Single Crystal Graphite Research Articles

Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part I. Optical emission spectroscopy

A comparative study of ablation plasma plumes originated from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum, the emission lines of carbon neutral C and ions of C+, C2+, and C3+ can be observed from both the SCG and a-C plasma plumes. The emission intensity from C atoms increases with laser energy density (EL) increase for both cases. The C2 emission intensity from the SCG plasma plume changes drastically with EL, while that from the a-C plasma plume is almost constant. The C2/C emission intensity ratio for the a-C case decreases with EL increase. As for the SCG case, the C2/C ratio decreases with EL increase up to 3.0 J/cm2, and increases slightly with further EL increase. Nanohardness of the deposited films decreases with the increase of the C2/C emission intensity ratio. It is suggested that for both the SCG and a-C target cases, the C2 molecule in the ablated plasma plume may not play an important role in producing high quality DLC films. It is further proposed that the threshold of laser fluence for the formation of diamond-like character film using KrF excimer PLD is 2.1 J/cm2(0.84×108 W/cm2) for the a-C target and 3.0 J/cm2(1.2×108 W/cm2) for the SCG target. The C2 vibrational temperature of the SCG and the a-C plasma plumes show different features on both the laser energy density and nitrogen pressure dependencies. Through optical emission spectroscopy and Langmuir probe measurements in vacuum and nitrogen background, it is concluded that there are many particles with higher mass in the SCG plasma plume, especially at relatively lower laser energy density below 3.0 J/cm2.

Temperature effect of electron-irradiation-induced structural modification in graphite

The structural change of graphite under high-energy electron irradiation at a wide range of temperatures was investigated by means of conventional transmission electron microscopy (CTEM), transmission electron diffraction (TED), high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). The experimentally observed result at lower temperatures (<600 K) could be basically interpreted by the local formation of non-hexagonal atomic rings (loss of the local hexagonal symmetry) in the graphite planes associated with the change in chemical bonding states. At higher temperatures (>650 K), a graphite single crystal was polycrystallized into nano-size crystallites with the original hexagonal network locally retained within each crystallite. The apparent activation energy derived from the present data over the whole temperature range was estimated to be 0.014 eV, which is in agreement with a reported migration energy of a self-interstitial atom between the basal planes.

Ultrahigh-Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry.

Ultrahigh-temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite-graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000 degrees C. The delta(13)C and delta(18)O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the delta(13)C values of single graphite crystals show variations in the order of 1 per thousand within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time-integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher delta(13)C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060 degrees C, which matches the petrologically inferred temperature estimates in the high-magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750 degrees C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite-graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.

Anisotropic growth of single-crystal graphite plates by nickel-assisted microwave-plasma chemical-vapor deposition

Growth of single-crystal graphite free-standing plates has been achieved by a microwavehydrogen-plasma etching of graphite powder and nickel mesh. The plates resemble a knife blade and grow in the 〈11̄00〉 direction with long crystals exceeding 100 μm. Hexagonal growth features at the edges and electron diffraction patterns confirm the single-crystal nature of these ultrathin plates. Electron microprobe and Raman spectroscopy indicate the presence of graphite. Diamond crystals nucleate on these plates and they grow simultaneously. We suggest that the paradoxical growth of graphite in a hydrogen plasma, under conditions in which graphite is usually etched away, is possible because of a protective coating by a Ni–C–H phase. This thin coating allows for transport of carbon atoms from the gas phase to the growing graphite surface.

Plasma and DLC Film Characteristics from Pulsed Laser Ablation Of Single Crystal Graphite and Amorphous Carbon: A Comparative Study Employing Electrostatic Probe Measurements

AbstractIn an ongoing effort to investigate plasma plume features yielding high quality DLC films, we have applied plasma plume diagnosis and film characterization to examine plume character distinction from KrF laser ablation of both amorphous carbon (a-C) and single crystal graphite (SCG) targets- The advancing plasma plume produced by these structurally different targets are observed to possess quantitatively similar total heavy particle inventory, ionized fraction, and electron thermal content, yet quite different ion kinetic energy, plume profile, C2 formation mechanism, and concentration of complex molecules. All data support the conclusion that the SCG target plasma plume is populated with heavier, more complex molecules than those in a-C which have been shown to be predominantly comprised of C and C+ under vacuum conditions with the addition of C2 at high fill pressure. Significantly smaller plume profile peaking factor, less energetic and slightly lower temperature plume conditions, laser energy (E1) dependent plume peaking, harder films produced at lower El strongly heterogeneous films, and lesser plume energy attenuation in high pressure background fill in SCG target plumes all support the conclusion of more massive plume species in SCG target plumes. Energy balance estimates indicate that ion kinetic energy dominates and that SCG target ablation liberates about twice the number of 12C atoms per unit El.

Instant diamond

Both graphite and diamond can be found in single-crystal form, so Daedalus proposes to flip one crystal form into the other by explosive forming. His idea is to lay an explosive next to a carefully shaped piece of single-crystal graphite, then to detonate it. The shock-wave running through the graphite should collapse its lattice into the denser structure of diamond.

Exceptional anisotropy in conductivity and mechanical properties of poly-3-octylthiophene films

Poly-3-octylthiophene (P3OT) films electrodeposited onto stainless steel substrates at a current density of 0.5 mA/cm 2 were found to exhibit ultra high tensile strength of 127 MPa. The outstanding anisotropy in conductivity of the P3OT film was also observed, and the conductivity along the direction parallel to the plane was about 10 5 times as much as that measured along the direction perpendicular to the plane of the P3OT film, which was higher than that (10 4) of single crystals of graphite. Various experimental parameters affecting the anisotropy in conductivity were studied.

Preparation and structure of nitrogen-substituted, highly oriented graphite thin film prepared by chemical vapor deposition

Highly-oriented, nitrogen-substituted graphite (C/N graphite) thin films were prepared by chemical vapor deposition (CVD) using pyrrole as a starting material and palladium deposited single crystal sapphire as a substrate material whose temperature was kept at 1273K. The interlayer spacing of C/N graphite films, determined by x-ray diffraction, was close to that of single-crystal graphite. The orientation of crystalline of C/N graphite films, evaluated by rocking angle measurement, was higher compared with that of highly oriented pyrolytic graphite. XPS and Raman spectroscopy were also measured for the characterization of these films.

Synchrotron X-ray Diffraction Studies of Multilayer Nitrogen Physisorbed on Porous Graphite Foam

We use synchrotron X-ray diffraction for structural analysis of the behavior of multilayer nitrogen films physisorbed on graphite foam. We provide structural information and concentrations of two- and three-dimensional solid phases at a coverage of ϑ = 8 ML (ϑ ≡ 1 ML for a √3 × √3 monolayer structure) for temperatures from below the bulk α−β transition temperature [Tα-β = 34 ± 0.5 K] to above the bulk triple point [Ttp = 63 K]. Our data indicate that layering begins near Tα-β, with subsequent layering occurring as the temperature is raised; all bulk nitrogen forms disordered film layers by 48 K at ϑ = 8 mL. Our results are consistent with ellipsometry studies of nitrogen on highly oriented pyrolytic graphite which found multilayer nitrogen on single-crystal graphite to undergo a series of layering transitions above the bulk nitrogen α−β structural transition. [Volkmann, U. G.; Knorr, K. Phys. Rev. Lett. 1991, 66, 473.] The effect of adsorption on a graphite foam substrate, which results in capillary condensation and finite size effects, is limited to a broadening and overlap of the discrete transition temperatures observed on a single-crystal substrate. A phase diagram for coverages above 2 ML is proposed, summarizing this and previous work.

Plasma Diagnostics During Pulsed Laser Deposition of Diamond-Like Carbon Using Single Crystal Graphite and Amorphous Carbon

ABSTRACTOptical emission study of ablation plasma plumes from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been investigated. The C I emission intensity increases with laser energy density (EL) increase, while the C2 emission changes drastically with EL for both ablated plasma plumes. The C2/C emission intensity ratio for the a-C plume decreases with EL increase, while the C2/C ratio for the SCG plume decreases with EL increase up to 3.0 J/cm2, then increases slightly with further EL increase Nanohardness of the DLC films decreases with the C2/C ratio increase. It is suggested that the C2 molecule in the plasma plume does not play an important role in producing high quality DLC films, and that the C2/C ratio is a good parameter to monitor the PLD process.

Properties of Carbon Films Prepared by Magnetron Sputtering of Woodceramics.

Highly resistant, high-transmittance woodceramic thin films were prepared using rf magnetron sputtering of a woodceramic disk in argon plasma. A film series was deposited based on substrate temperature, which was varied from 50 to 500°C. The film's electrical and optical properties depended on substrate temperature. Films deposited below 300°C were insulative, ρ>1010Ω·cm. Films deposited at 50°C had a density of 1.9-2.2g/cm3 comparable to that of single crystal graphite. Below 200°C, films had higher transmittance than typical DLC films in the visible and infrared region. Infrared C-H absorption spectrum was observed by FT-IR and there exist two types of bonding corresponding to sp2 or sp3.

Raman spectroscopic studies on well-defined carbonaceous materials of strong two-dimensional character

Using 514 nm radiation and a careful experimental protocol allowing us to make quantitative intensity analyses, we have examined the first- and second-order Raman spectra of five varieties of graphite. These include single-crystal graphite and highly oriented pyrolytic graphite as references, then two with a significant content of c-axis translation faults introduced through grinding and exfoliation, and a last sample which is purely turbostratic. We show that full c-axis stacking disorder results in a strong increase of the scattered E2g-mode intensity. We have further shown that even in those strongly disordered stacking sequences, but in which the AB sequence persists over only a few layers, the doublet around 2700 cm−1 is resolved. We argue against assigning the peak around 1355 cm−1 to “disorder,” as is so often done, but to the finite in-plane domain size as proposed a number of years ago; moreover, we suggest that this peak is a more sensitive probe of such small domains than is x-ray diffraction.

Atomic and electronic structures of heat treated 6H–SiC surface

We have studied heat-treated (950–1300°C) 6H–SiC(0001)Si and (0001̄)C face with photoemission spectroscopy using synchrotron radiation (SR-PES) and low energy electron diffraction (LEED). We observed LEED patterns of SiC 1×1, 3 × 3 , 3 × 3 +6 3 ×6 3 and graphite 1×1 sequentially with increasing heating temperature for (0001)Si face and SiC 1×1 for (0001̄)C face, respectively. We have measured Si(2p) spectra and valence band energy distribution curves (VB-EDCs). The trend of sublimation of Si atoms from surface is different between Si- and C-face. 3 × 3 superstructure must be Si-derived. The 6 3 ×6 3 structure could be explained as a moiré pattern caused by monolayer-graphite sitting on SiC surface. Si 3s-derived state of SiC 1×1 is different between SiC 1×1 for Si- and C-face. It is suggested that a single crystal graphite layer grows on Si-face and a polycrystalline graphite is formed on C-face for heated specimens above 1150°C.

Electrical resistivity of single-crystal graphite under pressure: An anisotropic three-dimensional semimetal

The in-plane (ρa) and c-axis (ρc) resistivities of natural single-crystal graphite have been measured from 4 to 450 K at zero pressure and from 77 to 450 K under pressures up to 500 MPa. Data for ρ ...

The electronic structure of different forms of solid carbon studied by electron momentum spectroscopy

Abstract The purpose of this paper is to first give an overview of the information obtained by electron momentum spectroscopy (EMS) on the electronic structure of different forms of solid carbon (single crystal graphite, polycrystalline carbon, different forms of amorphous carbon, and fullerenes). We show for sp2 bonded carbon that, with decreasing order, the structures observed change but certain basic features remain. sp3-bonded carbon films and fullerene films both have electronic structures different from these graphitic films. The results of EMS are compared with the results of other spectroscopic and scattering techniques. It is argued that the combination of energy and momentum resolution is crucial in order to get a clear picture of the changes in the electronic structure due to the changes in atomic arangements. Only EMS resolves both energy and momentum for all these solids.

Chemical Vapor Deposition Based Synthesis of Carbon Nanotubes and Nanofibers Using a Template Method

We have developed a new approach for preparing graphitic carbon nanofiber and nanotube ensembles. This approach entails chemical vapor deposition (CVD) based synthesis of carbon within the pores of an alumina template membrane with or without a Ni catalyst. Ethylene or pyrene was used in the CVD process with reactor temperatures of 545 °C for Ni-catalyzed CVD and 900 °C for the uncatalyzed process. The resultant carbon nanostructures were uniform hollow tubes with open ends. Increasing the deposition time converted the carbon nanotubes into carbon nanofibers. Transmission electron microscopy and electron diffraction data show the as deposited graphitic carbon nanofibers synthesized with the Ni catalyst were not highly ordered. Heating the carbon-containing membrane at 500 °C for 36 h, however, converts the carbon nanofibers into highly ordered graphite. The electron diffraction data show a spotted diffraction pattern characteristic of single-crystal graphite with the graphitic planes parallel to the long axis of the nanofibers.

Electrical Conductivity of Single Crystal Pyrolytic Graphite under Multiple Shock-Wave Compression.

Electrical Conductivity of Single Crystal Pyrolytic Graphite under Multiple Shock-Wave Compression.

Investigation of the relative stability of solid phases in the La-graphite system based on the method of multicenter atom-atom potentials

The relative stability of crystalline phases of the lanthanum-carbon system with metal contents up to 33.3 at. % is investigated using the method of semiempirical atom-atom potentials. It is shown that the stability of these phases depends on the stoichiometry, the structure and degree of order of the lattice, and also on the crystal size. Among the bulk phases, the most stable is a phase with the calcium carbide structure, followed by phases with the metal graphitide and fluorite structures. In layers with thicknesses of a few nanometers, including 1–4 monolayer films, the energies of formation of lanthanum graphitide and carbide approach one another. This makes the phase transition from carbide to graphitide easier, especially when the film of lanthanum-carbon compound is grown on the surface of single-crystal graphite.

Deuterium retention in beryllium, molybdenum and tungsten at high fluences

The retention of 1 keV D in room-temperature beryllium, molybdenum and tungsten has been measured as a function of fluence, for incident D 3 + fluences ranging from 10 21 to 10 25 D/m 2. The amount of D retained in Be tends to saturation (∼ 2.7 × 10 21 D/ m 2) for incident fluences > 10 22 D/ m 2, with an estimated saturation concentration (based on TRIM vectorized for multicomponent targets, TRVMC, range calculations) of ∼ 0.39 D/ Be within the implantation zone; this retention level is similar to what has been observed for single-crystal graphite, viz, ∼ 2.2 × 10 21 D/ m 2 or ∼ 0.4 D/ C. For fluences greater than about 10 22 D/m 2, the amount of D retained in tungsten also tends to saturation at ∼ 6 × 10 20 D/ m 2. Assuming a maximum ion implantation depth of ∼ 40 nm (as calculated by TRVMC), this amounts to a concentration of ∼ 0.24 D/ W. The amount of D retained in Mo does not appear to reach saturation over the fluence range studied. D retention in Mo is lower than that in W for incident D fluences ≲ 10 25 D/ m 2, at which point the two retention curves cross.

Graphitization behavior of Kapton-derived carbon film related to structure, microtexture and transport properties

Graphitization behavior of the carbonized film of commercially available aromatic polyimide film Kapton 25 μm in thickness was studied by X-ray diffractometry, measurements of the transport properties such as the electrical resistivity, Hall coefficient and magnetoresistance, and scanning electron microscopy. The specimens were films carbonized at 900 °C and heat-treated at temperatures between 1000 and 3000 °C with a step of 100 °C. For the specimens carbonized and heat-treated below 2000 °C, the average lattice constant a 0 was measured to be lower than that of the single crystal of graphite. Changes related to the transformation from the turbostratic structure to the graphitic occurred at a heat treatment temperature around 2200 °C in the microtexture on the fracture surface and the resistivity at liquid nitrogen temperature. The changes also appeared in the ratio of the resistivity at room temperature to that at liquid nitrogen temperature, and the Hall coefficient and magnetoresistance at liquid nitrogen temperature. The dependences of the resistivity ratio and also the Hall coefficient on HTT for the specimens with turbostratic structure can be related to the average crystallite size L 0(110) along the a-axis. The results of the resistivity, Hall coefficient and magnetoresistance were analyzed by a simple model.