Electronic structure and arrangement of purified HiPco carbon nanotubes

The electronic structure of purified single-wall carbon nanotubes obtained from HiPco process has been probed by X-ray fluorescent spectroscopy. The CKα spectrum of the HiPco nanotubes revealed a significant enhancement of the high-energy maximum intensity compared with the graphite spectrum. Quantum chemical calculations of carbon nanotube models show the spectral variations could be caused by vacancies in the tube walls developed with the thermal and acid treatment of the sample. © 2003 Elsevier Ltd. All rights reserved

Topology and electronic structure of onion-like carbon and graphite/diamond nanocomposites

Annealing of nanodiamond at moderate temperature makes it possible to produce structures being intermediate in the carbon transformation from sp3 - to sp2-state (graphite/diamond nanocomposites) and onion-like carbon (OLC). Electron microscopy shows such structures involve cage shells with spacing close to graphite. X-ray emission spectroscopy has been applied to examine the electronic structure of OLC and graphite/diamond nanocomposites. The CKα-spectra of OLC produced in the temperature range of 1600-1900 K were found to be markedly different from the spectrum of particles formed at 2140 K and characterized by better ordering of graphitic shells. The latter spectrum was shown to be very Similar to the CKα- spectrum of polycrystalline graphite, while the former ones exhibited a significant increase of high-energy maximum that might be caused by the holed defect structure of graphitic networks forming at the intermediate annealing temperature. To interpret experimental spectra, the quantum-chemical semiempirical AM1 calculation of icosahedral C%40 cage and that with holed defects was carried out. The lack of at least 22% atoms in an internal carbon cage was found to be essential to provide an increase of density of high-energy electronic states similar to that observed in the spectrum of OLC produced at 1900 K

Topology and electronic structure of onion-like carbon and graphite/diamond nanocomposites

Annealing of nanodiamond at moderate temperature makes it possible to produce structures being intermediate in the carbon transformation from sp3 - to sp2-state (graphite/diamond nanocomposites) and onion-like carbon (OLC). Electron microscopy shows such structures involve cage shells with spacing close to graphite. X-ray emission spectroscopy has been applied to examine the electronic structure of OLC and graphite/diamond nanocomposites. The CKα-spectra of OLC produced in the temperature range of 1600-1900 K were found to be markedly different from the spectrum of particles formed at 2140 K and characterized by better ordering of graphitic shells. The latter spectrum was shown to be very Similar to the CKα- spectrum of polycrystalline graphite, while the former ones exhibited a significant increase of high-energy maximum that might be caused by the holed defect structure of graphitic networks forming at the intermediate annealing temperature. To interpret experimental spectra, the quantum-chemical semiempirical AM1 calculation of icosahedral C%40 cage and that with holed defects was carried out. The lack of at least 22% atoms in an internal carbon cage was found to be essential to provide an increase of density of high-energy electronic states similar to that observed in the spectrum of OLC produced at 1900 K

X-ray emission studies of the valence band of nanodiamonds annealed at different temperatures

X-ray emission spectroscopy has been applied to examine the electronic structure of onion-like carbon (OLC) generated by the annealing treatment of nanodiamonds (ND). The C Kα spectra of OLC produced in the temperature range of 1600-1900 K were found to be markedly different from the spectrum of particles formed at 2140 K and to be characterized by better ordering of graphitic shells. The latter spectrum was shown to be very similar to the C Kα of polycrystalline graphite, while the former ones exhibited a significant increase of the high-energy maximum that might be caused by the defect structure of graphitic networks forming at the intermediate temperatures. The experimental spectra were compared with the theoretical spectra from quantum-chemical semiempirical AM1 calculation of several models: a fullerene molecule, C240, having icosahedral structure, a C240 molecule incorporating a greater number of nonhexagonal rings, and a holed structure formed by removing pentagons from the icosahedral molecule. The density of high-energy electronic states in the valence band of the graphitic cage was found to be practically invariant to a change in ring statistics but to significantly increase because of localization of electrons on the zigzag sites of a hole boundary

X-ray emission studies of the valence band of nanodiamonds annealed at different temperatures

X-ray emission spectroscopy has been applied to examine the electronic structure of onion-like carbon (OLC) generated by the annealing treatment of nanodiamonds (ND). The C Kα spectra of OLC produced in the temperature range of 1600-1900 K were found to be markedly different from the spectrum of particles formed at 2140 K and to be characterized by better ordering of graphitic shells. The latter spectrum was shown to be very similar to the C Kα of polycrystalline graphite, while the former ones exhibited a significant increase of the high-energy maximum that might be caused by the defect structure of graphitic networks forming at the intermediate temperatures. The experimental spectra were compared with the theoretical spectra from quantum-chemical semiempirical AM1 calculation of several models: a fullerene molecule, C240, having icosahedral structure, a C240 molecule incorporating a greater number of nonhexagonal rings, and a holed structure formed by removing pentagons from the icosahedral molecule. The density of high-energy electronic states in the valence band of the graphitic cage was found to be practically invariant to a change in ring statistics but to significantly increase because of localization of electrons on the zigzag sites of a hole boundary

Electron spectroscopy of carbon materials: Experiment and theory

We present a comparative spectroscopic study of carbon as graphite, diamond and C60 using C1s K-edge electron energy-loss spectroscopy (EELS), X-ray emission spectroscopy, and theoretical modelling. The first principles calculations of these spectra are obtained in the local density approximation using a self-consistent Gaussian basis pseudo-potential method. Calculated spectra show excellent agreement with experiment and are able to discriminate not only between various carbon hybridisations but also local variation in environment. Core-hole effects on the calculated spectra are also investigated. For the first time, the EEL spectrum of carbyne is calculated. © 2006 IOP Publishing Ltd

Ultradispersity of diamond at the nanoscale

Nanometre-sized diamond has been found in meteorites, protoplanetary nebulae, and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of diamond nanoparticles seems to be peaked around 2-5 nm, and to be largely independent of preparation conditions. We have carried out ab initio calculations of the stability of nanodiamond as a function of surface hydrogen coverage and of size. We have found that at about 3 nm, and for a broad range of pressures and temperatures, particles with bare, reconstructed surfaces become thermodynamically more stable than those with hydrogenated surfaces, thus preventing the formation of larger grains. Our findings provide an explanation of the size distribution of extraterrestrial and of terrestrial nanodiamond found in ultradispersed and ultracrystalline diamond films. They also provide an atomistic structural model of these films, based on the topology and structure of 2-3 nm diamond clusters consisting of a diamond core surrounded by a fullerene-like carbon networ