Etching mechanisms of graphene nanoribbons in downstream H2 plasmas: insights from molecular dynamics simulations

Lateral etching mechanisms of graphene nanoribbons (GNRs) with zigzag (ZZ) edges in downstream H-2 plasmas are investigated using molecular dynamics simulations. A new etching mechanism is found, which occurs in three consecutive phases and requires a continuous exposure of GNRs to H atoms and high substrate temperatures (similar to 800 K). Full hydrogenation of GNR free edges during phase 1 reduces the potential barriers to H chemisorption on near-edge C atoms from the basal plane. Subsequent hydrogenation of near-edge C-C dimers creates mechanical stress between C atoms (due to local sp(2)-to-sp(3) rehybridizations) which leads to the rupture of C-C dimers bonds, unzipping locally the 1st and 2nd edge carbon rows. The unzipping then propagates randomly along the GNR edges and creates suspended linear carbon chains (phase 2). Weakened by their exposure to continuous H bombardment and strong thermal vibrations, the suspended carbon chains may then rupture, leading to the sputtering of their carbon atoms as single C atoms or C-2 molecules (phase 3). Thus no formation of volatile hydrocarbon etching products is observed in this three-phase mechanism, which explains why the ribbon edges can be sharp-cut without generation of line-edge roughness, as also observed experimentally. Influence of substrate temperature on ZZ-GNRs etching is investigated and suggests the dominant mechanisms for understanding the temperature dependence of the etch rate observed experimentally (peaks at 800 K and decreases for lower or higher temperatures).