Single Crystal Graphite Research Articles

Elastic instability in graphite single crystal under dynamic triaxial compression: Effect of strain-rate on the resulting microstructure

We focus on the behavior of graphite under triaxial loading at a constant strain-rate using large scale molecular dynamics simulations. Buckling patterns (chevrons) in graphite nucleate from an elastic instability strongly related to the material anisotropy and subsequently grow until the first diamond nuclei appear. We show that the phase transition completely inhibits the growth of chevrons in buckled graphite, the diamond grain size being determined by the size of chevrons at the onset of nucleation. Cubic-diamond clusters nucleate within chevrons of buckled graphite and grow until the parent phase is entirely transformed. This phenomenon leads to nano-structured diamond polycrystals, with orientations of interfaces given by those of the buckled material right before the nucleation process. The buckled graphite microstructure is shown to strongly influence the final microstructure/size of nano-diamonds.

Intrinsic Capacitance of Molybdenum Disulfide.

The metallic, 1T polymorph of molybdenum disulfide (MoS2) is promising for next-generation supercapacitors due to its high theoretical surface area and density which lead to high volumetric capacitance. Despite this, there are few fundamental works examining the double-layer charging mechanisms at the MoS2/electrolyte interface. This study examines the potential-dependent and frequency-dependent area-specific double-layer capacitance (Ca) of the 1T and 2H polymorphs of MoS2 in aqueous and organic electrolytes. Furthermore, we investigate restacking effects and possible intercalation-like mechanisms in multilayer films. To minimize the uncertainties associated with porous electrodes, we carry out measurements using effectively nonporous monolayers of MoS2 and contrast their behavior with reduced graphene oxide deposited layer-by-layer on atomically flat graphite single crystals using a modified, barrier-free Langmuir-Blodgett method. The metallic 1T polymorph of MoS2 (Ca,1T = 14.9 μF/cm2) is shown to have over 10-fold the capacitance of the semiconducting 2H polymorph (Ca,2H = 1.35 μF/cm2) near the open circuit potential and under negative polarization in aqueous electrolyte. However, under positive polarization the capacitance is significantly reduced and behaves similarly to the 2H polymorph. The capacitance of 1T MoS2 scales with layer number, even at high frequency, suggesting easy and rapid ion penetration between the restacked sheets. This model system allows us to determine capacitance limits for MoS2 and suggest strategies to increase the energy density of devices made from this promising material.

Scanning tunneling microscopy of the tetraphenylporphyrin thin films on a graphite single-crystal substrate

Features of the surface topography of a tetraphenylporphyrin film on a graphite single crystal (HOPG) substrate and the local current-voltage characteristics were studied using a scanning tunneling microscope under ambient conditions. STM images of surface were acquired on a scale from a few nanometers to 4 micrometers. Local tunnelling current-voltage characteristics in different ranges of bias voltage and in different directions of voltage scanning can have hysteresis due to the hopping electron conductivity in porphyrin.

Study on Controllable growth of graphene prepared by CVD

Graphene is a two-dimensional honeycomb crystal of monoatomic thickness composed of hexagonal sp2 hybrid carbon atoms. Since the mechanical separation of Andre Gamm and Konstantin Novoselov in 2004 to obtain a single layer of graphene, this unique two-dimensional structure makes graphene excellent physical, chemical, mechanical, electrical and other properties. It has become the focus of much research in recent years. These properties include exceptionally high charge, high mechanical strength and elasticity, optical clarity and a variety of possible chemical modifications that make graphene promising in micro-nanoelectronics, photodetection and conversion materials, structural and functionally enhanced composites and storage. It can be applied in a wide range of fields, but graphene is different from graphite in nature, and its size is small and the yield is low. So how to prepare high quality graphene is the key to study its application prospects. The emergence of chemical vapor deposition (CVD) technology solves these problems and has evolved into an important method for preparing large-area, large-size, multi-purpose graphene. Large-sized graphite thin single crystals can be obtained on a substrate by controlling the growth mode and different treatments for the substrate. Due to the strong interaction between carbon atoms and the substrate, there are still many difficulties and challenges in directly growing graphite on the insulating substrate. This topic aims to prepare graphene by chemical vapor deposition (CVD) and prepare it. The method is improved to obtain graphene with excellent performance, which provides more feasibility for the practical application of graphite thinning in semiconductor devices.

Ultimate High Conductivity of Multilayer Graphene Examined by Multiprobe Scanning Tunneling Potentiometry on Artificially Grown High-Quality Graphite Thin Film

With the discovery of graphene, the excellent characteristics of graphite (multilayer graphene), such as high thermal and electric conductivities, heat resistance, and chemical stability, have also been attracting considerable attention again. However, the production of a large-area (>0.1 × 0.1 m2) graphite thin film with uniformly high electric conductivity, which will be indispensable in industrial applications, has been difficult because of its breakage occurring during the growth process. Recently, we have succeeded, for the first time, in growing large-area high-quality graphite films with thicknesses from ∼0.5 to ∼3 μm by an industrial method, providing a fundamental platform for examining and utilizing the intrinsic and ultimate characteristics of such a material. Here, as a step toward realizing such efforts, we performed microscopic measurements of electric conductivity by the method of multiprobe scanning tunneling potentiometry we have developed. Using the resistivity along the c-axis, ρc = 1.72 ± 0.04 mΩ·m, which was directly measured for the first time, we determined the resistivity in the ab-plane, ρab, to be 0.30 ± 0.01 μΩ·m, indicating that the conductance of the graphite thin film we grew is about 1.3 times the previous highest reported value for the highest grade single-crystal natural graphite with an ideal structure without grain boundaries or wrinkles (0.38 μΩ·m). Both the advantage of multilayer graphene, which is considered to reduce the effect of grain boundaries on conductance, and the decrease in conductance as an effect of wrinkles were directly evaluated.

Extreme stretching of high G:D ratio carbon nanotube fibers using super-acid

Few-wall carbon nanotube (CNT) textiles with unparalleled graphitic perfection and a solitary, prominent radial breathing mode (RBM) associated with metallic chirality have been mechanically stretched in chlorosulfonic acid (CSA) to a degree so far unseen in CNT textiles (150–250% of original length) with notably little tension required. This dramatically enhanced their microstructural alignment and density and, after most of the residual CSA was removed by a vacuum bake, the de-doped fiber's electrical conductivity was found to be 45% greater than single-crystal graphite – a significant milestone for CNT conductor development towards graphitic intercalation compounds (GICs) and traditional metals. Correlation tables and validated, multivariate statistical models show that conductivity is overwhelmingly linked to stretching degree, although eventually saturates near single-crystal graphite levels, implying the existence of a maximum undoped conductivity. The degree of stretching within CSA is correlated with the original mechanical properties (tenacity, elongation-to-break, and linear density); the Raman G’:G ratio and the upper-end oxidation temperature in thermogravimetric analysis also predict the best results. Less graphenically pristine CNT materials stretch to a lower degree in CSA, similar to previous reports. This study highlights the importance of post-synthesis processing to achieve superior performance in carbon nanotube textile materials.

Graphite in rocks of the Popigai impact crater: residual or retrograde?

The concentration of diamond-bearing tagamite from the Popigai impact crater produces large amounts of graphite in addition to impact diamonds (1:100, respectively). The question arises of whether this is residual graphite not converted to diamond at the time of the Popigai impact or is a retrograde form resulting from back-conversion of impact diamond to graphite in a high-temperature tagamite melt. Experiments show that graphite from tagamite is a residual phase. Coexistence of lonsdaleite, cubic diamond, and single-crystal graphite within a limited volume may be due to different orientations of the graphite base plane relative to the impact stress direction. Thus, the diamond-bearing rocks may contain significant amounts of residual graphite, which is consistent with published evidence.

Crystallographic orientation errors in mechanical exfoliation

We evaluate the effect of mechanical exfoliation of van der Waals materials on crystallographic orientations of the resulting flakes. Flakes originating from a single crystal of graphite, whose orientation is confirmed using STM, are studied using facet orientations and electron back-scatter diffraction (EBSD). While facets exhibit a wide distribution of angles after a single round of exfoliation (), EBSD shows that the true crystallographic orientations are more narrowly distributed (), and facets have an approximately error from the true orientation. Furthermore, we find that the majority of graphite fractures are along armchair lines, and that the cleavage process results in an increase of the zigzag lines portion. Our results place values on the rotation caused by a single round of the exfoliation process, and suggest that when a 1–2 degree precision is necessary, the orientation of a flake can be gauged by the orientation of the macroscopic single crystal from which it was exfoliated.

Modelling the coefficient of thermal expansion in graphite crystals: implications of lattice strain due to irradiation and pressure.

Theoretical models for the coefficient of thermal expansion (CTE) first proposed in the 1970s are expanded upon, allowing them, for the first time, to be implemented over a wide temperature range. The models are of interest because they predict the effects of the changes in the crystal lattice spacing and crystallite modulus on the CTE. Hence, they can in turn be used to investigate the influence of pressure and irradiation on the CTE. To date, typographical and mathematical errors and incomplete or conflicting assumptions between the various papers had made the complex mathematical formulations difficult, if not impossible, to follow and apply. This paper has two main aims: firstly to revisit and review the CTE models, correcting the errors and compiling and updating various input data, secondly to use the revised models to investigate the effect of loading and irradiation on the CTE. In particular, the models have been applied to data for natural and highly orientated pyrolytic graphite and compared with experimental data, giving an insight into the influence of temperature, loading and irradiation on both single crystal and polycrystalline graphite. The findings lend credence to postulated microstructural mechanisms attributed to the in-reactor behaviour of nuclear graphite, which finds a wide use in predictive multiscale modelling.

Tensile properties of carbon nanotube fibres described by the fibrillar crystallite model

This work presents a model that successfully describes the tensile properties of macroscopic fibres of carbon nanotubes (CNTs). The core idea is to treat such a fibre as a network of CNT bundles, similar to the structure of high-performance polymer fibres, with tensile properties defined by the CNT bundle orientation distribution function (ODF), shear modulus and shear strength. Synchrotron small-angle X-ray scattering measurements on individual fibres are used to determine the initial ODF and its evolution during in-situ tensile testing. This enables prediction of tensile modulus, strength and fracture envelope, with remarkable agreement with experimental data for fibres produced in-house with different constituent CNTs and for different draw ratios, as well as with literature data. The parameters extracted from the model include: CNT bundle shear strength, shear modulus and tensile strength. These are in agreement with data for commercial high-performance fibres, although high compared with values for single-crystal graphite and short individual CNTs. The manuscript also discusses the unusually high fracture energy of CNT fibres and exceptionally high figure of merit for ballistic protection. The model predicts that small improvements in orientation would lead to superior ballistic peformance than any synthetic high-peformance fiber, with values of strain wave velocity (U1/3) exceeding 1000m/s.

Surface structure modification of single crystal graphite after slow, highly charged ion irradiation

Abstract Single crystal graphite was irradiated by slow, highly charged ions. The modification of the surface structure was studied by means of Low-Energy Electron Diffraction. The observed damage cross section increases with the potential energy, i.e. the charge state of the incident ion, at a constant kinetic energy. The potential energy is more efficient for the damage production than the kinetic energy by more than a factor of twenty. Comparison with earlier results hints to a strong link between early electron creation and later target atom rearrangement. With increasing ion fluence, the initially large-scale single crystal is first transformed into μ m-sized crystals, before complete amorphisation takes place.

Wide-angle display-type retarding field analyzer with high energy and angular resolutions.

Deployments of spherical grids to obtain high energy and angular resolutions for retarding field analyzers (RFAs) having acceptance angles as large as or larger than ±45° were explored under the condition of using commercially available microchannel plates with effective diameters of approximately 100 mm. As a result of electron trajectory simulations, a deployment of three spherical grids with significantly different grid separations instead of conventional equidistant separations showed an energy resolving power (E/ΔE) of 3200 and an angular resolution of 0.6°. The mesh number of the wire mesh retarding grid used for the simulation was 250. An RFA constructed with the simulated design experimentally showed an E/ΔE of 1100 and an angular resolution of 1°. Using the RFA and synchrotron radiation of 900 eV, photoelectron diffraction (PED) measurements were performed for single-crystal graphite. A clear C 1s PED pattern was observed even when the differential energy of the RFA was set at 0.5 eV. Further improvement of the energy resolution was theoretically examined under the assumption of utilizing a retarding grid fabricated by making a large number of radially directed cylindrical holes through a partial spherical shell instead of using a wire mesh retarding grid. An E/ΔE of 14 500 was predicted for a hole design with a diameter of 60 μm and a depth of 100 μm. A retarding grid with this hole design and a holed area corresponding to an acceptance angle of ±7° was fabricated. An RFA constructed with this retarding grid experimentally showed an E/ΔE of 1800. Possible reasons for the experimental E/ΔE lower than the theoretical values are discussed.

Photonic Sorting of Aligned, Crystalline Carbon Nanotube Textiles

Floating catalyst chemical vapor deposition uniquely generates aligned carbon nanotube (CNT) textiles with individual CNT lengths magnitudes longer than competing processes, though hindered by impurities and intrinsic/extrinsic defects. We present a photonic-based post-process, particularly suited for these textiles, that selectively removes defective CNTs and other carbons not forming a threshold thermal pathway. In this method, a large diameter laser beam rasters across the surface of a partly aligned CNT textile in air, suspended from its ends. This results in brilliant, localized oxidation, where remaining material is an optically transparent film comprised of few-walled CNTs with profound and unique improvement in microstructure alignment and crystallinity. Raman spectroscopy shows substantial D peak suppression while preserving radial breathing modes. This increases the undoped, specific electrical conductivity at least an order of magnitude to beyond that of single-crystal graphite. Cryogenic conductivity measurements indicate intrinsic transport enhancement, opposed to simply removing nonconductive carbons/residual catalyst.

The structure of an in-situ formed titanium-boron-carbon coating on a graphite substrate

A titanium-boron-carbon coating was fabricated on a graphite substrate by heating TiB2 powder on a graphite surface above the eutectic temperature. The coating consisted of a pure graphite layer on the outer surface and a TiB2-C alloy layer inside. The graphite layer had many wrinkles due to the difference in the thermal expansion coefficients of TiB2 and graphite. The TiB2-C alloy layer had a continuous three-dimensional interpenetrating network microstructure. The d002 value of the graphite in the alloy layer was 0.335 6 nm, which was quite close to that of single crystal graphite (0.335 4 nm). Raman and X-ray photoelectron spectroscopy indicated that the graphite in both layers was doped substitutionally with boron atoms. A water quench thermal shock test verified a high adhesion strength between the coating and the substrate. This method is promising for the fabrication of thermal barrier coatings on carbon materials.

Tuneling Spectroscopy Study Of The Local Conductivity Features Of The Solid State Surface

The procedure of the scanning tunneling spectroscopy experiments on the base of air multimicroscope SMM2000T is developed. Highly oriented pyrolytic graphite single crystal was chosen as model sample. Proposed approach allows investigation of the local nonuniformity of the surface properties. The statistical confirmed difference of volt-ampere curves recorded over pyrolytic graphite terrace and steps determined by the structure features is shown

Preliminary investigation of electrical conductivity of monolithic biochar

Monolithic biochar is explored as electrode material in supercapacitor – a fast-charging, long-lasting energy storage device. Electrical conductivity of electrode is critical to supercapacitor's performance. Traditionally, biochar is used as a solid fuel for combustion where electrical conductivity is largely irrelevant and ignored. This work explores electrical conductivity of monolithic biochar and elucidates its dependence on micro and macro structures of biochar. Electrical conductivity of biochar was found to be highly dependent on its degree of carbonization. Bulk conductivity of biochar can increase by over six orders of magnitudes when its carbon content changes from 86.8 to 93.7 wt%. Transmission electronic microscope and x-ray diffraction analyses revealed the presence of graphite nanocrystals (∼3 nm) and the growth of biochar crystallinity after heat treatment at 950 °C. The highest skeletal conductivity of carbon was 343.2 S/m, found in a heat-treated sugar maple biochar with 96.2 wt% of carbon. It is higher than that of graphite single crystal in direction perpendicular to graphene plane (333.3 S/m). Moreover, it was observed conductivity of monolithic biochar increased with compressive loading and dropped to the pre-compression level when the loading was released - a new phenomenon termed “elastic behavior of electrical conductivity of biochar”.

Образование графита в алмазоподобных тонких углеродных пленках

In the present work, particularities of the formation and structure of a graphite phase appearing in a thin amorphous diamond-like carbon (ta-C) film were studied using the method of high resolution electron microscopy. In order to obtain a thin carbon film, a neodymium laser NTS300 with a wavelength of 1064 nm was used. The film was deposited on a glass substrate in the process of a direct evaporation of a graphite target. Study of the graphite phase formation in the film was carried out using the high resolution transmission electron microscopy that allowed for a direct observation of the thin film atomic structure. Electron diffraction analysis shows that in local areas of a thin film a phase transition from amorphous diamond-like ta-C state to graphite occurs. Single crystals of graphite are oriented so that their basic hexagonal planes are parallel to the film surface. This is due to the fact that sp3 bonds are metastable and can be unstable, particularly on the film surface. Therefore, the transition of sp3 bonds to sp2 ones with the formation of planar complexes with a ring structure initially occurs on the film surface. The growth of a graphite crystal nucleus in the ta-C matrix increases the stress and stabilizes the diamond state. Due to this, the graphite phases form dominantly on the film edges. Analysis of electron diffraction patterns has shown that the interplanar spacings depend on the orientation of the planes relative to the film edge. Measurements of interplanar spacings reveals a complicated character of the stress state of the graphite lattice in the thin film. Along with tension-compression stresses, shear stresses are present and they result in a change of the angles between carbon interatomic bonds.

ChemInform Abstract: Synthesis of Ultra‐Incompressible sp3‐Hybridized Carbon Nitride with 1:1 Stoichiometry.

AbstractSuperhard CN is produced from pre‐compressed single crystal natural graphite powder positioned on top of a thin Ir or Pt spacer loaded in a diamond anvil cell with an initial 200 MPa N2 pressure.

GRAPHITE REVISITED

ABSTRACT Laboratory measurements are used to constrain the dielectric tensor for graphite, from microwave to X-ray frequencies. The dielectric tensor is strongly anisotropic even at X-ray energies. The discrete dipole approximation is employed for accurate calculations of absorption and scattering by single-crystal graphite spheres and spheroids. For randomly oriented single-crystal grains, the so-called approximation for calculating absorption and scattering cross sections is exact in the limit and provides better than ∼10% accuracy in the optical and UV even when is not small, but becomes increasingly inaccurate at infrared wavelengths, with errors as large as ∼40% at . For turbostratic graphite grains, the Bruggeman and Maxwell Garnett treatments yield similar cross sections in the optical and ultraviolet, but diverge in the infrared, with predicted cross sections differing by over an order of magnitude in the far-infrared. It is argued that the Maxwell Garnett estimate is likely to be more realistic, and is recommended. The out-of-plane lattice resonance of graphite near 11.5 μm may be observable in absorption with the MIRI spectrograph on James Webb Space Telescope. Aligned graphite grains, if present in the interstellar medium, could produce polarized X-ray absorption and polarized X-ray scattering near the carbon K edge.

Resistivity of Rotated Graphite-Graphene Contacts.

Robust electrical contact of bulk conductors to two-dimensional (2D) material, such as graphene, is critical to the use of these 2D materials in practical electronic devices. Typical metallic contacts to graphene, whether edge or areal, yield a resistivity of no better than 100 Ω μm but are typically >10 kΩ μm. In this Letter, we employ single-crystal graphite for the bulk contact to graphene instead of conventional metals. The graphite contacts exhibit a transfer length up to four-times longer than in conventional metallic contacts. Furthermore, we are able to drive the contact resistivity to as little as 6.6 Ω μm(2) by tuning the relative orientation of the graphite and graphene crystals. We find that the contact resistivity exhibits a 60° periodicity corresponding to crystal symmetry with additional sharp decreases around 22° and 39°, which are among the commensurate angles of twisted bilayer graphene.