Single Crystal Graphite Research Articles

Peeling Single-Stranded DNA from Graphite Surface to Determine Oligonucleotide Binding Energy by Force Spectroscopy

We measured the force required to peel single-stranded DNA molecules from single-crystal graphite using chemical force microscopy. Force traces during retraction of a tip chemically modified with oligonucleotides displayed characteristic plateaus with abrupt force jumps, which we interpreted as a steady state peeling process punctuated by complete detachment of one or more molecules. We were able to differentiate between bases in pyrimidine homopolymers; peeling forces were 85.3 - 4.7 pN for polythymine and 60.8 +/- 5.5 pN for polycytosine, substantially independent of salt concentration and the rate of detachment. We developed a model for peeling a freely jointed chain from the graphite surface and estimated the average binding energy per monomer to be 11.5 +/- 0.6 k(B)T and 8.3 +/- 0.7 k(B)T in the cases of thymine and cytosine nucleotides, respectively. The equilibrium free-energy profile simulated using molecular dynamics had a potential well of 18.9 k(B)T for thymidine, showing that nonelectrostatic interactions dominate the binding. The discrepancy between the experiment and theory indicates that not all bases are adsorbed on the surface or that there is a population of conformations in which they adsorb. Force spectroscopy using oligonucleotides covalently linked to AFM tips provides a flexible and unambiguous means to quantify the strength of interactions between DNA and a number of substrates, potentially including nanomaterials such as carbon nanotubes.

Preparation and electronic properties of potassium doped graphite single crystals

AbstractHere we revisited the electronic structure of potassium doped graphite using angle‐resolved photoemission spectroscopy (ARPES). We perform an analysis of the charge transfer from potassium to graphite employing a tight‐binding model for the band structure. Our ARPES measurements clearly show that the π* band of graphite becomes occupied upon deposition and intercalation of small amounts of potassium.The observed shift in the Fermi level of ∼ 400 meV is well below the appearance of staged graphite compounds and is described by a rigid band shift model. The π* band in pristine graphite is unoccupied and can therefore not be measured by ARPES. Thus potassium doping in the dilute limit provides a key for measuring the π* bands close to the Fermi level by ARPES. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Extraction and identification of elemental carbon in sediment and its environmental and climatic implications

Elemental carbon was extracted from sediments by chemical oxidation and identified by Raman spectroscopy. The experimental results indicate that elemental carbon, defined by processing, is a continuum ranging from single crystal graphite to amorphous carbon. Chemical oxidation is a feasible method to extract elemental carbon from sediments, and when experimental condition and time are properly controlled, it can remove organic carbon without any loss of elemental carbon. However, due to the differences in experimental conditions and methods, different results have been obtained. Therefore, we believe that the determination of standard sample and its concentration as well as the unification of the experimental method are of great importance. Also, the unification and definition of all these carbon particulates can facilitate transect comparison and further research in related fields. In profile research, elemental carbon was extracted and determined by using the same method. The elemental carbon curve tends to change with other climatic proxies, indicating elemental carbon can provide another new proxy for climatic and environmental changes. Based on the analysis, we deduce that the elemental carbon record has different implications for climate and environmental changes on different time-scales.

Carbonaceous substance of the Sukhoi Log gold deposit ( East Siberia)

Insoluble carbonaceous substance (ICS) at the Sukhoi Log gold deposit has been studied following the scheme: source shales — concentrates — residual substance of rocks. Examination by electron microscopy, transmission electron microscopy with electron microdiffraction, thermography, and mass spectrometry revealed several morphogenetic ICS varieties: dot-drop-like, honeycomb, single graphite crystals, and spherical graphite crystals. Study of source shales and concentrates has shown their similar carbon isotope compositions (δ 13C av = 18.03–17.54‰). Residual carbonaceous substance is characterized by a heavy carbon isotope composition (δ 13C av = 10‰). Its enrichment with heavy carbon isotope, as compared with the source rocks, calls for special geochemical studies. The ICS heterogeneity in the Sukhoi Log shales points to variations in the physicochemical settings of ore formation at the deposit.

Effect of Linear Density of States on the Quasiparticle Dynamics and Small Electron-Phonon Coupling in Graphite

We obtained the spectral function of very high quality natural graphite single crystals using angle-resolved photoelectron spectroscopy. A clear separation of nonbonding and bonding bands and asymmetric lineshape are observed. The asymmetric line shapes are well accounted for by the finite photoelectron escape depth and the band structure. The extracted width of the spectral function (inverse of the photohole life time) near the K point is, beyond the maximum phonon energy, approximately proportional to the energy as expected from the linear density of states near the Fermi energy. The upper bound for the electron-phonon coupling constant is about 0.2, a much smaller value than the previously reported one.

High pressure transformation of single-crystal graphite to form molecular carbon–onions

Carbon onions were formed in graphite in the diamond anvil high pressure cell under axial pressurecombined with shear deformation at room temperature. The onions were ranging in size from 1–2 nmto 1–2 µm and HREM studies have shown that the amount of their concentric shellstended to increase with pressure and deformation growth. Raman spectrawere taken from large-size onions. Narrow bands found between 1400 and1550 cm−1 correspond to fullerene-related carbon materials. The splitting of the pentagonal pinch mode1446–1489 cm−1 is considered to be connected with the formation of heptagons.

Origin of localized states in graphite: Indirect photoemission processes or impurities?

The electronic band structure of different types of graphite samples have been investigated in order to identify the origin of non-dispersive density of states recently reported in the literature. A systematic series of synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements on graphite single crystal, highly oriented graphite (HOPG) and epitaxial grown graphite single crystal on 6H–SiC(0 0 0 1) samples, have been carried out as well as compared with theoretical tight binding calculations. Our results indicate that these localized states are present in all the graphite-investigated samples showing the same non-dispersive character and at the same binding energies. The photoemission data taken at several photon energies demonstrate that these states are not surface states nor due to indirect photoemission processes. It seems that they are closely related to the level of impurities present in the studied samples.

The aqueous electrochemistry of carbon-based surfaces-investigation by scanning tunneling microscopy

Electro-oxidation of carbon-based materials will lead to conversion of the solid to CO2/CO at the anode, with H2 being produced at the cathode. Recent voltammetric investigations of carbon nano-tubes and single crystal graphite have shown that only edge sites and other defect sites are electrochemically active. Local oxidation of diamond-like carbon films (DLC) by an STM tip in moist air followed by imaging allows correlation of topographical change with electro-chemical conditions and surface reactivity. The results may have implications for lithographic processing of carbon surfaces, and may have relevance for electrochemical H2 production.

Anomalous Quasiparticle Lifetime and Strong Electron-Phonon Coupling in Graphite

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on high-quality single crystals of graphite to elucidate the character of low-energy excitations. We found evidence for a well-defined quasiparticle (QP) peak in the close vicinity of the Fermi level comparable to the nodal QP in high-T(c) cuprates, together with the mass renormalization of the band at an extremely narrow momentum region around the K(H) point. Analysis of the QP lifetime demonstrates the presence of strong electron-phonon coupling and linear energy dependence of the QP scattering rate indicative of a marked deviation from the conventional Fermi-liquid theory.

Direct synthesis of SiC nanowires by multiple reaction VS growth

β-SiC nanowires were directly synthesized by heating single-crystal silicon wafer and graphite without metal catalysts. The diameter of SiC nanowires is in the range of 10–30 nm, and the length is up to a few millimeters. Two kinds of SiC nanowires, namely pure SiC nanowires and SiC/SiO 2 composite nanowires, formed at higher temperature (the holding stage) and lower temperature (the cooling stage), respectively. A multiple-reaction model was proposed to explain the formation of SiC nanowires.

Direct Experimental Evidence of Free-Fermion Antibunching

Fermion antibunching was observed on a beam of free noninteracting neutrons. A monochromatic beam of thermal neutrons was first split by a graphite single crystal, then fed to two detectors, displaying a reduced coincidence rate. The result is a fermionic complement to the Hanbury Brown and Twiss effect for photons.

Fermi surface and edge-localized states in graphite studied by high-resolution angle-resolved photoemission spectroscopy

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on graphite single crystal (kish graphite). We have successfully determined the electronic band structure in the close vicinity of the Fermi level and found an extremely small hole-like Fermi surface centered at the K(H) point. We also found a weakly dispersive band near the Fermi level around the K(H) point, which is not predicted by the bulk band calculation. The origin of this anomalous feature is discussed in relation to the electronic states characteristic of the stepped surfaces.

Anomalous electronic states in graphite studied by angle-resolved photoemission spectroscopy

We report high-resolution angle-resolved photoemission spectroscopy (ARPES) on high quality single crystal of graphite (kish graphite) to elucidate the origin of anomalous physical properties of carbon-based materials. We found an almost flat band in the vicinity of the Fermi level around the K(H) point which is not predicted by the bulk band calculation. We discuss the origin of this anomalous structure in relation to the edge-localized stateon the step edges of cleaved surface.

Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps

The Raman mapping technique provides a non-destructive means of studying internal growth textures and other micro-structural heterogeneity inside diamond single-crystals. Raman maps showing distribution patterns of the bandwidth (FWHM) of the main first-order lattice vibration of diamond ( LO=TO phonon at ~1332 cm −1 ) along two-dimensional planes inside diamond crystals may reveal the internal growth zoning of these crystals. The observed zoning is affected, and in some cases even obscured in micro-areas adjacent to inclusions, by patterns of heterogeneous strain in the diamond. We present Raman maps obtained from diamond crystals containing large, single-crystal graphite inclusions, from the Panda kimberlite, Ekati Diamond Mine, Canada. The diamond growth texture was always found to start from the graphite inclusion. This result implies that graphite must have been the primary phase and was overgrown by diamond, whereas syngenetic growth of diamond and graphite was unlikely.

Low-energy electron diffraction study of potassium adsorbed on single-crystal graphite and highly oriented pyrolytic graphite

Potassium adsorption on graphite has been a model system for the understanding of the interaction of alkali metals with surfaces. The geometries of the $(2\ifmmode\times\else\texttimes\fi{}2)$ structure of potassium on both single-crystal graphite (SCG) and highly oriented pyrolytic graphite (HOPG) were investigated for various preparation conditions for graphite temperatures between 55 and 140 K. In all cases, the geometry was found to consist of $\mathrm{K}$ atoms in the hollow sites on top of the surface. The $\mathrm{K}$-graphite average perpendicular spacing is $2.79\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, corresponding to an average $\mathrm{C}\text{\ensuremath{-}}\mathrm{K}$ distance of $3.13\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and the spacing between graphite planes is consistent with the bulk spacing of $3.35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. No evidence was observed for a sublayer of potassium. The results of dynamical LEED studies for the clean SCG and HOPG surfaces indicate that the surface structures of both are consistent with the truncated bulk structure of graphite.

Significance of graphite occurrences in the Aracena Metamorphic Belt, Iberian Massif

The significance of syngenetic and epigenetic graphite occurrences from the Variscan high-temperature/low-pressure Aracena Metamorphic Belt is discussed in the framework of the tectono-thermal evolution of this southern zone of the Iberian Massif. Syngenetic graphite is associated with both low- to medium-grade metamorphic rocks (La Umbría series, Precambrian in age) and high-grade, granulite facies rocks (the Precambrian Fuente del Oro series and a Cambrian calc-silicate series). Epigenetic, fluid-deposited occurrences correspond to overgrowths on existing metamorphic graphite grains and vein-type mineralization. Two types of graphitized particles with remarkable differences in reflectance, anisotropy and size can be distinguished in the Precambrian metapelites of the La Umbría series. Large, >150 μm length, platy crystals with high reflectance and anisotropy are interpreted as detrital and are considered indirect evidence of an old orogenic cycle prior to the Cadomian Orogeny, during which metamorphism exceeded greenschist facies. The coexistence of two types of particles explains the scattering of values of the c parameter of graphite determined by XRD (c = 6.72–6.74 Å), and the anomalously high temperatures of the DTA exothermic peak (close to 600 °C) of graphite with respect to that inferred from mineral assemblages in these rocks. The presence of graphite-rich quartzites and gneisses within the Fuente del Oro series and the calc-silicate series is evidence of sedimentation under reducing conditions in a continental shelf. The characteristics of graphite reflect the high-grade metamorphic conditions attained in the southern area of the Aracena Metamorphic Belt. Pervasive flow of fluids related to a major Variscan extensional event resulted in overgrowths on the pre-existing graphite in the gneisses and quartzites of the calc-silicate series, as evidenced by the heterogeneous isotopic composition of graphite single crystals in these rocks. A later stage of graphite precipitation is represented by scarce vein-type occurrences in mafic granulites that document channelled flow of fluids.

A LEED and STM study of H(D) adsorption on C(0 0 0 1) surfaces

Adsorption of H (D) atoms on natural graphite (NG) single crystal and highly oriented pyrolytic graphite (HOPG) surfaces was studied with low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). No superstructure was observed in LEED, which suggests random adsorption of H on C(0001). Loss of diffraction at increasing H(D) coverage indicates loss of translational symmetry on the surface. STM data suggest that on D covered surfaces randomly distributed C atoms exist which pucker out of the basal plane. Puckered atoms are those which contribute to the STM profile of clean HOPG, i.e., which have no neighbour in the plane below.

Sprout-like growth of carbon nanowires on a carbon-doped Ni(1 1 1) surface

Sprout-like growth of carbon nanowires on single-crystal graphite (0 0 0 1) terraces on a carbon-doped Ni(1 1 1) substrate has been observed for the first time using only a heat treatment in ultrahigh vacuum. Nanometer-scale morphology and chemistry have been clarified by low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy (STM). The growth mechanism is based on a bulk-to-surface precipitation process of internal carbon atoms that were doped in a high-purity Ni(1 1 1) substrate in advance. The observed carbon nanowires are single wires and their bundles, which have a metallic conductivity. The structures have some similarity to those of single-wall and multi-wall carbon nanotubes. Simple manipulation of a single carbon nanowire is demonstrated by STM.

Diamond syntheses in multicomponent carbonate–carbon melts of natural chemistry: elementary processes and properties

It was found in experiments at pressures of 5.5–8.5 GPa that diamond effectively crystallizes in melt-solutions of the multicomponent carbonate–carbon system. By chemical mineralogy evidence, components of the system are among the compositions of the natural media for diamond formation. Carbon solutions oversaturated to diamond form in carbonate–carbon melts for two reasons: (1) the difference of solubilities of diamond and starting graphite (a thermodynamically instable phase at the diamond stability conditions) and (2) temperature gradient over the melts. The PT-boundary conditions for both spontaneous diamond crystallization and seeded diamond growth from, correspondingly, labile and metastable multicomponent carbonate–carbon melt-solutions oversaturated to diamond are experimentally determined. The processes of spontaneous re-crystallization of starting polycrystalline graphite into graphite single crystals (blocks, plates, spherules) in the metastable carbonate–carbon melt-solutions oversaturated to diamond is a phenomenal peculiarity of the diamond-forming systems. For the processes of diamond growth in the carbon solution-melts of natural chemistry, diffusion mass transfer mechanism reinforced by the convection one for the low-viscous carbon solution-melts is of main importance. Kinetics properties of carbon mass transfer as diamond crystallizes in the systems of natural chemistry depend on chemical composition of a solvent and carbon solubility therein, pressure and temperature of diamond growth processes. Growth rate is strongly variable within the range of several millimeters per minute to several micrometers per minute. In case of seeded diamond growth, this locates structurally the crystallization fronts and imposes morphological peculiarities of the layers overgrown: from polycentric and roughly blocked to smooth surfaced with nano-dimensional growth steps. For spontaneous crystallization, the estimated density of nucleation is not less than 3–5×10 2 (for individual octahedral crystals to 200 μm-sized, spinel-law and polysynthetic twins, aggregates) to 1×10 5 nuclea of diamond phase in 1 mm 3 (dense polycrystalline diamond aggregates of the natural diamondite type with microcrystals measuring from 10–20 nm to 50–100 μm).

Kish graphite as a magnetic field sensor

Magnetoresistance effect (MRE) and Hall effect have been applied to magnetic field sensors. As materials for the field sensor due to MRE, semiconductor compounds and ferromagnetic metals have been used, but graphite materials have not, though graphite exhibits positive magnetoresistance. A purified high quality Kish graphite (KG) exhibits a large transverse magnetoresistance ∆ρ/ρ even at room temperature. KG is a graphite flake precipitated from molten iron at high temperatures [1]. When the flakes are made at a very high temperature above boiling point of iron and purified carefully, they have higher crystallinity than that of HOPG and exhibit a nature very similar to single crystal graphite. In the present study, therefore, a sensitive magnetic field sensor was tried to prepare using such KG flakes. On the MRE sensors, a magnetic field is measured as a voltage change ∆V induced by the magnetic field under a constant electric current: ∆ρ/ρ = ∆V/V. ∆V increases with the increase of the sample voltage V under zero magnetic field, if the defect concentration in the sample does not increase with increasing V. V increases with the increase of current, sample length and sample thickness. However, the power W increases with increasing current I as W = IR, where R is the sample resistance, i.e. Joule heat generates and temperature of the sample should rise under a large current flow. In the case of KG, sample sizes are limited because of flakes. Therefore, reducing the sample thickness is important factor to increase V and ∆V in the present study, but a careful treatment for reducing the thickness is needed to increase ∆ρ/ρ, i.e. not to increase the defect concentration.