Search

Ab initio calculations of dielectric function and electron energy loss (EEL) function of periodically rippled armchair-edged graphene were performed in the framework of the random-phase approximation. The bending of graphene was found to remove restrictions on the electron transitions being forbidden in the flat graphene for certain light polarization. As a result, new peaks appear in the optical absorption spectrum and EEL spectrum of rippled graphene. Energy position, intensity, and width of the peaks are sensitive to the height of out-of-plane graphene bending that can be used in construction of graphene-based materials with variable transparency window. [ABSTRACT FROM AUTHOR]

Effect of Ar+ ion irradiation on the structure of pristine and fluorinated single-wall carbon nanotubes (SWCNTs) was examined using transmission electron microscopy (TEM), Raman, and x-ray photoelectron spectroscopy (XPS). The TEM analysis revealed retention of tubular structures in both irradiated samples while Raman spectroscopy and XPS data indicated a partial destruction of nanotubes and formation of oxygen-containing groups on the nanotube surface. From similarity of electronic states of carbon in the irradiated pristine and fluorinated SWCNTs observed by XPS, it was suggested that defluorination of nanotubes proceeded with breaking of C-F bonds. [ABSTRACT FROM AUTHOR]

This paper presents an experimental and theoretical study of the electronic structure of the fluorinated fullerene C60F36. UV photoemission spectroscopy (UPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy have been used for probing the density of electronic states in the valence and conduction bands of the compound. An assignment of spectral features was carried out using the results of ab initio B3LYP ground-state calculations of the electronic structure of C60F36. The sample of C60F36 is a mixture of three isomers. The calculations of the density of occupied states of these isomers revealed only a small effect of the π-system organization on the UPS profile. It was demonstrated that the CK-edge NEXAFS spectrum of the fluorinated fullerene can be successfully modeled using the (Z+1) approach properly treating the core hole impact on the spectral profile. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) were synthesized by thermal decomposition of methane at 900 °C using Co-Mo/MgO, Fe-Mo/MgO, and Ni-Mo/MgO catalysts. To obtain metallic nanoparticles, polyoxomolybdate clusters of Co, Ni, and Fe deposited on MgO were thermally decomposed at 700 °С, and the obtained oxides were heated in a carbon-containing atmosphere. The method of transmission electron microscopy (TEM) testified formation of one to ten walled CNTs with the average outer diameter depending on the catalyst used. Raman spectroscopy data confirmed the presence of single-walled CNTs in the samples obtained with Co-Mo/MgO and Fe-Mo/MgO catalysts. The electrochemical properties demonstrated by the obtained materials in supercapacitors are shown to be functions of their structural and compositional features. [ABSTRACT FROM AUTHOR]

Few-layer graphene samples have been obtained by a thermal expansion of bromine-intercalated fluorinated graphite with a ∼C2F matrix composition at 600, 700, and 800 °C. X-ray photoelectron spectroscopy detected that the products contained almost the same quantities of foreign atoms independently on the exfoliation temperature. However, the Raman spectra showed a progressive increase in the relative intensity of the D mode with the temperature growth. This was attributed to more edge states developed in graphene planes at the higher temperature. Thin films from obtained samples were tested as sensors using 1000 ppm nitrogen dioxide (NO2) in argon. The film from few-layer graphene produced at 800 °C had the largest relative response, while its recovery was quite low at room temperature. The sensor performances were related with the structural characteristics of the exfoliated graphenes. Left: Relative response of gas sensors based on few-layer graphene products of an expansion of fluorinated graphite at 600, 700, and 800 °C toward 1000 ppm NO2. Right: Proposed structures of the products. [ABSTRACT FROM AUTHOR]

Graphene is an exciting material for optoelectronics and plasmonics. Its optical response may be changed under mechanical action, such as stretching or corrugation, and with confinement of a size in a certain direction. Theoretical investigations play an important role in interpretation of experimental data and stimulating a search for novel graphene architectures. Thereby, it is important to analyze restrictions of modern approaches in calculation of optical properties of graphene-based objects and, particularly, to reveal an impact of electron-electron and electron-hole interactions on the position and shape of optical features. Here, we review the recent progress in quantum-chemical calculations of monolayer and few-layer graphenes, graphene ripples, and dots in a light of optical excitations. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Nitrogen-containing carbon (CNx) nanotubes have been grown vertically on silicon substrates by decomposition of a ferrocene/acetonitrile aerosol at temperatures 750, 800, 850, and 900 °С. A CNx nanotube array, produced at 850 °С, had the greatest height ∼220 μm and the smallest nitrogen content ∼3.8 at%. The content of graphitic nitrogen form, referred to a nitrogen atom with three carbon neighbors in a hexagonal lattice, increased gradually with the synthesis temperature. Field electron emission measurements showed that the CNx nanotube arrays have better characteristics than aligned non-doped carbon nanotubes. To reveal effect of nitrogen on the filed threshold, we calculated current-voltage dependencies for CNx nanotube models containing different amount of graphitic and pyridinic nitrogen species. Influence of the synthesis temperature on the height and total nitrogen content in CNx nanotube array and relation of these parameters with the threshold of field emission appearance. [ABSTRACT FROM AUTHOR]

Nitrogen-containing porous carbon has been produced by acetonitrile vapor deposition at 700 and 800 °C on the products of thermolysis of calcium tartrate doped by iron. The template was removed by dilute HCl acid treatment, and the samples obtained were comparatively studied using transmission electron microscopy, N2 adsorption measurements, thermogravimetric analysis (TGA), X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. It was found that increase in the synthesis temperature results in an extension of the specific surface area from ∼640 to ∼670 m2/g, enlargement in the average pore size from ∼9 to ∼13 nm, and a decrease in the nitrogen content from ∼4.4 to 2.7 at%. TGA revealed that the samples differ by ∼80 °C in the temperature of nitrogen loss, which was desorbed as the NO species mainly. Scheme of nitrogen-doped porous carbon synthesis. [ABSTRACT FROM AUTHOR]

Two few-layer graphene materials, strongly distinguished in structure, were compared as electrode materials for supercapacitor. The stacks of flat sheets with basal size of a few microns have been obtained as the result of ultrasonication of bromine intercalated graphite dispersion. The highly crumpled large-size layers have been produced by rapid heating of bromine intercalated graphite fluoride. The average number of layers and in-plane crystalline size in the materials were evaluated from Raman spectroscopy. The functional composition was studied using X-ray photoelectron and Infrared spectroscopy. Electrochemical testing of the graphene materials detected re-stacking of thin crumpled layers during the electrode preparation. Furthermore, graphene material produced from the fluorinated graphite showed poor wettability. The mild oxidation of the material in a mixture of sulfuric and nitric acids allowed increasing the capacitance in several times. [ABSTRACT FROM AUTHOR]

Effect of corrugation of hexagonal carbon network on the collective electron excitations has been studied using optical absorption and X-ray photoelectron spectroscopy in conjunction with density functional theory calculations. Onion-like carbon (OLC) was taken as a material, where graphitic mantle enveloping agglomerates of multi-shell fullerenes is strongly curved. Experiments showed that positions of π and π + σ plasmon modes as well as π → π* absorption peak are substantially redshifted for OLC as compared with those of highly ordered pyrolytic graphite and thermally exfoliated graphite consisted of planar sheets. This effect was reproduced in behavior of dielectric functions of rippled graphite models calculated within the random phase approximation. We conclude that the energy of electron excitations in graphitic materials could be precisely tuned by a simple bending of hexagonal network without change of topology. Moreover, our investigation suggests that in such materials optical exciton can transfer energy to plasmon non-radiatively. [ABSTRACT FROM AUTHOR]

Arrays of nitrogen-doped carbon nanotubes (N-CNTs) have been grown on Si substrates by thermolysis of vapors of acetonitrile and ferrocene. To change the content of incorporating nitrogen, carbonyls of Mo, W, and Cr were added in the reaction mixture. The products were comparatively studied using electron microscopy, Raman scattering, X-ray photoelectron spectroscopy, and charge/discharge cycling in an acidic electrolyte. Electrochemical characteristics of N-CNTs were correlated with nanotube length and chemical state of nitrogen. It is found that pyridinic nitrogen has determining effect on the N-CNT performance at low scan rates of potential. The density functional theory calculations of N-CNT models reveal that protonation of pyridinic nitrogen induces a net of negative charges on the neighboring carbon atoms that could increase the electrode capacitance. Cyclic voltammogram of nitrogen-doped carbon nanotube array at a scan rate of 2 mV s−1 in a 1 M H2SO4 aqueous solution and possible assignment of redox peaks. [ABSTRACT FROM AUTHOR]

CdS nanoparticles have been grown on multiwalled carbon nanotubes (CNTs) using a chemical bath deposition. Transmission electron microscopy and Raman spectroscopy revealed that crystalline ordering of nanoparticles depends on the duration of synthesis. Moreover, X-ray photoelectron spectroscopy showed that the surface of CdS nanoparticles contains ammonia used as a complexing agent in the reaction. Application of voltage to the substrate with array of aligned CNTs resulted in luminescence of CdS nanoparticles. Efficiency of the electroluminescence depends on the CdS crystallinity and size. Large well-crystallized nanoparticles give bright spots on the cathode image, while the glow from smaller, poor ordered nanocrystalls is strongly scattered. [ABSTRACT FROM AUTHOR]

Optical properties of periodically rippled graphene depending on the direction of hexagonal layer bending were investigated using density functional theory within the random-phase approximation. The corrugation was demonstrated to allow transitions being forbidden in the flat structure, which produce new resonances in the optical spectra of graphene. The most significant distinction between dielectric functions of flat graphene and rippled models was detected in low-energy region for light polarization perpendicular to the layer surface. The structure and positions of resonances were almost independent on the direction of graphene corrugation that can be useful for application of rippled graphene in optics. [ABSTRACT FROM AUTHOR]

Electronic structure of the chlorinated fullerene D-CCl has been studied using X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), and quantum chemical B3LYP calculations. The calculated shift of the core C 1s lines corresponding to the chlorinated carbon atoms and bare carbon atoms was shown to well agree with the experimental value obtained from the XPS data. This shift was taken into account in the construction of the theoretical XANES spectra near the C K-edge of the molecule. Ground-state calculation of D-CCl and calculations of CNCl and CClAr ions simulating the excited states of the chlorinated fullerene ( Z + 1 approximation) were used for modeling of C K-edge and Cl L-edge XANES data. A satisfactory agreement between experiment and theory was obtained in the case of Z + 1 approximation. Distinctions between the structure of the lowest unoccupied levels in the ground state and the C 1s excited state of D-CCl have been revealed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

X-ray absorption near-edge structure (XANES) spectroscopy has been used for comparative investigation of the electronic structure of pristine and fluorinated single-walled carbon nanotubes (SWCNTs) before and after annealing. The SWCNTs were produced by catalytic chemical vapor deposition method and fluorinated using a gaseous mixture of BrF3 and Br2 at room temperature. The samples were annealed at 250°C for 0.5 hour directly in the vacuum chamber of the X-ray spectrometer. C K-edge XANES spectra showed increase in the π* resonance intensity after the treatment of as-produced and fluorinated SWCNTs that was attributed to the detachment of adsorbed molecules and defunctionalization of nanotubes with the heating. Furthermore, analysis of the spectra revealed the annealing of the SWCNT samples produces topological defects in the nanotube walls. [ABSTRACT FROM AUTHOR]

The static polarizability of the C60, C70, C80, and C186 fullerenes has been calculated within the semiempirical MNDO approximation implemented in the MOPAC quantum-chemical program package. It is demonstrated that the results obtained are comparable with experimental data and the results of the ab initio B3LYP method using the 6–31 G( d, p) basis set. The influence of topological defects (five-, seven-, and eight-membered rings), vacancies produced by removing pentagons, and nitrogen and boron atoms on the geometric parameters and the polarizability of the C60, C240, and C540 fullerenes has been investigated by the MNDO method. It is revealed that the polarizability of the fullerene with topological defects is higher than the polarizability of the perfect icosahedral fullerene. The formation of vacancies in the carbon cage leads to a linear decrease in the polarizability of the fullerene and an increase in the specific polarizability. The polarizability of the heterofullerene with nitrogen or boron atoms spaced apart in the carbon cage is higher than that of the fullerene with heteroatoms located adjacent to each other. [ABSTRACT FROM AUTHOR]

Double‐wall carbon nanotube (DWNT) samples have been fluorinated at room temperature with varied concentration of a fluorinating agent BrF 3 . Content of the products estimated from X‐ray photoelectron data was equal to CF 0.20 and CF 0.29 in the case of deficit and excess of BrF 3 . Raman spectroscopy showed considerable decrease of carbon nanotube amount in the fluorinated samples. Analysis of optical absorption spectra measured for pristine and fluorinated DWNT samples revealed a selectivity of carbon nanotube fluorination. Nanotubes with large chiral angle are more inert to the fluorinating agent used. [ABSTRACT FROM AUTHOR]

Nitrogen‐doped carbon nanotubes have been synthesized by thermal decomposition of acetonitrile vapor over Ni/Co catalysts with varied ratio of metals. X‐ray photoelectron spectroscopy revealed that nitrogen atoms are embedded into tube walls in two different forms at least: (1) three‐coordinated nitrogen and (2) pyridinic‐like nitrogen. The ratio between these forms depends on the catalyst composition. Electronic structure of nitrogen‐doped carbon nanotubes was examined by means of photoemission, X‐ray photoelectron, X‐ray emission and X‐ray absorption spectroscopy. Field electron emission characteristics of samples produced were correlated with nitrogen content. The experimental data was interpreted using results of quantum‐chemical semiempirical AM1 calculations on models of nitrogen‐containing carbon tubes. Incorporation of three‐coordinated nitrogen was found from calculation of the models using the transfer matrix approximation to improve the current‐voltage characteristics of carbon nanotubes. [ABSTRACT FROM AUTHOR]

The electronic structure of tetragonal and rhombohedral polymers of the C60 fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C60 molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C60 fullerene polymers leads to a broadening of the maxima in the C K α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C60 fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C60 molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level. [ABSTRACT FROM AUTHOR]

The electronic state of nanodiamond/graphite interfaces in samples prepared by annealing of nanodiamonds (ND) at 1150-1600 K has been probed using X-ray fluorescence spectroscopy and field-emission measurements. Comparison between[C]Ka spectra of ND before and after annealing revealed an enhancement of density of high-energy occupied states in the products. A quantum-chemical calculation using a carbon model showed that the observed states could originate from the electrons of dangling bonds produced by peeling of a graphitic shell from the (111) surface of a diamond particle. The developed graphitic layers screen the weakly bonding electrons, which results in a lowering of the efficiency of field-electron emission from the samples with an increase of annealing temperature. [ABSTRACT FROM AUTHOR]

The electronic structure of samples produced by nanodiamond annealing has been examined using ultra-soft X-ray emission and X-ray absorption spectroscopy. Analysis of spectra of diamond/graphite composites showed that carbon atoms constituting the nanoparticles are at least in three states: diamond-like state, graphitic-like state and interface carbon, characterized by high electron localization. Comparison between theoretical spectra of the models and experimental spectra suggested the latter states correspond to three-coordinated carbon atoms from diamond surface. [ABSTRACT FROM AUTHOR]

CNx nanotubes have been prepared by acetonitrile decomposition over Ni, Co and Ni/Co catalysts. X-ray photoelectron spectroscopy study on the samples revealed a change of nitrogen concentration and shape of N 1s line with variation of the catalyst used. Quantum-chemical calculations on tube fragments showed the energy of N 1s level depends on the atomic structure of carbon tube and kind of incorporated nitrogen. The largest binding energies were found to be characteristic of three-coordinated nitrogen atoms doping the zigzag and chiral carbon nanotubes. [ABSTRACT FROM AUTHOR]

A carbonaceous material containing single-wall carbon nanotubes (SWNTs) has been synthesized by arc-discharge evaporation of graphite with a catalytic additive of nickel and cobalt powders. The synthesized SWNTs were purified from an amorphous carbon component (soot) and the catalyst particles by boiling in nitric acid. A comparison of the X-ray fluorescence spectra measured before and after this treatment showed that acid etching significantly decreased the content of soot in the material. The material enriched with SWNTs is characterized by a reduced threshold for the appearance of the field emission current, which is explained by a decrease in the screening effect of soot. The current–voltage characteristics of SWNTs exhibit a hysteresis, which is suggested to be due to the adsorption of molecules and radicals on the surface and at the ends of carbon nanotubes. © 2004 MAIK “Nauka/Interperiodica”. [ABSTRACT FROM AUTHOR]

The influence of different types of atomic vacancies on the electronic structure of carbon nanotubes (20, 0) and (12, 12) was investigated using an empirical tight-binding method. The models with vacancies randomly and symmetrically arranged in an extended unit cell were constructed. The symmetrical distribution of defects was obtained by rotating a pore, obtained by removing a hexagonal ring from a graphitic shell around the tube axis. The random defects were generated by a specially developed algorithm, which eliminates a given number of atoms from the unit cell so that the remaining carbon atoms have no more than one dangling bond. The presence of vacancies in carbon nanotube causes an enhancement of density of occupied states that is near the Fermi level and whose localization increases with the number of two-coordinated atoms at the pore boundaries. It was found that the occurrence of atomic vacancies in the carbon nanotube walls might result in the narrow-gap semiconductors and ferromagnetic materials. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [ABSTRACT FROM AUTHOR]

Heteroatom doping is a widely used method for the modification of the electronic and chemical properties of graphene. A low-pressure chemical vapor deposition technique (CVD) is used here to grow pure, nitrogen-doped and phosphorous-doped few-layer graphene films from methane, acetonitrile and methane-phosphine mixture, respectively. The electronic structure of the films transferred onto SiO2/Si wafers by wet etching of copper substrates is studied by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy using a synchrotron radiation source. Annealing in an ultra-high vacuum at ca. 773 K allows for the removal of impurities formed on the surface of films during the synthesis and transfer procedure and changes the chemical state of nitrogen in nitrogen-doped graphene. Core level XPS spectra detect a low n-type doping of graphene film when nitrogen or phosphorous atoms are incorporated in the lattice. The electrical sheet resistance increases in the order: graphene < P-graphene < N-graphene. This tendency is related to the density of defects evaluated from the ratio of intensities of Raman peaks, valence band XPS and NEXAFS spectroscopy data. [ABSTRACT FROM AUTHOR]

Detonation nanodiamonds (NDs) were deposited on the surface of aligned carbon nanotubes (CNTs) by immersing a CNT array in an aqueous suspension of NDs in dimethylsulfoxide (DMSO). The structure and electronic state of the obtained CNT-ND hybrid material were studied using optical and electron microscopy and Infrared, Raman, X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. A non-covalent interaction between NDs and CNT and preservation of vertical orientation of CNTs in the hybrid were revealed. We showed that current-voltage characteristics of the CNT-ND cathode are changed depending on the applied field; below ~3 V/µm they are similar to those of the initial CNT array and at the higher field they are close to the ND behavior. Involvement of the NDs in field emission process resulted in blue luminescence of the hybrid surface at an electric field higher than 3.5 V/µm. Photoluminescence measurements showed that the NDs emit blue-green light, while blue luminescence prevails in the CNT-ND hybrid. The quenching of green luminescence was attributed to a partial removal of oxygen-containing groups from the ND surface as the result of the hybrid synthesis. [ABSTRACT FROM AUTHOR]

Hybrid systems based on graphene and semiconductor quantum dots are prospective materials for optoelectronics and photonics. In this work, electronic structure and dielectric properties of small particles of cadmium sulfide on the surface of graphene were studied using the density functional theory. The optical spectrum of this hybrid structure depends on the orientation of the nanoparticle relative to graphene due to the interaction between electrons of sulfur atoms on the surface of the CdS particle and π-orbitals of carbon atoms. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) properties of CdS nanoparticles were studied depending on spacing between the nanoparticles and carbon nanotube (CNT) surface. Using a chemical bath deposition procedure the nanoparticles were grown directly on CNT arrays or arrays covered by a polystyrene layer. The polymer thickness was changed by varying the polystyrene concentration in a solution and the duration of CNT immersing. Transmission electron microscopy and atomic force microscopy were used for estimation of CdS nanoparticle size. An increase in the separation between CdS and CNTs was found to enhance the PL intensity and contribute to the exciton lifetime. PL spectra of CdS nanoparticles grown on a CNT surface and polystyrene layer coating the CNTs. [ABSTRACT FROM AUTHOR]

Polyaniline (PANI) was electrochemically deposited onto aligned carbon nanotube (CNT) arrays during 500 and 2500 sec. The electronic state of nitrogen in the composites was investigated by near-edge X-ray absorption fine structure spectroscopy. The N K-edge spectra revealed that mainly emeraldine base was developed in the former composite, while emeraldine was highly protonated in the latter composite. Electrochemical measurements showed that the CNT/PANI composites have significantly better performance than the non-coated nanotubes. The higher specific capacity in the composite obtained during the shorter time was related with the thinner PANI layer deposited on the CNT surface. [ABSTRACT FROM AUTHOR]

Molybdenum(VI) sulfide deposited on the surface of thermally exfoliated graphite has been reduced at 1273 K in dynamic vacuum conditions. High resolution transmission electron microscopy showed formation of layered particles with an average diameter of 20 nm which, using the Fourier analysis of images, were attributed to Mo3S4 phase. Graphite surface more likely promotes the transformation of MoS3 to Mo3S4 because the unsupported particles occurred in the sample have the MoS2 structure. [ABSTRACT FROM AUTHOR]

Arrays of aligned multi-walled nitrogen-doped carbon (CNx) nanotubes were synthesized by the catalytic chemical vapor deposition method using acetonitrile and a mixture of acetonitrile and toluene taken in equal ratio as sources for nanotube growth. The structure of arrays was investigated by means of scanning electron microscopy (SEM) and angle-resolved X-ray absorption spectroscopy near the CK- and NK-edges. The width of angular distribution of CNx nanotubes in the arrays was determined from the Fourier analysis of SEM images, and the ordering of graphitic layers of nanotubes was evaluated from angular dependence of the π*/σ*-intensity ratio near the CK-edge. It was shown that relative intensity of π*-resonance corresponding to N2 molecules embedded into CNx nanotubes can be used as a marker for definition of the deviation of straight CNx nanotubes from a normal direction. [ABSTRACT FROM AUTHOR]

The impedance of polystyrene-onion-like-carbon (PS-OLC) composites in the low-frequency (10–50 kHz) range has been studied as a function of the OLC weight fraction in the material. The composites were fabricated by rolling of PS filled with OLC powder obtained through the annealing of detonation nanodiamonds at 2140 K. The homogeneity of OLC distribution in the PS matrix has been studied as dependent on the number of rolling stages. It is established that the percolation threshold in PS-OLC composites is achieved at an OLC content of 35–40 wt %. [ABSTRACT FROM AUTHOR]

The properties of materials containing carbon nanotubes depend on the degree of alignment and the internal structure of nanotubes. It is shown that the degree of misorientation of carbon nanotubes in samples can be evaluated from the measurements of the angular dependences of the carbon X-ray emission and carbon X-ray absorption spectra. The CK α emission and CK X-ray absorption spectra of the array of multiwalled carbon nanotubes synthesized by catalytic thermolysis of a mixture of fullerene and ferrocene are measured. A comparison of the calculated model dependences of the relative intensities of the π and σ bands in the spectra with the experimental results makes it possible to evaluate the degree of misorientation of nanotubes in the sample and their internal texture. [ABSTRACT FROM AUTHOR]

Angular-resolved X-ray absorption spectra were measured for pyrolytic graphite samples of various quality. A new approach to determining the misorientation of graphite grains in polycrystalline samples is proposed, which is based on calculations of the angular dependence of the relative intensity of a peak corresponding to the π* state for a normal distribution of grains. The experimental values are used to construct theoretical angular dependences using partial densities of the π* and σ* states determined from the nonempirical calculations for graphite. [ABSTRACT FROM AUTHOR]

Nitrogen-doped and pure carbon nanotube (CNT) based composites were fabricated for investigating their dielectric properties in static regime as well as electromagnetic response properties in microwave frequency range (Ka-band). Two classes of host matrix-polystyrene and phosphate unfired ceramics-have been used for composites fabrication. The study reveals miscellaneous effect of nitrogen doping on the dielectric permittivity, dc conductivity and electromagnetic interference shielding efficiency of CNT-based composites, produced with both polymer and ceramic matrices. The high-frequency polarizability, estimated for different-length CNTs, and static polarizability, calculated for nitrogen-containing CNT models using a quantum-chemical approach, show that this effect results from a decrease of the nanotube defect-free-length and deterioration of the polarizability with incorporation of nitrogen in pyridinic form. [ABSTRACT FROM AUTHOR]

It is demonstrated that the CK[sub α] x-ray fluorescence spectra of carbon nanotubes synthesized by electric-arc evaporation of graphite and the spectra of carbon nanotubes produced using catalytic decomposition of hydrocarbons differ in the intensity of the short-wavelength lines. In order to interpret the obtained data, the electronic structures of perfect and defect-containing nanotubes are calculated in the framework of the tightbinding method with optimized parameters. The density of localized states that correspond to the observed increase in the intensity of the fluorescence spectrum of catalytic nanotubes can be obtained in the case when 15-20% of the total number of carbon atoms are absent in the carbon network. It is shown that defects of this type can bring about a decrease in the band gap of the nanotubes. [ABSTRACT FROM AUTHOR]

In the present work, we compare the structure and transport properties of carbon nanohorns (CNHs) synthesized by arc evaporation of graphite alone and with the addition of some portion of toluene. The materials have been investigated using transmission electron microscopy, Raman and infrared spectroscopies, thermogravimetric and BET analyses. The addition of a small amount of toluene during the evaporation of graphitic rod increases the length of CNHs, affects their hierarchical arrangement in aggregates and results in surface functionalization. All these features significantly enhance the conductivity of CNHs obtained with toluene additive in comparison with the pristine CNHs. [ABSTRACT FROM AUTHOR]

Nitrogen-doped carbon nanomaterials have greater capacity and better cycling stability for Li-ion batteries as compared to undoped carbon materials. In situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in combination with quantum chemical modeling has been applied to determine the chemical states of the incorporated nitrogen after interaction with lithium. NEXAFS N K-edge spectra of nitrogen-doped porous carbon were measured before and after thermal deposition of Li vapors. The simulation and interpretation of NEXAFS data were carried out based on density functional theory calculations of initial and lithiated graphene fragments that contained different nitrogen species. The preferable interactions of Li with pyridinic and hydrogenated pyridinic nitrogen which are located at edges of atomic vacancies and graphene planes were revealed. [ABSTRACT FROM AUTHOR]

The valence electronic structure of rhombohedrally polymerized C[sub 60], synthesized at 6 GPa and 725 °C, has been studied using x-ray emission spectroscopy. The C Kα spectrum of the polymer was found to differ from that of the C[sub 60] fullerite especially in the high-energy region. The observed spectra were compared to densities of states, calculated using a tight-binding approximation for three two-dimensional hexagonal C[sub 60] networks, distinguished by the nature of intermolecular bonding. Theoretical spectra of the polymers agree well with experiment and indicate that changes in the valence band of C[sub 60] upon polymerization are mainly due to the formation of intermolecular bonds, and only to a negligible degree due to a distortion of the C[sub 60] cage. Rotation of C[sub 60] within the hexagonal layer changes dramatically the electronic properties of the polymer from a semiconducting behavior for the structure with 66/66 connections to a metallic behavior for a layer containing C[sub 60] molecules that are linked through 56/65 bonds. The occurrence of the latter configuration may explain the observed metalliclike in-plane conductivity of the rhombohedral phase of C[sub 60]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this report, we use optical absorption spectroscopy and density functional theory simulations to investigate the optical behavior of a graphitic material with nanoscale holes. The material, produced by heating of graphite oxide in concentrated sulfuric acid followed by annealing at 1000 °C, demonstrated enhanced near-infrared absorption as compared to the pristine graphitic material. The computational study of graphene models containing holes of different sizes and different edge terminations revealed the major interband transitions defining the peaks in the absorption spectra. Our results suggest that the enhancement of near-infrared absorption of the material is caused by electron excitations involving hole edge states. The optical spectrum is strongly dependent on the distance between the holes and almost independent of both hole sizes and the functionalization family. [ABSTRACT FROM AUTHOR]

A method has been developed for continuous synthesis of carbon nanotube (CNT) arrays on large-area metal substrates using a reactor that moves above the substrate surface with a given scan rate. The reagent vapors pass through the region of plasma laser discharge towards the substrate surface screened by an argon flow from the external atmosphere. Vertically aligned multi-walled CNTs (MWCNTs) are formed on copper as a result of decomposition of ethylene and the vapor of iron pentacarbonyl. The main parameters of laser plasma discharge, gas jets, and concentrations of feed compounds have been estimated in terms of their effect on the growth rate and MWCNT morphology. The measurements of MWCNTs/Cu field emission characteristics have been used to determine optimal synthesis parameters for the fabrication of effective field emission cathodes. [ABSTRACT FROM AUTHOR]

Graphene films on copper foils were synthesized using low-pressure (2200-2800 Pa) chemical vapor deposition (CVD) from methane/hydrogen mixtures. The number of graphene layers is shown to be dependent on the composition of gas mixture and synthesis parameters. The annealing procedure of copper foils used as substrates was optimized to obtain high quality graphene. Atomic and electronic structures of graphene on copper and SiO2/Si substrates were studied by Raman, X-ray photoelectron, and near-edge X-ray absorption fine structure spectroscopy methods. [ABSTRACT FROM AUTHOR]

Fluorinated graphene (FG) has been obtained by a fluorination of thermally exfoliated graphite fluoride C2F layers using a gaseous mixture of BrF3 and Br2 at room temperature. Comparative study of electromagnetic (EM) response of thin FG films after recovering their conductivity via different methods is presented in microwave (GHz) and terahertz (THz) frequency ranges. We discovered that fluorination can be used as promising tool for producing tunable EM interference (EMI) shielding materials, combining reflective and absorptive layers. [ABSTRACT FROM AUTHOR]

Multiwall carbon nanotubes (MWCNTs) have been synthesized by aerosol-assisted catalytic chemical vapor deposition method from a 2% ferrocene solution in toluene at temperatures of 720 and 800 °C. After an oxidation by a HNO3/H2SO4 mixture, MWCNTs were dispersed in water and deposited in form of buckypapers with a thickness of ∼150 μm. The obtained materials were used as working electrodes of supercapacitors without adding of binder. The electrode from MWCNTs synthesized at lower temperature showed the larger capacitance in 1 M H2SO4 and 6 M KOH electrolytes, especially at low scan rates. An increase of double-layer capacitance was due to larger specific surface and microporosity of the material; a gain in pseudocapacitance was attributed to larger quantity of oxygenated groups developed on the defective surface of nanotubes. The electrode showed a good cyclic performance during a repetitive charge/discharge test. A larger specific capacitance of MWCNTs synthesized at 720 °C is related with etching of nanotube tips and higher oxygenation of nanotube surface as the result of binder-free buckypaper electrode preparation. [ABSTRACT FROM AUTHOR]

Vertically aligned multi-wall carbon nanotube (VA-MWCNT) arrays have been used for deposition of molybdenum sulphide from a water solution of ammonia molybdate and thiourea. A hydrothermal synthesis at 175 °C resulted in formation of quasi-spherical nanoparticles inside and on the top surface of arrays without destruction of the array structure. X-ray photoelectron spectroscopy revealed a molybdenum disulfide (MoS2) composition of the nanoparticles, while Raman spectroscopy detected the bands characteristic of light scattering in the MoS2 crystallites only after a treatment of synthesis products in hydrogen flow. As compared to the initial VA-MWCNTs, the composites showed enhanced specific capacitances in 1M H2SO4 electrolyte especially at low scan rates due to redox reactions involving MoS2. The reactions were more pronounced for the hydrogen treated composite. Cyclic voltammetry curves demonstrate an enhanced specific capacitance of MoS2/MWCNT array (fragment of the structure is shown in the insert) as compared to initial array due to redox reactions. [ABSTRACT FROM AUTHOR]