Weak Field Gauge-Invariant Planar Magnetoresistance of Doped Cuprate Superconductors

We have calculated the planar magnetoresistance of the doped cuprate superconductor in the ‘normal’ state; the results are in agreement with the divergence of the magnetoresistance near T _c , and other unexplained data. Argument and evidence have been known for assuming that the ‘normal’ state in the doped cuprate is mainly a two-channel Kondo lattice. In the common moderately disordered nonstoichiometrically doped cuprate, in contrast with the fully ordered crystal, the two-channel Kondo fixed point is effectively stable.      

Tension effect on the absorbance of a graphene layer

The effect of uniform planar tension on the absorbance of a graphene layer located between air and a semi-infinite dielectric medium has been investigated. The results show that the magnitude of the Van Hove peaks in the absorbance of the graphene depends on the tension direction, the strain modulus and the angle between the tension direction and the tangential electric field. While, the frequency location of the Van Hove peaks only depends on the tension direction and the strain modulus and is independent from the angle between the tension direction and the tangential electric field. When the tangential electric field is perpendicular to one of the vectors connecting a given carbon atom to its nearest neighbours, the related peak disappears. While, for the tangential electric field parallel to those vectors, the related peak acquires its largest value. Moreover, it is shown that the absorbance of the graphene layer at the low photon energies doesn’t noticeably depend on the tension direction, the strain modulus and the angle between the tension direction and the tangential electric field.     

Dependence of Magnetoresistance on the Thickness of a Dusted Al Spacer Inserted in CoFe／Cu／CoFe Sandwiches

A series of CoFe(4nm)/Cu(X nm)Al(Y nm)/CoFe(6 nm） samples have been prepared at room temperature.An exponential decay of the GMR (Giant Magnetoresistance) with Y was observed for fixed X=2nm.The characteristic decay parameter of Al is obtained to be about 0.26nm,which is rather close to 1 monolayer for Al.A coexistant state of GMR and AMR (anisotropic magnetoresistance) was observed when Y=2nm.As the Cu spacer is replaced by Al layer,only AMR effect dominates.The experimental data further underline the important role played by the nonmagnetic spacers.     

1.38 MW peak power dual-loss modulated sub-nanosecond green laser with EO and graphene

By simultaneously employing electro-optic (EO) modulator and Graphene saturable absorber (SA) in a dual-loss-modulated Q-switched and mode-locking (QML) Nd:Lu_0.15Y_0.85VO₄/KTP green laser, the sub-nanosecond single mode-locking green laser is demonstrated with high peak power, low repetition rate and high stability. The monolayer and 3-layer graphene sheets grown by chemical vapor deposition (CVD) method were used as SAs in the experiment. When the pump power reached 10.72 W, the maximum peak power obtained from the doubly QML laser with EO and monolayer graphene-SA was 1.38 MW, corresponding to a pulse duration of 480 ps. The shortest pulse width of 340 ps was obtained with a 3-layer graphene-SA.     

Sawtooth-like graphene nanoribbon

Motivated by recent successful synthesize of segmented graphene nanoribbons (GNRs) with junctions, we explore electronic properties of a novel form of GNR with sawtooth-like structure using the density-functional theory method. It is found that the unique edge structures of the sawtooth-like GNR induce richer band-gap features than the straight GNR counterpart with either armchair or zigzag edges. The effect of external electric field on the electronic properties of the sawtooth-like GNR is also studied. The theoretical results may be useful for designing GNR-based fi eld-effect transistors.     

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.      

Oxide and nitride encapsulation of large-area graphene field effect devices

Room temperature magnetron sputtering is used to deposit SiO₂ and Si₃N₄ encapsulation layers on top of back gated graphene transistors, which are fabricated with CVD grown graphene transferred onto SiO₂ and Si₃N₄ substrates. Raman spectroscopy with 514 nm laser excitation was performed on bare and encapsulated devices. A small increase in D peak of the encapsulated spectrum indicates minimal increase in defect density for both SiO₂ and Si₃N₄ deposition. Graphene on Si₃N₄ exhibits an average mobility of ~ 4000 cm²/V.s at a carrier density of 10¹² cm^− 2 and up to 80% mobility is retained upon encapsulation with Si₃N₄, while on SiO₂ the average mobility is ~ 2000 cm²/V.s with mobility retention of up to 55% with SiO₂ encapsulation.     

Photonic realization of the deformed Dirac equation via the segmented graphene nanoribbons under inhomogeneous strain

ABSTRACT

Starting from an engineered periodic optical structure formed by waveguide arrays comprised of two interleaved lattices, we simulate a deformed Dirac equation. We show that the system also simulates graphene nanoribbons under strain. This optical analogue allows us to study the phenomenon of Zitterbewegung for the modified Dirac equation. Our results show that the amplitude of Zitterbewegung oscillations changes as the deformation parameter is changed.     

化学镀镍法制备Ni/石墨烯(英文)

采用化学镀镍方法,以氧化石墨烯薄片为基体、NaBH4为还原剂,在NiSO4溶液中制备了Ni/石墨烯。通过X射线衍射、场发射扫描电镜和透射电镜对样品进行了表征,结果表明:沉积在石墨烯片表面Ni的的质量分数高达32.9%时,仍具有高度的分散性;相互堆积Ni/石墨烯片形成了介孔和大孔。氮气等温吸附表明:其介孔和大孔为狭缝型结构,孔的Brunauer-Emmett-Teller比表面积为91 m2/g;吸附支的Barret-Joyner-Halenda平均孔径为3.83 nm,孔容为0.28 cm3/g。     

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     

化学还原氧化石墨烯制备高性能石墨烯自组装水凝胶

提出了一种以抗坏血酸钠为还原剂,通过化学还原氧化石墨烯制备高性能石墨烯自组装水凝胶的方法.用扫描电镜,流变及电导率测试,光电子能谱,X-射线晶体衍射和拉曼光谱等手段对该石墨烯水凝胶的结构与性能进行了表征.结果表明:化学还原氧化石墨烯对形成石墨烯水凝胶具有决定性作用.该石墨烯水凝胶具有优异的导电性(1 S·m-1),机械强度和电化学性能.在1 mol·L-1的硫酸电解质溶液中,通过1.2A·g-1恒电流允放电测试,石墨烯水凝胶电极的比电容高达240F·g-1.

Abstract：

Three-dimensional self-assembled graphene hydrogels (SGHs)have been fabricated by chemical reduction of graphene oxide (GO)with sodium ascorbate. The SGHs were characterized by scanning electron microscopy,rheological tests,electrical conductivity measurements,X-ray photoelectron spectroscopy,X-ray diffraction,and Raman spectroscopy. Results indicate that the reduction of GO promotes the assembly of graphene sheets. The SGHs are electrically conductive(1s·m-1)and mechanically strong and exhibit excellent electrochemical performance.In 1 mol·L-1 aqueous solution of H2SO4,the specific capacitance of SGHs was measured to be about 240F·g-1 at a discharge current density of 1.2·-1.     

Rashba effect induced magnetoresistance in an InAs heterostructure

The Rashba field in a quantum channel produces spin splitting and population imbalance between spin-up and -down electrons. The channel resistance depends on the alignment between the applied field and the Rashba field because the applied magnetic field causes different mobilities for two types of spins. With an applied field of 2 T, the mobility difference between spin-up and -down electrons is 0.75%.     

Low field magnetoresistance, temperature coefficient of resistance and magnetocaloric effect in Pr2/3Ba1/3MnO3:PdO composites

Electrical resistivity, magnetoresistance (MR), temperature coefficient of resistance (TCR) and magnetocaloric effect of (1 − x) Pr_2/3Ba_1/3MnO₃:x PdO (x = 0-30 mol% PdO) composite manganites are reported here. Pristine sample Pr_2/3Ba_1/3MnO₃ (PBMO) shows two insulator-metal like transitions (T_P1 ~ 194 K and T_P2 ~ 160 K) in the electrical resistivity behavior. With PdO, T_P1 becomes sharper whereas T_P2 disappears beyond 10 mol% PdO addition. The intrinsic MR gets enhanced from 22% for the pristine sample to ~ 42% for 27% PdO sample. However, the extrinsic MR is found to decrease in the composites. The TCR also increases from a negligible value for PBMO to 8% for 25 mol% PdO sample. These features have been explained on the basis of opening of new conducting channels and decrease in spin dependent scattering and the overall decrease in electrical resistivity. The magnetic entropy change and relative cooling power (RCP) for the PBMO sample are 5.3418 J.Kg-K and 304.5428 J/Kg respectively. However, these values decrease in the composites.     

Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir-Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air-water interface and transferred to silicon substrates by Langmuir-Blodgett technique as closely spaced monolayers of 20-40 μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp² bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8-12 cm^− 1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3 ± 0.3 to 0.8 ± 0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.     

Features of structures obtained by graphene nanoplatelets treatment in a diamond anvil high-pressure cell

Multilayer graphene nanoplatelets were treated in a shear deformation diamond anvil high-pressure cell. The resulting material was investigated by HRTEM and Raman spectroscopy. Onions, carbon nanotubes, and diamond fragments were detected in the treated material.     

Graphene fluoride: a stable stoichiometric graphene derivative and its chemical conversion to graphene

Stoichoimetric graphene fluoride monolayers are obtained in a single step by the liquid-phase exfoliation of graphite fluoride with sulfolane. Comparative quantum-mechanical calculations reveal that graphene fluoride is the most thermodynamically stable of five studied hypothetical graphene derivatives; graphane, graphene fluoride, bromide, chloride, and iodide. The graphene fluoride is transformed into graphene via graphene iodide, a spontaneously decomposing intermediate. The calculated bandgaps of graphene halides vary from zero for graphene bromide to 3.1 eV for graphene fluoride. It is possible to design the electronic properties of such two-dimensional crystals.     

The effect of Ag loading on supercapacitor performance of graphene based nanocomposites

In the present study, we prepared reduced graphene oxide (rGO) decorated with Ag nanoparticles by a one pot, simultaneous reduction method. The effect of AgNO₃ amount on the chemical, morphological and electrochemical properties of binary rGO-Ag nanocomposite for supercapacitor application was investigated. The chemical and morphological characterization of prepared rGO-Ag nanocomposites was realized with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). For supercapacitor application, electrochemical performance of the nanocomposites was investigated with cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. As a result of their excellent conductivity and spacer role which prevent aggregation of rGO nanosheets and maintain the electroactive surface area, Ag nanoparticles significantly enhance the electrochemical performance of the nanocomposite. The rGO-Ag nanoparticle nanocomposite exhibited a maximum specific capacitance of 34.2?mF?cm^?2 at 0.6?A?cm^?2 current density. The nanocomposite electrode also has excellent rate capability and cycle life. The capacitance retention of rGO-Ag electrode is 98% after 1000 charge-discharge cycle. The results showed that rGO-Ag nanocomposite is a building block for ternary or other multicomponent nanocomposites.     

Formation of graphene by the thermal annealing of a graphite layer on silicon substrate in vacuum

In this letter we present our preliminary results about the preparation of graphene and carbon nanosheets by thermal annealing of graphite layer on silicon substrate in vacuum and at temperature 900 °C. The surface area of about several tens of micrometers and curved at the ends. The carbon nanosheets are very clean and look like a piece of paper. Investigations with Raman showed three main peaks D, G, G′ and small peak at 2420 cm⁻¹. This method is considered to be a simple and cheap method for preparing the carbon nanosheets directly on the silicon substrate for the application in the electronics.