Diamond-like Phases Research Articles

First-principle calculations on the structural stability and electronic properties of superhard BxCy compounds

With first-principle calculations, we studied the structural stability and electronic properties of the BxCy compounds based on three kinds of phases including diamond-like, C20-like and B15-like phases. The C20-like structure B8C12 is found to be a new stable structure with relatively low formation energy in middle boron concentration and is expected to be synthesized experimentally. Combined with a microscopic model, the Vickers hardness of the different configurations of BxCy compounds is analyzed with the change of boron concentration. It is found that the hardness of the B–C system has a decreasing trend with the increase of boron concentration. In addition, all the structures have metallic properties, except B12C3 and B14C. With the analysis of Mulliken bond population and charge distribution, the bonds with high electron density and short bond length have an important contribution to the hardness in the B–C system, while the effect of metallicity to hardness can be ignored.

Features of the formation of cubic BCN phases in comparison with natural and synthetic polycrystalline diamonds

A study of the boron-carbon-nitrogen system was carried out to synthesize new materials and examine phase relations. Crystalline graphite- and diamond-like BCN phases were synthesized by thermobaric treatment of mixtures of boron with carbon nitride or melamine powders. These BCN phases crystallized in the basic lattice of boron nitride with statistically distributed carbon atoms. New structural modifications were discovered by studying the kinetics of phase formation. The hexagonal lattice of a graphite-like phase is transformed by a diffusionless mechanism into a cubic structure at pressures of around 6.5 GPa. A diamond-like BCN phase was synthesized in the form of individual crystals or polycrystalline aggregates. The microstructure of these aggregates is comparable to natural and synthetic carbonado-type diamond. The obtained results argue in favour of the hypothesis that natural carbonados formed in the Earth’s mantle.

Classification schemes for carbon phases and nanostructures

New schemes of structural classification for carbon phases and nanostructures have been proposed, which are based on the types of chemical bonds formed and the numbers of the nearest neighbors with which each atom forms covalent bonds. The classification schemes can describe not only the known phases, but also new phases and nanostructures. New phases can be derived by linking, superpositioning or cutting precursor structures. The classification scheme has been used to predict diamond polymorphs, yielding thirty diamond-like phases that consist of atoms in equivalent crystallographic positions and eighteen of which were predicted for the first time. [New Carbon Materials 2013, 28(4): 273–283]

Classification schemes for carbon phases and nanostructures

New schemes of structural classification for carbon phases and nanostructures have been proposed, which are based on the types of chemical bonds formed and the numbers of the nearest neighbors with which each atom forms covalent bonds. The classification schemes can describe not only the known phases, but also new phases and nanostructures. New phases can be derived by linking, superpositioning or cutting precursor structures. The classification scheme has been used to predict diamond polymorphs, yielding thirty diamond-like phases that consist of atoms in equivalent crystallographic positions and eighteen of which were predicted for the first time.

Graphite to diamond-like carbon phase transformation by high-pressure torsion

Diamond-like carbon (DLC) with significant fraction of tetrahedral sp3 bonds and amorphous structure is generally produced in the form of thin films by rapid cooling of high-energy carbon atoms in vacuum. This study shows that DLC can be directly formed from bulk samples of graphite by application of severe plastic deformation under high pressures. The formation of DLC is enhanced with increasing the shear strain, pressure, and temperature. It is suggested that the high pressure thermodynamically stabilize DLC and formation of high density of lattice defects by straining reduces the energy barrier for DLC formation.

Molecular dynamics simulation of orientation dependency in the shock-induced phase transition of C60 fullerene single crystals into amorphous diamond

A molecular dynamics simulation of the shock-wave propagation in the face-centered cubic (FCC) structured C60 fullerene along the <100>, <110> and <111> crystallographic directions is performed. For this purpose, the response of the material under different shock-wave loadings is studied through Hugoniot curves. Three regimes of the material behavior have been observed from fully elastic to elastic–plastic to plastic. The Hugoniot elastic limit and the phase transition are also investigated along different crystallographic directions. It is shown that the shock wave travels faster along the <110> and <111> directions than in the <100> direction in the material. Comparing the results with the experimental data, it is found that the trends of the Hugoniot data are the same as the experiments. Also, the piston velocities corresponding to different structures, such as the FCC C60 fullerene, graphite like and final diamond like, are determined based on the Hugoniot results. Moreover, it is shown that after the phase-transition occurrence, at high piston velocities the material responses along the low-index directions merge with each other; thus, an amorphous structure is expected for the final diamond-like phase.

Multiphase density functional theory parameterization of the interatomic potential for silver and gold

The ground state energies of Ag and Au in the face-centered cubic (FCC), body-centered cubic (BCC), simple cubic (SC) and the hypothetical diamond-like phase, and dimer were calculated as a function of bond length using density functional theory (DFT). These energies were then used to parameterize the many-body Gupta potential for Ag and Au. We propose a new parameterization scheme that adopts coordination dependence of the parameters using the well-known Tersoff potential as its starting point. This parameterization, over several phases of Ag and Au, was performed to guarantee transferability of the potentials and to make them appropriate for studies of related nanostructures. Depending on the structure, the energetics of the surface atoms play a crucial role in determining the details of the nanostructure. The accuracy of the parameters was tested by performing a 2 ns MD simulation of a cluster of 55 Ag atoms – a well studied cluster of Ag, the most stable structure being the icosahedral one. Within this time scale, the initial FCC lattice was found to transform to the icosahedral structure at room temperature. The new set of parameters for Ag was then used in a temperature dependent atom-by-atom deposition of Ag nanoclusters of up to 1000 atoms. We find a deposition temperature of 500 ± 50 K where low energy clusters are generated, suggesting an optimal annealing temperature of 500 K for Ag cluster synthesis. Surface energies were also calculated via a 3 ns MD simulation.

Phonon-Like Light Scattering in Polycrystalline Carbon Structures

A new type of inelastic light scattering in a disordered condensed matter is considered, in which, contrary to the conventional Raman scattering with fixed frequencies of vibrations, a shift of vibrational band positions is observed under variation of the excitation radiation frequency. This phonon-like scattering is inherent to intermediate states of the matter characterized by a ``smeared'' zone structure at the transition from individual non-correlated molecules of an amorphous material to a highly correlated structural state in single crystals. In particular, for different carbon structures, the intermediate structural statesbetween graphite and diamond include some shares of sp2 and sp3 hybridizations. A defect band and additional ones (D and Ad bands, respectively) have been clearly detected in the Raman spectra of such carbon forms. The bands have a doublet frame with the "redistribution" of intensities between components of the doublet (at 1350 and 1375 cm–1) in different structural modifications of graphite sensitive to the local environment of the correspondingchemical groups and to physical factors like pressure, temperature, and ionic implantation. The deconvolution of the composite bands into individual components has been performed through the subtraction spectra calculation. The origin of doublets and limitations of the applicability of a coupled double resonance concept are discussed. To determine the nature of the considered Raman bands, their fine structure and asymmetry are analyzed and compared with the behavior of similar bands in diamond-like hydrocarbon films and carbon metastable phases under high pressure.

Influence of organic solvent on optical and structural properties of ultra-small silicon dots synthesized by UV laser ablation in liquid

Ultra small silicon nanoparticles (Si-NPs) with narrow size distribution are prepared in a one step process by UV picosecond laser ablation of silicon bulk in liquid. Characterization by electron microscopy and absorption spectroscopy proves Si-NPs generation with an average size of 2 nm resulting from an in situ photofragmentation effect. In this context, the current work aims to explore the liquid medium (water and toluene) effect on the Si-NPs structure and on the optical properties of the colloidal solution. Si-NPs with high pressure structure (s.g. Fm3m) and diamond-like structure (s.g. Fd3m), in water, and SiC moissanite 3C phase (s.g. F4[combining macron]3m) in toluene are revealed by the means of High-Resolution TEM and HAADF-STEM measurements. Optical investigations show that water-synthesized Si-NPs have blue-green photoluminescence emission characterized by signal modulation at a frequency of 673 cm(-1) related to electron-phonon coupling. The synthesis in toluene leads to generation of Si-NPs embedded in the graphitic carbon-polymer composite which has intrinsic optical properties at the origin of the optical absorption and luminescence of the obtained colloidal solution.

ChemInform Abstract: High‐Pressure Route to Superhard Boron‐Rich Solids

AbstractReview: 43 refs.

Structures of diamond-like phases

The diamond-like phases containing carbon atoms with the same degree of hybridization, which is close to sp 3, are classified. It is found that twenty such phases can exist, and ten of them are described for the first time. Molecular mechanics and semi-empirical quantum-mechanical methods are used to calculate the geometrically optimized structures of diamond-like phase clusters and to determine their structural parameters and properties, such as the density, the bulk modulus, and the sublimation energy. The difference between the properties of the diamond-like phases and those of diamond is found to be determined by the difference between the structures of these phases and diamond.

High-pressure route to superhard boron-rich solids

Novel superhard phases are expected to be found among various high-pressure polymorphs of light element compounds. Besides diamond-like phases, the icosahedral boron-rich solids are of particular interest because they could combine high hardness with advanced electronic and phonon transport properties, lightness, high thermal and chemical stability. Here we review some recent results on high-pressure synthesis of novel boron-rich solids.

Submicrosecond polymorphic transformations accompanying shock compression of graphite

The effect of the structure factors and temperature on the transformation of shock-compressed graphite to diamond or a diamond-like phase has been investigated. It has been revealed a significant effect of the shock compression direction with respect to the crystallographic planes of graphite on the detected pressure and rate of the transformation. The effect is more pronounced in more ordered graphite. The main structure parameter determining the transformation rate is the degree of the three-dimensional ordering of graphite. The transformation pressure decreases with heating. The extrapolation of the results to the high-temperature region and their comparison with the data on the direct transformation of statically compressed graphite to diamond indicate that the “above-barrier” state should be apparently reached to ensure the irreducible transformation to cubic diamond.

Room temperature pulsed laser deposition of Si x C thin films in different compositions

Amorphous silicon–carbon alloy films in different compositions were prepared by pulsed laser deposition from two-component targets containing pure silicon and carbon parts. The silicon–carbon ratio in the films was varied by adjusting the number of laser shots on the constituent silicon and carbon targets. The composition, optical properties, thickness, and bonding structure of the films were determined by backscattering spectrometry, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy, respectively. Backscattering spectrometry data were used to determine the deposition rate of silicon and carbon. This enabled the calculation of the number of the shots onto each target to reach a predefined composition. As the film composition changed from carbon to silicon, it was shown that the microscopic and macroscopic properties of the films also changed from a diamond-like carbon phase to an amorphous silicon phase via graphite- and silicon-carbide-like composite.

Fe–Si core/Si-shell clusters prepared by double glow discharge sources

Using an improved plasma gas condensation cluster deposition system, Fe and Si clusters are prepared by a single glow discharge source and Fe/Si hybrid clusters by double glow discharge sources without inserting a partition plate. Transmission electron microscope observations indicate the following characteristic structures and morphologies. There are a bcc phase in Fe clusters and a diamond-like phase in Si clusters. In Fe/Si hybrid clusters, bcc Fe–Si cores are covered with crystalline and/or amorphous-like Si shells. Fe and Si cluster nuclei collide with each other at an initial stage, and Si cluster nuclei cover Fe cluster nuclei probably because the surface energy of Si is much lower than that of Fe. In a clean background condition, Fe and Si atoms mix and/or internally diffuse at the contact interface between juxtaposing Fe and Si cluster nuclei, and form bcc Fe–Si and crystalline or amorphous Si phases, whereas no intermetallic compound phase is formed due to the limited diffusion condition.

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.

Diamond-like phase formation in an amorphous carbon films prepared by periodic pulsed laser deposition and laser irradiation method

Diamond-like carbon (DLC) films were fabricated by pulsed laser ablation of a liquid target. During deposition process the growing films were exited by a laser beam irradiation. The films were deposited onto the fused silica using 248 nm KrF eximer laser at room temperature and 10 −3 mbar pressure. Film irradiation was carried out by the same KrF laser operating periodically between the deposition and excitation regimes. Deposited DLC films were characterized by Raman scattering spectroscopy. The results obtained suggested that laser irradiation intensity has noticeable influence on the structure and hybridization of carbon atoms deposited. For materials deposited at moderate irradiation intensities a very high and sharp peak appeared at 1332 cm −1, characteristic of diamond crystals. At higher irradiation intensities the graphitization of the amorphous films was observed. Thus, at optimal energy density the individual sp 3-hybridized carbon phase was deposited inside the amorphous carbon structure. Surface morphology for DLC has been analyzed using atomic force microscopy (AFM) indicating that more regular diamond cluster formation at optimal additional laser illumination conditions (∼20 mJ per impulse) is possible.

Reversible phase transformation in boron nitride under pulsed mechanical action

It is established that hexagonal boron nitride (h-BN) exhibits transformation to high-pressure diamondlike phases (w-, c-BN) under conditions of treatment in a planetary ball mill. After a 12-h processing, the yield of these phases (≃20%) no longer grows with further increasing the process duration, which is evidence for a reversible character of the transformation. This conclusion is confirmed by the w-BN → h-BN transformation observed under the same process conditions. In addition to h-, w-, and c-BN, we have also found several new modifications of boron nitride.

Orientation effect on the parameters of the polymorphic transformation of graphite under shock compression

A large effect of the shock compression direction with respect to the crystallographic planes of graphite on the detected pressure and rate of the transformation of graphite to a diamond or diamond-like high-pressure phase has been revealed. The effect is pronounced in more ordered graphite. The results indicate that shifts in basal planes complicate high-rate graphite-diamond transformation.

Deposition of fullerene films from the ablation plasma generated by high-power ion beams on graphite targets

A possibility of deposing carbon films with a high content of C60 and C70 fullerenes from an ablation plasma generated as a result of irradiation of graphite targets by pulsed high-power ion beams is shown. The relative contents of the crystalline diamond-like carbon phase, crystalline fullerene phase, and amorphous carbon phase have been determined by X-ray diffraction analysis for different deposition conditions. The nanohardness and Young’s modulus of the deposited films and their adhesion to the single-crystal silicon substrate have been measured.