B2 Modes Research Articles

Analysis method and experimental study of ice accumulation detection signal based on Lamb waves

A quantitative identification method for in-flight icing has the capability to significantly enhance the safety of aircraft operations. Ultrasonic guided waves have the unique advantage of detecting icing in a relatively large area, but quantitative identification of ice layers is a challenge. In this paper, a quantitative identification method of ice accumulation based on ultrasonic guided waves is proposed. Firstly, a simulation model for the wave dynamics of piezoelectric coupling in three dimensions is established to analyze the propagation characteristics of Lamb waves in a structure consisting of an aluminum plate and an ice layer. The wavelet transform method is utilized to extract the Time of Flight (ToF) or Time of Delay (ToD) of S0/B1 mode waves, which serves as a characteristic parameter to precisely determine and assess the level of ice accumulation. Then, an experimental system is developed to evaluate the feasibility of Lamb waves-based icing real-time detection in the presence of spray conditions. Finally, a combination of the Hampel median filter and the moving average filter is developed to analyze ToF/ToD signals. Numerical simulation results reveal a positive correlation between geometric dimensions (length, width, thickness) of the ice layer and ToF/ToD of B1 mode waves, indicating their potential as indicators for quantifying ice accumulation. Experimental results of real-time icing detection indicate that ToF/ToD will reach greater peak values with the growth of the arbitrary-shaped ice layer until saturation to effectively predict the simulation results. This study lays a foundation for the practical application of quantitative icing detection via ultrasonic guided waves.

Polarized-SERS of non-isotropic molecules on thermally-induced corrugated plasmonic surface supporting a NIR-SPP mode

Thermally-induced surface reconstruction of α-Al2O3 was used to fabricate corrugated templates of different periods to support percolated ultrathin metal film. Reflectance analysis and simulations revealed the coupling of a hybrid NIR-SPP mode along its corrugation vector (kcorrugation). The fabricated samples enhanced Raman signals of phenyl thiolates (PT) with reduced photo-induced disadvantages at λexc = 785 nm. Polarized SERS were performed to study adsorbed molecule – polariton interaction on these corrugated thin film surface. A twofold SERS anisotropy with a cos2(σ) dependency (σ = azimuthal angle) was observed. SERS dichroism of PT Raman modes of different polarizability tensors were analysed. A more pronounced dependence on corrugation period was observed for totally symmetric a1 modes than the non-totally symmetric b1 and b2 modes. Apart from the relative SERS intensity, their SERS dichroism was determined mostly by the excitation of SPP along the kcorrugation rather than the orientation of PT on sample surface. Edge-on orientation of adsorbed PT was predicted due to the absence of a2 modes with both tensor components parallel to the surface. Shorter corrugation periods fabricated were reproducible and ample for uniform, large-area sensing due to effective SPP coupling at σ = 0° resulting in a spatially extended SERS excitation volume.

Quasi-phase-matched nonlinear Lamb waves in composite laminates for material degradation monitoring

Monitoring material degradation and in-service damage in fibrous composites at an early stage is challenging, yet critical for many industrial applications. Second harmonic Lamb waves, which result from the interaction of fundamental waves with material microstructural defects, have great potential for Structural Health Monitoring (SHM) applications. However, understanding of the second harmonic generation in composite laminates remains largely insufficient, which hinders practical SHM applications. This issue is investigated in this study to highlight the quasi-cumulative second harmonic B2 mode Lamb wave in the low frequency range. Theoretical analyses are conducted to ascertain how anisotropy affects the quasi-cumulative effect, followed and validated by numerical simulations and analyses considering different wave propagation angles. The characteristics of the second harmonic B2 mode Lamb waves are further explored in terms of the cumulative feature, robustness to wave beam divergence, and excitability. Experiments are finally carried out to monitor material degradation during the thermal aging of a carbon fiber reinforced panel. The results confirm the omnidirectional cumulative effect of quasi-phase-matched second harmonic B2 mode Lamb waves. It is revealed that the 0° propagating Lamb waves exhibit superior cumulative effect, high robustness to wave beam divergence, and good excitability, rendering them an ideal tool for SHM applications. Furthermore, the high sensitivity of the second harmonic B2 mode waves to material degradation is demonstrated, confirming their promise for material degradation monitoring applications in composite structures.

Investigation of selective SERS enhancement mechanism of Au nanospheres and Au nanorods based on 2T2D-SERS correlation spectroscopy

The selective enhancement mechanism in surface-enhanced Raman scattering (SERS) is demonstrated. Two different types of single nanoparticles (Au nanosphere and Au nanorod) were used to investigate the role of the localized surface plasmon resonance (LSPR) in SERS spectra by using the two-trace two-dimensional (2T2D) correlation spectroscopy. The SERS intensities of three probe molecules, 4-mercaptobenzoic acid (4-MBA), 4-aminothiophenol (4-ATP), and 4-bromobenzenethiol (4-BBT), respectively, were enhanced but slightly different when adsorbed on Au nanospheres and Au nanorods. 2T2D correlation SERS spectra clearly showed that even with the same shape of Au nanoparticles, the main factors influencing the SERS enhancement can vary depending on the specific type of SERS tags used. Such subtle difference could not be clearly identified by the conventional spectral analysis. This result sheds light on potential applications of 2T2D correlation spectroscopy. For 4-MBA molecules, the a1 and b2 modes are mainly affected by the Au nanospheres and Au nanorods. For 4-ATP molecules, the a1 and b2 modes related to C–S stretching combined with C–C stretching band are mainly affected by Au nanorods and Au nanospheres. For 4-BBT molecules, the a1 and b2 modes of C–C (aromatic ring) stretching band are mainly affected by Au nanorods and Au nanospheres. This study offers valuable insights into the relationship between nanoparticle shape and SERS enhancement.

Numerical Simulation of Lamb Wave Propagation in Ice

Lamb wave is one of the most used ultrasonic guided waves, which has a wide application prospect in the field of aircraft icing detection. The platform of Lamb wave propagation based on piezoelectric wafer was used as the physical model, and COMSOL software was used as the numerical calculation tool to simulate the propagation of Lamb wave in different thickness, length and types of ice. The results showed that the attenuation of voltage amplitude increases with the thickness of ice in a certain range. The time delay of Lamb wave B1 mode at the receiving end increases linearly with the increase of ice length. As the mechanical parameters of ice layer decrease, the attenuation of voltage amplitude and the time delay of Lamb wave B1 mode increase gradually. The propagation characteristics of Lamb wave in ice are preliminarily proved, which provides a theoretical reference for the subsequent propagation experiments of Lamb wave in ice.

Development of a novel single-mode miniature standing wave ultrasonic motor

A novel single-mode miniature standing wave ultrasonic motor (SUSM) is proposed. The SUSM uses the first extension vibration mode and second extension vibration mode of the stator to generate different diagonal vibration trajectories and drive the rotor to rotate in different directions through a single-phase signal. The bidirectional motion of the SUSM can be realised by switching the two vibration modes and excitation methods. The motor is designed and the operating principle is analysed. The resonant frequencies of the two vibration modes are determined by the finite element simulated analysis, and the transient analysis is adopted to obtain the diagonal trajectories of the driving part. Finally, a prototype of the single-mode SUSM is developed and the mechanical performance of the prototype is measured and evaluated. According to the experimental results, the motor has a no-load speed of 265.37 rpm (B1 mode) and 320.20 rpm (B2 mode) in each direction under 200 Vp−p and 100 Vp−p voltage excitation, and the maximum load is 60 mN and 70 mN, respectively.

Resonant Charge Transport Assisted by the Molecular Vibration in Single-Molecule Junction from Time-Domain ab initio Nonadiabatic Molecular Dynamics Simulations

Using ab initio nonadiabatic molecular dynamics simulation, we study the time-dependent charge transport dynamics in a single-molecule junction formed by gold (Au) electrodes and a single benzene-1,4-dithiol (BDT) molecule. Two different types of charge transport channels are found in the simulation. One is the routine non-resonant charge transfer path, which occurs in several picoseconds. The other is activated when the electronic state of the electrodes and that of the molecule get close in energy, which is referred to as the resonant charge transport. More strikingly, the resonant charge transfer occurs in an ultrafast manner within 100 fs, which notably increases the conductance of the device. Further analysis shows that the resonant charge transport is directly assisted by the B 2 and A 1 molecular vibration modes. Our study provides atomic insights into the time-dependent charge transport dynamics in single-molecule junctions, which is important for designing highly efficient single-molecule devices.

Ag-Coated Ternary Layered Double Hydroxide as a High-Performance SERS Sensor for Aldehydes.

Volatile organic compounds (VOCs) are common environmental pollutants and important biomarkers for early diagnosis of lung cancer. However, aldehydes are difficult to detect directly due to their small Raman scattering cross-section and gaseous phase. Here, a Ag-coated ternary layered double hydroxide (LDH) was designed for the detection and identification of various aldehydes. The specific surface area of CoNi-LDH was increased by doping Fe3+, which provides abundant active sites to capture gas molecules. Furthermore, the energy band gap (Eg) was decreased due to the local amorphous FeCoNi-LDH with an extended band tail, promoting the excitonic transition of Fe0.07(CoNi)0.93-LDH. In addition, the Fermi level of Ag prevented the recombination of electron-hole pairs of Fe0.07(CoNi)0.93-LDH, providing a new bridge for charge transfer between the substrate and the molecule. Ag/Fe0.07(CoNi)0.93-LDH presented excellent surface-enhanced Raman scattering (SERS) performance for aldehyde VOCs by modification with 4-aminothiophenol (4-ATP) to capture aldehydes and realized the detection of benzaldehyde (BZA) at 10 ppb. The enhancement and Raman shift of the b2 mode indicated the contribution of chemical enhancement to the SERS system, so the substrate presented good uniformity. The recycling of the SERS substrate is realized based on the reversibility of the Schiff base reaction. These results manifested that Ag/FeCoNi-LDH has a wide prospect in the application in the trace detection of aldehydes.

Tunable Intrinsic Phonon Mode versus Anomalous Thermal Transport in Two-Dimensional Strongly Anharmonic Group IB Chalcogenides A2IBSe1/2Te1/2 (AIB = Cu, Ag, or Au).

Group IB chalcogenides, as promising thermoelectric materials, have ultralow thermal transport. Here, we propose a peculiar intrinsic B2 phonon mode that includes the in-plane rotational and stretching vibrations of metal atoms in two-dimensional A2IBSe1/2Te1/2 (AIB = Cu, Ag, or Au). The B2 mode is sensitive to the metal-atom mass, temperature, and strain for effectively tuning the lattice thermal conductivity. The in-plane stretching vibration leads to an unexpected increase in the lattice thermal conductivity from Cu to Ag and to Au systems, in contrast to Keyes' theory. The s(I) phase can be stabilized by the temperature-hardened B2 mode to reduce the lattice thermal conductivity, following the ∼T-0.59 instead of the traditional ∼T-1 trend. The s(II)-to-s(I) phase transition is driven by the strain-softened B2 mode to greatly enhance thermal transport via weakening the anharmonicity. Our work establishes the relationship of tunable intrinsic phonon mode versus thermal transport in two-dimensional group IB chalcogenides.

Development of a resonant screw-driven piezoelectric motor operating in single-mode vibration.

A resonant screw-driven piezoelectric motor operating in single-mode vibrations is proposed, designed, manufactured, and studied. The motor is constructed with a stator and a threaded rotor. The stator consists of a hollow parallelogram metal elastomer and two piezoelectric ceramic plates. The motor is excited by a single-phase signal to produce two separate vibration modes: the first expansion mode (B1 mode) and the second expansion mode (B2 mode). Each mode drives the threaded rotor in one direction, and the bidirectional motion is achieved by switching the two modes. The construction is designed, and modal simulation is performed using finite element software to determine the structural parameters. A frequency-domain analysis is performed to obtain the frequency response characteristics, and the motion trajectories of the stator are obtained using transient analysis. Finally, a prototype is produced, and experiments are conducted. Experimental results indicate that the no-load speeds of the motor under the 200 Vp-p voltage excitation are 1.67 and 1.04 mm/s in the two modes, which correspond to maximum loads of 35 and 20mN, respectively.

A rabbit shaped bidirectional linear ultrasonic motor driven by single-phase signal

A rabbit shaped block, single-phase, linear, ultrasonic motor is proposed. The motor is 17.7 × 9.1 × 6 mm3 in size, and the overall mass is 2.3 g. The motor is designed by referring to the rabbit running mode and the principle of a Langevin vibrator concentrator. It can realize bidirectional driving without changing the constraint position. First, the basic structure of the motor is introduced, and the working principle of the motor is discussed. Then, the motion trajectory and driving force of the motor driving foot are theoretically deduced and simulated, and the simulation results are verified by the modal impedance characteristic test. Finally, the mechanical characteristics of the motor prototype are tested. The results show that under the excitation voltage of 5 N preload and 200 Vpp, the motor is driven to the right in B1 mode, the no-load speed is 42.3 mm/s, the maximum driving force is 0.28 N, and the stall torque is 0.27 Nm. Correspondingly, the motor is driven in reverse in B2 mode, the no-load speed is 118 mm/s, the maximum driving force is 1.48 N, and the stall torque is 1.41 Nm.

Computational underpinnings for the dimerization of para-aminothiophenol to dimercaptoazobenzene on copper surface

The chemical transformation of PATP (para-aminothiophenol) adsorbed on copper to its dimer DMAB (p, p’-dimercaptoazobenzene) upon laser irradiation was computationally studied using the generalized gradient approximation for exchange–correlation functionals PW91PW91. Copper atoms were treated by relativistic effective core potentials (RECP) with (LANL2DZ) and for C, H, N, and S atoms 6–311 + G(d,p) basis set was used. The simulated SERS (surface enhanced Raman spectroscopy) spectrum of DMAB on copper cluster was observed in perfect match with the experimentally observed SERS of PATP on copper. Thus the enhanced signals observed in SERS of PATP were not the enhanced b2 modes of PATP but these were the ag modes of DMAB, which is formed due to the surface catalytic coupling reaction.

RF sputtered AZO thin films: A potential TCO for various opto-electronic applications

AZO films are grown using rf magnetron sputtering technique at different deposition time. Structural properties of AZO films are analysed using XRD and Raman technique. All the films are observed to be crystalline in nature with preferential growth towards (002) plane. Average crystallite size increases with subsequent increase in deposition time. Raman spectra analysis revealed the presence of B1 modes (B1Low, 2B1Low, B1High) which confirms the oxygen vacancies on ZnO sites along with E2HIGH mode indicating wurtzite crystal structure of the AZO film. Sheet resistance decreased from 2.4 Ω/□ to 0.2 Ω/□ and carrier concentration increased from 5.69 × 1020 to 1.5 × 1021 with increase in deposition time leading to increased film thickness. Optical transmittance of AZO films was 92% with the band gap 3.2 eV. Highest figure of merit (φ) for 1 hr. film is 2.02 × 10−2 Ω−1 which is close to ITO films.

Surface-enhanced Raman Scattering and Localized Surface Plasmon Resonance Detection of Aldehydes Using 4-ATP Functionalized Ag Nanorods

Formaldehyde, acetaldehyde, and benzaldehyde are well-known carcinogens affecting human health adversely. Thus, there is a need for efficient detection of these aldehydes. This work uses 4-aminothiophenol (4-ATP) functionalized silver nanorods (Ag NRs) to detect these three aldehydes. The detection mode includes localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS). The LSPR band of 4-ATP functionalized Ag NRs shows a linear decrease in absorbance with the increase in formaldehyde and acetaldehyde concentrations. A sensitivity of 0.96 and 0.79 ΔA/mM for formaldehyde and acetaldehyde were obtained. In the case of benzaldehyde, a nearly exponential decrease in absorbance with the increase in concentrations was observed. Above 98.4 μM concentration, the absorbance diminishes completely. The LoD for formaldehyde and acetaldehyde detection using LSPR is 33.8 and 24.6 μM, respectively. The SERS studies reveal that the 4-ATP binds to Ag NRs through both –SH and –NH2 groups and facilitates the inter-particle charge transfer process. The appearance of b2 modes of vibration for 4-ATP evidences this charge transfer process. In the presence of aldehydes, the change in the band shape, relative intensities, and band position were observed primarily in b2 modes of vibration, evidencing the modulation in the charge transfer process. These remarkable changes were seen in μM concentration of aldehydes. Therefore, detection of these aldehydes with 4-ATP functionalized Ag NRs using SERS is possible in concentrations as low as ~ 1 μM.

Radiation mediated oxidation processes for the conversion of 4-aminothiophenol in to 4, 4′-dimercaptoazobenzene

Gold (Au) nanoparticles of average size ∼ 4 nm were in-situ synthesized and simultaneously embedded on SiO2 nanospheres in a single step process by irradiating a solution (HAuCl4, PVP and SiO2 nanospheres) with 6 MeV electrons at a fluence of 1.0 × 1015 e−/cm2. Substrates were prepared by coating silicon wafers with the solution of SiO2 nanospheres embedded with Au nanoparticles and used for recording Surface-enhanced Raman scattering (SERS) spectra of 4-aminothiophenol (4-ATP) and 4-ATP along with NaOH with a Raman spectrometer. The appearance of characteristic SERS peaks of both 4-ATP and 4, 4′-DMAB in Raman spectra confirmed conversion of a fraction of 4-ATP into 4, 4′-DMAB in the presence of Au nanoparticles, and the intensity of SERS peaks increased in the presence of NaOH. In the recorded SERS spectra, the prominent peaks at around 1140 cm−1, 1390 cm−1 and 1438 cm−1 were observed and assigned respectively to β(C–H), ν(NN) and ν(NN) chemical bonds, which are called b2 modes. The results of the present work revealed that in the presence of Au nanoparticles, the OH− radicals produced from NaOH play an important role in the conversion of 4-ATP into 4, 4′-DMAB, and enhancing the intensities of b2 peaks. The Au nanoparticles of size ≤5 nm have inherent nanoscale defects and therefore surface with nanoscale roughness. All these together greatly enhanced electric field which brought out the strong Raman intensity and catalytic oxidation reaction leading to conversion of 4-ATP into 4,4′-DMAB on the Au–SiO2 substrates.

Investigation of lattice evolutions of tetragonal (Na0.5Bi0.5)TiO3-BaTiO3 single crystal by phonon modes in Raman scattering

Relaxor ferroelectrics (1-x%)(Na0.5Bi0.5)TiO3-x%BaTiO3 (NBT-xBT) have been widely studied in recent years for the high electrostrains and special structural features. However, there are some structure evolutions still unclear, such as the transition occurring at the temperature of dielectric frequency dispersion vanishes. Here, in-situ polarized Raman scattering was utilized to investigate the evolutions of the lattice structure induced by temperature and electric field. The Raman-active modes for P4bm NBT-8BT were separated and identified under the framework of group theory. By observing A1 modes, it is found that electric field cannot induce ferroelectric state after the frequency dispersion vanishes, demonstrating a competition between relaxor characteristics and long-range ferroelectric order which evolves with temperature. According to B2 modes, the P4bm-P4mm transition was not observed under extra-high electric fields. This work interpretates the Raman scattering and lattice evolutions of NBT-8BT, and proposes a method to analyze Raman spectrum which is generally applicable to other ferroelectrics.

Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group under ambient conditions. PbVO3 is isostructural with the model soft mode-driven ferroelectric PbTiO3, but it differs due to the so-called “supertetragonal” elongation of its unit cell. In this study, we investigated the lattice dynamics of PbVO3 based on Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical phonon modes were identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374, and 508 cm−1; A1 modes at 188, 429, and 874 cm−1, and B1 mode at 319 cm−1. The calculations confirmed the experimental symmetry assignment and allowed us to obtain the longitudinal optical phonon wavenumbers. In addition, we analyzed the mode eigenvectors in detail in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. Despite differences in chemistry and strain, the phonon eigenvectors were found to be highly comparable in both compounds. We investigated the position of the ferroelectric soft mode in PbVO3 compared with PbTiO3. Sizeable splitting of the B1+E modes appeared as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V–O bond frequency in PbVO3 was also addressed.

Controllable hot electron transfer in the Ag/MoO3 layer by layer system: Thickness-dependent MoO3 layer

The Ag and MoO3 layer-by-layer sputtering method was employed to fabricate Ag/MoO3 coated on a polystyrene (PS) array (Ag/MoO3@PS), which exhibited excellent surface-enhanced Raman scattering (SERS) activity. The thickness of the MoO3 layer was controlled by changing the sputtering power. The SERS intensity of 4-aminothiophenol (PATP) on Ag/MoO3@PS with a 2 nm thickness of the MoO3 layer was comparable to that on pure Ag coated on the PS array (Ag@PS). This is possible because hot electrons were injected from Ag into the MoO3 layer to enhance the photocatalyst reaction; thus, the SERS spectra of coupled PATP were obtained. The transport of hot electrons rapidly decayed and was blocked with increasing thickness of the MoO3 layer from 2 nm to 9 nm. Thus, the SERS intensity decreased, and interestingly, the b2 mode of PATP decreased and nearly disappeared. This study provides new insight into the control of hot electron reduction for catalytic reduction process monitoring.

Excitation-Dependent Anisotropic Raman Response of Atomically Thin Pentagonal PdSe2.

The group-10 noble-metal dichalcogenides have recently emerged as a promising group of two-dimensional materials due to their unique crystal structures and fascinating physical properties. In this work, the resonance enhancement of the interlayer breathing mode (B1) and intralayer Ag 1 and Ag 3 modes in atomically thin pentagonal PdSe2 were studied using angle-resolved polarized Raman spectroscopy with 13 excitation wavelengths. Under the excitation energies of 2.33, 2.38, and 2.41 eV, the Raman intensities of both the low-frequency breathing mode B1 and high-frequency mode Ag 1 of all the thicknesses are the strongest when the incident polarization is parallel to the a axis of PdSe2, serving as a fast identification of the crystal orientation of few-layer PdSe2. We demonstrated that the intensities of B1, Ag 1, and Ag 3 modes are the strongest with the excitation energies between 2.18 and 2.38 eV when the incident polarization is parallel to PdSe2 a axis, which arises from the resonance enhancement caused by the absorption. Our investigation reveals the underlying interplay of the anisotropic electron-phonon and electron-photon interactions in the Raman scattering process of atomically thin PdSe2. It paves the way for future applications on PdSe2-based optoelectronics.

Synergistic contribution to the enhanced charge transfer of the silver/4-mercaptobenzoic acid/polyaniline (Ag/MBA/PAN) system: thickness-dependent of PAN

The purpose of this paper is to study the charge transfer (CT) process of a Ag/MBA/PAN system with different thicknesses of PAN and explore its SERS enhancement mechanism. It is found that as the PAN thickness is increased, the frequency and intensity of the characteristic MBA band gradually increases. The intensity of the b2 mode increases significantly, which is attributed to the CT mechanism. As the PAN thickness is increased, the degree of CT (ρCT) gradually increases, which is due to the excited SPR from Ag. The electromagnetic enhancement mechanism, which occurs at the interface between PAN and Ag, generates hot electrons and rearranges the interface between PAN and Ag, thereby promoting electron transfer for the whole CT in Ag/MBA/PAN. These results show that CT occurs from the Fermi level of Ag to the LUMO level of MBA and then to the LUMO level of PAN. Additionally, the direction of CT is verified by 2D-COS. Therefore, this study not only reveals the ultrahigh SERS activity of the substrate but also explains the detailed process of regulating the substrate through the PAN thickness, which affects the SERS mechanism. This conductive polymer will open up a new horizon of SERS research and applications.