Graphene unrolled from 'cup-stacked' carbon nanotubes

High quality graphene with a large area and smooth edges has been obtained by unrolling the so-called 'cup-stacked' carbon nanotubes (CSCNTs) by the solution-phase oxidation and reduction. Atomic force microscopy and transmission electron microscopy observations reveal that the obtained graphene layers can even have a size of 20 μm in width and 100 μm in length, much larger than that of graphene unzipped from multi-walled carbon nanotubes. The low ratio of the D to G band intensities (within the 0.15-0.20 range) in Raman spectra indicates high quality of the obtained graphene, when compared to other graphene produced by the solution-phase oxidation. A formation mechanism is suggested for the graphene unrolled from the CSCNTs, providing an insight into the real microstructure of the CSCNTs, which are essentially continuous graphene layers rolled along the tube axis, yielding a pseudo cup-stacked like structure.