Inverse and reversible switching gradient surfaces from mixed polyelectrolyte brushes

We report on a thin polyelectrolyte film (mixed polyelectrolyte brush) with a gradual change of the composition (ratio between two different oppositely charged surface-grafted weak polyelectrolytes) across the sample. The gradient of surface composition creates a gradient in surface charge density and, consequently, a gradient of the wetting behavior. The gradient film is sensitive to a pH signal and can be reversibly switched via pH change.     

Chlorine-induced corrosion of silver surfaces studied by ultraviolet photoemission

Summary The corrosion of Ag(100), Ag(110) and Ag(111) single crystal surfaces by chlorine has been investigated at sample temperatures T = 90 K. After exposures to typically 30 to 50 l Cl₂ in all three cases one and the same disordered corrosion product AgCl is observed covered by condensed molecular Cl₂. Obviously, the underlying corrosion mechanism is not face-specific and may be generalized to arbitrary Ag surfaces, including polycrystalline films. It is demonstrated that ultraviolet photoemission spectroscopy is a unique tool for identifying the reaction products of halogenation processes and for elucidating elementary steps of the underlying corrosion mechanism.

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UV-Photoemissionsstudien zur chlorinduzierten Korrosion von Silberoberflächen

     

Spectroscopy of metal surface states by inverse photoemission

In the past decade inverse photoemission has proved to be a technique well suited to study unoccupied bulk and surface electronic states of solids. This paper presents a brief review focusing on the spectroscopy of surface states on metal surfaces and how these states are influenced by simple adsorbates, surface reconstructions or the presence of a ferromagnetic substrate.

The experimental observations are discussed within the framework of a nearly free electron model of surface states.     

Vibrationally promoted electron emission from low work-function metal surfaces

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9 < or = v < or =18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10(-2) electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces.     

Transition metal compound surfaces

Studies of transition metal compound surfaces using modern surface science techniques are reviewed. Studies of the surface structure and composition of model transition metal compound surfaces are emphasized. The growth of the transition metal compound surface from a chemisorbed layer is used as an introduction to investigations of the surface properties of macroscopic single crystals of transition metal compounds. Examples of both binary and tenary compound systems are examined in relation to chemisorbed layer studies. Although only a few systems are chosen to illustrate work in this field, extensive references to other studies and other systems are included.     

Preexponential factors for hydrogen desorption from single crystal metal surfaces

Partition functions for adsorbed hydrogen atoms have been evaluated for Ni(100), Ni(111), Pd(111) and Pt(111) surfaces. These account for the relative order of magnituted of the preexponential factors for associative desorption, ν_d, including the abnormally low value for the Pt(111)/H system. Furthermore application of transition state theory yields numbers for ν_d which are in good agreement with experimental data.     

Photoinitiated polymerization on metal surfaces

Irradiation of acrolein vapors in the presence of films of nickel and other metals results in the formation of thin films of polyacrolein on the metal surface. The polyarcrolein film protects the metal surface from abrasion and corrosion. Polymer growth is more rapid on oxidized vs. unoxidized metal surfaces indicating a role for surface oxide sites in the polymerization process. Several other monomers investigated fail to form adhesive polymer films on metal films, but can be copolymerized with acrolein.     

Carbon on transition metal surfaces

The review surveys the conditions of formation and properties of four forms of surface carbon on transition metals, to wit, adsorbed atoms and clusters, surface carbide and graphite, and their role in the physical and chemical processes on the surface. The first-order phase transition in the adlayer, when graphite islands coexist with carbon gas, are considered. The effect of intercalation, when atoms (Cs, K, Na, Ba, Pt, Si) penetrate spontaneously under the graphite islands physisorbed on the metal, and its mechanism are discussed. An analysis is made of the poisoning of platinum-group metal catalysts in the reaction of dissociation, when graphite islands characterized by extreme adsorption and catalytic passivity form in the adlayer. The method of CsCl dissociation to probe the surface carbon is treated. Attention is drawn to the adsorption of a number of atoms (Cs, K, Ba, Pt) on a graphite monolayer on metals, and the properties of such systems are discussed. The effects observed in coadsorption of CsCl molecules with K, Na, Ba, Tm atoms on a graphite monolayer on metals are covered. By analogy with the bulk carbides, surface carbides of fixed stoichiometry and very strong metal-carbon bonding have been revealed to form on the surface of transition metals (W, Re, Mo). The effect of displacement of surface carbon into the bulk of the metal stimulated by the adsorption of some atoms (Si, S, O) is discussed. The carbon clusters adsorbed on metals are considered. The transport of surface carbon, its desorption and diffusion between the surface and the bulk of the metal with a single- and double-phase adlayer are reviewed.     

Wettability of clean metal surfaces

Experimental and theoretical investigations bearing on the question of the wettability, by water, of clean oxygen-free metal surfaces are reviewed. Results on gold, silver, and copper are discussed in terms of surface cleanliness, surface structure, and extent of dispersion (London) force interaction. It is concluded that clean solid metal surfaces are hydrophilic. They will yield a zero degree contact angle when prepared in the amorphous state and possibly in the perfect crystalline state as well. These results do not necessarily preclude the possibility that physical interaction at the metal-water interface consists solely of dispersion forces.     

Near-field focused photoemission from polystyrene microspheres studied with photoemission electron microscopy

We use photoemission electron microscopy (PEEM) to image 3 μm diameter polystyrene spheres supported on a metal thin film illuminated by 400 nm (～3.1 eV) and 800 nm (～1.5 eV) femtosecond (fs) laser pulses. Intense photoemission is generated by microspheres even though polystyrene is an insulator and its ionization threshold is well above the photon energies employed. We observe intense photoemission from the far side (the side opposite the incident light) of the illuminated microsphere that is attributed to light focusing within the microsphere. For the case of p-polarized, 800 nm fs laser pulses, we observe photoemission exclusively from the far side of the microsphere and additionally resolve sub-50 nm hot spots in the supporting Pt∕Pd thin film that are located only within the focal region of the microsphere. We compare the PEEM images with finite difference time domain (FDTD) electrodynamic simulations to model our experimental results. The FDTD simulations predict light focusing in the microsphere and subsequent interaction with the supporting metal surface that is consistent with the experimental observations.     

A classical dynamics method for H2 diffraction from metal surfaces

We present a discretization method that allows one to interpret measurements on diffraction of diatomic molecules from solid surfaces using six-dimensional (6D) classical trajectory calculations. It has been applied to the D2NiAl(110) and H2Pd(111) systems (which are models for activated and nonactivated dissociative chemisorption, respectively) using realistic potential energy surfaces obtained from first principles. Comparisons with experimental results and 6D quantum dynamical calculations show that, in general, the method is able to predict the relative intensity of the most important diffraction peaks. We therefore conclude that classical mechanics can be an efficient guide for experimentalists in the search for the most significant diffraction channels.     

Study on functionalization of metal surfaces. III. Photopolymerization of monomer films on metal surfaces

The three kinds of monomer films on metal surfaces were deposited by adsorption from a solution of 6-polymerizable substituents-1,3,5-triazine-2,4-dithiol monosodium salts (RTDN); the polymerizable substituents such as cis-9-octadecenylamino, di(cis-9-octadecenyl)amino, and p-vinylbenzyl(cis-9-octadecenyl)amino groups were selected in view of the polymerization activity of unsaturated groups in the substituents and the packing degree of monomer molecules. The monomer films were estimated to consist of mainly 6-substituents-1,3,5,-triazine-2,4-dithione (3H, 5H) and to be multimolecular layers that are considerably cross-packed and ordered. The monomer films on metal surfaces were polymerizable under a UV light irradiation in air atmosphere to give polymer films. In the photopolymerization, azobis(isobutyronitrile) (AIBN) was very effective for increasing the monomer conversion and the polymerization rate. The optimum concentration of AIBN in monomer films was very small, about 0.025 mol %. The monomer conversion was influenced by the kind of monomers, namely, the polymerization activity and the packing degree. The effect of the packing degree was especially remarkable. The monomer conversion decreased with an increase in the thickness of monomer films. This is because the polymerization was initiated by oxygen and AIBN, which were diffused into the inner of monomer films. The possibility of polymerization of the unsaturated groups and the thione groups in monomer molecules under UV light irradiation is discussed.     

Reconstructions and relaxations on metal surfaces

On metal surfaces the top layer of atoms usually relaxes inwards, and several metal surfaces reconstruct, particularly W and Mo (001) and Ir, Pt and Au (001) and (110). This paper reviews the electronic origins of these atomic displacements, and the physical effects which they lead to. The surface relaxations are explained in terms of the change in balance between electronic forces at the surface. The W (001) surface reconstructs from (1 × 1) to (?2 × ?2)R45° in a continuous phase transition on cooling below room temperature, and the structure of these phases and the nature of the phase transition are discussed. The driving force for the reconstruction is discussed in terms of calculations and photoemission studies of the surface electronic structure. Phenomenological Landau theory is used to describe the effect of surface steps on the transition, and the origin of the long wavelength modulation in the Mo (001) reconstruction. Finally the reconstructions of Ir, Pt and Au (001) and (110) are briefly described; these involve a more substantial movement of atoms than in the W and Mo reconstructions.     

Self-assembled monolayers at electrode metal surfaces

Neutral organic compounds, dissolved in an electrolyte in contact with an electrode, adsorb and form different monolayers which may range from dilute to compact films. In some instances, non-electroactive organic molecules are highly associated and form 2D condensed phases which are characterized by the presence of phase transitions. The occurence of these self-assembled monolayers is discussed on the basis of experimental results obtained at equilibrium as well as under dynamic conditions. Self-assembling depends on the relative magnitude of the interactions involving the surfactant, the solvent and the electrode. Adequate potential-step programmes have been successfully used to trigger the formation of the ordered phase. It is found that the kinetics are controlled by a nucleation and growth mechanism. According to the experimental conditions, a deterministic or stochastic behaviour is observed. The amplitude of the supersaturation, given by the surface free energy gap between the final and metastable states, is independently controlled by the potential, temperature and surfactant concentration. The classical nucleation theory allows the determination of key parameters such as the line tension, the radius and free energy of formation of the critical nucleus. Ion and electron transfer processes through condensed monolayers are also briefly described.     

Thin metal films on quasicrystalline surfaces

Thin metal films with a thickness of one or over one monolayer formed on quasicrystalline surfaces were studied using reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, X-ray photoelectron diffraction and scanning tunneling microscopy. The substrates were the 10f surface of d-Al–Ni–Co and the 5f surface of i-Al–Pd–Mn. The metals deposited were Au, Pt, Ag and In. None of these metals forms any ordered layer by deposition onto clean quasicrystalline surfaces. However, if a submonolayer of In is present atop the 10f surface, an epitaxial layer of multiply-twinned AuAl₂ crystals is formed by Au deposition and subsequent annealing. This is also the case for Pt deposition, but not for Ag deposition. Although the surfactant effect of In is also observed in the case of Au deposition on the 5f surface of i-Al–Pd–Mn, the ordered layer formed is a film of Au–Al alloy with icosahedral symmetry. No ordered films are formed by Pt or Ag deposition onto the 5f surface, regardless of the presence of an In-precovered layer. A Sn film monolayer induced by surface diffusion was also studied.     

Electromagnetic interactions with rough metal surfaces

When surfaces are structured on the scale of the wavelength, we can expect incident light to be strongly modified by the surface. This is especially the case when the surface is metallic. We have developed a formalism for computing these modifications, closely analogous to electron scattering theory, which we briefly review and present some results for optical properties of, and electron energy loss in, colloids. Our main theme is another effect associate with rough or structured metallic surfaces: Surface Enhanced Raman Scattering, or SERS. We model the rough surface by a periodic array of spheres and obtain the correct magnitude for the enhancement and for the frequency shifts observed.     

Hydrogen-induced reconstruction of transition metal surfaces

This study compares the results of a number of recent papers on hydrogen adsorption on Rh(110), Rh(311) and Fe(211) as well as on Ni(111) and Fe(110) surfaces. It particularly deals with the structural aspect of these low energy electron diffraction (LEED) investigations and correlates them, if available, with respective thermodesorption data. Upon dissociative adsorption by a non activated process hydrogen induces local displacements of the atoms about the adsorption sites. With increasing coverage these displacements order to form a sequence of weakly reconstructed phases and gradually lift the surface layer relaxation of the formerly clean surface. Along close packed rows of metal surface atoms hydrogen atoms tend to occupy threefold coordinated adsorption sites which, in turn, arrange in single or double chains. The coverage dependent periodicity of these adlayer structure elements together with the respective shift buckling of the substrate surface generates the observed superstructures. Since not only open but also close packed surfaces show this weak (and sometimes strong) reconstruction upon hydrogen adsorption it should be generally considered in all adsorption systems.     

Long-lived adsorbate states on metal surfaces

It has been shown recently that the peculiarities of the band structure of a metal can qualitatively influence the electron tunnelling between an adsorbate and a metal surface, the so-called resonant charge transfer (RCT). The presence of a projected band gap along the normal to the surface in the case of Cu(111) has been shown to lead to a blocking of the RCT in the case of Cs/Cu(111), resulting in the existence of a very long-lived excited state. Such long-lived states are potentially very important for surface reaction mechanisms invoking a transient state as an intermediate. Various systems: Cs, model M- negative ion of p pi symmetry, CO adsorbed on Cu(111), are investigated in order to determine the conditions for the blocking of the RCT and the existence of long-lived states.