Graphite-like Phase Research Articles

Crystal Structure and Electronic Properties of Graphite-Like C<sub>7</sub>N Phase from First-Principles Calculations

The structural and electronic properties of graphite-like C7N compound have been calculated by using first-principles pseudopotential density functional method for ten possible C7N configurations, which are deduced from graphite and hexagonal boron nitride unit cell. The calculated total energy results show that the configuration C7N-I with AA stacking sequence along the c-axis based on hexagonal BN structure has been shown to be the most stable structure. From the calculated electronic band structures and electron density of states, the monolayer and bulk phase of C7N are expected to show insulating and metal states, respectively. The graphite-like C7N phases have been predicted to be a stable phase at ambient conditions by formation energy and elastic constant calculations. A critical pressure of about 41 GPa is expected for a synthesis of cubic C7N phase from this graphite-like C7N.

Emission Properties from Induced Structural Degradation of a-C:H Thin Film

Hydrogenated amorphous carbon (a-C:H) films were deposited by plasma enhanced chemical vapor deposition on silicon substrates. a-C:H thin film was irradiated to a typical He-Cd laser to study its emitting properties. The photoluminescence (PL) intensity during the irradiation achieved a maximum value when 2,000 seconds elapsed. Fourier transform infrared measurement revealed a-C:H thin film suffered transformation from a polymer-like to graphite-like phase during laser irradiation. Thermal annealing was done at various temperatures, ranging from room temperature to <TEX>$400^{\circ}C$</TEX> in the atmosphere, to investigate structural changes in a-C:H film by heat generation during the emission. PL intensity of a-C:H thin film increased 1.5 times without apparent structural change, as annealing temperature increased up to <TEX>$200^{\circ}C$</TEX>. However, a-C:H film above <TEX>$200^{\circ}C$</TEX> exhibited significant decrease of PL accompanying dehydrogenation. This led to a red shift of the PL peak.

Ab initio vibrational and thermal properties of AlN nanowires under axial stress

► Structural phase transitions occur in AlN nanowires by applying stress. ► The thermal conductance is enhanced in the graphite-like (GL) phase, compared to the wurtzite (WZ) phase. ► In the GL phase, the phonon spectrum indicates a larger phonon group velocity. ► At low temperatures, the heat capacity is larger for the WZ phase. The vibrational and thermal properties of small diameter AlN nanowires are investigated using first principles calculations. Upon applying external pressure, the nanowires suffer a stress induced phase transition from a würtzite (WZ) to a graphite-like (GL) phase. The thermal conductance displays a nearly identical behavior for all systems in the temperature regime governed by acoustical modes, while at higher temperatures the conductance is systematically enhanced for nanowires in the GL phase. The heat capacity points out the different phonon group velocities for the acoustical modes in the two structural configurations. Our DFT-based calculations are consistent with experimental data for bulk AlN, in terms of phonon spectrum and temperature dependent heat capacity in the large nanowire limit.

The induction of a graphite-like phase on diamond films by a Fe-coating/post-annealing process to improve their electron field emission properties

The electron field emission (EFE) process for diamond films was tremendously enhanced by Fe-coating and post-annealing processes. Microstructural analysis indicates that the mechanism for the improvement in the EFE process is the formation of nanographites with good crystallinity that surround the Fe (or Fe3C) nanoclusters. Presumably the nanographites were formed via the reaction of Fe clusters with diamond films, viz. by the dissolution of carbons into Fe (or Fe3C) clusters and the reprecipitation of carbon species to the surface of the clusters, a process similar to the growth of carbon nanotubes via Fe clusters as catalyst. Not only is a sufficiently high post-annealing temperature (900°C) required but also a highly active reducing atmosphere (NH3) is needed to give a proper microstructure for enhancing the EFE process. The best EFE properties are obtained by post-annealing the Fe-coated diamond films at 900°C in an NH3 environment for 5 min. The EFE behavior of the films can be turned on at E0 = 1.9 V/μm, attaining a large EFE current density of 315 μA/cm2 at an applied field of 8.8 V/μm (extrapolation using the Fowler–Nordheim model leads to Je = 40.7 mA/cm2 at a 20 V/μm applied field).

Pressureless synthesis of fully dense and crack-free SiOC bulk ceramics via photo-crosslinking and pyrolysis of a polysiloxane

This paper presents the pressureless preparation of fully dense and crack-free SiOC ceramics via direct photo-crosslinking and pyrolysis of a polysiloxane. Elemental analysis revealed the presence of high levels of carbon in the SiOC ceramics. Thus, the samples showed the highest content (78–86 mol%) of segregated “free” carbon reported so far. XRD investigations indicated that the materials prepared at 1100 °C were X-ray amorphous, whereas the sample prepared at 1400 °C contained a turbostratic graphite-like phase and silicon carbide as crystalline phases, as additionally confirmed by TEM and Raman spectroscopy. Vickers hardness was measured to be 5.5–8.6 GPa. The dc resistivity of the prepared material at 1100 °C was 0.35 Ω m, whereas the ceramic pyrolyzed at 1400 °C showed a value of 0.14 Ω m; both values are much lower than those of other known SiOC materials. This latter feature was attributed to the presence of a percolating carbon network in the ceramic.

On nature of bimodal release of noble gases during pyrolysis of the meteoritic nanodiamonds

Analysis of noble gas proportions and their release kinetics during stepped pyrolysis and oxidation of meteoritic nanodiamonds, as well as their core-shell structure led to the following conclusions: (1) Noble gases of HL component with anomalous isotopic composition were presumably formed prior to implantation in the nanodiamonds owing to mixing of nucleosynthetic products of p- and r- process associated with explosion of type-II supernova with noble gases having “normal” isotopic composition; (2) isotopically normal P3 noble gases in the nanodiamonds grains are confined to the nondiamond (for instance, graphite-like) phase in the surface layer. The “layer” structure of nanodiamonds grains resulted from heating up to 800–900°C. Observed increase in contents of P3 noble gases with increasing grain sizes of meteoritic nanodiamonds is caused by the dependence of the degree of graphitization of the superfical layer at given temperature on the grain size and surface defect density; (3) bimodal release of noble gases during pyrolysis of the meteoritic nanodiamonds from weakly metamorphosed meteorites was caused by P3 and HL components, which are comparable in abundance but sharply differ in their release temperature.

Structural changes and phase stability of graphitelike BC3 under explosive shock-wave loading

The response of graphitelike BC3 phases (t-BC3) to shock-wave loading has been studied using two types of high explosives, in order to investigate the possible routes to synthesize via dynamic compression superhard materials in the form of high-pressure phases such as the B-doped diamond produced recently under high static pressures and temperatures. The loading conditions resulting from wave propagation in the shock recovery setup have been determined from theoretical predictions confirmed by numerical simulations and velocity measurements. Over the explored range of shock pressure (from 10 to 30 GPa), no detectable diamond phase could be quenched, probably because of insufficient temperature, but Raman and x-ray diffraction studies of the recovered samples indicate permanent structural changes that have been compared to those observed after shorter, laser driven shock compression. These changes include local phase segregation of t-BC3 and the production of highly disordered phases.

Band gap tuning in GaN through equibiaxial in-plane strains

Structural transformations and the relative variation in the band gap energy (ΔEg) of (0001) gallium nitride (GaN) films as a function of equibiaxial in-plane strains are studied by density functional theory. For relatively small compressive misfits (−6%–0%), the band gap is estimated to be around its strain-free value, while for small tensile strains (0%–6%), it decreases by approximately 45%. In addition, at large tensile strains (&amp;gt;14.5%), our calculations indicate that GaN may undergo a structural phase transition from wurtzite to a graphitelike semimetallic phase.

Epitaxial Growth of Hexagonal Boron Nitride and the Ultraviolet Luminescence Properties

Hexagonal BN (h-BN) is graphite-like sp2-bonded crystal phase, which is the most stable among crystalline BN phases, and has a potential for optical device applications in the deep ultraviolet spectral region. Crystalline phase of BN film grown on a foreign substrate is turbostratic BN (t-BN) or mixed phase with t-BN, h-BN and cubic BN due to the large lattice mismatch and its various crystalline BN phases. Therefore, control of the crystalline phase in the BN film is challenging for single-phase BN growth. Here, we demonstrate that single-phase h-BN epitaxial films can be grown on nearly lattice-matched substrates by optimizing growth conditions during metalorganic vapor phase epitaxy and molecular beam epitaxy (MBE). We also investigate the ultraviolet luminescence properties of near band-gap emission in the h-BN epitaxial films grown by MBE.

Characteristics of Carbon Films Deposited by Magnetron Sputtering

Carbon thin films are often called in the literature, “diamond-like carbon” films. They consist of two basic allotropic forms of carbon, which are graphite and diamond. Carbon atoms with sp bonds form after deposition of a graphite-like phase. Atoms with sp bonds form a diamond-like phase. Diamond-like crystallites are built into a graphite-like phase matrix. In this paper there are presented experimental results of deposition of carbon films by the magnetron sputtering method and the results of analysis of the surface and phase structures of the deposited films. The amorphous carbon films were deposited from graphite targets on 316L steel substrates. The films were deposited at room temperature, in vacuum. The deposition time was 3 h; the depositions were conducted at two different distances between the substrate and the magnetron target.

Effects of flow ratios on surface morphology and structure of hydrogenated amorphous carbon films prepared by microwave plasma chemical vapor deposition

A series of hydrogenated amorphous carbon (a-C:H) films were deposited on silicon substrates by microwave plasma chemical vapor deposition technique with a mixture of hydrogen and acetylene. The effects of flow ratio of hydrogen to acetylene on surface morphology and structure of a-C:H films were investigated using surface-enhanced Raman spectroscopy and scanning probe microscope (SPM) in the tapping AFM mode. Raman data imply a transition from graphite-like phase to diamond-like bonding configurations when the flow ratio increases. AFM measurements show that the increase in hydrogen content, to some extent, can smoothen the surface morphology and decrease the RMS roughness. Excessive hydrogen is found to cause the formation of polymeric hydrocarbon clusters in the films and reduce deposition rate.

Origin of the phase transition of AlN, GaN, and ZnO nanowires

The stabilities of AlN, GaN, and ZnO nanowires/nanorods with different structures and sizes are investigated using first-principles calculations. We found a structure transformation from the graphitelike phase to wurtzite phase as the diameter and length of the nanowire increases. We show that this is due to the competition between the bond energy, the Coulomb energy, and the energy originating from the dipole field of the wurtzite structure. A mechanism of growing uniform nanowires using a graphitelike structure as a precursor is proposed through analyzing the phase diagram of these materials.

Modification of the electrical properties of polyimide by irradiation with 80 keV Xe ions

We modify the electrical properties of polyimide (PI) films by irradiation with 80 keV Xe ions. The surface resistivity of irradiated PI film at room temperature decreases remarkably from 1.2 × 10 14 Ω/□ for virgin PI film to 3.15 × 10 6 Ω/□ for PI film irradiated by 5.0 × 10 16 ions/cm 2 , and the temperature dependence of the resistivity of the treated films is well-fit using Mott's Equation. The irradiated PI film structure is studied using Raman spectroscopy , X-ray diffraction, and Rutherford Backscattering Spectrometry . The concentration of O in the irradiated layer decreases with increasing fluence , while the variation of N concentration is negligible. Graphite-like carbon-rich phases are created in the irradiated layers, leading to the modification of the electrical properties.

Structure Characterization of Modified Polyimide Films Irradiated by 2 MeV Si Ions

Structures of polyimide (6051) films modified by irradiation of 2.0 MeV Si ions with different fluences are studied in detail. Variations of the functional groups in polyimide are investigated by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy. The results indicate that the functional groups can be destroyed gradually with the increasing ion fluence. The variations of structure and element contents are characterized by x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy (XPS). The results indicate that the contents of N and O decrease significantly compared with the original samples, some graphite-like and carbon-rich phases are formed in the process of irradiation.

Formation of Pt–C films by ion beam assisted deposition for the application of suppression thermo-electron emission

Abstract Platinum and carbon were deposited onto the surface of molybdenum grids simultaneously by ion beam assisted deposition. The structure of the Pt–C films was studied by XRD and Raman spectroscopy. The XRD results showed that Pt exhibited mixed strong (1 1 1) and weak (2 0 0) orientations. The Raman spectra showed that the carbon existed in the form of graphite-like phase. Electron emission characteristics from the Mo grid with and without Pt–C films were measured using analogous diode method. The results showed that electron emission from the Mo grid coated with Pt–C films was much less than that from the Mo grid without Pt–C films. The obtained results demonstrated that the Pt–C films are effective grid-coating materials for the application of suppression thermo-electron emission.

Catalytic synthesis of nanosized feathery carbon structures via the carbide cycle mechanism

The morphology of carbon nanostructures obtained by 1,2-dichloroethane decomposition on the 90% Ni/Al2O3 catalyst under different reaction conditions was studied by high-resolution transmission electron microscopy. A new carbon product was discovered, which received the name of feathery carbon. The product has an extremely loose disordered structure consisting of separate fragments of a graphite-like phase. The structural disordering is assumed to be caused by the variation of chlorohydrocarbon decomposition conditions on the frontal face of the metal particle. This changes the character of carbon atom diffusion from the frontal face to the backside face of the nickel particles and finally results in a feathery morphology of the carbon phase. The specific surface area of feathery carbon is 300–400 m2/g.

Effect of load triaxiality on polymorphic transitions in zinc oxide

Molecular dynamics (MD) simulations and first-principles calculations are carried out to analyze the stability of both newly discovered and previously known phases of ZnO under loading of various triaxialities. The analysis focuses on a graphite-like phase (HX) and a body-centered-tetragonal phase (BCT-4) that were observed recently in [ 0 1 1 ¯ 0 ] - and [0 0 0 1]-oriented nanowires respectively under uniaxial tensile loading as well as the natural state of wurtzite (WZ) and the rocksalt (RS) phase which exists under hydrostatic pressure loading. Equilibrium critical stresses for the transformations are obtained. The WZ → HX transformation is found to be energetically favorable above a critical tensile stress of 10 GPa in [ 01 1 ¯ 0 ] nanowires. The BCT-4 phase can be stabilized at tensile stresses above 7 GPa in [0 0 0 1] nanowires. The RS phase is stable at hydrostatic pressures above 8.2 GPa. The identification and characterization of these phase transformations reveal a more extensive polymorphism of ZnO than previously known. A crystalline structure–load triaxiality map is developed to summarize the new understanding.

Raman scattering from turbostratic graphitelike BC4 under pressure

Raman spectra of turbostratic graphitelike BC4(t-BC4) were measured at room temperature and pressures up to 45 GPa on both compression and decompression. At ambient conditions, these Raman spectra contain lines that were not previously observed in Raman spectra of other graphitelike phases. Raman bands of the starting BC4 were assigned on the basis of the lattice dynamics of the ideal graphite structure and relaxation of the q-vector selection rule. The frequency of the E2g(2) in-plane mode at 1561 cm−1 exhibits nonlinear behavior under pressure with the initial pressure coefficient of 7.3 cm−1/GPa. The general behavior of the E2g(2) band at high pressures is consistent with the model of pressure-induced buckling of layers.

Synthesis and characterization of super hard, self-lubricating Ti–Si–C–N nanocomposite coatings

Abstract Quaternary super hard Ti–Si–C–N coatings with different carbon contents were deposited on high-speed steel substrates by pulsed direct current plasma-enhanced chemical vapor deposition (PECVD) technology, using a gaseous mixture of TiCl 4 /SiCl 4 /N 2 /H 2 /CH 4 /Ar. A variety of technologies have been employed to characterize the coatings, including X-ray diffraction, scanning and transmission electron microcopies, X-ray photoelectron spectroscopy, energy dispersive X-ray analysis, automated load–depth sensing and pin-on-disc. The super hard Ti–Si–C–N coatings were found to have unique nanocomposite structures composed of nanocrystallite and amorphous nc-Ti(C,N)/a-Si 3 N 4 /a-C and/or nc-Ti(C,N)/nc-TiSi 2 /nc-Si/a-Si 3 N 4 /a-C, depending on the carbon contents in the coatings. The friction coefficient of the Ti–Si–C–N coatings with a higher carbon content (nc-Ti(C,N)/a-Si 3 N 4 /a-C nanocomposites) were found to be much lower than those of the Ti–Si–N coatings both at room and elevated temperatures, suggesting the formation of a graphite-like lubricious phase of amorphous carbon. However, they are still super hard (32–48 GPa) in spite of the carbon incorporation. This is due to a strong, thermodynamically driven and diffusion-rate-controlled (spinodal) phase segregation that leads to the formation of a stable nanostructure by self-organization. The energy difference between the grain boundary and the crystallite/amorphous phase interface hinders grain boundary mobility, leading to a gradual decrease in the grain size of the nanocrystallites. As a result, nanocomposite Ti–Si–C–N coatings with high hardness and a low friction coefficient can be produced. The coatings are foreseen to have high potential in dry and high-speed cutting tool applications, thus providing for cleaner, healthier and more pleasant machining conditions.

Preparation of nitrides and carbides from g-C 3N 4

By a facile solid-state reaction process using graphite-like phase of C3N4 (g-C3N4) as a precursor with another one being corresponding elemental powders, we synthesized four technologically important nitrides and two carbides including cubic VN, CrN, hexagonal GaN, BN, WC and MoC at moderate temperatures. The products were characterized by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The XRD results show that g-C3N4 is a highly efficient nitridizing and carbonizing reagent and the elemental powders are completely converted into the corresponding nitrides and carbides at lower temperatures than reported in the conventional methods. This novel route may provide new insights into the synthesis of other nitrides and carbides.