Surface Vibrational Modes Research Articles

Vibrational Raman scattering from surfaces of III-V semiconductors: Microscopic and macroscopic surface modes

With ongoing instrumental improvements Raman spectroscopy (RS) is advancing into the study of surface vibrational modes of semiconductors. On compound semiconductors, like the III–V's, two distinct types of surface modes occur, microscopic modes being vibrations confined to the surface and macroscopic modes penetrating much deeper into the bulk, dependent on the electromagnetic boundary conditions. Both mode types depend on surface properties, the microscopic ones on the atomic scale, and the macroscopic ones on the scale of the surface polariton wavelength. While the former one delivers surface atomic structure information, the latter one may be useful for instance to study the morphology of surfaces, such as the shape of semiconductor nanowires. We discuss Raman spectra obtained on the atomically well-defined III–V(110) model surfaces and recent results obtained on isolated III–V nanowires. The comparison of both gives insight in the capabilities of Raman scattering from surface phonons: The contributions of surface phonons, surface resonances, and macroscopic modes (Fuchs–Kliewer modes, surface phonon polaritons) to the Raman spectra become evident.

Comparing ultrafast surface and bulk heating using time-resolved electron diffraction

From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time of the random atomic motion resulting from the electronic relaxation after short pulse laser excitation. The surface sensitive reflection high energy electron diffraction and transmission electron diffraction yield a time constant of about 12 ps and 3 ps, respectively. The different energy transfer rates indicate relatively weak coupling between bulk and surface vibrational modes.

Surface vibrational modes in disk-shaped resonators

The natural frequencies and distributions of displacement components for the surface vibrational modes in thin isotropic elastic disks are calculated. In particular, the research is focused on even solutions for low-lying resonant vibrations with large angular wave numbers. Several families of modes are found which are interpreted as modified surface modes of an infinitely long cylinder and Lamb modes of a plate. The results of calculation are compared with the results of the experimental measurements of vibrational modes generated by means of resonant excitation in duraluminum disk with radius of ≈90mm and thickness of 16mm in the frequency range of 130–200kHz. An excellent agreement between the calculated and measured frequencies is found. Measurements of the structure of the resonant peaks show splitting of some modes. About a half of the measured modes has splitting Δfsplit/fmode at the level of the order of 10−5. The Q-factors of all modes measured in vacuum lie in the interval (2…3)×105. This value is typical for duraluminum mechanical resonators in the ultrasonic frequency range.

Hydrogen Chemisorption on Co(0001) Surface: A DFT Study

In this work the total energy calculations on the atomic geometry and chemisorption properties of Co(0001)/H surface are performed by using the density-functional theory in the approach of the projector-augmented wave (PAW) method. For a clean Co(0001) surface the atomic relaxations of the top three Co(0001) layers are obtained. The adsorptions of atomic Hydrogen have been investigated in the surface periodicities of p(1×1), p(1×2), p(2×2), and (  )R30º combined with hcp hollow and fcc hollow sites. For the cases studied in this work, the atomic hydrogen occupies fcc hollow site preferably drawn from the total energies. In these surface structures the perpendicular distances between the adsorbate and the first layer of substrate (DH-Co) range from 0.93 Å to 0.99 Å. The calculations on the surface vibrational modes proved the frequencies of atomic hydrogen along substrate surface displayed strong dependence on the adsorption geometries.

Temperature diffuse scattering of nanocrystals

The effects of thermal vibrations on X-ray powder diffraction patterns are discussed. Special considerations for extremely small crystallites are described, including the occurrence of surface and edge vibrational modes, and a restriction on the maximum phonon wavelength. In doing so, a complete temperature diffuse scattering (TDS) model is presented, which includes the influence of these features on: the Debye-Waller parameter; first-order TDS; and higher-order TDS terms. The importance of using an accurate TDS representation is studied as a function of temperature and crystallite size. It is found that a misrepresentation of the TDS for small crystallites can lead to an error in the determined Debye-Waller parameter on the order of 20-40% and a slight overestimation of the peak broadening. While the presented theory is primarily developed considering X-ray scattering, the same expressions are expected to describe the TDS in faster-than-sound neutron powder diffraction measurements.

Features of the formation of silicon nanocrystals upon the annealing of SiO2 layers implanted with Si ions

The effect of annealing on the ion-beam synthesis of silicon nanocrystals in Si layers was investigated by low-frequency Raman scattering (RS). The occurrence of crystal nuclei in a matrix of glass results in an additional contribution to density of the acoustic vibrational states associated with the surface vibrational modes of nanocrytals. The low-frequency RS caused by interaction of light with acoustic vibration modes of nanoparticles is an effective method of research. The low-frequency Raman spectra show that the samples do not have a smooth distribution of nanoparticle size, but have two specific sizes of nanoparticles, 3 and 6 nm.

Attachment cross sections of protonated water clusters

The attachment of water molecules onto size selected protonated water clusters has been experimentally investigated. Absolute attachment cross sections are measured as a function of cluster size, collision energy, and initial cluster temperature. Although thermal evaporation is ruled out in our experiment, attachment cross sections become significantly smaller than hard sphere cross sections as the collision energy increases. This feature is attributed to a transition from adiabatic to nonadiabatic regime. It is shown to be due to a dynamical effect: as the collision duration becomes shorter than the typical time required for collision energy redistribution into clusters internal energy, the attachment probability is reduced. We relate this typical time to the period of the main surface vibrational mode excited by the collisions. This hypothesis is further supported by results obtained with deuterated water clusters.

Metal–insulator transition in Si(111)‐(4 × 1)/(8 × 2)‐In studied by optical spectroscopy

AbstractMeasurements of the surface vibrational modes and optical response of Si(111)‐(4 × 1)/(8 × 2)‐In are compiled and a comparison to ab initio calculations performed within DFT‐LDA formalism is given. Surface resonant Raman spectroscopy allows identifying a number of surface phonons with high spectral precision. The phase transition of the (4 × 1)–(8 × 2) surface structure is found to be accompanied by characteristic changes of the surface phonons, which are discussed with respect to various structural models suggested. The optical anisotropy of the (8 × 2) phase shows that the anisotropic Drude tail of the (4 × 1) phase is replaced by two peaks at 0.50 and 0.72 eV. The spectroscopic signatures of the (4 × 1) and (8 × 2) phases agree with a metal–insulator transition. The mid‐IR‐anisotropic optical response of the insulating (8 × 2) phase is interpreted in terms of electronic single particle excitations between surface electronic bands related to the In‐nanowire surface. Comparison of the measured optical transitions with DFT ab initio calculations for the hexagon model and the trimer model of the (8 × 2) structure shows evidence for the existence of the hexagon structure.

On the robustness of surface vibrational modes: case studies in the Cd region

Recent studies of the stable Cd isotopes, using inelastic neutron scattering, radioactive decay and nucleon-transfer reactions, have resulted in the need to fundamentally reassess the concept of low-energy vibrational modes in these isotopes. We present a synopsis of the nuclear physics that has led to this situation, followed by the present status of spectroscopy for and against vibrational modes in nuclei neighbouring the Cd isotopes. We continue with some details of the spectroscopic techniques used and examples of the data obtained which lead to the above statements. We provide further perspective with some details of octupole degrees of freedom. We conclude with an outlook of where spectroscopic data are needed to further assess the presence or absence of low-energy vibrations in nuclei; and we provide some reflections on where future theoretical investigations should be directed.

Superconductivity of p-type diamond (001) and (111) thin films: Ab initio calculations

The surface structure, electronic properties, lattice dynamics, and the electron–phonon coupling for p-doped diamond (001)-(2 × 1) and (111)-(2 × 1) thin films have been extensively investigated using ab initio methods within the virtual crystal approximation. The calculations of p-doped diamond thin films strongly favor dimer reconstruction of diamond surfaces. The physical origin of superconductivity of diamond (001)-(2 × 1), respectively, and (111)-(2 × 1) thin films which is higher than that of bulk diamond is systematically studied. It is showed that surface vibrational modes of diamond (001)-(2 × 1) surfaces give main contributions to superconducting transition temperature T c , while T c of diamond (111)-(2 × 1) surfaces is attributed to the combined action of surface and bulk vibrational modes. Therefore, at the highest concentration (13.98 × 10 21 cm − 3 ) T c ≈ 56.5 K of diamond (111)-(2 × 1) surfaces is about twice as high as that of bulk and diamond (001)-(2 × 1) surfaces.

Structure and composition of aluminum oxide films in contact with the liquid In-Ga eutectic

The oxide layers on the surface of aluminum metal and the aluminum alloys AD-1 and A-5 have been studied by attenuated total reflection infrared spectroscopy. The Al-O and Al-OH surface vibrational modes and A-O modes of the AlO4 and AlO6 groups have been identified. The structural inhomogeneity of the surface oxide layers is shown to contribute to their disruption when the sample is brought into contact with the liquid In-Ga eutectic. In situ scanning electron microscopy and X-ray microanalysis have been used to follow the dynamics of oxide layer disruption and the morphological and compositional changes in the eutectic alloy and the surface layer of aluminum.

Optical generation of long-lived surface vibrations in a periodic microstructure

We use the laser-induced transient grating technique for the excitation and detection of surface vibrational modes of a periodic microstructure on a silicon substrate forming a one-dimensional phononic crystal. Two standing wave eigenmodes with zero-group velocity corresponding to the top and bottom of the bandgap in the dispersion of the zone-folded Rayleigh waves are produced by setting the spatial period of the excitation pattern to twice the structure period. These modes do not radiate acoustic energy into the substrate, yielding an enhanced lifetime. We show that the relative amplitude of the two modes is controlled by the spatial phase of the excitation pattern, and discuss the dependence of the confinement time of the acoustic oscillations within the excitation area on the curvature of the dispersion surface.

Ionic Binding of Na+ versus K+ to the Carboxylic Acid Headgroup of Palmitic Acid Monolayers Studied by Vibrational Sum Frequency Generation Spectroscopy

Ionic binding of alkali ions Na(+) and K(+) to the carboxylic acid headgroups of fatty acid monolayers is studied as a proxy toward understanding the fundamental chemistry in cell biology. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation and complex formation of fatty acid headgroups. Palmitic acid (C(15)H(31)COOH) exists as a monolayer on aqueous surfaces. Surface vibrational stretch modes of palmitic acid from 1400 cm(-1) to 3700 cm(-1) were observed (nu(s)-COO(-), nu-C horizontal lineO, nu-C-H, nu-O-H of -COOH, free OH). Palmitic acid is mostly protonated at the aqueous surface at neutral pH (approximately 6). However, various degrees of deprotonation are initiated by the introduction of Na(+) and K(+) that results in the complexation of K(+):COO(-) and solvent separated Na(+):COO(-). Evidence in several spectral regions indicates that K(+) exhibits stronger ionic binding affinity to the carboxylate anion relative to Na(+).

Nano size effects in the high resolution electron energy loss spectra and excitation function of hydrogenated diamond films

The present work studies the high resolution electron energy loss spectra (HR-EELS) and excitation function of hydrogenated diamond films composed of diamond grains of varying size of ∼ 300, ∼ 20 nm and ∼ 5 nm. It is reported that HR-EELS modes associated with C–C lattice phonons are comparably more intense than modes associated with the C–H surface vibrational modes in the case of nano-crystalline compared to micro-crystalline diamond films. Excitation function measurements of the different modes show a pronounced maximum at ∼ 8 eV for the nano-diamond films, whereas for micro-crystalline films similar modes showed maximum intensities at 13 eV with a smaller contribution at 8 eV. It is reported that the elastic electron reflectivity is substantially larger for the nano-diamond films, showing broad maxima at ∼ 8 and ∼ 13 eV in both cases. It is suggested that these effects are associated with the larger probability for inelastic scattering of electrons by lattice phonons mediated by scattering with internal grain boundaries and defects in the case of nano-crystalline diamond films.

Polymer surface and thin film vibrational dynamics of poly(methyl methacrylate), polybutadiene, and polystyrene

Inelastic helium atom scattering has been used to investigate the vibrational dynamics at the polymer vacuum interface of poly(methyl methacrylate), polystyrene, and polybutadiene thin films on SiO(x)Si(100). Experiments were performed for a large range of surface temperatures below and above the glass transition of these three polymers. The broad multiphonon feature that arises in the inelastic scattering spectra at surface temperatures between 175 and 500 K is indicative of the excitation of a continuum of surface vibrational modes. Similarities exist in the line shapes of the scattering spectra, indicating that helium atoms scatter from groups of similar mass on the surface of these polymer thin films. The line shapes obtained were further analyzed using a semiclassical scattering model. This study has shown that quite different polymer thin films can have similar interfacial dynamics at the topmost molecular layer.

Vibrational analysis of per-fluorinated-triamantane

Triamantane was isolated from petroleum, per-fluorinated and the vibrational behaviour investigated by experimental and theoretical methods. Close agreement was found between the experimental and calculated Raman spectrum. The surface vibrational modes are shifted to lower wavenumbers relative to hydrogenated triamantane, due to the increased mass of the surface groups. The Raman spectrum of the fluorinated molecule more closely resembles that of pure diamond compared with hydrogenated triamantane. The absence of a peak at 1150 cm −1 for fluorinated triamantane suggests that this peak, often seen in the spectrum of nanocrystalline diamond, cannot be attributed to vibrations of diamond nanocrystals.

Sum-frequency generation: Polarization surface spectroscopy analysis of the vibrational surface modes on the basal face of ice Ih

In recent years, sum-frequency generation (SFG) has been used to investigate numerous interfaces including aqueous interfaces. A longstanding challenge to interpretation of the SFG results, along with the related aqueous-solution infrared and Raman spectra, is a lack of connection between features in the broad hydrogen-bonded region and molecular-level interactions or configurations. This paper reports results of a newly developed polarization analysis of the generated sum-frequency signal as a function of wavelength both to deconvolute spectral resonances and to characterize the dynamic polarization associated with the resonances. Operationally, the polarization angle of the generated sum frequency is determined by identifying the null angle. The technique is hence termed polarization-angle null analysis or PAN. PAN applied to ice is very powerful; it reveals that the hydrogen-bonded region of the basal face of ice I(h) contains at least five oscillators, each with a distinct polarization. The dynamic polarizability of the longest wavelength oscillator is nearly entirely transverse (perpendicular to the surface normal, i.e., in the surface plane); in contrast, the shortest wavelength oscillator is almost entirely longitudinal (along the surface normal).

Heat capacity of silicon carbide at low temperatures

The heat capacity of biomorphic silicon carbide, a high-porosity material with specific cellular pores, is measured in the temperature range 3.5–60 K. Biomorphic silicon carbide is prepared by the chemical removal of excess silicon from the SiC/Si biomorphic composite, a product of eucalyptus wood. It is shown that the major contribution to the heat capacity of biomorphic SiC comes from surface vibrational modes.

The role of quantum confinement and surface chemistry in silicon nanocrystals at the strong confinement regime

AbstractUsing infrared photo‐induced absorption (PIA) spectroscopy, we have found a remarkable correlation between inter‐sub‐level (ISL) optical transitions and the photoluminescence (PL) from silicon nanocrystals. Our results indicate that the quantized electronic sublevels of the nanocrystals are resonantly coupled to surface vibrational modes via a polarization field produced by coherent longitudinal polar vibrations. The model and the experimental results support our findings that the mechanism responsible for the efficient PL from silicon nanocrystals should be assigned to inhibition of nonradiative channels rather than enhancement of radiative transitions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Derivative Spectra of Very Low Energy Photoelectrons from CO/Cu(001) Surface Obtained by a Lock-in Technique

We have constructed a system for measuring the derivative spectra of very low energy photoelectrons using a lock-in technique. The main advantage of this system lies in determining the energy position of steps in the photoelectron spectra that arise from inelastic scattering of photoelectrons through the excitation of surface vibrational modes. With this system we have measured the derivative spectra of two isotopes, 12 C 16 O and 13 C 18 O adsorbed on Cu(001), and observed the small isotopic shift of ∼1 meV. This result demonstrates that one can precisely determine the vibrational energy of surface adsorbates by using the present measurement system.