Raman-assessed structural evolution of as-deposited few-layer graphene by He/H2 arc discharge during rapid-cooling thinning treatment

We report a novel method to reduce layer number and structural defects of few-layer graphene (FLG) in He/H2 arc discharge via rapid cooling treatment. By adjusting the parameters of He/H2 mixtures appropriately, the effects of incremental cooling rates on the formation, morphology and structural defects of the raw soot have been investigated. According to observations of transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM), such FLG affected by rapid cooling tends to be thinner than untreated counterpart. TEM and HRTEM observations show that such sample contains a large amount of FLG with a fraction of single-layer graphene (SLG). AFM results show the thickness of the thinnest flake in the soot equals 0.5 nm, indicating the presence of SLG. With the increasing of cooling rate, Raman-based estimations not only show the reduction of defect density by calculating the nanocrystallite size La, but confirm the thinning trend for layer number via Raman-assessed layer number. The possible reason for thinning effect is discussed on both aspects of crystal growth and fluid dynamics.