Fabrication of graphene and graphite films on the Ni(111) surface

The growth of graphene and graphite films on nickel surface under conditions for ultrahigh-vacuum carburization and subsequent annealing is studied at film thicknesses ranging from a single layer to ≈1000 layers. The cooling of nickel carburized at a temperature of 900–1500 K leads to the growth of graphene and thin graphite films the thickness of which depends on the carburization temperature and the growth temperature of the films. Dissolution of nickel with graphite film in diluted sulfuric acid makes it possible to separate the film from the sample. The graphite film thickness amounts to ?0.4 µm at carburization and growth temperatures of 1500 and 1100 K, respectively.     

Phonon anharmonicities in graphite and graphene

We determine from first principles the finite-temperature properties-linewidths, line shifts, and lifetimes-of the key vibrational modes that dominate inelastic losses in graphitic materials. In graphite, the phonon linewidth of the Raman-active E(2g) mode is found to decrease with temperature; such anomalous behavior is driven entirely by electron-phonon interactions, and does not appear in the nearly degenerate infrared-active E(1u) mode. In graphene, the phonon anharmonic lifetimes and decay channels of the A(1)' mode at K dominate over E(2g) at Gamma and couple strongly with acoustic phonons, highlighting how ballistic transport in carbon-based interconnects requires careful engineering of phonon decays and thermalization.     

Magnetism in disordered graphene and irradiated graphite

The magnetic properties of disordered graphene and irradiated graphite are systematically studied using a combination of mean-field Hubbard model and first-principles calculations. By considering large-scale disordered models of graphene, I conclude that only single-atom defects can induce ferromagnetism in graphene-based materials. The preserved stacking order of graphene layers is shown to be another necessary condition for achieving a finite net magnetic moment of irradiated graphite. Ab initio calculations of hydrogen binding and diffusion and of interstitial-vacancy recombination further confirm the crucial role of stacking order in pi-electron ferromagnetism of proton-bombarded graphite.     

Thermal properties of compressed expanded graphite: photothermal measurements

The thermal diffusivity and the thermal conductivity of compressed expanded graphite (CEG) samples were investigated by photothermal measurements in two geometries differing by a place of temperature disturbance detection. This disturbance can be detected on a surface opposite to the one at which the disturbance was generated (rear detection) or on the same surface (front detection). A measurement based on the rear detection allowed us to determine the effective thermal diffusivity of the sample, while the method with front detection gives the possibility of analysis of homogeneity of the sample. It is shown that the thermal diffusivity of CEG strongly depends on its apparent density. Moreover, CEG samples reveal anisotropy of the thermal properties. The thermal diffusivity in the direction parallel to the compacting axis is lower than the one in the direction perpendicular to it. The parallel thermal diffusivity decreases with growing apparent density, while the perpendicular thermal diffusivity significantly grows when the apparent density grows. The perpendicular thermal conductivity exhibits the same behavior as the perpendicular thermal diffusivity. The parallel thermal conductivity slightly grows with growing density and then reaches a plateau. The anisotropy of CEG samples grows with growing apparent density and vanishes for low-density samples. The photothermal measurement with front signal detection revealed that the CEG samples are non-homogeneous in the direction of the compacting axis and can be modeled by a two-layer system.     

Variational analysis of propagation characteristics in thermally diffused expanded core fibers

We present a scalar variational method for the analysis of light-propagation characteristics in thermally diffused expanded core (TEC) fibers. The method leads to simple closed-form expressions regarding the mode field diameter, numerical aperture, waveguide dispersion as well as coupling losses produced by radial, longitudinal and angular misalignments. The dependence of coupling losses on the ‘taper ratio’ and normalized frequency is investigated and previous predictions that TEC fibers transmit light more effectively in free space over the order of millimeters, is confirmed. The coupling losses between small core diameter step-index fibers typically used in most erbium-doped fiber amplifiers and TEC-fibers is also considered.     

膨胀石墨 3 mm波消光数值计算

为探讨膨胀石墨作为3 mm波干扰材料的消光、散射特性及其影响因素, 基于有限长度、有限电导率圆柱状导体的电磁散射, 利用矩量法建立了膨胀石墨的消光、散射、吸收及后向散射截面(雷达散射截面RCS)的计算式. 运用Mathematica编程计算并分析了膨胀石墨长度、半径、电导率、磁导率等因素与膨胀石墨消光、散射、吸收截面及RCS的关系. 结果表明: 当膨胀石墨的长度为1.5 mm、半径为0.05 mm时, 具有较好的消光、散射效果; 适当增大膨胀石墨的电导率、磁导率, 有利于提高其消光、散射能力. 本研究为探索增强膨胀石墨干扰3 mm波效果的技术途径提供了有价值的参考.关键词：膨胀石墨石墨层间化合物消光截面矩量法     

Magneto-spectroscopy studies of graphite nanoplatelet films

The magneto-optical properties of graphite nanoplatelet films in the THz frequency range have been investigated. The room-temperature THz spectrum of graphite nanoplatelets shows a free carrier absorption at zero frequency with an electronic scattering rate of 175 cm⁻¹ (3.3×10¹³ rad/s) and plasma frequency of 1675 cm⁻¹. The lack of a major change in Drude plasma frequency down to 4.2 K implies that any band gaps in graphite nanoplatelets are less than 1 meV. The 300 K magneto-transmission contrast is as large as 60% near 1 THz at 10 T. The results are potentially useful for magnetic memory applications away from the dc limit.     

Anisotropic laser-induced evaporation of graphite films

An experimental investigation of the effect of linearly polarized high-energy pulsed laser light, normally incident on a carbon thin film, is reported. The material under study consists of platelike graphite crystallites with basal crystallographic planes mostly oriented perpendicular to the substrate surface. An increase is revealed in the fraction of the graphite crystallites oriented perpendicular to the polarization plane. Laser light is found to cause significant anisotropy in diffuse scattering by the film surface. Experimental observations are explained by a model of anisotropic evaporation of graphite-like carbon material due to polarization dependence of the absorption and reflection coefficients for a rough surface.     

Raman spectroscopy of magneto-phonon resonances in graphene and graphite

The magneto-phonon resonance or MPR occurs in semiconductor materials when the energy spacing between Landau levels is continuously tuned to cross the energy of an optical phonon mode. MPRs have been largely explored in bulk semiconductors, in two-dimensional systems and in quantum dots. Recently there has been significant interest in the MPR interactions of the Dirac fermion magneto-excitons in graphene, and a rich splitting and anti-crossing phenomena of the even parity $E_{2g}$ long wavelength optical phonon mode have been theoretically proposed and experimentally observed. The MPR has been found to crucially depend on disorder in the graphene layer. This is a feature that creates new venues for the study of interplays between disorder and interactions in the atomic layers. We review here the fundamentals of MRP in graphene and the experimental Raman scattering works that have led to the observation of these phenomena in graphene and graphite.     

Curcumin-assisted ultrasound exfoliation of graphite to graphene in ethanol

In this paper, we demonstrated a simple and cost-effective method to produce graphene from graphite in ethanol using ultrasound assisted with curcumin. The influence of curcumin concentration, starting graphite amount, sonication power, and sonication time on the graphene concentration was studied schematically. The π-π interaction between curcumin and graphene, being confirmed by FTIR spectrum, facilitate the exfoliation of the graphite into graphene. The concentration of the graphene in the ethanol reached up to 1.44 mg mL⁻¹ and the exfoliated suspension was relatively stable. The content of monolayer, bilayer, and multilayer in the exfoliated graphene suspension were 21%, 37%, and 42%, respectively. The as-prepared graphene sheets were free-defect. This novel approach may not only enable to exfoliate the graphite into graphene but also to make the graphene-curcumin hybrid which might find applications in pharmaceutical industry.     

Free vibrations and buckling of graphene sheets

The molecular mechanics method is used to determine both eigenfrequencies and modes of vibrations and the critical compressing loads and buckling modes of graphene sheets. To simulate interatomic interactions in graphene, the DREIDING field of potential forces is used. This field includes four types of potential energies of covalent atomic interactions such as central forces, the variations in the angle between the neighboring bonds, the dihedral angle that is responsible for the torsion of the covalent bond, and the inversion angle (the angle corresponding to the retiring of the atom from the plane relative to three neighboring atoms).     

Charge carriers in few-layer graphene films

The nature of the charge carriers in 2D few-layer graphites (FLGs) has been recently questioned by transport measurements [K. S. Novoselov, Science 306, 666 (2004)10.1126/science.1102896] and a strong ambipolar electric field effect has been revealed. Our density functional calculations demonstrate that the electronic band dispersion near the Fermi level, and consequently the nature of the charge carriers, is highly sensitive to the number of layers and the stacking geometry. We show that the experimentally observed ambipolar transport is only possible for an FLG with a Bernal-like stacking pattern, whereas simple-carrier or semiconducting behavior is predicted for other geometries.     

Electrical properties of thermally evaporated nickel-dimethylglyoxime thin films

Thin Bis-(dimethylglyoximato)nickel(II) [Ni(DMG)₂] films of amorphous and crystalline structures were prepared by vacuum deposition on Si (P) substrates. The films were characterised by X-ray fluorescence and X-ray diffraction. The constructed Al/Ni(DMG)₂/Si(P) metal-insulator-semiconductor devices were characterised by the measurement of the gate-voltage dependence of their capacitance and ac conductance, from which the surface states density D_it of insulator/semiconductor interface and the density of the fixed charges in the oxide were determined. The ac electrical conduction and dielectric properties of the Ni(DMG)₂-Silicon structure were studied at room temperature. The data of the ac measurements of the annealed films follow the correlated barrier-hopping CBH mode, from which the fundamental absorption bandgap, the minimum hopping distance, and other parameters of the model were determined.     

Effect of the structure of a graphite target on the parameters of carbon films obtained by the laser plasma method

We have used Raman light scattering and electron paramagnetic resonance methods to study carbon films obtained by laser plasma deposition, using different types of graphite targets. We have established that the films deposited in this way have a diamond-like structure and are a nanostructured composite containing clusters of both sp² and sp³-hybridized carbon. We have shown that an increase in structural perfection of the graphite target causes an improvement in the structure of the carbon films obtained from it and an increase in the content of sp³-hybridized carbon in it. Thermal stimulation of the substrate during application of a coating leads to the same effect. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 539–546, July–August, 2008.     

Properties of low-refractive-index films obtained by the close-spaced vapor transport technique under the sublimation of graphite in a quasi-closed volume

The properties of low-refractive-index carbon films obtained by close-spaced vapor transport at graphite sublimation are studied. The optical properties of the films are investigated by monochromatic multiple-angle ellipsometry, and their morphology is examined by AFM. It is found that the films have a columnar structure with a background surface roughness of about 1 nm. In addition, the surface of the film contains islands up to 50 nm in height with a footprint of ≈200 nm. A low-refractive-index carbon film deposited by close-spaced vapor transport on silicon tips is found to decrease the field emission threshold and drastically raise the current.     

Stable electron field emission from carbon nanotubes emitter transferred on graphene films

Carbon nanotubes (CNTs) arrays grown by microwave plasma enhanced chemical vapor deposition (MPCVD) method was transferred onto the substrate covered with graphene layer obtained by thermal chemical vapor deposition (CVD) technology. The graphene buffer layer provides good electrical and thermal contact to the CNTs. The field emission characteristics of this hybrid structure were investigated in this study. Compared with the CNTs arrays directly grown on the silicon substrate, the hybrid emitter shows better field emission performance, such as high emission current and long-term emission stability. The presence of this graphene layer was shown to improve the field emission behavior of CNTs. This work provides an effective way to realize stable field emission from CNTs emitter and similar hybrid structures.