Low-frequency Shoulder Research Articles

Water Structure at the Hydrophobic Nanodroplet Surface Revealed by Vibrational Sum Frequency Scattering Using Isotopic Dilution.

The water structure at the hydrophobic/water interface is key toward understanding hydrophobicity at the molecular level. Herein, we characterize the hydrogen-bonding network of interfacial water next to sub-micron-sized hydrophobic oil droplets dispersed in water using isotopic dilution vibrational sum frequency scattering (SFS) spectroscopy. The relative intensity of different modes, the frequency shift of the uncoupled O–D spectrum, and a low-frequency shoulder (2395 cm–1) reveal that water forms an overall stronger hydrogen-bonding network next to hydrophobic droplets compared to bulk water and the air/water interface. Half of the spectral width of the oil droplet SFS spectrum is determined by inter- and intramolecular coupling of water molecules. Isotopic dilution also confirms the presence of a broad distribution (ca. 2640–2745 cm–1) of non-water-hydrogen-bonded O–D modes that are red-shifted and broadened compared to similar species at the air/water interface. This band corroborates the presence of charge transfer between water and oil.

Modification of elastic deformations and analysis of structural and optical changes in Ar+-implanted AlN/GaN superlattices

The effects of built-in deformation and stress relaxation on the structural and optical properties for a 21-period AlN/GaN superlattice (SL) after implantation with argon ions have been studied. Upon implantation, the satellites’ peaks in the X-ray diffraction spectra shift toward the smaller angles which indicate decreasing of tensile deformation in SL by the value of Δc/c = 0.28%. The SIMS analysis showed that the sputtering rate for the initial sample decreases with depth, which is explained by the effect of varying deformation fields in SL and a faster etching of SL layers with a larger deformation magnitude. The change in the sputtering rate of individual layers of SL after implantation also correlates well with the magnitude of deformation. Implantation of Ar+ ions into SL leads to relaxation of the system and more homogeneous distribution of bond energy in individual layers of superlattice. The Raman spectra of the implanted SL are characterized with a low-frequency shoulder in the most intense band of the GaN layer E2 (high), which testifies partial stress relaxation in SL. Thus, based on a comprehensive study of the initial and implanted SL, it has been concluded that point defects that occur during implantation activate the process of dislocations nucleation in the superlattice/GaN buffer layer interface, which reduces the compressive stresses in the GaN layers of SL.

Temperature Dependence of the Air/Water Interface Revealed by Polarization Sensitive Sum-Frequency Generation Spectroscopy.

The temperature dependence of the vibrational sum-frequency generation (vSFG) spectra of the air/water interface is investigated using many-body molecular dynamics (MB-MD) simulations performed with the MB-pol potential energy function. The vSFG spectra calculated for different polarization combinations are then analyzed in terms of molecular autocorrelation and cross-correlation contributions. To provide molecular-level insights into interfacial hydrogen-bonding topologies, which give rise to specific spectroscopic features, the vSFG spectra are further investigated by separating contributions associated with water molecules donating zero, one, or two hydrogen bonds to neighboring water molecules. This analysis suggests that the low frequency shoulder of the free OH peak which appears at ∼3600 cm-1 is primarily due to intermolecular couplings between both singly and doubly hydrogen-bonded molecules.

Rheology in shear and elongation and dielectric spectroscopy of polystyrene-block-poly(4-vinylpyridine) diblock copolymers

We elucidate the influence of composition (weight ratios of 89/11, 76/24 and 49/51) and morphology (spherical, cylindrical and lamellar) on the dielectric and viscoelastic properties in shear and elongation of anionically synthesized polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers in the microphase-separated state. The temperature dependence of the response in both experiments is compared. The analysis of the linear regime shows the appearance of composition (superposition of moduli) and interfacial effects caused by microphase separation (low frequency shoulder, plateau and ω1/2 regime for dynamic moduli and Maxwell-Wagner-Sillars polarization in dielectric experiments). In shear and extensional flows with a constant deformation rate, a pronounced strain-softening behavior in case of a cylindrical and a lamellar morphology appears. For a high weight fraction of the majority phase and a spherical morphology, respectively, strain-softening is observed to a lesser extent. Consequently, strain-softening of diblock copolymer melts can be tuned by the weight/volume ratio of the two blocks.

Heavy strain conditions in colloidal core-shell quantum dots and their consequences on the vibrational properties fromab initiocalculations

We preform large-scale \emph{ab initio} density functional theory calculations to study the lattice strain and the vibrational properties of colloidal semiconductor core-shell nanoclusters with up to one thousand atoms (radii up to 15.6~\AA). For all the group IV, III-V and II-VI semiconductors studied, we find that the atom positions of the shell atoms, seem unaffected by the core material. In particular, for group IV core-shell clusters the shell material remains unstrained, while the core adapts to the large lattice mismatch (compressive or tensile strain). For InAs-InP and CdSe-CdS, both the cores and the shells are compressively strained corresponding to pressures up to 20 GPa. We show that this compression, which contributes a large blue-shift of the vibrational frequencies, is counterbalanced, to some degree, by the undercoordination effect of the near-surface shell, which contributes a red-shift to the vibrational modes. These findings lead to a different interpretation of the frequency shifts of recent Raman experiments, while they confirm the speculated interface nature of the low-frequency shoulder of the high frequency Raman peak.

The “Surface Optical” Phonon in CdSe Nanocrystals

The origin of the ubiquitous low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature is usually assigned as a "surface optical" (SO) phonon, but it is only slightly affected by modifying the surface through exchanging ligands or adding a semiconductor shell. Here we present excitation profile data showing that the low-frequency shoulder loses intensity as the excitation is tuned to longer wavelengths, closer to resonance with the lowest-energy 1Se-1S3/2 excitonic transition. Calculations of the resonance Raman spectra are carried out using a fully atomistic model with an empirical force field to calculate the phonon modes and the standard effective mass approximation envelope function model to calculate the electron and hole wave functions. When a force field of the Tersoff type is used, the calculated spectra closely resemble the experimental ones in showing mainly the higher-frequency LO phonon with 1Se-1S3/2 resonance but showing intensity in lower-frequency features with 1Pe-1P3/2 resonance. These calculations indicate that the main LO phonon peak involves largely motion of the interior atoms, while the low-frequency shoulder is more equally distributed throughout the crystal but not surface-localized. Interestingly, very different results are obtained with the widely used Coulomb plus Lennard-Jones force field developed by Rabani, which predicts far more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.

Nuclear quantum effects on the structure and the dynamics of [H2O]8 at low temperatures

We use ring-polymer-molecular-dynamics (RPMD) techniques and the semi-empirical q-TIP4P/F water model to investigate the relationship between hydrogen bond connectivity and the characteristics of nuclear position fluctuations, including explicit incorporation of quantum effects, for the energetically low lying isomers of the prototype cluster [H2O]8 at T = 50 K and at 150 K. Our results reveal that tunneling and zero-point energy effects lead to sensible increments in the magnitudes of the fluctuations of intra and intermolecular distances. The degree of proton spatial delocalization is found to map logically with the hydrogen-bond connectivity pattern of the cluster. Dangling hydrogen bonds exhibit the largest extent of spatial delocalization and participate in shorter intramolecular O-H bonds. Combined effects from quantum and polarization fluctuations on the resulting individual dipole moments are also examined. From the dynamical side, we analyze the characteristics of the infrared absorption spectrum. The incorporation of nuclear quantum fluctuations promotes red shifts and sensible broadening relative to the classical profile, bringing the simulation results in much more satisfactory agreement with direct experimental information in the mid and high frequency range of the stretching band. While RPMD predictions overestimate the peak position of the low frequency shoulder, the overall agreement with that reported using an accurate, parameterized, many-body potential is reasonable, and far superior to that one obtains by implementing a partially adiabatic centroid molecular dynamics approach. Quantum effects on the collective dynamics, as reported by instantaneous normal modes, are also discussed.

Phonon Raman spectra of colloidal CdTe nanocrystals: effect of size, non-stoichiometry and ligand exchange

Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs.PACS: 63.20.-e, 78.30.-j, 78.67.-n, 78.67.Bf

A FTIR search for linkage isomerism of CN− ions adsorbed on oxides and zeolites

Adsorption of HCN and DCN was studied at 150-600 K on TiO(2), ZnO, MgO and a set of X zeolites by means of FTIR spectroscopy. At the lowest temperatures adsorption is non-dissociative and occurs on surface OH groups of oxides, cations and coordinately unsaturated oxygen atoms. Bands at 2243-2147 cm(-1) assignable to CN(-) ions start to grow in the spectra of ZnO at 170 K, at 250 K for MgO and at 300 K for TiO(2), where bands of polymeric species appear simultaneously. The structure of bands near 2200 cm(-1) in the spectra of the studied oxides does not display a temperature dependence characteristic of isomeric forms. Reversible temperature changes of the band of CN(-) ions in Na-, K-, Rb- and CsX zeolites exposed to HCN provide evidence for linkage isomerism, when the ions bound to alkali cations via C- or N- atoms co-exist in the thermodynamic equilibrium. C-Bonded species account for the stronger band at 2200-2140 cm(-1), while the low-frequency shoulder of this band, which appears and grows at elevated temperatures, is due to the energetically less favorable N-bonded form. The enthalpy of isomerization, estimated from the temperature dependence of band intensities, decreases from ca 20 kJ mol(-1) or more for Na(+) and K(+) to 7.6 kJ mol(-1) for Cs(+), in accordance with the electrostatic model.

Polarization-Resolved Ultrafast Polarizability Relaxation in Polar Aromatic Liquids

The isotropic part of the polarizability relaxation of three polar aromatic liquids has been measured as a function of temperature using a diffractive optic based heterodyne detection method. The isotropic spectral density, which contains only interaction induced components of the liquid's collective polarizability, is contrasted with the anisotropic response to unravel the interaction induced and molecular contributions to the latter. Very similar behavior was observed for the three different aromatic liquids. The fast damping of the isotropic response in the time domain confirms the assignment of the prominent slow exponential relaxation in the anisotropic response to diffusive molecular orientational relaxation. The isotropic response in the frequency domain is found to be asymmetric and peaked at low frequency but extended to higher frequencies, thus contributing over a rather wide frequency range. The relatively low mean frequency of the isotropic spectral density supports the assignment of the highest frequency part of the anisotropic response to molecular librational motion. Finally, a number of affinities are noted between the interaction-induced (isotropic) response and the low frequency shoulder in the anisotropic spectral density, which is so characteristic of the dynamics of aromatic liquids. These data are discussed in relation to some existing molecular dynamics simulations.

Charge and Anion Ordering in (TMTTF)2X Quasi-One-Dimensional Conductors

We present results of measurements of dielectric spectroscopy concerning the 4kF charge density wave associated with the charge ordering (CO) of Wigner type in (TMTTF)2X conductors. This CO transition is due to correlated electron interactions on the molecular chains and to the appropriate displacement of the anion chains (transition at q=0). Recent experiments on the substitution of hydrogen by deuterium in TMTTF molecules will be reported. In the vicinity of TCO it has also been found the divergence of the relaxation time and the development of a low frequency shoulder in the frequency dependence of the imaginary part of the dielectric permittivity, which are typical features for classical ferroelectrics.

Spectroscopic studies on self-supporting multi-wall carbon nanotube based composite films for sensor applications

Self-supporting composite buckypapers containing 50 wt% multi-wall carbon nanotubes (MWCNTs) and SnO x or VO x were prepared by filtration using two different methods. Either the metal-oxides were pre-prepared and mixed with the nanotubes only before filtration (co-filtration method), or the MWCNTs were placed into the synthesis autoclaves of the oxides (single-step method) and the buckypaper was filtered from the product of this synthesis. The resulting composites were characterized by high resolution scanning electron microscopy, XPS, IR and Raman spectroscopy. Whereas the co-filtered samples are simple mechanical mixtures of their individual components, in case of the single-step composites we have found evidence for the existence of an interfacial region between the carbon nanotubes and the metal oxides. This is verified by the appearance of Sn and V in lower oxidation state from the metal-oxide side as well as by the presence of a low-frequency shoulder of the Raman G-band from the carbon nanotube side. The latter feature resembles a Breit–Wigner–Fano type line which is characteristic for phonon coupling to an electronic continuum.

4KFCDW induced by long range Coulomb interactions

By method of dielectric spectroscopy we have obtained experimental data concerning the 4k F charge density wave (CDW) associated with the charge ordering (CO) state of Wigner type in (TMTTF) 2 X conductors. A divergence of the relaxation time was found at the critical transition temperature T CO into the CO state. Below T CO a low frequency shoulder appears in the frequency dependence of the imaginary part of the dielectric permittivity. Under a moderate pressure, the peak in the real part of the dielectric permittivity, corresponding to the transition into the CO state, shifts to low temperature and broadens. The substitution of hydrogen by deuterium in TMTTF molecules results in an increase of T CO (isotopic effect). The results obtained confirm the ferroelectric character of the CO state; they also favour long range Coulomb interactions as the main driven force for the formation of this state.

Surface-state origin for the blueshifted emission in anodically etched porous silicon carbide

Anodic etching of SiC yields a highly monodisperse distribution of nanometer dimension porous structures which extend to a significant depth. Cathodoluminescence (CL) studies of the porous layers yield luminescence peaks in the UV region, above the band gap energy of bulk SiC. Higher etching current densities produce porous silicon carbide (PSiC) with peak CL emission wavelengths deeper in the ultraviolet. Photoluminescence (PL) is also blueshifted in anodically etched PSiC, although not to the extent of the CL emission, suggesting that different emissive states are accessed in CL and PL. Raman investigations of the polar A1 LO mode, which couples strongly to the macroscopic electric field accompanying the LO phonon, were conducted in an attempt to discern whether quantum confinement effects could effectively explain the blueshifted emission. The principal feature of the Raman spectra was a significant low-frequency shoulder on the A1 LO mode, the magnitude of which correlates with the magnitude of the blueshift in PL and the intensity of the blueshifted CL emission. The shoulder was fit quantitatively with a model incorporating the effects of extraordinary LO modes and longitudinal and transverse Fröhlich modes. The Fröhlich mode widths derived from the fit are too wide to be due solely to Fröhlich modes and likely indicate the combined effects of diffuse scattering, broadening of spectral lines, and violation of the symmetry selection rules. The preponderance of the evidence, especially the inability to fit the low-frequency shoulder in the Raman spectra with a phonon confinement model, support an interpretation in which defect structures or surface states are responsible for the UV emission.

Resonance peak in underdoped cuprates

The magnetic susceptibility measured in neutron-scattering experiments in underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ is interpreted based on the self-consistent solution of the $t\ensuremath{-}J$ model of a Cu-O plane. The calculations reproduce correctly the frequency and momentum dependencies of the susceptibility and its variation with doping and temperature in the normal and superconducting states. This allows us to interpret the maximum in the frequency dependence---the resonance peak---as a manifestation of the excitation branch of localized Cu spins and to relate the frequency of the maximum to the size of the spin gap. The low-frequency shoulder well resolved in the susceptibility of superconducting crystals is connected with a pronounced maximum in the damping of the spin excitations. This maximum is caused by intense quasiparticle peaks in the hole spectral function for momenta near the Fermi surface and by the nesting.

Chemical composition and vibrational spectra of tungsten-bearing goethite and hematite from Western Rhodopes, Bulgaria

The incorporation of tungsten in goethite and hematite as well as the relationships between the two minerals in a supergene environment are considered on the basis of a representative sample from the Grantcharitza tungsten deposit (Western Rhodopes, Bulgaria), studied by scanning electron microscopy, electron probe microanalysis, X-ray powder diffraction and micro-Raman spectroscopy. The sample is a colloform black material consisting predominantly of goethite ( G-I ) with 6.83–10.85 wt.% WO3 and of several products of its alteration, including W-poor goethite ( G-II ) with 1.35–5.72 wt.% WO3, a W-rich ferric iron oxide phase ( Fw ) with 22.19–25.12 wt.% WO3, and hematite ( H-I ) with 1.21–1.72 wt.% WO3. It is shown that G-I is an aggregate of proper goethite and of a W-rich ferric iron oxide phase ( Sw ). The alteration of G-I aggregates includes: (i) selective dissolution of Sw which either re-precipitates, thus forming a separate W-rich phase ( Fw ), or which is almost entirely removed from the aggregate; (ii) replacement of the residual goethite G-II by hematite H-I . An atomic ratio W/Fe of about 0.006 is supposed to correspond to the isomorphic (Fe3+ ⟷ W6+) incorporation of tungsten into goethite and hematite in the sample studied. The Raman spectrum of G-II resembles the already published spectroscopic data on goethite, whereas H-II shows spectral features typical of a highly disordered hematite structure. The presence of W atoms in the structures of hematite H-I and goethite G-II causes the appearance of a low-frequency shoulder of the Raman peak near 400 cm−1. According to the Raman spectroscopy data, the W-rich constituent Sw in G-I aggregates as well as its re-precipitated variety Fw both have a Fe-O network similar to that of goethite. In both materials only a small amount of W atoms occupies Fe-positions, while the major part of tungsten forms W-O self-assembled clusters generating a broad band between 580 and 750 cm−1 in the Raman spectra. As shown by model calculations, tungsten is organised in chain-like clusters of corner-sharing WO6 octahedra with an average W-O-W bond angle of about 150°. In addition, Fw contains isolated WO6 octahedra or corner-sharing-WO6 clusters with an average W-O-W angle of about 180°.

Local field distribution in YNi 2B 2C superconductor

Local field distribution in the mixed state of type II superconductors has been numerically calculated and compared with 11B NMR spectra for YNi 2B 2C single crystals. We find that only small distortion of vortex positions from the perfect lattice points is enough to smear out the low frequency shoulder. It is found that the second moment of the field distribution has a major contribution from the high frequency tail. Consequently, a linewidth calculated from the second moment assuming for a Gaussian line shape is overestimated.

Hybrid Thermal‐Nonthermal Synchrotron Emission from Hot Accretion Flows

We investigate the effect of a hybrid electron population, consisting of both thermal and nonthermal particles, on the synchrotron spectrum, image size, and image shape of a hot accretion flow onto a supermassive black hole. We find two universal features in the emitted synchrotron spectrum: (1) a prominent shoulder at low (≲1011 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (2) an extended tail of emission at high (≳1013 Hz) frequencies whose spectral slope depends on the slope of the power-law energy distribution of the electrons. In the low-frequency shoulder, the luminosity can be up to 2 orders of magnitude greater than with a purely thermal plasma even if only a small fraction (<1%) of the steady state electron energy is in the nonthermal electrons. We apply the hybrid model to the Galactic center source, Sgr A*. The observed radio and IR spectra imply that at most 1% of the steady state electron energy is present in a power-law tail in this source. This corresponds to no more than 10% of the electron energy injected into the nonthermal electrons and hence 90% into the thermal electrons. We show that such a hybrid distribution can be sustained in the flow because thermalization via Coulomb collisions and synchrotron self-absorption are both inefficient.

Infrared reflectivity of La2CuO4: Anharmonic shell-model analysis.

The infrared reflectivity (IR) of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ is calculated by means of an anharmonic shell-model (SM) theory developed recently. The phonon frequencies and eigenvectors are taken from a SM fit of the experimental data in the principal symmetry directions. The phonon linewidths \ensuremath{\Gamma} are obtained first from the anharmonic terms of the potentials determined by fitting the phonon frequencies. The corresponding IR spectra show fairly good agreement with measurements, but this is remarkably improved by suitably varying the anharmonic terms. The frequency dependence of \ensuremath{\Gamma} explains certain shapes of the reflectivity bands, which led to erroneous assignments of phonons in a previous analysis of the spectra with ad hoc fitted \ensuremath{\Gamma}. The measurements show a low-frequency shoulder of the IR band corresponding to the stretching mode in the Cu-${\mathrm{O}}_{2}$ plane (\ensuremath{\simeq} 672 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). We conclude that this feature cannot be interpreted as a peculiar shape of the band due to the frequency dependence of \ensuremath{\Gamma}, and should be ascribed to an additional IR-active, possibly polaronic excitation.

A High‐resolution 15N Solid‐state NMR Study of Collagen and Related Polypeptides

High-resolution solid-state 15N-NMR was used to clarify the effect of hydration on the stability of the coiled-coil triple-helix conformation of Gly-Xaa-Yaa repeating units in collagen and the collagen-like polypeptides, (Pro-Ala-Gly)n and (Pro-Pro-Gly)10, because the stability is thought to be related to the presence of (Gly)NH ... O = C(Xaa or Pro) hydrogen bonds. The 15N-NMR signals of these samples were narrowed upon hydration, mainly due to hydration-induced conformational change or rearrangement of the repeating units. In particular, the 15N chemical shifts of the Gly N-H group and the high-field (low-frequency) shoulder peak of Pro nitrogen signals in (Pro-Pro-Gly)10 were shifted downfield (4.9 ppm and 6.8 ppm, respectively) with increasing relative humidity, while the corresponding peaks for collagen and (Pro-Ala-Gly)n were unchanged and close to the 15N shift of (Pro-Pro-Gly)10 in the hydrated state. Such downfield shifts are consistent with the formation of N-H ... O = C hydrogen bonds. In agreement with the NMR results, it was found that the (Gly)NH ... O = C (Xaa or Pro) hydrogen bonds are retained in dehydrated collagen fibrils but not in partially dehydrated (Pro-Pro-Gly)10. No evidence was obtained for the partial formation of the 3(1) helix form (Pro)n or (Gly)n either under hydrated or dehydrated conditions. It is concluded that the Gly 15N chemical shift is a very sensitive probe for studying supercoiling in collagen and collagen-like polypeptides.