Single Crystal Graphite Research Articles

Properties of a-C/B : H films relevant to plasma—surface interactions

Thin boron doped carbonaceous films have been deposited on metal alloys (stainless steel, Inconel), graphite and silicon single crystals during the boronization procedures in TEXTOR.The deposits were investigated by X-ray- and electron-diffraction (XRD, RHEED), transmission electron microscopy (TEM), nuclear reaction and backscattering techniques (NRA, ERD, RBS), electron microprobe (EPMA) and surface analysis techniques combined with ion sputtering (AES, XPS, SNMS). The films show a very good adhesion to all the substrates investigated, they are hard, semitransparent, amorphous and homogeneous down to about 3 nm. Colour-thickness correlation implies a refractive index n of about 2.4. The density of the film is ≈ 1.2 g cm −3, the average atomic distance d ≈ 0.23 nm (C-C ≈ 0.25 nm, C-B ≈ 0.15 nm, B-B ≈ 0.31 nm). The energy shift of the B(ls) core level in XPS indicates a covalent character of the B-C bond in the deposit. No indication of B-O bond has been found.

Low-energy helium scattering from ordered physisorbed layers of polar molecules

Atomic beam diffraction is discussed as a useful method for the structural characterization of mono- and multilayers of gases physisorbed on single-crystal surfaces with some emphasis on experimental techniques. The method is applied to the study of physisorbed phases of polar molecules on the (0001) face of single crystal graphite. On bare graphite CH{sub 3}F forms a commensurate {radical}3 {times} {radical}3R30{degree} structure with all the molecular dipoles parallel to each other while CH{sub 3}Cl and CH{sub 3}Br form uniaxially incommensurate phases with two molecules per unit cell that have the dipoles antiparallel to each other. If Xe-covered graphite is used as a substrate, the unit cell of CH{sub 3}F doubles with the two dipoles becoming also antiparallel, while the study of CH{sub 3}Cl and CH{sub 3}Br is complicated by layer mixing phenomena. HCl on graphite forms an incommensurate closed packed structure, rotated by 30{degree} with respect to the substrate lattice, with a lattice parameter of 3.80 {angstrom}. On the same substrate ammonia shows instead ringlike diffraction patterns indicative of a two-dimensional polycrystalline phase in which the crystalline domains have a large degree of azimuthal disorder.

X-ray study of the liquid and solid phases of the alkali metals in KC24- and RbC24-intercalated graphite single crystals.

We report a detailed x-ray scattering study of the structure of stage-2 K- and Rb-intercalated graphite single crystals, as a function of temperature. The graphite-host samples were characterized by a low level of stage impurities but the degree of stacking disorder was high. The diffuse scattering from the room-temperature two-dimensional (2D) liquidlike structure of the alkali metals has been measured. It reveals subtle but meaningful differences between K and Rb. The in-plane liquid-solid transitions and the associated growth of interlayer correlations (2D-3D crossover) have been studied through scans of the main and satellite reflections. The stacking faults of the graphite host appear to be transferred to the intercalate. In the case of ${\mathrm{KC}}_{24}$, a model calculation based on an average hexagonal K layer, with a subsequent partial relaxation of the K atoms in the graphite sites, provides a good fit to the observed reflection intensities. At lower temperatures, the K layers are confirmed to undergo a hexagonal-oblique symmetry-breaking transition at ${\mathit{T}}_{\mathit{L}}$\ensuremath{\sim}95 K, while the Rb layers stay hexagonal down to 10 K. Moreover, complex stacking sequences of the intercalate develop as the temperature is reduced, and in ${\mathrm{RbC}}_{24}$ one observes a clear loss of the stacking correlations. This is due presumably to a frustration of the K sublattice coming from interlayer and intralayer interactions.

Anomalous superperiodicity in scanning tunneling microscope images of graphite

An anomalously large periodicity has been observed in scanning tunneling microscope images of a (00.1) graphite sample. The unusual contrast has hexagonal symmetry, a periodicity of 7.7±0.2 nm and is superimposed on the usual a=0.246 nm atomic spacing of graphite. Current-voltage curves recorded in the scanning tunneling spectroscopy mode from the region showing the superperiodicity exhibit slightly more metallic behavior than those from neighboring normal regions. One possible explanation for the observed periodicity is that it is a rotational moiré pattern resulting from the overlap between a misoriented layer of graphite and the underlying graphite single crystal.

X-Ray Diffuse Scattering by Highly Oriented Pyrolytic Graphite

The X-ray diffuse scattering intensity was measured for a highly oriented pyrolytic graphite (HOPG) sample as well as for a single crystal kish graphite sample. The measurements were carried out both at 10 K and at room temperature. The HOPG sample gives rise to a temperature independent diffuse scattering along the c * -direction. This scattering was analyzed as that due to the crystallite boundaries. The thermal diffuse scattering exhibits unusual features which could be interpreted to be a result of the coupling of low frequency phonons with an excitation mode which is yet to be identified.

The structure of ammonia overlayers physisorbed onto the surface of single crystal graphite, determined by means of atomic beam diffraction

The structure of NH3 monolayers and submonolayers physisorbed on graphite C(0001) has been determined by means of low energy helium atom beam diffraction experiments. The measured diffraction pattern is found to consist of diffraction ‘‘rings,’’ indicating a large degree of orientational disorder among the adsorbed domains. We observe up to fourth order diffraction and periodicities which can be generated from a rectangular 3.53×6.11 Å unit cell or by a 7.06×7.06 Å hexagonal mesh. Both of these possibilities are characteristic of a 3.53 Å nearest neighbor separation. These findings suggest that, because of the relatively strong molecule–molecule interactions, the degree of order within a given domain is relatively high, while the domain’s orientational disorder may be due to the insensitivity of the adlayer to the corrugation and anisotropy of the graphite substrate. This behavior is atypical, since HCl, CH3F, CH3Cl, and CH3Br all form well organized monolayers which have a single well defined orientation with respect to the graphite substrate.

Anisotropic effects in x-ray Raman scattering from graphite

X-ray Raman scattering spectra from single crystal graphite were recorded. By making use of the relative directions of the scattering vector (s) and an axis of graphite crystal (c), anisotropic effects in the x-ray Raman scattering spectrum are observed; the onset of the scattering shifts by about 7 eV and the extended fine structures show significant differences in the spectra obtained for s∥c and s⊥c. Analyses by the uses of the equation employed for EXAFS show that interlayer or intralayer C–C distances can be obtained separately from the spectra for s∥c and s⊥c, respectively. This is the first observation of the anisotropy effect in the extended structure in x-ray Raman spectra.

Damage mechanisms and characteristics of the cavity reflectors for vacuum ultraviolet argon excimer lasers.

For high-power outputs from rare-gas excimer lasers, cavity reflectors having high reflectivity and free from radiation damage areprimarily required. We measure reflectivities and surface topographic features for some reflectors made of Alfilms deposited on SiO2 substrates, SiC films deposited on graphite substrates, SiO2 glasses, and single crystals of W, Mo, Si and Cr. The results of surface damageobserved on Al and Si mirrors as well as a thermodynamical calculation show that the surface damage is caused mainly by surfacemelting due to strong absorption of laser energy.

Stable isotope analysis of small amount of carbon dioxide and its application to the microscale isotopic zoning of graphite crystals in metamorphic rocks.

A conventional dynamic mass spectrometer with an inlet system specially modified for microscale samples was used to measure carbon isotopic ratio of subsamples within individual graphite crystals collected from Hida metamorphic belt, central Japan. The graphite crystals with three different occurrences i.e., in pelitic gneiss, skarn deposit, and crystalline limestone were examined for their isotopic zonation. Single graphite crystals collected from pelitic gneiss and hedenbergite skarn deposit has homogeneous isotopic distribution, whereas composite graphites of marble show complicated zonation which may imply the evolution of the metamorphic fluid during metamorphism.

Absorption of cesium by polycrystalline graphite-sticking coefficient studies

The interaction of metal vapors with graphite is a subject of considerable scientific and technological importance. Graphite is known to getter cesium (Cs) atomic vapor and this has found important space applications. Results of our experimental investigations of the absorption of Cs by polycrystalline graphite are reported. We measure the sticking coefficient γ of Cs on graphite using an atomic beam apparatus. For polycrystalline graphite γ starts with the initial value of 1 (every impinging Cs atom sticks to the surface) and decreases slowly with time (~days), for continuous exposure to the Cs atomic beam. This is in sharp contrast to single crystal graphite where γ is essentially 0. For polycrystalline graphite, the rate of decline of γ with time is found to be essentially independent of the incident Cs beam flux over the range of fluxes investigated experimentally. A theoretical model for the rate of decline of γ based on the diffusion of Cs into polycrystalline graphite is developed. Polycrystalline graphite consists of crystallites graphitized in a rather porous structure. In general, for such an inhomogeneous medium several types of diffusion with different activation energies come into play. We assumed that under our experimental conditions grain boundary diffusion (surface diffusion along the grains) is the single dominant mechanism of migration of Cs through polycrystalline graphite. The experimental results show a good fit with the predictions of this model. This represents the first quantitative study of the Cs gettering property of graphite at room temperature.

Scanning tunneling microscopy and transmission electron microscopy studies of Au and Pd clusters grown on a clean graphite surface

Scanning tunneling microscopy (STM) has been used to image gold and palladium clusters deposited under ultrahigh vacuum (UHV) conditions on clean cleaved surfaces of natural graphite single crystals. Extensive transmission electron microscopy (TEM) studies were performed to determine the growth conditions. Moreover each sample was checked by ex situ TEM after each in situ STM study. The overall shape of the largest isolated clusters imaged by STM (100–200 Å) is consistent with TEM observations: triangular or hexagonal outlines and clusters sitting along surface steps are elongated by coalescence. STM also reveals that they have a height versus diameter ratio about 0.1. Smallest clusters (10–20 Å) are imaged with atomic resolution simultaneously with the graphite lattice. They are close packed and 2–3 atomic layers high. On occasion, two-dimensional Pd islands are observed. They have an hexagonal non-close-packed arrangement.

Disorder in the layer stacking in carbon fibers

The stacking disorder of pitch-based carbon fibers has been characterized by the wide-angle X-ray measurement. The distribution function of the interlayer spacing has been deduced on a reasonable base, and the variation in the stacking disorder with the mean interlayer spacing is discussed. It is obtained that the distribution of the interlayer spacings in a crystallite is represented by a skewed distribution function having a longer tail at larger spacings, and that the stacking disorder diminishes while the asymmetry of the distribution increases, when the mean interlayer spacing approaches that of the graphite single crystal.

Low energy helium atom scattering from HCl monolayers physisorbed on graphite

Low energy helium atom scattering has been used to characterize monolayers of HCl adsorbed onto single crystals of graphite. The unit cell of this system is described by real space lattice vectors 3.80 Å in length, separated by 120°. The monolayer's lattice is rotated 30° with respect to the substrate. The attenuation of the He beam scattered in the specular direction due to the thermal motion of the HCl molecules perpendicular to the surface, can be described as due to a collection of Einstein oscillators with a common, characteristic frequency of 7.8 × 10 12 rad s . A polar plot i.e. a plot of the specular intensity as a function of the incident angle, has been compared to the predictions of a close-coupled scattering program, that uses realistic He-surface interaction potentials which include the contributions from the atom-adlayer and atom-substrate interactions, along with the relevant three-body interaction terms. The existing HeHCl spherically averaged potentials do not give good agreement with our data although reducing the well depth by 8.5% improves the agreement slightly. Since the HeHCl interaction in the “T” geometry is less strong than that along the molecular axis (and therefore than the spherically averaged interaction) our measurements may suggest that the HCl molecules are preferentially aligned with their dipole axes parallel to the surface.

X-ray diffraction study of the structure of xenon multilayers on single crystal graphite

The structures of xenon multilayers (1 to 44 layers) physisorbed onto a graphite single crystal surface have been studied using x-ray scattering techniques. Both the intra-planar and inter-planar structures could be examined by measurements of the (1 0l) diffraction rods. The (0 0l) diffraction provides direct information about the thickness of the multilayer. We find two principal and surprising results. First, the xenon does not form an infinite number of layers at low temperatures. This disagrees with a number of previous reports which suggest complete wetting of xenon on graphite. Second, the structures of the adsorbed layers turn out to be rather elaborate. Instead of uniform, defect free layers, the xenon multilayers exhibit stacking disorder and a commensurate-incommensurate transition in the first layer.

Far-infrared Alfven waves in graphite

Alfven-wave propagation in single-crystal graphite has been studied at 4.2 K with wavelengths of 392.5, 433 and 512.5 mu m in magnetic fields up to 8.5 T applied along the c axis in the Faraday geometry. The mass density function obtained from the magnetic field dependence of interference maxima is in excellent agreement with the predicted value from the parameters of graphite. The amplitude of the propagating wave is larger with the circular polarisation of the infrared set to sense holes than it is to sense electrons and the amplitude is reduced when the hole Landau levels coincide with the Fermi surface. The propagating wave has a higher amplitude for the hole polarisation at a given magnetic field because the holes have a smaller mass than the electronics but the dispersion of the propagating wave is given by the linear wave approximation for Alfven waves.

Synchrotron x-ray study of the structures and phase transitions of monolayer xenon on single-crystal graphite.

The phases and phase transitions of solid xenon monolayers on a graphite single crystal were investigated using high-resolution synchrotron x-ray scattering. As the temperature is lowered a sequence of incommensurate solid phases (aligned\ensuremath{\rightarrow}rotated\ensuremath{\rightarrow}reentrant aligned) occurs. This sequence is followed by an incommensurate-commensurate transition. Zero-temperature mean-field theories are able to explain the C-IC transition but not the reentrant rotational transitions. The corrugation of the adsorption potential and the width of domain walls are estimated using relaxation simulations at zero temperature.

Helium scattering studies of the dynamics of a xenon-monolayer-covered graphite single-crystal surface.

The dispersion curves of the surface modes of a monolayer of xenon adsorbed on a single crystal of graphite have been measured by helium-atom scattering. A strong hybridization-induced splitting is observed between the localized adsorbate mode and the substrate Rayleigh mode, indicating a stronger adsorbate-substrate interaction than that for the case of rare gases on metals.

Microcalorimetry study of the monolayer4He ordering transition on single crystal graphite

We have developed a low-temperature, ac microcalorimeter for exploring adsorption on a single 2.25 mm2 graphite leaf that is capable of 10 picoJoule/degree resolution. The microcalorimeter was used to determine the phase diagram and heat capacity critical exponents α of monolayer4He films at the commensurate ordering transition. After in situ baking at 600 K, we reproduced the narrow-ordered phase region (≃1% in coverage) reported by Campbell and Bretz for HOPG, but find quasilogarithmic, rather than power law, heat capacity divergences. We argue that the disappearance of the Potts-like exponent in the heat capacity is attributable to the geometry of nucleation along cleavage edge planes present on single crystal graphite surfaces.

The neutron diamagnetic form factor of graphite

The form factor of the diamagnetic moment induced by a field of 4.62 T in a single crystal of graphite has been measured by polarised neutron diffraction. The data are consistent with a pi -electron orbital current having a radius of 1.38(8) AA. A slightly smaller than that of the carbon rings in the planar sheets of the graphite structure (1.42 AA). The corresponding effective mass ratio for the pi -electrons is m*/m=0.019(3).

X-ray determination of the modulation potential in a stage-2 potassium-graphite intercalation compound.

By use of both synchrotron and conventional x-ray sources, the HK.L Bragg peaks have been measured at room temperature on a single crystal of graphite, intercalated with potassium to stage 2. For these reflections there is a contribution of the two-dimensional alkali metal liquid due to the modulation induced by the host graphite lattice. Using the theory of Reiter and Moss, we have extracted the modulation potential by fitting the integrated intensities for the H-K=3m reflections and the intensity profiles along L for those reflections where H-K=3m\ifmmode\pm\else\textpm\fi{}1. The potential is similar to that derived for Rb in graphite but with enhanced local minima at the carbon atoms. The stacking faults in this crystal are found to occur at random.