Diamond Bonds Research Articles

Shear-promoted graphite-to-diamond phase transition at the grain boundary of nanocrystalline graphite

High pressure has traditionally been considered essential for the transformation of graphite into diamond. However, reducing the transition pressure required for this graphite-to-diamond (G2D) conversion holds significant appeal in both scientific research and engineering applications. In this study, we conducted large-scale molecular dynamics (MD) simulations using an environment-dependent interaction potential (EDIP) to examine the shear deformation of nanocrystalline graphite (n-graphite) with a grain size of approximately 6.5 nm. We discovered that the G2D transition pressure in n-graphite can be reduced to 2–3 GPa, significantly lower than the ∼90 GPa uniaxial stress required in crystalline graphite. This reduction is primarily due to concentrated local shear stresses at grain boundaries (GBs), which induce substantial rotations of graphite layers. These rotations facilitate the initial formation of diamond bonds at sites of pre-existing imperfections at the GBs, assisted by shear. Once initiated at the GBs, the G2D transition rapidly propagates within grains aligned parallel to the shear components, resulting in the formation of nanocrystalline diamond. Our findings underscore the critical roles of GBs and shear stress in enabling the G2D transition in n-graphite.

High-temperature wear mechanism of diamond at the nanoscale: A reactive molecular dynamics study

Diamond is highly wear-resistant at room temperature, while it suffers from rapid wear under the high-temperature tribological conditions. In this paper, we present a reactive molecular dynamics study to unveil the nanoscale wear mechanism of diamond with the evolution of temperatures. We find a critical temperature over which the mechanical failure and wear of diamond will be significantly accelerated. The diamond structure fails when the total stress reaches 157–165 GPa, which is composed of thermal stress and friction-induced stress. The thermal stress due to the restriction of thermal expansion substantially increases with temperature and contributes a major part to the total stress. At the critical temperature, the interfacial chemical bonding and the frictional contact are strongly intensified, with substantially increased contact quality and contact area. On the other hand, the temperature elevated to the critical value induces a drastic increase of sp and dangling bonds in diamond, leading to the internal collapse of the mechanical strength. Ultimately, the synergy of the enhanced frictional contact and decreased mechanical strength leads to the failure and wear of diamond. This work provides a novel insight into the wear mechanism of diamond under elevated temperatures, and contributes to the wear theory in diamond materials.

Study on the Growth Mechanism of Chlorocarbon Radicals on the Surface of CVD Diamond (100)

In the CH3Cl/H2 atmosphere, the adsorption process of various active chlorocarbon groups (CH2Cl, CHCl, CCl) and CH3 on the surface of clean diamond used density functional theory (DFT) calculations during CVD process. The reaction heat and activation energy of the active sites on the adsorption reconstituted surface of D-CH3 and D-CH2Cl were calculated by transition state search to explore the actual effect of the carbon chloride active group on the surface of CVD diamond (100).The results showed that the adsorption capacity of CHCl, CH2Cl and CCl on the substrate was gradually weakened and the adsorption energy of CH2Cl and CH3 was close. Both CHCl and CH2Cl could form diamond bonds with the substrate carbon atoms to directly promote the growth of the diamond coating. Since the C-Cl bond was weaker than the C-H bond, the adsorption recombination surface of CH2Cl generated an energy barrier of the active site lower than the adsorption reconstitution surface of CH3. Therefore, using CH3Cl/H2 as a gas source could effectively reduce the energy required for diamond coating growth.

Atomic-Scale Visualization of Ultrafast Bond Breaking in X-Ray-Excited Diamond.

Ultrafast changes of charge density distribution in diamond after irradiation with an intense x-ray pulse (photon energy, 7.8keV; pulse duration, 6fs; intensity, 3×10^{19} W/cm^{2}) have been visualized with the x-ray pump-x-ray probe technique. The measurement reveals that covalent bonds in diamond are broken and the electron distribution around each atom becomes almost isotropic within ∼5 fs after the intensity maximum of the x-ray pump pulse. The 15fs time delay observed between the bond breaking and atomic disordering indicates nonisothermality of electron and lattice subsystems on this timescale. From these observations and simulation results, we interpret that the x-ray-induced change of the interatomic potential drives the ultrafast atomic disordering underway to the following nonthermal melting.

Metastability of diamond ramp-compressed to 2 terapascals.

Carbon is the fourth-most prevalent element in the Universe and essential for all known life. In the elemental form it is found in multiple allotropes, including graphite, diamond and fullerenes, and it has long been predicted that even more structures can exist at pressures greater than those at Earth's core1-3. Several phases have been predicted to exist in the multi-terapascal regime, which is important for accurate modelling of the interiors of carbon-rich exoplanets4,5. By compressing solid carbon to 2 terapascals (20 million atmospheres; more than five times the pressure at Earth's core) using ramp-shaped laser pulses and simultaneously measuring nanosecond-duration time-resolved X-ray diffraction, we found that solid carbon retains the diamond structure far beyond its regime of predicted stability. The results confirm predictions that the strength of the tetrahedral molecular orbital bonds in diamond persists under enormous pressure, resulting in large energy barriers that hinder conversion to more-stable high-pressure allotropes1,2, just as graphite formation from metastable diamond is kinetically hindered at atmospheric pressure. This work nearly doubles the highest pressure at which X-ray diffraction has been recorded on any material.

Fabrication of Transparent Very Thin SiOx Doped Diamond-Like Carbon Films on a Glass Substrate

A novel direct current (DC) magnetron sputtering system via radio frequency (RF) bias with hexamethyldisiloxane (HMDSO) plasma polymerization was developed for the deposition of SiOx-doped diamond-like carbon (DLC) films on a glass substrate. As the RF bias increased, the ratio of intensity of D peak and G peak (I(D)/I(G)) decreased and the G peak shifted to a low position, leading to high hardness and a large portion of sp3 bonds. Additionally, weak sp2 graphite bonds were broken and sp3 diamond bonds formed because the RF bias attracted hydrogen ion bombarding the DLC films. Increasing DC power was helpful to improve the hardness of the DLC films because the proportion of sp3 bonds and the I(D)/I(G) ration was increased. HMDSO was introduced into this process to form SiOx:DLC films with enhanced optical performance. The average transmittance in the visible region of these very thin SiOx:DLC films with a thickness of 37.5 nm was 80.3% and the hardness of the SiOx:DLC films was increased to 7.4 GPa, which was 23.3% higher than that of B270 substrates.

Influence of diamond and graphite bonds on mechanical properties of DLC thin films

Mechanical properties of diamond-like carbon thin films with various ratios of sp3/sp2 bonds were studied. The films were prepared in argon atmosphere (0.25 Pa) by laser deposition method for laser energy densities from 4 J·cm−2 to 14 J·cm−2. The sp2 and sp3 bonds were calculated by X-ray photoelectron spectroscopy. Films contained sp3 bonds up to 70 %. Surface properties as roughness and atomic force microscopy topology were measured. Hardness (and reduced Young's modulus) were determined by nanoindentation and reached of 30 GPa (203 GPa). Films adhesion was studied using scratch test and was up to 12 N for biomedical alloy (titanium substrates – Ti-6Al-4V). Relations among deposition conditions and measured properties are presented.

Misfit accommodation mechanism at the heterointerface between diamond and cubic boron nitride.

Diamond and cubic boron nitride (c-BN) are the top two hardest materials on the Earth. Clarifying how the two seemingly incompressible materials can actually join represents one of the most challenging issues in materials science. Here we apply the temperature gradient method to grow the c-BN single crystals on diamond and report a successful epitaxial growth. By transmission electron microscopy, we reveal a novel misfit accommodation mechanism for a {111} diamond/c-BN heterointerface, that is, lattice misfit can be accommodated by continuous stacking fault networks, which are connected by periodically arranged hexagonal dislocation loops. The loops are found to comprise six 60° Shockley partial dislocations. Atomically, the carbon in diamond bonds directly to boron in c-BN at the interface, which electronically induces a two-dimensional electron gas and a quasi-1D electrical conductivity. Our findings point to the existence of a novel misfit accommodation mechanism associated with the superhard materials.

Effect of the pulsed laser deposition conditions on the tribological properties of thin-film nanostructured coatings based on molybdenum diselenide and carbon

The structural state and tribological properties of gradient and composite antifriction coatings produced by pulsed laser codeposition from MoSe2(Ni) and graphite targets are studied. The coatings are deposited onto steel substrates in vacuum and an inert gas, and an antidrop shield is used to prevent the deposition of micron-size particles from a laser jet onto the coating. The deposition of a laser jet from the graphite target and the application of a negative potential to the substrate ensure additional high-energy atom bombardment of growing coatings. Comparative tribological tests performed at a relative air humidity of ∼50% demonstrate that the “drop-free” deposition of a laser-induced atomic flux in the shield shadow significantly improves the antifriction properties of MoSe x coatings, decreasing the friction coefficient from 0.07 to 0.04. The best tribological properties, which combine a low friction coefficient and high wear resistance, are detected in drop-free MoSe x coatings additionally alloyed with carbon (up to ∼55 at %) and subjected to effective bombardment by high-energy atoms during growth. Under these conditions, a dense nanocomposite structure containing the self-lubricating MoSe2 phase and an amorphous carbon phase with a rather high concentration of diamond bonds forms.

Compounded effect of vacancy on interfacial thermal transport in diamond–graphene nanostructures

By employing molecular dynamics simulations, it is observed that the distance of the vacancy(s) from the diamond–graphene interface is a determinant of interfacial resistance. In this study, we explain this relationship in two inter-related approaches. (1) A vacancy situated close to the interface reduces the interfacial resistance, suggesting that phonon dynamics around the vacancy contribute to reduce the interfacial barrier. Vibrational density of state calculations show that phonon scattering processes at an interfacial vacancy(s) influence the interfacial transmission significantly. This is attributed to inelastic mode conversion processes at the vacancy(s) that create modes with frequencies and velocities which match well with those in the interfacial diamond, resulting in the drop of interfacial thermal resistance as the location of vacancy approaches the interface. (2) Radial distribution function analyses indicate the conformity nature of interfacial diamond bonds and the contraction/lengthening of interfacial graphene bonds as the vacancy position is varied. These structural changes improve/weaken the matching of the interfacial bond length and result in mode conversion processes that modify the amount of mismatch in the vibrational density of states and phonon velocities in the two media.

Diamond membrane surface after ion-implantation-induced graphitization for graphite removal: Molecular dynamics simulation

Fabrication of diamond membranes, wherein photonic crystals and other nanosized optical devices can be realized, is of great importance. Many spintronic devices are based on specific optically active atomic structures in diamond, such as the nitrogen-vacancy center, and rely on the membrane's performance. One promising approach for realizing such membranes is by creating a heavily damaged layer (rich in broken bonds) in diamond by ion implantation. Following annealing, this layer converts to graphite, which can be chemically removed, leaving a free-standing diamond membrane. Unfortunately, the optical properties of the exposed diamond surface (the diamond-vacuum interface) of such membranes currently are insufficient for high-quality photonic devices. We present molecular dynamics studies of the atomic structure of the etchable graphite/diamond interface. Different implantation and annealing conditions are simulated. The results show that cold implantation, followed by high-temperature annealing ($>1500 {}^{\ifmmode^\circ\else\textdegree\fi{}}\mathrm{C}$) leads to the creation of the sharpest diamond-etchable graphite interface, which should exhibit optimal optical properties among diamond membranes created by the implantation/graphitization method.

Spin Configuration in the1/3Magnetization Plateau of Azurite Determined by NMR

High magnetic field (63,65)Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu3(CO3)(2)(OH)(2), which is a model system for the distorted diamond antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the Cu2 (dimer) sites is found to be field independent, negative and strongly anisotropic, corresponding to approximately 10% of fully polarized spin in a d orbital. This is close to the expected configuration of the quantum plateau, where a singlet state is stabilized on the dimer. However, the observed nonzero spin polarization points to some triplet admixture, induced by strong asymmetry of the diamond bonds J1 and J3.

Stress in Diamond Coatings on Cutting Tools Deposited by Hollow Substrate Holder

Chemical vapor deposition (CVD) diamond coatings were deposited on milling cutter substrate using a hollow substrate holder. The substrate is WC–Co cemented carbide contained 6% of cobalt concentration. Structures and stress state of diamond films were analyzed by scanning electron microscopy (SEM) and Raman spectroscopy. It was found that the diamond coating is of the same quality at the same cutting tool deposited on a hollow substrate holder. Diamond (sp3) bonds are better developed with substrate temperature of ~760°C. A higher or lower substrate temperature could lead to a higher non-diamond carbon content in the films. A higher substrate temperature could lead to a higher thermal stress. The compressive stress increases when the substrate temperature is higher or lower than 760°C. The concentration of amorphous phase in the coatings is low with CH4 concentration of 1.0% and 1.5%. A higher non-diamond carbon content and defects in the diamond coatings increase with the increase of CH4 concentration, which leads to the compressive stress value does not increase significantly under a high CH4 concentration.

Field emission from diamond nanotips treated with nitrogen plasma immersion ionimplantation

Diamond nanotips were synthesized on diamond/Si substrates by using a microwaveplasma-enhanced chemical vapor deposition process. The diamond nanotips are∼1 µm in height and the diameter at the bottom of a nanotip is∼200 nm. The as-grown diamond nanotips exhibit excellent field emission characteristics after treatingwith nitrogen plasma immersion ion implantation (PIII) for 10 min. A low turn-on field of3 V µm−1 and a highcurrent density of 4 mA cm−2 at 9 V µm−1 are achieved. The morphologies of diamond nanotips are unchanged after thenitrogen PIII treatment. The diamond quality reduces with treatment time. Thesp3 bonds in diamond can be broken by the nitrogen ions accelerated by the pulse voltage of−25 kV, which results in G peak broadening in Raman spectra. Several nanoscale amorphousregions are generated in a crystalline diamond nanotip, observed by the high resolutiontransmission electron microscope. The improvement of field emission properties iscorrelated to the amorphous regions and possible implantation-induced atomicdefects.

Triboluminescence from diamond

The process by which diamonds wear when they are facetted using traditional diamond polishing has been the subject of much debate. The absence of a plausible wear mechanism to describe the process has been due in part to the lack of reliable data; a multitude of variables are involved with the process. In recent years, the enhancement of sophisticated analytical techniques has led to the discovery that diamond undergoes a phase transformation during polishing, although the process by which this occurs is unclear. One of the more interesting observations seen during diamond polishing is the occasional emission of light from a polishing diamond. Since this emission is a direct consequence of physical processes at the polishing interface, it provides us with detailed and direct information about electronic processes occurring as diamond bonds that are broken and/or transformed. These light emissions have loosely been described as triboluminescence, although no detailed study of the phenomenon occurring in diamond has ever been published. This paper describes an investigation into the light emitted during fracture (such as that obtained when diamond is scratched) and during polishing. In each case, the spectral emissions are different. During fracture, the overall spectral envelope is dominated by light emission due to frictional sliding and to a lesser extent by photoluminescence, demonstrated by the appearance of the ‘N3’ centre. During polishing, it is suggested that the mechanism by which light emission occurs is via electroluminescence.

Ultra hydrophobic wetting behaviour of amorphous carbon films

Carbon films are distinguished by their broad structural variability and a correspondingly wide range of properties varying from diamond-like to graphite-like. This variability is determined by the relation of diamond bonds (sp3) to graphitic bonds (sp2) and can be influenced by addition of other elements. The highest degree of diamond-likeness, e.g. expressed by high hardness, is achieved in pure carbon films. Such super hard films, so-called tetrahedral bonded amorphous carbon films (ta-C), reach a hardness from 40 GPa up to 80 GPa and a Young's modulus from 400 GPa up to 800 GPa.By incorporation of additional elements, not only the mechanical properties but also the surface energy can be varied. PTFE-like properties can be obtained by addition of fluorine or other elements. At Fraunhofer IWS Dresden such doped amorphous carbon films (a-C:X) have been deposited by a special pulsed vacuum-arc technique, the laser-induced pulsed arc (Laser-Arc). The deposition process is characterized by producing a high activated carbon plasma from a pure or doped graphite cathode and its deposition on substrates under vacuum or reactive conditions.The mechanical properties of the doped a-C:X-films were investigated in dependence on the content of the doped elements. Furthermore, the surface energy was determined by means of contact angle measurements.To get ultra hydrophobic properties of a surface additionally to a low surface energy a defined structuring of the surface topography is necessary. Different kinds of such structuring methods and their effects in conjunction with a doped a-C:X-film on the super hydrophobic behaviour are shown.

The action of iron particles at catalyzed hydrogenation of {100} and {110} faces of synthetic diamond

Results of treatment of {100} and {110} faces of synthetic diamond with dispersed iron at catalyzed hydrogenation are presented. The experiments were held at a temperature of 900 °C in flow hydrogen medium. The regular behavior of iron particles on the surface of diamond crystals was established. It was found that breaking of –C–C- bonds in diamond during its dissolution in the metal-catalyst is the limiting stage of the process.

Chemical Vapor Deposition of Diamond Films in Hot Filament Reactor

Diamond films of different quality have been synthesized by hot filament chemical vapor deposition (HF-CVD) methods from a mixture of hydrogen and hydrocarbon gases. Thin polycrystalline films deposited on silicon substrate were studied using Raman spectroscopy, scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) technique. Nonuniform distribution of substrate temperatures yields to growth of diamond films in a variety of ways and affects on various types of carbon structures and different kind of defects. The Raman peak shifts to wave number higher than 1332 cm -1 what corresponds to a compressive stress in the range of -0.49 to -1.87 GPa. Two EPR centres with g = 2.003 and ΔH pp equal to 0.65 and 1.2 mT originate from carbon dangling bonds in diamond and in non-diamond phase, respectively.

The electrical and optical properties of thin film diamond implanted with silicon

The superb mechanical and electrical properties of diamond make it an attractive material for use in extreme conditions. Diamond devices have been fabricated, but the combination of diamond with other materials to form alloys is not yet well-understood. We have investigated the electrical and optical properties of diamond implanted with Si, which is in principle isoelectronic in diamond. The diamond layers were 23μm thick p-type layers grown on Si(100) substrates. Silicon was implanted at room temperature, at energy of 300keV with doses of up to 8.0×1016cm−3. A two-stage post-implant anneal process was then performed at 500°C for 30min, then 800°C for 30min. Rutherford backscattering spectrometry (RBS) measurements indicated Si concentrations up to 2at.%, in agreement with simulations of implant profiles. X-ray measurements indicated that about 60–70% of the Si was incorporated substitutionally, corresponding to up to 1.5at.% from linear interpolation of lattice constants. Raman spectroscopy measurements confirmed that the diamond structure was reconstructed with the post-implant anneal treatment. NEXAFS measurements confirmed the reconstruction of the sp3 diamond bonds from sp2 graphite after annealing. Electrical measurements indicated an increase in diode current density with increased Si dose and a decrease in contact resistance with contact anneals. These novel diamond alloys may be useful for electronic and optical devices. We report on the preparation and properties of these alloys.

High Resolution TEM Investigations of Nanostructures in Hard Amorphous Carbon Films

Amorphous carbon films allow the simultaneous realization of sp 3 (diamond) bonds and sp 2 (graphite) bonds. By adjusting the sp 3 : sp 2 ratio, the film properties may be varied over a broad range and in this way be adapted to the special demands of applications. Amorphous films appear featureless, but thorough investigations by means of electron microscopy reveal two kinds of structurization on the nanometer scale [CDJ 91, DJJ 90, SXT 96, VD 96], which open a wide field for Structural optimization and furthermore lead to new applications. By periodic alteration of the deposition conditions the sp 3 : sp 2 ratio (and hence density, Stiffness and so on) may be changed from layer to layer. Such density modulated multilayer stacks of the same material can e.g. be used for X-ray mirrors of high stability against diffusional degradation [DHM 95]. Presupposition for such an application are a very regular stacking sequence and very smooth interfaces. Both aspects may be controlled by special electron microscopic techniques. Under constant (special) deposition conditions a lateral structurization on the nanometer level was found consisting of patches of fullerene-like nanoparticles (nanoonions or nanotubes) dispersed in an amorphous matrix [AKZ 97, SSB 95]. The large interest in such Structures stems from their possible application as electron emitters in flat panel displays. For these TEM investigations, foils for observation parallel and perpendicular to the carbon film were prepared. In dependence on the different structural details to be investigated, optimized electron microscopic methods have been applied: - Scattering absorption contrast allows a rough visualization of density variations (e.g. for carbon investigation with Philips CM200 FEGIST: Δp ≃ 0.5 g/cm 3 at lateral resolution of Δx ≃ 1 nm). - Phase contrast shows a much higher sensitivity (for the same microscope: Δp ≃ 0.05 g/cm 3 ). It may be adapted to the characteristic Structural length: With strong underfocussing a large area phase contrast is visible (2 - 10 nm). For higher resolution the Scherzer focus is recommended; then Δx ≃ 0.24 nm is achievable with the Philips CM200 FEGIST electron microscope. - High resolution electron microscopy allows the direct observation of the atomic arrangement, e.g. with Philips CM30FEG/UT special Tubingen electron microscope: Δx ≃0.17 nm. - Off-axis electron holograms contain the complete information of the complex image wave allowing the reconstruction of both phase and amplitude of the electron object wave. By computer correction of aberrations, resolution can be further increased (for the Philips CM200 FEGIST: down to 0.13 nm)[H 97, ORL 95]. Important tools for the evaluation of the electron micrographs are Fourier techniques for image analysis and image interpretation by accompanying computer simulations.