Anomalous two-phonon absorption in diamond nanocrystals embedded in amorphous carbon

Observation of anomalously strong IR absorption are reported in the region of two-phonon diamond frequencies in amorphous hydrogenated carbon films grown by cosputtering graphite and copper in a plasma. Up to five structural elements of bands having frequencies close to or coinciding with the figures quoted in the literature for two-phonon absorption bands in bulk diamond were observed. This suggests the presence of diamond nanocrystals in the grown layers. Observation of two-phonon absorption in thin films is in itself remarkable because of the smallness of its coefficient in bulk diamond. An estimate of the absorption coefficient in the dominant band at 2140 cm^?1 yields about 200 cm^?1, which exceeds by more than an order of magnitude that for bulk diamond. This can be assigned to an anomalous enhancement of the two-phonon absorption coefficient due to phonon confinement in the small diamond particles nucleated in the amorphous carbon matrix. An estimate of these inclusions from the band width yields about 20 Å. The reasons for the catalytic activity of copper in diamond nucleation are analyzed.     

Critical-point analysis of the two-phonon Raman spectrum of silicon

Two-phonon Raman spectrum of silicon has been measured in back-scattering configuration at room temperature using Argon ion laser. It reveals many critical points and consists mainly of two-phonon overtone states. The agreement between the calculated density of states from neutron diffraction data and the present Raman spectrum is excellent except the region where combination spectrum appears. Polarization measurement shows that the structure centered at 300 cm^?1 is not the one-phonon fine structure, predicted by Cowley and observed in part and identified as such by Wright and Mooradian, but the two-phonon overtone spectrum of acoustic phonons. Critical-point analysis has been done on the two-phonon overtone and combination spectra, and phonon frequencies of points of high symmetry in the Brillouin zone are determined and compared with other experimental results. In contrast to the Raman spectrum of diamond, no peak was observed near twice the frequency of zone-center optical phonon. This fact is explained by the difference of phonon dispersion relation between diamond and silicon.     

Copper-oxygen substructures in carbon allotropes (graphite and fullerites)

A comparative study of crystalline graphite and copper-doped fullerite membranes is reported. It is assumed that C₆₀ clusters form complexes with oxygen and copper similar to those known to exist in graphite. Above room temperature, these complexes, first, change the symmetry of the fullerite lattice and, second, are responsible for the nonmonotonic temperature dependence of the electrical resistance. Fiz. Tverd. Tela (St. Petersburg) 41, 748–750 (April 1999) Deceased.     

Novel techniques for the prepartion of synthetic diamond

Abstract

The objectives of this work are two fold: (1) to study the effect of using oxygen-acetylene flame grown synthetic diamond as seed crystals for the high pressure-high temperature conversion of graphite into diamond and (2) to demonstrate the ability to produce small crystallites of diamond by a simple, electron beam evaporation technique. In each case, the production of diamond from graphite was confirmed.     

Shock metamorphism of the graphite quasimonocrystal

Abstract

Micro structure examination of graphite quasimonocrystal recovered after dynamic loading to pressure of 35-45 GPa was carried out. Only a small amount of cubic diamond and recrystalized graphite was detected. Most of the graphite (～80 vol.%) remained in initial high orientation, but transformed to fine, grained phase with crystalite size 0.1-1 microns. Relaxation time of the transformation (～ 10 ns) and the degree of the transformation (～ 70-80 vol.%) were determined by means of measurements of the electrical resistivity during loading up to 26 GPa and following computer simulation of the results. We proposed that two simultaneous processes take place at pressures higher than 20 GPa: i) relatively slow diffusive graphite to diamond transformation localized in the zones with defect structure: ii) highly oriented graphite transforms to a diamond like phase with density of about 3.2 g/cm³ at zero pressure. This, transformation has fast, martensitic kinetics and is reversible.     

Two-phonon bound states in imperfect crystals

The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover, the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared absorption and Raman scattering from mixed crystals of GA_1−c Al _c P and Ge_1−c Si _c are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency (difference spectrum) peak to the central peak observed in structural phase transitions, is conjectured; This work is a part of the thesis to be submitted by one of the authors (SS) in partial fulfilment of the degree of Doctor of Philosophy to Utkal University, Bhubaneswar, India.     

Electronic structure of graphite nanopipes

A recursion method is used for calculating the electron-state density n(E) of carbon in diamond, graphite, C₆₀ fullerene, and graphite nanopipes of various structures and diameters. The calculated n(E) for diamond, graphite, and fullerene are compared with experimental data. The distinctive features of the electron-state density n(E) in graphite nanopipes are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1118–1121 (June 1997)     

Melting behaviors of carbon underhigh pressures

Abstract

The melting behaviors of graphite and diamond were investigated at pressures up to 25 GPa using flash-heating method. By rapid heating, the metastable graphite was melted in the diamond stable P-T field, competing with its conversion to diamond in the rate of reaction. For the diamond the pressure dependence of inserted energy required to reach the molten state suggested that the melting temperature of diamond increases with pressure.     

Enhanced diamond nucleation on copper substrates by graphite seeding and CO2 laser irradiation

Diamond nucleation on copper (Cu) substrates was investigated by graphite seeding and CO₂ laser irradiation at initial stages of the combustion-flame deposition. A graphite aerosol spray was used to generate a thin layer of graphite powders (less than 1 μm) on Cu substrates. The graphite-seeded Cu substrates were then heated by a continuous CO₂ laser to about 750 °C within 1 min. It was found that diamond nucleation density after this treatment was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4 μm were obtained in 5 min. The enhancement of diamond nucleation on the graphite-seeded Cu substrates was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO₂ laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth.     

Electrical properties of Cu-O monolayers intercalated into crystalline graphite

Electrical properties of Cu-O monolayers intercalated into crystalline graphite have been studied at microwave frequencies (up to 1 GHz) in the temperature range 80–400 K. As the temperature increases above 300 K, the resistance of the starting graphite samples increases because of oxygen desorption. Heating a sample containing intercalated Cu-O layers results in a transition from the metallic to the semiconducting (or insulating) state in the 95–130 K interval. At T=8 K, the samples exhibit microwave absorption typical of superconductors (this effect is not observed at T=260 K). Fiz. Tverd. Tela (St. Petersburg) 39, 97–100 (January 1997)     

Influence of the nucleation behaviour on the synthesis of diamond

Abstract

Using a solid state nucleation model the influence of different carbon types on the nucleation number of diamond has been studied, keeping constant the synthesis parameters p, T, t. The carbon types used have been investigated concerning their mechanical, electrical and chemical properties; the graphite content and the lattice perfection of the graphite are also determined. As a experimental result the nucleation number shows a significant dependence on the carbon type used, particularly on the amount of crystalline graphite.

The results show the possibility to influence certain steps of the diamond synthesis process.     

Laser Induced Damage in CVD Diamond Films

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A Combined Petrographic and Micro-Raman Study of Meteoritic Microdiamond in ALH-77257 Ureilite and ALH-78113 Aubrite

ABSTRACT

Two diamond-bearing meteorites (ALH-77257 and ALH-78113) were investigated by petrographic microscope and micro-Raman spectroscopy. The meteoritic diamonds can be found in fractures or veins cross-cutting the original minerals (olivine, pyroxene, feldspar) and inside the host minerals. The micro-Raman spectral features of microdiamonds in two studied meteorites are very similar to each other. The diamond peak was found at 1329 in ALH-77257 and 1332 cm^?1 in ALH-78113. Hexagonal diamond was not present. However, we observed the D and G bands of graphite or amorphous carbon at 1322 and 1582 cm^?1 wavenumbers, indicating an origin of the meteoritic microdiamonds by magmatic processes.     

Photoacoustic spectroscopy of diamond powders and polycrystalline films

Photoacoustic spectroscopy is used to study optical absorption in diamond powders and polycrystalline films. The photoacoustic spectra of diamond powders with crystallite sizes in the range from ∼100 μm to 4 nm and diamond films grown by chemical vapor deposition (CVD) had a number of general characteristic features corresponding to the fundamental absorption edge for light with photon energies exceeding the width of the diamond band gap (∼5.4 eV) and to absorption in the visible and infrared by crystal-structure defects and the presence of non-diamond carbon. For samples of thin (∼10 μm) diamond films on silicon, the photoacoustic spectra revealed peculiarities associated with absorption in the silicon substrate of light transmitted by the diamond film. The shape of the spectral dependence of the amplitude of the photoacoustic signal in the ultraviolet indicates considerable scattering of light specularly reflected from the randomly distributed faces of the diamond crystallites both in the polycrystalline films and in the powders. The dependence of the shape of the photoacoustic spectra on the light modulation frequency allows one to estimate the thermal conductivity of the diamond films, which turns out to be significantly lower than the thermal conductivity of single-crystal diamond. Fiz. Tverd. Tela (St. Petersburg) 39, 1787–1791 (October 1997)     

Resonant Raman scattering at the indirect exciton in silver bromide

By excitation in the indirect exciton absorption of AgBr at low temperatures selectively enhanced two-phonon Raman scattering is observed. Using different excitation wavelengths the resonance enhancement is found to be associated with the Г-L exciton as intermediate state for the resonant scattering process. The resonant phonons involved are pairs of LA and TA phonons with opposite wave vectors near L. Measurements in the temperature range 2 K ? T ? 40 K show a decrease of the scattering intensity with increasing temperature. The origin of this temperature dependence is due to lifetime broadening of the scattering state. Several features of the indirect exciton absorption of AgBr are discussed.     

The laboratory confirmed natural crystallization of diamond from carbonized organic compounds

Abstract

In the paper the conception of the natural diamond growth as a result of the organic compounds reaction in the presence of noncompensated spin was checked. They might have played the essential role in the nucleation of diamond crystals. In the experiments, the partly carbonized phenolformal-dehyde resin instead of graphite was used as a carbon source. The final result of the process of the diamond growth depended on the temperature of resin carbonization as well as on the temperature and pressure of synthesis. The 3,8 GPa pressure, which is less than needed for graphite to diamond transformation in the classic industrial process, was sufficient to produce transparent and colourles diamond crystal of the size up to 0,7 mm.     

High-pressure synthesis of a novel form of endohedral Li diamond from Li graphite intercalation compound

A novel form of hexagonal diamond containing Li atoms in the open rooms surrounded by sp³-bonded carbon atoms was successfully synthesized from a Li graphite intercalation compound under high pressure, as had been predicted by theoretical studies. High-pressure experiments with LiC₆ were performed in the pressure range from 0.1 MPa to 43 GPa using a diamond-anvil cell. In situ X-ray diffractometry and optical microscopy revealed that LiC₆ was transformed to a hexagonal-diamond form without losing Li atoms. The c-axis of the hexagonal-diamond form was considerably longer than that of the hexagonal diamond transformed from pure graphite, which was consistent with the predicted structure of the endohedral Li diamond. The observed high-pressure form exhibited a golden metallic gloss, which was also consistent with the calculated metallic band structure.     

Direct observation of split-off exciton and phonon structures in absorption spectrum of silicon

The split-off band exciton of silicon has been observed in the absorption spectrum by using a wavelength modulation technique. The spin-orbit splitting of the valence band is determined to be 44.1 ± 0.3 meV at 1.8 °K. The structures associated with some two-phonon indirect transitions have also been observed in the absorption spectrum.     

Structural changes in graphite-to-diamond transition on the P-T link of minimum concentration jump in carbon

Abstract

Structural aspects of the direct graphite-diamond transition have been studied along with the reaction P-T-hysteresis arising during phase transformations in carbon. The direct transition has been found to be a high-molecular reaction of cross-linking of two-dimensional graphite networks resulting in their transformation into three-dinensions1 covalent networks of diamond. Crystal-oriented barochemical reaction of cross-linking is described by two fundamental matrices: the matrix of atomic coordinates transformation and the matrix of atomic concentration Jump. A method of hi&-molecular thermomechanics developed in this paper is used to discuss P-T-hysteresis that was experimentally observed in, the direct and inverse transitions in carbon.     

X-ray absorption spectroscopy under high pressures in diamond anvil cells

Abstract

Although potentially extremely important for understanding the high-pressure microscopic behaviour of materials, over the years the area of high-pressure EXAFS in particular using diamond anvil cells has proved to be technically difficult. This has significantly hampered its development. The interference of X-ray dimaction from the diamonds in the diamond anvil cell with the absorption signal has proved to be a challenging problem to tackle, restricting the use of high-pressure EXAFS to energies below about 11 key Below 11 keV however the technique is also limited due to absorption of incident X-rays by the diamonds making it virtually impossible to conduct X-ray absorption experiments below about 9keV In this paper we describe in detail the methodology for scanriirig high-pressure EXAFS in diamond anvil cells and examine the origins of the associated problems and ways of dealing with them. We also demonstrate that it is possible to extend the useful range of studied absorption edges from 7keV up to at least 30keV This brings about new opportunities for high pressure EXAFS using diamond anvil cells.