Single Crystal Graphite Research Articles

A synchrotron X-ray diffraction study of the structural phase behaviour of multilayer xenon on single-crystal graphite

We present a high-resolution synchrotron X-ray diffraction study of xenon adsorbed on single-crystal graphite for coverages of two, three and six monolayers. We observe the evolution of the lattice mismatch between the xenon overlayer and a square root 3* square root 3R30 degrees structure commensurate with the underlying substrate. In the case of the bilayer we see a first-order incommensurate-commensurate (IC-C) phase transition at 62.0 K+or-1.2 K, which we demonstrate is consistent with the layer closest to the substrate having locked into the commensurate sites. The role of substrate quality is discussed and we conclude that previous studies on relatively poorer substrates may have favoured equilibrium structures strongly influenced by the surface imperfections. The risk of an insufficiently equilibrated adsorbate is discussed in the context of the very slow kinetics we observe for multilayer xenon on high-quality substrates. For the three- and six-layer adsorbates we observe a systematic increase in lattice mismatch with film thickness and no indication of an incommensurate-commensurate transition. We conclude that the trilayer system consists of many distinct domains approximately 500 AA across, each of which has a random close packed structure.

Simultaneous angle-resolved measurement of the band structure of single-crystal graphite by an improved two-dimensional display analyzer

We have embodied a new version of two-dimensional display-type spherical mirror analyzer [H. Daimon and S. Ino, Rev. Sci. Instrum. 61, 57(1990)]. A couple of concentric hemispherical small grids and 12 pairs of obstacle rings have been attached to the original type of analyzer in order to operate with a constant pass-energy mode and improve the uniformity of the energy resolution of the analyzer all over the detection area, respectively. We performed a test on the resolution of the analyzer, and applied it to the simultaneous angle-resolved photoelectron spectroscopy on the band structures of single-crystal graphite (Kish graphite) within the acceptance cone of ±50°. The observed band structure is in good agreement with those reported by other investigators. We observed a ring-like pattern which enlarged as the binding energy approached the Fermi level. This ring is ascribed to the cross section of the π band. Although the designed energy resolution is 1% of the pass energy, the minimum value realized reached in this study was 0.2 eV for the pass energy of 10 eV [H. Daimon, Rev. Sci. Instrum. 59, 545 (1988); 61, 205 (1990)].

Giant and supergiant lattices on graphite.

Anomalous giant lattices have been observed on four separate samples of highly oriented pyrolytic graphite with scanning tunneling microscopy. They exhibit hexagonal symmetry with lattice constants of 1.7, 2.8, 3.8, and 6.6 nm. Atomic resolution of graphite was obtained simultaneously. Kuwabara, Clarke, and Smith [Appl. Phys. Lett. 56, 2396 (1990)] have suggested that those superperiodic features may be Moire patterns due to rotational misorientation of the top layer relative to the underlying graphite single crystal. In this paper we present (1) evidence for the misorientation of the graphite top layer which causes the observed giant lattices, (2) a complete description to account for detailed features of these giant lattices, (3) observation of a supergiant lattice superimposed on the atomic and giant lattices, and (4) adsorption of cobalt particles on the giant lattice.

Catalyzed Carbon Gasification Studied by Scanning Tunneling Microscopy and Atomic Force Microscopy

Several important catalyzed gas-carbon reactions are studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM): C/H2/Pt, C/H2/Pd, C/O2/V2O5, C/CO2/K2CO3, and C/ NO/Rh. Single-crystal graphite is used as a prototype of carbon in the reactions. STM and AFM provide atomic resolution in three dimensions, making possible the direct identification of monolayer and multilayer structures on the reacted graphite surface. Reactions take place on the basal plane of graphite through monolayer and multilayer channeling, pitting. and edge recession. Therefore, by examining the etched surface structures of graphite after different reactions, reaction pathways and reaction parameters can be determined. It has been found that channeling is the primary contribution to the gasification rate in all these reactions. Two different modes of channeling are found. For reactions in C/H2/Pt, C/H2/Pd, C/O2/V2O5 systems, catalyst particles remain unchanged in size. Thus, the resulting channels either have constant-width or form zig-zag structures due to the changes of the motion of catalyst particles. However, for reactions in the C/CO2/K2CO3 and C/NO/Rh systems, wedge-shaped channels are produced. This indicates that the catalyst particles are shrinking in size, either because of volatilization or deposition of catalyst at the step edge.

Interaction of Hydrogen with Metal Sulfide Catalysts - Direct Observation of Spillover

A combination of controlled-atmosphere electron microscopy and in-situ electron diffraction techniques have been used to study the manner by which certain metal sulfides interact with 0.2 Torr hydrogen. In these experiments single crystal graphite was used as a probe material since its reactivity in both molecular and atomic hydrogen is well characterized. When the metal sulfide was in direct contact or physically separated from the graphite probe, pitting of the basal plane regions was observed even at room temperature. This unusual behavior is believed to result from the action of atomic hydrogen which is produced via reversible dissociation of molecular hydrogen on the metal sulfide particles. These species are extremely reactive towards the π-electrons present on the graphite basal planes and this action leads to the creation of pits. At the low pressures used in this work, it is clear that the atomic species can migrate not only by surface diffusion processes (spillover) but also by transport through the gas phase.

X-ray-diffraction and scanning-tunneling-microscopy studies of a liquid-crystal film adsorbed on single-crystal graphite.

Synchrotron x-ray-diffraction and scanning-tunneling-microscopy (STM) experiments reveal a new commensurate monolayer structure of 10CB (decylcyanobiphenyl) molecules adsorbed on the (0001) graphite surface. Our results are consistent with two generic structures for nCB monolayers on surfaces of hexagonal symmetry. The monolayer d spacing of the new phase inferred by STM is 10% larger than that obtained by x-ray diffraction on the same sample. We suggest that part of this discrepancy results from a systematic error introduced in calibration of the STM length scale against the graphite substrate.

Steps of Surface Etching and Carbon Deposition on the Graphite Basal Plane

AbstractThe reactions of O2, H2O, CO2, NO2, NO, and N2O with single crystal graphite between 400 and 700°C have been studied by STM to obtain quantitative kinetics by measuring the number and size of monolayer pits on the basal plane versus temperature and time. At low temperature the reaction initiates exclusively from the point defects on the basal plane to form monolayer pits. The shape of the monolayer pits vary from nearly triangular to hexagonal to circular depending on the rate of the reaction and the reacting gases. The sizes of the monolayer pits grow linearly with reacting time. The monolayer reaction rates follow the order of RNO2 > RN2O > RNO > RO2 > RH2O > RCO2. The activation energies for reactions with O2, H2O, NO2, NO, and N2O, are determined to be 127, 205, 60, 89, and 74 kJ/mol respectively.Carbon deposition from hydrocarbons onto surfaces of single crystal graphite has been examined to study the fundamental steps of chemical vapor deposition. Uniform monolayer pits on graphite surface were first produced by reactive etching of freshly cleaved single crystal of graphite in oxygen and carbon was then made to deposit exclusively on these defects in the basal plane. Carbon vapor deposition forms unique structures around the monolayer steps. By measuring the sizes of structures on steps in various gases versus temperature and pressure, the kinetics of hydrocarbon decomposition and the role of surface diffusion can be determined.

Experimental determination of the atomic quadrupole moment of graphite

The symmetry of the graphite crystal implies that each carbon atom has an electric quadrupole moment associated with it. This atomic moment may have a significant influence on the location of adsorbed molecules on a graphite surface and on the adsorption energy. A novel experiment, involving the measurement of the effect of a laboratory electric field gradient on the period of oscillation of a small single crystal of graphite mounted on a glass torsion fibre, has been carried out. The quadrupole moment of a carbon atom within the sample is found to be (–3.03 ± 0.10)× 10–40 C m2; however, this value may include a contribution from surface effects.

On the crystal orientation of graphite lamellae in unidirectionally solidified Fe-C eutectic alloy

As is well known, the crystal structure of graphite is hexagonal, and the atoms in the (0001) face are strongly combined by covalent bonds, while the atoms between the (0 0 01) faces are linked weakly by the Van der Waals force. Because the strength of a covalent bond is much greater that that of a Van der Waals bond [1, 2] the mechanical properties of single-crystal graphite are strongly anisotropic. It was thought that, by using the unidirectional solidification technique, the Fe-C eutectic in situ composite should be able to be produced, and its mechanical properties also would be strongly influenced by the crystal orientation of the graphite flakes. Therefore, it is important to determine which crystal face of graphite is parallel to the growth direction. In the present work, with the help of scanning electron microscopy (SEM) and X-ray diffractometry, we examined the crystal orientation of graphite lamellae in Fe-C eutectic in situ composite. In addition, the longitudinal ultimate tensile strength of this material was investigated. The chemical composition of the eutectic Fe-C alloy prepared from commercial pure iron and pyrolytic graphite was (wt %): C 4.35, Si 0.040, Mn 0.010, P 0.010 and S 0.012. The experiments were conducted using the Bridgman technique with a temperature gradient (G) in the liquid of 110 Kcm -I and growth velocities (R) ranging from 0.278 to 62.0/xms -1. Pure argon gas was introduced into the crucible to protect the molten alloy from oxidation during the experiment. At the end of solidification cylindrical specimens about 120 mm long and 12 mm in diameter were

STM study of single-crystal graphite

Freshly cleaved surfaces of single-crystal graphite (SCG) are shown to have very large expenses of step-free surface; however, the tunnelling signal is found to be considerably noisier than with HOPG. We speculate that the noise is due to carbon adatoms hopping onto the STM tip. Carbon adatoms are normally immobilized at steps. The lower step density on SCG results in an increased average surface concentration of carbon adatoms as compared with HOPG. Atomically resolved topographic images of the (0001) surface of SCG were also obtained. They show the same trigonal symmetry that characterizes the analogous images of HOPG, but with a two- to three-fold increase in corrugation depth. In addition to the low density of steps, SCG is relatively devoid of graphitic artifacts and pseudo-periodic features that have been mistaken for DNA and other genuine molecular structures, implying that SCG may be a useful substrate for STM studies of biological samples.

In-situ electron microscopy studies of surface segregation in bimetallic catalyst particles

We have used various techniques including in-situ transmission electron microscopy to examine the manner by which the catalytic action of platinum is modified by the introduction of either iron, cobalt, or nickel into the metal. Changes in the behavior of the noble metal have been investigated by the use of the probe reactions, catalytic oxidation, and hydrogenation of single-crystal graphite, which are known to be extremely sensitive to the chemical state of the catalyst surface. In-situ electron diffraction measurements showed that in all cases alloys were formed when the mixed system was treated in hydrogen. When such specimens were subsequently heated in oxygen the composition and chemical state of the particles changed significantly, the ferromagnetic metal tending to segregate to the catalyst-gas interface in the form of an oxide leaving platinum and residual alloy at the graphite interface. Under these conditions the catalyst was observed to undergo a spreading action along the graphite edge regions and it was interesting to find that in this bimetallic state there was a significant decrease in the catalytic activity of the noble metal toward graphite oxidation.

High-field phase in the magnetic-field-induced electronic phase transition of graphite.

The magnetoresistance of single-crystal graphite has been measured at low temperatures in pulsed high magnetic fields up to 52 T applied parallel to the c axis. Many features were found in the high-field phase after the field-induced electronic phase transition. After the sharp rise at the phase transition, the resistance shows a dramatic decrease with increasing field, indicating the reentrant character of the phase boundary. The phase transition was no longer observed at the elevated temperature of 10.8 K in a field up to 48 T. Fine structures consisting of local maxima and an inflection point were observed in the high-field phase. Although the temperature dependence of the critical field is qualitatively in accord with Yoshioka and Fukuyama's charge-density-wave model, a more detailed theory is required to explain most of the present observation.

Kinetics of coverage loss for H2, HD, and D2 physisorbed on graphite

Low-energy electron diffraction was used to study the kinetics of the coverage loss effect for H2, HD, and D2 physisorbed on graphite single crystals at 5 K. The present study shows striking isotope dependence of the rate of coverage loss: 4.1 × 1011 cm−2 s−1 for D2, 6.5 × 1011 cm−2 s−1 for HD and 2.8 × 1012 cm−2 s−1 for H2 in the incommensurate monolayer regime. Below monolayer completion, the rates of coverage loss depend upon coverage very weakly. The rates become larger in the multilayer regime. We found the coverage loss effect is qualitatively consistent with the infrared induced desorption effect that was found by others for hydrogen and isotopes on several metal and insulator surfaces at low temperature. The possibility of coverage loss induced by surface diffusion is also discussed and ruled unlikely.

New features of the electronic phase transition observed for graphite in high magnetic fields

We have measured the magneto-resistance in single crystal graphite in pulsed high magnetic fields up to 52 T at low temperatures. When the magnetic field was applied parallel to the c-axis, many new features were found in the high field phase after the field-induced electronic phase transition. After a sharp rise at the phase transition, the resistance shows a dramatic decrease with increasing field, indicating the re-entrant character of the phase boundary. Fine structures consisting of local maxima were observed in the high field phase. Also observed was a prominent frequency dependence of the real and imaginary parts of the AC magneto-resistance, suggesting the validity of the pinned CDW model.

High-quality and highly oriented graphite block from polycondensation polymer films

High-quality and highly oriented graphite was produced in the form of a thick block having physical properties close to those of single-crystal graphite. It was prepared from polyimide films of 25 μm thick with high molecular orientation. A sheaf of the films was heat treated in an argon atmosphere at temperatures up to 1000°C (carbonization) and then heated for graphitization up to 3000°C under a pressure between 100 and 300 kg/cm 2. Graphite blocks as large as 150 × 50 × 13 mm 3 were prepared, and the mosaic spread (the degree of preferred orientation) attained was 0.4°.

Interaction of deuterium atoms with radiation defects in graphite

A thermal desorption mass spectrometry (TDS) study of deuterium trapping in single crystal graphite as a result of ion irradiation at low doses is presented. TDS spectra reveal several desorption peaks attributed to the dissociation of multiply occupied deuterium-vacancy complexes. A mechanism for the dissociation of these complexes is proposed. The resulting dissociation energy values are in fairly good agreement with those determined experimentally.

Light-Transparent Phase Formed by Room-Temperature Compression of Graphite

Single-crystal graphite has been compressed at room temperature and found to undergo a transformation at 18 gigapascals as indicated by a drastic increase in the optical transmittance. This high-pressure phase is unquenchable; a more transparent phase formed by the heating of this phase can be quenched at ambient conditions. This latter phase is probably "hexagonal diamond," and the present observations suggest that the structure of the transparent phase obtained by the room-temperature compression is different from the hexagonal diamond structure.

Ethylene on graphite: A low-energy electron-diffraction study.

Several phases of monolayer ethylene on single-crystal graphite have been studied using low-energy electron diffraction. We have determined the unit cell and orientation with respect to the graphite substrate of the orientationally ordered and disordered low-density phases (OLD and DLD), in which molecules are believed to lie with the C?C bond parallel to the surface. Based on published neutron-scattering results, we propose a basis for the OLD phase. The low-coverage DLD phase, believed to undergo a continuous melting transition at 70 K, appears to be a higher-order commensurate structure at this transition. We also report observations of ordered and disordered high-density phases in which molecules stand on end, in both commensurate and incommensurate epitaxies.

The Effect of Argon Addition on the Microstructure, Texture and Phases of Silicon Carbide Prepared by Chemical Vapor Deposition

Silicon carbide was chemically deposited on the (111) surface of silicon single crystal and isotropic pyrolytic graphite by using methyltrichlorosilane as source gas. The deposits were examined by X-ray diffractometry and electron scanning microscopy (SEM) and found to be composed of β-SiC, free Si and a trace of 2H-SiC. The coatings yielded the less free silicon in it using graphite as substrate and at a higher argon flow rate. X-ray analysis indicated that the degree of preferred orientation varied with the kinds of substrate materials and the amount of argon added.

Growth and Microstructure of Aluminum Nitride Thin Films

AbstractThe synthesis of aluminum nitride thin films by pulsed-laser ablation is demonstrated. The films were formed on single-crystal sapphire and graphite substrates. A number of techniques were used to characterize the films: transmission and scanning electron microscopy, Rutherford backscattering spectrometry and x-ray diffraction.