Electronic excitations by chemical reactions on metal surfaces

Dissipation of chemical energy released in exothermic reactions at metal surfaces may happen adiabatically by creation of phonons or non-adiabatically by excitation of the electronic system of the metal or the reactants. In the past decades, the only direct experimental evidence for such non-adiabatic reactions has been exoelectron emission into vacuum and surface chemiluminescence which are observed in a special class of very exothermic reactions. The creation of e–h pairs in the metal has been discussed in many theoretical models but it was only recently that a novel experimental approach using Schottky diodes with ultrathin metal films makes direct measurement of reaction-induced hot electrons and holes possible. The chemical reaction creates hot charge carriers which travel ballistically from the metal film surface toward the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and NO₂ molecules and of certain hydrocarbons on Ag. This paper reviews briefly exoelectron and chemiluminescence experiments and the concept of the Nørskov–Newns–Lundqvist model. The major part is devoted to the detection of chemically induced e–h pairs with thin metal film Si Schottky diodes by discussing the different influences on the chemicurrent magnitude and presenting experimental results predominantly with hydrogen and deuterium atoms. The experiments introduce a new method to investigate surface reaction kinetics and dynamics by use of an electronic device. In addition, the diodes may be used as selective reactive gas sensors.     

Femtosecond dynamics of chemical reactions at surfaces

One of the major goals in physical chemistry is to obtain a microscopic understanding of chemical reactions. Recent developments in femtosecond laser techniques provide the opportunity to resolve the timescale of elementary steps of chemical reactions at surfaces. This is exemplified for the femtosecond laser-induced oxidation of CO on Ru(001). Among other adsorbate-specific probes vibrational sum-frequency generation spectroscopy offers the possibility to monitor adsorbates or reaction intermediates directly at the surface. Recently, we have employed this technique to investigate the dynamics of the CO-stretch vibration of CO adsorbed on Ru(001) after optical excitation leading to CO desorption. Received: 4 August 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Self-organized nanostructures in surface chemical reactions: Mechanisms and mesoscopic modeling

Nanoscale patterns can form in reactive adsorbates on catalytic surfaces as a result of attractive lateral interactions. These structures can be described within a mesoscopic theory that is derived by coarse graining the microscopic master equation thus providing a link between microscopic lattice models and reaction-diffusion equations. Such mesoscopic models allow to systematically investigate mechanisms responsible for the formation of nanoscale nonequilibrium patterns in reactive condensed matter. We have found that stationary and traveling nanostructures may result from the interplay of the attractive lateral interactions and nonequilibrium reactions. Besides reviewing these results, a detailed investigation of a single reactive adsorbate in the presence of attractive lateral interactions and global coupling through the gas phase is presented. Finally, it is outlined how a mesoscopic theory should be constructed for a particular scanning tunneling microscopy experiment [the oxidation of hydrogen on a Pt(111) surface] in order to overcome the failure of a corresponding reaction-diffusion model to quantitatively reproduce the experiments. (c) 2002 American Institute of Physics.     

Strain-driven self-organization of nanostructures on semiconductor surfaces

Received: 14 April 1998/Accepted: 23 October 1998     

Secondary effects on solid surfaces stimulated by the cathode plasma flux in vacuum arcs

The conducting plasma region between the electrodes of a vacuum arc contains single-and multiple-charged ions of the cathode material. Investigation into the behaviour of this flux when it impinges upon a surface should take into account secondary processes. From among various processes on the surface in vacuum arc, particular attention is paid to the sputtering of the surface and to the secondary electron emission by positive ions emitted from the cathode spots. On the basis of the sputtering yield characteristicsS _r as a function of ion kinetic energyE _i and their energy distribution in the cathode flux, the total sputtering yieldsS _re for atomicaly clean surface have been determined. Experimental verification ofS _re was performed by measuring the ratio of mass deposited on the collectors to mass carried by the particle incident flux. It was found that the calculated value ofS _re is comparable to the evaluated one from the experiments. The emission of secondary electrons from clean metal surfaces due to a bombardment by multiple-charged ions is considered. The secondary electron emission _c depends on excess energyE _ex of incident ion and its average value _av for multiple-charged ions depends both on the fractional distribution of ion flux _zi and on the excess energyE _ex of each ion. It is proposed that the relationship derived is applicable to most combinations of atomic ions and pure metal surfaces.Presented at 17^th Symposium Plasma Physics and Technology, Prague, June 13–16, 1995.This work was supported by the State Committee for Scientific Research within the research project No. 3P40101507.     

Acoustic wave effects on catalysis: design of surfaces with artificially controllable functions for chemical reactions

The effects of surface acoustic wave (SAW) and resonance oscillation (RO) of bulk acoustic waves on the catalysis of metals were studied in an attempt to design a catalyst surface with artificially controllable functions for chemical reactions. In ethanol decomposition on a thin Cu film catalyst deposited on the propagation path of a shear horizontal leaky SAW, the SAW-on increased the activity for ethylene production remarkably but a little for acetaldehyde production. A poled ferroelectric z-cut LiNbO₃ with a thickness extensional mode RO (TERO) and a x-cut LiNbO₃ with a thickness shear mode RO (TSRO) were employed as a substrate, on which a thin Ag film catalyst was deposited. For ethanol decomposition, TERO increased ethylene production activity and the selectivity for ethylene production from 79 to 96%, whereas TSRO caused little activity enhancement for both ethylene and acetaldehyde production. The combination with the results of laser Doppler measurements showed that the activity enhancement and selectivity changes with SAW and RO of the acoustic waves are associated with dynamic large lattice displacement vertical to the surface.     

Use of molecular beams for kinetic measurements of chemical reactions on solid surfaces

In this review we survey the contributions that molecular beam experiments have provided to our understanding of the dynamics and kinetics of chemical interactions of gas molecules with solid surfaces. First, we describe the experimental details of the different instrumental setups and approaches available for the study of these systems under the ultrahigh vacuum conditions and with the model planar surfaces often used in modern surface-science experiments. Next, a discussion is provided of the most important fundamental aspects of the dynamics of chemical adsorption that have been elucidated with the help of molecular beam experiments, which include the development of potential energy surfaces, the determination of the different channels for energy exchange between the incoming molecules and the surface, the identification of adsorption precursor states, the understanding of dissociative chemisorption, the determination of the contributions of corrugation, steps, and other structural details of the surface to the adsorption process, the effect to molecular steering, the identification of avenues for assisting adsorption, and the molecular details associated with the kinetics of the uptake of adsorbates as a function of coverage. We follow with a summary of the work directed at the determination of kinetic parameters and mechanistic details of surface reactions associated with catalysis, mostly those promoted by late transition metals. This discussion we initiate with an overview of what has been learned about simple bimolecular reactions such as the oxidation of CO and H₂ with O₂ and the reaction of CO with NO, and continue with the review of the studies of more complex systems such as the oxidation of alcohols, the conversion of organic acids, the hydrogenation and isomerization of olefins, and the oxidative activation of alkanes under conditions of short contact times. 6 Reactions on supported nanoparticles: Materials gap, 7 Low-probability reactions: Pressure gap of this review deal with the advances made in the use of molecular beams with more realistic models for catalysis, using surfaces comprised of metal nanoparticles dispersed on the oxide surfaces used as catalyst support and high-flux beams to approach the pressures used in catalysis. The next section deals with the study of systems associated with fields other than catalysis, mainly with the etching and oxidation of semiconductor surfaces and with the chemistry used to grow thin solid films by chemical means (chemical vapor deposition, CVD, or atomic layer deposition, ALD). We end with a personal assessment of the past accomplishments, present state, and future promise of the use of molecular beams for the study of the kinetics of surface reactions relevant to practical applications.     

Dynamics and stability of nanostructures on metal surfaces

Ultrathin metal films consisting of two-dimensional clusters are typically unstable: the cluster ensemble has the tendency to reduce its total free energy via Ostwald ripening or dynamic coalescence of mobile clusters. In this paper we give an overview of recent model experiments addressing these coarsening mechanisms. The experiments have been performed using STM on ensembles consisting of adatom or vacancy clusters with typical diameters in the nanometer range on fcc(111)-metal surfaces. Agreement with and deviations from conventional theories are discussed. Received: 29 March 1999 / Accepted: 17 August 1999 / Published online: 30 September 1999     

Controllable growth of low-dimensional nanostructures on well-defined surfaces

The controllable growth of nanostructures with desired geometric order and well-defined shapes has stimulated great interest due to its applicability in the development of microelectronic devices. Self-assembly is an efficient and versatile way to guide the atoms or molecules into low-dimensional nanostructures as a consequence of balancing complex interplay between adsorbate-adsorbate and adsorbate-substrate interfacial interactions. The tailoring of low-dimensional nanostruc- tures by control of inter-adsorbate and adsorbate-substrate interfacial interactions is reviewed. Such inherent interactions greatly influence not only the size and shape of the growing nanostructures, but also their chemical identity. The degree of interaction between adsorbates can be controlled via preparation procedures, opening up the study of the influence of this phenomenon with respect to reactivity and catalytic behavior. The formation of well-defined molecular layers can be controlled not only by repulsive molecule-molecule interaction but also by symmetry matching or charge transfer be- tween adsorbed molecules and the substrate. It has become obvious that inter-adsorbate and adsorbate-substrate interfacial interactions can be tuned to fabricate diverse surface nanostructures from semiconductor, metallic, and molecular materials.     

Chemical reactions on palladium surfaces studied with Pd-MOS structures

It is pointed out that metal-oxide-semiconductor structures can be used to stfidy catalytic reactions on metal surfaces (like Pd and Pt surfaces). The flatband voltage shift induced by hydrogen at the metal-oxide interface is a measure of the amount of hydrogen in the metal, which in turn reflects the chemical reactions on the surface. Experimental results on hydrogen in argon and in air detected with Pd-MOS structures are compared with absolute reaction rate theory. The agreement between theory and experiments is surprisingly good.     

Coulomb sink effect on coarsening of metal nanostructures on surfaces

We discuss Coulomb effects on the coarsening of metal nanostructures on surfaces. We have proposed a new concept of a “Coulomb sink” [Phys. Rev. Lett., 2004, 93: 106102] to elucidate the effect of Coulomb charging on the coarsening of metal mesas grown on semiconductor surfaces. A charged mesa, due to its reduced chemical potential, acts as a Coulomb sink and grows at the expense of neighboring neutral mesas. The Coulomb sink provides a potentially useful method for the controlled fabrication of metal nanostructures. In this article, we will describe in detail the proposed physical models, which can explain qualitatively the most salient features of coarsening of charged Pb mesas on the Si(111) surface, as observed by scanning tunneling microscopy (STM). We will also describe a method of precisely fabricating large-scale nanocrystals with well-defined shape and size. By using the Coulomb sink effect, the artificial center-full-hollowed or half-hollowed nanowells can be created.      

Coulomb sink effect on coarsening of metal nanostructures on surfaces

We discuss Coulomb effects on the coarsening of metal nanostructures on surfaces. We have proposed a new concept of a “Coulomb sink” [Phys. Rev. Lett., 2004, 93: 106102] to elucidate the effect of Coulomb charging on the coarsening of metal mesas grown on semiconductor surfaces. A charged mesa, due to its reduced chemical potential, acts as a Coulomb sink and grows at the expense of neighboring neutral mesas. The Coulomb sink provides a potentially useful method for the controlled fabrication of metal nanostructures. In this article, we will describe in detail the proposed physical models, which can explain qualitatively the most salient features of coarsening of charged Pb mesas on the Si(111) surface, as observed by scanning tunneling microscopy (STM). We will also describe a method of precisely fabricating large-scale nanocrystals with well-defined shape and size. By using the Coulomb sink effect, the artificial center-full-hollowed or half-hollowed nanowells can be created.     

Supramolecular architectures and nanostructures at metal surfaces

The controlled formation of non-covalent bonds (H-bonding, metal–ligand interactions) is the key ingredient for the fabrication of supramolecular architectures and nanostructures. Upon deposition of molecular building blocks at well-defined surfaces, this issue can be directly addressed. Scanning tunneling microscopy observations are presented, which provide insight into the interaction of functional groups on metal substrates at the molecular level. In particular, carboxylic acids were employed: (4-[(pyrid-4-yl-ethynyl)]-benzoic acid (PEBA), 4-[trans-2-(pyrid-4-yl-vinyl)]-benzoic acid (PVBA) and trimesic acid (1,3,5-benzenetricarboxylic acid, TMA), which could be stabilized in a flat geometry at the surface. By choosing the appropriate substrate material and symmetry, the sensitive balance of intermolecular and molecule–substrate interactions can be tuned to obtain well-defined supramolecular architectures and nanostructures. The head-to-tail hydrogen bonding of the related rod-like species PEBA and PVBA stabilizes molecular rows on Ag(111). The subtle difference in the molecular geometries is reflected in the lateral ordering: While 2-D islanding is encountered with PEBA, 1-D nanogratings of supramolecular chiral H-bonded twin chains evolve for PVBA. The threefold symmetry of TMA in conjunction with the self-complementarity of its exodentate groups accounts for the formation of H-bonded honeycomb networks on Cu(100) at low temperatures. Metal–ligand interactions were probed with PVBA and TMA at Cu surfaces at ambient temperature. Deprotonation of the carboxyl moiety takes place, which readily interacts with Cu adatoms evaporated from step edges. This leads to a head-to-head pairing of PVBA on Cu(111) and cloverleaf-shaped Cu–TMA coordination compounds on Cu(001). Received: 4 June 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-21/693-3604, E-mail: johannes.barth@epfl.ch     

Biomimetic compound eye with a high numerical aperture and anti-reflective nanostructures on curved surfaces

Biomimetic compound eyes with a high numerical aperture on a curved surface were successfully fabricated by intelligent integration of traditional top-down and bottom-up micro- and nanofabrication methods together. In addition, the new hybrid micro- and nanofabrication method allows us to fabricate the antireflective nanostructures on each ommatidium to increase its vision sensitivity by improving the light transmission. The fabricated compound eye was optically characterized and was shown to have a numerical aperture of 0.77 for each ommatidium. Furthermore, it is shown that the transmission of the compound eye can be improved by 2.3% for the wavelength of 632.8 nm and a clearer image can be formed by the fabricated compound eye with antireflective nanostructures compared with that without antireflective nanostructures. In addition, the developed hybrid manufacturing method can be adapted to the fabrication of other complex micro- and nanodevices for photonics or other research areas.     

Soft chemical routes to semiconductor nanostructures

Soft chemistry has emerged as an important means of generating nanocrystals, nanowires and other nanostructures of semiconducting materials. We describe the synthesis of CdS and other metal chalcogenide nanocrystals by a solvothermal route. We also describe the synthesis of nanocrystals of AlN, GaN and InN by the reaction of hexamethyldisilazane with the corresponding metal chloride or metal cupferronate under solvothermal conditions. Nanowires of Se and Te have been obtained by a self-seeding solution-based method. A single source precursor based on urea complexes of metal chlorides gives rise to metal nitride nanocrystals, nanowires and nanotubes. The liquidliquid interface provides an excellent medium for preparing single-crystalline films of metal chalcogenides.     

Slow ion scattering by crystal surfaces and nanostructures

This review is dedicated to considering issues of low-energy ion scattering by solid surfaces. In addition to general issues, original results obtained by researchers at the department of physical electronics (Faculty of Physics, Moscow State University) are considered. They have initiated new directions in the study of slow ion scattering. Attention is focused on experimental data and the results of computer simulation. The section dedicated to discussing the charge exchange between the scattered particles and the surface, and nanostructures is an important part of this review.     

Surface engineering with ion beams: from self-organized nanostructures to ultra-smooth surfaces

Low-energy ion-beam sputtering, i.e. the removal of atoms from a surface due to the impact of energetic ions or atoms, is an inherent part of numerous surface processing techniques. Besides the actual removal of material, this surface erosion process often results in a pronounced alteration of the surface topography. Under certain conditions, sputtering results in the formation of well-ordered patterns. This self-organized pattern formation is related to a surface instability between curvature-dependent sputtering that roughens the surface and smoothing by different surface relaxation mechanisms. If the evolution of surface topography is dominated by relaxation mechanisms, surface smoothing can occur. In this presentation the current status of self-organized pattern formation and surface smoothing by low-energy ion-beam erosion of Si and Ge is summarized. In detail it will be shown that a multitude of patterns as well as ultra-smooth surfaces can develop, particularly on Si surfaces. Additionally, the most important experimental parameters that control these processes are discussed. Finally, examples are given for the application of low-energy ion beams as a novel approach for passive optical device engineering for many advanced optical applications. PACS 81.16.Dn; 81.16.Rf; 81.65.Cf; 81.65.Ps; 68.35.Ct     

MBE fabrication of self-assembled Si and metal nanostructures on Si surfaces

Two types of fairly regular distributions of Si nanostructures, of interest as templates to grow spatially controlled ensembles of metal (Co, Fe, Ag, etc.) nanostructures, are presented in this paper. Both of them are achieved by self-assembling processes during Si homoepitaxy. One corresponds to films grown by molecular beam epitaxy (MBE) on Si(0 0 1)-2 × 1 surfaces with low (<1°) miscut angles. In this case, arrays of 3D Si-islands displaying well defined pyramid-like shapes can be obtained, as evidenced by Scanning Force Microscopy (SFM) and Scanning Transmission Electron Microscopy (STEM). Such arrays exhibit strong similarities with those reported for Ge and SiGe islands on Si(0 0 1), and may thus serve as a simpler route to produce ordered distributions of metallic nanodots. On the other hand, on Si(1 1 1)-7 × 7 vicinal substrates misoriented 4° toward the direction, step rearrangement during homoepitaxy permits to produce nanopatterned surfaces, the building-blocks of which are triangular (1 1 1) platforms, with lateral dimensions of hundreds of nanometers, bound by step bunches about 30 nm high. Furthermore, different Ag deposition experiments support this spontaneous patterning on Si(1 1 1) as a promising approach to achieve regular distributions of metallic nanocrystals with an overall homogeneity in sizes, shapes and spacing.