Lattice Vibrational Properties Research Articles

Study on lattice vibrational properties and Raman spectra of Bi2Te3 based on density-functional perturbation theory

We present a variational density-functional perturbation theory (DFPT) to investigate the lattice dynamics and vibrational properties of single crystal bismuth telluride material. The phonon dispersion curves and phonon density of states (DOS) of the material were obtained. The phonon dispersions are divided into two fields by a phonon gap. In the lower field, atomic vibrations of both Bi and Te contribute to the DOS. In the higher field, most contributions come from Te atoms. The calculated Born effective charges and dielectric constants reveal a great anisotropy in the crystal. The largest Born effective charge generates a significant dynamic charge transferring along the c axis. By DFPT calculation, the greatest LO—TO splitting takes place in the infrared phonon modes and reaches 1.7 THz in the Brillouin zone center. The Raman spectra and peaks corresponding to respective atomic vibration modes were found to be in good agreement with the experimental data.

Electron–Phonon Coupling and Superconducting Critical Temperature of the YIr2Si2 and LaIr2Si2 High-Temperature Polymorphs from First-Principles

Ab initio calculations of the electronic structure and lattice-vibrational properties were performed on the superconducting high-temperature polymorphs of YIr2Si2 and LaIr2Si2. The electron–phonon coupling constants λep were found to be 0.61 and 0.56 for YIr2Si2 and LaIr2Si2, respectively. The superconducting critical temperatures estimated from the Allen–Dynes formula agree well with the available experimental data and indicate that the superconductivity in these compounds can be explained by intermediate strength electron–phonon coupling.

Bismuth-induced effects on optical, lattice vibrational, and structural properties of bulk GaAsBi alloys

Bulk GaAs1 - xBix/GaAs alloys with various bismuth compositions are studied using power- and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni’s law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm-1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm-1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.PACS78.55Cr 78.55-m 78.20-e 78.30-j

Raman scattering from superhard rhenium diboride under high pressure

Lattice vibrational properties of superhard rhenium diboride (ReB2) were examined up to 8 GPa in a diamond anvil cell using Raman spectroscopy techniques. Linear pressure coefficients and mode Grüneisen parameters are obtained. Good agreement is found between the experimental and theoretical calculated Grüneisen parameters. Examination of the calculated mode Grüneisen parameters reveals that both B-B and Re-B covalent bonds play a dominant role in supporting the applied load under pressure. A comparison of vibrations parallel and perpendicular to the c-axis indicates that bonds along the c-axis tend to take greater loads. Our results agree with observations of elastic lattice anisotropy obtained from both in situ X-ray diffraction measurements and ultrasonic resonance spectra.

Effects of substrate and compositional disorder upon optical and lattice vibration properties of quaternary semiconductor GaxIn1−xAsyP1−y

The present paper deals with a theoretical study of optical and phonon properties of quaternary semiconductor GaxIn1−xAsyP1−y alloys lattice matched to InP, GaAs and ZnSe in the zinc-blende phase. The calculations are primarily based on the pseudopotential formalism under the virtual crystal approximation (VCA) that incorporates the disorder effect. Special attention has been devoted to the effects of compositional disorder and substrate upon the features of interest. The numerically calculated values show reasonable agreement with the experiment. It is found that the compositional disorder has an important effect on the studied properties and should be included in the calculations. The substrate effect is also found to change the properties of interest suggesting thus that more diverse opportunities regarding optical and lattice vibration properties may be provided by a proper choice of the substrate and the composition y.

The Structural, Electronic and Elastic Properties, and the Raman Spectra of Orthorhombic CaSnO3 through First Principles Calculations

First principles calculations based on the density functional theory of the electronic structure, elastic and lattice vibrational properties of orthorhombic CaSnO3 are carried out using standard functional approximation and density functional perturbation theory. The results show that CaSnO3 is an insulator with an indirect local density approximation and generalized gradient approximation gap of 3.10(2.69) eV. In addition, the Raman vibration modes of CaSnO3 are determined by the calculated phonon frequencies at the gamma point, where the prominent peaks of the Raman spectra of CaSnO3 coinciding with the calculated frequencies can be assigned.

Structural and vibrational stability of M and Z phases of silicon and germanium from first principles

First-principles calculations were performed to investigate the structural feasibility of M and Z phases (novel monoclinic and orthorhombic structures recently reported for carbon) for silicon and germanium. The lattice parameters, bulk modulus, vibrational properties, and elastic constants are calculated using the local density approximation to describe the exchange-correlation energy, while the optical properties are calculated by using Many-Body Perturbation Theory in the G0W0 approximation. Our results indicate that silicon and germanium with the proposed crystal symmetries are elastically and vibrationally stable and are small band-gap semiconductors. We discuss the possible synthesis of such materials.

First-principles study of thermoelectric and lattice vibrational properties of chalcopyrite CuGaTe2

The electronic structure of CuGaTe2 has been investigated using first-principles calculations, and it was discovered that it has a mixture of heavy and light bands near the valence band maximum and a combination of electronically conducting and insulating units near the gap edges, highly desirable for good thermoelectric performance. Semi-classic Boltzmann transport theory was then used to calculate the thermoelectric properties of CuGaTe2, and the optimal p- or n-type doping concentrations have been estimated based on the predicted maximum power factors. The phonon dispersion, phonon density of states, and specific heat of CuGaTe2 were evaluated by density functional perturbation theory in combination with the quasi-harmonic approximation, and the calculated phonon frequencies at the Γ point as well as the specific heat are in agreement with experimental data. The relatively high thermal conductivity of CuGaTe2 at low temperature is attributed to the lack of low-frequency vibrational modes, suggesting that its thermal conductivity can be reduced by introducing additional phonon scattering.

Electronic and lattice vibrational properties of cubic BaHfO3 from first principles calculations

The electronic structure and lattice vibrational properties of cubic SrHfO3 were investigated by first-principles calculations based on density functional theory in the framework of the local density approximation (LDA), generalized gradient approximation (GGA), and density functional perturbation theory (DFPT), respectively. The calculated equilibrium lattice constant of cubic SrHfO3 is in good agreement with available experimental and theoretical results. The results show that cubic SrHfO3 is an insulator with an indirect LDA (GGA) band gap of 3.6 (3.7) eV. Use of the screened exchange local density approximation (sX-LDA) as a functional in successive band calculation has also been performed. The band gap is predicted to be 6.27 eV within sX-LDA, in excellent agreement with the gap value of 6.1 ± 0.1 eV obtained from x-ray photoelectron spectroscopy. The phonon dispersion curves and LO–TO splitting of cubic SrHfO3 were also calculated. Negative phonon frequencies were observed along the M–C–R–M line in the Brillouin zone, indicating instability of the SrHfO3 structure, consistent with previous theoretical investigation.

Electronic and Lattice Vibrational Properties of Cubic SrHfO3 from First-Principles Calculations

The electronic structure and lattice vibrational properties of cubic SrHfO3 were investigated by first-principles calculations based on density functional theory in the framework of the local density approximation (LDA), generalized gradient approximation (GGA), and density functional perturbation theory (DFPT), respectively. The calculated equilibrium lattice constant of cubic SrHfO3 is in good agreement with available experimental and theoretical results. The results show that cubic SrHfO3 is an insulator with an indirect LDA (GGA) band gap of 3.6 (3.7) eV. Use of the screened exchange local density approximation (sX-LDA) as a functional in successive band calculation has also been performed. The band gap is predicted to be 6.27 eV within sX-LDA, in excellent agreement with the gap value of 6.1 ± 0.1 eV obtained from x-ray photoelectron spectroscopy. The phonon dispersion curves and LO–TO splitting of cubic SrHfO3 were also calculated. Negative phonon frequencies were observed along the M–Γ–R–M line in the Brillouin zone, indicating instability of the SrHfO3 structure, consistent with previous theoretical investigation.

Phonons and related crystal properties in indium phosphide under pressure

Based on the pseudopotential approach, the lattice vibration and polaron properties in InP under pressure up to 8GPa have been investigated. At zero pressure, our calculated values for all features of interest are in reasonable agreement with data available in literature. Upon compression, the optical phonon modes at Γ point in the Brillouin zone are shifted towards high energies and their frequencies display a blueshift with rising pressure. The splitting of the longitudinal and transverse optical phonons narrows under pressure, indicating thus a decrease of the ionicity of InP crystal with pressure. It is observed that all physical quantities being studied in this contribution behave monotonically upon compression. To the best of the author's knowledge, the pressure dependence of the polaron properties for InP in the present study is reported for the first time.

Piezoreflectance and Raman Characterization of Mo<sub>1−x</sub>W<sub>x</sub>S<sub>2</sub> Layered Mixed Crystals

A systematic optical characterization of a series of Mo1-xWxS2 (0 ≤ x ≤ 1) layered mixed crystals grown by chemical vapour transport method were carried out by using piezoreflectance (PzR) and Raman scattering measurements. From a detailed lineshape fit of the PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband transitions were determined accurately. The transition energies and their splittings vary smoothly with the tungsten composition x indicating that the nature of the band structure is similar for the Mo1-xWxS2 series compounds. The peaks of the two dominant first-order Raman-active modes, and , and several second-order bands are observed in the range of 250-450 cm-1. The peaks corresponding to mode show a one-mode type behavior, while the peaks of mode demonstrate two-mode type behavior for the entire series of Mo1-xWxS2 crystals. These behaviors were discussed on the basis of the lattice vibrational properties of 2H-MoS2 and 2H-WS2 compounds.

Electronic and Lattice Vibrational Properties of BaSi2 from Density Functional Theory Calculations

BaSi2 is a potential thermoelectric material because of its very low thermal conductivity. Using the full-potential linearized augmented plane-wave method and semiclassical Boltzmann theory, thermoelectric transport properties of BaSi2 have been investigated. The calculations show that the thermoelectric properties can be remarkably improved by optimizing the carrier concentration. The linear response method within the framework of density functional theory was employed to investigate the underlying physics of heat transport. There are rather flat optical dispersion curves and low frequency of acoustic phonon modes in the phonon band structure of BaSi2. The low-lying optical phonon branch at the Γ point of the Brillouin zone (BZ) corresponds to rigid-unit vibration of the Si tetrahedron. The rigid-unit vibration mode confines the acoustic phonon modes and scatters the heat-carrying acoustic modes, leading to the low lattice thermal conductivity.

Local lattice dynamics and isotope effect in yttrium diboride probed by extended x-ray absorption fine structure spectroscopy

The lattice vibration properties and the boron isotope effect have been studied in YB2 by using temperature-dependent extended x-ray absorption fine structure (EXAFS). The data show anomalous behavior of the Debye–Waller factor of the Y–B pair due to the superposition of an optical mode associated with the boron sublattice and an acoustic mode corresponding to the yttrium sublattice. We claim that the observed decoupling between metal and boron vibrations is responsible for the lower transition temperature compared to MgB2. The analysis of the boron isotope effect confirms also that the B–B vibration mode plays a key role in the electron–phonon coupling.

Electronic, lattice vibration and mechanical properties of CdTe, ZnTe, MnTe, MgTe, HgTe and their ternary alloys

The main representative telluride family, CdTe, ZnTe, MnTe, MgTe and HgTe, and their mixed ternary alloys have been extensively studied, but some points bearing on phonon frequencies properties have never been previously reported or are still not clear. In this paper, we report results on the electronic, lattice dynamical and mechanical properties for this family in the zinc–blende (ZB) structure. Numerical calculations based on the empirical pseudo-potential method under the virtual crystal approximation combined with the Harrison bond orbital model are used to investigate these properties. Our results are found to be generally in good agreement with the available experimental data. Other case, our results are predictions and may serve as a reference. The information derived from the present study demonstrates the potential of the materials of interest for optoelectronic and photovoltaic device applications from the ultraviolet to infrared and mid-infrared.

Nanocarbon materials: probing the curvature and topology effects using phonon spectra

AbstractMuch 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.

Lattice vibration spectrum of GaN from first-principle calculations

We report first-principle calculations in the framework of the density functional perturbation theory of the lattice vibration properties of GaN in the wurtzite and rocksalt phases at zero and under pressures ranging from 0 to 40 GPa. The calculations predict the full phonon spectrum throughout the Brillouin zone and phonon density of states. Generally, the calculated zone-center modes for the wurtzite structure are found to be in good agreement with previous theoretical and experimental results. For the phonons of rocksalt GaN in the presence of hydrostatic pressure, these are the first results to our knowledge. The pressure dependence of the zone-center optical frequencies is examined and discussed which allowed the calculation of the pressure coefficients and mode Grüneisen parameters.

The influence of nitrogen plasma treatment on the lattice vibrational properties of hydrothermally grown ZnO nanorods

In this study, the authors have investigated the optical properties of hydrothermally grown ZnO nanorods subjected to the combination of thermal annealing and nitrogen plasma treatments. In particular, ultraviolet-visible micro-Raman scattering has been used to study the influence of nitrogen incorporation in ZnO nanorods grown on GaN/sapphire templates. The band-edge photoluminescence spectra show significant changes due to nitrogen plasma treatment. In addition, visible Raman spectra show intensity enhancement of the disorder-activated vibrational modes from plasma-treated ZnO nanorods. Multiple longitudinal optical (LO) phonons are observed under ultraviolet resonant Raman excitation from these nanorods. The first-order resonant LO phonon line shape fitting is correlated to the nitrogen-induced lattice disorder.

Structural, elastic, and electronic properties of Fe3C from first principles

Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010], and [001] directions. The three methods agree well with each other; the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

A lattice dynamical treatment for the total potential energy of single-walled carbonnanotubes and its applications: relaxed equilibrium structure, elastic properties, andvibrational modes of ultra-narrow tubes

In this paper, we propose a lattice dynamic treatment for the total potential energy ofsingle-walled carbon nanotubes (SWCNTs) which is, apart from a parameter for thenonlinear effects, extracted from the vibrational energy of the planar graphene sheet. Theenergetics, elasticity and lattice dynamics are treated in terms of the same set offorce constants, independently of the tube structures. Based upon this proposal,we have investigated systematically the relaxed lattice configuration for narrowSWCNTs, the strain energy, the Young’s modulus and Poisson ratio, and the latticevibrational properties with respect to the relaxed equilibrium tubule structure. Ourcalculated results for various physical quantities are nicely in consistency with existingexperimental measurements. In particular, we verified that the relaxation effectmakes the bond length longer and the frequencies of various optical vibrationalmodes softer. Our calculation provides evidence that the Young’s modulus ofan armchair tube exceeds that of the planar graphene sheet, and that the largediameter limits of the Young’s modulus and Poisson ratio are in agreement withthe experimental values of graphite; the calculated radial breathing modes forultra-narrow tubes with diameters ranging between 2 and 5 Å coincide with theexperimental results and the existing ab initio calculations with satisfaction. Fornarrow tubes with a diameter of 20 Å, the calculated frequencies of optical modes inthe tubule’s tangential plane, as well as those of radial breathing modes, arealso in good agreement with the experimental measurements. In addition, ourcalculation shows that various physical quantities of relaxed SWCNTs can actually beexpanded in terms of the chiral angle defined for the corresponding ideal SWCNTs.