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Phonon Spectra (phonon + spectrum)
Selected AbstractsNanocarbon materials: probing the curvature and topology effects using phonon spectraJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2009Sanju Gupta Abstract Much has been learned from the use of resonance Raman spectroscopy and high-resolution transmission electron microscopy techniques about the micro-/nanoscopic structure of various nanostructured carbons. However, they still possess some features that are not entirely understood particularly in terms of topological characteristics, which go beyond making a distinction with just the geometrical structure at nanoscale. To effectively utilize the potential of these materials for technological needs, understanding both the geometrical and topological structure and perhaps relating these attributes to physical (optical/electronic, lattice vibrational) properties become indispensable. Here, we make an attempt to describe the differences between various nanostructures and provide geometrical and topological property assessment semiquantitatively by monitoring the phonon spectra using resonance Raman spectroscopy thereby also capturing the electronic spectra. We elucidate the notion of global topology and curvature for a range of technologically important nanoscale carbons including tubular (single-, double- and multiwalled nanotubes, peapod), spherical (hypo- and hyperfullerenes, onion-like carbon) and complex (nanocones, nanohorns, nanodisks and nanorings) geometries. To demonstrate the proof-of-concept, we determined the variation in the prominent Raman bands of the respective materials, represented as D, G and D* (the overtone of D) bands, as a possible topological or curvature trend due to their sensitivity toward structural modification. The latter arises from local topological defects such as pentagons giving rise to curved nanocarbons. In this study, we provide systematics of their variation with respect to their geometric forms and compare with highly oriented pyrolytic graphite and monolayer graphene since the nanocarbons discussed are their derivatives. Once established, this knowledge will provide a powerful machinery to understand newer nanocarbons and indeed point to an unprecedented emergent paradigm of global topology/curvature , property , functionality relationship. We emphasize that these concepts are applicable to other topologically distinct nanomaterials, which include boron-nitride (BN) nanotubes and nanotori, helical gold nanotubes and Möbius conjugated organics. Copyright © 2009 John Wiley & Sons, Ltd. [source] Isotope and disorder effects in the Raman spectra of LiHxD1,x crystalsJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2001V. G. Plekhanov Most of the physical properties of a solid depend on its isotopic composition in some way or another. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. A great number of stable isotopes and well-developed methods for their separation have made it possible to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu2O, Si, Ge and ,-Sn with a controllable isotopic composition. Among these compounds, LiH possesses the largest value of the isotope effect. The great number of theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the vibrational properties of crystals. These effects are fairly large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering and neutron scattering). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between an LiH crystal and its deuterated analogue exceeds a 100 K. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm,1) but are also broadened. This broadening is related to the isotopic disorder of the crystal lattice. It is shown that the degree of a change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to apply successfully the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. The common nature of the isotopic and disorder effects in a wide range of crystals is emphasized. Copyright © 2001 John Wiley & Sons, Ltd. [source] Lattice dynamics of chalcopyrite semiconductors LiAlTe2, LiGaTe2 and LiInTe2PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2009A. V. Kosobutsky Abstract Within density functional perturbation theory using norm-conserving pseudopotentials and a plane-wave basis set calculations of phonon dispersion relations and densities of states of LiAlTe2, LiGaTe2 and LiInTe2 compounds being crystallized into the tetragonal chalcopyrite structure have been performed. Theoretical values of phonon mode frequencies in LiGaTe2 and LiInTe2 are in good agreement with the experimental data available for these crystals obtained by the methods of Raman and infrared spectroscopies. The similarity of the physical and chemical properties of the crystals concerned manifests itself in the similarity of their phonon spectra that are especially close to each other in low- and high-frequency ranges. Phonon modes of the upper phonon band are predominantly caused by the lithium sublattice vibrations and have an upper bound of 350,370 cm,1. In a mid-frequency range a significant downshift of the vibrational frequencies is observed on going from LiAlTe2 to LiGaTe2 and LiInTe2 that is a consequence of the third group cation mass reduction. From calculated electron density maps it follows that Li-containing chalcopyrites are characterized by a less pronounced bond between the first group cation and anion as compared with the Cu- and Ag-based analogs due to the absence of pd-hybridization. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Thermal conductance of the AlN/Si and AlN/SiC interfaces calculated with taking into account the detailed phonon spectra of the materials and the interface conditionsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2010M. Kazan Abstract We present a calculation of the thermal conductance (TC) of the interface between aluminium nitride (AlN) and silicon (Si) and that between AlN and silicon carbide (SiC) with taking into account the detailed phonon spectra of the materials, as obtained from first principles calculations, and the interface conditions. On the basis of the results obtained, we discuss the relation between the interface TC, the interface conditions, and the mismatches between the acoustic waves velocities and the phonon densities of states of the materials in contact. Our calculation method is expected to provide a reliable tool for thermal management strategy, independently from the substrate choice (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phonons in InAs quantum dot structuresPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2009Alexander Milekhin Abstract We present a Raman study of the phonon spectra of periodical structures with (In,Ga)As QDs in (Al,Ga)As matrix as well as AlAs QDs embedded in InAs grown by molecular beam epitaxy. Raman scattering by optical, interface and acoustic phonons was observed in the QD structures. TO and LO phonons in the QDs are strongly affected by both strain and confinement. The Raman study reveals a two-mode behavior of optical phonons in the whole composition range for both InGaAs QDs and the AlGaAs matrix. Raman scattering by InAs- and GaAs-like LO phonons in InGaAs QDs shows a size-selective resonant behaviour. Interface phonons were investigated in InGaAs QDs and the AlGaAs matrix. Their frequency positions were analyzed as a function of the alloy content within the dielectric continuum model. The positions of IF phonons in the QD structures observed in the experiment agree well with calculated ones assuming that the QDs have the shape of oblate ellipsoids. Multiple phonon Raman scattering involving both pure overtones of the first-order InAs, GaAs and AlAs optical and interface phonons and combination of phonons from the materials is observed in the vicinity with E0 resonance in QDs. Possible mechanisms of these processes are discussed. Low frequency resonant Raman scattering by acoustic phonons was observed in the QD structures. The periodic oscillations seen in the Raman spectra are well described by the elastic continuum model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Raman spectra and elastic properties of KPb2Cl5 crystalsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004A. N. Vtyurin Abstract Raman scattering spectra and elastic constants of KPb2Cl5 crystals have been studied. The results obtained are interpreted in terms of the ab initio lattice dynamics model taking into account multipole moments of ionic electron envelopes. The experimental results have been found to be in good agreement with numerical simulation; the narrow phonon spectra are shown to be due to a considerable contribution of heavy cations into the eigenvectors of the higher frequency lattice modes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] X-ray diffraction by a crystal in a permanent external electric field: general considerationsACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2005Semen V. Gorfman The variations of X-ray diffraction intensities from a crystal in the presence of a permanent external electric field is modeled analytically using a first-order stationary perturbation theory. The change in a crystal, induced by an external electric field, is separated into two contributions. The first one is related to a pure polarization of an electron subsystem, while the second contribution can be reduced to the displacements of the rigid pseudoatoms from their equilibrium positions. It is shown that a change of the X-ray diffraction intensities mainly originates from the second contribution, while the influence of the pure polarization of a crystal electron subsystem is negligibly small. The quantities restored from an X-ray diffraction experiment in the presence of an external electric field were analyzed in detail in terms of a rigid pseudoatomic model of electron density and harmonic approximation for the atomic thermal motion. Explicit relationships are derived that link the properties of phonon spectra with E -field-induced variations of a structure factor, pseudoatomic displacements and piezoelectric strains. The displacements can be numerically estimated using a model of independent atomic motion if the Debye,Waller factors and pseudoatomic charges are known either from a previous single-crystal X-ray diffraction study or from density functional theory calculations. The above estimations can be used to develop an optimum strategy for a data collection that avoids the measurements of reflections insensitive to the electric-field-induced variations. [source] Structural and dynamical properties of the Ge(001)/Sb(2×1) surfacePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004H. M. Tütüncü Abstract The structural properties of the Ge(001)/Sb(2×1) surface were investigated by using the density functional theory. The calculated structural parameters for this surface are in good agreement with previous experimental and theoretical results. In addition to structural properties, we present the complete phonon spectrum of this surface as calculated from first principles employing density functional theory. We have compared the phonon spectrum of this surface with that of the clean Ge(001)(2×1) surface in detail. We have found that adsorption of Sb results in several characteristic phonon modes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||