Pulsed-laser-induced transformation path of graphite to diamond via an intermediate rhombohedral graphite

The new phase transformation of hexagonal graphite to cubic diamond was experimentally produced without catalyst, using a high-power pulsed laser. Interestingly, by the X-ray diffraction spectra, it was proved that this transition was not direct, but through an intermediate rhombohedral phase. Furthermore, it is important that the rhombohedral phase, as the theoretical transformation path of hexagonal graphite to cubic diamond, was first truly substantiated by our experimental results. The transformation mechanism was suggested that diamond with hexagonal structure was obtained by the direct transforming of hexagonal graphite to hexagonal diamond, and diamond with cubic structure was formed by the indirect transforming, i.e., hexagonal graphite to rhombohedral graphite to cubic diamond. Received: 7 February 2000 / Accepted: 28 March 2000 / Published online: 9 November 2000     

Hall crystal states at nu = 2 and moderate landau level mixing

The structure and orientational ordering of N2O molecules physisorbed on graphite (0001) is investigated applying x-ray, neutron, and low-energy electron diffraction techniques. Combining the results of the three techniques, we find that N2O forms a highly ordered, hexagonal, commensurate (sqrt[21]xsqrt[21])R10.89 degrees phase. The unit cell contains seven molecules which are arranged in a seven-sublattice pinwheel structure, unexpected for linear molecules on a hexagonal lattice. Potential energy calculations corroborate these results.     

X-ray study of the order—disorder transition in high-stage alkali metal intercalated graphite single crystals

Graphite intercalation compounds display a variety of structural properties because of their composite nature (graphite + intercalate) and their layered arrangement. Alkali metal intercalated graphite compounds undergo order-disorder phase transitions when the temperature is varied in the range 300–10 K. The disordered state shows true two-dimensional character, whereas three-dimensional coupling takes place on ordering. Results of single-crystal X-ray diffractometric and photographic studies of stage-2 KC₂₄ single crystals are presented. The positional and orientational correlations of the modulated liquid phase have been studied from 300 K down to the temperature transition T_u = 123.5° K. At the transition, the hexagonal incommensurate solid structure of the alkali metal is modulated by the graphite potential. This transition is discussed in terms of the relaxed-close packed structure model (Dicenzo, 1982). At low temperature a second transition takes place at T_L ≈ 95 K. It is found to correspond to the breaking of the 2D hexagonal symmetry of the K layer.     

Energetic and electronic structure of BC2N compounds

The stability and electronic structure of BC2N compounds are studied using first-principle calculations. The investigated structures have the topology of graphite layers with either carbon, nitrogen or boron atoms at each site. The calculations show that stabler structures are obtained by increasing the number of C-C and B-N bonds. On the other hand, less stable structures result from increasing the number of N-N and B-B bonds. The energy gap of the stablest compounds varies from 0.0 to 1.62 eV, depending on the distribution of B, C, and N atoms in the unit cell. The electronic properties of BC2N layered materials strongly depend on their atomic arrangements. The observed changes in energy gaps do not simply follow a symmetry-based argument proposed earlier.     

STUDY ON THE CATHODE DEPOSIT PRODUCED IN THE PROCESS OF CARBON ARC DISCHARGE

The cathode deposit produced by carbon are discharge wae studied by scanning electron microscopy, high resolution electroa microscopy, X-ray diffraction and Raman spectroscopy. The results indicate that the stacking of hexagonal carbon layer in the discharged graphite is different from the ABAB stacking sequence in graphite, and that carboy nanotube is a kind of graphite-like micro-crystal with a low-dimensional structure.     

Observation of coexistence of 1D and 2D nanostructures in cobalt dipyrromethene trimer complexes adsorbed on a graphite surface

We report the direct observation of 1D and 2D nanostructures of cobalt dipyrromethene trimer complexes adsorbed on a highly oriented pyrolytic graphite surface using scanning tunneling microscopy (STM). STM images showed two types of ordered structures coexisting on the surface: long 1D molecular chains isolated on the terraces, and 2D hexagonal patterns confined by a 1D chain and/or a graphite step edge. These 1D and 2D structures are attributed to ‘edge-on’ and ‘face-on’ complex alignments on the surface, respectively. In both configurations, substrate-mediated molecule-molecule interactions may play a significant role in stabilizing the nanostructures.     

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.     

Charge distribution of a potassium-doped combined system of graphene and hexagonal boron nitride

By using first-principles density functional theory, we investigate the charge distribution of a potassium-doped layered combined system of graphene and hexagonal boron nitride. Two configurations of potassium-doped hexagonal boron nitride layers on graphenes and the reverse geometry of graphenes on hexagonal boron nitride layers are considered. We find that the charge distribution exhibits different features in these two situations. In the former case, the outmost hexagonal boron nitride layer cannot screen the external charges offered by potassium atom completely and most of the transferred charges reside on the two bounding layers. In contrary, the outmost graphene layer near the potassium atom can accept almost all of the transferred charges and only a few of them stay at interior layers in the latter case. A more amazing result is that the characteristics of charge transfer are independent of the number of hexagonal boron nitride layers and graphenes.     

脉冲激光诱导液-固界面反应合成金刚石纳米晶中的结构相变模型

 探讨了脉冲激光诱导液-固界面反应法（PLIR: Pulsed-Iaser Induced Liquid-Solid Interface Reaction ）制备金刚石纳米晶的物理化学机制,提出了金刚石纳米晶的成核机理,即由激光诱导石墨六方结构原子团过渡到石墨菱方结构、然后转变成立方金刚石晶核,以及由石墨六方结构直接转变成六方金刚石结构的相变模型,并讨论了基于液-固界面反应的纳米晶生长动力学,较好地从动力学上解释了合成金刚石纳米晶的物理化学机制。     

On the imaging mechanism of monatomic steps in graphite

The shape and the atomic arrangement of monolayer steps of graphite have been characterized by STM. The origin of the appearance of the imaged features along the steps is discussed, addressing for the first time both morphological and electronic considerations. Extended Hückel theoretical calculations of nanotubes are used to identify the contribution of the electronic structure to the STM image of monolayer steps. We show that mechanical tip–sample interactions dominate the imaging process of graphite, leading to step deformation during scanning and negative STM contrast of the atom positions in the hexagonal unit cell. Received: 11 April 2000 / Accepted: 18 April 2000 / Published online: 23 August 2000     

Investigation on the formation of lonsdaleite from graphite

Structural stability and the possible pathways to experimental formation of lonsdaleite—a hexagonal 2H polytype of diamond—have been studied in the framework of the density functional theory (DFT). It is established that the structural transformation of orthorhombic Cmmm graphite to 2H polytype of diamond must take place at a pressure of 61 GPa, while the formation of lonsdaleite from hexagonal P6/mmm graphite must take place at 56 GPa. The minimum potential barrier height separating the 2H polytype state from graphite is only 0.003 eV/atom smaller than that for the cubic diamond. The high potential barrier is indicative of the possibility of stable existence of the hexagonal diamond under normal conditions. In this work, we have also analyzed the X-ray diffraction and electron-microscopic data available for nanodiamonds found in meteorite impact craters in search for the presence of hexagonal diamond. Results of this analysis showed that pure 3C and 2H polytypes are not contained in the carbon materials of impact origin, the structure of nanocrystals found representing diamonds with randomly packed layers. The term “lonsdaleite,” used to denote carbon materials found in meteorite impact craters and diamond crystals with 2H polytype structure, is rather ambiguous, since no pure hexagonal diamond has been identified in carbon phases found at meteorite fall sites.     

Microstructure and wear property of carbon nanotube carburizing carbon steel by laser surface remelting

Carbon nanotube was used to carburize the surface of medium carbon steel and mild steel, respectively, by means of laser surface remelting. The slurry of carbon nanotube of ethanol was coated on the surface of the materials prior to laser irradiation. Microstructures, microhardness and wear property of the surface layers treated by different laser performance parameters were studied. Graphite coating was also used for carburizing. The results showed that both carbon nanotube and graphite were dissolved in the surface molten layer, leading a carburized hardening layer on the surface of the substrate. However, different microstructures formed in the carburizing layers, depending mainly on the type of carburization materials, carbon nanotube or graphite. The carbon nanotube hardening layer exhibits a little higher hardness than the graphite hardening layer. The carburized layer greatly increases the wear resistance of the base material.     

石墨-六角氮化硼微晶混合物与水高压反应产物分析

 利用球磨法制备石墨-六角氮化硼微晶混合物，并在6.1 GPa、800~1 500 ℃条件下与水进行高压反应，以便研究用水作触媒合成B-C-N三元化合物的可能性。通过对反应产物的XRD、XPS谱分析发现：高压下随着温度的升高，反应产物中出现再结晶石墨，其晶化程度逐渐提高；但没有出现再结晶六角氮化硼，也未出现立方氮化硼。在球磨不充分条件下，石墨-六角氮化硼混合物的XRD谱没有完全弥散，它们与水高压反应时，能观察到石墨与立方氮化硼分别结晶的现象，但都没有形成B-C-N晶化结构。     

Structural relation and epitaxial properties of hexagonal InN and oxidized cubic In2O3

The epitaxial properties and structural relation between hexagonal InN and cubic In₂O₃ phases were studied by synchrotron X-ray scattering and X-ray photoelectron spectroscopy. The cubic bixbyite In₂O₃ phase on the sapphire(0001) substrate was formed after an annealing time of 10 min at 10⁻⁵ Torr after the hexagonal InN film was grown at 550 °C, above the dissociation temperature of InN, by RF-magnetron sputtering. The crystal orientation was cubic In₂O₃(222), parallel to Al₂O₃(0001) and parallel to hexagonal InN(0002) before the oxidation process. The cubic In₂O₃ phase was believed to be formed layer by layer by the oxidation of the hexagonal InN phase.     

Structural properties of a monolayer graphite film on the (111)Ir surface

The structural properties of a monolayer graphite film prepared on the (111)Ir surface through thermal decomposition of benzene molecules were studied. The study was carried out in ultrahigh vacuum using scanning tunneling microscopy, which allowed observation of the atomic structure of the film. It is shown that, on extended smooth regions of the Ir surface, a continuous graphite film with a regular arrangement of carbon atoms in a planar hexagonal lattice is formed. The orientation of zigzag carbon atom chains coincides with the 〈110〉 direction on the Ir surface. Structural defects of the (5, 7) configuration were revealed in the film. A comparison of the topographies of the film and the (111)Ir surface shows that the graphite layer smoothly (without discontinuities) flows over subnanometer topographical features existing on the Ir surface and that the distance between the graphite film and the metal surface in this case can reach 1 nm.     

High-quality ZnO nanorods grown on graphite substrates by chemical solution method

ZnO nanorods (NRs) were grown perpendicularly on graphite substrates using low-temperature wet chemical bath deposition (CBD) with sputtered ZnO film as seed layer. The individual ZnO NRs exhibit single-crystalline feature with well defined hexagonal prism shape and smooth side facets. The high optical qualities of ZnO NRs on graphite substrates were demonstrated by the dominant near-band edge emission and nearly undetectable deep level emissions in room-temperature photoluminescence spectra. The extremely low average reflectance of 0.45 % was obtained for the ZnO NRs/graphite structure in the spectra range from 200 to 1100 nm, indicating that the reported ZnO NRs/graphite structures have significant opportunity for potential application in high-performance photovoltaic devices. Considering the excellent material characteristics of ZnO NRs and the versatile and fascinating features of graphite substrates, the achievements make it possible for the development of high-performance ZnO-based nano-devices even in transferable, flexible, or stretchable forms.     

Structure modification of highly ordered pyrolytic graphite by Ar plasma beam scanning at different incident angles

The surface of highly ordered pyrolytic graphite (HOPG) was modified by Ar plasma beam scanning at a controllable angle of incidence. The characteristics of plasma modified HOPG were investigated by atomic force microscope (AFM), micro-Raman, X-ray photoemission spectroscopy (XPS), and grazing incident angle of X-ray diffraction (GIAXRD). A smooth surface of HOPG can be obtained by adjusting the incident angles of Ar plasma beam scanning. The surfaces of HOPG become smoother with increasing angle of incidence after Ar plasma beam scanning. Raman spectra indicate that the plasma beam scanning breaks the hexagonal structures of sp² C=C bonds near the surface of HOPG. The broken hexagonal network structures can form C–O bonds that increase the amount of oxygen on the surface of HOPG, supported by C_1s and O_1s XPS spectra. GIAXRD data support that the co-existence of both crystalline structures of 2H and 3R in HOPG. The carbon bond breaking in 2H and 3R is different and depends on the angle of incidence. Most broken carbon bonds form damaged aromatic rings near the surface of HOPG.     

Several large-scale superperiodicities on highly oriented pyrolytic graphite observed by scanning tunneling microscopy

On freshly cleaved highly oriented pyrolytic graphite we observed large-scale superperiodicities by a scanning tunneling microscope at room temperature in air. Several hexagonal superstructures with periods of 30 nm, 4.2 nm, 2.4 nm, and 2.0 nm, respectively, and a strip-like superstructure with a period of 1 nm were obtained. With exception of the largest hexagonal superperiodicity (30 nm spacing), all other superstructures are superimposed on the atomic corrugation of graphite. The origin of these superstructures is not clear yet. We assume that they arise from crystal defects in graphite. The hexagonal superstructure may be caused by the Moiré effect due to the rotational misorientation of the two top layers or of two successive layers near the surface.     

Effect of silicon on the formation of graphitic polyhedra and balloon-like particles

We conducted laser vaporization of graphite and graphite containing 1 at. % silicon in Ar gas atmosphere. Comparison of the products from the laser vaporization indicated that the coexistence of Si promoted graphitization in grown carbon particles of 90–1500 nm. Polyhedral graphite (PG) particles and balloon-like carbon (BC) particles with shells of graphitic layers were grown under control of Ar gas pressures of 0.1–0.7 MPa. We discuss possible roles of Si in graphitic structure growth and the formation mechanisms of the PG and BC particles. PACS 81.16.Mk; 61.46.Df; 68.37.Lp     

Prediction of high T(c) superconductivity in hole-doped LiBC

The layered lithium borocarbide LiBC, isovalent with and structurally similar to the superconductor MgB2, is an insulator due to the modulation within the hexagonal layers (BC vs B2). We show that hole doping of LiBC results in Fermi surfaces of B-C p sigma character that couple very strongly to B-C bond stretching modes, precisely the features that lead to superconductivity at T(c) approximately equal to 40 K in MgB2. Comparison of Li(0.5)BC with MgB2 indicates the former to be a prime candidate for electron-phonon coupled superconductivity at substantially higher temperature than in MgB2.