磁性石墨

德国来比锡大学的EsquinaziP教授及其同事们利用质子对石墨样品进行辐射,可使石墨在室温下转变为一种纯碳的、不含金属的、轻型的磁性材料.众所周知,碳元素具有许多值得注意的固体形态:一种是粉末状的石墨,由于它在二维层间只有松散的键约束,因此可用来作为润滑油或铅笔芯;第二种是钻石,由于它在三维空间内都有紧密的约束而成为坚硬的宝石;第三种就是著名的富勒球;其他还有碳纳米管等.所有这些固态碳都具有重要的电性质,但都没有磁性.迄今为止还没有任何一种由纯碳组成的样品具有磁性,当然除了保持温度在绝对零度附近的掺杂的石墨样品.来比…     

Superlubricity of graphite

Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]].     

Graphite on graphite

We propose potential geometry for fabrication of the graphite sheets with atomically smooth edges. For such sheets with Bernal stacking, the electron–electron interaction and topology should cause sufficiently high density of states resulting in the high temperature of either spin ordering or superconducting pairing.     

Electron spectroscopy of graphite,graphite oxide and amorphous carbon

Core-level and valence-band photoelectron spectra and Auger spectra for the basal plane of highly ordered graphite, the edge plane of highly ordered graphite, the basal plane of graphite oxide, and for the basal plane of disordered graphite are compared in an effort to determine spectral features that may be used to identify these chemical species in carbon-rich specimens. The photoelectron spectra were recorded using 1253.6 eV X-ray excitation. The Auger spectra were obtained using both X-ray (1253.6 eV) and electron (3 keV) excitation. The differential X-ray excited C-KVV spectra were the most useful in distinguishing the different types of carbon. In particular, the plasmon structure and the energy separation between the two major excursions were very sensitive to changes in the surface chemistry and structure. Changes in peak position and peak width observed in the valence band and C(1s) photoelectron spectra were also very helpful in distinguishing the pristine, structurally damaged, and the oxidized graphite surfaces. Much of the structural and chemical information apparent in the X-ray excited C-KVV spectra was lost upon electron beam exposure. Differences in the C-KVV Auger lineshapes for X-ray and electron excitation were attributed to both structural and chemical changes induced by electron beam exposure.     

Diamagnetism of graphite

The temperature-dependent diamagnetism of graphite has been accounted for using band parameters agreeing with magnetoreflection, optical, and de Haas-van Alphen results. The agreement requires inclusion of the effects of trigonal warping and addition of a small paramagnetic constant.     

K/graphite: Uniform energy shifts of graphite valence states

Ultra-thin graphite overlayers prepared by heating SiC crystals can give highly resolved valence band photoemission spectra. This is exploited to monitor K deposition induced energy shifts of graphite valence band states. States near the corners of the Brillouin zone, responsible for the semimetal character of graphite, are observed to show shifts that are nearly equal to the shifts of the uppermost filled σ state and of an empty state 7.6 eV above E_F. The results give credence to the rigid band shift model often used to discuss the electronic structure and charge transfer for graphite ad- and absorption systems.     

Pulsed-laser-induced transformation path of graphite to diamond via an intermediate rhombohedral graphite

The new phase transformation of hexagonal graphite to cubic diamond was experimentally produced without catalyst, using a high-power pulsed laser. Interestingly, by the X-ray diffraction spectra, it was proved that this transition was not direct, but through an intermediate rhombohedral phase. Furthermore, it is important that the rhombohedral phase, as the theoretical transformation path of hexagonal graphite to cubic diamond, was first truly substantiated by our experimental results. The transformation mechanism was suggested that diamond with hexagonal structure was obtained by the direct transforming of hexagonal graphite to hexagonal diamond, and diamond with cubic structure was formed by the indirect transforming, i.e., hexagonal graphite to rhombohedral graphite to cubic diamond. Received: 7 February 2000 / Accepted: 28 March 2000 / Published online: 9 November 2000     

Laser nanoablation of graphite

Experimental data on laser ablation of highly oriented pyrolitic graphite by nanosecond pulsed UV ( $\lambda =193$  nm) and green ( $\lambda =532$  nm) lasers are presented. It was found that below graphite vaporization threshold $\approx $ 1 J/cm $^{2}$ , the nanoablation regime can be realized with material removal rates as low as 10 $^{-3}$  nm/pulse. The difference between physical (vaporization) and physical–chemical (heating + oxidation) ablation regimes is discussed. Special attention is paid to the influence of laser fluence and pulse number on ablation kinetics. Possibility of laser-induced graphite surface nanostructuring has been demonstrated. Combination of tightly focused laser beam and sharp tip of scanning probe microscope was applied to improve material nanoablation.     

Transverse transport in graphite

Graphite is a layered material showing a strong anisotropy. Among the unconventional properties reported by experiments, the electronic transport along the c-axis, which has direct implications in order to build graphitic devices, remains a controversial topic. We study the influence of inelastic scattering on the electron tunnelling between layers. In the presence of electron electron interactions, tunnelling processes are modified by inelastic scattering events.     

Intercalation compounds of graphite

A broad review of recent research work on the preparation and the remarkable properties of intercalation compounds of graphite, covering a wide range of topics from the basic chemistry, physics and materials science to engineering applications.     

Radiation defects in graphite

This article discusses the nature of radiation defects in graphite, reviewing past and recent developments in understanding their structure, interactions and effect on physical properties. The principal focus is on behaviour at the atomic and microstructural level, with an interest both in understanding graphite moderator damage in nuclear reactors and building a foundation for the range of emerging technological applications of defect-engineered graphitic materials. It is hoped that this article will both clarify the picture that has emerged over the last 50 years and provide a useful background to ongoing efforts.     

Influence of graphite size on the synthesis and reduction of graphite oxides

We investigated the influence of the precursor, graphite, size on the synthesis and reduction of graphite oxide. Three precursors of graphite with different size were used to synthesize the graphite oxide which was consecutively reduced by hydrazine of different concentration ratios. Size dependent effect on the reduction of the graphite oxide was found, and the graphite oxide of the smallest size provided the best reduction result. Electrochemical properties of the reduced graphene oxide were investigated in both of the base and acid electrolytes, finding the reduced graphene oxide of the smallest size gives the best electrochemical performance due to the high reduction. Therefore, the precursor size is a very important factor in the synthesis and reduction of graphite oxide, affecting the electrochemical performance considerably for the energy storage applications.     

人工裁剪制备石墨纳米结构

采用不同的方法裁剪高定向热解石墨（HOPG），制备纳米尺寸的石墨条.首先,发现用聚焦离子束（镓离子）刻蚀高定向热解石墨，可以得到边缘整齐程度在几十纳米的石墨条,另外,用 电子束曝光和反应离子刻蚀的工艺，可以得到最小尺寸为50 nm的纳米石墨图型 (nano-size d graphite pattern,纳米尺寸的多层石墨结构).采用了三种不同的方案制备反应等离子刻 蚀过程中需要的掩膜，分别是PECVD生长的SiO₂掩膜，磁控溅射的方法生长的Si O₂掩膜和PMMA光刻胶掩膜，并将三种方案的刻蚀结果做了对比. 关键词： 高定向热解石墨 聚焦离子束刻蚀 电子束曝光 反应离子刻蚀     

Plasma bands in graphite

The π + σ plasmons in a strongly oriented pyrolitic graphite were studied experimentally using the characteristic electron energy loss technique with an angular resolution in the reflection mode. It was established that the bulk intralayer π + σ plasma bands have two branches with a positive and a negative dispersion, respectively. Each dispersion branch of the π + σ plasmons is a doublet. The interlayer plasmons are characterized by a positive dispersion coefficient only. The results are interpreted qualitatively based on the special features of the electronic band spectrum of graphite. The doublet arises because both dipole and multipole interband transitions similar in intensity are excited in graphite exposed to electron bombardment.     

Phonon dispersion in graphite

We measured the dispersion of the graphite optical phonons in the in-plane Brillouin zone by inelastic x-ray scattering. The longitudinal and transverse optical branches cross along the Gamma-K as well as the Gamma-M direction. The dispersion of the optical phonons was, in general, stronger than expected from the literature. At the K point the transverse optical mode has a minimum and is only approximately 70 cm(-1) higher in frequency than the longitudinal mode. We show that first-principles calculations describe very well the vibrational properties of graphene once the long-range character of the dynamical matrix is taken into account.     

Magnetic structure of graphite ribbon

Anoble mechanism of spin polarization is proposed for finite graphite sheet with edge. For graphite ribbon with zigzag edge, there appear peculiar ‘edge states’. These localized states comprise nearly flat band at the Fermi level, which easily causes magnetic instability. Magnetic structure is suggested from Hartree-Fock analysis of the Hubbard model, where huge magnetic moments are induced at around both of edges by weak HubbardU and are coupled antiferromagnetically with each other.     

Electronic structure of lithium graphite

X-ray photoemission spectra of vacuum cleaved LiC₆, prepared from highly oriented pyrolytic graphite, provide a direct measure of the filling of the graphite π bands by electrons from Li. The resulting Fermi energy shift is in agreement with recent band structure calculations and indicates near unity charge transfer from Li. Core level spectra exhibit shifts compatible with the expected charge transger and line shapes showing strong asymmetries resulting from the metallic character of this compound.     

Anisotropy of graphite optical conductivity

The graphite conductivity is evaluated for frequencies between 0.1 eV, the energy of the order of the electronhole overlap, and 1.5 eV, the electron nearest hopping energy. The in-plane conductivity per single atomic sheet is close to the universal graphene conductivity e ²/4ħ and, however, contains a singularity conditioned by peculiarities of the electron dispersion. The conductivity is less in the c direction by the factor of the order of 0.01 governed by electron hopping in this direction.     

Universal optical conductance of graphite

We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to (pi/2)e2/h, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-Weiss-McClure model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The f-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.     

Backscattered electron spectra from graphite

The backscattered electron spectra from graphite sample were studied both experimentally and theoretically at impact energies between 500 and 5000 eV. The angle of the incident electron beam was 50° and the detection angle was 0° with respect to the surface normal, respectively. Monte Carlo (MC) simulations were performed based on the Classical Transport Theory (CTT) model to mimic the experimental spectra. In our simulations, both elastic and inelastic scattering of primary electrons and secondary electron emission from graphite are taken into account. There is found satisfactory agreement between measured and calculated electron spectra.