[Show abstract][Hide abstract] ABSTRACT: Graphene free-standing film-like or paper-like materials have attracted great attention due to their intriguing electronic, optical and mechanical properties and potential application in chemical filters, molecular storage and supercapacitors. Although significant progress has been made in fabricating graphene films or paper, there is still no effective method targeting ultrathin free-standing graphene films (UFGFs). Here, we present a modified filtration assembly method to prepare these ultrathin films. With this approach, we have fabricated a series of ultrathin free-standing graphene oxide films and UFGFs, up to 40 mm in diameter, with controllable thickness from micrometre to nanoscale (approx. 40 nm) dimensions. This method can be easily scaled up and the films display excellent optical, electrical and electrochemical properties. The ability to produce UFGFs from graphene oxide with a scalable, low-cost approach should take us a step closer to real-world applications of graphene.
Preview · Article · Mar 2014 · Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences
[Show abstract][Hide abstract] ABSTRACT: Via a small-carbon-flow, long-time, near-equilibrium chemical vapor deposition method, single-crystal graphene grains larger than 10 μm are grown directly on various dielectric substrates, successfully practicing the Chinese saying of (slow work yields a fine product). The high mobility, exceeding 5000 cm(2) V(-1) s(-1) , is comparable to that of metal-catalyzed graphene. Further details can be found in the article by Y. Liu and co-workers on page 1348.
No preview · Article · Mar 2014 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: 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 cm2 V−1 s−1, which is comparable to those of some metal‐catalyzed graphene crystals, reflecting the good quality of the graphene lattice.
No preview · Article · Mar 2014 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: By using tapping-mode AFM with a biased tip, the type of material (e.g.,metallic, n-, or p-type semiconductor) can be distinguished. Taking p-doping of graphene as an example, Y. Q. Liu, H. J. Gao and co-workers, on page 7015, observe a series of false apparent heights of graphene flakes when applying a series of different voltages on the tip. However, the heights under negative bias are higher than those under positive bias at the same value. This is caused by an asymmetric polarization effect, which is related to the p-doping nature of graphene.
No preview · Article · Dec 2013 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Gram-scale amounts of high-quality graphene sheets are synthesized on page 1330 through an arc-discharge method. An arc, which is generated between two graphite rods when a high voltage is applied by Y. Liu and co-workers, can cause the graphite layers to separate into graphene sheets via a high-temperature exfoliation, or even broken into small clusters/atoms, which then form graphene sheets by catalytic growth, demonstrating that two mechanisms exist in an arc discharge process simultaneously.
[Show abstract][Hide abstract] ABSTRACT: An anisotropic etching mode is commonly known for perfect crystalline materials, generally leading to simple Euclid geometric patterns. The principle is also proved to apply to the etching of thinnest crystalline material, i.e., graphene, resulting in hexagonal holes with zigzag edge structures. Here we demonstrate for the first time that graphene etching mode can be significantly deviated from the simple anisotropic etching. By virtue of an as-grown graphene film on a liquid copper surface as a model system, we show that the etched graphene patterns can be modulated from simple hexagonal to complex fractal geometric patterns with a six-fold symmetry by varying the flow rate ratios of Ar/H2. The etched fractal patterns are formed by a 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 the material etching.
No preview · Article · Apr 2013 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Feb 2013 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: The phenomenon of ordered pattern formation is universal in nature but involves complex non-equilibrium processes that are highly important for both fundamental research and applied materials systems. Among countless pattern systems, a snowflake is possibly the most fascinating example offered by nature. Here, we report that single-layered and single-crystalline graphene flakes (GFs) with highly regular and hexagonal symmetric patterns can be grown on a liquid copper surface using a CH4 chemical vapor deposition (CVD) method. The different morphologies of these GFs can be precisely tailored by varying the composition of the inert gas/H2 carrier gas mixture used to produce the GFs, and the GF edges can be continuously tuned over the full spectrum from negative to zero to positive curvature in a controllable way. The family of GF crystal patterns is remarkably analogous to that of snowflakes, representing an ideal two-dimensional (2D) growth system. Pattern formations from compact to dendritic GFs can be explained by the continuous modulation of the competition between adatom diffusion along island edges or corners and surface diffusion processes.
[Show abstract][Hide abstract] ABSTRACT: Multilayer graphene tips are developed for conducting probe atomic force microscopy for the formation of molecular junctions. Molecular junctions using graphene tips show very small tip-to-tip variance, excellent operational stability, good endurance, and long shelf-life. These properties, together with high yield and the simple processing involved, suggest that commercial mass-production of graphene tips is viable for molecular electronic applications.
No preview · Article · Jul 2012 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: A reliable molecular junction with reproducible electronic behavior is fabricated by Y. Liu and co-workers on page 3482 using multilayer graphene-coated AFM tips. Molecular junctions using graphene tips show very small tip-to-tip variance, excellent operational stability, good endurance, and long shelf-life. These properties, together with a high yield and the simplicity of the process involved, suggest that mass production of graphene tips is viable for future practical molecular electronic and AFM-based applications.
No preview · Article · Jul 2012 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Jun 2012 · Journal of the American Chemical Society
[Show abstract][Hide abstract] 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.
Preview · Article · Apr 2012 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Efficient field-effect mobilities measured from both p- and n-type organic monolayer field-effect transistors that utilize graphene electrodes permit quantitative analysis of the role of the first layer. Information about the relationship between the kinetics of the diffusion process and film morphology of the first layer during the increasing substrate temperature, and correlations of field-effect mobilities of the first monolayer and the substrate temperature is obtained.
Full-text · Article · Mar 2012 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a simple and controllable way to synthesize large-area, few-layer graphene on iron substrates by an optimized chemical vapor deposition (CVD) method using a mixture of methane and hydrogen. Based on an analysis of the Fe-C phase diagram, a suitable procedure for the successful synthesis of graphene on Fe surfaces was designed. An appropriate temperature and cooling process were found to be very important in the synthesis of highly crystalline few-layer graphene. Graphene-based field-effect transistor (FET) devices were fabricated using the resulting few-layer graphene, and showed good quality with extracted mobilities of 300–1150 cm2/(V·s).
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Nov 2011 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Single-walled carbon nanotubes (SWCNTs) with high purity and very narrow diameter distribution have been synthesized using the dc arc-discharge method with Y–Ni alloy as catalyst and selenium (Se) as promoter. The SWCNTs show a very narrow diameter distribution mainly at about 1.5nm, and can further be readily purified up to >99% purity with traditional purification including HNO3 reflux and air oxidation. The key factor of the wetting effect of Se in the SWCNTs growth improvement process is proposed and discussed. Moreover, a new less-destructive purification method including electrolysis, air-oxidation and centrifugation has been introduced, and SWCNTs with semiconducting content up to 94% have been produced through density gradient ultracentrifugation method.
[Show abstract][Hide abstract] ABSTRACT: Controlling the densities of aligned single-walled carbon nanotube arrays (SWNTs) on ST-cut quartz is a critical step in various
applications of these materials. However the growth mechanism for tuning SWNT density using the chemical vapor deposition
(CVD) method is still not well understood, preventing the development of efficient ways to obtain the desired results. Here
we report a general “periodic” approach that achieves ultrahigh density modulation of SWNT arrays on ST-cut quartz substrates—with
densities increased by up to ∼60 times compared with conventional methods using the same catalyst densities—by varying the
CH4 gas “off” time. This approach is applicable to a wide range of initial catalyst densities, substrates, catalyst types and
growth conditions. We propose a general mechanism for the catalyst size-dependent nucleation of SWNTs associated with different
free carbon concentrations, which explains all the observations. Moreover, the validity of the model is supported by systematic
experiments involving the variation of key parameters in the “periodic” CVD approach.
[Show abstract][Hide abstract] ABSTRACT: Hexagonal graphene flakes: Equiangular hexagon-shaped graphene domains are synthesized by methane chemical vapor deposition on copper surface in a controlled manner. These graphene domains possess either a armchair or zigzag edge, and are formed via a nucleation and growth mechanism. The left image is a typical AFM image of one graphene flake, and the right image indicates the armchair or zigzag edge of hexagonal graphene.
No preview · Article · Aug 2011 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: A general and useful method has been developed to evaluate the metallic-to-semiconducting (M/S) ratio for separated single-walled carbon nanotubes (SWNTs). By virtue of measuring UV−vis−NIR spectra of a variety of solutions with different ratios of metallic-rich to semiconducting-rich SWNTs, the commercial IsoNanotubes samples as well as metallic-rich HiPCO SWNTs (HiPCO-M) separated by an Agarose gel method have been evaluated. Values of 99.5% metallic contents for IsoNanotubes-M, 98.9% semiconducting contents for IsoNanotubes-S, and 1.24 for the absorption coefficient of IsoNanotubes, whereas 80.4% metallic contents for HiPCO-M and 1.05 for the absorption coefficient of HiPCO SWNTs were obtained. This method does not need pure metallic (M-) or semiconducting (S-) SWNTs as references. Furthermore, we found that this method can also be applied to evaluate the M/S ratio for any SWNT samples.
No preview · Article · Jul 2010 · The Journal of Physical Chemistry C