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.      

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.      

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.      

Facile synthesis and application of Ag-chemically converted graphene nanocomposite

An in situ chemical synthesis approach has been employed to prepare an Ag-chemically converted graphene (CCG) nanocomposite. The reduction of graphene oxide sheets was accompanied by generation of Ag nanoparticles. The structure and composition of the nanocomposites were confirmed by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction. TEM and AFM results suggest a homogeneous distribution of Ag nanoparticles (5–10 nm in size) on CCG sheets. The intensities of the Raman signals of CCG in such nanocomposites are greatly increased by the attached silver nanoparticles, i.e., there is surface-enhanced Raman scattering activity. In addition, it was found that the antibacterial activity of free Ag nanoparticles is retained in the nanocomposites, which suggests they can be used as graphene-based biomaterials.      

A facile one-step synthesis of TiO2/graphene composites for photodegradation of methyl orange

TiO₂/graphene composite photocatalysts have been prepared by a simple liquid phase deposition method using titanium tetrafluoride and electron beam (EB) irradiation-pretreated graphene as the raw materials. The products were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The effects of varying the synthesis parameters such as graphene content, concentration of titanium tetrafluoride solution and irradiation dose were investigated. It was found that the preparation conditions had a significant effect on the structure and properties of the final products. The irradiated graphene was covered with petal-like anatase TiO₂ nanoparticles, which were more uniform and smaller in size than those in products synthesized without EB irradiation-pretreated graphene. The photocatalytic activities of the products were evaluated using the photocatalytic degradation of methyl orange as a probe reaction. The results showed that the products synthesized using EB irradiation-pretreated graphene exhibited higher photocatalytic activities than those using graphene without EB irradiation pretreatment.      

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.      

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.      

<Emphasis Type="Italic">In situ</Emphasis> etching for total control over axial and radial nanowire growth

We report a method using in situ etching to decouple the axial from the radial nanowire growth pathway, independent of other growth parameters. Thereby a wide range of growth parameters can be explored to improve the nanowire properties without concern of tapering or excess structural defects formed during radial growth. We demonstrate the method using etching by HCl during InP nanowire growth. The improved crystal quality of etched nanowires is indicated by strongly enhanced photoluminescence as compared to reference nanowires obtained without etching.      

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     

An exchange intercalation mechanism for the formation of a two-dimensional Si structure underneath graphene

A two-dimensional (2D) Si film can form between a graphene overlayer and a Ru(0001) substrate through an intercalation process. At the graphene/2D-Si/Ru(0001) surface, the topmost graphene layer is decoupled from the Ru substrate and becomes quasi-freestanding. The interfacial Si layers show high stability due to the protection from the graphene cover. Surface science measurements indicate that the surface Si atoms can penetrate through the graphene lattice, and density functional theory calculations suggest a Si-C exchange mechanism facilitates the penetration of Si at mild temperatures. The new mechanism may be involved for other elements on graphene, if they can bond strongly with carbon. This finding opens a new route to form 2D interfacial layers between graphene and substrates.      

Shape control of CoO and LiCoO<Subscript>2</Subscript> nanocrystals

Shape control of nanocrystals has become a significant subject in materials science. In this work, we describe a convenient way to achieve morphology-controllable synthesis of CoO nanocrystals including octahedrons and spheres as well as LiCoO₂ polyhedrons and spheres. In particular, we explain the formation of CoO octahedrons exposing only high-energy (111) facets using theoretical calculations; these should also be a useful tool for directing future face-controlled preparation of other nanocrystals. More importantly, the as-obtained LiCoO₂ nanocrystals showed different electrochemical performance depending on their morphology, indicating that Li-insertion/deintercalation dynamics might be crystal face-sensitive.      

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.      

Direct evidence for lip-lip interactions in multi-walled carbon nanotubes

The stability of open edged multi-walled carbon nanotubes has been investigated by using in situ high resolution transmission electron microscopy at elevated temperatures. Formation of inter-shell structures was experimentally observed for the first time and attributed to a robust interaction between adjacent concentric shells (so-called lip-lip interaction). The fl uctuating behavior of the inter-shell structures suggests a mechanism by which the carbon atoms can pass in or out through the inter-shell edges during carbon nanotube growth or shrinkage processes. This article is published with open access at Springerlink.com     

Holed nanostructures formed by aluminum droplets on a GaAs substrate

We have studied the morphology evolution of holed nanostructures formed by aluminum droplet epitaxy on a GaAs surface. Unique outer rings with concentric inner holed rings were observed. Further, an empirical equation to describe the size distribution of the outer rings in the holed nanostructures has been established. The contour line generated by the equation provides physical insights into quantum ring formation by droplets of group III materials on III–V substrates.      

Photoluminescence of CdSe nanowires grown with and without metal catalyst

We present temperature and power dependent photoluminescence measurements on CdSe nanowires synthesized via vapor-phase with and without the use of a metal catalyst. Nanowires produced without a catalyst can be optimized to yield higher quantum efficiency, and narrower and spatially uniform emission, when compared to the catalyst-assisted ones. Emission at energies lower than the band-edge is also found in both cases. By combining spatially-resolved photoluminescence and electron microscopy on the same nanowires, we show that catalyst-free nanowires exhibit a low-energy peak with sharp phonon replica, whereas for catalyst-assisted nanowires low-energy emission is linked to the presence of nanostructures with extended morphological defects.      

Microwave synthesis of large few-layer graphene sheets in aqueous solution of ammonia

Few-layer graphene (FLG) sheets with sizes exceeding several micrometers have been synthesized by exfoliation of expanded graphite in aqueous solution of ammonia under microwave irradiation, with an overall yield approaching 8 wt.%. Transmission electron microscopy (in bright-field and dark-field modes) together with electron diffraction patterns and atomic force microscopy confirmed that this graphene material consisted mostly of mono-, bi- or few-layer graphene (less than ten layers). The high degree of surface reduction was confirmed by X-ray photoelectron and infrared spectroscopies. In addition, the high stability of the FLG in the liquid medium facilitates the deposition of the graphene material onto several substrates via low-cost solution-phase processing techniques, opening the way to subsequent applications of the material.      

Projected performance advantage of multilayer graphene nanoribbons as a transistor channel material

The performance limits of a multilayer graphene nanoribbon (GNR) field-effect transistor (FET) are assessed and compared with those of a monolayer GNRFET and a carbon nanotube (CNT) FET. The results show that with a thin high dielectric constant (high-κ) gate insulator and reduced interlayer coupling, a multilayer GNRFET can significantly outperform its CNT counterpart with a similar gate and bandgap in terms of the ballistic on-current. In the presence of optical phonon scattering, which has a short mean free path in the graphene-derived nanostructures, the advantage of the multilayer GNRFET is even more significant. Simulation results indicate that multilayer GNRs with incommensurate non-AB stacking and weak interlayer coupling are the best candidates for high-performance GNRFETs.      

Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanostructures: A new photocatalyst

Uniform colloidal Bi₂S₃ nanodots and nanorods with different sizes have been prepared in a controllable manner via a hot injection method. X-ray diffraction (XRD) results show that the resulting nanocrystals have an orthorhombic structure. Both the diameter and length of the nanorods increase with increasing concentration of the precursors. All of the prepared Bi₂S₃ nanostructures show high efficiency in the photodegradation of rhodamine B, especially in the case of small sized nanodots—which is possibly due to their high surface area. The dynamics of the photocatalysis is also discussed.      

Field effects on the statistical behavior of the molecular conductance in a single molecular junction in aqueous solution

We have combined molecular dynamics simulations with first-principles calculations to study electron transport in a single molecular junction of perylene tetracarboxylic diimide (PTCDI) in aqueous solution under external electric gate fields. It is found that the statistics of the molecular conductance are very sensitive to the strength of the electric field. The statistics of the molecular conductance are strongly associated with the thermal fluctuation of the water molecules around the PTCDI molecule. Our simulations reproduce the experimentally observed three orders of magnitude enhancement of the conductance, as well as the temperature dependent conductance, under the electrochemical gates. The effects of the molecular polarization and the dipole rearrangement of the aqueous solution are also discussed.      

Shedding light on the crystallographic etching of multi-layer graphene at the atomic scale

The controlled etching of graphite and graphene by catalytic hydrogenation is potentially a key engineering route for the fabrication of graphene nanoribbons with atomic precision. The hydrogenation mechanism, though, remains poorly understood. In this study we exploit the benefits of aberration-corrected high-resolution transmission electron microscopy to gain insight to the hydrogenation reaction. The etch tracks are found to be commensurate with the graphite lattice. Catalyst particles at the head of an etch channel are shown to be faceted and the angles between facets are multiples of 30°. Thus, the angles between facets are also commensurate with the graphite lattice. In addition, the results of a post-annealing step suggest that all catalyst particles—even if they are not involved in etching—are actively forming methane during the hydrogenation reaction. Furthermore, the data point against carbon dissolution being a key mechanism during the hydrogenation process.