described as a function of the different plasma irradiation conditions.

In the last part of the talk, a few interesting features arising from the graphene GB resilience under plasma irradiation are discussed. For example, this phenomenon leads to the formation of hexagonal nano-pyramids during argon plasma treatment of graphite substrates (Fig. 2 [5]). Whilst adatoms/interstitials instigate the synthesis of these new structures, they might also feed the formation of the loops zipping the basal planes observed along the pyramidal facets (Fig. 3 [5]). Additionally, the boundaries plays an important role in graphene functionalization: one may observe different doping behaviours when treated in nitrogen or diborane plasmas; N-incorporation is inhibited at GBs in the former while B-incorporation is favoured at the domain boundaries in the latter.

• [1] Glad & Vinchon et al. (2020) “Self-Healing at Grain boundaries in plasma-treated graphene” currently under major revision in Nature Materials
• [2] Bai et al. (2010) “Efficient annealing of radiation damage near grain boundaries via interstitial emission” Science 327
• [3] Zhang et al. (2018) “Radiation damage in nanostructured materials” Progress in Materials Science 96
• [4] Gaufrès et al. (2018) “Hyperspectral Raman imaging using Bragg tunable filters of graphene and other low‐dimensional materials” Journal of Raman Spectroscopy 49
• [5] Glad et al. (2014) “Plasma synthesis of hexagonal-pyramidal graphite hillocks” Carbon 76