H. Ueba

Heteroepitaxial growth of Ge films on the Si(100)‐2×1 surface

The initial stage of Ge heteroepitaxy on a Si(100)‐2×1 surface has been investigated by low‐energy electron diffraction (LEED) and Auger‐electron spectroscopy (AES). The growth mode of the Ge films was studied by measuring the decrease in the Si(LVV) AES line at 92 eV with an increase in the Ge overlayer thickness. The Ge films deposited at room temperature exhibit layer‐by‐layer growth up to at least six monolayers. When the substrate is heated up to 350 °C, the growth mode is characterized by the Stranski–Krastanov type; i.e., the first three monolayers of growth is followed by island formation. Although these characteristics of the growth mechanism are similar to the case of Ge on Si(111)‐7×7 surfaces, annealing behavior of the Ge films suggests that the bond strength between Ge and Si is stronger on Si(100) than on Si(111) surfaces. In contrast to the case of Ge on Si(111) surfaces, where the original 7×7 superstructure of the Si surface is replaced by a new 5×5 pattern at about two‐monolayer coverage...

Crack propagation in rubber-like materials

Crack propagation in rubber-like materials is of great practical importance but still not well understood. We study the contribution to the crack propagation energy (per unit area) G from the viscoelastic deformations of the rubber in front of the propagating crack tip. We show that G takes the standard form G(v,T) = G0[1+f(v,T)] where G0 is associated with the (complex) bond-breaking processes at the crack tip while f(v,T) is determined by the viscoelastic energy dissipation in front of the crack tip. As applications, we discuss the role of crack propagation for adhesion, rolling resistance and sliding friction for smooth surfaces, and for rubber wear.

Unified description of inelastic propensity rules for electron transport through nanoscale junctions

We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic and vibrational symmetries in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states--namely those belonging to the most transmitting eigenchannels--need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.

H-atom relay reactions in real space

Hydrogen bonds are the path through which protons and hydrogen atoms can be transferred between molecules. The relay mechanism, in which H-atom transfer occurs in a sequential fashion along hydrogen bonds, plays an essential role in many functional compounds. Here we use the scanning tunnelling microscope to construct and operate a test-bed for real-space observation of H-atom relay reactions at a single-molecule level. We demonstrate that the transfer of H-atoms along hydrogen-bonded chains assembled on a Cu(110) surface is controllable and reversible, and is triggered by excitation of molecular vibrations induced by inelastic tunnelling electrons. The experimental findings are rationalized by ab initio calculations for adsorption geometry, active vibrational modes and reaction pathway, to reach a detailed microscopic picture of the elementary processes.

Vibrational relaxation and pump-probe spectroscopies of adsorbates on solid surfaces

Abstract Recent experimental and theoretical progress in nonlinear pump-probe spectroscopies for vibrational relaxation of adsorbates on solid surfaces is reviewed. Among the extensive body of experimental works, typical examples of conventional infrared absorption spectra of hydrogen atoms on Si surfaces and carbon monoxide molecules chemisorbed on metal surfaces are presented to demonstrate how the mechanism of the vibrational relaxation and the corresponding relaxation time are inferred from the observed linewidth. The theoretical foundation of the pump-probe surface vibrational spectroscopy, which allows a direct determination of the relaxation time is presented on the basis of the density-matrix formulation. In the limit, where the time envelope of the pump and probe field is given by the 5-function, the analytical expressions are derived for the so-called transient absorption difference spectrum and sum-frequency-generation, which directly measure the energy (population) relaxation time T 1 and the total dephasing time T 2 of the vibrational excited states of adsorbates. Effects of nonlinear coherent interaction between pump and probe pulses are discussed for pulse durations shorter than the vibrational dephasing time. Experimental results of various transient adsorbate vibrational spectra in frequency and time domains are presented and discussed in comparison with the relaxation time estimated from the analysis of infrared absorption spectra of adsorbates. Vibrational dynamics of the adsorbate low-frequency mode, which plays an important role as an exchange partner in the dephasing process of high-frequency mode, is also reviewed in conjunction with transient vibrational response to ultrafast substrate heating by visible pulse irradiation.

Theory of charge transfer excitation in surface enhanced Raman scattering

Abstract On the basis of a recently proposed energy diagram of pyridine chemisorbed on Ag, a simple theory is presented to calculate the Raman scattering polarizability of an adsorbed molecule on a metal surface, where charge transfer excitation participates as intermediate state of the Raman process. The charge transfer excitation is assumed to occur from the metal to an unoccupied electron affinity level of the adsorbed molecule. Using a simple version of the Newns-Anderson model of chemisorption, it is found that the Raman polarizability is substantially enhanced when the frequencies of the incident or Raman scattered photons become resonant with the charge transfer excitations of the coupled system. The degree of enhancement is found to be sensitive to position and broadening of the charge transfer state and also to the width of the electronic band of the metal. A broadened Raman line is also expected due to the modified electron-phonon interaction of the adsorbed molecule.

Molecular Beam Epitaxy of SrTiO3 Films on Si(100)-2×1 with SrO Buffer Layer

Molecular beam epitaxy of SrTiO3 thin films on a Si(100)-2×1 surface has been studied using reflection high-energy electron diffraction (RHEED), an atomic force microscope (AFM) and X-ray diffraction (XRD) as a function of the thickness of the SrO buffer layer and growth temperature. Epitaxial SrTiO3 films do not grow directly on Si(100). Therefore, a SrO buffer layer was applied to grow SrTiO3 on Si(100). It was found that the SrO layer with a thickness of 100 A grown at 300 – 400°C in oxygen atmosphere of 5 ×10-8 Torr was sufficient to grow epitaxial SrTiO3 on Si(100). Then SrTiO3 thin films with a thickness of 2000 A were grown on the SrO(100A)/Si surface at 400 – 700°C using codeposition of strontium and titanium in oxygen atmosphere of 8×10-8 Torr. At 500°C, the best-quality SrTiO3(100) film grew parallel to Si(100), and numerous rectangular platelike crystals were observed on the surface in the AFM image. The crystallinity of the STO films was improved with increasing thickness of the SrO layer. The epitaxial relation between SrTiO3 and SrO/Si(100)-2×1 is also discussed using the RHEED patterns which show streaks and spots.

Chemical effects on vibrational properties of adsorbed molecules on metal surfaces: Coverage dependence

Vibrational properties of chemisorbed molecules on metal surfaces are studied with a focus on the coverage dependent chemical shift of the frequencies. Available experimental data of a CO adsorption on transition metal and noble metal surfaces are analyzed in the light of the coverage dependent back-donation into the 2 π * orbitals of chemisorbed CO molecules. The vibrational frequency ω CO of the intramolecular stretching mode exhibits a downward shift of varying magnitude, depending on the amount of back-donation into the 2 π * orbitals of the chemisorbed CO. On increasing the coverage θ, ω CO usually increases due to the dipole-dipole interaction. On Cu surfaces, however, the shifts are relatively small, or in some cases, negative. So far, this anomalous frequency shift with θ is understood as a result of competitive effect between the upward dipole Ω dip and the downward chemical shift Ω chem associated with back-donation. The purpose of this paper is to establish the possible origin of the downward frequency shift through the electronic properties of an incomplete monolayer of adsorbates. The adsorbate density of states ρ a is calculated by means of the coherent potential approximation, in which the electron hopping between the adsorbates (band formation effect) and the depolarization effect due to the proximity of ionized adsorbed molecules are taken into account. The change of the occupied portion of ρ a and ρ a ( e F ) at the Fermi level e F with increasing θ then manifests itself in the coverage dependent Ω chem not only due to the static back-donation, but also due to the dynamical charge fluctuation during vibrational excitation. It is found that in a weakly chemisorbed system, such as CO/Cu, the negative Ω chem amounts to Ω dip at low θ . Consequently the apparent total frequency shift remains almost constant. As the coverage increases, Ω chem becomes larger than Ω dip due to the band effect. It is also shown that the variation of the back-donated charge with θ gives rise to the coverage dependent polarizability, which in turn influences the frequency shift estimated by the preiously dipole coupling theory. In the case of a strongly chemisorbed system, the molecules become negatively charged as a result of large back-donation. Depolarization fields, due to the increasing number of the adsorbed molecules carrying a negative charge, shift up the adsorbate energy level so that the occupation in ρ a decreases with θ . This leads to the positive Ω chem for strongly chemisorbed molecules, and explains why the coverage dependence of Ω CO of strongly chemisorbed CO on transition metal surfaces can be satisfactorily understood in terms of the dipole coupling theory. The coverage dependent back-donation also plays a significant role in the work function change Δ φ of the substrate upon CO adsorption. The polarity of a weakly chemisorbed CO molecule remains unchanged compared to free CO( − CO + ) so that Δ φ exhibits the initial lowering in the presence of the positive dipoles. The increase in the back-donated charge with θ causes the decrease in the effective dipole moment towards a compensation of the positive hole due to 5 σ donation. On the other hand, for a strongly chemisorbed CO, the polarity changes reversely as + CO − due to large back-donation, and Δ φ increases almost linearly up to a certain coverage where the sticking probability drops rapidly. A simple explanation is offered to clarify the characteristic difference of the work function change between the strongly and weakly chemisorbed CO molecules on metal surfaces. In particular, a possible origin of the work function minimum observed for CO/Cu systems is discussed in terms of the coverage dependent backdonation.

Electronic friction and liquid-flow-induced voltage in nanotubes

A recent exciting experiment by Ghosh et al. [Science 299, 1042 (2003)] reported that the flow of an ion-containing liquid such as water through bundles of single-walled carbon nanotubes induces a voltage in the nanotubes that grows logarithmically with the flow velocity

Vibrational state of the chemisorbed molecule on metal surfaces: Role of electron‐hole pair excitation

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