Efficient synthesis of graphene nanoribbons sonochemically cut from graphene sheets

We report a facile approach to synthesize narrow and long graphene nanoribbons (GNRs) by sonochemically cutting chemically derived graphene sheets (GSs). The yield of GNRs can reach ∼5 wt% of the starting GSs. The resulting GNRs are several micrometers in length, with ∼75% being single-layer, and ∼40% being narrower than 20 nm in width. A chemical tailoring mechanism involving oxygen-unzipping of GSs under sonochemical conditions is proposed on the basis of experimental observations and previously reported theoretical calculations; it is suggested that the formation and distribution of line faults on graphite oxide and GSs play crucial roles in the formation of GNRs. These results open up the possibilities of the large-scale synthesis and various technological applications of GNRs.      

Chemical self-assembly of graphene sheets

Chemically derived and noncovalently functionalized graphene sheets (GS) were found to self-assemble onto patterned gold structures via electrostatic interactions between the functional groups and the gold surfaces. This afforded regular arrays of single graphene sheets on large substrates, which were characterized by scanning electron microscopy (SEM), Auger microscopy imaging, and Raman spectroscopy. This represents the first time that self-assembly has been used to produce on-substrate and fully-suspended graphene electrical devices. Molecular coatings on the GS were removed by high current “electrical annealing”, which restored the high electrical conductance and Dirac point of the GS. Molecular sensors for highly sensitive gas detection using the self-assembled GS devices are demonstrated. Electronic Supplementary Material  Further characterization of GS and self-assembled structures by AFM, Raman, and Auger can be found in the ESM with nine figures and one table which are available in the online version of this article at http://dx.doi.org/ and accessible free of charge.     

Field emission from vertical graphene sheets formed by screen-printing technique

Vertical graphene field emission cathodes were fabricated using screen-printing and the following selective photoetching techniques. Excellent field emission properties of the screen-printed vertical graphene film were obtained with low turn on field, high maximum current density, and large field enhancement factor. The organic layer anchoring the vertical graphene sheets acted as negative feedback layer, which contributed to the high uniformity and stability of the field emission device. The results pave the low-cost way for applications of graphenes in large-scale field emission devices.     

Substrate-free gas-phase synthesis of graphene sheets

We present a novel method for synthesizing graphene sheets in the gas phase using a substrate-free, atmospheric-pressure microwave plasma reactor. Graphene sheets were synthesized by passing liquid ethanol droplets into an argon plasma. The graphene sheets were characterized by transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, and electron diffraction. We prove that graphene can be created without three-dimensional materials or substrates and demonstrate a possible avenue to the large-scale synthesis of graphene.     

Vibrational characteristics of bilayer graphene sheets

Bilayer graphene sheets (BLGSs) are currently receiving increasing attention. In this paper, the vibration characteristics of BLGSs are investigated using analytical and atomistic finite element approaches. Various possible scenarios, namely different geometrical configuration (armchair and zigzag), boundary conditions, and aspect ratio are considered in the present study. The dynamic characteristics of BLGS studied have shown dependence on aspect ratio and the boundary conditions. The unique vibrational properties and large stiffness of BLGS identified in the present work make them suitable candidates for manufacturing nanosensors; electromechanical resonators also will aid the nanomaterials research community to design nanodevices.     

Vibrational analysis of single-layered graphene sheets

A molecular structural mechanics method has been implemented to investigate the vibrational behavior of single-layered graphene sheets. By adopting this approach, mode shapes and natural frequencies are obtained. Vibrational analysis is performed with different chirality and boundary conditions. Numerical results from the atomistic modeling are employed to develop predictive equations via a statistical nonlinear regression model. With the proposed equations, fundamental frequencies of single-layered graphene sheets with considered boundary conditions can be predicted within 3% difference with respect to the atomistic simulation.     

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.     

由聚丙烯腈基碳纤维制备石墨烯薄膜的探索研究

为了拓展生产石墨烯薄膜的方法,对不具有皮芯结构的T700和T300聚丙烯腈基碳纤维进行了超薄切片,分别制备了超薄切片试样BM1和BM2,借助于高分辫、XRD、拉曼光谱等表征方法对石墨烯薄膜进行了表征和研究.结果发现超薄切片石墨烯薄膜BM1和BM2相比较,BM1中碳原子网络石墨烯平面部分平直,部分扭曲,网平面上碳原子之间...     

Functionalized graphene sheets filled isotactic polypropylene nanocomposites

The graphene oxide sheets (GOs) reacted with 4,4′-diphenylmethane diisocyanate (MDI) and then stearic acid to form the functionalized graphene sheets (FGs), in order to improve their compatibility with isotactic polypropylene (iPP). The iPP incorporated with FGs were adequately mixed in a Haaker mixer and then compression molded to obtain the iPP/FGs nanocomposites. The crystallization, thermal stability and mechanical properties of the nanocomposites together with iPP/graphite sheets (Gs) and iPP/GOs composites were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and tensile test. The FGs achieved good dispersion with exfoliated and intercalated nanostructure and strong interfacial adhesion with iPP, which made the nanocomposites have a significant enhancement of thermal stability and mechanical properties at low FGs loadings.     

Highly conducting graphene sheets and Langmuir-Blodgett films

Graphene is an intriguing material with properties that are distinct from those of other graphitic systems. The first samples of pristine graphene were obtained by 'peeling off' and epitaxial growth. Recently, the chemical reduction of graphite oxide was used to produce covalently functionalized single-layer graphene oxide. However, chemical approaches for the large-scale production of highly conducting graphene sheets remain elusive. Here, we report that the exfoliation-reintercalation-expansion of graphite can produce high-quality single-layer graphene sheets stably suspended in organic solvents. The graphene sheets exhibit high electrical conductance at room and cryogenic temperatures. Large amounts of graphene sheets in organic solvents are made into large transparent conducting films by Langmuir-Blodgett assembly in a layer-by-layer manner. The chemically derived, high-quality graphene sheets could lead to future scalable graphene devices.     

Imaging mechanical vibrations in suspended graphene sheets

We carried out measurements on nanoelectromechanical systems based on multilayer graphene sheets suspended over trenches in silicon oxide. The motion of the suspended sheets was electrostatically driven at resonance using applied radio frequency voltages. The mechanical vibrations were detected using a novel form of scanning probe microscopy, which allowed identification and spatial imaging of the shape of the mechanical eigenmodes. In as many as half the resonators measured, we observed a new class of exotic nanoscale vibration eigenmodes not predicted by the elastic beam theory, where the amplitude of vibration is maximum at the free edges. By modeling the suspended sheets with the finite element method, these edge eigenmodes are shown to be the result of nonuniform stress with remarkably large magnitudes (up to 1.5 GPa). This nonuniform stress, which arises from the way graphene is prepared by pressing or rubbing bulk graphite against another surface, should be taken into account in future studies on electronic and mechanical properties of graphene.     

Configuration-sensitive molecular sensing on doped graphene sheets

We show by molecular dynamics simulations that configuration-sensitive molecular spectroscopy can be realized on optimally doped graphene sheets vibrated by an oscillatory electric field. High selectivity of the spectroscopy is achieved by maximizing Coulombic binding between the detected molecule and a specific nest, formed for this molecule on the graphene sheet by substituting selected carbon atoms with boron and nitrogen dopants. One can detect binding of different isomers to the nest from the frequency shifts of selected vibrational modes of the combined system. As an illustrative example, we simulate detection of hexanitrostilbene enantiomers in chiral nests formed on graphene.      

Free-standing graphene at atomic resolution

Research interest in graphene, a two-dimensional crystal consisting of a single atomic plane of carbon atoms, has been driven by its extraordinary properties, including charge carriers that mimic ultra-relativistic elementary particles. Moreover, graphene exhibits ballistic electron transport on the submicrometre scale, even at room temperature, which has allowed the demonstration of graphene-based field-effect transistors and the observation of a room-temperature quantum Hall effect. Here we confirm the presence of free-standing, single-layer graphene with directly interpretable atomic-resolution imaging combined with the spatially resolved study of both the pi --> pi* transition and the pi + sigma plasmon. We also present atomic-scale observations of the morphology of free-standing graphene and explore the role of microstructural peculiarities that affect the stability of the sheets. We also follow the evolution and interaction of point defects and suggest a mechanism by which they form ring defects.     

Hydrothermally grown ZnO nanostructures on few-layer graphene sheets

This study describes the hydrothermal growth of ZnO nanostructures on few-layer graphene sheets and their optical and structural properties. The ZnO nanostructures were grown on graphene sheets of a few layers thick (few-layer graphene) without a seed layer. By changing the hydrothermal growth parameters, including temperature, reagent concentration and pH value of the solution, we readily controlled the dimensions, density and morphology of the ZnO nanostructures. More importantly, single-crystalline ZnO nanostructures grew directly on graphene, as determined by transmission electron microscopy. In addition, from the photoluminescence and cathodoluminescence spectra, strong near-band-edge emission was observed without any deep-level emission, indicating that the ZnO nanostructures grown on few-layer graphene were of high optical quality.     

Coating graphene oxide sheets with luminescent rare-earth complexes

Rare-earth (RE) complexes can be easily adsorbed on the surface of graphene oxide sheets (GOSs) by a simple noncovalent approach. The resulting nanocomposite materials, RE complexes functionalized GOSs, have been comprehensively characterized via transmission electron microscopy, X-ray photoelectron spectroscopy, FT-IR spectra, X-ray diffraction, and thermogravimetric analysis. The luminescence properties of GOSs-RE were also investigated. Under the excitation of UV light, these samples display the characteristic emissions of Eu³⁺ ions. The photophysical investigations revealed that the presence of GOSs do not quench the lanthanide-centered luminescence stemming from the characteristic electronic transitions within the 4f shell of the Eu³⁺ ions. The GOSs–RE complex hybrids and their dispersion can emit bright red luminescence. Moreover, the presence of GOSs can elongate the fluorescence lifetime of the RE complexes to 983 μs. These properties make them useful in many practical fields, such as in the lighting and biological labeling.     

Centimeter-scale high-resolution metrology of entire CVD-grown graphene sheets

A high-throughput metrology method for measuring the thickness and uniformity of entire large-area chemical vapor deposition-grown graphene sheets on arbitrary substrates is demonstrated. This method utilizes the quenching of fluorescence by graphene via resonant energy transfer to increase the visibility of graphene on a glass substrate. Fluorescence quenching is visualized by spin-coating a solution of polymer mixed with fluorescent dye onto the graphene then viewing the sample under a fluorescence microscope. A large-area fluorescence montage image of the dyed graphene sample is collected and processed to identify the graphene and indicate the graphene layer thickness throughout the entire graphene sample. Using this metrology method, the effect of different transfer techniques on the quality of the graphene sheet is studied. It is shown that small-area characterization is insufficient to truly evaluate the effect of the transfer technique on the graphene sample. The results indicate that introducing a drop of acetone or liquid poly(methyl methacrylate) (PMMA) on top of the transfer PMMA layer before soaking the graphene sample in acetone improves the quality of the graphene dramatically over immediately soaking the graphene in acetone. This work introduces a new method for graphene quantification that can quickly and easily identify graphene layers in a large area on arbitrary substrates. This metrology technique is well suited for many industrial applications due to its repeatability and flexibility.     

Temperature-dependent elastic properties of single layer graphene sheets

Elastic properties of single layer graphene sheets (SLGSs) with different values of aspect ratio are presented by using molecular dynamics simulation. SLGSs subjected to uniaxial tension, shear load and transverse uniform pressure are simulated under temperature varying from 300 K to 700 K. Based on the classical plate theory, an individual orthotropic plate model is adopted for SLGSs. By direct measuring the bending deflections, the effective thickness of SLGSs is determined uniquely. It is found that SLGSs exhibit anisotropic, size-dependent and temperature-dependent properties. The results reveal that Young’s modulus decreases with increasing in temperature, whereas the shear modulus depends weakly on temperature change. The results also show that the effective thickness of zigzag sheets is larger than that of armchair sheets.     

Elastic properties of chemically derived single graphene sheets

The elastic modulus of freely suspended graphene monolayers, obtained via chemical reduction of graphene oxide, was determined through tip-induced deformation experiments. Despite their defect content, the single sheets exhibit an extraordinary stiffness ( E = 0.25 TPa) approaching that of pristine graphene, as well as a high flexibility which enables them to bend easily in their elastic regime. Built-in tensions are found to be significantly lower compared to mechanically exfoliated graphene. The high resilience of the sheets is demonstrated by their unaltered electrical conductivity after multiple deformations. The electrical conductivity of the sheets scales inversely with the elastic modulus, pointing toward a 2-fold role of the oxygen bridges, that is, to impart a bond reinforcement while at the same time impeding the charge transport.     

Static and dynamic response of sector-shaped graphene sheets

Nonlocal elasticity theory is presented for the free vibration and bending analysis of nano-scaled graphene sheets having a sector shape. An eight-node curvilinear domain is used for transformation of the governing equation of motion of sector graphene from physical region to computational region in conjunction with the Kirchhoff plate theory. The discrete singular convolution method is employed for numerical solutions of resulting nonlocal governing differential equations and related boundary conditions. Then, the effects of nonlocal parameters, mode numbers, sector angles, and radius ratios on the static and vibration results of nano-scaled sector-shaped graphene sheets are discussed.