An electrochemical route to graphene oxide

Large-scale graphene oxide (GO) with adjustable resistivity was synthesized from graphite via an electrochemical method using KCl solution as an effective electrolyte. During the exfoliation process, electrostatic force intercalates chloride ions between the expanded graphite layers on the anode. These chloride ions form small gas bubbles between the graphite layers in the electrochemical reaction. It is believed that the gas bubbles expand the gap between graphite sheets and produce a separating force between adjacent graphene layers. This separating force overcomes the Van der Waals force between adjacent sheets and exfoliates graphene layers from the starting graphite. Because the graphene is electrochemically oxidized by chorine during the exfoliation, the exfoliated GO sheets are hydrophilic and easily dispersed in the electrolyte solution. The GO solution prepared by the electrochemical exfoliation can be simply sprayed or spin-coated onto any substrate for device applications. The measured average thicknesses of a monolayer, bilayer, and trilayer exfoliated GO on SiO2 substrate were 1.9, 2.8, and 3.9 nm, respectively. It was observed that the measured resistance of the exfoliated GO sheets increases due to electrochemical oxidation in the solution. This electrochemical approach offers a low-cost and efficient route to the fabrication of graphene based devices.     

Spin in organics: a new route to spintronics

New developments in the nascent field of organic spintronics are discussed. Two classes of phenomena can be discerned. In hybrid organic spin valves (OSVs), an organic semiconducting film is sandwiched between two ferromagnetic (FM) thin films, aiming at magnetoresistive effects as a function of the relative alignment of the respective magnetization directions. Alternatively, organic magnetoresistance (OMAR) is achieved without any FM components, and is an intrinsic property of the organic semiconductor material. Some of the exciting characteristics of OMAR, in both electrical conductance and photoconductance, are presented. A systematic, combined experimental-theoretical study of sign changes between positive and negative magnetoresistance is shown to provide important insight about the underlying mechanisms of OMAR. A simple explanation of experimental observations is obtained by combining a 'spin-blocking' mechanism, an essential ingredient in the recently proposed bipolaron model, with specific features of the device physics of space charge limited current devices in the bipolar regime. Finally, we discuss possible links between the physics relevant for OMAR and that for OSVs. More specifically, weak hyperfine fields from the hydrogen atoms in organic materials are thought to be crucial for a proper understanding of both types of phenomena.     

A chemical route to graphene for device applications

Oxidation of graphite produces graphite oxide, which is dispersible in water as individual platelets. After deposition onto Si/SiO2 substrates, chemical reduction produces graphene sheets. Electrical conductivity measurements indicate a 10000-fold increase in conductivity after chemical reduction to graphene. Tapping mode atomic force microscopy measurements show one to two layer graphene steps. Electrodes patterned onto a reduced graphite oxide film demonstrate a field effect response when the gate voltage is varied from +15 to -15 V. Temperature-dependent conductivity indicates that the graphene-like sheets exhibit semiconducting behavior.     

The synthesis of graphene oxide nanostructures for supercapacitors: a simple route

In this work, we report a simple strategy for synthesis of graphene oxide nanostructures with various morphologies including single-, few-layer, and three-dimensional networks. Morphology control is achieved by adding different amounts of Ni²⁺ into a one-step hydrothermal process. The involved growth mechanisms for the morphology control are discussed. A random arrangement of graphene oxide nanosheets is suggested to induce the networks’ formation. Ni²⁺ facilitates the formation of graphene oxide’s preferential face-to-face overlapping structure, and high Ni²⁺ concentrations render adjacent graphene oxide sheets to combine each other tightly to form closely packed, layered structures. Compared with single-, few-layer graphene oxide, the electrode prepared by three-dimensional networks has a mass specific capacitance of 352 F g^?1 at v = 5 mV s^?1, which is much higher than that of recently reported three-dimensional graphene oxide nanostructures (240 F g^?1).     

Novel growth mechanism of epitaxial graphene on metals

Graphene, a hexagonal sheet of sp(2)-bonded carbon atoms, has extraordinary properties which hold immense promise for nanoelectronic applications. Unfortunately, the popular preparation methods of micromechanical cleavage and chemical exfoliation of graphite do not easily scale up for application purposes. Epitaxial graphene provides an attractive alternative, though there are many challenges, not least of which is the absence of any understanding of the complex atomistic assembly kinetics of graphene layers. Here, we present a simple rate theory of epitaxial graphene growth on close-packed metal surfaces. On the basis of recent low-energy electron-diffraction microscopy experiments, our theory supposes that graphene islands grow predominantly by the attachment of five-atom clusters. With optimized kinetic parameters, our theory produces a quantitative account of the measured time-dependent carbon adatom density. The temperature dependence of this density at the onset of nucleation leads us to predict that the smallest stable precursor to graphene growth is an immobile island composed of six five-atom clusters. This conclusion is supported by a recent study based on temperature-programmed growth of epitaxial graphene, which provides direct evidence of nanoclusters whose coarsening leads to the formation of graphene layers. Our findings should motivate additional high-resolution imaging experiments and more detailed simulations which will yield important input to developing strategies for the large-scale production of epitaxial graphene.     

Aryl functionalization as a route to band gap engineering in single layer graphene devices

Chemical functionalization is a promising route to band gap engineering of graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer graphene sheets in the form of substrate-supported or free-standing films. Our transport measurements demonstrate that nonsuspended functionalized graphene behaves as a granular metal, with variable range hopping transport and a mobility gap ～0.1 eV at low temperature. For suspended graphene that allows functionalization on both surfaces, we demonstrate tuning of its electronic properties from a granular metal to a semiconductor in which transport occurs via thermal activation over a transport gap ～80 meV from 4 to 300 K. This noninvasive and scalable functionalization technique paves the way for CMOS-compatible band gap engineering of graphene electronic devices.     

Impurity band Mott insulators: a new route to high Tc superconductivity

Last century witnessed the birth of semiconductor electronics and nanotechnology. The physics behind these revolutionary developments is certain quantum mechanical behaviour of ‘impurity state electrons’ in crystalline ‘band insulators’, such as Si, Ge, GaAs and GaN, arising from intentionally added (doped) impurities. The present article proposes that certain collective quantum behaviour of these impurity state electrons, arising from Coulomb repulsions, could lead to superconductivity in a parent band insulator, in a way not suspected before. Impurity band resonating valence bond theory of superconductivity in boron doped diamond, recently proposed by us, suggests possibility of superconductivity emerging from impurity band Mott insulators. We use certain key ideas and insights from the field of high-temperature superconductivity in cuprates and organics. Our suggestion also offers new possibilities in the field of semiconductor electronics and nanotechnology. The current level of sophistication in solid state technology and combinatorial materials science is very well capable of realizing our proposal and discover new superconductors.     

Impurity band Mott insulators: a new route to high Tc superconductivity

Abstract

Last century witnessed the birth of semiconductor electronics and nanotechnology. The physics behind these revolutionary developments is certain quantum mechanical behaviour of ‘impurity state electrons’ in crystalline ‘band insulators’, such as Si, Ge, GaAs and GaN, arising from intentionally added (doped) impurities. The present article proposes that certain collective quantum behaviour of these impurity state electrons, arising from Coulomb repulsions, could lead to superconductivity in a parent band insulator, in a way not suspected before. Impurity band resonating valence bond theory of superconductivity in boron doped diamond, recently proposed by us, suggests possibility of superconductivity emerging from impurity band Mott insulators. We use certain key ideas and insights from the field of high-temperature superconductivity in cuprates and organics. Our suggestion also offers new possibilities in the field of semiconductor electronics and nanotechnology. The current level of sophistication in solid state technology and combinatorial materials science is very well capable of realizing our proposal and discover new superconductors.     

Universal segregation growth approach to wafer-size graphene from non-noble metals

Graphene has been attracting wide interests owing to its excellent electronic, thermal, and mechanical performances. Despite the availability of several production techniques, it is still a great challenge to achieve wafer-size graphene with acceptable uniformity and low cost, which would determine the future of graphene electronics. Here we report a universal segregation growth technique for batch production of high-quality wafer-scale graphene from non-noble metal films. Without any extraneous carbon sources, 4 in. graphene wafers have been obtained from Ni, Co, Cu-Ni alloy, and so forth via thermal annealing with over 82% being 1-3 layers and excellent reproducibility. We demonstrate the first example of monolayer and bilayer graphene wafers using Cu-Ni alloy by combining the distinct segregation behaviors of Cu and Ni. Together with the easy detachment from growth substrates, we believe this facile segregation technique will offer a great driving force for graphene research.     

Fabrication of ridge waveguides: a new solgel route

Passive ridge waveguides can be deposited on silicon by a solvent-assisted lithographic process incorporating simple mask technology and photosensitive solgel-derived glasses. Thick films (~4 mum) are dip coated in one step, and channel waveguides and power splitters are imprinted in them by UV light through appropriate masks. Unexposed regions of the glass are removed by soaking of the films in n-propanol. The remaining ridges are then treated at 200 degrees C and planarized with a solgel cladding layer. Circular mode profiles are observed from ridge guides covered with the cladding. The waveguides are characterized with scanning electron microscopy, atomic force microscopy, surface profilometry, ellipsometry, and fiber end coupling. Overall, the procedure is simple and reproducible and leads to waveguides with low loss, of the order of 0.13 dB/cm.     

Localized electro-thermal processing: a new route to the patterning of magnetic recording media

Previous reports have detailed the fabrication of media able to support high density magnetic recording in both longitudinal and perpendicular formats by the global rapid thermal processing of sputtered non-magnetic precursor films. During processing in this manner a magnetic element is released from its nitride and agglomerates to form a random near mono-dispersion of magnetic nano-particles. Here we explore, primarily through modelling and simulation, the feasibility of processing similarly formulated precursor media not globally but locally. We investigate the potential of using conducting nano-probe tips to produce, via electro-thermal (Joule) heating, a nano-patterned recording medium in the form of regular arrays of magnetic islands in a non-magnetic host. In the first instance we concentrate on the simplest cobalt based precursor medium for which both initial simulation and experimental studies indicate the formation of magnetic islands with dimensions of the order of the tip diameter; this is relatively straightforward. The results signify that if practical production scenarios can be devised to produce technologically significant areas of recording media by the rapid multi-probe repetition of this technique, then processing in this manner offers a promising route to areal recording densities of perhaps 5 Terabit/in(2) even with the simplest cobalt media. We also note that the electro-thermal processing method is potentially extendable to the production of a wide variety of magnetic materials (e.g.?PtCo, FeCo, NiFe alloys) and, applied via electrical nano-imprinting type techniques, to the production of a wide variety of patterned structures.     

Structure and chemical reactivity of lithium-doped graphene on hydrogen-saturated silicon carbide

Herein, we studied the structure and hydrogenation of graphene supported on hydrogen-terminated SiC, with intercalated Li atoms. Strong nonbonded interactions occur between graphene and the Si–H groups. The latter were found to be significantly augmented by the intercalation of Li atoms, which enhanced the SiH::pi interactions. Although the electronic structure of graphene did not experience significant changes when supported on hydrogen-terminated SiC, a small gap of 0.04 eV was computed at the HSEH1PBE/6-31G* level. The clustering of Li atoms over graphene was prevented by the SiC support. A significant enhancement of reactivity could be corroborated due to the presence of intercalated Li atoms, given that the C–H binding energy was increased by almost 1 eV with respect to pristine graphene. We expect that hydrogen-terminated SiC with intercalated Li atoms can be used to enhance the chemical reactivity of graphene, given that it strongly interacts with graphene but does not compete for the electrons donated by Li.     

Spinodal decomposition of mono- to few-layer graphene on Ni substrates at low temperature

Mono to few-layer graphene were prepared on pre-annealed polycrystalline nickel substrates by chemical vapor deposition at a relatively low temperature of 800 degrees C using fast cooling rate. It was observed that the reduced solubility of Carbon in Ni at low temperature and an optimum gas mixing ratio (CH4:H2 = 60/80 (sccm)) can be used to synthesize mano-layer graphene that covers about 100 microm2 area. The number of graphene layers strongly depends upon the hydrogen and methane flow rates. An increase in the methane flow is found to increase the growth density of the single-layer graphene. The number of graphene layers was identified from micro-Raman spectra. The thinnest areas containing mono-layer graphene formed at small Ni grains surrounded by large Ni Grains can be explained in terms of Spinodal decomposition. Scanning tunneling microscopy observations of the graphene samples indicate that the graphene structure exhibits no defects, and extremely symmetry hexagon carbon at flat graphene surface is observed.     

A facile low temperature solvothermal route to copper monosulfide submicrotubes

The synthesis of copper monosulfide (CuS) submicrotubes under mild conditions has been achieved by two simple steps: firstly, a precursor (copper-thiosulfate complexes) was prepared through the precipitation reaction of CuSO₄·5H₂O and Na₂S₂O₃·5H₂O in distilled water at room temperature; then, in the absence of any extra catalyst, surfactant or template, pure submicrometer-sized CuS crystallites with a tubular morphology could be obtained just via solvothermal treatment of the precursor in ethanol at 150-180 °C for 12-24 h. Also, the effects of the reaction temperature and reaction time on the phase and morphology of the resultant products were investigated and the possible formation mechanism of tubular structure CuS crystallites in this solvothermal system was discussed.     

Interfacially organized DNA/polycation complexes: a route to new planar polymeric and composite nanostructures

Effects of nano-scale supramolecular organization and patterning in amphiphilic polycation monolayer formed by poly-4-vinilpyridine with 16% cetylpyridinium groups and in novel planar DNA/amphiphilic polycation complexes formed at the air-aqueous DNA solution interface and deposited on the solid substrates have been studied using AFM. Stable ordered quasi-crystalline planar polymeric monolayer structures were formed by amphiphilic polycation molecules. Extended net-like and quasi-circular toroidal condensed conformations of deposited planar DNA/amphiphilic polycation complexes were obtained in dependence on the amphiphilic polycation Langmuir monolayer state during the DNA binding. Those monolayer and multilayer DNA/polycation complex Langmuir–Blodgett films were used as templates and nanoreactors for generation of inorganic nanostructures. As a result, ultrathin polymeric nanocomposite films with integrated DNA building blocks and inorganic semiconductor (CdS) and iron oxide nanoparticle quasi-linear arrays or aggregates (nanorods) were formed successfully and characterized by TEM. The data obtained give evidence for effectiveness of the monolayer techniques for study mechanisms of DNA structural transformations caused by complexation with cationic compounds and demonstrate its perspectives for creation of new planar DNA-based self-organized stable polymeric complex nanostructures and nanocomposites with nano-scale structural ordering.