Surface Vibration Modes Research Articles

Gas‐Lubricated Vibration‐Based Adhesion for Robotics

Controllable adhesion has the capability to enable mobile robots to move freely across vertical and inverted surfaces for applications such as inspection, exploration, and cleaning. Previous methods for generating controllable adhesion have relied on fluidic adhesion through suction forces, electromagnetic adhesion through magnetic or electrical interactions, or dry fibrillar structures. Herein, a new method for achieving controllable adhesion by vibrating a flexible plate near a surface, which generates a strong and controllable attraction force, is presented. This adhesion mechanism has the unique property of providing strong adhesion normal to a surface, but very low resistance to motion parallel to the surface, making it attractive for mobile robots. Adhesive capabilities of vibration‐based adhesion (VBA) to characterize adhesive force dependence on vibration frequency and surface size are studied. Spatial pressure measurements within the adhesive zone, in combination with visualization of surface vibration modes, demonstrate that adhesion is localized to the center of the disk and decreases radially. A mobile robot to highlight the capabilities and robustness of VBA for payload transport, climbing to inversion transitions, and adhesion control is developed. Overall, a novel physical mechanism for robot‐surface adhesion that is robust, controllable, and enables rapid low‐friction locomotion is presented herein.

The effect of Si impurities on the transport properties and the electron-surface phonon interaction in single layer graphene deposited on polar substrates

We investigated theoretically the effect of introducing Si impurities in a single layer graphene (1LG) that had been deposited on a polar substrate on the transport properties of the graphene layer. We consider in our analysis the scattering effects due to the surface optical (SO) phonons located at the interface of the 1LG with various polar substrates such asSiC, hexagonal BN,SiO2andHfO2. Our results demonstrate a reduction of SO phonon-limited (SOPL) mobility, and SOPL conductivity as well as an increase of the SOPL resistivity and of the scattering rate in the presence of Si impurities in the 1LG. Further, we studied the effect of Si impurities on the electron-surface phonon interaction. For our analysis we used the eigenenergies aquired from the tight-binding Hamiltonian in 1LG. Indeed the presence of the Si impurities induces a decrement in the resonant coupling between the electronic sub-levels and the surface vibration modes in monolayer graphene deposited on polar substrates. Finally, we investigated the effect of Si impurities on the Auger scattering process which affects the carriers relaxation. Our results show an enhancement of the Auger scattering rate in the case of the Si-doped 1LG compared to the undoped 1LG.

Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy

We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi$_2$Se$_3$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 cm$^{-1}$ are associated with Raman active $A_{1g}$ bulk phonon modes, the other two at 136 and 158 cm$^{-1}$ are associated with infrared active bulk phonons with $A_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from $D_{3d}$ in the bulk to $C_{3v}$ on the crystal surface. In particular, the 158 cm$^{-1}$ surface phonon mode shows a Fano lineshape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

Effect of crystallite size, Raman surface modes and surface basicity on the gas sensing behavior of terbium-doped SnO2 nanoparticles

In the present work, we report the structural, optical and gas sensing properties of Tb3+-doped SnO2 nanoparticles. XRD results confirmed tetragonal rutile structure of both undoped and Tb3+-doped SnO2 nanoparticles which was further confirmed from Raman results. The increase of dopant concentration resulted in decrease in crystallite size which has been confirmed from XRD and TEM results. Raman spectra exhibited bands positioned at 562 and 487cm−1 whose contribution has been found to increase with decrease in crystallite size. The shifting and broadening of Raman active bands has been explained by well-known phonon confinement model. EDS analysis inveterate presence of terbium in the doped SnO2 nanoparticles. It has been observed that 3% doped samples exhibited optimum sensor response towards ethanol vapor. The optimum operable temperature of doped samples has been reduced as compared to undoped samples. The enhanced sensor response of doped nanoparticles is attributed to the small crystallite size, high surface basicity and enhanced contribution of Raman surface vibration modes of nanoparticles.

Surface phonons of the Si(111)-(7×7) reconstruction observed by Raman spectroscopy

We have studied the surface phonon modes of the reconstructed Si(111)-($7\ifmmode\times\else\texttimes\fi{}7$) surface by polarized Raman spectroscopy. Six surface vibration modes are observed in the frequency range between 62.5 and 420.0 cm${}^{\ensuremath{-}1}$. The mode frequencies agree very well with reported calculation results. This enables their attribution to calculated eigenmodes, whose elongation patterns are dominated by specific atomic sites: the two most characteristic novel fingerprints of the ($7\ifmmode\times\else\texttimes\fi{}7$) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm${}^{\ensuremath{-}1}$, and collective vibrations of the adatoms and first- and second-layer atoms, located at 420.0 cm${}^{\ensuremath{-}1}$. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110 to 140 cm${}^{\ensuremath{-}1}$, and finally the backfolded acoustic Rayleigh wave at 62.5 cm${}^{\ensuremath{-}1}$, which coincides with helium atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (${A}_{1}$ and/or $E$ symmetry) are deduced for each mode.

Unexpected features of the formation of Si and Ge nanocrystals during annealing of implanted SiO2 layers: Low frequency Raman spectroscopic characterization

The present paper reports a study of the influence of heat treatments on the ion-beam synthesis of Si and Ge nanocrystals in SiO2 layers by low-frequency Raman scattering. Low-frequency Raman scattering is used just because the appearance in the glass matrix of crystal nuclei leads to an additional contribution to the density of only low-frequency acoustic vibrational states due to surface vibration modes of the nuclei. Electron microscopy, contrary to expectations, revealed a decrease rather than increase in the size of the crystal nucleus during annealing. Additionally, low-frequency Raman spectra show that the samples do not have a smooth distribution of nanoparticle sizes, as expected, but two different sizes of Si and Ge nanocrystals. This similarity is surprising because Si and Ge have different diffusion coefficients, temperatures of crystallization, meltings, and binding energies. Despite this, in both cases the same mechanism operates during the growth of Si and Ge nanocrystals.

Novel Kinetic Model for Chemical Energy Accommodation on Silica Surface

A new phenomenological catalysis model for silica surfaces has been developed, the main feature of which is the capability of accounting for chemical energy accommodation, i.e., the effective transfer of chemical energy to the catalyst surface by catalytically formed molecules. In this new model, the interaction time between just-formed molecules and the surface is comparable with the time of other processes. The efficiency of the interaction depends on the coupling between the molecules interaction time prior to desorption and the phonons frequency distribution (characteristic period of surface vibration). The features of this interaction should lead to coupling with surface vibration modes and thus, transfer energy from just-recombined species to the surface lattice via phonons. To this aim, a new three-dimensional model of the interaction potential between the surface atoms and the impacting atom has been built and used inside the bulk structure of crystalline silica. The evaluation of a suitable potential function is of fundamental importance; it defines the surface sites in correspondence of which the gas atoms will probably create a chemical bond with the surface, and most of the kinetic model parameters depend on them. Finally, the model has been validated using data collected during a test campaign in the SCIROCCO plasma wind tunnel on a full-scale spacecraft component.

Imaging the surface stress and vibration modes of a microcantilever by laser beam deflection microscopy

There is a need for noninvasive techniques for simultaneous imaging of the stress and vibration mode shapes of nanomechanical systems in the fields of scanning probe microscopy, nanomechanical biological and chemical sensors and the semiconductor industry. Here we show a novel technique that combines a scanning laser, the beam deflection method and digital multifrequency excitation and analysis for simultaneous imaging of the static out-of-plane displacement and the shape of five vibration modes of nanomechanical systems. The out-of-plane resolution is at least 100 pm Hz−1/2 and the lateral resolution, which is determined by the laser spot size, is 1–1.5 μm. The capability of the technique is demonstrated by imaging the residual surface stress of a microcantilever together with the shape of the first 22 vibration modes. The vibration behavior is compared with rigorous finite element simulations. The technique is suitable for major improvements in the imaging of liquids, such as higher bandwidth and enhanced spatial resolution.

Role of surface vibration modes in Si nanocrystals within light emitting porous Si at the strong confinement regime

In this work, we study theoretically the resonant coupling between longitudinal optical surface vibrations of Si-OH and/or Si-O-Si and electron and hole states in the silicon nanocrystals (Si NCs) within light emitting porous Si (PSi) thin films in the framework of the Fröhlich interaction. The results of this analysis are compared with experimental results, which show considerable enhancement and a redshift of the photoluminescence (PL) spectrum of a fresh as-grown PSi thin film after prolonged laser irradiation or after aging in air. These effects coincide with the formation of Si-OH and Si-O-Si bonds on the surface of PSi. The redshift of the PL spectrum is due to the pinning of the bandgap of the light emitting Si NCs, as both oxidation via laser irradiation in air and aging in air introduce energy states in the Si NC band gaps. According to the theoretical analysis, the PL enhancement is assigned to inhibition of nonradiative channels rather than to an enhancement of radiative channels in the Si NCs within the PSi film, due to a strong coupling of the surface Si-OH and/or Si-O-Si vibrational modes to the electronic sublevels in the Si NCs within the PSi layer.

Tin oxide nanocrystals: controllable synthesis, characterization, optical properties and mechanistic insights into the formation process

A novel, surfactant-free, solution-phase method has been successfully developed for the synthesis of SnO2 nanocrystals using a solvothermal route. The nanocrystals having average diameters in the range 4–8 nm, have been synthesized by a non-aqueous sol–gel reaction using tin(IV) bis(acetylacetonate)dichloride, [(Sn(acac)2Cl2)] with benzyl alcohol as the reaction medium at 200 °C. The crystal structure, morphology, and sizes of the SnO2 nanocrystals have been determined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman studies. Raman peaks at 627, 768 cm−1 characteristic of the rutile phase of bulk SnO2 are observed along with broad surface vibration modes in the range 400–600 cm−1. Optical properties of the nanocrystals have been explored by optical absorption and photoluminescence (PL). A blue shift of the optical band gap of the nanocrystals is observed due to size effects. The estimated band gap of the SnO2 nanocrystals from optical absorption data is found to be 3.81 eV. The photoluminescence spectrum showed broad UV as well as visible emission. Based on the GC-MS and carbon-13 NMR analysis of the final reaction solution, a formation mechanism encompassing the ether elimination and solvolysis of acetylacetonate ligand is proposed.

Raman and photoluminescence of Er 3+-doped SnO 2 obtained via the sol–gel technique from solutions with distinct pH

Raman and infrared photoluminescence measurements are carried out for Er 3+ -doped SnO 2 pellets (hard-pressed powders) obtained by the sol–gel technique from colloidal suspensions with distinct pH. Raman data indicate that bulk vibration modes are dominating for neutral pH. When the pH is modified, there is an intensity increase of acoustic surface modes. The evaluation of bulk and crystallite surface vibration modes indicates that the contribution of the bulk decreases for pH above the neutral and decreases even further for acid pH, becoming the surface vibration modes as dominating in this case. Infrared photoluminescence data indicate that the intensity of the emission generated from the 4I 13/2 → 4I 15/2 transition is strongly dependent on the pH of colloidal suspension, which may broaden the emission peaks due to overlapping of Stark levels.

Analysis of 1-3 piezocomposite and homogeneous piezoelectric rings for power ultrasonic transducers

Some power ultrasonic transducers, such as Tonpilz transducers, require high-power transmitting capability as well as broadband performance. Optimized vibrational modes can achieve these requirements. This work compares the resonant characteristics and the surface vibration modes between a homogeneous piezoelectric ring and a 1-3 piezocomposite ring, both used in power ultrasonic transducers. This is the first step in the design of power transducers. Analytical models and finite element results are validated by electrical impedance measurements and the surface acoustic spectroscopy method. Excellent agreement between theoretical and experimental results was obtained. Results show that using piezocomposite ceramics minimize superposition of undesirable modes and increase the bandwidth, as shown in sonograms.

Enhancement and red shift of photoluminescence (PL) of fresh porous Si under prolonged laser irradiation or ageing: Role of surface vibration modes

We study the effect of a red shift and a considerable enhancement of photoluminescence (PL) intensity from a freshly etched porous Si (PS) thin film after prolonged laser irradiation or after aging in atmosphere. Both effects coincide with the appearance of Si–OH and Si–O–Si vibration modes in the Fourier transform infrared (FTIR) absorption spectra. The red shift is attributed to a pinning of the band gap of the light-emitting Si nanocrystals (NCs) due to the formation of Si–OH and Si–O–Si bonds. Using theoretical calculations, we estimated the electron and hole energy shifts caused by the interaction of the electronic states of the Si NCs with the surface vibrations, and correlated the observed PL enhancement with resonant coupling between the quantized valence sub-levels in the Si NCs and surface vibration modes.

Influence of hierarchical nanostructures on the gas sensing properties of SnO 2 biomorphic films

SnO 2 hierarchical films with interwoven tubular structures assembled by nanocrystallites have been achieved through a sol–gel approach combined with calcination treatment and involving eggshell membrane as a template. The gas-sensing property and the sensing mechanism have been studied from the sensor responses to ethanol, formaldehyde, liquefied petroleum gas, gasoline, and H 2S species, based on the investigation of the crystallite size, specific surface area, and Raman surface vibration modes. It is investigated that as-synthesized SnO 2 as a gas sensor has significant advantages attributable to the small nanocrystallite units and hierarchical assembly, which are mainly responsible for the surface activity of the SnO 2 films and further influence on the gas sensing properties.

Nonlinear capillary vibration of a charged bubble in an ideal dielectric liquid

The problem of nonlinear radial pulsations and surface vibrations of a charged bubble placed in an ideal incompressible dielectric liquid is asymptotically solved up to the second order of smallness by the method of many scales. It is shown that, in the case of nonlinear vibrations, resonance energy exchange may take place not only between surface modes but also between the radial mode and a surface mode. A new type of instability (other than Rayleigh instability against the self-charge), instability against the excess vapor pressure in the bubble, is discovered. The new type of instability shows up as energy transfer from the centrosymmetric pulsation mode to all initially excited surface vibration modes simultaneously.

Raman Surface Vibration Modes in Nanocrystalline SnO2: Correlation with Gas Sensor Performances

Raman surface vibration modes have been measured for SnO2 nanocrystalline powders with grain sizes of 3−36 nm and a specific surface area up to 180 m2 g-1, which were prepared by four different routes of chemical synthesis. The influence on these surface vibration modes of the treatment temperature, the crystallite size, and the specific surface area has been studied and bands at 245, 257, 286, 310−350, and 400−700 cm-1 have been identified. The 400−700 cm-1 band intensity has been found proportional to the surface active area. Likewise, the correlation of the 400−700 cm-1 band intensity with the sensing mechanisms have been analyzed from the sensor response of the prepared thick-film gas sensors against reducing CO and oxidizing NO2 species diluted in a N2 carrier. The influence of the nanostructure surface on the sensor signal exhibits opposite trends for CO than for NO2 detection. As the Raman surface vibration modes, 400−700 cm-1, band intensity increases, the sensor response for CO increases too, whi...

Structural analysis of pure and LiCF3SO3-doped amorphous WO3 electrochromic films and discussion on coloration kinetics

Structural differences between pure and LiCF3SO3-doped amorphous WO3 thin films were studied by Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Based on our interpretation of reports in the literature on Raman spectra of WO3 thin films, we found that the LiCF3SO3-doped amorphous WO3 film presented greater amounts of oxygen vacancies, W+4 valence state and W+6=O surface vibration modes. These structural differences served as the basis to make inferences about the dissimilarities also found in the kinetic coloration observed during the electroinsertion reaction, focusing particularly on the dynamics of Li+ insertion into the two different structures.

Dynamic stability of liquid‐filled cylindrical shells under vertical excitation, Part I: Experimental results

AbstractExperimental studies have been carried out on the dynamic stability of a cantilever cylindrical shell partially filled with liquid, under vertical excitation. Two polyester test cylinders with radius 100 mm, thickness 0.25 mm, and lengths 113 and 227 mm were used. The test cylinder was harmonically excited with constant acceleration‐ or displacement‐amplitude. It was found that not only the parametric principal instability resonance but also the parametric combination involving two natural vibrations, each of which has the same circumferential wave number but different axial mode numbers, could occur. The latter type of vibration apparently has not been previously studied. By varying the dimensionless water height from 0.25 to 1.0 stepwise by 0.25 increments, the instability regions and vibration modes were determined for the two test cylinders. The response waves of shell wall and liquid free surface, and axial and circumferential vibration modes were also observed.

Acoustic Field Analysis Using Superposed Sound Sources

We developed a practical computer program to calculate the stationary sound pressure level by using wall or machine surface vibration modes and frequencies. This approach is named the method of superposed sound sources. In this paper, we first describe the theoretical background of this method, and next show that the numerical results obtained by this method agree with the experimental or analytical results. We can predict the sound level by using this approach.

Acoustic field analysis by superposed sound sources.

The authors developed a practical computer program to calculate the stationary sound pressure level by using wall or machine surface vibration modes and frequencies. This approach is named the method of superposed sound sources. In this paper, we first describe the theoretical background of this method, and next show that the numerical obtained results by this method agree with the experimental or analytical results. We can predict the sound level by using this approach.