Single Crystal Graphite Research Articles

A new adsorption substrate: Single crystal exfoliated graphite

We describe the controlled exfoliation of single crystal graphite flakes intercalated to stage eleven with SbCl5. The resulting exposed surface remains well oriented in the basal plane. Our best sample to date has an internal surface area of 6 Torr cc and an in-plane mosaic of 2.3° HWHM (half-width at half-maximum). High resolution x-ray scattering from adsorbed krypton reveals an average step-free surface size of at least 2000 Å. Experiments which probe orientational information in the layer planes should now be possible.

Catalysis of carbon oxidation by transition metal carbides and oxides

The technique of etch decoration/transmission electron microscopy has been employed to study the catalysis of the carbon-oxygen reaction by seven group VB and VIB metal carbides and oxides. Turnover frequencies for carbon gasification were measured at 680 °C from the monolayer pit expansion rate on the basal plane of single-crystal graphite, with and without catalyst particles deposited on the surface. Among the catalysts used in this study, only MoO 3 followed the wellestablished mode of channeling, while all others catalyzed the reaction at the edges of the etch pits which were distant from the catalyst by apparently a long-range action. The etch pits formed in the presence of carbides (WC, TaC, and Mo 2C) were all hexagonal, unlike the uncatalyzed reaction in which pits were circular, and the edges of the hexagonal pits were composed of the zigzag {101̄0} faces. These pits were similar to those created directly by atomic oxygen. The results suggested that the transition metal carbides served as dissociation centers and dissociation was followed by spillover and reaction on the edges of the pits. Deformed circular pits were formed in the presence of the oxide catalysts (Cr 2O 3, WO 3, and Ta 2O 5), and the oxide catalyst particles disintegrated and dispersed on the surface. Because the reaction temperature was near or above the Tammann temperatures, it appeared that the catalysts dispersed by emission of molecular species or clusters which migrated on the basal plane of graphite and were subsequently trapped on the edges of the etch pits. Gasification of the edge carbon atoms was thus catalyzed by the trapped catalyst through direct contacts.

Intercalate structure, melting, and the commensurate-incommensurate transition in bromine-intercalated graphite

In situ high-resolution x-ray scattering experiments have been carried out to study in-plane intercalate structure and phase transitions as a function of temperature in a single crystal graphite host. For the case of bromine-intercalated graphite the intercalate plane has three sublattices and each sublattice has a centered ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}7$) rectangular structure with four ${\mathrm{Br}}_{2}$ molecules per two-dimensional unit cell in the commensurate phase. The coherently ordered in-plane bromine regions exceed 10 000 \AA{} in size. Above the commensurate-incommensurate transition (342.20\ifmmode\pm\else\textpm\fi{}0.05 K), a stripe domain phase becomes established in a single domain of a sublattice along the sevenfold direction. The incommensurability as a function of reduced temperature exhibits a power law with an exponent of 0.50\ifmmode\pm\else\textpm\fi{}0.02, confirming the existing theories. The relative shifts observed for the various harmonics are accurately predicted by a sharp-domain-wall model with $\frac{4\ensuremath{\pi}}{7}$ phase shifts. A power-law line shape is observed for the incommensurate intercalate layer, yielding values for the exponent $\ensuremath{\eta}$ consistent with model calculations. Results on the temperature dependence of the intensities, linewidths, and line shapes of several Bragg peaks around the melting transition are presented. The intercalate layer exhibits a continuous melting transition from a two-dimensional solid phase to an anisotropic fluid phase, occurring at 373.41\ifmmode\pm\else\textpm\fi{}0.10 K for a stage-4 compound.

Exfoliation of single crystal graphite and graphite fibers intercalated with halogens

The in-plane unit cell and the twinned domain structure were found by X-ray diffraction to be preserved after exfoliation and collapse of single crystal graphite intercalated with bromine. The fractional expansion due to exfoliation was comparable for single crystals and highly oriented pyrolytic graphite (HOPG), except that the fractional expansion in the first exfoliation was larger for single crystals. The degree of hysteresis was less for single crystals than for HOPG. The first observation was made of the exfoliation of graphite fibers.

Angle-resolved photoemission and secondary electron emission from single-crystal graphite

Angle-resolved photoemission and secondary electron emission from single-crystal graphite

Two-dimensional structure of bromine intercalated graphite

Synchrotron and sealed tube x-ray diffraction studies on the quasi two-dimensional stage-2 graphite-bromine commensurate phase, prepared with single crystals of graphite, are presented. The two-dimensional structure of the intercalate bromine and the carbon plane has been derived from intensity data measured with a four-circle diffractometer. The bromine structure, having nearest neighbor distances of 2.25, 2.38, 2.55 and 2.86 Å in the (001) plane, exhibits a novel long-chain polymeric form. The distances are comparable to the interatomic bond lengths in the existing polymeric bromides.

Valence and conduction band dispersions of single crystal graphite studied by angle resolved photoelectron and secondary electron spectroscopy

Valence and conduction band dispersions of single crystal graphite studied by angle resolved photoelectron and secondary electron spectroscopy

Synchrotron X-ray diffraction study of the room temperature incommensurate phase in graphite-bromine intercalation compound

Synchroton X-ray and a four-circle diffractometer were used to study the room temperature incommensurate phase in stage-2 graphite-bromine prepared with single crystal graphite in bromine liquid. The two strongest in-plane superlattice reflections and the strongest satellite peak were scanned along the most incommensurate direction of the lattice between 302 K and 324 K. The incommensurate-commensurate transition at 319±1 K caused disappearance of the incommensurate peaks and appearance of the commensurate one. Possible models of the incommensurate phase are presented Etude a l'aide d'un diffractometre a 4 cercles sur le compose d'insertion au stade 2, prepare a partir d'un monocristal de gaphite, entre 302 et 324 K. Observation de la transformation incommensurable-coummensurable a 319 K. Proposition de modeles pour la phase incommensurable

Electrical and mechanical properties of copper chloride-intercalated pitch-based carbon fibers

Using chlorine vapor transport we have intercalated pitch-based fibers, highly oriented pyrolytic graphite (HOPG), and natural single crystals of graphite with CuCl 2. In this paper we report mainly on the electrical properties of pitch-based fibers heat treated to 2500, 2750 and 3000 °C and then intercalated. The electrical resistivity both above and below room temperature, the tensile strength, and Young's modulus are reported. In very high magnetic fields at 4 K the fibers exhibit Shubnikov-de Haas oscillations, which means this powerful technique is now available as a diagnostic tool for intercalated fibers.

An experimental study of the energy dependence of the total yield due to the incidence of low-energy electrons onto graphite surfaces

The energy dependence of the total yield resulting from the incidence of low-energy electrons in the energy range of 0 to 30 eV above the vacuum level on various graphite surfaces: (A) the basal plane of single-crystal graphite, (B) the basal plane of highly oriented pyrolytic graphite and (C) polycrystalline graphite, is presented. The authors have also explored (D) vapour-deposited carbon film on polycrystalline graphite surfaces. The spectra are discussed partly in terms of the onset of inelastic electron-electron scattering accompanied by interband transitions and partly by excitation of collective modes. A peak at 14.2 eV is ascribed to the Q1u+ to Q2g- transition while features at 26 and 21.2 eV have been ascribed to the excitation of volume plasmons and surface plasmons, respectively.

Orientational Ordering of Nitrogen Molecular Axes for a Commensurate Monolayer Physisorbed on Graphite

Low-energy electron diffraction has been used to study the orientational ordering of the molecular axes of nitrogen molecules physisorbed on graphite single crystals. A 2 \ifmmode\times\else\texttimes\fi{} 1 superlattice structure with in-plane orientations is inferred for the monolayer where the molecular centers have the commensurate ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$) 30\ifmmode^\circ\else\textdegree\fi{} structure. The superlattice intensity decreases significantly near 30 K but persists up to 40 K, consistent with a first-order transition with finite-size effects.

Commensurate-Incommensurate and Melting Transitions in Bromine-Intercalated Single Crystal Kish Graphite

ABSTRACTWe summarize the results of an extensive x-ray scattering study of the phase transitions in intercalated graphite-Br2 compounds. The experiments are aimed at understanding the intraplanar and interplanar correlations of the intercalate bromine as a function of temperature. Using in-situ high resolution x-ray scattering techniques, we have studied stage-4 graphite-Br2 under equilibrium conditions, where the intercalant is in the commensurate phase, incommensurate phase, and fluid phase. We demonstrate that the transitions between these phases are model examples of phase transitions in quasi two-dimensional systems. The coherently ordered in-plane bromine regions exceed 10000 Å in size in both commensurate and incommensurate phases. A commensurateincommensurate transition in the 7–fold direction is observed from a centered (√3 × 7) phase to a stripe domain phase. The domain wall density exhibits a power-law behavior in temperature with an exponent of 0.50 ± 0.02. We have also observed power-law lineshapes for bromine superlattice peaks in the incommensurate phase due to the rigorous absence of true long range order. The intercalate layer appears to exhibit an anisotropic melting transition from an incommensurate solid to a novel fluid phase.

Orientational epitaxy of an incommensurate neon monolayer adsorbed on graphite

The structure of a monlayer of neon adsorbed on the (0001) surface of a graphite single crystal has been studied at T ⋍ 10 K by using high resolution Low Energy Electron Diffraction. The incommensurate monlayer is rotated by ⋍17° with respect to the (√3 × √3) 30° commensurate structure for a mean overlayer interatomic spacing d ⋍ 3.24 Å. Thus result agrees with the prediction of Novaco-McTague theory.

Orientational epitaxy of an incommensurate neon monolayer adsorbed on graphite

The structure of a monlayer of neon adsorbed on the (0001) surface of a graphite single crystal has been studied at T≅ 10 K by using high resolution Low Energy Electron Diffraction. The incommensurate monlayer is rotated by ≅17° with respect to the (√3 × √3) 30° commensurate structure for a mean overlayer interatomic spacing d≅ 3.24 Å. Thus result agrees with the prediction of Novaco—McTague theory.

Single crystal structure studies of C 16AsF 5

X-ray diffraction studies at 23 °C have been made for the second stage AsF 5 intercalated graphite of nominal composition C 16AsF 5. Single crystals of graphite, obtained by dissolving a marble matrix, were intercalated to stage 2 by exposure to AsF 5 vapor at a pressure of ∼ 170 mm. The diffraction pattern consists of Bragg peaks corresponding to a hexagonal unit cell with a = 2.46 Å (as in graphite) and c = 11.50 Å and a relatively uniform high background of diffuse scattering. The mosaic spread of Bragg peaks is essentially the same as in the single crystal of graphite before intercalation. Integrated intensities ( θ − 20 θ scans) were measured for the Bragg peaks with monochromated Mo Kα radiation (0.710 Å) on a single crystal diffractometer. The intercalant molecules are disordered in the intercalant layer with the consequence that except for 00 l reflections the intensities of Bragg reflections are quantitatively accounted for by the arrangement of the graphite layers. An investigation into the nature of the intercalating species, and their location, has been made by least squares fitting of calculated to observed intensities for 19 00 l reflections. Similar studies on an HOPG sample are also reported. The presence of both AsF 5 and AsF 6 − seems to be needed for a satisfactory fit, the amount of AsF 3 being indeterminate. The arrangement of graphite layers is in the same AB sequence as in ordinary graphite, although twinning is present. The CC bond distance, the interlayer spacing, and the carbon thermal parameters are essentially the same as for pure graphite.

Studies of the nickel-catalyzed hydrogenation of graphite

Controlled atmosphere electron microscopy has been used to study the effects of small nickel particles on the hydrogenation of single crystal graphite. These particles propagated channels in the graphite basal plane, the majority of these channels being parallel to the 〈112̄0〉 directions. Increase in channel propagation rate occurred with increase in temperature, but at temperatures above 1075 K nickel particle disappearance was frequent, believed to be due to diffusion of metal into the graphite structure. At higher temperatures graphite platelets were formed from immobile nickel particles. Quantitative analysis also revealed that the rate of carbon gasification was proportional to the external surface areas of individual particles and not to the leading catalyst/graphite interfacial area. All these effects and possible reaction mechanisms are discussed.

Vibrational spectrum of H and D on the (0001) graphite surface from scattering experiments

We have investigated the vibrational spectrum of hydrogen and deuterium atoms adsorbed on the (0001) surface of a graphite single crystal by means of atomic beam scattering in UHV conditions. Bound state resonances, which appear as minima in the specularly reflected beam have been detected over a wide range of incidence and azimuthal angles. Energy levels at 31.6±0.2 and 15.3±0.3 meV for 1H and at 35.4 ±2; 21.35±0.1; 12.0±0.1, and 5.9±0.3 meV for 2H have been determined. By fitting the experimental energy levels to those calculated with the potential the following values of the parameters are obtained: D=43.3±0.5 meV; λ=1.36±0.03 Å−1; p=4.4±0.6. The hypothesis that the interaction potential is the sum over all crystal layers of an atom–single layer potential V*(z) is used. V*(z) has a well depth D*=39.2±0.5 meV and a long range behavior given by C*4/z4 with C*4=5.2±0.3 eV Å4.

High resolution electron microscopy of some carbonaceous materials

SUMMARYThe image contrast of extremely thin films of amorphous carbonaceous materials has been studied experimentally using thin supporting films of single crystal graphite. The use of thin specimens reduces the ‘overlap’ effect in imaging amorphous materials. Image contrast of their images was therefore interpreted by the ‘projected potential approximation’.Single sheets of graphitic carbon crystal were imaged with c perpendicular to the beam and their image details were compared with those of the amorphous carbon films. This comparison demonstrated that amorphous carbon films prepared by evaporation contained microcrystalline regions having graphitic structure.

Catalytic gasification of graphite by bimetallic particles

Controlled-atmosphere electron microscopy was used as a microanalytical probe of Pt: Ir (80:20 atomic ratio) on single-crystal graphite. The test reaction for this probe was the catalytic gasification in 5 Torr oxygen. Continuous observation of samples prepared by vacuum evaporation of a Pt: Ir alloy wire onto graphite showed the particles propagated shallow channels emanating from edges and steps on the graphite surface for temperatures near 670 °C; increasing the temperature up to 950 °C led to increased channeling activity; near 1000 °C the catalytic gasification ceased; finally, at higher temperatures (ca. 1030 °C), channeling recommenced and particle mobility was observed. This behavior is characteristic of Ir particles, and no particles showing Pt behavior were seen. Thus, Pt: Ir on single-crystal graphite forms alloy particles, the surface of which is enriched with Ir in an oxidizing atmosphere. A similar series of experiments performed with a Pt: Rh alloy showed that the Rh component preferentially segregates to the surface of the particles.

Crystal growth and graphitization

A survey of the research concerning crystal growth with respect to the calcination of lime-stone and the preparation of synthetic mica cryatals and also those on carbon and graphite is described. Starting with X-ray diffraction investigation of carbon brushes, development of crystallinity in carbon, such as the composite structure of 00 l diffraction profile of carbon products, crystallization of graphite from carbon melt, graphitization under high pressure as observed by X-ray diffraction technique and by microscopy, and growth of single crystals of graphite are discussed in some detail.