Uniform hexagonal graphene flakes and films grown on liquid copper surface.
ABSTRACT Unresolved problems associated with the production of graphene materials include the need for greater control over layer number, crystallinity, size, edge structure and spatial orientation, and a better understanding of the underlying mechanisms. Here we report a chemical vapor deposition approach that allows the direct synthesis of uniform single-layered, large-size (up to 10,000 μm(2)), spatially self-aligned, and single-crystalline hexagonal graphene flakes (HGFs) and their continuous films on liquid Cu surfaces. Employing a liquid Cu surface completely eliminates the grain boundaries in solid polycrystalline Cu, resulting in a uniform nucleation distribution and low graphene nucleation density, but also enables self-assembly of HGFs into compact and ordered structures. These HGFs show an average two-dimensional resistivity of 609 ± 200 Ω and saturation current density of 0.96 ± 0.15 mA/μm, demonstrating their good conductivity and capability for carrying high current density.
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ABSTRACT: We investigate the effect of mechanical strain on graphene synthesized by chemical vapor deposition (CVD) transferred onto flexible polymer substrates by observing the change in the Raman spectrum and then compare this to the behavior of exfoliated graphene. Previous studies into the effect of strain on graphene have focused on mechanically exfoliated graphene, which consists of large single domains. However, for wide scale applications CVD produced films are more applicable, and these differ in morphology, instead consisting of a patchwork of smaller domains separated by domain boundaries. We find that under strain the Raman spectra of CVD graphene transferred onto a silicone elastomer exhibits unusual behavior, with the G and 2D band frequencies decreasing and increasing respectively with applied strain. This unusual Raman behavior is attributed to the presence of domain boundaries in polycrystalline graphene causing unexpected shifts in the electronic structure. This was confirmed by the lack of such behavior in mechanically exfoliated large domain graphene and also in large single-crystal graphene domains grown by CVD. Theoretical calculation of G band for a given large shear strain may explain the unexpected shifts while the shift of the Dirac points from the K point explain the conventional behavior of a 2D band under the strain.ACS Nano 10/2012; 6(11):10229. · 10.77 Impact Factor