<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.      

Computational model of edge effects in graphene nanoribbon transistors

We present a semi-analytical model incorporating the effects of edge bond relaxation, the third nearest neighbor interactions, and edge scattering in graphene nanoribbon field-effect transistors (GNRFETs) with armchair-edge GNR (AGNR) channels. Unlike carbon nanotubes (CNTs) which do not have edges, the existence of edges in the AGNRs has a significant effect on the quantum capacitance and ballistic I-V characteristics of GNRFETs. For an AGNR with an index of m=3p, the band gap decreases and the ON current increases whereas for an AGNR with an index of m=3p+1, the quantum capacitance increases and the ON current decreases. The effect of edge scattering, which reduces the ON current, is also included in the model. This article is published with open access at Springerlink.com     

Strain effects in graphene and graphene nanoribbons: The underlying mechanism

A tight-binding analytic framework is combined with first-principles calculations to reveal the mechanism underlying the strain effects on electronic structures of graphene and graphene nanoribbons (GNRs). It provides a unified and precise formulation of the strain effects under various circumstances-including the shift of the Fermi (Dirac) points, the change in band gap of armchair GNRs with uniaxial strain in a zigzag pattern and its insensitivity to shear strain, and the variation of the k-range of edge states in zigzag GNRs under uniaxial and shear strains which determine the gap behavior via the spin polarization interaction.      

Recovery of edge states of graphene nanoislands on an iridium substrate by silicon intercalation

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate.We used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

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Vapour-phase graphene epitaxy at low temperatures

We report an epitaxial growth of graphene, including homo- and hetero-epitaxy on graphite and SiC substrates, at a temperature as low as ∼540 °C. This vapour-phase epitaxial growth, carried out in a remote plasma-enhanced chemical vapor deposition (RPECVD) system using methane as the carbon source, can yield large-area high-quality graphene with the desired number of layers over the entire substrate surfaces following an AB-stacking layer-by-layer growth model. We also developed a facile transfer method to transfer a typical continuous one layer epitaxial graphene with second layer graphene islands on top of the first layer with the coverage of the second layer graphene islands being 20% (1.2 layer epitaxial graphene) from a SiC substrate onto SiO₂ and measured the resistivity, carrier density and mobility. Our work provides a new strategy toward the growth of graphene and broadens its prospects of application in future electronics.      

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.      

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.      

Inorganic nanoribbons with unpassivated zigzag edges: Half metallicity and edge reconstruction

We have investigated the electronic and structural properties of inorganic nanoribbons (BN, AlN, GaN, SiC, and ZnO) with unpassivated zigzag edges using density functional theory calculations. We find that, in general, the unpassivated zigzag edges can lead to spin-splitting of energy bands. More interestingly, the inorganic nanoribbons AlN and SiC with either one or two edges unpassivated are predicted to be half metallic. Possible structural reconstruction at the unpassivated edges and its effect on the electronic properties are investigated. The unpassivated N edge in the BN nanoribbon and P edge in the AlP nanoribbon are energetically less stable than the corresponding reconstructed edge. Hence, edge reconstruction at the two edges may occur at high temperatures. Other unpassivated edges of the inorganic nanoribbons considered in this study are all robust against edge reconstruction.      

Band gap of strained graphene nanoribbons

The band structures of strained graphene nanoribbons (GNRs) are examined using a tight-binding Hamiltonian that is directly related to the type and magnitude of strain. Compared to a two-dimensional graphene whose band gap remains close to zero even if a large strain is applied, the band gap of a graphene nanoribbon (GNR) is sensitive to both uniaxial and shear strains. The effect of strain on the electronic structure of a GNR depends strongly on its edge shape and structural indices. For an armchair GNR, a weak uniaxial strain changes the band gap in a linear fashion, whereas a large strain results in periodic oscillation of the band gap. On the other hand, shear strain always tends to reduce the band gap. For a zigzag GNR, the effect of strain is to change the spin polarization at the edges of GNR, and thereby modulate the band gap. A simple analytical model, which agrees with the numerical results, is proposed to interpret the response of the band gap to strain in armchair GNRs.      

Edge stability of boron nitride nanoribbons and its application in designing hybrid BNC structures

First-principles investigations of the edge energies and edge stresses of single-layer hexagonal boron nitride (BN) are presented. The armchair edges of BN nanoribbons (BNNRs) are more stable in energy than zigzag ones. Armchair BNNRs are under compressive edge stress while zigzag BNNRs are under tensile edge stress, due to the edge reconstruction effect and edge coulomb repulsion effect. The intrinsic spin-polarization and edge saturation play important roles in modulating the edge stability of BNNRs. The edge energy difference between BN and graphene can be used to guide the design of specific hybrid BNC structures as the hybrid BNC systems prefer the low-energy edge configurations: In an armchair BNC nanoribbon (BNCNR), BN domains are expected to grow outside of C domains, while the opposite occurs in a zigzag BNCNR. More importantly, armchair BNCNRs can reproduce unique electronic properties of armchair graphene nanoribbons (GNRs), which are expected to be robust against edge functionalization or disorder. Within a certain range of C/BN ratios, zigzag BNCNRs may exhibit intrinsic half-metallicity without any external constraints. These diverse electronic properties of BNCNRs may offer unique opportunities to develop nanoscale electronics and spintronics beyond individual graphene and BN. More generally, these principles for designing BNC can also be extended to other hybrid nanostructures.      

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.      

Fabrication of high-quality all-graphene devices with low contact resistances

All-graphene devices are new class of graphene devices with simple layouts and low contact resistances. Here we report a clean fabrication strategy for all-graphene devices via a defect-assisted anisotropic etching. The as-fabricated graphene is free of contamination and retains the quality of pristine graphene. The contact resistance at room temperature (RT) between a bilayer graphene channel and a multilayer graphene electrode can be as low as ～5 Ω·μm, the lowest ever achieved experimentally. Our results suggest the feasibility of employing such all-graphene devices in high performance carbon-based integrated circuits.      

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.      

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.      

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).      

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.      

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.      

Monolayer graphene as a saturable absorber in a mode-locked laser

We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers when compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, or functionalized graphene. Monolayer graphene has a remarkably large modulation depth of 65.9%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picosecond ultrafast laser pulses (1.23 ps) can be generated using monolayer graphene as a saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability, and output energy.      

Raman spectroscopy and imaging of graphene

Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications. Here we review recent results on the Raman spectroscopy and imaging of graphene. We show that Raman spectroscopy and imaging can be used as a quick and unambiguous method to determine the number of graphene layers. The strong Raman signal of single layer graphene compared to graphite is explained by an interference enhancement model. We have also studied the effect of substrates, the top layer deposition, the annealing process, as well as folding (stacking order) on the physical and electronic properties of graphene. Finally, Raman spectroscopy of epitaxial graphene grown on a SiC substrate is presented and strong compressive strain on epitaxial graphene is observed. The results presented here are highly relevant to the application of graphene in nano-electronic devices and help in developing a better understanding of the physical and electronic properties of graphene. This article is published with open access at Springerlink.com     

A facile tool for the characterization of two-dimensional materials grown by chemical vapor deposition

The metrology of two-dimensional (2D) materials such as graphene, boron nitride or molybdenum disulfide grown by chemical vapor deposition (CVD) is critical for the optimization of their synthesis. We demonstrate the use of film-induced frustrated etching (FIFE) as a facile, scalable method to reveal and quantify structural defects in continuous thin sheets. The sensitivity of the analysis technique to intentionally induced lattice defects in graphene compares favorably to the sensitivity of Raman spectroscopy. A strong correlation between the measured defectiveness and the maximum carrier mobility in graphene emphasizes the importance of the technique for growth optimization. Due to its ease and widespread availability, we anticipate that FIFE will find wide application in the characterization of CVD-synthesized 2D materials.