Adsorbate-covered Surfaces Research Articles

Tracking the surface atomic motion in a coherent phonon oscillation

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft x-ray pulses from the free electron laser FLASH. We collect Se $3d$ photoelectron diffraction patterns over a wide angular range from optically excited $\mathrm{Bi}{}_{2}\mathrm{Se}{}_{3}$ with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the ${A}_{1g}$ optical phonons. Using a fluence of $4.2\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$ from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 0.01 \AA{}.

Local effects during ion scattering on adsorbate-covered surfaces

This study addresses the importance of adsorbate-induced local effects during collisions between projectiles and adsorbate-covered surfaces. Various systems are investigated: H− on Na/Ag(111); H− on Na/Cu(111); and Li+ on K/Al(100), by using wave packet propagation methods. From the projectile's perspective, the adsorbate's presence on the surface can be felt from up to a few tens of atomic units away when the projectile is close to the surface, and up to 100 a.u or more when the projectile is on top of adsorbate. This is a well-defined bell-shape region around the adsorbate. However, the scattered ion/neutral fractions are not significantly affected unless the projectile impacts the surface in the close 10–15 a.u. vicinity of the adsorbate. Scattering from adsorbate sites compared to scattering from substrate sites produces very different ion/neutral fractions, that vary not only with the surface coverage, but especially with the distance from the projectile's point of impact on the surface to the adsorbate. Our calculated Li neutral fractions obtained after scattering Li+ from K/Al(100) are in satisfactory agreement with the experimental results.

Sipanje atomov helija – vsestranska tehnika za študij nanostruktur

In this paper we present helium atom scattering – a non-destructive technique used to study the surface structure and lattice dynamics (phonons) of clean and adsorbate-covered surfaces. We review the fundamentals of elastic and inelastic helium scattering theory and describe the basic components of the He scattering apparatus as well as its operation. We then demonstrate the use of the technique in three areas of surface analysis: clean surface structure determination of Ge(001) surface, study of overlayer lattice dynamics in Pb/Ge(001) thin film, and morphology/structure examination of organic overlayers in pentacene/Au(110).

Quantum-mechanical simulation of attosecond streaked photoemission from Mg-covered W(110) surfaces

SynopsisWe apply a quantum-mechanical model to simulate infrared-streaked photoelectron emission by an ultrashort extreme ultraviolet pulse from adsorbate-covered metal surfaces. Incorporating effects of energy- dependent electron mean-free paths, the properties of initial states, photoelectron energy dispersion, and the screening of the streaking field, this model is able to reproduce recently measured photoelectron spectrograms and adsorbate-thickness-dependent photoemission time delays.

Tunneling spectroscopy of clean and adsorbate-covered gold surfaces in humid air, measured with fast bias voltage ramps

The noise level of tunneling spectroscopic data can be significantly reduced by averaging the tunneling current over a large number of short bias voltage ramps, instead of recording over a single slow ramp. This effect is demonstrated for tunneling spectra of Au(111) by averaging over 200 consecutive bias voltage ramps, each 500μs long. We attribute the improvement of the data quality to the frequency dependence of the current noise spectral density. Due to mechanical vibrations and tip instabilities the noise density is usually much higher for low frequencies ca. <1kHz than for the high frequencies relevant for measuring with fast bias ramps. The high data quality allowed for the routine detection of the Au(111) surface state and the investigation of the influence of steps in humid air, i.e., with a water-covered tunneling gap. For a CN covered Au surface in the presence of water we unexpectedly found additional electronic density of states at positive energies, around 0.6eV, i.e., for unoccupied states. STS spectra of a 3×3R30° Cu-UPD layer, formed by adsorbed sulfate and Cu species, indicate tunneling via the sulfate electronic density of states.

Molecular recognition of PTCDI–C8 molecules on the Si(110)–(16×2) surface

Mechanisms of PTCDI–C8 adsorption and thin films growth on Si(1 1 0)–(16 × 2) have been studied using a combination of X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. Oxygen atoms of the PTCDI–C8 molecule interact with the substrate increasing substantially the adsorption energy. Adsorbed molecules do not form long-range ordered structures on the surface but local chain-like arrangements. A good agreement between the calculated isosurfaces of electronic charge density and the scanning tunneling microscopy images of the filled electron states of clean and adsorbate-covered surfaces allows to recognize the PTCDI–C8 molecules adsorbed on Si(1 1 0)–(16 × 2).

Simple model explaining and predicting coverage-dependent atomic adsorption energies on transition metal surfaces

The adsorption energies of simple atomic adsorbates are coverage dependent. We use density-functional theory to show that the coverage dependence is due to an adsorbate-induced modification of the surface $d$-band structure. We developed a simple model for predicting the $d$-band widths of clean and adsorbate-covered metallic surfaces using a tight-binding formalism. The new model can be used in conjunction with the $d$-band adsorption model to estimate adsorption energies as a function of coverage.

Jürgen Behm: Building Bridges from Surface Science to Catalysis and Fuel Cell Research

Jürgen Behm, to whom this special issue of ChemPhysChem is dedicated on the occasion of his 60th birthday, 1 started his career at one of the best places imaginable for a young surface scientist in the late seventies—the laboratory of 2007 Nobel Laureate Gerhard Ertl at the Ludwig Maximilians University in Munich. Jürgen Behm′s early research focused on the structure of H and CO adsorbate layers on transition metals and remains a classic work in surface science. However, early on he also developed a major interest in surface reactions, which he pursued during his post-doc sabbatical with Dick Brundle at the IBM Almaden Research Center. Although having worked successfully with a wide variety of techniques, he is arguably best known for scanning tunneling microscopy, notably building up one of the two first instruments of this type in Germany after returning to the Ertl group in 1984. After Jürgen Behm had accepted a professor position in Munich in 1987, the STM technique became his major work horse. In the following five years, a stream of high-impact publications on atomic-resolution studies of clean and adsorbate-covered metal surfaces as well as on chemical processes at semiconductor surfaces and electrochemical interfaces emerged. These publications have been cumulatively cited more than 5000 times by now. In 1992 Jürgen Behm was ready for the next big leap. Moving to the newly created chair of surface science and catalysis at the University Ulm, he set out to build a group that covered all aspects of heterogeneous reactions, from atomic-scale studies of the surface reactivity of planar model systems up to real supported catalysts. In particular, he focused on electrochemical and gas-phase reactions relevant for low-temperature fuel cells—a topic for which he is nowadays as well known as for STM. Throughout his career, Jürgen Behm’s work has been characterized by utmost scientific rigor as well as by a great willingness to cross boundaries between different approaches, such as fundamental studies and applied research, and different disciplines, such as UHV surface science and electrochemistry, resulting in more than 300 publications. The contributions in this issue, coming from the large international community of his former and current collaborators, reflect the breadth of his interests.

Electron–phonon coupling at surfaces and interfaces

Over the recent years, electronic surface states have been used for a detailed spectroscopic study of the electron–phonon (e–ph) interaction, both experimentally and theoretically. This review discusses the basic physics of e–ph coupling and how information can be obtained from angle-resolved photoemission experiments and first principles calculations. Several recent results for clean and adsorbate-covered surfaces, quantum wells and free-standing monolayers are also discussed.

Laser-assisted photoemission from adsorbate-covered metal surfaces: Time-resolved core-hole relaxation dynamics from sideband profiles

Illumination of an adsorbate-covered metal surface with an xuv and a delayed ir laser pulse can result in sidebands in the photoelectron (PE) spectra. We present a theoretical model for the delay-dependent PE spectra and show how the relaxation dynamics of xuv-induced core-level holes in adsorbate atoms can be deduced from the temporal shift between sideband peaks in the spectra of secondary adsorbate (Auger) electrons and conduction-band PEs from the substrate. Furthermore, in comparison with gaseous targets, we find a characteristic sideband-intensity enhancement in the laser-assisted photoemission from the substrate core-level bands. This sideband enhancement effect can be tested in experiments with tunable xuv wavelength. Our calculated PE spectra support time-resolved experiments for Xe-covered Pt(111) surfaces, promoting the direct analysis in the time domain of surface dynamical processes.

Effect of adsorbates on the formation of doubly excited He atoms during impact of He 2+ ions on a Ni(1 1 0) surface

The formation of doubly excited states of He atoms during impact of He 2+ ions with projectile energies of 60–1000 eV under near-grazing angles of incidence of 5°–20° on clean and adsorbate-covered Ni(1 1 0) surfaces is studied by means of Auger electron spectroscopy. Pronounced dependencies of electron spectra from autoionization of atoms in doubly excited 2 s 2, 2 s2 p and 2 p 2 configurations on the coverage of the target surface with adsorbates are observed. These are directly related to work function changes, which are studied for the controlled adsorption of oxygen. Changes of the electron spectra on the target temperature are found for adsorbate-covered surfaces only, which puts into question recent interpretations of similar electron spectra in terms of a high local electron spin polarization of Ni(1 1 0) by an alternative interpretation based on thermal desorption or dissolution into bulk of surface contaminations. The formation of doubly excited states is studied for the oxygen p(2 × l) and p(3 × l) superstructures on Ni(1 1 0) in order to provide well-defined experimental data for theoretical investigations.

Effect of thermally induced surface defects on the optical anisotropy of Ag(110)

We show that the temperature dependence of the intensity of the $\ensuremath{\sim}1.7\text{ }\text{eV}$ peak in the reflection anisotropy spectrum of clean Ag(110) can be explained by considering the effect of thermally induced surface defects and the thermal shift in binding energy of the occupied surface state involved. Two simple models are used to quantify the defect formation energy and the length scale over which the contribution to the intensity of this reflection anisotropy spectroscopy (RAS) feature is quenched due to the presence of thermal defects. We have simulated the decrease in the RAS peak intensity with increased temperature, achieving consistency with related measurements on ion-bombarded and adsorbate-covered surfaces, and previous estimates of the defect activation barrier for the Ag(110) surface. This study demonstrates the potential of RAS as a tool for monitoring surface kinetic behavior.

Electron dynamics in H −/Na/Cu(1 1 1) collisions

Surface adsorbates induce strong local perturbations in the electronic structure and potentials in their surroundings. Consequently, charge transfer processes between projectiles and adsorbate-covered surfaces are strongly affected. The theoretical calculations and experiment measurements reported herein are focused on the H −/Na/Cu(1 1 1) system. The electron dynamics at the Na/Cu(1 1 1) surface and the influence of Na adsorbates on the H −–Cu(1 1 1) charge transfer are treated and discussed in detail. The ion fractions are mainly influenced by the ion exit trajectories. At low Na coverage, they exhibit a maximum near the 60° exit angle from surface. The calculations and experimental data are in good agreement.

Raman spectroscopy of surfaces

Raman scattering has usually a very low efficiency. Therefore, during the first five decades after its discovery, Raman spectroscopic investigations of adsorbate-covered surfaces (except surfaces of highly porous samples) were out of reach. This changed in 1970s, when for molecules adsorbed on some surfaces, very large increase of the intensity of Raman spectrum (denoted as surface-enhanced Raman spectroscopy – SERS) was reported. In the past decade, two other very important achievements in surface Raman spectroscopy have been made: observation of SER spectrum of a single molecule and coupling of Raman spectroscope with the scanning probe microscope (STM or AFM) allowing a significant increase in the spatial resolution of Raman measurements in so-called tip-enhanced Raman spectroscopy (TERS). In the latter approach, fine tip made of a metal that supports surface plasmon resonances (such tip may be treated as a very local electromagnetic resonator) is brought at the nanometer distance above the surface, which induces large increase of the Raman scattering from molecules adsorbed at a surface located underneath the tip. This short review presents an overview of the state of the art and further possible applications of Raman spectroscopy in surface analysis. We mainly focus on SERS and TERS. Future prospects in these fields are also discussed.

Morphological Instability of Metallic Surfaces: Adsorbate Induced Nanoscale Faceting

AbstractHeterogeneous catalysis is a field of major interest in the surface science and development of new supported model catalysts with a narrow size distribution. The possibility to create narrow size distributions is to use the faceted surface structures as self assembled templates on which metal nanoclusters can grow. In the present study, new aspects of adsorbate induced faceting and nanoscale phenomena on adsorbate-covered metallic surfaces are studied on atomically-rough morphologically unstable metallic fcc (Rh) and hcp (Re) surfaces. Formation of oxygen induced faceting of atomically-rough Rh (210) surface has been studied and characterized by means of LEED, STM and AES. The LEED studies confirm the formation of three sided faceted (nanoscale pyramidal structure) Rh(210) surface when the oxygen covered Rh surface annealed to temperature higher than 550K. The facet orientations of the nanopyramid are characterized as two {731} face and a reconstructed (110) face. The excess oxygen overlayer can be removed from pyramidal faceted Rh(210) surface via catalytic reaction at low temperature using CO oxidation or H2 reaction while preserving the pyramidal structure. The average size of the nanopyramids is observed to be dependent on annealing temperature and vary from 12nm to 21nm. The atomically resolved STM images confirm the LEED observations and also revealed that (110) face of nanopyramids exhibits various reconstructions (1×n, n = 2-4) depending on oxygen coverage. Further, oxygen induced nanoscale faceted Re (12-31) surface has been formed which consist of nano-trenches (two sided ridges) with facets in (11-21) and (01-10) direction. These two- sided faceted Re (12-31) surface is used as a template to grow gold nanostructures. Under controlled growth conditions, the gold induced nanostructures are formed on oxygen induced nano-trenched Re (12-31) surface. It is observed that gold grows in the form of islands on the faceted rhenium substrate i.e. at lower coverages (≥0.8ML), it forms 2D islands whereby for higher coverages 3D islands are formed on top of the nano-ridges. The surfaces morphology of the gold covered faceted Re surface changes drastically on annealing, for the temperature &gt;870 K, gold atoms wet the rhenium template while annealing to higher temperature (˜970K) led to the development of a three sided nanopyramids. On further annealing to higher temperatures (&gt;1100K) cause the complete destruction of the faceted structure. LEED and STM studies revealed that the gold induced three sided nanopyramids consists of two faces of original two sided ridges with (11-20), (01-10) orientation and an additional facet in (12-32) direction. The high resolution STM images revealed that the (12-32) facet of the nanopyramid is fully decorated with single atomic zigzag gold nanochain. These faceted surfaces can be potential template for heterogeneous catalysis and development of new supported model catalysts with a narrow size distribution.

Nanoscale surface chemistry over faceted substrates: structure, reactivity and nanotemplates

Faceting is a form of self-assembly at the nanometre-scale on adsorbate-covered single-crystal surfaces, occurring when an initially planar surface converts to a "hill and valley" structure, exposing new crystal faces of nanometre-scale dimensions. Planar metal surfaces that are rough on the atomic scale, such as bcc W(111), fcc Ir(210) and hcp Re(1231), are morphologically unstable when covered by monolayer films of oxygen, or by certain other gases or metals, becoming "nanotextured" when heated to temperatures above approximately 700 K. Faceting is driven by surface thermodynamics (anisotropy of surface free energy) but controlled by kinetics (diffusion, nucleation). Surfaces can spontaneously rearrange to minimize their total surface energy (by developing facets), even if this involves an increase in surface area. In this critical review, we discuss the structural and electronic properties of such surfaces, and first principles calculations are compared with experimental observations. The utility of faceted surfaces in studies of structure sensitive reactions (e.g., CO oxidation, ammonia decomposition) and as templates for growth of metallic nanostructures is explored (122 references).

From atom-resolved scanning tunneling microscopy (STM) studies to the design of new catalysts

The scanning tunneling microscope (STM) is today established as a unique tool for resolving the atomic-scale structure of surfaces. STM studies of model systems relevant to heterogeneous catalysis have made it possible to address and resolve many important, fundamental questions related to catalytic processes by imaging, in direct space, the atomic-scale structure of catalytically relevant model systems, e.g. adsorbate-covered single crystal surfaces or nanoclusters supported either on metals or oxide surfaces. These studies are normally carried out under well-controlled vacuum or pressure conditions. Here we discuss three recent STM studies of model catalyst systems, all illustrating how the insight gained from fundamental studies of idealized model systems has been successfully linked to studies on real catalyst systems operating under realistic conditions, and how this interplay has facilitated the development of new and superior high surface area catalysts.

Dynamics of excited electronic states at clean and adsorbate-covered insulator surfaces

We present an ab initio approach to electronic excitations of surface systems and to their dynamics. Based on a density-functional calculation for the electronic ground state, excitations are described by many-body perturbation theory, taking electronic correlation effects and electron–hole interaction into account. This approach yields highly reliable results for electronic spectra, optical excitations, and excited-state total energies. In addition, the femtosecond dynamics (like, e.g., charge-transfer processes) of excited states can be evaluated. As examples, we discuss surface excitons at the LiF(0 0 1)-(1 × 1) surface, as well as the initial decay of the first excited state of CO molecules due to adsorption of the molecules on the insulator surface MgO(0 0 1)-(1 × 1).

Electronic structure studies of adsorbate-induced surface reconstructions: oxygen on Rh(1 0 0)

Solid-state Fenske–Hall band structure calculations have been used to study the electronic structure and bonding that occur on an “asymmetric” clock reconstructed Rh(1 0 0) surface with a half-monolayer of O atom adsorbates. The displacement of the top-layer Rh atoms on reconstructed O/Rh(1 0 0) is similar to that observed when a half-monolayer of C or N atoms adsorb onto clean Ni(1 0 0). Unlike the five-coordinate C or N adsorbates that adsorb into effectively coplanar sites on the Ni(1 0 0) surface, however, O atoms sit well above the Rh surface plane and occupy three-coordinate adsorption sites. The results of these calculations show that the asymmetric clock reconstruction of O/Rh(1 0 0) increases the negative charge localized on the highly electronegative O atoms and strengthens the O–Rh bonding relative to an unreconstructed surface. This suggests that, in contrast to the C(N)/Ni(1 0 0) clock, which appears to be driven primarily by the restoration of metal–metal bonding, the asymmetric O/Rh(1 0 0) clock reconstruction is driven by the optimization of the O atom bonding environment. Comparisons of the O/Rh(1 0 0) and C(N, O)/Ni(1 0 0) surfaces further indicate that the electronegativity and electron count of the adsorbed species, as well as the electron count and physical size of the metal, all play a role in determining the preferred atomic geometries of these adsorbate-covered transition metal surfaces.

Surface stress: implications and measurements

Recent advances in the understanding of the role of surface stress for surface reconstruction, self-assembled pattern formation, alloying, shape and structural transitions, and adsorbate-induced stress are discussed. An understanding of the implications of surface stress requires a thorough theoretical investigation of the relevant processes on the atomic and mesoscale. Progress in the theoretical description of surface stress of clean and adsorbate-covered surfaces is described. A presentation of an optical deflection technique for stress measurements concludes the article.