Single Crystal Graphite Research Articles

Influence of surface defects on superlattice patterns in graphene on graphite

Superstructures observed by scanning tunnelling microscopy on graphite have been reported several decades ago, but the interest in these superstructures recently intensified due to their occurrence in graphene grown on different substrates. Generally accepted explanation of origin of these superstructures is an overlap of disoriented top layer of graphite and the underlying graphite single crystal, which causes moiré pattern. Here we present experimental findings that the orientation of the superstructure is influenced by surface defects and edges of graphene. Superstructures in graphene put on graphite exist even if the graphene is not supported by graphite over its entire area. The modulation of the density of states influences the strength of intra-layer carbon bonds in such a way that the graphene breaks along the superstructure minima. The tunnelling conductance of the areas with superstructures is enhanced with regard to bulk graphite.

Measurements of the in-plane coefficient of thermal expansion of freestanding single-crystal natural graphite

The in-plane coefficient of thermal expansion (CTE) of freestanding single-crystal natural graphite was directly determined using the thermal bulge method. The Scotch tape method was used to obtain 40-nm-thick graphite flakes by the micromechanical cleavage of high-quality graphite flakes. The naturally bulged shape of the samples enabled us to easily measure the CTE without any need for transfer or additional processes. The measured CTE was negative and gradually decreased in magnitude from 1.8×10−6/°C to 0.7×10−6/°C when the samples were heated from 25°C to 225°C.

Reconstruction of low-index graphite surfaces

The low-index graphite surfaces (101-0), (101-1), (112-0) and (112-1) have been studied by density functional theory (DFT) including van-der-Waals (vdW) corrections. Different from the (0001) surface which has been extensively investigated both experimentally and theoretically, there is no comprehensive study on the (101-0)- (101-1)-, (112-0)- and (112-1)-surfaces available, although they are of relevance for Li insertion processes, e.g. in Li-ion batteries. In this study the structure and stability of all non-(0001) low-index surfaces were calculated with RPBE-D3 and converged slab models. In all cases reconstruction involving bond formation between unsaturated carbon atoms of two neighboring graphene sheets reduces the surface energy dramatically. Two possible reconstruction patterns have been considered. The first possibility leads to formation of oblong nanotubes. Alternatively, the graphene sheets form bonds to different neighboring sheets at the upper and lower sides and sinusoidal structures are formed. Both structure types have similar stabilities. Based on the calculated surface energies the Gibbs–Wulff theorem was applied to construct the macroscopic shape of graphite single crystals.

Molecular dynamics simulations of irradiation defects in graphite: Single crystal mechanical and thermal properties

Molecular dynamics simulations of single crystal graphite have been performed, incorporating point and extended defect structures formed as a result of thermally annealed radiation damage. These AIREBO potential calculations have simulated the changes to crystal dimensions, elastic moduli and thermal expansion due to the formation of different representative morphologies of extended irradiation-induced defects (vacancy and interstitial aggregates) and over a range of atomic displacement fractions. We find that, in addition to the simulation method reproducing the important mechanical and thermal properties of virgin graphite, the property changes caused by the formation of extended defects in the graphite crystal structure qualitatively agree with experimental observations at different irradiation/annealing temperature regimes. The results of these calculations provide a direct insight into how the underlying atomic scale defects and dislocations created by radiation damage can lead to material property changes, and demonstrate how this computationally efficient simulation method can be employed to reproduce crystallite changes in large-scale models of polygranular graphite.

Natural occurrence of pure nano-polycrystalline diamond from impact crater.

Consolidated bodies of polycrystalline diamond with grain sizes less than 100 nm, nano-polycrystalline diamond (NPD), has been experimentally produced by direct conversion of graphite at high pressure and high temperature. NPD has superior hardness, toughness and wear resistance to single-crystalline diamonds because of its peculiar nano-textures, and has been successfully used for industrial and scientific applications. Such sintered nanodiamonds have, however, not been found in natural mantle diamonds. Here we identified natural pure NPD, which was produced by a large meteoritic impact about 35 Ma ago in Russia. The impact diamonds consist of well-sintered equigranular nanocrystals (5–50 nm), similar to synthetic NPD, but with distinct [111] preferred orientation. They formed through the martensitic transformation from single-crystal graphite. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD. A huge amount of natural NPD is expected to be present in the Popigai crater, which is potentially important for applications as novel ultra-hard material.

The Nature of Graphene Surfaces As Determined from the Wettability Studies of Basal and Edge Planes

Electrochemical studies of two-atom thick graphene sheet have attracted significant attention from the research community due to the attractive properties of its basal and edge surfaces. The graphene edge surface contains dangling bonds, unsaturated valencies, and can favorably attach to a plethora of functional groups. It has been demonstrated that the graphene edge surface has several orders of magnitude higher specific capacitance, rapid electron transfer rates and much stronger electrocatalytic activity than those of graphene basal surfaces. The graphene basal plane has been known to possess a high conductivity. Coupling these properties of the graphene basal and edge surfaces presents graphene or high quality single crystal graphite as potential electronic material. But the utilization of this versatile material in aquatic conditions is still limited due to unknown key properties of the basal and edge surfaces such as interaction of these surfaces with water and the isoelectric point. In this paper, we have tried to throw some light on the wettability characteristics of graphite basal and edge surfaces using carbon nanoparticles (fullerene and fullerol) and other characterization techniques. Highly oriented pyrolytic graphite (HOPG) is used as a model surface for fundamental studies of the wetting characteristics of graphite surfaces. HOPG is a highly anisotropic material, exhibiting different properties on its face and edge surfaces. The essential aim of this work was to provide surface chemistry information for HOPG surfaces. Fundamental wetting studies were accomplished through contact angle measurements of the as-received HOPG single crystal, which was characterized by its distinct face and edge surfaces. To study the effect of functional groups, contact angle measurements were repeated on the HOPG face and edge surfaces oxidized with different concentrations of hydrogen peroxide. Further, Raman spectroscopic studies were performed on the as-received and oxidized HOPG basal and edge surfaces. Atomic force microscopy (AFM) was also used a tool to image the HOPG surfaces and for surface force measurements to determine the iso-electric point (IEP) of the face and edge surface of HOPG. Zeta potential measurements of graphite powder in solutions of varying pH were also done to determine its iso-electric point and compare to the values reported in literature for graphitic carbon materials. Further AFM was used to study the interaction of the fullerene and fullerol nanoparticles on the graphite face and edge surfaces. The zeta potential measurements of the above mentioned carbon nanoparticles were also carried out to confirm that the interaction was due to hydrophobic or hydrophilic forces and not due to electrostatic interactions. In addition molecular dynamics simulations were also carried out to verify the interactions of the carbon nanoparticles with the graphite surfaces. Results indicate interesting charge properties of the graphite basal and edge surfaces and that the basal surface is highly hydrophobic whereas the edge is less hydrophobic (not completely hydrophilic), which makes it a promising electronic material for aquatic use.

Adhesive tape exfoliation: Why it works for graphene

Single-crystal graphite can be cleaved by the use of an adhesive tape. This was also the initial route for obtaining graphene, a one-layer thick graphite slab. In this letter a few simple and fun considerations are presented in an attempt to shed some light on why this procedure is successful. In particular on the nature of the surprisingly small number of repetitive steps that are needed in order to obtain a single-layer slab. Two frameworks for exfoliation are investigated: parallel exfoliation involving repetitive simultaneous cleaving, the other, serial exfoliation, which involves the repetitive cleaving of a single chunk of graphite. For both cases, parallel and serial exfoliation, it is investigated how many generations of cleavages are needed. An approximate model with the probability distribution expressed as a simple closed form is presented and compared with the simulations.

The structure of MB2MCC (MZr, Hf, Ta) multi-phase ceramic coatings on graphite

Abstract A new class of MB2 MC C multi-phase ceramic coatings were fabricated via pilling up MB2 (M Zr, Hf, Ta) powders on the surface of graphite plates followed by heating at 2823 K. Results showed that with the increase of atomic number of Zr, Hf, and Ta, the wettability of the ceramics on graphite plates increased. The surface and inside morphology of the ceramic coatings were different from each other. The graphite inside these ceramic coatings was highly ordered with graphite interlayer distance closing to that of single crystal graphite (0.3354 nm) and some B of MB2 diffused into graphite causing the decrease of local symmetry without disrupting the graphite structure. This approach is envisaged as a promising path to develop new kinds of ultra-high temperature material coatings for carbon materials.

On the energy spectra of secondary ions emitted from silicon and graphite single crystals

Secondary ion emission from silicon and graphite single crystals bombarded by argon ions with energies E 0 varied from 1 to 10 keV at various angles of incidence α has been studied. The evolution of the energy spectra of C+ and Si+ secondary ions has been traced in which the positions of maxima (E max) shift toward higher secondary-ion energies E 1 with increasing polar emission angle θ (measured from the normal to the sample surface). The opposite trend has been observed for ions emitted from single crystals heated to several hundred degrees Centigrade; the E max values initially remain unchanged and then shift toward lower energies E 1 with increasing angle θ. It is established that the magnitude and position of a peak in the energy spectrum of secondary C+ ions is virtually independent of E 0, angle α, and the surface relief of the sample (in the E 0 and α intervals studied). Unusual oscillating energy distributions are discussed, which have been observed for secondary ions emitted from silicon (111) and layered graphite (0001) faces. Numerical simulations of secondary ion sputtering and charge exchange have been performed. A comparison of the measured and calculated data for graphite crystals has shown that C+ ions are formed as a result of charge exchange between secondary ions and bombarding Ar+ ions, which takes place both outside and inside the target. This substantially differs from the ion sputtering process in metals and must be taken into account when analyzing secondary ion emission mechanisms and in practical applications of secondary-ion mass spectrometry.

Comparison of graphite materials for targets of laser ion source

To investigate efficient graphite material for carbon ion production in laser ion source, the plasma properties produced from these materials are measured. Comparing acquired current profile and charge state distribution, the distributions of ions in laser induced plasma from isotropic graphite and single crystal of graphite are different. The produced quantity of C(6+) from isotropic materials is larger than that from single crystal.

In-situ TEM studies of ion-irradiation induced bubble development and mechanical deformation in model nuclear materials

ABSTRACTThe MIAMI* facility at the University of Huddersfield is one of a number of facilities worldwide that permit the ion irradiation of thin foils in-situ in a transmission electron microscope. MIAMI has been developed with a particular focus on enabling the in-situ implantation of helium and hydrogen into thin electron transparent foils, necessitating ion energies in the range 1 – 10 keV. In addition, however, ions of a variety of species can be provided at energies of up to 100 keV (for singly charged ions), enabling studies to focus on the build up of radiation damage in the absence or presence of implanted gas.This paper reports on a number of ongoing studies being carried out at MIAMI, and also at JANNuS (Orsay, France) and the IVEM / Ion Accelerator Facility (Argonne National Lab, US). This includes recent work on He bubbles in SiC and Cu; the former work concerned with modification to bubble populations by ion and electron beams and the latter project concerned with the formation of bubble super-lattices in metals.A study is also presented consisting of experiments aimed at shedding light on the origins of the dimensional changes known to occur in nuclear graphite under irradiation with either neutrons or ions. Single crystal graphite foils have been irradiated with 60 keV Xe ions in order to create a non-uniform damage profile throughout the foil thickness. This gives rise to varying basal-plane contraction throughout the foil resulting in almost macroscopic (micron scale) deformation of the graphite. These observations are presented and discussed with a view to reconciling them with current understanding of point defect behavior in graphite.*Microscope and Ion Accelerator for Materials Investigations

Secondary particle emission from semiconductor crystals

Sputtering and secondary ion emission (SIE) from semiconductors with different values of the band gap have been studied experimentally and by means of computer simulation. The particular oscillations of secondary ion energy distribution for graphite, sapphire, and silicon single crystals were observed under irradiation by 1 and 10 keV Ar+ ions. This result cannot be interpreted using existing theories of SIE. We have explained them by the interplay of the charge-exchange processes between emitted and incident particles. Comparing the evidence from our experiments and computer simulation, we have drawn the conclusion that secondary ions emerge as a result of charge exchange with Ar+ ions not only when exiting the target, but also in the bulk. This is very different from what happens in metals, and needs to be taken into account when describing the processes in SIE, as well as in practical applications in mass spectrometry of secondary ions.

Crystal-grain size, phonon and carrier mean free paths in the basal plane, and carrier density of graphite films prepared from aromatic polyimide films

The electrical and thermal conductivities in the basal plane of graphite films prepared from aromatic polyimide films were evaluated with reference to previously measured electrical and thermal conductivities along their longitudinal directions at room temperature with the aid of magnetoresistance anisotropy measurement at 77 k. The average crystal-grain size of each graphite film was obtained from the relation between thermal conductivity and grain size originally derived by Klemens and Pedraza. It was found that at room temperature the crystal-grain boundary scattering of phonons could dominantly occur for crystal-grains with sizes smaller than ∼520 nm or the average d 002 value of grains larger than ∼0.3357 nm. When the grain size exceeds the intrinsic phonon mean free path in a single graphite crystal, i.e. ∼1760 nm, or the average d 002 value of the crystal-grains is less than ∼0.3357 nm, the three phonon process in thermal conduction becomes dominant. The mean free paths of carriers were found to be much shorter than those of phonons and it was shown that at 77 k grain sizes smaller than ∼220 nm could cause boundary scattering of carriers. The total carrier density at 77 k evaluated from the magnetoresistance and electrical conductivity data shows a decrease from the value of a single crystal of graphite as the crystallinity of the graphite film is reduced. [TANSO 2012 (No. 254) 176–86.]

Photoelectron Diffraction and Holographic Reconstruction of Graphite

A series of two-dimensional photoelectron angular distributions from a graphite single crystal for different kinetic energies from 311 to 908 eV using circularly polarized soft X-ray was measured. We realized that the directions of the prominent peaks arranged in a hexagon appearing in the photoelectron diffraction patterns and the directions from a photoelectron emitter atom to the surrounding atoms, which we previously assigned as the corresponding scatterer atoms [Appl. Phys. Lett. 85 (2004) 3737], do not match. From the comparison of the photoelectron diffraction patterns with multiple scattering calculations for each atomic site, we corrected our earlier assignments and revealed that the signatures of such peaks are even found in the photoelectron diffraction pattern of monolayer graphene. In the case of graphite that has a large interlayer separation, the diffraction rings due to in-plane scattering become dominant over the forward focusing peaks owing to interlayer scattering. Furthermore, a holographic reconstruction algorithm based on the fitting of elemental diffraction patterns for various C–C bond distances was used to analyze the full set of measured data. Clear nanometer-scale real-space images of several graphite layers were reconstructed.

Structure and electrochemical applications of boron-doped graphitized carbon nanofibers

Boron-doped graphitized carbon nanofibers (CNFs) were prepared by optimizing CNFs preparation, surface treatment, graphitization and boron-added graphitization. The interlayer spacing (d002) of the boron-doped graphitized CNFs reached 3.356 Å, similar to that of single-crystal graphite. Special platelet CNFs (PCNFs), for which d002 is less than 3.400 Å, were selected for further heat treatment. The first heat treatment of PCNFs at 2800 °C yielded a d002 between 3.357 and 3.365 Å. Successive nitric acid treatment and a second heat treatment with boric acid reduced d002 to 3.356 Å. The resulting boron-doped PCNFs exhibited a high discharge capacity of 338 mAh g−1 between 0 and 0.5 V versus Li/Li+ and 368 mAh g−1 between 0 and 1.5 V versus Li/Li+. The first-cycle Coulombic efficiency was also enhanced to 71–80%. Such capacity is comparable to that of natural graphite under the same charge/discharge conditions. The boron-doped PCNFs also exhibited improved rate performance with twice the capacity of boron-doped natural graphite at a discharge rate of 5 C.

Note: Adhesive stamp electrodes using spider silk masks for electronic transport measurements of supra-micron sized samples

A procedure for fabricating adhesive stamp electrodes based on gold coated adhesive tape used to measure electronic transport properties of supra-micron samples in the lateral range 10-100 μm and thickness >1 μm is described. The electrodes can be patterned with a ~4 μm separation by metal deposition through a mask using Nephila clavipes spider dragline silk fibers. Ohmic contact is made by adhesive lamination of a sample onto the patterned electrodes. The performance of the electrodes with temperature and magnetic field is demonstrated for the quasi-one-dimensional organic conductor (TMTSF)(2)PF(6) and single crystal graphite, respectively.

Crystal-grain size, phonon and carrier mean free paths in the basal plane, and carrier density of graphite films prepared from aromatic polyimide films

The electrical and thermal conductivities in the basal plane of graphite films prepared from aromatic polyimide films were evaluated with reference to previously measured electrical and thermal conductivities along their longitudinal directions at room temperature with the aid of magnetoresistance anisotropy measurement at 77 k. The average crystal-grain size of each graphite film was obtained from the relation between thermal conductivity and grain size originally derived by Klemens and Pedraza. It was found that at room temperature the crystal-grain boundary scattering of phonons could dominantly occur for crystal-grains with sizes smaller than ∼520 nm or the average d 002 value of grains larger than ∼0.3357 nm. When the grain size exceeds the intrinsic phonon mean free path in a single graphite crystal, i.e. ∼1760 nm, or the average d 002 value of the crystal-grains is less than ∼0.3357 nm, the three phonon process in thermal conduction becomes dominant. The mean free paths of carriers were found to be much shorter than those of phonons and it was shown that at 77 k grain sizes smaller than ∼220 nm could cause boundary scattering of carriers. The total carrier density at 77 k evaluated from the magnetoresistance and electrical conductivity data shows a decrease from the value of a single crystal of graphite as the crystallinity of the graphite film is reduced. [TANSO 2012 (No. 254) 176–86.]

Graphene production by etching natural graphite single crystals in a plasma-chemical reactor based on beam-plasma discharge

Graphene production by etching natural graphite single crystals in a plasma-chemical reactor based on beam-plasma discharge

Estimating the Thermal Conductivities and Elastic Stiffness of Carbon Nanotubes as a Function of Tube Geometry

The thermal conductivities and elastic properties of carbon nanotubes (CNTs) are estimated by using the double‐inclusion model, which is based on rigorous elasticity approach. The model regards a CNT as one inclusion (the inner cylindrical void) embedded in the other (the outer coaxial single‐crystal graphite shell). The concept of homogenization is employed, and vital microstructural parameters, such as CNT diameter, length, and aspect ratio, are included in the present model. The relationship between microstructure and thermal conductivities and elastic stiffness of CNTs is quantitatively characterized. Our analytical results, benchmarked by experimental data, show that the thermal conductivities and elastic stiffness of CNTs are strongly dependent on the diameter of CNT with little dependence on the length of CNT.

Electronic structure of single crystal and highly oriented pyrolytic graphite from ARPES and KRIPES

We present a comparative study of the near fermi-level electronic structure of single crystal and highly oriented pyrolytic graphite (HOPG). Angle resolved photoelectron spectroscopy and angle resolved inverse photoelectron spectroscopy have been used to probe the occupied and unoccupied electronic states, respectively. The band dispersions showed by single crystal graphite along its Γ K and Γ M symmetry directions were found to be in agreement with calculated band structure of graphite. The π bands of single crystal graphite were found to have a splitting of ∼ 0.5 eV at the K-point. We also observe the presence of a quasiparticle peak below E F at the K point at low temperature which indicates a strong electron–phonon coupling in graphite. In HOPG, the M and K points like features were found to be present in the same radial direction due to the superposition of the Γ M and Γ K directions. Results from our angle resolved inverse photoemission spectroscopy present the dispersion of the conduction band states, particularly the lower π ⁎ band. We have also found the presence of some non-dispersive features in both the valence and the conduction bands.