Hexagonal Carbon Network Research Articles

Observation of Nanosized Cap Structures on 6H–SiC(0001) Substrates by Ultrahigh-Vacuum Scanning Tunneling Microscopy

Nanosized cap structures on a thermally treated 6H–SiC(0001) substrate were investigated using atomic-resolution ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). Hexagonal carbon networks, partly composed of pentagons, were clearly observed on the surface of the cap structures for a sample annealed at 1250°C, in which carbon nanotubes (CNTs) had negligibly grow into the SiC substrate. Comparing their sizes and shapes with those annealed at 1350°C, the cap structures were considered to be the initial state of carbon nanotube (CNT) growth which determines the tube diameters of the final grown CNTs.

Molecular Dynamics of Nucleation Process of Single-Walled Carbon Nanotubes in Catalytic CVD Method

Nucleation process of single-walled carbon nanotubes (SWNTs) in the catalytic chemical vapor deposition method is studied using molecular dynamics simulations. A nanotube cap structure is generated when pieces of the hexagonal network structure extending from inside the cluster merged above the metal surface. Furthermore, interaction between catalytic metals and carbon atoms on formation process of SWNTs are studied using the home-made many-body potentials based on density functional theory calculations of small metal-carbon binary clusters. The Co cluster has a partially crystal structure where metal atoms are regularly allocated and embedded in the hexagonal carbon network. On the other hand, carbon atoms cover the entire surface of the Fe cluster. This implies stronger interaction between the graphitic lattice and Co atoms than Fe atoms. The difference of graphitization ability may reflect the ability as a catalyst on the formation process of an SWNT.

STM observation of formation process of carbon nanotube from 6H-SiC (000-1)

AbstractFormation process of nanosized cap structures on a thermally treated 6H-SiC(000-1) substrate was investigated using atomic-resolution ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). After formation of clusters of carobon particles 1-2 nanometer in diameter at 1150°C, these nanoparticles merged, forming nanosized cap structures. Hexagonal carbon networks, partly composed of pentagons, were clearly observed on the surface of the cap structures for a sample annealed above 1200°C. A model for the formation of carbon nanocaps on 6H-SiC(000-1) was proposed.

Molecular dynamics simulations of formation process of single-walled carbon nanotubes

Formation process of single-walled carbon nanotubes (SWNTs) by the catalytic chemical vapor deposition method is studied by molecular dynamics simulations. The random deposition process of carbon atoms to a nano-scale metal catalyst is realized with the multi-body potentials based on density functional theory calculations of small metal-carbon and metal-metal clusters A nanotube cap structure is generated when pieces of the hexagonal network structure emerges from the metal-carbon binary cluster and then coalesces. Furthermore, the tendency of graphitization of carbon atoms is compared for Ni, Co, Fe metal clusters. The Co cluster has a pseudo crystal structure where metal atoms are regularly allocated and embedded in the hexagonal carbon network. On the other hand, carbon atoms cover the entire surface of the Fe cluster. This implies stronger interaction between the graphitic lattice and Co atoms, hence higher graphitization ability. Finally, the mechanism of chirality determination by the stability of nanotube cap structure is discussed in order to explain the recent experimental finding that the near-armchair nanotubes are more dominantly produced for small diameter nanotubes.

Excited-state vibrations of benzene and polycyclic aromatic hydrocarbons: simple force field models based on molecular orbital characteristics of hexagonal carbon networks

Simple force field models for excited-state vibrations of benzene and polycyclic aromatic hydrocarbons (PAH) have been established. Root-mean-square frequency errors by the present MO/8 ΔE and MO/8 ΔΠ methods were found to be comparable with or even better than the conventional CCSD(T) and CIS calculations. The very strong vibronic coupling of the ν 8 mode for the S 2( 1 B 1 u ) and T 1( 3 B 1 u ) states of benzene proposed in jet-spectroscopic studies by Hiraya and Shobatake and in EELS studies by Swiderek et al. has given confirmation by the present calculations. The present methods hold promise for predicting excited-state vibrations for PAH in the accuracy of 40 cm −1.

Scanning tunneling microscope study of boron-doped highly oriented pyrolytic graphite

Atomically resolved scanning tunneling microscopy (STM) results are shown for substitutionally doped boron atoms in the hexagonal carbon network of highly oriented pyrolytic graphite (HOPG). STM images of boron-doped HOPG reveal not only a clear change in the electronic structure of the surface graphene network, but also one that directly affects the electronic structure of the graphene layer from the HOPG surface which is very exceptional for STM measurements. The boron atom site in the graphene network appears as the brightest area in the image including the six adjacent carbon atoms which have relatively higher intensity than normal carbon atoms in the STM image. The average boron-to-boron distance in the basal plane is consistent with Raman spectroscopy results. These structural results suggest that the graphite planes can be tailor made both atomically and electronically, and that boron doping can contribute to controlling the properties of the hexagonal carbon network in order to modify its properties relative to those of ideal graphite.

Structural change at the carbon-nanotube tip by field emission

Carbon-nanotube tips are plastically deformed during field emission. High-resolution transmission electron microscopy and structural simulations suggest that the deformed structure of the closed nanotube is explained by heterogeneous nucleation of the pentagonal and heptagonal carbon ring pairs, and that of the opened one is represented by sp3-like line defects in the hexagonal carbon network. It is considered that the changing of the inclination of the Fowler–Nordheim plots corresponds to the structural change in which a tip becomes sharp. The field ion microscope image and the corresponding field-emission pattern suggest that the electron emission from a closed nanotube is not necessarily from pentagonal carbon rings, but from the protrudent carbon network sites on the tip.

KCl crystallization within the space between carbon nanotube walls

The arc discharge method has been used hitherto to encapsulate metal-containing structures within carbon nanotubes [1, 2], the arrangement of the nanotube walls playing an important role in the process. We show that KCl can be generated in the space between nanotube walls by treating potassium-intercalated uncapped tubes with CCl 4. A relationship between the KCl crystal domain and the hexagonal carbon network is thought to give rise to preferential formation of a 100 plane perpendicular to the tube axes.

Interlayer structure changes of graphite after hydrogen ion irradiation

Changes in graphite interlayer orderings due to keV energy hydrogen irradiation were investigated as a basic step for studies of plasma-wall interactions in fusion reactors. Distribution of interplanar spacings within a 0–45 nm deep implantation layer was measured using Fourier transforms of the (002) lattice fringe images from HRTEM taken at side facets of 700 nm diameter graphite whiskers after 1 keV H + irradiation. Discrete increases of the interplanar spacing from 0.34 nm to 0.38, 0.41 and 0.43 nm were observed, with other indications at 0.47, 0.56, 0.62 and 0.69 nm. A sub-peak formation at 0.376, and at 0.402 nm was also observed in (002) spectra of X-ray diffraction (XRD) of 200 nm diameter whiskers after 1–6 keV H + irradiations at 673 K, and at 623 K, respectively. The increases to 0.38 and 0.41–0.43 nm were explained through sp 3 type C H bond formation in hexagonal carbon networks at the ‘low’ trapped H densities (H/C < 0.4), and the ‘high’ trapped H densities (H/C > 0.4), respectively, while those at 0.47–0.69 nm were attributed to interlayer, axial CH 3-bond formations and C x H y molecule insertions.

High resolution electron microscopy of graphite defect structures after KeV hydrogen ion bombardment

Structure changes of graphite due to keV-energy hydrogen ion bombardment are studied by means of high resolution electron microscopy (HREM). Highly graphitized vapor grown graphite fibers of diameters around 700 nm are irradiated side on with 1 keV H + ion beam to various fluences ranging from 1 × 10 14 to 1 × 10 18 H +/cm 2. The (002) lattice fringe images from the fiber side edges give “edge-on” projections of several tens of nanometers deep layers of the H + implanted graphite basal face, thus enabling direct observation of defect profiles in the surface layer. In a 1 keV H + ion bombardment at ambient temperature, interlayer spacing, d 002, within 12 nm from the graphite surface increases from the initial value of 0.336 nm to larger values ranging from 0.40 to 0.53 nm at an ion fluence of 1 × 10 17 H +/cm 2. The d 002 value ranges from 0.45 to 0.58 nm at an ion fluence of 1 × 10 18 H +/cm 2. Mean crystallite size, L a, on the other hand, decreases with fluence from the initial value above 100 nm to an equilibrium at 0.5 nm after a 1 keV H + irradiation to a fluence of1 × 10 17 H +/cm 2. The increase in the interlayer spacing is attributed at least to an increased van der Waals radius of the hexagonal carbon networks due to chemical bonding between lattice carbon atoms and hydrogen atoms at interlayer positions.

Anisotropy of Synthetic Metals

SYNTHETIC metals can be derived from graphite by intercalation of either electron donor or electron acceptor molecules between the carbon hexagon networks. In graphite, these networks are in effect macromolecules with aromatic character. Planar sheets of carbon hexagon networks lie parallel to the crystallographic a-axis of graphite, which has hexagonal symmetry. Physical properties in directions parallel to the networks are thus “macro-aromatic” properties. Perpendicular to these sheets, in the direction of the crystallographic c-axis of graphite, physical properties involve much weaker physical interaction between the layers; probably only van der Waals dispersion forces hold neighbouring sheets together in graphite crystals. It is comparatively easy to account for the high n or p electrical conductivities observed in the direction of the a-axis (as a consequence of charge transfer to or from the graphite electron bands1) but effects of such charge transfer on electrical properties in the direction of the c-axis of the crystals cannot be predicted as readily. Even in the parent graphite, the anisotropy ratio of electrical conductivity σa/σc is about 4 × 103 for well aligned stress recrystallized material2. It is possible that the relatively poor conduction of electricity in the c-axis direction in pure graphite involves a different kind of mobility, akin to electron hopping in crystals between molecules of poor conductors. In the synthetic metals the charged carbon hexagon networks are separated by positively charged layers of potassium atoms as in C8K (ref. 3) or by negatively charged layers whose molecular composition is given by formulae such as C24+ HSO4− 2H2SO4, C24+ NO3− 3HNO3, C8Br, C8ICl and so on. As far as conduction mechanisms in the c-axis direction are concerned, it is significant that considerable crystal swelling accompanies this intercalation; the repeat distance between successive sheets of carbon atoms increases from 3.35 A in the parent graphite to a much larger separation, which is about 8 A for graphite bisulphate4.

Electronic anomalies in dilute synthetic metals

Extended families of synthetic p metals can be prepared by controlled intercalation of electron acceptor molecules between the carbon hexagon networks of well oriented graphite. Resistivities and Hall coefficients have been determined down to 1.8 K for graphite bisulphates and (to a more limited extent) for graphite perchlorates. For many of these dilute synthetic metals, peak anomalies are found in Hall effects at around 20 K . These are discussed in relation to possible changes in the structure of the intercalate layers with temperature.

Electronic properties of some synthetic metals derived from graphite

Detailed measurements have been made of the temperature coefficient of electrical resistance for metallic solids with the approximate composition C 8 K, C 8 Rb and C 8 Cs, derived from near-ideal graphite of high quality. These data and other electronic properties of the solids confirm their n-type behaviour as metallic conductors with the carbon-hexagon networks acting as macro-anions. Comparisons are also discussed with p-type synthetic metals derived from graphite by intercalation of anions, in which the carbon hexagon networks act as macro-cations.

Overpotentials for Intercalation into Graphite

BEFORE intercalation of other molecules between the carbon hexagon networks in graphite can be started, they must first be given a sufficient chemical or electrical overpotential. The excess potential needed depends in a sensitive way on the textural perfection of the graphite used, which makes measurements of overpotential a valuable diagnostic test with near ideal graphites1.

Charge transfer effects in acid salts of graphite

Specimens of well oriented stress-annealed pyrolytic graphite mounted as electrical conductors have been oxidized under controlled conditions to give the acid salts, graphite nitrate and graphite bisulphate. Progressive intercalation in forming these compounds has been followed by measurements of the electrical resistivity, the magneto-resistance, and the Hall effect. Various electronic properties confirm a model for the acid salts in which layers of carbon hexagon networks act as macro-cations, separated by layers in which acid anions are linked to additional molecules of acid by hydrogen bonds. In conformity with this model, as the concentration rises, the resistance drops, at first steeply, finally asymptoting to a limiting value characteristic of a synthetic metal. The Hall effect which is negative for the near-ideal graphites used as starting materials passes through zero and finally becomes positive, confirming the general nature of the charge transfer hitherto assumed. The magneto-resistance measured a t 6 kG decreases, to become practically zero as intercalation proceeds.

Structural, thermodynamic and electronic aspects of the lambda transformation in graphite nitrates

Graphite nitrates of several sequences have been prepared by vapour phase nitration. Properties measured over a wide range of temperatures include thermal expansions, X-ray diffractions, electrical resistivities and thermoelectric power (TEP). The good metallic conduction and strongly positive TEP of these nitrates points to structures with carbon hexagon networks as macro-cations. Nitrate anions and nitric acid molecules are intercalated to an approximate formula C 24+ NO 3−3 HNO 3 for the first sequence. On cooling several of these compounds pass through a lambda type transformation at around −21°C. From X-ray data, the intercalated layers pass from liquid like disorder to crystalline order over a range of temperatures. Electrical resistivities decrease with falling temperatures more steeply in the lambda region where there is marked hysteresis, as is characteristic of lambda transformations generally. A peak value of TEP is observed in the lambda region; this novel effect is attributed to scattering of charge carriers by domains in “hybrid” crystal structures formed in this region.

Electrical properties and phase transformations of graphite nitrates

Methods are described for converting pieces of well alined graphite into nitrates of the first and second sequences, using vapour-phase nitration. In general, the more highly pinned starting materials require a higher threshold vapour pressure of nitric acid, before onset of conversion to the nitrates. Measurements of electrical resistivity and of thermo-electric power in the a -axis direction have been made over a range of temperatures, including temperature cycles around the λ point. Electronically, the solid nitrates of both first and second sequences behave as good p -conductors, in accordance with a structural model in which carbon hexagon networks act as macro-cations, with intercalations of nitrate anions, and of molecules of nitric acid. Resistivity measurements down to about 64 °K show that the well ordered graphite nitrates below the λ point have electrical conductivities and temperature coefficients of conductivity comparable with natural elemental conductors such as silver or copper. Resistivity data and thermo-electric power (t. e. p.) measurements both reveal novel features about the order–disorder transformation taking place around –20 °C. Definite hysteresis is found, and details of the hysteresis loops depend on the original texture of the specimen of near-ideal graphite used, as well as on nitration procedures. Furthermore, a large positive excess t. e. p. is observed at the λ peak, flanked by premonitory effects extending on either side of it. Both the hysteresis and the excess t. e. p. at the λ point are interpreted in terms of a general theory of phase transformations, which involve intermediate hybrid structures, with domains of the two forms coexistent in the λ region of temperatures.

Electron microscope study of interlamellar compounds of graphite with bromine, iodine monochloride and ferric chloride

Evidence is given of the existence of isolated dislocations in graphite-bromide and graphite-iodine monochloride, which are bounding the intercalated reactant layers. These boundary dislocations show stacking fault contrast to one side. The basal component of the Burgers vector lies along one of the three projections of the {112̄0} planes on the basal plane, compatible with the Burgers vectors of normal partial dislocations. It is therefore thought that the carbon hexagon networks on either side of a reactant layer are shifted in identical positions. Evidence is further given of a mechanism by which moving boundary dislocations leave numerous loops of occluded reactant behind on their way through the crystal. The islands of occluded reactant may account for at least part of the residual reactant retained in the crystal upon decomposition. Investigated and discussed are interactions of a boundary dislocation and a normal dislocation ribbon as well as the widening of dislocation ribbons presumably by preferential diffusion of reactant along the ribbon. The situation in the graphite-ferric chloride compound proved to be more complicated and the results not consistent. Evidence is given of isolated dislocations, which show no stacking fault contrast at one side. They separate however regions, resembling normal graphite, where normal dislocation ribbons are still present from regions where the contrast of the partials has become more diffuse, the partial dislocations assuming equidistant positions. It is therefore thought that these dislocations bound the intercalated ferric chloride layers. The widening of the contrast of the partials and their behaviour upon intercalation of FeCl 3 is interpreted as a result of the expansion of the interlayer spacing.

Pyrolytic Graphites: Their Description as Semimetallic Molecular Solids

This paper is concerned with an investigation of the electrical,the galvanomagnetic, and the thermoelectric properties of pyrolytic graphites whose morphological features are conditioned by the deposition temperature, the heat treatment, and the doping level. (1) Basal-plane magnetoresistance and c-direction specific resistance of deposits prepared at temperatures ranging from 1900°to 2500°C point to a remarkable improvement of the crystallites'alignment with rising deposition temperature. In both crystallographic directions the Seebeck coefficient closely follows semi-empirical predictions based on the two-dimensional model of theπ-electron bands. The Fermi level of a standard deposit (2100°C)is at 0.025 eV below the valence-band edge and thus indicates that crystal defects trap about 7.5×1018 electrons/cm3 at room temperature;this figure is in accord with a Hall coefficient of 0.33 cm3/C. The average in-plane mobility (930 cm2/V-sec) corresponds to a mean free path of the order of the crystallite diameter (250 Å). (2) Post-deposition treatment at temperatures above 2500°C results in (a) a rapid drop of the room-temperature basal-plane resistivity down to 50 μΩ-cm or less, (b) a Hall effect conversion from p to n type in the early stages of graphitization, and (c) a trend toward negative Seebeck coefficients in the layer planes. In conjunction with low-field magnetoresistance measurements these results can be described in terms of semimetallic concepts, the simultaneous presence of holes and electrons with equal concentrations(6×1018 cm−3 at room temperature) stemming from a slight band overlap. Average mobilities imply that the carrier behavior approaches single-crystal characteristics ( ≈ 104 cm2/V-secat room temperature) after heat treatment above 3000°C. Normal to the layers, the specific resistance always exceeds 0.1 Ω-cm, which points to a molecular conduction process in this direction. (3) An incorporation of boron into the carbon-hexagon networks lowers the electrical resistance of graphite particularly in the c direction (twenty-fold decrease at a composition of 0.6 at.%B); concurrently the two temperature coefficients become approximately equal to zero. In the rigid-lattice approximation band-population figures derived from the resistivity temperature dependence reflect the Hall coefficient behavior, the peak occurring at an equivalent boron content of 0.04%. The ionization efficiency is of the order of 50% with a Fermi level depressed by more than 0.1 eV. Thermoelectric power measurements in the c direction accord with the view that charge transport across the layer planes involves most of the excess holes, and reveal that boron enhances the Seebeck anisotropy of graphite.

Metallic Conduction in the Crystal Compounds of Graphite

Tests were made on samples of graphite in which the preferred orientation of the carbon hexagon networks was so good that the results permit definite inferences about the behavior of single crystals, and in particular about the anisotropy of the electronic properties along the a and c axes of graphite. Also, tests were made on samples of graphite in which individual crystallitcs show excellent alignment of their a-axes parallel to a plane, and of the c-axis normal to it. It was found that because of the excellent alignment, these specimens could take up materials such as bromine, iodine monochloride, or potassium up to the limiting values characteristic of the most highly charged crystal compounds, with cracking due to uneven expansion of individual regions in the specimen. Data are presented on the relative change of resistance in a and c axis directions on forming crystal compounds of graphite. Comparisons were made of the resistivity of wellaligned graphite and its crystal compounds with that of iron, nickel, and aluminum. (J.H.M.)