Electron microscopy analyses of natural and highly oriented pyrolytic graphites and the mechanically exfoliated graphenes produced from them

The microstructure of natural graphite (NG) and highly ordered pyrolytic graphite (HOPG) blocks, and their mechanically exfoliated graphenes, prepared by only one stamping on a meshed Cu grid, was investigated using dual beam scanning electron microscope/focus ion beam (SEM/FIB) and transmission electron microscopy (TEM). Channeling contrast enhanced SEM/FIB images and cross section TEM micrographs showed that HOPGs have very shallow grains, less than 30 nm in thickness and 6–30 μm in size, while NGs have grains larger than mm in size with some folded layers. In order to understand the structure of exfoliated graphene, an observation along the z  -axis of lattice on suspended membranes was conducted. Through the aid of diffraction pattern (DP) simulations, demonstrating the dependency of the intensity ratio between {112¯0} and {11¯00} on the number of layers and stacking sequences, a suspended monolayer could be distinguished from a multilayer. With the observations of unfolded graphene edges, we found that the graphene edges always have a roughness below a nanometer despite their straight feature in a low magnification view and have no preferential direction. DP and HREM analyses of folded edges in NG membranes also revealed that the folding direction is arbitrary and graphite is mostly stacked with AB sequence.