Structural characterization of individual graphene sheets formed by arc discharge and their growth mechanisms

Abstract

Graphene sheets formed by arc discharge are hard to characterize in detail due to their complex pristine states in raw soot, which always exhibit an overall morphology of overlapping aggregation, together with other carbonaceous by-products. Here we used an improved arc method and simple separation procedure to obtain a large number of individual graphene sheets with single- to few-layers, and further probed their structural details using optical microscopy, transmission electron microscopy (TEM), atomic force microscopy and Raman spectroscopy. By TEM characterization, two major types of graphene sheets are shown; one is observed with folded fringes and polycrystalline structure, whereas the other is with an even graphene plane and single crystalline structure. In contrast to that of supported graphene, the Raman spectra of these graphene sheets show some different characteristics such as opposite shift of G band frequency as the layers increase. With increasing layers, the frequencies of G and G′ bands and the full width at half maximum (FWHM) of the G′ band totally exhibit layer-dependence. According to the FWHM of G′ bands, the folding within graphene sheets is also discussed. In addition, the defect types for arc graphene are analysed based on the D and D′ bands. Our results suggest that the D bands of such graphene sheets result from edges, rather than topological defects or disorder. Based on the findings, a new growth mechanism of arc graphene is proposed rationally responsible for the difference of two types of graphene sheets.