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   

Synthesis and characterization of graphene oxide,reduced graphene oxide and their nanocomposites with polyethylene oxide

This work describes the synthesis of GO, rGO and their nanocomposites with PEO. GO and rGO were prepared by the modified Hummers method and in-situ reduction of GO utilizing green reductant L (+) Ascorbic acid. The nanocomposites were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Thermogravimetric Analysis (TGA), and Universal Testing Machine (UTM). FT-IR and XRD confirmed the synthesis of GO and rGO. FE-SEM confirmed the uniformly exfoliated GO and rGO nanosheets in the polymer matrix. Hydrogen bonding was the main interaction mechanism for GO with PEO while no interaction was detected by FT-IR for rGO. Enhanced thermal stability was observed for both GO/PEO and rGO/PEO nanocomposites. The mechanical analysis showed an increase in Young's modulus, tensile strength, and elongation at break for GO/PEO nanocomposites, which is attributed to the homogeneous dispersion and hydrophilic hydrogen bonding interaction of GO with PEO.     

Kondo效应对磁杂质碳纳米管电输运特性的影响

从碳纳米管中Kondo效应的影响出发, 在有限温度下采用Anderson模型表征碳纳米管/磁杂质系统, 利用Landauer公式对磁杂质碳纳米管的电导和热电势进行研究, 得出和实验结果一致结论. 关键词： Kondo效应 碳纳米管 电输运特性     

Kondo effect in a deformed molecule coupled asymmetrically to ferromagnetic electrodes

The nonequilibrium Kondo effect is studied in a molecule quantum dot coupled asymmetrically to two ferromagnetic electrodes by employing the nonequilibrium Green function technique. The current-induced deformation of the molecule is taken into account, modeled as interactions with a phonon system, and phonon-assisted Kondo satellites arise on both sides of the usual main Kondo peak. In the antiparallel electrode configuration, the Kondo satellites can be split only for the asymmetric dot-lead couplings, distinguished from the parallel configuration where splitting also exists, even though it is for symmetric case. We also analyze how to compensate the splitting and restore the suppressed zero-bias Kondo resonance. It is shown that one can change the TMR ratio significantly from a negative dip to a positive peak only by slightly modulating a local external magnetic field, whose value is greatly dependent on the electron--phonon coupling strength.     

Magnetocaloric effect in the ferromagnetic Kondo lattice model

The Kondo lattice model describes a lattice of localized spins S_i interacting with the conduction electrons via a local exchange coupling J. Assuming a ferromagnetic Hund's rule coupling J>0, the model can be used to describe some itinerant magnetocaloric materials such as Gd(Si_xGe_1-x)₄, La(Fe_1-xSi_x)₁₃, and LaCa_1-xMn_xO₃, which are important for magnetic refrigeration near room temperature. The localized magnetic moments are described in the model Hamiltonian by spin operators, and the conduction electrons by fermionic operators. To study the magnetocaloric effect, a uniform external magnetic field is added through a Zeeman term. By averaging the fermionic degrees of freedom, one obtains an indirect exchange coupling between spins at sites i and j, which corresponds to the RKKY interaction. The self-consistent mean value is evaluated in the effective Heisenberg Hamiltonian within the random phase approximation (RPA). The conduction electron magnetization for a given value of is obtained from the corresponding Green's functions through the equation of motion method. The pressure and doping dependence of the Curie temperature are taken into account in the evaluation of . The magnetocaloric effect is characterized by the isothermal entropy change ΔS and the adiabatic temperature change ΔT_ad upon magnetic field variations in the neighborhood of the ferromagnetic phase transition. The results are obtained for and compared to measurements with Gd compounds.     

堆叠石墨片对锯齿型石墨纳米带电子输运的影响

采用格林函数方法研究了堆叠石墨片对锯齿型石墨纳米带电子输运性质的影响,计算了两种不同堆叠方式下锯齿型石墨纳米带的电导.研究发现,由于堆叠石墨片与石墨纳米带的耦合作用,锯齿型石墨纳米带的电导谱出现了电导谷.在远离费米能处,两种堆叠方式下的电导谷位置相近甚至重合;而在费米能附近,两种堆叠方式下的电导谷存在差异.此外,讨论了堆叠石墨片的几何尺寸对锯齿型石墨纳米带电子输运的影响.结果显示,随石墨片几何尺寸的增大,锯齿型石墨纳米带在两种堆叠方式下远离费米能处的电导谷逐渐向费米能方向移动,同时其费米能附近的电导谷在两种堆叠方式下的差异随石墨片尺寸的增大变得更为明显.研究结果表明,堆叠石墨片能够有效地调制锯齿型石墨纳米带的电子输运性质.     

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.     

扶手椅型石墨纳米带的双空位缺陷效应研究

采用基于密度泛函理论的第一性原理电子结构和输运性质计算,研究了扶手椅型石墨纳米带（具有锯齿边缘）的双空位缺陷效应.研究发现：双空位缺陷的存在并没有改变石墨纳米带的金属特性,但改变了费米面附近的能带结构.同时,双空位缺陷的取向对石墨纳米带的输运性质有很重要的影响.对于奇数宽度的纳米带,斜向双空位缺陷使得石墨带导电性能减弱,而垂直双空位能基本保留原有的线性伏安特性,导电性能降低较少;对于偶数宽度的纳米带,斜向双空位缺陷会使石墨带导电性能明显增强,而垂直双空位缺陷则具有完整石墨带的输运性质. 关键词： 石墨纳米带 585双空位缺陷 电子结构 输运性质     

Interplay between chiral magnetic and Kondo effects in Weyl metal phase

We investigate the Kondo effect in a Weyl metal state, which occurs from a spin-orbit coupled Dirac metal phase under magnetic fields. We start from an effective field theory in terms of low-energy fermions on a pair of chiral Fermi surfaces, which takes into account both the Berry curvature and chiral anomaly. Resorting to the U(1) slave-boson mean-field theory, we find that the effective Kondo temperature increases monotonically as a function of the external magnetic field due to enhancement of the density of states. The enhancement is originated from the chiral magnetic effect which is novel feature of Weyl metals. This leads to the prediction of the magnetic-field dependence in the logarithmic temperature dependence of the longitudinal magnetoconductivity.     

Hydrothermal method for the production of reduced graphene oxide

Reduced graphene oxide, RGO (also called chemically modified graphene, CMG) was synthesized by a simple hydrothermal method, with graphite oxide (GO), prepared by the modified Hummers method, served as the raw material. Structural and morphological studies indicate the degree of reduction is dependent on the temperature, which is also verified by Raman analysis. The variation in interlayer distance and the intensity ratio of the D to G Raman modes (I_D/I_G) indicates higher reaction temperature can accelerate the reduction of GO. The conductivity also varies with the degree of reduction, as verified by electrochemical analyzer. Moreover, the reaction process affects organic functional groups, the mechanism during the reaction process is discussed.     

The metal‐insulator transitions of VO2: A band theoretical approach

The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide are presented. In addition, the insulating phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal orbitals, which fall into the one‐dimensional band, and the isotropically dispersing bands. Hybridization of both types of bands is weak. In the phase splitting of the band due to metal‐metal dimerization and upshift of the bands due to increased p‐d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal‐insulator transition can be understood as a Peierls‐like instability of the band in an embedding background of electrons. In the phase, the metal‐insulator transition arises as a combined embedded Peierls‐like and antiferromagnetic instability. The results for VO₂ fit into the general scenario of an instability of the rutile‐type transition‐metal dioxides at the beginning of the d series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO₂ and NbO₂. In the compounds, the and bands can be completely separated, which leads to the observed metal‐insulator transitions.     

Deposition of hybrid structures of reduced graphene oxide and tin dioxide thin films,and persistent photoconductivity observation

Reduced graphene oxide (rGO) is deposited on glass substrate by dripping and sol-gel-coating methods giving rise to nanostructures. When in combination with thin films of SnO₂, they form a heterostructure SnO₂:2 at% Eu/rGO, which alters the surface electrical conductivity. SnO₂ and rGO were also combined as a composite, with conductivity strongly affected by ultraviolet excitation, and shows persistent photoconductivity (PPC) phenomenon even very close to room temperature. Both sort o hybrid structures can be applied in electronic devices. The SnO₂ films are deposited via chemical route by sol-gel or by a mixed technique that combines powders generated by drying the sol-gel solution with resistive evaporation of this powder. Resistivity measured as a function of temperature show that the SnO₂:2 at%Eu sample behaves very similarly to the SnO₂:2 at%Eu/rGO heterostructure sample, with the same energy level for the dominant defect, 172 meV, coincident with ionization of oxygen vacancies. Despite not changing the position of this level, the presence of rGO on the surface of the SnO₂ film induces a decrease in conductivity in vacuum, demonstrating the surface interaction.     

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.     

Splitting and Restoration of Kondo Peak in a Deformed Molecule Quantum Dot Coupled to Ferromagnetic Electrodes

We adopt the nonequilibrium Green＇s function method to theoretically study the Kondo effect in a deformed molecule, which is treated as an electron-phonon interaction （EPI） system. The self-energy for phonon part is calculated in the standard many-body diagrammatic expansion up to the second order in EPI strength. We find that the multiple phonon-assisted Kondo satellites arise besides the usual Kondo resonance. In the antiparallel magnetic configuration the splitting of main Kondo peak and phonon-assisted satellites only happen for asymmetrical dot-lead couplings, but it is free from the symmetry for the parallel magnetic configuration. The EPI strength and vibrational frequency can enhance the spin splitting of both main Kondo and satellites. It is shown that the suppressed zero-bias Kondo resonance can be restored by applying an external magnetic field, whose magnitude is dependent on the phononic effect remarkably. Although the asymmetry in tunnel coupling has no contribution to the restoration of spin splitting of Kondo peak, it can shrink the external field needed to switch tunneling magnetoresistance ratio between large negative dip and large positive peak.     

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.     

The mechanism of photocurrent enhancement of ZnO ultraviolet photodetector by reduced graphene oxide

An ultraviolet (UV) photodetector based on ZnO-reduced graphene oxide (ZnO-rGO) composites have been successfully fabricated. A pure ZnO photodetector was also fabricated by similar method. In comparison with the pure ZnO UV photodetector, the ZnO-rGO photodetector exhibits a much larger photocurrent and a better light-to-dark-current-ratio. The mechanism of photocurrent enhancement was investigated using I-V characteristics, photoluminescence (PL) spectra, transmittance spectra and time-dependent photocurrent analysis. Results show that the photocurrent enhancement of the ultraviolet photodetector is due to the improvement of the carrier lifetime, because the carrier recombination of ZnO were reduced by rGO. It provides a potential way to fabricate high-response UV photodetectors.     

电场诱导氧化石墨烯的极化动力学特性研究

本文通过外加电场改变氧化石墨烯团簇分子的共振能量,利用激光激发氧化石墨烯产生的共振荧光特性测量氧化石墨烯在电场作用下的极化动力学特性. 发现存在外加电场使得荧光共振峰的半高全宽趋于饱和的时间特性,而不同的氧化石墨烯团簇分子的荧光共振峰的暂态特性同时反映了电场对氧化石墨烯产生定向极化和变形极化的动力学特性. 关键词： 氧化石墨烯 团簇分子 电场 极化动力学     

Carbon nanotube transistors with graphene oxide films as gate dielectrics

Carbon nanomaterials,including the one-dimensional(1-D) carbon nanotube(CNT) and two-dimensional(2-D) graphene,are heralded as ideal candidates for next generation nanoelectronics.An essential component for the development of advanced nanoelectronics devices is processing-compatible oxide.Here,in analogy to the widespread use of silicon dioxide(SiO2) in silicon microelectronic industry,we report the proof-of-principle use of graphite oxide(GO) as a gate dielectrics for CNT field-effect transistor(FET) via a...     

The effect of electrode/graphene interfaces and dephasing processes on conductance and giant magnetoresistance

This study is devoted to potential applications of graphene nanostructures (GNRs) in future electronics and spintronics. This problem is related with the impact of geometrical and magnetic features of interfaces between contacts and the GNR on electrical performance of the device. The present approach combines the tight‐binding model and Green's function technique. It makes possible to put end‐contacted and side‐contacted arrangements on an equal footing, and understand their effect on conductance and giant magnetoresistance (GMR), depending on: aspect ratio, current orientation, gate voltage and dephasing. It turns out that at low gate voltages, dephasing processes may enhance the GMR effect, and that for short nanoribbons the end‐contacted geometry leads to the highest conductance. Long semiconducting armchair graphene nanoribbons, in turn, may reveal a sizeable inverse GMR effect. Some additional insight into the scalability of GNRs, for the spin‐dependent transport, is also provided.

     

Structural and optical properties of thermally reduced graphene oxide for energy devices

Natural intercalation of the graphite oxide, obtained as a product of Hummer's method, via ultra-sonication of water dispersed graphite oxide has been carried out to obtain graphene oxide(GO) and thermally reduced graphene oxide(RGO).Here we report the effect of metallic nitrate on the oxidation properties of graphite and then formation of metallic oxide(MO) composites with GO and RGO for the first time. We observed a change in the efficiency of the oxidation process as we replaced the conventionally used sodium nitrate with that of nickel nitrate Ni(NO_3)_2, cadmium nitrate Cd(NO_3)_2,and zinc nitrate Zn(NO_3)_2. The structural properties were investigated by x-ray diffraction and observed the successful formation of composite of MO–GO and MO–RGO(M = Zn, Cd, Ni). We sought to study the effect on the oxidation process through optical characterization via UV-Vis spectroscopy and Fourier Transform Infrared(FTIR) spectroscopy.Moreover, Thermo Gravimetric Analysis(TGA) was carried out to confirm 90% weight loss in each process thus proving the reliability of the oxidation cycles. We have found that the nature of the oxidation process of graphite powder and its optical and electrochemical characteristics can be tuned by replacing the sodium nitrate(NaNO_3) by other metallic nitrates as Cd(NO_3)_2, Ni(NO_3)_2, and Zn(NO_3)_2. On the basis of obtained results, the synthesized GO and RGO may be expected as a promising material in antibacterial activity and in electrodes fabrication for energy devices such as solar cell, fuel cell,and super capacitors.