Local Vibrational Modes Research Articles

Perovskite titanates at the nanoscale: Tunable luminescence by energy transfer and enhanced emission with Li+ co-doping

Three series of highly luminescent Ca1-xTiO3:xDy3+, Ca0.98-yDy0.02TiO3:yEu3+ and Ca0.91-zDy0.02Eu0.07TiO3:zLi+ nanophosphors have been synthesized by solution combustion method. X-ray diffraction analysis and Rietveld refinements of these phosphors confirms the pnma (62) structure and the occupancy of substituent ions in the matrix. Increase in intensity of the Raman modes viz., O–Ti–O bending mode (249 ​cm−1) and local vibrational mode (800 ​cm−1) signifies the occupancy of rare earth ions in the lattice. XPS analysis verifies the change in coordination environment around the RE3+ ions due to the variation in electron density by the observed shift in binding energy corresponding to the chemical states of Dy 3d and Eu 3d when Li+ ions are codoped. Ca1-xTiO3:xDy3+ exhibits a luminescence close to standard white and tuning of correlated color temperature is achieved by varying the excitation wavelength. In Ca0.98-yDy0.02TiO3:yEu3+ phosphors, co-doping of Eu3+ enables red component in white emission and at higher concentration of Eu3+, emission changes from orange red to purplish red, when wavelength of excitation is altered from 366 to 388 ​nm by virtue of energy transfer mechanism. An enhancement of luminescence intensity by 2.1 times has been observed due to Li+ substitution at optimum concentration in Ca0.91-zDy0.02Eu0.07TiO3:zLi+. This evidences the role of Li+ ions in modifying the crystal field interaction in perovskite luminescent materials.

Decoding chemical information from vibrational spectroscopy data: Local vibrational mode theory

AbstractModern vibrational spectroscopy is more than just an analytical tool. Information about the electronic structure of a molecule, the strength of its bonds, and its conformational flexibility is encoded in the normal vibrational modes. On the other hand, normal vibrational modes are generally delocalized, which hinders the direct access to this information, attainable only via local vibration modes and associated local properties. Konkoli and Cremer provided an ingenious solution to this problem by deriving local vibrational modes from the fundamental normal modes, obtained in the harmonic approximation of the potential, via mass‐decoupled Euler–Lagrange equations. This review gives a general introduction into the local vibrational mode theory of Konkoli and Cremer, elucidating how this theory unifies earlier attempts to obtain easy to interpret chemical information from vibrational spectroscopy: (a) the local mode theory furnishes bond strength descriptors derived from force constant matrices with a physical basis, (b) provides the highly sought after extension of the Badger rule to polyatomic molecules, (c) and offers a simpler way to derive localized vibrations compared to the complex route via overtone spectroscopy. Successful applications are presented, including a new measure of bond strength, a new detailed analysis of infrared/Raman spectra, and the recent extension to periodic systems, opening a new avenue for the characterization of bonding in crystals. At the end of this review the LMODEA software is introduced, which performs the local mode analysis (with minimal computational costs) after a harmonic vibrational frequency calculation optionally using measured frequencies as additional input.This article is categorized under: Structure and Mechanism > Molecular Structures Theoretical and Physical Chemistry > Spectroscopy Software > Quantum Chemistry Electronic Structure Theory > Ab Initio Electronic Structure Methods

Mid-IR photothermal beam deflection technique for fast measurement of thermal diffusivity and highly sensitive subsurface imaging

The resonances of phonon and plasmon modes make the absorbance coefficient of materials tremendously high in the mid-infrared spectral range, which allows for a mid-infrared excitation laser to heat the surface layers of these materials with high efficiency. Furthermore, phonon scattering by defects and defect-induced localized vibrational modes affect the local infrared dielectric function and, hence, the local infrared absorption coefficient. In this paper, we present a mid-infrared photothermal beam deflection technique that takes advantage of the strong interaction between infrared optical excitations and vibrational modes to measure the thermal diffusivity of materials without any sample preparation and takes advantage of the strong dependence of the infrared complex dielectric function on defects to detect subsurface defects with high sensitivity. We demonstrate the importance of the developed technique by measuring the thermal properties of highly transparent and reflective samples and detecting defects undetectable with any of the existing optical methods. Namely, using the developed technique, we find that the thermal diffusivities of high-quality Si, crystalline AlN, and crystalline α-SiO2 substrates are 1.00 ± 0.05, 0.67 ± 0.02, and 0.09 ± 0.01, respectively, and we record highly resolved images of structural subsurface defects as well as defects produced by ion-implantations at a depth of 62 μm under the surfaces of 4H-SiC substrates.

Two-dimensional discrete breathers in fcc metals

The interest in discrete breathers (DB), i.e. a time-periodic and spatially localized vibrational mode in a defect free nonlinear lattice, is related to their ability to localize vibrational energy of the order of several eV. In the present work, for the first time, a systematic study of eight nonlinear vibrational modes localized in one spatial dimension and delocalized in the two other dimensions is performed by means of molecular dynamics simulations in defect-free single crystals of fcc metals (Al, Cu and Ni). For this goal, the standard embedded atom method potentials are employed. Calculations are performed at a zero temperature in three-dimensional computational cells. Excitation of DBs takes place via displacement of the specified atoms from their equilibrium lattice sites according to the patterns corresponding to the delocalized nonlinear vibrational modes (DNVMs) found earlier for a two-dimensional triangular lattice. It is revealed that only four out of the eight studied DNVMs can support stable two-dimensional DBs with the lifetimes in the range of 24–47 ps and accumulate vibrational energy of the order of 2 eV per atom. All excited DBs are characterized by a hard type of nonlinearity, i.e. the frequency increases with the increasing oscillation amplitude. Obtained results indicate the presence of a wide variety of nonlinear, spatially localized vibrational modes in three-dimensional lattices of fcc metals.

Local vibrational modes of Si vacancy spin qubits in SiC

Silicon carbide is a very promising platform for quantum applications because of extraordinary spin and optical properties of point defects in this technologically-friendly material. These properties are strongly influenced by crystal vibrations, but the exact relationship between them and the behavior of spin qubits is not fully investigated. We uncover the local vibrational modes of the Si vacancy spin qubits in as-grown 4H-SiC. We apply the resonant microwave field to isolate the contribution from one particular type of defects, the so-called V2 center, and observe the zero-phonon line together with seven equally-separated phonon replicas. Furthermore, we present first-principles calculations of the photoluminescence lineshape, which are in excellent agreement with our experimental data. To boost up the calculation accuracy and decrease the computation time, we extract the force constants using machine learning algorithms. This allows us to identify dominant modes in the lattice vibrations coupled to an excited electron during optical emission in the Si vacancy. The resonance phonon energy of 36 meV and the Debye-Waller factor of about 6% are obtained. We establish experimentally that the activation energy of the optically-induced spin polarization is given by the local vibrational energy. Our findings give insight into the coupling of electronic states to vibrational modes in SiC spin qubits, which is essential to predict their spin, optical, mechanical and thermal properties. The approach described can be applied to a large variety of spin defects with spectrally overlapped contributions in SiC as well as in other 3D and 2D materials.

Substructure Identification for an Axially Loaded Member Using Local Vibration Modes

AbstractAxially loaded elements play an important role in various civil structures. Knowledge of these crucial members is critical to health monitoring and performance assessment of a structure sys...

A Critical Evaluation of Vibrational Stark Effect (VSE) Probes with the Local Vibrational Mode Theory.

Over the past two decades, the vibrational Stark effect has become an important tool to measure and analyze the in situ electric field strength in various chemical environments with infrared spectroscopy. The underlying assumption of this effect is that the normal stretching mode of a target bond such as CO or CN of a reporter molecule (termed vibrational Stark effect probe) is localized and free from mass-coupling from other internal coordinates, so that its frequency shift directly reflects the influence of the vicinal electric field. However, the validity of this essential assumption has never been assessed. Given the fact that normal modes are generally delocalized because of mass-coupling, this analysis was overdue. Therefore, we carried out a comprehensive evaluation of 68 vibrational Stark effect probes and candidates to quantify the degree to which their target normal vibration of probe bond stretching is decoupled from local vibrations driven by other internal coordinates. The unique tool we used is the local mode analysis originally introduced by Konkoli and Cremer, in particular the decomposition of normal modes into local mode contributions. Based on our results, we recommend 31 polyatomic molecules with localized target bonds as ideal vibrational Stark effect probe candidates.

Observation of a 1.979-eV spectral line of a germanium-related color center in microdiamonds and nanodiamonds

Color centers in diamond are considered as a platform for quantum computing and communications, biomedical markers, and nanosensors. Negatively charged split-vacancy centers have outstanding properties due to bright and almost monochromatic luminescence, but they have poor spin coherence and relaxation times. This drawback is believed to be absent in the neutral charge state of the defects. So far only the neutral silicon-vacancy center has been observed in luminescence and absorption spectra. Here we report the observation of its germanium-based analog in luminescence spectra with the zero-phonon line (ZPL) at 1.979 eV in samples containing negatively charged ${\mathrm{GeV}}^{\ensuremath{-}}$ centers. The relationship between the center and Ge dopant is unambiguously confirmed by studies of $^{12}\mathrm{C}$ diamonds containing $^{70}\mathrm{Ge}$, $^{73}\mathrm{Ge}$, or $^{76}\mathrm{Ge}$ isotopes. The intensity ratio of the ZPL of these centers varies with the crystal size, and the intensity of the ${\mathrm{GeV}}^{0}$ centers reaches its maximum in samples 150 nm in size. In the vibronic sideband of this center the local vibrational mode with an energy of 23 meV was identified. The density functional theory calculations yield a ZPL energy value of the ${\mathrm{GeV}}^{0}$ center which is 150 meV higher than the experimentally observed one and matches the values of its relaxational energy as well as the local phonon mode frequency.

Phonon-Induced Optical Dephasing in Single Organic Molecules.

We present a joint experiment-theory analysis of the temperature-dependent emission spectra, zero-phonon linewidth, and second-order correlation function of light emitted from a single organic molecule. We observe spectra with a zero-phonon line together with several additional sharp peaks, broad phonon sidebands, and a strongly temperature dependent homogeneous broadening. Our model includes both localized vibrational modes of the molecule and a thermal phonon bath, which we include nonperturbatively, and is able to capture all observed features. For resonant driving we measure Rabi oscillations that become increasingly damped with temperature, which our model naturally reproduces. Our results constitute an essential characterization of the photon coherence of molecules, paving the way to their use in future quantum information applications.

Improved genetic algorithm with two-level multipoint approximation for complex frame structural optimization

In this paper, an improved structural topology and sizing optimization method is developed for the fast and efficient engineering design of complex frame structures where beam elements are mainly used in the structures. Discrete and continuous variables are included that the elimination or existence of beam elements are treated as discrete variables (0,1), and the continuous sizing variables of beam cross sections are considered to be continuous variables. To solve the mixed variable problem, the paper introduces a two-level multipoint approximation strategy (TMA). The first-level approximate problem is established by using the branched multipoint approximate function, which includes both two types of variables. Genetic algorithm (GA) is used to determine the absence or presence of beam members. The second-level approximate problem that only involving retained continuous size variables is made on this basis, which uses Taylor expansion and dual methods to solve the inner layer continuous optimization problem. Meanwhile, a strategy of adding a new complementary design point is adopted to expend the search scopes and improve the precision. Temporal deletion techniques are used to temporarily remove redundant constraints and local vibration modes processing techniques are used for continuum topology optimization under frequency constraints. Several representative examples are investigated to validate the effectiveness of the improved method.

In Situ Assessment of Intrinsic Strength of X-I⋯OA-Type Halogen Bonds in Molecular Crystals with Periodic Local Vibrational Mode Theory.

Periodic local vibrational modes were calculated with the rev-vdW-DF2 density functional to quantify the intrinsic strength of the X-I⋯OA-type halogen bonding (X = I or Cl; OA: carbonyl, ether and N-oxide groups) in 32 model systems originating from 20 molecular crystals. We found that the halogen bonding between the donor dihalogen X-I and the wide collection of acceptor molecules OA features considerable variations of the local stretching force constants (0.1–0.8 mdyn/Å) for I⋯O halogen bonds, demonstrating its powerful tunability in bond strength. Strong correlations between bond length and local stretching force constant were observed in crystals for both the donor X-I bonds and I⋯O halogen bonds, extending for the first time the generalized Badger’s rule to crystals. It is demonstrated that the halogen atom X controlling the electrostatic attraction between the -hole on atom I and the acceptor atom O dominates the intrinsic strength of I⋯O halogen bonds. Different oxygen-containing acceptor molecules OA and even subtle changes induced by substituents can tweak the (X-I) charge transfer character, which is the second important factor determining the I⋯O bond strength. In addition, the presence of the second halogen bond with atom X of the donor X-I bond in crystals can substantially weaken the target I⋯O halogen bond. In summary, this study performing the in situ measurement of halogen bonding strength in crystalline structures demonstrates the vast potential of the periodic local vibrational mode theory for characterizing and understanding non-covalent interactions in materials.

Thermal conductivity and its relation to atomic structure for symmetrical tilt grain boundaries in silicon

We perform a systematic study of thermal resistance/conductance of tilt grain boundaries (GBs) in Si using classical molecular dynamics. The GBs studied are naturally divided into three groups according to the structural units forming the GB core. We find that, within each group, the GB thermal conductivity strongly correlates with the excess GB energy. All three groups predict nearly the same GB conductivity extrapolated to the high-energy limit. This limiting value is close to the thermal conductivity of amorphous Si, suggesting similar heat transport mechanisms. While the lattice thermal conductivity decreases with temperature, the GB conductivity slightly increases. However, at high temperatures it turns over and starts decreasing if the GB structure undergoes a premelting transformation. Analysis of vibrational spectra of GBs resolved along different directions sheds light on the mechanisms of their thermal resistance. The existence of alternating tensile and compressive atomic environments in the GB core gives rise to localized vibrational modes, frequency gaps creating acoustic mismatch with lattice phonons, and anharmonic vibrations of loosely-bound atoms residing in open atomic environments.

توسعه الگوریتم بهینهسازی توپولوژی تکاملی دوسویه برای سازههای پیوسته با تابع هدف سختی و فرکانس طبیعی پایه با اعمال قید تقارن هندسی

Topology optimization of structures, seeking the best distribution of mass in the design space to improve the performance and weight of a structure, is one of the most comprehensive issues raised in the field of structural optimization. In addition to the structure stiffness as the most common objective function, frequency optimization is of great importance in automotive and aerospace industries achieved by maximizing the fundamental frequency or the gap between two consecutive eigenfrequencies. The phenomenon of multiple frequencies, mesh dependency of topology responses, checkerboarding, geometric symmetry constraint, and occurrence of artificial localized vibration modes in low-density regions are the most important challenges faced by the designer in stiffness and frequency optimization problems which influence the manufacturability of the design too. In this paper, Bidirectional Evolutionary Structural Optimization (BESO) method which is a successful approach in stiffness problems is applied for a frequency and stiffness problem separately via creating a software package including a Matlab code and Abaqus FE solver linked to each other. Also, in this paper, the effect of geometric symmetry constraint is considered on resulted topologies from stiffness and frequency problems. So the BESO method is applied for modeling a 2D beam and its stiffness and frequency optimization and finally, the optimization results of both objective functions will be compared with the initial structure.

Local vibrational force constants – From the assessment of empirical force constants to the description of bonding in large systems

The local vibrational mode analysis, originally introduced by Konkoli and Cremer, provides a physically sound platform for a comprehensive analysis of calculated or measured vibrational spectra and for providing detailed insights into chemical bonding and other structural features. In this work, we summarize the essentials of the local vibrational mode theory with a focus on local vibrational force constants and their relationship with compliance and relaxed force constants. Furthermore, we discuss how local vibrational force constants can be used (i) to assess the quality of empirically derived force constants, (ii) to disclose bonding features in large molecules, and (iii) to provide metal ligand force field parameters. Future applications will be suggested.

Infrared Absorption Study of a Dihydrogen Complex in Anatase TiO2

An infrared absorption study of natural anatase TiO2 is performed. A complex with absorption lines at 3355.9 and 3357.3 cm−1 is shown to consist of at least two equivalent hydrogen atoms, whereby these features are due to the stretch local vibrational modes of two OH bonds located in the basal plane of the anatase crystal, forming an angle of approximately 90°. The complex is shown to be electrically active with a level in the upper part of the bandgap. The microscopic model of the defect is discussed.

Demonstration of n-type behavior in catalyst-free Si-doped GaAs nanowires grown by molecular beam epitaxy

The realization of n-type conduction in directly bottom-up grown Si-doped GaAs nanowires (NWs) by molecular beam epitaxy has remained a long-standing challenge. Unlike the commonly employed vapor–liquid−solid growth, where the amphoteric nature of Si dopants induces p-type conduction, we report a completely catalyst-free, selective area molecular beam epitaxial growth that establishes n-type behavior under Si doping. The vapor–solid selective area growth on prepatterned Si (111) substrates is enabled by an important in situ substrate pretreatment to create an As-terminated 1 × 1-Si(111) substrate necessary for the growth of [111]-oriented GaAs:Si NWs with a large aspect ratio and high yield. Correlated resonant Raman scattering and single-NW micro-photoluminescence (μPL) experiments confirm the n-type nature of the Si-doped GaAs NWs evidenced by a dominant SiGa local vibrational Raman mode, a distinct band filling effect (up to > 10 meV) along with increased PL peak broadening upon increased Si concentration. Excessive Si doping is further found to induce some auto-compensation evidenced by red-shifted PL and the appearance of minor SiAs and SiGa–SiAs pair-like local vibrational Raman modes. Employing excitation power dependent μPL, we further discern signatures in below-gap defect luminescence (∼1.3–1.45 eV) arising from structural defects and Si dopant-point defect complexes.

Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

Optical Activity from the Exciton Aharonov–Bohm Effect: A Floquet Engineering Approach

Floquet engineering is a convenient strategy to induce nonequilibrium phenomena in molecular and solid-state systems, or to dramatically alter the physicochemical properties of matter, bypassing costly and time-consuming synthetic modifications. In this article, we investigate theoretically some interesting consequences of the fact that an originally achiral molecular system can exhibit nonzero circular dichroism (CD) when it is driven with elliptically polarized light. More specifically, we consider an isotropic ensemble of small cyclic molecular aggregates in solution whose local low-frequency vibrational modes are driven by an elliptically polarized continuous-wave infrared pump. We attribute the origin of a nonzero CD signal to time-reversal symmetry breaking due to an excitonic Aharonov-Bohm (AB) phase arising from a time-varying laser electric field, together with coherent interchromophoric exciton hopping. The obtained Floquet engineered excitonic AB phases are far more tunable than analogous magnetically-induced electronic AB phases in nanoscale rings, highlighting a virtually unexplored potential that Floquet engineered AB phases have in the coherent control of molecular processes and simultaneously introducing new analogues of magneto-optical effects in molecular systems which bypass the use of strong magnetic fields.

Revisiting the Theory of Radiative and Nonradiative Transitions of a Polyatomic System

We have revisited the theory of radiative and nonradiative transitions using the crude adiabatic approximation and methods of quantum electrodynamics in order to calculate the transition rates for these processes. All the integrals involving vibrational wave functions have been calculated analytically using Laguerre functions. It has been argued that the radiative transitions will be mediated by the local vibrational modes, referred to as active modes. In the case of nonradiative transitions both local and nonlocal modes contribute to the transition rates. The multidimensional theory involving all the active normal coordinates has been applied to interpret the vibronic spectra of yttrium garnet doped with Pr3+(YAG:Pr3+) at low temperature. The Huang-Rhys parameters have been obtained for the normal frequencies observed for YAG. This approach provides a powerful technique to determine the Huang-Rhys parameters from the low temperature data, which can be used to predict the temperature dependence of luminescence from an optical ion in solid.

Crystallographic Characterisation of Ultra-Thin, or Amorphous Transparent Conducting Oxides-The Case for Raman Spectroscopy.

The electronic and optical properties of transparent conducting oxides (TCOs) are closely linked to their crystallographic structure on a macroscopic (grain sizes) and microscopic (bond structure) level. With the increasing drive towards using reduced film thicknesses in devices and growing interest in amorphous TCOs such as n-type InGaZnO (IGZO), ZnSnO (ZTO), p-type CuCrO, or ZnRhO, the task of gaining in-depth knowledge on their crystal structure by conventional X-ray diffraction-based measurements are becoming increasingly difficult. We demonstrate the use of a focal shift based background subtraction technique for Raman spectroscopy specifically developed for the case of transparent thin films on amorphous substrates. Using this technique we demonstrate, for a variety of TCOs CuO, a-ZTO, ZnO:Al), how changes in local vibrational modes reflect changes in the composition of the TCO and consequently their electronic properties.