Phonon Vibrational Modes Research Articles

Overlayer Surface-Enhanced Raman Spectroscopy for Studying the Electrodeposition and Interfacial Chemistry of Ultrathin Ge on a Nanostructured Support

Ultrathin films of germanium (Ge) have been electrodeposited onto surface-enhanced raman spectroscopy (SERS)-active, polycrystalline gold (Au) nanoparticle film electrodes from aqueous solutions containing dissolved GeO2. An overlayer SERS strategy was employed to use the SERS-activity of the underlying Au electrode to enhance the Raman signatures separately for the Ge phonon mode and vibrational modes of surface groups. Electrochemical and spectroscopic data are presented that demonstrate monolayer-level detection of the electrodeposited material and the preparation of crystalline Ge films exhibiting quantum-confinement effects. Potential-dependent Raman spectra are shown that identify electrodeposition conditions where Ge films can be deposited with either long- or short-range crystalline order. Raman spectra collected with electrodeposited Ge films immersed in solutions containing CN-(aq) did not indicate a significant presence of pinholes that exposed the underlying Au(s) substrate. Raman spectra were also collected that identified a potential-dependence for Ge hydride formation at the interface of these films. Separate spectra were collected for the oxidative dissolution of Ge in solution and the complete dry oxidation of Ge to GeOx in air. These data sets cumulatively represent the first demonstration of the overlayer SERS strategy to follow surface chemical processes at crystalline, nanostructured, Ge materials in situ and in real time.

Proton transfer in the hydrogen-bonded chains of lepidocrocite: a computational study

A responsible approach to the development of alternative energy sources, storage and conversion systems is to utilize abundant materials, with minimal or no negative environmental impacts. Here we report that lepidocrocite (γ-FeOOH, a naturally occurring metastable phase of iron oxyhydroxide) shows great promise as a proton conductor, based on sophisticated first-principles calculations that include the important corrections of on-site Coulomb interactions for this strongly correlated material. Our results show how proton transfer is facilitated by phonon vibration modes and relatively low transition energy barriers.

水热合成ZnTe纳米粉的发光性能

ZnTe nanopowder was synthesized at 160℃by hydrothermal method,in which the zinc and tellurium powders were used as the precursors.The powder was characterized by X-ray diffraction,X-ray energy dispersive spectroscopy,transmission electron microscopy and micro-Raman spectroscopy.The X-ray diffraction pattern indicates that the synthesized ZnTe particles are zine blend structure.The X-ray energy dispersive spectrum exhibits that the main elements in the ZnTe nanopowder are zine and tellurium and the impurity is oxygen. The image of transmission electron microscopy shows that the particles in the powder are diversified in size ranging from about 8 to 160 nm and the small particles aggregate easily.Raman spectrum shows there are three longitudinal optical phonon vibration modes of ZnTe centered at about 206,411,615 cm~(-1).The photoluminescence of ZnTe nanopowders was investigated at room temperature.The photoluminescence spectrum shows the weak recombination emission of donor-acceptor pairs presented at about 535.6 nm and the strong emission bands related to the complex defects containing Zn vacancies and impurity as well as isoelectronic oxygen trap at about 581,699 nm,respectively.The recombination emission of donor-acceptor pairs at room temperature is reasonably explained by the related theory.

Phase Mixture and Reliability of BaTiO3-Based X7R Multilayer Ceramic Capacitors: X-Ray Diffractometry and Raman Spectroscopy

BaTiO3-based multilayer ceramic capacitors (MLCCs) using Ni base-metal electrodes (BME) exhibiting EIA-X7R dielectric behavior are represented by the characteristic microstructure of core–shell grains. In the present study, the crystalline phase contents in the mixture were determined quantitatively for two groups of experimental BME-MLCCs, one with CaO doping and the other with (CaO+MgO) codoping by Rietveld refinement using the software package TOPAS®. X-ray diffractometry was used for the identification of crystalline phases whose quantities were determined for cubic (C), tetragonal (T), and rhombohedral (R)–BaTiO3. Phase identification was corroborated by Raman spectroscopy where the characteristic phonon vibration modes corresponding to the C-, T-, and R-phase were assigned. It is suggested that the C-phase content, corresponding to the shell grains containing intragranular voids and the shell region of the core–shell grains, and the R-phase content, corresponding to the modulated core region, not only determine the diffusiveness of the temperature coefficient of capacitance curve but also serve as measuring indices for evaluating the resistance of MLCC formulations against dc degradation.

Optical and acoustic phonon modes confined in gallium phosphide nanoparticles

The main aim of this paper is to discuss the confinement effects on the optical and acoustic phonon vibrational modes in gallium phosphide (GaP) nanoparticles (cylindric grain). The Raman scattering from the GaP nanoparticles was investigated. It was found that the red-shifts of the longitudinal optical (LO) mode and transverse optical (TO) mode were 15 cm−1 and 13.8 cm−1, respectively. It is generally accepted that the red-shifts of the optical phonon modes are due to the presence of smaller nanosized particles (∼1.2 nm) acting as the nanoclustered building blocks of the GaP nanoparticles. In the low frequency Raman spectrum, a set of Stokes lines with almost the same spacing was clearly observed. The scattering feature originates from the discrete phonon density of states of the nanoclustered building blocks. According to Lamb’s vibrational theory, the Raman shift wavenumbers of the spheroidal mode and torsional mode of the lowest energy surface modes for the nanoclustered building blocks were calculated. Good agreement can be achieved between the calculated results and the observed scattering peaks. These results indicate that the corresponding Raman peaks are due to scattering from the localized acoustic phonons in the nanoclustered building blocks in the GaP nanoparticles.

Influence of Stress and Orientation on Magnetoelectric Coupling of Pb(Zr,Ti)O3-CoFe2O4 Bilayer Films

Multiferroic Pb(Zr,Ti)O3–CoFe2O4 (PZT–CFO) bilayer films with different residual stress status and oriented PZT layers were prepared on (111)Pt/Si substrate via a simple sol–gel spin-coating processing. Different heat-treatment processing resulted in different orientation growth of the PZT layer and stress status in the bilayer films, which affects the magnetoelectric (ME) coupling of the PZT–CFO bilayer films. Both the magnetic-field-induced shifts in the phonon vibration modes and the direct ME coupling effect observed in these bilayer films demonstrate stress-mediated ME coupling mechanism in the multiferroic bilayer films.

Narrow bandwidth and highly polarized ratio infrared thermal emitter

Polarized infrared thermal emitters consisting of a waveguide thermal emitter combined with silver grating structure were studied. For a device containing a perforated silver film under the grating, polarized infrared light was emitted only when the wavelength satisfied the standing wave condition, therefore the broad bandwidth SiO2 phonon vibration modes were suppressed in the thermal radiation spectrum. The polarized ratio with different grating thickness was investigated and the highly polarized ratio up to 0.875 was achieved. This study demonstrated that the integrated structure can be used as a narrow bandwidth and highly polarized ratio infrared light source.

Gap discrete breathers in two-component three-dimensional and two-dimensional crystals with Morse interatomic potentials

The properties of gap discrete breathers in three-dimensional and two-dimensional crystals of the composition A3B with interatomic bonds described by the Morse potential have been investigated by the molecular dynamics method for different ratios between the masses of components mA/mB. The transition to a thermal equilibrium from a state far from equilibrium has been studied for the two-dimensional crystal. In this case, a short-wavelength phonon vibrational mode is excited in the crystal. This mode appears to be modulationally unstable for not too small amplitudes. During the transition to the state characterized by a uniform energy distribution between all vibrational modes of the crystal, the energy is localized in the form of gap discrete breathers, which exist in time intervals that exceed their oscillation period by several orders of magnitude.

Raman scattering, electronic, and ferroelectric properties of Nd modified Bi4Ti3O12 nanotube arrays

Neodymium-modified bismuth titanate, Bi4−xNdxTi3O12 (BNdT), nanotube arrays were fabricated by sol-gel method utilizing experimentally prepared porous anodic aluminum oxide (AAO) templates with pore diameters of about 200 nm and 100 nm, respectively. The as-prepared nanotube arrays exhibit orthorhombic perovskite polycrystalline structure of BNdT, which have outer diameters of about 200 and 100 nm, corresponding to the pores diameters of the AAO templates employed, and with wall thicknesses of about 9.7 nm and 12 nm, respectively. The phonon vibration modes corresponding to the Bi atoms in the Bi2O2 layers weaken and broaden with increasing Nd content. The changes of Raman internal modes originated from the vibrations of atoms inside the TiO6 octahedral indicate the increase in octahedron tilting and structural distortion. The leakage current and polarization-electric field response curves of BNdT nanotube arrays were measured, and the hysteresis loop illustrates a good ferroelectric property of as-prepared BNdT nanotube array at room temperature. The dielectric constant and dissipation factor were measured in the frequency region from 1 kHz to 1 MHz indicating polarization relaxation phenomenon.

Vibrational spectroscopic study of Sr2ZnTeO6 double perovskites

AbstractSr2ZnTeO6 ceramics were prepared by the solid‐state route and their vibrational phonon modes were investigated using optical spectroscopic techniques, for the first time. X‐ray diffraction (XRD) and Raman and infrared spectroscopies were employed to investigate the structures of these perovskite materials and the results analysed together with group‐theoretical predictions. The number and behaviour of the first‐order modes observed in both spectroscopic techniques are in agreement with the calculations for a tetragonal I4/m space group. The complete set of the optical phonon modes was determined, and the intrinsic dielectric properties of the materials were evaluated, allowing us to discuss their potential application in microwave (MW) circuitry. Copyright © 2009 John Wiley & Sons, Ltd.

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu, Ag, and Au layers

This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.

Graphitic carbon growth on Si(111) using solid source molecular beam epitaxy

Solid source molecular beam epitaxy is used to explore the growth of carbon films directly on Si(111). It is shown that graphitic carbon is grown by the implementation of a thin amorphous carbon film that suppresses the formation of SiC precipitates. Raman scattering measurements show the D and G vibrational phonon modes, indicating graphitic ordering in the carbon film. X-ray photoelectron spectroscopy is used to verify the formation of sp2 bonds in the graphitic carbon films and confirms the suppression of SiC.

Optical properties and potential applications of ɛ-GaSe at terahertz frequencies

Ordinary and extraordinary refractive indices of an ɛ-GaSe crystal at terahertz (THz) frequencies are experimentally determined in this study. Fitting experimental data by THz time-domain spectroscopy (THz-TDS) and a Fourier transform infrared spectrometer (FTIR), we proposed revised complex dielectric functions and Sellmeier equations of GaSe for both ordinary and extraordinary waves from 0.2 to 100 THz. Phonon vibrational modes and overtones in the THz frequency range are examined in detail. The high magnitude of the figure of merit (FOM~103 at 1 THz), the large birefringence (Δn~0.76 at 1 THz). and the low absorption coefficient (α~0.2 cm−1 at 1 THz) of GaSe are also identified. Potential applications to practical photonic devices such as phase shifters at THz frequencies are proposed.

Raman spectroscopy of rare earth doped silver halide crystals

The phonon energy of rare earth doped silver halide crystals was investigated using Raman spectroscopy. An additional phonon vibrational mode was observed and attributed to localized defect vibrational modes. Good agreement was found between the measured and the predicted frequencies of the local modes. These additional low energy (65–80 cm−1) vibrational modes do not increase the thermal quenching of mid-IR luminescence of the rare earth ions in silver halide crystals. The rare earth doped silver halide crystals are suitable for the fabrication of mid-IR solid state lasers and fiber lasers due to their extremely low phonon energies.

Preparation of GaN Film and Investigation of Vibrational Spectrum of GaN by DFT

Using Ga_2O_3 as Ga source,the GaN films are successfully deposited on Si(111)substrates by two-step method of sol-gel and high nitridation temperature technique.The as-prepared films were confirmed as single crystalline GaN with wurtzite structure by X-ray diffraction(XRD).The dimension of GaN crystal particulates is smaller than 100nm under the observation of scanning electron micro- scope(SEM).Moreover,E_1(TO)vibrational mode of GaN are surveyed in IR spectrum of the GaN films.Vibrational frequency of GaN small clusters is calculated using density functional theory(DFT). The results show that the vibrational frequencies of the Ga reached clusters are close to the peaks of the phonon vibration modes of the wurtzite structure GaN,and 2200cm~(-1)is an intensive vibrational frequency of N-N bond stretch in N reached GaN.FTIR of the sample are analyzed in further.

Electron–phonon coupling and vibrational modes contributing to linear electro‐optic effect of the efficient NLO chromophore 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST) from their vibrational spectra

AbstractThe optimized geometry and structural features of the most prospective electro‐optic crystal 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST), and the vibrational spectral investigations have been comprehensively described with the near infrared Fourier transform (NIR FT) Raman and Fourier transform infrared (FT‐IR) spectra supported by the density functional theoretical (DFT) computations to elucidate the contribution of vibrational modes to the linear electro‐optic (LEO) effect. Mulliken population analysis and natural bond orbital (NBO) analysis have also been carried out to analyze the effects of intramolecular charge transfer (ICT), intramolecular hydrogen bonding and hyperconjugative interactions on the geometries. The influence of CT interaction between the phenyl ring and the dimethylamino group of the nonlinear optical (NLO) chromophore on the endocyclic and exocyclic angles, and the electronic effects such as hyperconjugation and back‐donation on the methyl hydrogen atoms have been examined. The concurrent intense activation of Raman and IR activities of the effective conjugation vibrational coordinate, which significantly contributes to the LEO effect resulting from the strong electron–phonon (e/ph) coupling, has been analyzed in detail. The effects of frontier orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), transition of electron density (ED) transfer and the influence of planarity in the stilbazolium ring on the first hyperpolarizability are also discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Raman Scattering and Infrared Spectroscopy of Chemically Substituted Sr2LnTaO6(Ln = Lanthanides, Y, and In) Double Perovskites

Sr2LnTaO6 (Ln = lanthanides, Y, and In) ceramics were prepared by a solid-state route, and their vibrational phonon modes were investigated using Raman and infrared (IR) spectroscopic techniques. Lorentzian lines were used to fit the Raman spectra, which showed 24 bands in perfect agreement with theoretical calculations. The IR spectra were fitted with a four-parameter semiquantum model, and the results showed 17 bands for the materials. The position and width of the phonon modes were determined and correlated to the ionic radii for the different atoms substituted in the B′-site. It was verified that all materials belong to the monoclinic P21/n (C2h5) structure.

Amplitude-squared and amplitude-cubed squeezing in stimulated Raman and in spontaneous Raman scattering

Coherent light coupled to a third-order nonlinear medium is responsible for spontaneous and for stimulated Raman processes. The analytical solutions up to second order in coupling constants of various field modes for spontaneous Raman and for stimulated Raman processes are utilized to obtain the amplitude-squared and amplitude-cubed squeezing of the input coherent light. The amplitude-squared and amplitude-cubed squeezing are completely ruled out for all modes for spontaneous Raman processes. For stimulated Raman processes, we observe that the squeezing for amplitude-squared and amplitude-cubed are possible for pump and for vibrational phonon modes. It is found that the percentage of higher ordered amplitude squeezing decreases with the increase of the orders. The squeezing in Stokes and in anti-Stokes modes are ruled out even for stimulated Raman processes.

Vibrational Studies and Microwave Dielectric Properties of A-Site-Substituted Tellurium-Based Double Perovskites

A2MgTeO6 (A = Ba, Sr, Ca) ceramics were prepared by solid-state route and their microstructure and microwave dielectric properties, as well as the vibrational phonon modes were investigated using the resonance method and spectroscopic techniques for the first time. For the optimized sintering temperatures, Ba2MgTeO6, Sr2MgTeO6, Ca2MgTeO6 materials presented the following properties: εr of 11, 14.3, and 13.2; Quxf of 25, 27.5, and 81 THz, and τf of −16, −61, and −81 ppm/°C, respectively. XRD besides Raman and infrared spectroscopies were employed to investigate the structures of these materials and the results showed that Ba- and Sr-based double perovskites belong to the tetragonal I4/m space group, while Ca-based materials exhibit a monoclinic P21/n structure. The phonon modes were evaluated and the intrinsic microwave dielectric properties were discussed in terms of the A-site substitution on tellurium-based ceramics. The number of bands observed in both spectroscopic techniques agrees well with the group-theoretical predictions for all materials, thus confirming our assumptions.

Electronic and vibrational properties ofγ−AlH3

Aluminum hydride (alane) $\mathrm{Al}{\mathrm{H}}_{3}$ is an important material in hydrogen storage applications. It is known that $\mathrm{Al}{\mathrm{H}}_{3}$ exists in multiply forms of polymorphs, where $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ is found to be the most stable with a hexagonal structure. Recent experimental studies on $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ or other polymorphs. Using density functional theory, we have investigated the energetics, and the structural, electronic, and phonon vibrational properties for the newly reported $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ structure. The current calculation concludes that $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ is less stable than $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ by $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{KJ}∕\mathrm{mol}$, with the zero-point energy included. Interesting binding features associated with the unique geometry of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ are discussed from the calculated electronic properties and phonon vibrational modes. The binding of $\mathrm{H}\text{\ensuremath{-}}s$ with higher energy $\mathrm{Al}\text{\ensuremath{-}}p,d$ orbitals is enhanced within the double-bridge arrangement, giving rise to a higher electronic energy for the system. Distinguishable new features in the vibrational spectrum of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ were attributed to the double-bridge and hexagonal-ring structures.