Surface Vibrational Modes Research Articles

High frequency surface vibrational modes and relaxation of MgO nanocrystals.

We present a computational proof for the puzzling experimental vibrational density of states in MgO nanocrystals measured by neutron scattering. For the first time, the experimental peak of longitudinal optical modes in the high frequency region is theoretically reproduced and traced back to surface inward relaxation. Practically perfect agreement of theory with the experiment demonstrates the importance of nanocrystal size effects. Dependence of the transversal optical model on the volume/surface ratio is also verified. Strong (up to 2%) buckling of nanocrystal faces is found. Our calculations show varieties of relaxation shapes for clusters, shedding light on contradictory data on nanocrystals and infinite surfaces.

Polar surface vibration strips on GaN/AlN quantum dots and their interaction with confined electrons

The conditions are found for the existence of polar surface vibrational modes on a spheroidal quantum dot. These conditions determine allowed windows in the frequency-surface coordinate plane. The modes found are either truly localized of leaky states. The latter can provide effective energy relaxation of the confined electrons.

Propagation of acoustic waves through finite solid superlattices

The transmission of longitudinal waves through solid superlattices is studied. The waves income from a fluid or a solid and the transmission medium is a fluid. It is found that the transmission spectrum is closely related to the free-stress surface vibrational modes in the superlattice slab. The correspondence of the transmission peaks with these modes is better when the impedance contrast between the external medium and the superlattice is high. Results for normal and oblique waves in a superlattice of Al/W embedded in Hg, water, or air are presented. As the impedance of the external medium changes it is found new minigaps for the wave transmission inside the allowed frequency region for bulk propagation. Additional structure to the transmission spectrum is produced by localized excitations. Transmission with frequencies in the gap is assisted by the occurrence of surface waves and enhanced transmission is observed inside the frequency bands due to the presence of surface resonances. Theoretical results for a Pt/W superlattice are presented. B.M-M. is grateful to CONACyT Mexico, for a doctorate scholarship. [Work supported by CONACyT Mexico, Grant No. 35541-E.]

Surface phonons and other localized excitations

The diatomic linear chain of masses coupled by harmonic springs is a textbook model for vibrational normal modes (phonons) in crystals. In addition to propagating acoustic and optic branches, this model is known to support a “gap mode” localized at the surface, provided the atom at the surface has light rather than heavy mass. An elementary argument is given which explains this mode and provides values for the frequency and localization length. By reinterpreting this mode in different ways, we obtain the frequency and localization length for three other interesting modes: (1) the surface vibrational mode of a light mass impurity at the surface of a monatomic chain; (2) the localized vibrational mode of a stacking fault in a diatomic chain; and (3) the localized vibrational mode of a light mass impurity in a monatomic chain.

Reconstruction of the (001) Surface of Potassium Tantalate

We investigate the atomic structure and vibrational states of the KTaO3(001) surface by means of density-functional calculations. To account for extended core states present in oxide perovskites, we use the full-potential linearized augmented plane-wave method. The (001) surface of KTaO3 is described within the slab-supercell approach. We perform computations for different structural models which are considered in the light of the autocompensation principle and electron-counting rules. A low-energy structure is identified, for which the atomic positions are entirely relaxed with respect to the forces acting on the atoms. Surface vibrational modes are studied with the help of the frozen-phonon method. Our investigations are focused on providing an interpretation for the data recently obtained in helium-atom scattering experiments. The measurements have revealed that the (001) surface of KTaO3 exhibits a higher-order reconstruction which shows surface vibrational features similar to those of the (1×1) surface.

Nanophase alumina synthesis in thermal arc plasma and characterization: correlation to gas-phase studies

Nanophase alumina (Al 2O 3) was synthesized in a d.c. arc plasma reactor under isochronal oxygen flow conditions. Transmission electron microscopy revealed spherical particles (50 nm) and the corresponding electron diffraction showed a δ-Al 2O 3 phase. Structural morphology by X-ray diffraction (XRD) evidenced the phase changes. These measurements confirmed the existence of the nanophase structure. X-ray photoelectron spectrsocopy (XPS) of the core levels of as-deposited, and calcined powders were carried out for comparative study. Zeta potential as determined from electrophoretic mobility measurements, at a pH value of 4, indicated a high value (+42.7 mV) for the nanophase alumina as compared to that for the commercial alumina (−14.1 mV). This is a measure of the state of agglomeration, which is higher for the nanophase alumina. Fourier transform infrared spectra of the alumina powder revealed a broad band from 500 to 1000 cm −1, indicative of the complex Al–O vibration due to interactions of the octahedral and tetrahedral coordination groups. The FTIR spectra also revealed the intermediate route leading to alumina formation, as seen from the presence of gas-phase type sub-oxide bands. In addition the presence of a strained surface vibrational mode retaining itself even after complete transformation to α-Al 2O 3 is confirmed. Optical absorption spectroscopy was studied yielding a band gap for the nanophase alumina of >5 eV. The Infrared and the absorption spectrum is characteristic of a gas-phase type of reaction. Thus a dimensional evolution starting from a gas-phase precursor, which is molecular in nature, leads to a collisionally quenched structure resulting in very fine particulates condensing from the plasma.

Optical investigation of CdS quantum dots in Langmuir-Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements.

Comparison of cluster and slab models of the surface structure of Cl-terminated Ge(111) and GaAs(111) surfaces

The surface reconstruction and bonding of Cl atoms on two semiconductor surfaces, namely, Ge(111) and GaAs(111), are studied using first-principles energy-minimization methods. We use (a) specially designed clusters which mimic the semiconductor-surface geometry and the GAUSSIAN quantum-chemistry program at various levels of basis-set refinement, and (b) slab calculations using plane-waves, pseudopotentials, and density-functional theory within the local-density approximation (LDA). We also calculate the vibrational spectra of the surface atoms and the Raman spectrum of Ge(111)Cl for comparison with future experiments. The surface vibrational modes predicted by the slab models are at least 30 ${\mathrm{cm}}^{\ensuremath{-}1}$ smaller than from the cluster models. The best cluster model tends to overestimate the Ge-Cl and Ga-Cl bond length by about 4%, while the LDA slab model underestimates the bond lengths by 1.8%. Very little reconstruction is found in the bond lengths of the ``substrate'' as modeled by the cluster, and in fact tend to overestimate the ``bulk'' bond lengths. In contrast, the slab calculation shows considerable reconstruction of the near-surface layers, shrinking the layer separations by pulling the surface toward the interior of the crystal.

Study of the chemistry of NH 3 on aluminum nitride and oxynitride under steady-state conditions using external-reflection infrared spectroscopy

Polarization-modulated Fourier-transform infrared reflection absorption spectroscopy has been applied to the characterization of thin films of AlN and Al oxynitride (‘AlON’) on NiAl(111) and to the observation of surface chemical processes under steady-state conditions in a static NH 3 ambient of up to 200 Torr. AlN films, grown by dosing NiAl(111) with NH 3 at ∼1230 K, are found to be structurally inhomogeneous and largely disordered, with no evidence of chemisorption at 300 K in a background of 200 Torr of NH 3. AlON films, grown by dosing NiAl(111) with NO near 300 K and annealing to 1000 K, appear more homogeneous and show distinct surface and bulk vibrational modes. The former responds reversibly to chemisorbed NH 3 which can be detected under steady-state conditions by the appearance of the δ s symmetric deformation mode. In comparison to similar results for Al 2O 3, the δ s frequency indicates a lesser degree of Lewis acidity for the AlON surface.

Surface vibrational modes and reflection times of phonons in finite-size superlattices

We study theoretically the interaction of vibrational modes localized at a surface of a superlattice with acoustic phonons injected from a substrate. We calculate both numerically and analytically the phase time of the phonons reflected from the superlattice. We find sharp enhancements in the phase time, suggesting their strong interactions. The results indicate that the existence of the surface vibrational modes can be examined by a time-resolved phonon reflection experiment.

Surface Vibrational Modes and Reflection Time of Phonons in a Finite-Size Superlattice.

We discuss the interaction of bulk phonons with surface vibrational modes in a finite-size superlattice. The incident phonons injected normally on the superlattice from a substrate are perfectly reflected, i.e., the reflection ratee is unity irrespective of frequency. However, these phonons come back to the substrate with a large time delay when the frequency coincides with an eigenfrequency of the surface mode. This is due to the resonant interaction of the incident phonons with a surface vibrational mode. The results suggest that the surface vibrational modes are detectable from the other side of the surface by a time-resolved phonon reflection experiment.

Surface Phonon Spectra of Mineral TiO2 Anatase (001) and (100)

The surface properties of TiO2 anatase are of great importance in chemical technology because this material is frequently used as a support for oxide catalysts. In the framework of an extensive study of the system V2O5/TiO2 anatase, the vibrational properties of single crystal anatase (001) and (100) have been examined by means of HREELS. Both of these surfaces are believed to provide a good lattice match with V2O5(001). Single crystal anatase samples were obtained by orienting and sawing natural minerals. The resulting samples were yellowish transparent and gave rise to a LEED image after adequate UHV cleaning. Despite the insulating character of TiO2 anatase, HREELS could be performed without charging problems, due to the natural contamination. Energy loss spectra were recorded in the range −140 to 40 meV, with an instrument resolution of about 3 meV, and displayed surface vibrational modes.

Surface phonons ofAs:Si(111)−(1×1)andAs:Si(001)−(2×1)

We present calculated surface-phonon spectra of $\mathrm{Si}(111)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ and $\mathrm{Si}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces covered by a full monolayer of As adatoms. The calculations are based on a semiempirical total-energy ansatz. We have extended our previous approach by determining the parameters of the ansatz using input data from our accompanying first-principles atomic and electronic structure calculations. Our analysis yields a number of salient vibrational surface modes. For the $\mathrm{A}\mathrm{s}:\mathrm{S}\mathrm{i}(111)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ surface, these are found to be in very good agreement with available experimental and other theoretical data. For the $\mathrm{A}\mathrm{s}:\mathrm{S}\mathrm{i}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface, respective data are still lacking. Our predictions, therefore, will hopefully stimulate future experimental investigations of the latter surface and are expected to turn out very useful for a quantitative interpretation of respective data.

Surface phonons of H:Si(110)-(1×1)

We present the surface-phonon spectrum of the hydrogen-terminated Si(110)-(1$\ifmmode\times\else\texttimes\fi{}$1) surface calculated using a semiempirical total-energy approach. We obtain a number of salient surface vibrational modes whose physical origin and nature is identified and which are found in gratifying agreement with recently reported experimental data.

Surface vibrational mode of ZnS nanoparticles

Raman scattering and infrared absorption from ZnS nanoparticles were measured. A broad peak was observed in Raman and infrared absorption spectra which can be attributed to the surface vibrational mode of ZnS nanoparticles.

Direct subsurface absorption of hydrogen on Pd(111): Quantum mechanical calculations on a new two-dimensional potential energy surface

We have calculated a two-dimensional (2D) potential energy surface (PES) for H2 interacting with a Pd(111) surface. The geometry considered is for H2 approaching a bridge site and dissociating into neighboring hollow sites and the subsurface sites directly below these. Density functional calculations were performed using both the local density approximation (LDA) and the generalized gradient approximation (GGA). The LDA PES gives the usual overbinding and shows no barrier (relative to the bottom of the H2 potential) to subsurface absorption, while the GGA PES agrees with the experimental adsorption energies and has a large barrier. We have performed quantum mechanical wave packet calculations on the GGA PES to obtain the direct subsurface absorption probability. We have also calculated the barrier height’s dependence on a coordinate that can be associated with a local surface vibrational mode and the results suggest that this degree of freedom should be taken into account in the dynamical calculations.

Vibrations localized near surfaces and interfaces in nontraditional crystals

This survey is devoted to studying some peculiar features of various surface and interface vibrational modes in strongly anisotropic and other nontraditional crystals. The group of the nontraditional crystals includes such anisotropic systems as layered and chain-type crystals, complex compounds of the ferroelastic and ferroelectric types in the vicinity of structural phase transitions and many compounds with weak interatomic bonds in some crystallographic directions or in certain planes. The majority of such materials can be considered as quasi-low-dimensional systems. It is shown that the presence of the weak interatomic bonds in the nontraditional crystals causes a considerable large effect of anharmonicity of the vibrations on properties of the longwavelength surface and interface modes. New types of deeply penetrating surface waves are analyzed in anisotropic crystals and crystals near structural phase transitions, and in nonlinear crystals. The contribution of capillary effects to the peculiarities of the surface and interface waves in the anisotropic crystals is estimated. The effect of the strong elastic anisotropy on the properties of the ordinary and generalized Rayleigh waves and the shear surface waves is described analitically. A possible connection of the geometry of the isofrequency surfaces (the inverse velocity surfaces) with the existence of the generalized surface waves is discussed. The longwave dynamics of twins in the matrix of layered crystals, such as high T c superconductors are also considered. In such systems, the dispersion relation for the interface elastic waves have the form of ω ~ √k, characteristic of the spectrum of two-dimensional plasmons.

Vibrational dynamics and thermodynamics of Ni(977)

We present an analysis of the vibrational density of states and characteristics of some surface vibrational modes of Ni(977), using a real space Green’s function approach with force constants derived from interaction potentials based on the embedded atom method. Changes in the force fields at and near the steps are found to lead to both softening and stiffening of the force constants between the surface atoms, as compared to those on Ni(111). Among the striking features of this vicinal, is a quasi-one-dimensional mode at 3.3 THz that is super localized at the step and a mode at 9.1 THz that has a propagation direction peculiar to this surface. Further, from an examination of the surface thermodynamics of Ni(977) it is seen that only the step and the corner atoms display features that are distinct from those on Ni(111).

STM-HREELS Investigation of C60 on Cu(111)

Using an STM/HREELS system, we have investigated the relationship between the surface morphology and vibrational modes of C 60 adsorbed on Cu(111). At 300K, C 60 is mobile on the Cu(111) terrace, but is more strongly bound at the step edge. Therefore, any C 60 which adsorbs on the terrace will migrate to the step edge where it is immobilized. Thus for very low coverages, the C 60 preferentially adsorbs at the step edge. Adsorption of the molecule on metal surfaces results in a significant amount of charge transfer from the substrate to the C 60 molecule. Two of the four t 1u infrared active modes are predicted by theory to shift to lower frequency as the amount of charge transfer increases. However, only a softening of the lowest frequency t 1u mode was observed. Furthermore, no further shifting of the frequency of this mode was seen for very low coverages, indicating that any additional charge transfer at the step edge is smaller than one electron per C 60 molecule.

Resonant interaction of phonons with surface vibrational modes in a finite-size superlattice.

We study theoretically the interaction of phonons with surface vibrational modes in a finite-size superlattice with a free surface. A phonon incident normally on the superlattice from a substrate is perfectly reflected, i.e., the reflection rate is unity irrespective of frequency. However, it comes back to the substrate with a large time delay when the frequency coincides with an eigenfrequency of the surface mode. This result is attributable to the resonant interaction of incident phonons with a vibrational mode localized near the surface. \textcopyright{} 1996 The American Physical Society.