Acoustic Vibrational Modes Research Articles

Photoluminescence Quenching Upon Growth of Metal Nanoparticles: Quantum-Mechanical Views.

In dictating the optical processes in metal nanoparticles, for instance, quantum nature of free electrons is significantly dominant and plays very crucial roles at the level of nanoscale dimensions of materials. As consequences of the quantum-confinement effects on the conduction electrons, surface-plasmon resonance induced optical absorption and light emission properties of metal nanoparticles are found to be strongly dependent on physical dimensions of the nanomaterials. In addition, surface-confined acoustic vibration (phonon) modes have been experimentally observed to depend on the sizes of the metal nanoparticles. Also, interestingly, tuning of the surface-plasmon resonance condition is found to enhance the intensity of the acoustic Raman modes in metal nanoparticles. The study highlights the role of plasmon-phonon coupling in Co metal nanoparticles embedded in a silica-glass. In the research field of nanosciences and nanotechnologies, extraordinary behaviour and properties of nanoscale matters are investigated. In this context, interesting studies have been discussed in this review article to elaborate optical, chemical and photoluminescence properties of nanoscale Ag metal particles. Subtle detection of optical phenomena associated with the excited many-body electronic processes in the metal nanoparticles, for example, are very interesting but definitely challenging. Here we make an attempt to find out how the thermal growth of Ag metal nanoparticles in a glass matrix snuffs out the light emission from the samples? Quantum mechanical interpretations of the underlying processes about the quenching of photoluminescence phenomena with the growth of the metal nanoparticles will help to fine tune the optical properties of plasmonic systems as well as to harness potential applications of the nanomaterials.

Goos–Hänchen shift for coupled vibrational modes in a semiconductor structure

We present a theoretical investigation of the Goös–Hanchen shift (GHS) experienced by acoustic and optical vibrational modes reflected and transmitted from the surfaces of a semiconductor thin film sandwiched between two semi-infinite media. Our study focuses on the impact of the incident angle on the GHS, considering the coupling between longitudinal and transverse modes. For acoustic vibrations, our findings reveal that the GHS can reach magnitudes up to seven times larger than the thickness of the thin film and up to 20 times larger than the incident wavelength. Besides, it is shown that this significant amplification of the GHS highlights the strong influence of the incident angle and the frequency of the modes involved. In the case of optical vibrations, we observe even more pronounced GHS values, exceeding 30 times the incident wavelength. This demonstrates the potential of GHS in acoustical systems, which opens up possibilities for applications in the design of acoustic devices.

Effect of heat treatment on the thermal stability and properties evolution of TiAlPN thin film prepared by reactive HiPIMS

The influence of post-coating heat treatment on the thermal stability and evolving properties of TiAlPN thin film is investigated. Three different annealing temperatures of 350 °C, 550 °C, and 750 °C are used for 1 h in an ambient air and argon atmosphere to study the thermal stability, mechanical, structural, tribological and corrosion properties of the TiAlPN thin films prepared by High Impulse Magnetron Sputtering (HIPIMS). The X-ray diffraction analysis reveals the thermal decomposition and kinetic transformation of fcc-TiAlPN into mixed phases of fcc – TiN and fcc – AlN. Additional peaks of TiO2 and Al2O3 are identified in all the thin films. Broad bands assigned to acoustic, second-order acoustic and optical phonon range vibration modes were observed from the Raman spectroscopy of the as-deposited thin film. The Raman spectroscopy of annealed samples showed additional broad bands associated with the oxide layer. The increase of the annealing temperature from 350 °C to 550 °C enhances the mechanical properties of the TiAlPN thin films, resulting in a maximum hardness of 33.10 GPa, while enhanced adhesion strength is obtained for thin films annealed at 550 °C in an argon atmosphere. The optimal corrosion resistance with corrosion efficiency of 90.11 % and tribology properties are noticed after annealing in ambient air at an annealing temperature of 750 °C.

Radiative heat exchange driven by acoustic vibration modes between two solids at the atomic scale

Radiative heat exchange driven by acoustic vibration modes between two solids at the atomic scale

Observation of thermal acoustic modes of a droplet coupled to an optomechanical sensor

The bulk acoustic modes of liquid droplets, well understood from a theoretical perspective, have rarely been observed experimentally. Here, we report the indirect observation of acoustic vibrational modes in a picoliter-scale droplet, extending up to ∼40 MHz. This was achieved by coupling the droplet to an ultra-sensitive optomechanical sensor, which operates in a thermal-noise limited regime and with a substantial contribution from acoustic noise in the ambient medium. The droplet vibrational modes manifest as Fano resonances in the thermal noise spectrum of the sensor. This is among the few reported observations of droplet acoustic modes and of Fano interactions in a coupled mechanical oscillator system driven only by thermal Brownian motion.

Radiative heat exchange driven by acoustic modes between two solids at the atomic scale

In the near field (for separation distances smaller than the thermal wavelength, of the order of some microns at ambient temperature), the radiative heat flow between two solids at different temperatures can exceed the blackbody limit by several orders of magnitude. Furthermore, at the atomic scale, close to contact, the vibrational modes of the crystal lattice are expected to play an important role in the heat exchange. While the contribution of acoustic phonons tunnel-ing due to Van der Waals forces and electrostatic interactions near the surface has been investigated [1–5], the radiative contribution of the acoustic modes has been neglected so far. Under the local assumption (wavevector k ≈ 0), optical modes are independent from the acoustic modes, and are the sole responsible for the thermal radiation for distances larger than a few nanometers. However at subnanometer distances, the electromagnetic response of solids is nonlocal (k ≠ 0) and both acoustic and optical modes are coupled, contributing together to the radiative heat exchange. In order to demonstrate the role of this contribu-tion, we calculate [6] the nonlocal dielectric permittivity of magnesium oxide (MgO) using molecular dynamics. In conjunction with fluctuational electrody-namics calculations, we are able to highlight the role of radiation between two polar crystals produced by acoustic modes compared to the expected results from the local theory. We show that this additional contribution can become the dominant channel for radiative heat exchanges at atomic scale in the cryo-genic regime (below 100 K). Since the acoustic vibration modes can be excited with the help of piezoelectric transducers, our work opens the possibility to the control of radiative heat exchanges at atomic scale using external mechanical actuation.

Acoustic Vibration Modes of Gold–Silver Core–Shell Nanoparticles

Bimetallic Au/Ag core–shell cuboid nanoparticles (NPs) exhibit a complex plasmonic response dominated by a dipolar longitudinal mode and higher-order transverse modes in the near-UV, which may be exploited for a range of applications. In this paper, we take advantage of the strong signature of these modes in the NP ultrafast transient optical response, measured by pump-probe transient absorption (TA) spectroscopy, to explore the NP vibrational landscape. The fast Fourier transform analysis of the TA dynamics reveals specific vibration modes in the frequency range 15–150 GHz, further studied by numerical simulations based on the finite element method. While bare Au nanorods exhibit extensional and breathing modes, the bimetallic NPs undergo more complex motions, involving the displacement of facets, edges and corners. The amplitude and frequency of these modes are shown to depend on the Ag shell thickness, as the silver load modifies the NP aspect ratio and mass. Moreover, the contributions of the vibrational modes to the experimental TA spectra are shown to vary with the probe laser wavelength at which the signal is monitored. Using the combined simulations of the NP elastic and optical properties, we elucidate this influence by analyzing the effect of the mechanisms involved in the acousto-plasmonic coupling.

Single-defect phonons imaged by electron microscopy.

Crystal defects affect the thermal and heat-transport properties of materials by scattering phonons and modifying phonon spectra1-8. To appreciate how imperfections in solids influence thermal conductivity and diffusivity, it is thus essential to understand phonon-defect interactions. Sophisticated theories are available to explore such interactions, but experimental validation is limited because most phonon-detecting spectroscopic methods do not reach the high spatial resolution needed to resolve local vibrational spectra near individual defects. Here we demonstrate that space- and angle-resolved vibrational spectroscopy in a transmission electron microscope makes it possible to map the vibrational spectra of individual crystal defects. We detect a red shift of several millielectronvolts in the energy of acoustic vibration modes near a single stacking fault in cubic silicon carbide, together with substantial changes in their intensity, and find that these changes are confined to within a few nanometres of the stacking fault. These observations illustrate that the capabilities of a state-of-the-art transmission electron microscope open the door to the direct mapping of phonon propagation around defects, which is expected to provide useful guidance for engineering the thermal properties of materials.

Высокочастотный резонансный магнитоэлектрический эффект в структуре FeCoSiB-AlN на диэлектрической подложке

A high-frequency magnetoelectric (ME) effect has been discovered in a planar structure containing a ferromagnetic FeCoSiB layer and a piezoelectric AlN layer produced by magnetron sputtering on a borosilicate glass substrate. The structure was excited by a magnetic field at a frequency of the thickness acoustic vibration mode of 32.4 MHz, and the electric voltage generated by the piezoelectric layer was measured. The coefficient of ME conversion at the frequency of the second thickness mode is αE ≈ 19 V / A at a bias magnetic field of 120 A / m, corresponding to the maximum of the piezomagnetic coefficient of the ferromagnetic layer.

Topology optimization of dynamic acoustic–mechanical structures using the ersatz material model

Topology optimization of dynamic acoustic–mechanical structures is challenging due to the interaction between the acoustic and structural domains and artificial localized vibration modes of structures. This paper presents a floating projection topology optimization (FPTO) method based on the mixed displacement/pressure (u/p) finite element formulation and the ersatz material model. The former is able to release the need for tracking the interface boundaries explicitly between the structural and acoustic domains during the optimization process. The ersatz material model enables us to entirely avoid artificial localized vibration modes caused by the extremely high ratio between mass and stiffness. The floating projection simulates the original 0/1 constraints, and it gradually pushes the design variables toward 0 or 1 at the desired level so that the optimized element-based design can be accurately represented by a smooth design. Some 2D and 3D numerical examples, including minimizing sound pressure at the designated domain, restraining structural vibration, and maximizing sound transmission loss, are presented to demonstrate the effectiveness of the proposed topology optimization algorithm. The optimized solutions achieve the consistency of the objective function between the element-based design using the mixed formulation and the smooth design using the segregated formulation. The study suggests that the FPTO method using the ersatz material model is a promising approach for optimizing dynamic acoustic-mechanical structures.

Pure mode axis and the electromechanical coupling coefficients of the 0.67Pb(Mg1/3Nb2/3) O3-0.33PbTiO3 single crystal

Pure mode axes of the 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-0.33PT) single crystals are investigated by slowness surfaces method. It is found only the Z-axis is a pure mode axis. The piezoelectricically stiffened Christoffel equation of the PMN-0.33PT crystal is solved in the rotating coordinate system. All of the single acoustic modes of thickness vibration are obtained. For the pure longitudinal mode of Z-cut crystal, the electromechanical coupling coefficient is up to 0.82. However, it is only 0.34 for the pure shear modes of X-cut or Y-cut crystal. It is along the pure mode axis that the maximum electromechanical coupling coefficients can be achieved.

Large and tunable negative thermal expansion induced by a synergistic effect in M2II[MIV(CN)8] Prussian blue analogues.

Designing negative thermal expansion (NTE) materials with a larger NTE coefficient and a wider temperature window is a great challenge nowadays, leading to the limitation of existing NTE materials such that only about 150 kinds of NTE materials have been discovered since 1996. Here, using first-principles calculations combined with the quasi-harmonic approximation (QHA), we find that the synergistic effect of different vibrational modes can significantly enhance the NTE in open framework compounds. We systematically investigate the NTE properties of the M2IIMIV(CN)8 (MII = Ni, Co, Fe, and Mn; MIV = Mo and W) family, which is the first kind of Prussian blue analogues (PBAs) with a 2D and 3D topology structure, to explore the synergistic enhancement effect in NTE. We reveal that both the optical modes of the rotational motion of [W(CN)8] and [Ni(NC)4] rigid units and the low frequency acoustic modes of the transverse vibration contribute significantly to the NTE. Furthermore, the coefficients of NTE increase monotonously with increasing ionic radius upon substituting Ni in M2IIW(CN)8 with Co, Fe, or Mn, respectively. Analyzing the vibrational modes of the substituted systems indicates that the dramatic changes in NTE originate from a highly synergistic effect, in which all the frequencies of these NTE modes have the same trend, i.e. the lower the frequencies, the larger the coefficient of NTE. This work clearly presents a synergistic mechanism of enhancing NTE in PBA materials, and sheds light on designing new materials with better properties.

Structural and optical study of alternating layers of In and GaAs prepared by magnetron sputtering

Currently, the obtention of nano-structures based on III-V materials is expensive. This calls for novel and inexpensive nanostructure manufacturing approaches. In this work we report on the manufacture of a nanostructures consisting of alternating layers of In and GaAs on a silicon substrate by magnetron sputtering. Furthermore, we characterized the produced nanostructures using secondary ion mass spectroscopy (SIMS), X-ray diffraction analysis, and Raman spectroscopy. SIMS revealed variation in the concentration of In atoms across In/GaAs/In interphases, and X-ray diffraction revealed planes corresponding to phases associated with GaAs and InAs due to In interfacial diffusion across GaAs layers. Finally, in order to study the composition and crystalquality of the manufactured nanostaructures, Raman spectra were taken using laser excitation lines of 452 nm, 532 nm, and 562 nm at different points across the nanostructures.This allowed to determine the transverse and longitudinal optical modes of GaAs and InAs,characteristic of a two-mode behavior. An acoustic longitudinal vibrational mode LA(Γ) of GaAs and an acoustic longitudinal mode activated by disorder (DALA) were observed. These resulted from the substitution of Ga atoms for In atoms in high concentrations due to the generation of Ga(VGa) and/or Arsenic(VAs) vacancies.This set of analyses show that magnetron sputtering can be aviable and relatively low-cost technique to obtain this type of semiconductors.

Vibrational spectroscopy at atomic resolution with electron impact scattering

Atomic vibrations control all thermally activated processes in materials including diffusion, heat transport, phase transformations, and surface chemistry. Recent developments in monochromated, aberration-corrected scanning transmission electron microscopy (STEM) have enabled nanoscale probing of vibrational modes using a focused electron beam. However, to date, no experimental atomic resolution vibrational spectroscopy has been reported. Here we demonstrate atomic resolution by exploiting localized impact excitations of vibrational modes in materials. We show that the impact signal yields high spatial resolution in both covalent and ionic materials, and atomic resolution is available from both optical and acoustic vibrational modes. We achieve a spatial resolution of better than 2 {\AA} which is an order of magnitude improvement compared to previous work. Our approach represents an important technical advance that can be used to provide new insights into the relationship between the thermal, elastic and kinetic properties of materials and atomic structural heterogeneities.

Making waves: Radiation damping in metallic nanostructures.

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

Superconducting Properties of LaSn3 Under Positive Hydrostatic Pressure

Superconducting properties of LaSn3 were calculated at ambient and applied positive hydrostatic pressure. The lattice structure of LaSn3 remained stable at ambient and all applied positive hydrostatic pressures due to the positive frequency of phonon dispersion plots for all modes of vibrations. The electron-phonon coupling constant (λep) and superconducting transition temperature (Tc) show an almost linear decrease with positive hydrostatic pressure. The majority of electron‑electron interaction is mediated by acoustic modes of vibration in comparison to optical modes of vibrations.

Sputter deposited chromium nitride thin electrodes for supercapacitor applications

Transition metal nitrides have attracted significant attention for energy storage applications due to their unique structural, electronic and catalytic properties. In this study, chromium nitride thin films have been deposited on 304L steel substrate by reactive magnetron DC sputtering and then it used as a supercapacitor electrode without additional processing. These films were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), Energy Dispersive X-ray analysis (EDAX, Oxford Instruments), and X-ray photoelectron spectroscopy (XPS) techniques. XRD spectra shows the (1 1 1), (2 0 0), and (2 2 0) planes of the cubic phase of CrN film. The Raman spectra of CrN thin film shows the acoustic vibrational mode at 238 cm−1, and optical mode at 619 cm−1. The FE-SEM images show the columnar structure of CrN thin films. The electrochemical properties were evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The working electrode showed remarkably high specific capacitance of 41.6 F/g at the scan rate of 5 mV/s. The working electrode depicted high capacitance retention of 87% up to 2000 cycles. The electrochemical performance of the CrN showed that they can be used as potential electrode material for electrochemical supercapacitors.

Experimental observation of parametric instability of downward propagating flame under higher acoustic vibration mode

Experimental observation of parametric instability of downward propagating flame under higher acoustic vibration mode

Tunable electron-phonon coupling superconductivity in platinum diselenide

The superconducting property of platinum diselenide $(\mathrm{PtS}{\mathrm{e}}_{2})$, being a type-II Dirac semimetal, doped with electron and hole carriers has been investigated by first-principles calculations. It is found that the superconducting transition temperature ${T}_{\mathrm{c}}$ of pristine $\mathrm{PtS}{\mathrm{e}}_{2}$ is very low $(\ensuremath{\le}0.002$ K), making it infeasible as a practical superconductor. The electron doping is an effective way to increases the ${T}_{\mathrm{c}}$ of $\mathrm{PtS}{\mathrm{e}}_{2}$ to 2.15 K at $0.5{e}^{\ensuremath{-}}$ per unit cell. We further find that the mechanism of superconducting transition is the acoustic branch vibration mode softening rather than the Fermi surface nesting. Our results provide an important clue to increase superconducting temperature in the heavy transition-metal dichalcogenides and shed light on searching for new superconducting topological semimetals.

Resonant Dipolar Coupling of Microwaves with Confined Acoustic Vibrations in a Rod-shaped Virus

In this letter, we treat a rod-shaped virus as a free homogenous nanorod and identify its confined acoustic vibration modes that can cause strong resonant microwave absorption through electric dipolar excitation with a core-shell charge distribution. They are found to be the n = 4N-2 modes of the longitudinal modes of the nanorods, where N is an integer starting from 1 and n is the mode order quantum number. This study was confirmed by measuring the microwave absorption spectra of white spot syndrome virus (WSSV), which is a rod-shaped virus. This is also the first study to identify the “dipolar-like” mode in a rod-shaped nano-object. Our study is not only an important step to achieve rapid and sensitive detection of rod-shaped viruses based on their microwave spectroscopic features and a non-contact method to measure the Young’s modulus of rod-shaped viruses, but also is critical to formulate an efficient epidemic prevention strategy to deactivate viruses with the structure-resonant microwaves.