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This volume contains review articles written by the invited speakers at the ninth International Summer Institute in Surface Science (ISISS 1989), held at the Uni­ versity of Wisconsin-Milwaukee in August of 1989. During the course of ISISS, invited speakers, all internationally recognized experts in the various fields of surface science, present tutorial review lectures. In addition, these experts are asked to write review articles on their lecture topic. Former ISISS speakers serve as advisors concerning the selection of speakers and lecture topics. Emphasis is given to those areas which have not been covered in depth by recent Summer Institutes, as well as to areas which have recently gained in significance and in which important progress has been made. Because of space limitations, no individual volume of Chemistry and Physics of Solid Surfaces can possibly cover the whole area of modern surface science, or even give a complete survey of recent progress in this field. However, an attempt is made to present a balanced overview in the series as a whole. With its comprehensive literature references and extensive subject indices, this series has become a valuable resource for experts and students alike. The collected articles, which stress particularly the gas-solid interface, have been published under the following titles: Surface Science: Recent Progress and Perspectives, Crit. Rev. Solid State Sci.



1. Reactivity of Surfaces

On July 29, 1823, J.W. Doebereiner, professor of chemistry at the University of Jena, informed his minister, J.W. Goethe, about his observation that finely divided platinum causes hydrogen to react with oxygen “by mere contact”, whereby the platinum even starts to glow due to the heat evolved in this process [1.1]. This discovery was a sensation in the scientific world (perhaps comparable to the turmoil about recent reports about the so-called “cold fusion”, with the essential difference that Doebereiner’s effect was real!) and prompted many researchers to further investigations in this field, for which somewhat later Berzelius coined the term “catalysis” [1.2]. A tentative explanation was offered by Faraday [1.3] who speculated:
“dependent upon the natural conditions of gaseous elasticity combined with the exertion of that attractive force possessed by many bodies …; by which they are drawn into association more or less close, … and which occasionally leads to the combination of bodies simultaneously subjected to this attraction”.
G. Ertl

2. New Mechanisms for the Activation and Desorption of Molecules at Surfaces

Many chemical reactions occurring on the surfaces of solid materials appear to proceed only under high pressures of the gaseous reactants but not at low pressures (< 10−4 Torr), despite favorable thermodynamics. This lack of reactivity at the low pressures where ultrahigh vacuum (UHV) surface science techniques are operable is known loosely as the pressure gap in the reactivity in heterogeneous catalysis [2.1,2]. Our group proposed that an origin of the pressure gap is the presence of a barrier to dissociative chemisorption of at least one of the reactants upon collision with the surface [2.3–6]. Since it is the translational or internal energy of the incident molecule that is important in surmounting this barrier and not the surface temperature, the rate of the reaction is limited by the flux of incident molecules with energies above the energy of the barrier. High pressures simply increase the absolute number of high energy molecules, thereby increasing the reaction rate sufficiently for the products to be detected.
S. T. Ceyer

3. Photochemistry at Adsorbate-Metal Interfaces: Intra-adsorbate Bond Breaking

While photochemistry in gases, liquids and solids has a long and well-documented record [3.1], and photochemistry, including photoelectrochemistry, at semiconductor-adsorbate interfaces is well known [3.2], photochemistry at adsorbate-metal interfaces is a newly emerging and rapidly expanding area of surface chemistry. There are also a number of recent interesting experiments involving photochemistry of molecules adsorbed at insulator surfaces [3.3]. In this paper we focus on bond cleavage within the first monolayer of adsorbate rather than on adsorbate-metal bond cleavage or photochemistry in multilayers. The latter topic has been the subject of a number of investigations and reviews [3.4–6], Table 3.1 lists a number of recent successful photochemical intraadsorbate bond cleavage experiments [3.7–27]. An interesting comparison is given in Table 3.2, which lists experiments where no photochemistry was observed [3.28–31].
J. M. White

4. Desorption Induced by Electronic Transitions

Bombardment of a surface by electrons or photons can cause rupture of surface bonds and desorption from the surface, by inducing transitions to repulsive electronic states. The phenomenon of desorption induced by electronic transitions (DIET) includes both electron stimulated desorption (ESD) and photon stimulated desorption (PSD) [4.1–3]. DIET processes are of widespread importance in many areas of science and technology, including beam damage in surface analysis using x-rays or electrons, in electron and photon beam lithography, and in radiation physics of interstellar space, to name a few.
Theodore E. Madey, S. A. Joyce, J. A. Yarmoff

5. Transition Metal Clusters and Isolated Atoms in Zeolite Cages

Surface science research has made much use of metal single crystals. Results have been obtained of high relevance to chemisorption and catalysis, in particular with respect to the nature of the chemisorption bonds, their dipole moments and their crystal face specificity. The coverage dependence of specific parameters and the important phenomenon of surface reconstruction have also been studied quantitatively. Among the methods used, only field ion and field electron microscopy make use of highly curved surfaces which expose a variety of crystal faces of high and low Miller indices, so that the surface migration can be studied across face boundaries [5.1–3]. Other surface science methods use macroscopic single crystals and focus on one large crystal face, usually of rather high atom density.
W. M. H. Sachtler

6. Studies of Bonding and Reaction on Metal Surfaces Using Second-Harmonic and Sum-Frequency Generation

Over the past 10 years there has been a resurgence in efforts to develop nonlinear optical techniques for studies of surfaces and interfaces. Chief among these techniques are second-harmonic generation (SHG) and ir-visible sum-frequency generation (SFG). These second-order nonlinear processes offer several advantages. They are inherently sensitive only to the interfacial region between centrosymmetric media. This is because, in the electric dipole approximation, these processes are forbidden in the bulk of such media but are allowed at the interface where the symmetry is broken. Thus, so-called buried interfaces, i.e. surfaces under liquids, high pressure gases, and even other solids, may be investigated. In addition, with sufficiently short-duration laser excitation, these techniques have the potential to provide sub-picosecond time resolution.
R. B. Hall, J. N. Russell, J. Miragliotta, P. R. Rabinowitz

7. Surface Physics and Chemistry in High Electric Fields

To specify the term “high electric fields” in the title of this chapter, we mention typical static field strengths encountered in a variety of situations. To start with, we note that the maximum field strength that can be maintained between two conductors in air is limited to less than about 104 V/cm above which dielectric breakthrough leads to the formation of an ionized plasma. In semiconductors, fields of the order of 106 V/cm can be maintained, whereas fields within the double layer at the electrolyte-electrode interface can reach 107 V/cm. Around localized charges in zeolite cavities, electric fields of the order of 108V/cm = 1V/Å have been estimated on the basis of Coulomb’s law
$$F = \frac{{3.4}}{{r^2 }}\frac{q}{e}[V/{\AA}]$$
at a distance r, measured in Ångstroms, away from a charge of magnitude q/e, measured in units of the elementary charge e. Fields of this order can also be established within 103 Å of a metal tip with a tip radius of less than 103 Å, provided dielectric breakthrough is avoided by working in ultrahigh vacuum. The upper limit of electric field strength that can be maintained over macroscopic distances is dictated by the onset of field emission and field evaporation, and is of the order of 6 V/Å.
H. J. Kreuzer

8. Chaos in Surface Dynamics

An undeniable reality of current thinking in many areas of intellectual inquiry, both at the “arm-chair physics” as well as serious-research level, is an awareness of the possibility of “chaotic behavior” in one’s favorite system. The reasons for such an awareness are many-fold due in part to: i) the easy availability of the entire spectrum of computers (from simple PCs to super computers) being utilized in state-of-the art numerical experiments characteristic of the field; ii) real cross-disciplinary studies in which the eccentricities of nonlinear systems form common links between previously unrelated areas [8.1–7]; iii) the appearance of many excellent review articles directed at the “non-involved but interested physical scientist layperson” [8.8–14], of which the one by Jensen is particularly lucid [8.13]; iv) the timely publication of the justifiably best selling book “Chaos” by James Gleick [8.15] which has gone a long way towards making the concept of chaos a household word, but in a scientifically and/or mathematically enlightened way. Still, one may wonder if and how this explosion of colorful pictures [8.16] and frequently humorous (e.g., “The Chaotic Behavior of the Leaky Faucet” [8.17] or the exposition of the “Ding-a-Ling Model” [8.18])but legitimate numerical studies impacts on ones own field of work. One asks, “Is it necessary for me to learn what is going on in chaos and is there anything that I would do or think differently if I was to avail myself of modern (chaotic) thinking?” [8.19] It is the intent of this chapter to provide some guidance to those physicists and chemists involved in surface studies such that they might better answer these questions for themselves.
J. W. Gadzuk

9. Ten Years of Low Energy Positron Diffraction

It was not until five years after the discovery of low-energy electron diffraction (LEED) [9.1] that the positron was first observed [9.2]. By the time it became possible to form controllable, albeit very weak, low-energy positron beams in the 1970s, LEED had become an ubiquitous and powerful tool in surface structure determination. Thus, when the first observation of low energy positron diffraction (LEPD) was made in 1979 [9.3] one could well question whether it was too late for LEPD to be of any practical value in surface physics. As this review will chronicle, during the past ten years, LEPD has evolved from being a novelty to becoming a valuable technique for surface structure determination. The differences between LEPD and LEED are nontrivial; they not only raise questions of fundamental interest but also suggest that in some cases LEPD ultimately may prove superior to LEED for quantitative surface structure determination.
K. F. Canter, C. B. Duke, A. P. Mills

10. Time-of-Flight Scattering and Recoiling Spectrometry (TOF-SARS) for Surface Analysis

Low energy (< 10 keV) ion scattering spectrometry [10.1] is becoming increasingly important as a surface analysis technique in three specific areas, i.e., surface elemental analysis [10.2–4], probing surface structure [10.5–16], and studying electronic transition probabilities [10.7,7–19] between ions or atoms and surfaces. This is largely due to the following recent advances: (i) impact collision ion scattering spectrometry [10.6] (ICISS) in which the scattering angle is close to 180°, thus simplifying the scattering geometry and allowing experimental determination of the shadow cone radii, (ii) the use of alkali primary ions [10.9, 10] which have low neutralization probabilities, leading to higher scattered ion fluxes, (iii) time-of-flight (TOF) techniques [10.20–23] with detection of both neutrals and ions in a multichannel mode in order to enhance sensitivity, (iv) scattered ion fractions [10.7,17] to probe the spatial distributions of electrons, and (v) the use of recoiling [10.24, 25] to determine the structure of light adsorbates on surfaces.
O. Grizzi, M. Shi, H. Bu, J. W. Rabalais

11. Scanning Electron Microscopy with Polarization Analysis: Studies of Magnetic Microstructure

When a beam of electrons with energies greater than several hundred eV is incident upon a ferromagnetic metal, spin polarized secondary electrons are emitted. The polarization of these secondary electrons is related to the polarization of the electrons in the ferromagnet. In the case of transition metal ferromagnets, the polarization of the secondary electrons is directly proportional to the magnetization. Spin polarization analysis of the secondary electrons, therefore, provides a direct measurement of the magnetization in the region probed by the incident electron beam. Scanning electron microscopy with polarization analysis (SEMPA), illustrated schematically in Fig. 11.1, combines the finely focused beam of the scanning electron microscope with secondary electron spin polarization analysis to obtain a technique that provides high resolution images of the surface magnetic microstructure of ferromagnetic materials. The purpose of this chapter is to review the SEMPA technique and to present several examples of magnetic microstructures that were studied using SEMPA.
J. Unguris, M. R. Scheinfein, R. J. Celotta, D. T. Pierce

12. Low Energy Electron Microscopy

Low energy electron microscopy (LEEM) is a surface imaging technique in which the surface is illuminated by an approximately parallel electron beam at near normal incidence. The image is formed with those electrons which are elas- tically backscattered into a small angular region around the surface normal. The limitation to a small angular region is necessary because of the large aberrations of the objective lens which produces the primary image. This lens is a so-called cathode lens which not only has imaging properties but at the same time decelerates the fast electrons of the illuminating beam to the desired low energy at the specimen and re-accelerates the backscattered electrons to high energies again. In order to achieve this, the specimen is at a high negative potential which differs from the potential of the emitter of the electron gun of the illumination system by V 0 = E 0/e, where E 0 is the energy of the electrons at the specimen. Typical energy values are E = eV = 15−20 keV for the fast electrons and 0 < E 0 < 50 eV at the specimen. There are three fundamental quantities which are important in LEEM: resolution, intensity and contrast. These will be discussed in Sect. 12.1. Section 12.1 also describes how LEEM can be combined with other surface characterization techniques, such as low energy electron diffraction (LEED), photoemission electron microscopy (PEEM) and other emission microscopies. Section 12.2 illustrates the applications of LEEM and of the associated techniques to the study of clean surfaces, while Sect. 12.3 presents examples of the power of LEEM in the study of surface layers. Section 12.4 gives an outlook for possible future developments. A brief summary (Sect. 12.5) concludes this chapter.
E. Bauer

13. Atomic Scale Surface Characterization with Photoemission of Adsorbed Xenon (PAX)

The physical and chemical properties of solid surfaces are strongly influenced — if not dominated — by structural and chemical heterogeneities. For example, structural surface defects like steps, kinks and vacancies act as heterogeneous nucleation centers and thereby affect the growth mode and the final structure of epitaxial films and adsorbed gas layers. Chemical defects like heteroatoms are the origin of local surface structure modifications or may even trigger structural phase transitions and surface reconstruction. Both surface topography and composition are decisive parameters for the chemical reactivity and catalytic activity of solid surfaces. Surface defects and dopants (heteroatoms) create surface states in the band gap of semiconductors, which in turn determine the electronic properties of semiconductor devices. A full understanding of all kinds of surface properties and surface processes ultimately requires a characterization of real surfaces on an atomic scale. A Kossel model of a real surface is depicted in Fig. 13.1, showing schematically a binary surface with several kinds of structural defects.
K. Wandelt

14. Theoretical Aspects of Scanning Tunneling Microscopy

Since its invention by Binnig, Rohrer, and coworkers [14.1], scanning tunneling microscopy (STM) has established itself as a remarkable tool for studying surfaces. This chapter reviews the present theoretical understanding of STM, with emphasis on the interpretation of atomic-resolution STM images. The basic ideas and instrumentation have already been described in detail elsewhere [14.1].
J. Tersoff

15. Proximal Probes: Techniques for Measuring at the Nanometer Scale

Innovation in analytical surface science has evolved a panoply of tools which are sensitive to nanometer scale lengths in one dimension (typically depth). A listing of those techniques would include Auger and photoelectron spectroscopies, low energy electron diffraction, secondary ion mass spectrometry, etc.; these tools have been under development since the late 1960s and are well covered in many textbooks and reviews [15.1–5]. They are surface specific, but probe relatively large areas (i.e., > 50 nm diameter). The continued scientific/technological evolution toward smaller structures raises the need for a different class of surface analytical tools which specifically probe nanometer volume elements. Even for larger structures, the analysis of defects requires the capability to examine properties of small volume elements.
James S. Murday, Richard J. Colton

16. Studying Surface Chemistry Atom-by-Atom Using the Scanning Tunneling Microscope

A solid surface provides a great variety of sites for a possible chemical reaction. A perfect surface may have several strucutrally inequivalent sites while a real life surface may have steps, a multitude of point and extended defect sites, adsorbed foreign atoms, etc. It is clear then that in order to even begin discussing the chemistry of a particular surface we have to have a good idea about its structure. This recognized need has led to the development of a great variety of surface structural techniques [16.1]. Most of these techniques provide information averaged over a macroscopic area of the sample defined by the size of the probe beam. Moreover, diffraction-based structural probes require long-range order which is not present in many cases of interest. Even when the structure of the surface is known, the understanding of the reactivities of the various sites requires that a correlation be made between the chemical behavior of a particular site and its local electronic structure. Thus, ideally one needs a surface technique or techniques which will allow the study of the topography and valence electronic structure of surfaces with atomic resolution.
Phaedon Avouris, In-Whan Lyo

17. Bonding and Structure on Semiconductor Surfaces

Since the early days of surface science and vacuum technology, it has been known that semiconductor surfaces exhibit a rich variety of reconstructed structures. Through the years, an assortment of surface analytical tools and theoretical methods have been developed to probe the arrangement and bonding of atoms on semiconductor surfaces [17.1].
S. Y. Tong, H. Huang, C. M. Wei

18. Tribology at the Atomic Scale

Tribology is the study of contacting surfaces rubbing against each other. It encompasses the topics of friction, lubrication, and wear, and historically has received the most attention from mechanical engineers. The study of tribology has generally been motivated by the need to optimize the efficiency, endurance, and precision of mechanical devices. There has been a gradually increasing appreciation that tribology presents interesting fundamental problems to materials scientists, physicists, chemists and particularly surface scientists, and that solving these fundamental problems could have striking ramifications for applied problems.
Gary M. McClelland, Sidney R. Cohen


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