Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.     

Model pseudopotential for the Cu(110) surface

A method has been proposed for constructing the two-dimensional pseudopotential describing the electronic structure of the Cu(110) surface. This method can also be applied to construct the corresponding pseudopotentials for the (110) surface of a number of other face-centered cubic metals, such as Ag, Au, Al, Pd, and Pt. The electronic structure obtained can be used for fast calculations of single-particle and collective electron excitations both on the pure Cu(110) surface and on the surface covered with adatoms or ultrathin films of other metals.     

Symmetric versus nonsymmetric structure of the phosphorus vacancy on InP(110)

The atomic and electronic structure of positively charged P vacancies on InP(110) surfaces is determined by combining scanning tunneling microscopy, photoelectron spectroscopy, and density-functional theory calculations. The vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge transfer level 0.75+/-0.1 eV above the valence band maximum. The scanning tunneling microscopy (STM) images show only a time average of two degenerate geometries, due to a thermal flip motion between the mirror configurations. This leads to an apparently symmetric STM image, although the ground state atomic structure is nonsymmetric.     

Quantum-well states and resonances in thin single-crystal layers of noble metals on W(110) substrates

This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy.     

Adsorbate dynamics induced by STM

Microscopic models to describe adsorbate dynamics induced by STM, e.g. the dynamics of STM-induced desorption of CO from Cu(111) and the dynamics of STM-induced rotation of acetylene on Cu(100) are presented. In these models, the relation between the change of the electronic state caused by an electron tunneling from an STM tip and the transition of the vibrational state for the molecular/intramolecular motion is taken into account from a microscopic point of view. Calculated probabilities for inducing desorption and rotation in these models agree with recent experimental results.     

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.     

Electron-vibron coupling in high-resolution X-Ray absorption spectra of organic materials: NTCDA on Ag(111)

We report additional rich fine structures in high-resolution near-edge x-ray-absorption fine structure (NEXAFS) spectra of large organic molecules using NTCDA on Ag(111) as an example. These fine structures are completely interpreted as vibronic coupling to electronic core excitations. The coupling is mode selective; predominantly one vibronic mode couples to each excitation. The fit results suggest the occurrence of a Davydov splitting, first observed for core excitons. Morphological differences substantially influence the electron-vibron coupling, indicating a strong intermolecular interaction. Thus NEXAFS becomes a more subtle probe for organic solids.     

Adsorbate motions induced by vibrational mode coupling

Adsorbate motions are discussed with a primary attention focused on the coupling between a vibrational mode excited by ultrafast laser heated hot-electrons or by inelastic tunneling electrons with scanning tunneling microscope and the reaction coordinate (RC) mode. Recent experimental results have demonstrated an efficient reaction pathways involving an indirect excitation of a frustrated translational mode, rather than its direct excitation for adsorbate hopping on surfaces. Elementary processes are briefly described for hopping of CO molecules on a laser heated stepped Pt surface, where excitation of the frustrated rotation mode has been found to plays an indispensable. Calculation of the inelastic tunneling current (ITC) for excitation of the C-O stretch mode of a CO molecule is combined with a theory of anharmonic mode coupling to activate the frustrated translation mode above the barrier. The hopping rate as a function of the bias voltage agrees with the experimental result. An unified theory of single-, and two-electron processes for ITC-induced motions induced by an indirect excitation of the RC-mode via mode coupling is also applied to reproduce a crossover from hopping to desorption of a single NH₃ molecule on Cu(1 0 0) with an increase in the tunneling current.     

Energy barriers of single-adatoms diffusion on unreconstructed and reconstructed (110) surfaces

The present paper is aimed mainly to investigate theoretically the diffusion of Ag, Cu, Au and Pt adatoms on the (1 × 1) unreconstructed geometry for Ag, Cu and Pt (110), and reconstructed geometries ((1 × 2), (1 × 3) and (1 × 4)) for Pt and Au (110) surfaces. We consider the single adatom diffusion when additional atoms are deposited in adjacent row. For this study, we have used the molecular statics simulations combined with the embedded atom method. For several systems, we have calculated the activation barriers for hopping mechanism. For the diffusion on the unreconstructed surfaces, the trends for the activation barriers are the same for all considered systems except for Cu/Ag (110) system, where the activation barrier do not change. Further, our results indicate that additional atoms lead to a small decreasing of activation barriers for diffusion on reconstructed surfaces for some systems, while for other systems; the activation barrier remains practically unchanged.     

New mechanism for single atom manipulation

Scanning tunneling microscope (STM) investigations of the step roughening of Ag(110) have shown that the STM tip extracts atoms from otherwise stable steps even at typical imaging conditions. Detailed analyses of single STM scans reveal that none of the so far known lateral manipulation mechanisms (pushing, pulling, sliding) account for the observed atom extraction. The Ag atoms rather follow the energetically favorable path of a tip induced exchange process, similar to the concerted motion proposed previously for the diffusion on fcc(110) surfaces including a metastable and thus experimentally detectable dumbbell transition state.     

On different mechanisms of electron-phonon scattering of electron and hole excitations on an Ag(110) surface

We present the results of a detailed theoretical study of the electron-phonon scattering of electron and hole excitations in the unoccupied and occupied surface states on an Ag(110) surface. We show that the electron-phonon coupling parameter λ in the unoccupied surface state is approximately three times smaller than that in the occupied one, because the scattering of these states is determined by different phonon modes. The difference in the phonon-induced decay mechanisms of electron and hole excitations is determined by different spatial localizations of the unoccupied and occupied surface states at the $ \overline Y $ \overline Y point of the two-dimensional Brillouin zone.     

Model studies of the structure and optical properties of the TiO2(110) surface with an adsorbed Ag atom

ABSTRACT

The present studies of the atomic Ag adsorbate on the substrate TiO₂(110) explore the importance of dispersion (or van der Waals) energies for determining the structure of the adsorbed Ag atom, using density functional theory (DFT) supplemented by a dispersion energy treatment, within the PBE-D3 treatment. It is also of interest to explore electronic excitation by light absorption. Electronic density of states (EDOS) are obtained without and with Ag adsorbed on the TiO₂(110), to find the extent of change on the density of valence, conduction and intraband states. This is done using the hybrid HSE06 functional, which is known to provide good values for the energy band gap of the substrate. A computationally efficient PBE?+?BG procedure for these structures, which corrects the PBE band gap, is implemented to generate accurate EDOSs and light absorption intensities versus photon energies. This is followed by a reduced density matrix treatment of the dissipative dynamics of light absorption, generating state-to-state oscillator strengths and photoabsorbances for the pure and nanostructured TiO₂(110) surfaces. Adsorption of Ag leads to a noticeable increase in light absorption at visible wavelengths, and very large increases in the UV region of the spectrum.     

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.     

Origin of spin-orbit splitting for monolayers of au and ag on w(110) and mo(110)

Spin-orbit coupling can give rise to spin-split electronic states without a ferromagnet or an external magnetic field. We create large spin-orbit splittings in a Au and Ag monolayer on W(110) and show that the size of the splitting does not depend on the atomic number of the Au or Ag overlayer but of the W substrate. Spin- and angle-resolved photoemission and Fermi-surface scans reveal that the overlayer states acquire spin polarization through spin-dependent overlayer-substrate hybridization.     

Substrate-induced spin-orbit splitting of quantum-well and interface states in Au,Ag, and Cu layers of different thicknesses on W(110) and Mo(110) surfaces

The substrate-induced spin-orbit splitting of interface and quantum-well states formed in Au, Ag, and Cu layers on W(110) and Mo(110) surfaces has been revealed using angle- and spin-resolved photoelectron spectroscopy. It has been shown that the magnitude of the splitting depends noticeably on the atomic number of the substrate material and is markedly larger for layers of these metals on W(110), i.e., on the surface of a metal with a larger atomic number (Z _W = 74), than on the surface of Mo(110), i.e., an element with a smaller atomic number (Z _Mo = 42), while depending only weakly on the atomic number of the adsorbed metal. Measurements of the dispersion of the formed quantum-well states have shown that the substrate-induced spin-orbit splitting increases with increasing parallel component of the photoelectron momentum (which correlates with the Rashba model) for all thicknesses of deposited films (up to 10 ML). The magnitude of induced spin-orbit splitting of the interface states evolving in monolayer Au, Ag, and Cu coatings on W(110) and Mo(110) decreases with increasing parallel component of the excited photoelectron momentum.     

The influence of oxygen on the growth of silver on Cu(110)

The influence of the (2 × 1)O reconstruction on the growth of Ag on a Cu(110) surface was studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). On the bare Cu(110) surface, Stranski–Krastanov growth of silver is observed at sample temperatures between 277 K and 500 K: The formation of a Ag wetting layer is followed by the growth of three-dimensional Ag wires. In contrast, on the oxygen-precovered Cu(110) surface, the growth of silver depends heavily on the substrate temperature. Upon Ag deposition at room temperature, a homogeneous, polycrystalline Ag layer is observed, whereas at 500 K, three-dimensional wires separated by (2 × 1)O reconstructed areas are formed. The behavior of a deposited Ag layer upon annealing is also influenced greatly by the presence of oxygen. On the bare surface, annealing does not change the Ag wetting layer and gives rise to Ostwald ripening of the Ag wires. On the oxygen-precovered surface, however, the initial polycrystalline Aglayer first transforms into Ag wires at around 500 K. Above this temperature, the depletion of the (2 × 1)O reconstructed areas due to Ag-induced O desorption is balanced by the formation of a Ag wetting layer. On both, the bare and the oxygen-precovered Cu(110) surface, the deposited silver diffuses into the Cu bulk at temperatures above 700 K.     

Ag-induced atomic structures on the Si(110) surface

We report the results of STM investigation of the initial stage of Ag adsorption on an Si(110) surface. At 0.21 ML Ag coverage, the size and orientation of the unit cell correspond to the parameters of a 16 × 2 unit cell of clean Si(110) surface. With increasing of the Ag coverage up to 0.42 ML, the type of surface reconstruction changes to a 4 × 1-Si(110)-Ag structure. The text was submitted by the authors in English.     

Adsorption of CO on Ni-promoted TiO2(110)

The valence-band structure and the vibrational modes of CO adsorbed on nickel-promoted TiO₂(110) surfaces as a function of CO exposure have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy-loss spectroscopy (HREELS). It is found that CO exists in molecular form at room temperature on the nickel-promoted TiO₂(110) surfaces and most likely binds to the Ni atoms or nickel-affected sites rather than to the substrate atoms. At saturation coverage, CO molecules adsorb simultaneously on the 2-fold bridge sites and terminal sites on the (111)-oriented Ni islands deposited upon TiO₂(110). The occupation of the edge sites of Ni islands gives rise to an anomalously low frequency of the C---O stretching vibration. This frequency, indicative of a weakened C---O bond, suggests existence of a precursor to the dissociated state.     

Photoinduced structural instability of the InP (110)-(1x1) surface

A scanning tunneling microscopy study reveals the removal of P and In atoms at intrinsic surface sites of InP (110)-(1x1) through an electronic mechanism under ns-laser excitation. Femtosecond nonresonant ionization spectroscopy detects desorption of P and In atoms associated directly with the bond rupture, and shows their translational energies characteristic of electronic bong breaking. The rate of P-atom removal is 4 times higher than that of In-atom removal, revealing a prominent species-dependent effect of structural instability under electronic excitation on semiconductor surfaces.     

Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver metal surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab initio potential energy surface. The model is applied to study energy dissipation in the scattering of N(2) on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel, whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.