Magnetic and electromagnetic absorption properties of FeNi alloy nanoparticles supported by reduced graphene oxide

FeNi alloy nanoparticles (NPs) supported by reduced graphene oxide (RGO) (FeNi/RGO nanocomposites) were successfully synthesized through in‐situ reduction. Large amounts of sphere‐like FeNi NPs are uniformly deposited on the RGO nanosheets. The magnetic hysteresis measurement reveals the ferromagnetic behavior of the nanocomposites at room temperature. According to the electromagnetic (EM) characteristics, the FeNi/RGO nanocomposites show outstanding EM absorption properties in the 2–18 GHz range, as evidenced by the wide effective absorption bandwidth (up to 3.3 GHz, with reflection loss R_L < –10 dB) and a minimal R_L (–32 dB) at 12.4 GHz with a thickness of 1.5 mm. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

Synthesis and Characterization of Poly(vinyl pyrrolidone)/Reduced Graphene Oxide Nanocomposite

Poly(vinyl pyrrolidone) (PVP)/reduced graphene oxide (RGO) nanocomposites were synthesized by reducing graphene oxide in the polymer matrix at different temperatures. The effects of the GO content on the properties of the nanocomposites were investigated by Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The degree of dispersion of GO in the PVP matrix was examined by field-emission scanning electron microscopy. The results showed that both GO and RGO were well dispersed in the PVP matrix. Under low filler content, the improvement of onset decomposition temperatures of PVP nanocomposites was not obviously observed, but the amounts of residual char of the PVP nanocomposites were clearly increased. In addition, the decomposition temperature peak values of the PVP nanocomposites were increased, while the peak was broadened.     

Phonon-limited transport coefficients in extrinsic graphene

The effect of electron-phonon scattering processes on the thermoelectric properties of extrinsic graphene was studied. Electrical and thermal resistivity, as well as the thermopower, were calculated within the Bloch theory approximations. Analytical expressions for the different transport coefficients were obtained from a variational solution of the Boltzmann equation. The phonon-limited electrical resistivity ρ(e-ph) shows a linear dependence at high temperatures and follows ρ(e-ph) ~T(4) at low temperatures, in agreement with experiments and theory previously reported in the literature. The phonon-limited thermal resistivity at low temperatures exhibits a ~T dependence and achieves a nearly constant value at high temperatures. The predicted Seebeck coefficient at very low temperatures is Q(T) ~ Π(2)k(2)_(B)/T(3eE_(F), which shows a n(-1/2) dependence with the density of carriers, in agreement with experimental evidence. Our results suggest that thermoelectric properties can be controlled by adjusting the Bloch-Grüneisen temperature through its dependence on the extrinsic carrier density in graphene.     

Enhanced rate capability of LiMn0.9Mg0.1PO4 nanoplates by reduced graphene oxide/carbon double coating for Li-ion batteries

The reduced graphene oxide (RGO)/carbon double-coated LiMn_0.9Mg_0.1PO₄ (LMP) nanoplates are introduced as a cathode material for Li-ion batteries with excellent rate capability. The double coating of RGO and carbon simultaneously brings the unique advantages of conformal carbon layer on each particle surface, and soft RGO sheets connecting the nanoplates to each other, thereby provides easy conduction pathways for the whole LMP aggregates. In particular, the simple self-assembly process driven by the electrostatic interactions enables conducting RGO sheets effectively to wrap the carbon-coated LMP, establishing three-dimensional RGO network. The RGO/C/LMP nanocomposites exhibit remarkably enhanced rate capability compared to the only C- or RGO-coated LMP, which is well explained by the reduced charge-transfer resistance achieved from electrochemical impedance spectroscopy.     

Complete coverage of reduced graphene oxide on silicon dioxide substrates

Reduced graphene oxide(RGO) has the advantage of an aqueous and industrial-scale production route. No other approaches can rival the RGO field effect transistor platform in terms of cost(相似文献   

Magnetoresistance of ferromagnetic tunnel junctions in the double-exchange model

We conduct a theoretical study of the temperature dependence of the spin polarization ( P) and the magnetoresistance (MR) ratio using the double exchange (DE) model for ferromagnetic tunnel junctions with half-metallic systems. It is shown that the strong exchange coupling in the DE model plays an important role in the temperature dependence of both P and the MR ratio; their values can be less than the maximum values expected for half-metallic systems at low temperatures, and the MR ratio decreases more rapidly than P with increasing temperature. The calculated results, however, indicate that the MR ratio may still be large at high temperatures near the Curie temperature.     

Oxygenated Functional Group Density on Graphene Oxide: Its Effect on Cell Toxicity

The association of cellular toxicity with the physiochemical properties of graphene‐based materials is largely unexplored. A fundamental understanding of this relationship is essential to engineer graphene‐based nanomaterials for biomedical applications. Here, an in vitro toxicological assessment of graphene oxide (GO) and reduced graphene oxide (RGO) and in correlation with their physiochemical properties is reported. GO is found to be more toxic than RGO of same size. GO and RGO induce significant increases in both intercellular reactive oxygen species (ROS) levels and messenger RNA (mRNA) levels of heme oxygenase 1 (HO1) and thioredoxin reductase (TrxR). Moreover, a significant amount of DNA damage is observed in GO treated cells, but not in RGO treated cells. Such observations support the hypothesis that oxidative stress mediates the cellular toxicity of GO. Interestingly, oxidative stress induced cytotoxicity reduces with a decreasing extent of oxygen functional group density on the RGO surface. It is concluded that although size of the GO sheet plays a role, the functional group density on the GO sheet is one of the key components in mediating cellular cytotoxicity. By controlling the GO reduction and maintaining the solubility, it is possible to minimize the toxicity of GO and unravel its wide range of biomedical applications.     

Reduced graphene oxide/MWCNT hybrid sandwiched film by self-assembly for high performance supercapacitor electrodes

A reduced graphene oxide/multiwalled carbon nanotube (RGO/MWCNT) hybrid sandwiched film with different MWCNTs content was prepared by vacuum-assisted self-assembly from a complex dispersion of graphene oxide (GO) and MWCNTs followed by heat-treating at 200 °C for 1 h in a vacuum oven to reduce the GO into RGO. The free-standing RGO/MWCNT hybrid sandwiched film before heat-treatment showed a layered structure with an entangled network of MWCNTs sandwiched between the GO sheets. This unique structure not merely contribute to remove the oxygen-containing groups in GO during the heat-treatment, but also decrease the defects for electron transfer between RGO layers, which enhances the electrochemical capacitive performances of graphene-based films. A specific capacitance up to 379 F/g was achieved based on RGO/MWCNT with 30 % MWCNTs mass fraction at 0.1 A/g in a 6 M KOH electrolyte. The excellent performance of RGO/MWCNT hybrid sandwiched film signifies the importance of controlling the surface chemistry and sandwiched nanostructure of graphene-based materials.     

Low-field carrier transport properties in biased bilayer graphene

Based on a semiclassical Boltzmann transport equation in random phase approximation, we develop a theoretical model to understand low-field carrier transport in biased bilayer graphene, which takes into account the charged impurity scattering, acoustic phonon scattering, and surface polar phonon scattering as three main scattering mechanisms. The surface polar optical phonon scattering of carriers in supported bilayer graphene is thoroughly studied using the Rode iteration method. By considering the metal–BLG contact resistance as the only one free fitting parameter, we find that the carrier density dependence of the calculated total conductivity agrees well with that observed in experiment under different temperatures. The conductivity results also suggest that in high carrier density range, the metal–BLG contact resistance can be a significant factor in determining the BLG conductivity at low temperature, and both acoustic phonon scattering and surface polar phonon scattering play important roles at higher temperature, especially for BLG samples with a low doping concentration, which can compete with charged impurity scattering.     

Spin transport and relaxation in graphene

We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications.     

Three‐Dimensional Multilayer Assemblies of MoS2/Reduced Graphene Oxide for High‐Performance Lithium Ion Batteries

Three‐dimensional (3D) multilayer molybdenum disulfide (MoS₂)/reduced graphene oxide (RGO) nanocomposites are prepared by a solution‐processed self‐assembly based on the interaction using different sizes of MoS₂ and GO nanosheets followed by in situ chemical reduction. 3D multilayer assemblies with MoS₂ wrapped by large RGO nanosheets and good interface are observed by transmission electron microscopy. The interaction of Na⁺ ions with oxygen‐containing groups of GO is also investigated. The measurement of lithium ion batteries (LIBs) shows that MoS₂/RGO anode nanocomposite with a weight ratio of MoS₂ to GO of 3:1 exhibits an excellent rate performance of 750 mAh g^?1 at 3 A g^?1 outperforming many previous studies and a high reversible capacity up to ≈1180 mAh g^?1 after 80 cycles at 100 mA g^?1. Good rate performance and high capacity of MoS₂/RGO with 3D unique layered‐structures are attributed to the combined effects of continuous conductive networks of RGO, good interface facilitating charge transfer, and strong RGO sheets preventing the volume expansion. Results indicate that 3D multilayer MoS₂/RGO prepared by a facile solution‐processed assembly can be developed to be an excellent nanoarchitecture for high‐performance LIBs.     

Selective Etching of N‐Doped Graphene Meshes as Metal‐Free Catalyst with Tunable Kinetics,High Activity and the Origin of New Catalytic Behaviors

New N‐doped reduced graphene oxide (N‐RGO) meshes are facile fabricated by selective etching of 3–5 nm nanopores, with controllable doping of N dopants at an ultrahigh N/C ratio up to 15.6 at%, from pristine graphene oxide sheets in one‐pot hydrothermal reaction. The N‐RGO meshes are illustrated to be an efficient metal‐free catalyst toward hydrogenation of 4‐nitrophenol, with new catalytic behaviors emerging in following three aspects: (i) tunable kinetics following pseudofirst order from commonly observed pseudozero order; (ii) strikingly improved activity with 26‐fold increased rate constant (1.0 s⁻¹ g⁻¹ L); (iii) no induction time required prior to reaction due to depressed back conversion, and dramatically decreased apparent activation energy (E_a) (17 kJ mol⁻¹). The origin of these new catalytic properties can be assigned to the synergetic effects between graphitic N doping and structural defects arising from nanopores. Deeper understanding unveils that the concentration of graphitic N is inverse proportion to E_a, while the pyrrolic N has no impact on this reaction, and oxygenate groups hampers it. The porous nature allows the N‐RGO meshes to conduct catalyze reactions in continuous flow fashion.     

Ultrasound assisted synthesis of Sn nanoparticles-stabilized reduced graphene oxide nanodiscs

Sn nanoparticles-stabilized reduced graphene oxide (RGO) nanodiscs were synthesized by a sonochemical method using SnCl₂ and graphene oxide (GO) nanosheets as precursors in a polyol medium. TEM and XPS were used to characterize the Sn-stabilized RGO nanodiscs.     

Reduced graphene oxide as saturable absorbers for erbium-doped passively mode-locked fiber laser

We demonstrate a nanosecond mode-locked erbium-doped fiber laser(EDFL)based on a reduced graphene oxide(RGO)saturable absorber(SA).The RGO SA is prepared by depositing the graphene oxide(GO)on fluorine mica through thermal reduction of GO.A scanning electron microscope(SEM),Raman spectrometer,and x-ray photoelectron spectroscopy(XPS)are adopted to analyze the RGO characteristics.The results show that the reduction degree of graphene oxide is very high.By embedding the RGO SA into the EDFL cavity,a stable mode-locked fiber laser is achieved with a central wavelength of 1567.29 nm and repetition rate of 12.66 MHz.The maximum output power and the minimum pulse duration are measured to be 18.22 mW and 1.38 ns respectively.As far as we know,the maximum output power of18.22 mW is higher than those of other nanosecond mode-locked oscillators reported.Such a nanosecond pulse duration and megahertz repetition rate make this mode-locked erbium-doped fiber laser a suitable seed oscillator for high-power applications and chirped pulse amplifications.     

Direct synthesis of graphene nanosheets support Pd nanodendrites for electrocatalytic formic acid oxidation

We report a solvothermal method preparation of dendritic Pd nanoparticles(DPNs) and spherical Pd nanoparticles(SPNs) supported on reduced graphene oxide(RGO). Drastically different morphologies of Pd NPs with nanodendritic structures or spherical structures were observed on graphene by controlling the reduction degree of graphene oxide(GO) under mild conditions. In addition to being a commonplace substrate, GO plays a more important role that relies on its surface groups, which serves as a shape-directing agent to direct the dendritic growth. As a result, the obtained DPNs/RGO catalyst exhibits a significantly enhanced electro-catalytic behavior for the oxidation of formic acid compared to the SPNs/RGO catalyst.     

State of the art and recent advances in the ultrasound-assisted synthesis,exfoliation and functionalization of graphene derivatives

Sonochemistry, an almost a century old technique was predominantly employed in the cleaning and extraction processes but this tool has now slowly gained tremendous attention in the synthesis of nanoparticles (NPs) where particles of sub-micron have been produced with great stability. Following this, ultrasonication techniques have been largely employed in graphene synthesis and its dispersion in various solvents which would conventionally take days and offers poor yield. Ultrasonic irradiation allows the production of thin-layered graphene oxide (GO) and reduced graphene oxide (RGO) of up to 1 nm thickness and can be produced in single layers. With ultrasonic treatment, reactions were made easy whereby graphite can be directly exfoliated to graphene layers. Oxidation to GO can also be carried out within minutes and reduction to RGO is possible without the use of any reducing agents. In addition, various geometry of graphene can be produced such as scrolled graphene, sponge or foam graphene, smooth as well as those with rough edges, each serving its own unique purpose in various applications such as supercapacitor, catalysis, biomedical, etc. In ultrasonic-assisted reaction, deposition of metal NPs on graphene was more homogeneous with custom-made patterns such as core-shell formation, discs, clusters and specific deposition at the edges of graphene sheets. Graphene derivatives with the aid of ultrasonication are the perfect catalyst for various organic reactions as well as an excellent adsorbent. Reactions which used to take hours and days were significantly reduced to minutes with exceedingly high yields. In a more recent approach, sonophotocatalysis was employed for the combined effect of sonication and photocatalysis of metal deposited graphene. The system was highly efficient in organic dye adsorption. This review provides detailed fundamental concepts of ultrasonochemistry for the synthesis of graphene, its dispersion, exfoliation as well as its functionalization, with great emphasis only based on recent publications. Necessary parameters of sonication such as frequency, power input, sonication time, type of sonication as well as temperature and dual-frequency sonication are discussed in great length to provide an overview of the resultant graphene products.     

Enhanced microwave absorption performance of MOF-derived hollow Zn-Co/C anchored on reduced graphene oxide

Composite materials assembled by metal/carbon nanoparticles and 2D layered flakes can provide abundant interfaces, which are beneficial for high-performance microwave absorbers. Herein, Zn-Co/C/RGO composites, composed of Zn-Co metal-organic framework-derived Zn-Co/C nanoparticles and reduced graphene oxide (RGO), were obtained through a facile method. The multilayer structure was due to the introduction of hollow Zn-Co/C nanoparticles in the RGO sheets. Zn-Co/C nanoparticles provided abundant polarization and dipole centers on the RGO surface, which enhanced the microwave absorption abilities. Different concentrations of RGO were introduced to optimize impedance matching. The minimum reflection loss (R_L) of Zn-Co/C/RGO with a thickness of 1.5 mm reached -32.56 dB with the bandwidth corresponding to R_L at -10 dB, which can reach 3.92 GHz, while a minimum R_L of -47.15 dB at 11.2 GHz was also obtained at a thickness of 2.0 mm. The electromagnetic data demonstrate that Zn-Co/C/RGO presented excellent absorption performance and has potential for application in the microwave absorption field.     

Atmospheric pressure chemical vapor deposition (APCVD) grown bi‐layer graphene transistor characteristics at high temperature

We report the characteristics of atmospheric chemical vapor deposition grown bilayer graphene transistors fabricated on ultra‐scaled (10 nm) high‐κ dielectric aluminum oxide (Al₂O₃) at elevated temperatures. We observed that the drive current increased by >400% as temperature increased from room temperature to 250 °C. Low gate leakage was maintained for prolonged exposure at 100 °C but increased significantly at temperatures >200 °C. These results provide important insights for considering chemical vapor deposition graphene on aluminum oxide for high temperature applications where low power and high frequency operation are required. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and electrochemical performance of LiNi0.6Co0.2Mn0.2O2/reduced graphene oxide cathode materials for lithium-ion batteries

A LiNi_0.6Co_0.2Mn_0.2O₂/reduced graphene oxide (RGO) composite with RGO content of 1.2 % was prepared by a simple spray-drying method instead of high-energy ball milling method. The composite has been characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, energy dispersive spectroscopy, and charge/discharge test. The X-ray diffractometry result showed that composite possessed a typical hexagonal structure. The RGO sheets served as efficient electronically conductive frameworks benefitting from its 2D structure and outstanding electronic conductivity. The scanning electron microscope and transmission electron microscopy verified that LiNi_0.6Co_0.2Mn_0.2O₂ particles were wrapped with RGO sheets, which facilitated electronic conductivity between particles. The electrochemical results indicated that composite delivered a higher discharge capacity at various discharge rates. The cycling performance was also evaluated. The composite exhibited better cycling performance than pristine sample. Electrochemical impedance spectroscopy showed that the RGO can greatly reduce the charge transfer resistance. The results here gave clear evidence of RGO to improve electrochemical performance.