The origin of wrinkles on transferred graphene

When two-dimensional graphene is exfoliated from three-dimensional highly oriented pyrolytic graphite (HOPG), ripples or corrugations always exist due to the intrinsic thermal fluctuations. Surface-grown graphenes also exhibit wrinkles, which are larger in dimension and are thought to be caused by the difference in thermal expansion coefficients between graphene and the underlying substrate in the cooling process after high temperature growth. For further characterization and applications, it is necessary to transfer the surface-grown graphenes onto dielectric substrates, and other wrinkles are generated during this process. Here, we focus on the wrinkles of transferred graphene and demonstrate that the surface morphology of the growth substrate is the origin of the new wrinkles which arise in the surface-to-surface transfer process; we call these morphology-induced wrinkles. Based on a careful statistical analysis of thousands of atomic force microscopy (AFM) topographic data, we have concluded that these wrinkles on transferred few-layer graphene (typically 1–3 layers) are determined by both the growth substrate morphology and the transfer process. Depending on the transfer medium and conditions, most of the wrinkles can be either erased or preserved. Our work suggests a new route for graphene engineering involving structuring the growth substrate and tailoring the transfer process.      

Different growth behaviors of ambient pressure chemical vapor deposition graphene on Ni(111) and Ni films: A scanning tunneling microscopy study

Graphene growth on the same metal substrate with different crystal morphologies, such as single crystalline and polycrystalline, may involve different mechanisms. We deal with this issue by preparing graphene on single crystal Ni(111) and on ∼300 nm thick Ni films on SiO₂ using an ambient pressure chemical vapor deposition (APCVD) method, and analyze the different growth behaviors for different growth parameters by atomically-resolved scanning tunneling microscopy (STM) and complementary macroscopic analysis methods. Interestingly, we obtained monolayer graphene on Ni(111), and multilayer graphene on Ni films under the same growth conditions. Based on the experimental results, it is proposed that the graphene growth on Ni(111) is strongly templated by the Ni(111) lattice due to the strong Ni-C interactions, leading to monolayer graphene growth. Multilayer graphene flakes formed on polycrystalline Ni films are usually stacked with deviations from the Bernal stacking type and show small rotations among the carbon layers. Considering the different substrate features, the inevitable grain boundaries on polycrystalline Ni films are considered to serve as the growth fronts for bilayer and even multilayer graphene.      

Raman scattering study of the phonon dispersion in twisted bilayer graphene

Bilayer graphene with a twist angle θ between the layers generates a superlattice structure known as a Moiré pattern. This superlattice provides a θ-dependent q wavevector that activates phonons in the interior of the Brillouin zone. Here we show that this superlattice-induced Raman scattering can be used to probe the phonon dispersion in twisted bilayer graphene (tBLG). The effect reported here is different from the widely studied double-resonance in graphene-related materials in many aspects, and despite the absence of stacking order in tBLG, layer breathing vibrations (namely ZO’ phonons) are observed.      

Is graphene aromatic?

We analyze the chemical bonding in graphene using a fragmental approach, the adaptive natural density partitioning method, electron sharing indices, and nucleus-independent chemical shift indices. We prove that graphene is aromatic, but its aromaticity is different from the aromaticity in benzene, coronene, or circumcoronene. Aromaticity in graphene is local with two π-electrons delocalized over every hexagon ring. We believe that the chemical bonding picture developed for graphene will be helpful for understanding chemical bonding in defects such as point defects, single-, double-, and multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities, and new materials that are derivatives of graphene.      

Synthesis of large-area, few-layer graphene on iron foil by chemical vapor deposition

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).      

Graphene-based conducting inks for direct inkjet printing of flexible conductive patterns and their applications in electric circuits and chemical sensors

A series of inkjet printing processes have been studied using graphene-based inks. Under optimized conditions, using water-soluble single-layered graphene oxide (GO) and few-layered graphene oxide (FGO), various high image quality patterns could be printed on diverse flexible substrates, including paper, poly(ethylene terephthalate) (PET) and polyimide (PI), with a simple and low-cost inkjet printing technique. The graphene-based patterns printed on plastic substrates demonstrated a high electrical conductivity after thermal reduction, and more importantly, they retained the same conductivity over severe bending cycles. Accordingly, flexible electric circuits and a hydrogen peroxide chemical sensor were fabricated and showed excellent performances, demonstrating the applications of this simple and practical inkjet printing technique using graphene inks. The results show that graphene materials-which can be easily produced on a large scale and possess outstanding electronic properties-have great potential for the convenient fabrication of flexible and low-cost graphene-based electronic devices, by using a simple inkjet printing technique.      

Production of graphene nanospheres by annealing of graphene oxide in solution

We report a simple method to produce graphene nanospheres (GNSs) by annealing graphene oxide (GO) solution at high-temperature with the assistance of sparks induced by the microwave absorption of graphite flakes dispersed in the solution. The GNSs were formed by rolling up of the annealed GO, and the diameters were mostly in the range 300–700 nm. The GNS exhibited a hollow sphere structure surrounded by graphene walls with a basal spacing of 0.34 nm. Raman spectroscopy and X-ray photoelectron spectroscopy of the GNSs confirmed that the GO was efficiently reduced during the fabrication process. The resulting GNSs may open up new opportunities both for fundamental research and applications, and this method may be extended to the synthesis of other nanomaterials and the fabrication of related nanostructures.      

Convective assembly of linear gold nanoparticle arrays at the micron scale for surface enhanced Raman scattering

A convective assembly technique at the micron scale analogous to the writing action of a “pipette pen” has been developed for the linear assembly of gold nanoparticle strips with micron scale width and millimeter scale length for surface enhanced Raman scattering (SERS). The arrays with interparticle gaps smaller than 3 nm are hexagonally stacked in the vicinity of the pipette tip. Variable numbers of stacked layers and clean surfaces of the assembled nanoparticles are obtained by optimizing the velocity of the pipette tip. The SERS properties of the assembled nanoparticle arrays rely on their stacking number and surface cleanliness.      

<Emphasis Type="Italic">In situ</Emphasis> imaging of layer-by-layer sublimation of suspended graphene

An individual suspended graphene sheet was connected to a scanning tunneling microscopy probe inside a transmission electron microscope, and Joule heated to high temperatures. At high temperatures and under electron beam irradiation, the few-layer graphene sheets were removed layer-by-layer in the viewing area until a monolayer graphene was formed. The layer-by-layer peeling was initiated at vacancies in individual graphene layers. The vacancies expanded to form nanometer-sized holes, which then grew along the perimeter and propagated to both the top and bottom layers of a bilayer graphene joined by a bilayer edge. The layer-by-layer peeling was induced by atom sublimation caused by Joule heating and facilitated by atom displacement caused by high-energy electron irradiation, and may be harnessed to control the layer thickness of graphene for device applications.      

One-step electrochemical approach to the synthesis of Graphene/MnO2 nanowall hybrids

We have demonstrated a one-step and effective electrochemical method to synthesize graphene/MnO₂ nanowall hybrids (GMHs). Graphene oxide (GO) was electrochemically reduced to graphene (GN), accompanied by the simultaneous formation of MnO₂ with a nanowall morphology via cathodic electrochemical deposition. The morphology and structure of the GMHs were systematically characterized by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. The resulting GMHs combine the advantages of GN and the nanowall array morphology of MnO₂ in providing a conductive network of amorphous nanocomposite, which shows good electrochemical capacitive behavior. This simple approach should find practical applications in the large-scale production of GMHs.      

Solvothermal-assisted exfoliation process to produce graphene with high yield and high quality

Monolayer and bilayer graphene sheets have been produced by a solvothermal-assisted exfoliation process in a highly polar organic solvent, acetonitrile, using expanded graphite (EG) as the starting material. It is proposed that the dipole-induced dipole interactions between graphene and acetonitrile facilitate the exfoliation and dispersion of graphene. The facile and effective solvothermal-assisted exfoliation process raises the low yield of graphene reported in previous syntheses to 10 wt%–12 wt%. By means of centrifugation at 2000 rpm for 90 min, monolayer and bilayer graphene were separated effectively without the need to add a stabilizer or modifier. Electron diffraction and Raman spectroscopy indicate that the resulting graphene sheets are high quality products without any significant structural defects.      

Sodium citrate: A universal reducing agent for reduction / decoration of graphene oxide with au nanoparticles

A facile method is proposed for the synthesis of reduced graphene oxide nanosheets (RGONS) and Au nanoparticle-reduced graphene oxide nanosheet (Au-RGONS) hybrid materials, using graphene oxide (GO) as precursor and sodium citrate as reductant and stabilizer. The resulting RGONS and Au-RGONS hybrid materials were characterized by UV-vis spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, atomic force microscopy, transmission electron microscopy, and X-ray diffraction. It was found that the RGONS and Au-RGONS hybrid materials formed stable colloidal dispersions through hydrogen bonds between the residual oxygen-containing functionalities on the surface of RGONS and the hydroxyl/carboxyl groups of sodium citrate. The electrochemical responses of RGONS and Au-RGONS hybrid material-modified glassy carbon electrodes (GCE) to three kinds of biomolecules were investigated, and all of them showed a remarkable increase in electrochemical performance relative to a bare GCE.      

Graphene-like bilayer hexagonal silicon polymorph

We present molecular dynamics simulation evidence for a freezing transition from liquid silicon to quasi-two-dimensional (quasi-2D) bilayer silicon in a slit nanopore. This new quasi-2D polymorph of silicon exhibits a bilayer hexagonal structure in which the covalent coordination number of every silicon atom is four. Quantum molecular dynamics simulations show that the stand-alone bilayer silicon (without the confinement) is still stable at 400 K. Electronic band-structure calculations suggest that the bilayer hexagonal silicon is a quasi-2D semimetal, similar to a graphene monolayer, but with an indirect zero band gap.      

Sequential assembly of metal-free phthalocyanine on few-layer epitaxial graphene mediated by thickness-dependent surface potential

Due to strong interactions between epitaxial graphene and SiC(0001) substrates, the overlayer charge density induced by the interface charging effect is much more attenuated than that of exfoliated graphene on SiO₂. We report herein a quantitive detection of the charge properties of few-layer graphene by surface potential measurements using electrostatic force microscopy (EFM). A minor difference in surface potential is observed to mediate a sequential assembly of metal-free phthalocyanine (H₂Pc) on monolayer, bilayer and trilayer graphenes, as demonstrated by scanning tunneling microscopy (STM). In order to understand this, we further executed density functional theory (DFT) calculations which showed higher adsorption energies for Pc on thinner graphenes. In this case, we attribute the unique growth behavior of Pc to its variable adsorption energies on few-layer graphene, and in turn the layer charge variations from the viewpoint of energy minimizations. This work is expected to provide fundamental data useful for related nanodevice fabrications.      

Fabrication of a low defect density graphene-nickel hydroxide nanosheet hybrid with enhanced electrochemical performance

The development of efficient energy storage devices with high capacity and excellent stability is a demanding necessary to satisfy future societal and environmental needs. A hybrid material composed of low defect density graphene-supported Ni(OH)₂ sheets has been fabricated via a soft chemistry route and investigated as an advanced electrochemical pseudocapacitor material. The low defect density graphene effectively prevents the restacking of Ni(OH)₂ nanosheets as well as boosting the conductivity of the hybrid electrodes, giving a dramatic rise in capacity performance of the overall system. Moreover, graphene simultaneously acts as both nucleation center and template for the in situ growth of smooth and large scale Ni(OH)₂ nanosheets. By virtue of the unique two-dimensional nanostructure of graphene, the as-obtained Ni(OH)₂ sheets are closely protected by graphene, effectively suppressing their microstructural degradation during the charge and discharge processes, enabling an enhancement in cycling capability. Electrochemical measurements demonstrated that the specific capacitance of the as-obtained composite is high as 1162.7 F/g at a scan rate of 5 mV/s and 1087.9 F/g at a current density of 1.5 A/g. In addition, there was no marked decrease in capacitance at a current density of 10·A/g after 2000 cycles, suggesting excellent long-term cycling stability.      

Assessment of high-frequency performance limits of graphene field-effect transistors

High frequency performance limits of graphene field-effect transistors (FETs) down to a channel length of 20 nm have been examined by using self-consistent quantum simulations. The results indicate that although Klein band-to-band tunneling is significant for sub-100 nm graphene FETs, it is possible to achieve a good transconductance and ballistic on-off ratio larger than 3 even at a channel length of 20 nm. At a channel length of 20 nm, the intrinsic cut-off frequency remains at a few THz for various gate insulator thickness values, but a thin gate insulator is necessary for a good transconductance and smaller degradation of cut-off frequency in the presence of parasitic capacitance. The intrinsic cut-off frequency is close to the LC characteristic frequency set by graphene kinetic inductance (L) and quantum capacitance (C), which is about 100 GHz·μm divided by the gate length.      

Advanced asymmetrical supercapacitors based on graphene hybrid materials

Supercapacitors operating in aqueous solutions are low cost energy storage devices with high cycling stability and fast charging and discharging capabilities, but generally suffer from low energy densities. Here, we grow Ni(OH)₂ nanoplates and RuO₂ nanoparticles on high quality graphene sheets in order to maximize the specific capacitances of these materials. We then pair up a Ni(OH)₂/graphene electrode with a RuO₂/graphene electrode to afford a high performance asymmetrical supercapacitor with high energy and power density operating in aqueous solutions at a voltage of ∼1.5 V. The asymmetrical supercapacitor exhibits significantly higher energy densities than symmetrical RuO₂-RuO₂ supercapacitors or asymmetrical supercapacitors based on either RuO₂-carbon or Ni(OH)₂-carbon electrode pairs. A high energy density of ∼48 W·h/kg at a power density of ∼0.23 kW/kg, and a high power density of ∼21 kW/kg at an energy density of ∼14 W·h/kg have been achieved with our Ni(OH)2/graphene and RuO₂/graphene asymmetrical supercapacitor. Thus, pairing up metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for asymmetrical supercapacitors represents a new approach to high performance energy storage.      

Chemical versus thermal folding of graphene edges

Using molecular dynamics (MD) simulations, we have investigated the kinetics of the graphene edge folding process. The lower limit of the energy barrier is found to be ∼380 meV/? (or about 800 meV per edge atom) and ∼50 meV/? (or about 120 meV per edge atom) for folding the edges of intrinsic clean single-layer graphene (SLG) and double-layer graphene (DLG), respectively. However, the edge folding barriers can be substantially reduced by imbalanced chemical adsorption, such as of H atoms, on the two sides of graphene along the edges. Our studies indicate that thermal folding is not feasible at room temperature (RT) for clean SLG and DLG edges and is feasible at high temperature only for DLG edges, whereas chemical folding (with adsorbates) of both SLG and DLG edges can be spontaneous at RT. These findings suggest that the folded edge structures of suspended graphene observed in some experiments are possibly due to the presence of adsorbates at the edges.      

Polarized light microscopy and spectroscopy of individual single-walled carbon nanotubes

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures.      

Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites

We have demonstrated a facile and efficient strategy for the fabrication of soluble reduced graphene oxide sheets (RGO) and the preparation of titanium oxide (TiO₂) nanoparticle-RGO composites using a modified one-step hydrothermal method. It was found that graphene oxide could be easily reduced under solvothermal conditions with ascorbic acid as reductant, with concomitant growth of TiO₂ particles on the RGO surface. The TiO₂-RGO composite has been thoroughly characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy, atomic force microscopy, and transmission electron microscopy) have been employed to probe the morphological characteristics as well as to investigate the exfoliation of RGO sheets. The TiO₂-RGO composite exhibited excellent photocatalysis of hydrogen evolution.