Changes In Force Constants Research Articles

Investigation of the efficacy of Zn/Ce-CuO nanoparticles for enhanced photocatalytic, antibacterial, and antioxidant activities.

The world is dealing with unprecedented environmental challenges, leading to a growing urgency to limit environmental damage. So, this study focuses on the synthesis of pure CuO, Zn, Ce, and Zn/Ce dual-doped CuO nanoparticles (NPs)using extract of Citrus limon leaves as reductant via simple co-precipitation method. The X-ray diffraction (XRD) characterization was employed to analyze structural characteristics of synthesized samples which confirm influence of Zn or Ce doping on crystallite size, dislocation density, and strain. The role of functional groups, changes in force constant, and bond length on addition of dopants was indicated by FTIR results. The SEM and TEM results showed variation in morphology from irregular to spherical. The zeta-potential and BET analysis confirmed surface potential as well as surface area characteristics. The change in energy gap values from 1.81 to 1.45eV of Zn/Ce-doped CuO NPscomputed from UV-vis analysis elevated its photocatalytic performance and reduced the chances of recombination of electron-hole pair due to presence of trapping levels between valence and conduction bands. The enhanced photo-degradation of Congo red (CR) and rhodamine B (RhB) with 91 and 94%, respectively, for Zn/Ce-doped CuO NPswas observed. The so-obtained samples have also exhibited good antibacterial and antioxidant activities.

Structural Stability of γ-Boron under High Pressure up to 126 GPa with Fine Pressure Increments

The structural stability of γ-boron is investigated using Raman spectra and DFT calculations under high pressures, up to 126 GPa, at ambient temperature. The pressure dependence of all the Raman-active modes of the γ-boron is reported. We also observe amusing changes within the B3g and B1g Raman-active vibrational modes, which result in the phenomenon of first merging and then separating the Raman peaks. In addition to the Raman measurements, the changes in crystal structure and force constants are calculated to reasonably explain the discrepancy between the two Raman modes in response to pressure. The results of the continuous shifts for all Raman modes and the unit-cell parameters, as well as volume with increasing pressure, indicate that there is no structural transformation of γ-boron below this pressure value, with no changes in either symmetry or structure.

DFT and TD-DFT Study of the Chemical Effect in the SERS Spectra of Piperidine Adsorbed on Silver Colloidal Nanoparticles.

The surface-enhanced Raman scattering (SERS) spectra of piperidine adsorbed on silver/chloride colloids were studied by a combined density functional theory (DFT)/time dependent DFT (TD-DFT) approach. The mechanism of chemical enhancement on the Raman signals is due to at least two contributions: the first comes from the changes in the molecular force constants and the dynamic polarizabilities of the normal modes, when the molecule is chemisorbed. DFT calculations satisfactorily reproduce the SERS spectra of piperidine adsorbed on silver, showing that the species formed on the silver particle is a complex formed by a deprotonated piperidine linked to a silver cation. A second contribution to the SERS chemical enhancement is due to a resonance Raman effect occurring when the wavelength of the Raman excitation falls within the electronic excitation band of the molecule/metal complex. Actually, the SERS spectra of piperidine show a significant dependence on the wavelength of the laser excitation, with a marked enhancement in the green-light region. TD-DFT calculations on the most-probable complex explain this behavior, because a strong excitation band of the complex is calculated in the green spectral region. This pinpoints that a resonance between the exciting radiation and the absorption band of this complex is responsible for this enhancement effect.

Symmetry adapted impurity modes in as grown n-type GaP:X and GaSb:X (X = S, Se and Te)

A comprehensive average-t-matrix Green’s function (ATM-GF) theory is used, in the framework of a realistic rigid-ion-model, to simulate the symmetry induced vibrational modes of different defect centers in GaP:X and GaSb:X (X = S, Se and Te) crystals. Explicit calculations are performed for isolated 32SP+ (34SP+) defects, nearest-neighbor 32SP+-CuGa2- (34SP+-CuGa2-) pairs in GaP, and next-nearest-neighbor complex 32SSb+-GaSb2-(34SSb+-GaSb2-) center in GaSb by using apposite perturbation (P↔) and Green’s function (G↔o) matrix elements. For the isolated closest mass isoelectronic and charged (donor and acceptor) defects, the study has provided a convincing empirical relationship associating the increase or decrease of force constant change between impurity-host atoms to the increase or decrease of covalency of impurity-host bond. The rule has accurately predicted the observed isotopic shifts of local vibrational and/or gap modes of isolated impurities and offered modes for different “donor–acceptor” pairs of reduced symmetry. We feel that the ATM-GF method will play a significant role identifying the site selectivity of defects for estimating their mode frequencies in the technologically important semiconductor materials.

Role of anharmonicity in the electronic heat capacity of superconductors

The role of anharmonicity in the electronic heat capacity (EHC) of high temperature superconductors (HTS) has been investigated via electron density of state (EDOS) using quantum dynamical approach. The EDOS may be obtained using correlation function with the help of Zubarev double time temperature dependent electron Green’s functions (GF) via a generalized Hamiltonian which consists of (i) unperturbed electron, (ii) unperturbed phonon, (iii) electron-phonon, (iv) anharmonic and (v) defect interactions. The model Hamiltonian includes the force constant changes and mass difference caused by the impurities along with the effects of cubic anharmonicity reveals some striking features of EHC of HTS.

Thermal conductivity of ThO2: Effect of point defect disorder

Thoria (ThO2) has lately gained attention due to its potential for use as a nuclear fuel. From a physics standpoint, ThO2 is an actinide-bearing material with no 5f electrons and is thus ideally suited as a baseline material for future studies of the physical properties of actinide systems with correlated electrons. Current investigations of ThO2 as a nuclear fuel focus on the influence of radiation-induced lattice defects on its thermal properties, especially the conductivity. This work presents a first investigation of the impact of point defect disorder on phonon thermal conductivity of ThO2 by solving the Boltzmann transport equation within the single-mode relaxation time approximation. The relaxation times of intrinsic, three-phonon scattering are calculated by a rigorous sampling of k-points within the irreducible Brillouin zone of the face-centered cubic crystal structure. The effect of point defects on the thermal conductivity of ThO2 is predicted using the classic model by Klemens for phonon relaxation times that result from the change in mass and induced lattice strain associated with point defects. Within this model, the change in force constants and atomic radii are computed using input from an atomistic model of ThO2. The defects considered are uranium substitution at a thorium site, oxygen vacancies and interstitials, and thorium vacancies and interstitials. The results show that the conductivity of ThO2 is highly sensitive to intrinsic point defects and less sensitive to U substitution on the cation sublattice.

Lattice dynamics of cobalt orthoborate Co3(BO3)2 with kotoite structure

Transition metal oxyborates M3(BO3)2 (M = Mn, Co, Ni) belong to a group of materials with the kotoite structure and many of them demonstrate complex magnetic behavior as a result of competing interactions between magnetic ions in nonequivalent crystallographic positions. In this paper, we report results of the detailed infrared and Raman spectroscopic studies of a Co3(BO3)2 single crystal. Most of the phonons are observed and identified. Unusually large phonon shifts, in comparison to isostructural compound Ni3(BO3)2, were registered in the low- and mid-infrared frequency range (100–700 cm−1) of the spectra. The experimental findings are supported by the ab initio simulations of the lattice dynamics, which allow us to propose the normal-mode assignments. We argue that the large phonon shift between cobalt and nickel compounds are associated with changes of force constants induced by the changes of M–O bond lengths within the two types of octahedral MO6 groups. In addition to the first-order Raman scattering, intense second-order Raman scattering was detected due to the resonant enhancement regime. Furthermore, we performed the single crystal XRD experiments which did not show any detected evidences of magneto-structural phase transition previously observed in Ni3(BO3)2.

Effect of carbon vacancies and oxygen impurities on the dynamical and thermal properties of uranium monocarbide

An influence of carbon vacancies and substitutional oxygen impurities on the phonon dynamics and thermal properties of uranium monocarbide is investigated within the density functional theory incorporating strong local Coulomb and spin-orbit interactions. Anharmonicity of lattice vibrations is taken into account via the quasi-harmonic theory. Results obtained for the modeled UC1−x and UC1−xOx compositions with x=3% show that the changes in atomic displacements and force constants induced by mono-vacancies and substitutional oxygen impurities in the carbon sublattice of UC remain restricted to the immediate neighborhood of the incorporated defects. The most sensitive to point defects in the carbon sublattice of UC are the optical phonon branches, while the phonon densities of states and the phonon-dependent thermodynamical properties remain affected to a lesser extent. The X-ray powder diffraction experiments have been performed to examine the temperature evolution of the lattice parameter of highly stoichiometric UC between 20 and 310 K. The present experimental studies supplement existing literature data by extending description of the uranium monocarbide structure below room temperature. They also support and validate our theoretical predictions about behavior of the investigated system at low temperatures.

Anisotropic optical phonons in MOCVD grown Si-doped GaN/Sapphire epilayers

Micro-Raman spectroscopy is employed to study the anisotropic optical phonons of Si-doped GaN/Sapphire epifilms grown by metal organic chemical vapor deposition method. In an undoped 3.6 μm thick sample – our polarized Raman measurements in the backscattering geometry revealed major first order modes of GaN and sapphire. Careful analyses of the second-order Raman spectra using critical-point-phonons from a rigid-ion-model fitted inelastic X-ray spectroscopy data with appropriate selection rules helped us attain expedient data for the lattice dynamics of GaN. In Si-doped films, a modified phonon confinement model is used for simulating Raman line shapes of E2high phonons to monitor crystalline quality. While the optical phonons in lightly doped samples are coupled to electron plasma– at higher carrier concentration the over-damped A1(LO) mode vanished in the background. For each sample we assessed the transport parameters by simulating Raman profiles of A1(LO) line shape with contributions from plasmon-LO-phonon and Lorentzian shaped Eg sapphire mode. A realistic Green’s function theory is adopted to study the vibrational modes of Si donors and Mg acceptors in GaN by including force constant changes estimated from lattice relaxations using first-principles bond-orbital model. Theoretical results of impurity-activated modes compared favorably well with the existing Raman scattering data.

Pharmaceutical property interpretation and biological property study on Ethionamide using quantum spectroscopic and computational tools

ABSTRACTIn this work, the molecular spectroscopic technique was used in a nonconventional way to predict the drug mechanism action of the drug compound, Ethionamide. The formatted molecular charge levels, due to the injection of substitutions in suitable positions, were measured. The molecular geometry distortion by the adoption of ligands in the ring system was noted at the nodal points of the core and the change of force constant of the heterogeneous intra-molecular assembly. The activity of the compositional parts of the molecule was purposively monitored from which the entire core and allied parts of the molecule were evaluated by the vibrational characteristics. The molecular reaction mechanism for saturating drug potential was observed from the chemical shift of core and substituted carbons. The fabricated charge transfer complex was identified and recognized from the base and ligand-binding structure. The major part of the exchange of electro-chemical quantized energy in non-bonding molecular orbitals for arranging biological activity in the compound was measured. The rule of five and Lipinski parameters to recognize the drug quality were checked and estimated.

(Invited) Mie-like Scattering of Phonons by a Single Vacancy in IV-VI Compounds

Defects engineering has been a widely used approach for developing low thermal conductivity materials. Also, for some materials (e.g., IV-VI compound), considerably large amount of vacancies was often observed in single crystalline samples. The atomic vacancy in dilute case was assumed as a point defect that can cause the Rayleigh scattering with its rate proportional to the fourth power of phonon frequency. Due to the large dependence of scattering rate on phonon frequency, a vacancy was considered to only weakly suppress the mode thermal conductivity of low frequency acoustic phonons. We show that this does not hold for IV-VI materials. We compare the phonon scattering by a single vacancy in group IV (diamond and Si) and IV-VI (PbTe and GeTe) crystals using an exact ab initio Green’s function approach. The phonon-vacancy scattering rates in diamond and Si follow the well-known fourth-power dependence on phonon frequency. However, PbTe and GeTe exhibit phonon scattering rates that have much weaker frequency dependence. As a result, a vacancy scatters low frequency acoustic phonons more strongly in PbTe and GeTe than in Si and diamond. This unusual behavior in PbTe and GeTe originates from the significant changes of interatomic force constants that extend up to the distance of 10 angstrom from the vacancy site. Therefore, a vacancy in IV-VI materials cannot be assumed as a point defect but its finite size should be considered for phonon scattering. In contrast, a vacancy in Si and diamond changes interatomic force constants only up to 3 angstrom range. Our study provides new insights on the phonon spectral contribution to thermal transport when vacancies exist in IV-VI materials and will help to develop low thermal conductivity materials by engineering vacancies.

Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces

Silicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi$_2$, CoSi$_2$, NiSi and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no difference in the thermal interface conductance of epitaxial and non-epitaxial silicide/silicon interfaces. The effect of substrate carrier concentration is also investigated over a wide range of p- and n-type doping, and is found to be independent of carrier concentration, regardless of whether the interface is epitaxial and regardless of silicide type. In the case of epitaxial CoSi$_2$, a comparison of temperature dependant experimental data is made with two detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain and (2) an atomistic Green' function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. At temperatures above 100K, the AGF approach greatly underpredicts the CoSi$_2$ data, while the DMM prediction matches the data well. The full-dispersion DMM is also found to closely predict the experimentally observed temperature-dependent interface conductance for epitaxial NiSi/Si and non-epitaxial TiSi$_2$/Si interfaces. In the case of epitaxial PtSi/Si interfaces, full dispersion DMM significantly overpredicts the experimental data.

The electronic heat capacity of YBa2Cu3O7−δ superconductor

The contributions due to the point defects or disorder and anharmonicities which play deterministic role in the understanding of electronic heat capacity (EHC) of high temperature superconductors (HTS) have been investigated via electron density of states (EDOS) approach on the basis of quantum dynamical many body theory. The evaluation of EDOS has been carried out with the help of most versatile method of double time temperature dependent electron Green’s functions (GF) via a Hamiltonian (non BCS type) which includes the effects of electrons, phonons, defects, anharmonicity, and electron-phonon interactions which enables to account the effects of cubic anharmonicity besides with both the force constant changes and mass difference caused by the impurities in developing the results for EDOS and EHC. The new results reveal some striking features of EHC of HTS.

Simulating far infrared spectra of Zn1-xMnxSe/GaAs epifilms, MnSe/ZnSe superlattices and predicting impurity modes of N, P defects in Zn1-xMnxSe

A comprehensive lattice dynamical study is reported to emphasize the vibrational behavior of perfect/imperfect zinc-blende (zb) ZnSe, MnSe and Zn1−xMnxSe alloys. Low temperature far-infrared (FIR) reflectivity measurements performed on a series of molecular beam epitaxy grown Zn1−xMnxSe/GaAs (001) epilayers have a typical ‘intermediate-phonon-mode’ behavior. Besides perceiving ZnSe- and MnSe-like TO-phonon resonances, the study also revealed a weak Mn alloy-disorder mode below MnSe band. A classical effective-medium theory of multilayer optics is used to evaluate dielectric tensors of both epilayers and substrate for simulating reflectivity and transmission spectra of ultrathin epifilms and superlattices at near normal and/or oblique incidence. In the framework of a realistic rigid-ion model and exploiting an average t-matrix Greens function (ATM-GF) theory we appraised the vibrational properties of nitrogen and phosphorous doped Zn-Mn chalcogenides. Lattice relaxations around isolated NSe (PSe) defects in ZnSe and zb MnSe are evaluated by first principles bond-orbital model that helped construct perturbation models for simulating the localized vibrational modes (LVMs). Calculated shift of impurity modes for isotopic 14NSe (15NSe) defects in ZnSe offered a strong revelation of an inflexible defect–host interaction. By retaining force constant change parameter of 14NSe (15NSe) in heavily N-doped ZnSe, the ATM-GF theory predicted (a) three non-degenerate LVMs for the photoluminescence defect center V Se-Zn-14NSe (V Se-Zn-15NSe) of C s symmetry, and (b) six impurity modes for the second nearest-neighbor NSe-Zn-NSe pair defect of C 2v symmetry. From the range of simulated defect modes, we have ruled out the possibility of N-pairs and justified the presence of VSe-Zn-NSe complex centers – likely to be responsible for the observed large absorption bandwidth in highly N-doped ZnSe. High resolution measurements of FIR absorption and/or Raman scattering spectroscopy are needed to validate the accuracy of our theoretical conjectures.

Modulation of mechanical resonance by chemical potential oscillation in graphene

The classical picture of the force on a capacitor assumes a large density of electronic states, such that the electrochemical potential of charges added to the capacitor is given by the external electrostatic potential and the capacitance is determined purely by geometry. Here we consider capacitively driven motion of a nano-mechanical resonator with a low density of states, in which these assumptions can break down. We find three leading-order corrections to the classical picture: the first of is a modulation in the static force due to variation in the internal chemical potential, the second and third are change in static force and dynamic spring constant due to the rate of change of chemical potential, expressed as the quantum (density of states) capacitance. As a demonstration, we study a capacitively driven graphene mechanical resonators, where the chemical potential is modulated independently of the gate voltage using an applied magnetic field to manipulate the energy of electrons residing in discrete Landau levels. In these devices, we observe large periodic frequency shifts consistent with the three corrections to the classical picture. In devices with extremely low strain and disorder, the first correction term dominates and the resonant frequency closely follows the chemical potential. The theoretical model fits the data with only one adjustable parameter representing disorder-broadening of the Landau levels. The underlying electromechanical coupling mechanism is not limited the particular choice of material, geometry, or mechanism for variation in chemical potential, and can thus be extended to other low-dimensional systems.

Remarkable suppression of thermal conductivity by point defects in MoS2 nanoribbons

By applying non-equilibrium molecular dynamics simulation, we investigate the effect of point defects on thermal conductivity of MoS2 nanoribbons, such as sulfur vacancies (VS) and oxygen substitutions to sulfur (SO). It is found that both VS and SO can significantly reduce thermal conductivity of monolayer MoS2 nanoribbons, but the suppression of thermal conductivity by vacancies is stronger than that by substitutions. For armchair MoS2 nanoribbon of 41.1nm length and 4.4nm width, when defect density is only 1.5%, the reduction of thermal conductivity at room temperature by VS defects and SO defects is 42.3% and 35.1%, respectively. We perform the vibrational eigenmodes analysis and find that the strong localization of phonons of all modes by defects results in the severe reduction of thermal conductivity of MoS2 nanoribbons. Further spectra analysis reveals that the localized modes are located in the sites of defects and the sites around defects, due to the change of force constant at these sites. Our findings are helpful for understanding and tuning the thermal conductivity of MoS2 nanoribbons by defect engineering.

Role of force-constant difference in phonon scattering by nano-precipitates in PbTe

We study the effect of nanoscale precipitates on lattice thermal conduction in thermoelectric PbTe using a combination of ab-initio phonon calculations and molecular dynamics. We take into account the effects of mass difference and change in force constants, and find an enhanced influence of the latter with increased precipitate concentration. As a consequence, our inclusion of the change in force constants in the calculation affords a smaller predicted optimal nano-precipitate size that minimizes the thermal conductivity. These results suggest that the phonon scattering by nanoprecipitates in thermoelectric composites could be stronger than previously thought.

Mechanical properties of atomic force microscopy probes with deposited thin films

Deposition of thin films on atomic force microscopy (AFM) probes is a common technique that is used to either improve the light reflectance of the back side of the probe cantilevers or to modify the surface properties of the probe tips. However, this technique also affects the force constant and resonance frequency of the AFM cantilevers. The present work investigates theoretically this effect in the approximation of very thin films. The cantilever force constant changes due to the contribution of bending moment of the elastic force in the deposited film, while the resonant frequency changes due to the film contributions to the bending force and inertia of the cantilever. It is found that the relative variations of cantilever force constant and resonance frequency depend on the film to cantilever thickness, density and elasticity modulus ratios. This theoretical prediction is confirmed by the experimental investigations on mechanical properties of silicon cantilevers covered by diamond like carbon (DLC) and fluorocarbon thin films obtained by high power impulse magnetron sputtering depositions. Moreover, the effect of film depositions on the cantilever mechanical properties is used to determine the elasticity modulus and mass density of the deposited thin films.

A bond-order theory on the phonon scattering by vacancies in two-dimensional materials.

We theoretically investigate the phonon scattering by vacancies, including the impacts of missing mass and linkages () and the variation of the force constant of bonds associated with vacancies () by the bond-order-length-strength correlation mechanism. We find that in bulk crystals, the phonon scattering rate due to change of force constant is about three orders of magnitude lower than that due to missing mass and linkages . In contrast to the negligible in bulk materials, in two-dimensional materials can be 3–10 folds larger than . Incorporating this phonon scattering mechanism to the Boltzmann transport equation derives that the thermal conductivity of vacancy defective graphene is severely reduced even for very low vacancy density. High-frequency phonon contribution to thermal conductivity reduces substantially. Our findings are helpful not only to understand the severe suppression of thermal conductivity by vacancies, but also to manipulate thermal conductivity in two-dimensional materials by phononic engineering.

Dynamics of impurity modes and electron–phonon interaction in Heavy Fermion (HF) systems

A theoretical explanation is provided to understand the effect of small concentration of impurities characterized by change in mass and nearest neighbor force constants on the phonon spectrum as well as on the electron–phonon interaction in some Heavy Fermion (HF) systems in the normal state within theoretical framework of the Periodic Anderson Model (PAM). Three different mechanisms of the electron–phonon interactions, namely, the usual interaction between the phonons with the electrons in the f-bands, electrons arising from that of hybridization term of PAM and the local electron–phonon coupling at the impurity sites are considered. Coherent Potential Approximation (CPA) is used to evaluate the configuration averaged self–energy and the total Green function. For simplicity of calculation the CPA self–energy is evaluated in Average t -matrix Approximation (ATA). The analytical analysis is carried out for finite T in the long wavelength limit. The influence of impurity mass parameter λ and other system parameters such as d, the position of f-level, the effective coupling strength g on the calculated re-normalized phonon frequency and the excitation spectrum through the spectral function is studied. The numerical analysis of the results does show the influence of impurities as evident from different plots in this paper.