S. N. Nesov

Effect of pulsed ion irradiation on the electronic structure of multi-walled carbon nanotubes

The effect of pulsed ion irradiation and vacuum annealing on the ratio of sp2- and sp3-hybridized orbitals of carbon atoms in the layers of oriented multi-walled carbon nanotubes has been studied by analyzing the photoemission spectra of the C1s core level and the valence band of carbon, which were obtained using the equipment of the BESSY II Russian-German beamline of synchrotron radiation and a Riber analytical system. It has been shown that the ion irradiation leads to a significant decrease in the fraction of atoms with the sp3 hybridization of electrons. On the contrary, the annealing reduces the fraction of the sp3-component in the spectra of carbon. Typical features of the valence band of multi-walled carbon nanotubes in the annealed and irradiated states have been established.

Transformation of the electronic structure of the SnO2 − x/MWCNT nanocomposite under high-vacuum annealing conditions

The transformation of the structural phase state and the electronic structure of the SnO2 − x/MWCNT composite has been studied using X-ray spectroscopy and high-resolution transmission electron microscopy. It has been shown that the character of the interaction of the metal-oxide component of the composite with the array of carbon nanotubes depends on the structural state of tin oxide in globules of the metal-oxide component. In the initial composite with a large content of amorphous tin oxide, covalent functionalization of the MWCNT surface occurs. High-vacuum annealing results in the formation of a nanocrystalline structure in globules of the metal-oxide component and is accompanied by changes in the character of its interaction with carbon tubes.

XANES and XPS studies of processes initiated by high-vacuum annealing in SnO x /MWCNT composite layers

The initial and thermally treated composites based on SnOx/MWCNT have been studied by the XANES and XPS methods using the equipment of the BESSY II Russian-German beamline of synchrotron radiation and by the AES and HRTEM methods. The characteristic mechanisms of chemical and structural transformations in the SnOx phase have been determined depending on the vacuum annealing temperature. It has been found that the basic process in the metal-oxide component at annealing temperatures not exceeding 500°C is the tin monoxide SnO disproportionation reaction with the formation of the dioxide SnO2 phase and metallic tin. An increase in the annealing temperature to 800°C results in the activation of carbothermal reduction of metallic tin in contact areas of oxide clusters and MWCNT, as well as in the formation of nanocrystalline structures in the metal-oxide component of composite.

Interfacial interaction in a composite based on multi-walled carbon nanotubes and amorphous tin oxide

The specific features of changes in the electronic structure of multi-walled carbon nanotubes (MWCNTs) due to the interaction with an amorphous tin oxide in the SnOx/MWCNT composite formed by magnetron sputtering have been investigated using X-ray spectroscopy. It has been shown that the formation of chemical bonds responsible for significant changes in the local and electronic structures of the outer layers of MWCNTs occurs at the boundaries of the “amorphous oxide/MWCNT” contacts. The vacuum annealing of the composite leads to the disturbance of the chemical interaction at interfaces of the composite and to a partial recovery of the local structure of the outer layers of MWCNTs. A decrease in the amount of oxygen in the tin oxide under vacuum annealing conditions causes an increase in the number of unpaired Sn 5s electrons, which, in turn, enhances the charge transfer through the interfaces in the composite and leads to a splitting of the π*-subsystem of the outer layers of MWCNTs.

An observation of the radial breathing mode in the Raman spectra of CVD-grown multi-wall carbon nanotubes

Abstract MWCNTs grown by chemical vapor deposition on SiO 2 /Si substrates were investigated by Raman spectroscopy, transmission electron microscopy (TEM), Auger spectroscopy, and X-ray photoelectron spectroscopy before and after an annealing at 390 °C for 120 min in air or chemical treatment with a HCl solution. The Raman spectroscopy was focused on the low-frequency (250-300 cm −1 ) band. It is found that the positions and full widths at half maximum of the peaks forming the 250-300 cm −1 Raman band change little with the annealing or chemical treatment. The measured inner diameters of small-diameter CNTs from TEM agree well with those from Raman spectroscopy. These indicate that the low-frequency band originates from the radial breathing oscillations of carbon atoms in the inner walls of small-diameter MWCNTs.

Electronic structure of nitrogen-containing carbon nanotubes irradiated with argon ions: XPS and XANES studies

Using the methods of X-ray photoelectron (XPS) and X-ray absorption near edge structure (XANES) spectroscopies with synchrotron radiation, data on changes in the electronic structure and chemical composition of nitrogen-containing multiwalled carbon nanotubes (N-MWCNTs) upon their exposure to the radiation of argon ions with an energy of 5 keV are obtained. It is found that the exposure leads to an increase in the degree of defectiveness of the N-MWCNTs structure and to the carbon oxidation with formation of various oxygen-containing groups (C–OH, C=O/COOH, C–O–C/O–C–O, and CO3). The presence of carbon–oxygen bonds on the surface of carbon nanotubes is associated with the formation of radiation defects. It is shown that an increase in the fraction of nitrogen atoms present in the substituting configuration in the N-MWCNTs wall structure due to the irradiation does not give rise to an increase in the density of the occupied states near the Fermi level against the background of an increase in the degree of structure defectiveness, carbon oxidation, and a decrease in the total nitrogen concentration. The obtained results show that the irradiation of N-MWCNTs with argon ions allows one to successfully functionalize their surface.

Oxidation of the porous silicon surface under the action of a pulsed ionic beam: XPS and XANES studies

The changes in the electronic structure and phase composition of porous silicon under action of pulsed ionic beams have been studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) using synchrotron radiation. The Si 2p and O 1s core photoemission spectra for different photoelectron collection angles, valence band photoemission spectra, and X-ray absorption near-edge fine structure spectrain the region of Si L2,3 edges of the initial and irradiated samples have been analyzed. It has been found that, as a result of the irradiation, a thin oxide film consisting predominantly of higher oxide SiO2 is formed on the porous silicon surface, which increases the energy gap of the silicon oxide. Such film exhibits passivation properties preventing the degradation of the composition and properties of porous silicon in contact with the environment.

AES and XPS studies of a por-Si/SnOx nanocomposite formed using a powerful ion beam of nanosecond duration

The results of the X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) investigations of tin-oxide nanolayers on samples of por-Si/SnOx composites with varying matrix porosity, formed using a powerful ion beam of nanosecond duration, are presented. It is shown that rapid melting and crystallization of the surface leads to the formation of Si nanoparticles with a maximal size of 200 nm. It is established that tin is included in the structure of the nanocomposite in an oxidized state with a small inclusion of metallic β tin. With increasing porosity, the phase composition of the tin nanolayers becomes closer to the state corresponding to the highest tin oxide (SnO2). It is also shown that, upon an increase in the porosity, the intensity of the tin 4d subvalent line increases, which is, apparently, associated with an enhanced degree of hybridization of tin and oxygen atoms. The changes in the elemental composition of the composite and the depth of tin penetration are estimated from the results of ion etching.

The formation of layers of porous crystalline tin dioxide from a composite on the basis of multiwalled carbon-nanotube arrays

A new method for the synthesis of porous crystalline tin-dioxide (SnO2) layers from composites on the basis of multiwalled carbon nanotubes (MWCNTs) and nonstoichiometric amorphous tin oxide (MWCNT/SnOx) is proposed. An MWCN/SnOx composite layer produced by magnetron sputtering is annealed in air atmosphere at 500°C for 30 min. A homogeneous porous layer comprised of crystalline SnO2 spherical particles with a size of about 0.1 μm is obtained as a result. In the process of annealing, nearly all the amount of carbon is removed in the form of gaseous oxides (only a small amount remains in the upper part of the porous SnO2 layer). The structural defectiveness of nanotube walls, which increases because of the magnetron deposition of tin, plays a crucial role in the carbon oxidation and destruction of MWCNTs.

Changes in the chemical state and concentration of iron in carbon nanotubes obtained by the CVD method and exposed to pulsed ion irradiation

Data on the distribution of iron in nitrogen-containing multiwall carbon nanotubes (N-MWCNTs) and changes in its chemical state and concentration under different parameters of irradiation by a pulsed ion beam are obtained by methods of transmission electron microscopy, X-ray photoelectron spectroscopy, and energy dispersion analysis. It is shown that the irradiation of N-MWCNTs with an energy density of 0.5 J/cm2 lead to the formation, on their lateral surfaces, of structures with a size of 2–10 nm, consisting of metallic iron encapsulated in a carbon shell. An increase in the energy density to 1–1.5 J/cm2 leads to a substantial removal of iron clusters from the tips of carbon nanotubes and a reduction in the amount of iron in the bulk of the N-MWCNT layer.