Article

Uniform hexagonal graphene flakes and films grown on liquid copper surface

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 04/2012; 109(21):7992-6. DOI: 10.1073/pnas.1200339109
Source: PubMed

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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    • "They showed that the continuous films are formed by connecting randomly oriented, irregular-shaped, and micrometre-sized graphene flakes. Employing a liquid matrix has been proposed as effective means for controlling the nucleation process in CVD graphene production [11]. It completely removes the grain boundaries in polycrystalline solid, allowing a uniform distribution of graphene nucleation sites, and enabling self-assembly of graphene flakes into compact and ordered structures. "
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    • "Our results are consistent with earlier reports indicating that preheating and annealing of Cu in presence of H 2 not only reduces surface irregularities (e.g. surface bumps and striation marks) [11] [17], but also promotes formation "
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    ABSTRACT: Understanding the mechanism of graphene synthesis by chemical vapor deposition and the effect of process parameters is critical for production of high-quality graphene. In the present work, we investigated the effect of H 2 concentration during annealing on evolution of Cu surface morphology, and on deposited graphene characteristics. Our results revealed that H 2 had a smoothening effect on Cu surface as its surface roughness was reduced significantly at high H 2 concentration along with the formation of surface facets, dents and nanometer-sized particles. Furthermore, H 2 content influenced the graphene morphology and its quality. A low H 2 concentration (0% and 2.5%) during annealing promoted uniform and good quality bilayer graphene. In contrast, a high concentration of H 2 (20% and 50%) resulted in multilayer, non-uniform and defective graphene. Interestingly, the annealed Cu surface morphology differed considerably from that obtained after deposition of graphene, indicating that graphene deposition has its own impact on Cu surface.
    Full-text · Article · Jul 2015 · Carbon
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    • "The continuity of polycrystalline graphene and the minimization of its grain boundaries (GBs) remains a big challenge towards its integration into large-scale applications [2] [3] [4] [5]. "
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    ABSTRACT: The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220 nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB-stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, as well as peak intensities depend on the width of the overlap. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped regions via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4 eV below the charge neutrality point. Our results suggest an enhancement of electrical conductivity across overlapped grain boundaries, similar to previously observed measurements (Tsen et al., 2012). The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity.
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