Dechao Geng

Uniform hexagonal graphene flakes and films grown on liquid copper surface

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 μm2), 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.

Oxygen-Aided Synthesis of Polycrystalline Graphene on Silicon Dioxide Substrates

We report the metal-catalyst-free synthesis of high-quality polycrystalline graphene on dielectric substrates [silicon dioxide (SiO(2)) or quartz] using an oxygen-aided chemical vapor deposition (CVD) process. The growth was carried out using a CVD system at atmospheric pressure. After high-temperature activation of the growth substrates in air, high-quality polycrystalline graphene is subsequently grown on SiO(2) by utilizing the oxygen-based nucleation sites. The growth mechanism is analogous to that of growth for single-walled carbon nanotubes. Graphene-modified SiO(2) substrates can be directly used in transparent conducting films and field-effect devices. The carrier mobilities are about 531 cm(2) V(-1) s(-1) in air and 472 cm(2) V(-1) s(-1) in N(2), which are close to that of metal-catalyzed polycrystalline graphene. The method avoids the need for either a metal catalyst or a complicated and skilled postgrowth transfer process and is compatible with current silicon processing techniques.

Low Temperature Growth of Highly Nitrogen-Doped Single Crystal Graphene Arrays by Chemical Vapor Deposition

The ability to dope graphene is highly important for modulating electrical properties of graphene. However, the current route for the synthesis of N-doped graphene by chemical vapor deposition (CVD) method mainly involves high growth temperature using ammonia gas or solid reagent melamine as nitrogen sources, leading to graphene with low doping level, polycrystalline nature, high defect density and low carrier mobility. Here, we demonstrate a self-assembly approach that allows the synthesis of single-layer, single crystal and highly nitrogen-doped graphene domain arrays by self-organization of pyridine molecules on Cu surface at temperature as low as 300 °C. These N-doped graphene domains have a dominated geometric structure of tetragonal-shape, reflecting the single crystal nature confirmed by electron-diffraction measurements. The electrical measurements of these graphene domains showed their high carrier mobility, high doping level, and reliable N-doped behavior in both air and vacuum.

Equiangular Hexagon‐Shape‐Controlled Synthesis of Graphene on Copper Surface

The electric properties and device performance are strongly dependent on the size, shape, crystallinity, layer numbers, and edge structures of pristine graphene. In general, imperfection in these parameters leads to undesired scattering of charge carriers that compromise the high intrinsic mobility of graphene. Controlling these parameters of graphene in synthesis or post-synthesis manipulation is thus critical to achieve tunable properties and optimized device performance. Post-synthesis methods including anisotropic etching, [ 3 , 4 ] scanning probebased lithography [ 5 ] and electron-beam induced edge reorganization of graphene [ 6 ] provide some levels of control on graphene geometric parameters. However, direct growth of graphene with controllable shape and edges by chemical vapor deposition (CVD) [ 7–10 ] or epitaxial growth on metal surfaces [ 11 , 12 ] has met with limited success. Here we report a large scale synthesis of equiangular hexagon-shaped single or multilayer graphene by methane CVD on Cu surface at ambient pressure. The shape refl ects the hexagonal graphene lattice, possessing either zigzag or armchair edges. The hexagon-shaped graphene shows no observable defects confi rmed by Raman spectra, and is formed by nucleation and growth mechanism, thus allowing control of both density and size. Moreover, the shape evolution follows an empirical rule that higher CH 4 fl ow rate leads to shorter nucleation time, higher growth rates and larger deviations from equiangular hexagon shape. Based on these observations, we proposed a growth model that qualitatively establishes a connection between various experimental conditions and the fi nal state of the grown graphene, and is in principle capable of predicting the results from different conditions in the Cu-methane CVD system. Moreover, this system provides direct evidence of layer spatial arrangement in the case of multi-layer graphene

Fractal etching of graphene.

An anisotropic etching mode is commonly known for perfect crystalline materials, generally leading to simple Euclidean geometric patterns. This principle has also proved to apply to the etching of the thinnest crystalline material, graphene, resulting in hexagonal holes with zigzag edge structures. Here we demonstrate for the first time that the graphene etching mode can deviate significantly from simple anisotropic etching. Using an as-grown graphene film on a liquid copper surface as a model system, we show that the etched graphene pattern can be modulated from a simple hexagonal pattern to complex fractal geometric patterns with sixfold symmetry by varying the Ar/H2 flow rate ratio. The etched fractal patterns are formed by the repeated construction of a basic identical motif, and the physical origin of the pattern formation is consistent with a diffusion-controlled process. The fractal etching mode of graphene presents an intriguing case for the fundamental study of material etching.

Near‐Equilibrium Chemical Vapor Deposition of High‐Quality Single‐Crystal Graphene Directly on Various Dielectric Substrates

By using near-equilibrium chemical vapor deposition, it is demonstrated that high-quality single-crystal graphene can be grown on dielectric substrates. The maximum size is about 11 μm. The carrier mobility can reach about 5650 cm(2) V(-1) s(-1) , which is comparable to those of some metal-catalyzed graphene crystals, reflecting the good quality of the graphene lattice.

Two‐Stage Metal‐Catalyst‐Free Growth of High‐Quality Polycrystalline Graphene Films on Silicon Nitride Substrates

By using two-stage, metal-catalyst-free chemical vapor deposition (CVD), it is demonstrated that high-quality polycrystalline graphene films can directly grow on silicon nitride substrates. The carrier mobility can reach about 1500 cm(2) V(-1) s(-1) , which is about three times the value of those grown on SiO(2) /Si substrates, and also is better than some examples of metal-catalyzed graphene, reflecting the good quality of the graphene lattice.

Graphene Single Crystals: Size and Morphology Engineering

Recently developed chemical vapor deposition (CVD) is considered as an effective way to large-area and high-quality graphene preparation due to its ultra-low cost, high controllability, and high scalability. However, CVD-grown graphene film is polycrystalline, and composed of numerous grains separated by grain boundaries, which are detrimental to graphene-based electronics. Intensive investigations have been inspired on the controlled growth of graphene single crystals with the absence of intrinsic defects. As the two most concerned parameters, the size and morphology serve critical roles in affecting properties and understanding the growth mechanism of graphene crystals. Therefore, a precise tuning of the size and morphology will be of great significance in scale-up graphene production and wide applications. Here, recent advances in the synthesis of graphene single crystals on both metals and dielectric substrates by the CVD method are discussed. The review mainly covers the size and morphology engineering of graphene single crystals. Furthermore, recent progress in the growth mechanism and device applications of graphene single crystals are presented. Finally, the opportunities and challenges are discussed.

Hierarchy of graphene wrinkles induced by thermal strain engineering

Here, we study hierarchy of graphene wrinkles induced by thermal strain engineering and demonstrate that the wrinkling hierarchy can be accounted for by the wrinklon theory. We derive an equation λu2009=u2009(ky)0.5, explaining evolution of wrinkling wavelength λ with the distance to the edge y observed in our experiment by considering both bending energy and stretching energy of the graphene flakes. The prefactor k in the equation is determined to be about 55u2009nm. Our experimental result indicates that the classical membrane behavior of graphene persists down to about 100u2009nm of the wrinkling wavelength.

Gram‐Scale Synthesis of Graphene Sheets by a Catalytic Arc‐Discharge Method

Flake graphite is used as carbon source and ZnO or ZnS as catalyst in the synthesis of high-quality graphene sheets. A catalytic growth mechanism for cathode-part graphene synthesis in the arc-discharge apparatus and an exfoliation mechanism for wall-part graphene synthesis are introduced. N-doped cathode-part graphene and undoped wall-part graphene are formed simultaneously.