Determination of SWCNT diameters from the Raman response of the radial breathing mode

We report on the evaluation of the distribution of diameters for nanotube samples with a wide variation of mean diameters. Such results were obtained from a detailed analysis of the radial breathing mode Raman response and compared to results obtained from an evaluation of optical spectra and X-ray diffraction pattern. The evaluation of the Raman data needs a well refined analysis as the experimental analysis exhibits a rather complicated and oscillating relation between response and exciting laser. Both, an exact calculation where the density of states was considered explicitly and an approximate calculation were applied. Both models used for the analysis are able to explain several unexpected results from the experiment such as the oscillating behavior of the spectral moments, unusual discontinuities in the first moments of the Raman response for excitation in the IR, a fine structure for the response in optics and Raman, and an up shift of the RBM frequency as compared to qualified ab initio calculations. In detail the first moment and the variance of the spectra were used for the evaluation of the diameter distribution. To obtain good results between experimental and theoretical oscillation pattern the transition energy between the first two van Hove singularities had to be scaled up which is considered as a result from coulomb interaction of the electrons in the tubular material. On the other hand the analysis does not only allow to determine the mean value and the width of the diameter distribution but yields also a value for the average bundle diameters or, alternatively, the strength of the tube-tube interaction. The model used for the analysis of the Raman data is also appropriate to analyze the optical response, at least for the spectral range from 0.5 eV to 3.5 eV. The fine structure in the response for the transitions between the three lowest van Hove singularities is well reproduced and the mean tube diameters and their distribution is obtained in very good agreement with the results from the Raman analysis. From the X-ray analysis the same mean values and comparable distributions for the tube diameters were received whereas the bundle diameters could not be retained with high precision in this case. Received 18 February 2001 and Received in final form 3 April 2001     

Resonance Raman excitation and electronic structure of the single bonded dimers (C ¯60)2 and (C 59N)2

Raman spectra are presented for the single bonded dimeric fullerene (C ₆₀ ^- ) ₂ and compared to optical spectra and Raman spectra of the isostructural and isoelectronic heterofullerene (C₅₉N)₂. The spectra of both materials exhibit strong correlations with respect to splitting, line position, and line intensity. This holds for non resonant excitation with blue and green lasers as well as for the strong resonances observed with red lasers. The latter observation is consistent with a downshift for the electronic transition energies as compared to C₆₀. The absorption edge of thin films of (C₅₉N)₂ was found at 1.4 eV. The three intercage modes were observed at 82, 103, and 111, and at 88, 98, and 106 cm^-1 for (C₅₉N)₂ and (C ₆₀ ^- ) ₂ , respectively. A surprising difference was found for the position of the pentagonal pinch modes in the two materials as they were observed at 1461 and at 1451 cm^-1, for (C₅₉N)₂ and (C ₆₀ ^- ) ₂ , respectively. This is interpreted as a consequence of some characteristic differences in the electronic structure of the two compounds. Received 25 January 2000 and Received in final form 10 April 2000     

Thermal conductance for single wall carbon nanotubes

We report a theoretical analysis of the phonon thermal conductance, κ(T), for single wall carbon nanotubes (SWCN). In a range of low temperatues up to 100 K, κ(T) of perfect SWCN is found to increase with temperature, approximately, in a parabolic fashion. This is qualitatively consistent with recent experimental measurements where the tube-tube interactions are negligibly weak. When the carbon-carbon bond length is slightly varied, κ(T) is found to be qualitatively unaltered which implies that the anharmonic effect does not change the qualitative behavior of κ(T). Received 12 June 2001     

Infrared spectra and lattice vibrations of the spin-chain compound LiCuVO 4

The infrared spectra of the one-dimensional antiferromagnet LiCuVO₄ are measured in the frequency range from 10 cm^-1 to 10 000 cm^-1 and at temperatures from 2 K to 300 K, for the electric field vector E of the radiation polarized either along the a- or along the b-crystallographic directions. For each polarization six infrared active phonon modes are observed in accordance with factor group analysis of the crystal structure of LiCuVO₄. The theoretical group analysis of the possible spinel low-symmetry phases is performed within the framework of Landau's theory of phase transitions. The parameters of several phonon lines show noticeable anomalies around 150 K where the magnetic correlations appear in the copper chains, which may indicate a finite interaction between the phonon and the magnon subsystems in LiCuVO₄. Received 19 February 2001 and Received in final form 26 June 2001     

Raman and infrared spectroscopical investigation of the optical vibrational modes in GaSb/AlSb superlattices

The vibrational spectrum of ultra-thin layer GaSb/AlSb superlattices was investigated in detail by infrared (IR) and Raman spectroscopies. The effect of confinement of the transverse and longitudinal optical phonons in both types of the layers was studied. The dispersions of optical phonons of the GaSb and the AlSb obtained from the analysis of the Raman and IR spectra are in a good accordance with the theoretical data and results of neutron scattering experiments. First- and second-order Raman spectroscopy indicates the presence of intermixture of atoms at the interfaces in the GaSb/AlSb superlattices. Received: 11 May 1998 / Accepted: 21 July 1998     

Sound modes in composite incommensurate crystals

We propose a simple phenomenological model describing composite crystals, constructed from two parallel sets of periodic inter-penetrating chains. In the harmonic approximation and neglecting thermal fluctuations we find the eigenmodes of the system. It is shown that at high frequencies there are two longitudinal sound modes with standard attenuation, while in the low frequency region there is one propagating sound mode and an over-damped phase mode. The crossover between these two regions is analyzed numerically and the dynamical structure factor is calculated. It is shown that the qualitative features of the experimentally observed spectra can be consistently described by our model. Received 28 November 2001 and Received in final form 23 January 2002     

Optical absorption and energy-loss spectra of aligned carbon nanotubes

Optical-absorption cross-sections and energy-loss spectra of aligned multishell carbon nanotubes are investigated, on the basis of photonic band-structure calculations. A local graphite-like dielectric tensor is assigned to every point of the tubules, and the effective transverse dielectric function of the composite is computed by solving Maxwell's equations in media with tensor-like dielectric functions. A Maxwell-Garnett-like approach appropriate to the case of infinitely long anisotropic tubules is also developed. Our full calculations indicate that the experimentally measured macroscopic dielectric function of carbon nanotube materials is the result of a strong electromagnetic coupling between the tubes. An analysis of the electric-field pattern associated with this coupling is presented, showing that in the close-packed regime the incident radiation excites a very localized tangential surface plasmon. Received 18 January 2001     

Composition dependence on dispersion of Au/Cu bimetallic nanoparticles in nylon 11 thin films

Effect of composition on the dispersion of Au_xCu_1-x bimetallic nanoparticles into nylon 11 matrix has been investigated. TEM, EDX, and XPS depth profiling were used for characterizing the changes in the composition of the bimetallic particles and in the depth distribution of the particles in the nylon 11 layer caused by heat treatment in N₂ atmosphere. The island-like bimetallic particles were found to be formed on the nylon 11 surface before heat treatment. The results of XPS depth profiling revealed that, by the heat treatment, the Au_xCu_1-x bimetallic particles with x? 0.55 were not dispersed into the nylon 11 layer while those with x≥ 0.70 were homogeneously dispersed in the films, indicating the existence of critical composition for penetration of the bimetallic particles. By comparing the composition and structure of the bimetallic particles, the cause of these finding is discussed in terms of surface free energy of the particles. Received 29 November 2000     

Theoretical study of small silicon clusters on a graphite layer

In a series of recent experiments, the HOMO-LUMO energy gaps of small Si clusters deposited on a graphite substrate have been determined by Scanning Tunneling Microscopy (STM). The values obtained were found to be substantially smaller than the energy gaps of corresponding passivated clusters. This work considers dimensional reduction as a possible mechanism for a sizeable energy gap narrowing by the example of the system Si₅. The impact of the graphite substrate on the deposited species is investigated in the framework of a pseudocluster model. Received 30 November 2000     

How accurate can the determination of chiral indices of carbon nanotubes be?

We present a general and systematic method for determining the chiral indices of carbon nanotubes. This method relies on the semi-quantitative analysis of experimental selected area diffraction pattern intensities, together with extensive comparison with kinematic theory. We show how to retrieve the chiral indices of single walled or multiwalled carbon nanotubes, even when their radii are large (up to approximately 40 ?). All theoretical and experimental sources of errors are discussed. By discussing the experimental case of a double-walled carbon nanotube, we show how it is possible to determine the chiral indices of each of its constituant tubes independently, by analyzing parts of the diffraction pattern where the contributions of these tubes do not interfere. Using the parts where all the contributions do interfere, we successfully crosschecked independently the preceding determination. Received 23 December 2003 Published online 7 May 2003     

Structures and energetics of carbon bridged C 60 clusters

The structures and energetics of carbon bridged C₆₀ clusters (C ₆₀ ) _n C_m have been studied by simulated annealing technique within the tight-binding molecular-dynamics. The “sp² addition” ball-and-chain dimers exhibit odd-even alternations over the number of chain atoms, with the dimers containing even chain atoms more stable against dissociation than their immediate neighbors containing odd chain atoms. In addition to the usual “sp² addition” dimers, a pentagon-linked C₁₂₁ isomer and a hexagon-linked C₁₂₂ isomer are also found to be stable. Based on our tight-binding calculations, trimers and larger clusters can be simply regarded as being made up of independent or weakly interacting dimers, if the C-C₆₀ joints on a single cage are not too close to each other. Large C₆₀ clusters connected by chains each containing only one or two carbon atoms have similar stability to that of constituent dimers, indicating the possibility to form stable C₆₀-carbon polymers. Received 17 January 2001 and Received in final form 26 February 2001     

Inelastic neutron scattering study of the dynamics of the AlNiCo decagonal phase

The dynamics of the decagonal AlNiCo phase has been investigated on a single-grain quasicrystalline sample using inelastic neutron scattering. The decagonal structure can be viewed as a periodic stacking of quasiperiodic planes. The anisotropy between the modes propagating in the periodic and quasiperiodic directions is found to be much weaker than theoretically predicted. A strong resonance splitting is observed at an energy transfer of 15 meV for transverse modes polarized in the quasiperiodic plane. Received: 18 November 1998 / Accepted: 27 November 1998     

Molecular dynamics simulations of the freezing of gold nanoparticles

A set of molten gold clusters, each with 1157 gold atoms, was studied by molecular dynamics simulations as the clusters underwent freezing at three different temperatures. Most of the clusters attained an icosahedral structure upon freezing, a structure found to be stable to mild annealing. Other structures observed were imperfect truncated decahedral, truncated octahedral and hexagonal close packed structures. The role of kinetics in the process of cluster solidification is discussed. Received 6 November 2000     

Preparation of Cu 2 O nanoparticles dispersed in NH 2 -terminated polyethyleneoxide matrix

Small Copper (I) oxide, Cu₂O, nanoparticles dispersed in diamine-terminated polyethyleneoxide (PEO-NH₂) matrix have been successfully prepared by vacuum evaporation of copper onto the molten PEO-NH₂. The obtained composite were characterized by TEM, electron diffraction, TG-DTA and FT-IR spectroscopy. The stable composite, in which the Cu₂O nanoparticles are stabilized through interaction between NH₂ chain end groups of PEO molecules and Cu₂O nanoparticles was obtained when the samples were heat-treated at 110 ^° C. The mean size of the Cu₂O nanoparticles increased from 2.5 to 3.5 nm in diameter upon increasing the amount of initial Cu deposition. The obtained composite material having a waxy texture was soluble in many solvents without aggregation and can be handled as a simple chemical compound for starting material in various applications. Received 29 November 2000     

Anharmonic dynamical behaviour in bcc zirconium

We present inelastic neutron scattering measurements of the low energy and strongly damped phonons in the high temperature bcc phase of zirconium. These phonons were investigated at different scattering vectors but equivalent phonon wave vectors in different Brillouin zones or along different but equivalent paths in the same Brillouin zone. Neither the observed differences in intensity nor in line shapes can be explained by the coherent one-phonon scattering law . This leads to an apparent violation of the fundamental symmetry of lattice dynamics. Taking into account the strong anharmonicity of these phonons, interferences between one- and multi-phonon scattering are held responsible for these effects. Measurements in different scattering planes reveal that due to the symmetry of the bcc lattice, these effects can only be observed in certain directions. Received: 24 December 1997 / Received in final form: 9 March 1998 / Accepted: 19 March 1998     

Ab initio vibrational and dielectric properties of chalcopyrite CuInS <Emphasis Type="Bold">2</Emphasis>

We have performed a first-principles study of structural, dynamical, and dielectric properties of the chalcopyrite semiconductor CuInS₂. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. Born effective charge tensors, dielectric permitivity tensors, the phonon frequencies at the Brillouin zone center and mode oscillator strengths are calculated using density functional perturbation theory. The calculated properties agree with infrared and Raman measurements. Received 12 December 2002 / Received in final form 17 March 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: resul@ibu.edu.tr     

Formation and stability of large B 6 O clusters with icosahedral structure

In this paper, the formation and the stability of large B₆O icosahedral particles was discussed on the basis of elastic deformation theory. Our calculation illustrate the stability of macroscopic Mackay packing B₆O icosahedral particles at high pressure. The transition pressure from rhombohedral structure of B₆O particles to macroscopic B₆O icosahedral ones was calculated to be 6 GPa, which is in good agreement with the experimental data (4.0-5.52 GPa). The maximum diameter of B₆O icosahedral particles at low pressure is estimated to be 200-300 nm. Received 30 November 2000     

High energy collisions of protonated water clusters

We have measured attenuation cross sections and fragmentation cross sections for protonated water clusters H(H₂O)_n ⁺ (n = 1 to 100) colliding with noble gas atoms (He and Xe) at a laboratory energy of 50 keV. In collisions with He, a transparency effect in the attenuation cross section was observed. For the case of fragmentation in collisions with Xe, a strong enhancement of small clusters was observed which we attribute to multifragmentation. Received 30 November 2000     

Phase behavior of organic-inorganic crystal

We have investigated the structure and phase behavior of nonmolecularly layered silver stearate by means of temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Upon heating the sample, remarkable spectral changes took place. The first phase transition took place that might be associated with a premelting event characterized by the formation of gauche conformers at 390-420 K. A second phase transition took place in which silver nanoparticles with a size of ∼4 nm were formed by thermal decomposition of silver stearate at 520-550 K. These silver nanoparticles, derivatized by stearate, were readily spread as a monolayer at air/water interface, and could be packed in 3-D assemblies by the Langmuir-Blodgett method. Received 29 November 2000     

Third-order nonlinear optical response of Au:SiO 2 thin films: Influence of gold nanoparticle concentration and morphologic parameters

We report a study on the third-order nonlinear optical properties of nanocomposite thin films composed of gold particles embedded in a silica host matrix. Samples of various metal volume fractions, ranging from 8 to 35%, are synthesized by the sputtering technique. Some of them are annealed. Nonlinear optical measurements, which are performed by using the z-scan technique, reveal both a very large nonlinear absorption and a weak nonlinear refraction close to the surface plasmon resonance frequency of the particles. We especially study the effect of the metal concentration and the influence of thermal treatment on the real and imaginary components of the third-order nonlinear susceptibility. Our results reveal that, as the metal concentration reaches a few percent, the mutual electromagnetic interactions between particles greatly enlarge the nonlinear optical response of the material and can not be neglected in the theoretical analysis. Moreover, the thermal treatment leads, for a given concentration, to a significant increase of the nonlinear response, which is ascribed to a modification of the material morphology. We finally point out that the material nonlinear properties are very sensitive to the incident wavelength through the local field enhancement phenomenon. Received 12 December 2001