Surfaces ultra-high vacuum

The picture therefore remains obscure. The degree of localization may well depend on variable factors such as the purity of the surface (ultra high vacuum is now known to be essential), the temperature, and the magnitude of the lattice parameters relative to the (rather large) size of the xenon atom. 

Surface defects (Section VII-4C) are also influenced by the history of the sample. Such imperfections may to some extent be reversibly affected by processes such as adsorption so that it is not safe to regard even a refractory solid as having fixed surface actions. Finally, solid surfaces are very easily contaminated detection of contamination is aided by ultra-high-vacuum techniques and associated cleaning protocols [24]. 

Soriaga M P 1992 Ultra-high vacuum techniques in the study of single-crystal electrode surfaces Prog. Surf. Sc/. 39 325... 

Ultra-high vacuum (UHV) surface science methods allow preparation and characterization of perfectly clean, well ordered surfaces of single crystalline materials. By preparing pairs of such surfaces it is possible to fonn interfaces under highly controlled conditions. Furthennore, thin films of adsorbed species can be produced and characterized using a wide variety of methods. Surface science methods have been coupled with UHV measurements of macroscopic friction forces. Such measurements have demonstrated that adsorbate film thicknesses of a few monolayers are sufficient to lubricate metal surfaces [12, 181. 

It has been suggested that gold does not have a stable surface oxide (227), and therefore, its surface can be cleaned simply by removing the physically and chemically adsorbed contaminants. However, more recently it has been shown that oxidation of gold by uv and ozone at 25°C gives a 1.7 0.4 — nm thick AU2O2 layer (228), stable to extended exposure to ultra high vacuum (UHV) and water and ethanol rinses. 

Some elements, such as the rare eartlrs and the refractory metals, have a high afflnity for oxygen, so vaporization of tlrese elements in a irormaT vacuum of about 10 " Pa, would lead to the formation of at least a surface layer of oxide on a deposited flhrr. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 Pa. 

Fig. 6. Lateral stiffness vs. load data for a silicon nitride tip vs. mica surface in ultra-high vacuum. Solid line is fit of the JKR model to the data. Reprinted with pennission from ref. [67].

In X-ray photoelectron spectroscopy (XPS), a beam of soft X-rays with energy hv s. focused onto the surface of a solid that is held under an ultra-high vacuum, resulting in the ejection of photoelectrons from core levels of the atoms in the solid [20]. Fig. 15 shows an energy level diagram for an atom and illustrates the processes involved in X-ray-induced photoelectron emission from a solid. 

In recent years the number of techniques available for analysis of metal surfaces has proliferated greatly " . Many of the new methods are ultra-high vacuum (UHV) techniques suitable for analyses of films ranging in... 

A big problem with /t-type materials is their laigc instability in oxygen. This is exemplified by CW), the mobility of which can be as high as 0.08 cm" V 1 s l in ultra-high vacuum, but falls by four or five orders of magnitude upon exposure to air [105]. This could be due to problems of contacts, as illustrated by the fact that modifying the surface of the source and drain electrodes with tetrakisdimethylami-noethylene (TDAE) leads to a substantial increase in the mobility [I05. ... 

While electron or ion beam techniques can only be applied under ultra-high vacuum, optical techniques have no specific requirements concerning sample environment and are generally easier to use. The surface information which can be obtained is, however, quite different and mostly does not contain direct chemical information. While with infra-red attenuated total reflection spectroscopy (IR-ATR) a deep surface area with a typical depth of some micrometers is investigated, other techniques like phase-measurement interference microscopy (PMIM) have, due to interference effects, a much better surface sensitivity. PMIM is a very quick technique for surface roughness and homogeneity inspection with subnanometer resolution. 

Figure 5.39a shows the residual O Is spectrum obtained in ultra-high-vacuum after repeated cleaning cycle at 350-400°C. It is clear that there is a significant amount of residual O on the Pt surface which cannot be removed with conventional cleaning procedures. This by itself suffices to prove the presence of the omnipresent backspillover-formed effective double layer on the vacuum exposed Pt surface. 

Since the pioneering work of Rohrer and Binning,77 scanning tunelling microscopy (STM) has been used to image atomic-scale features of electrically conductive surfaces under ultra-high-vacuum but also at atmospheric pressure and in aqueous electrochemical environments. The ability of STM to image chemisorption and surface reconstruction is well... 

Figure 6.16. Different modes of adsorption of CHjOH on Pt under ultra-high vacuum (left) and in aqueous solutions (right) showing the effect of local electrostatic field and surface work function on the mode of adsorption.100 Reprinted with permission from the American Chemical Society.

The physical methods mostly require ultra high vacuum conditions having the disadvantage of not being applicable directly to solvent swollen films, but recent developments of in situ measurements in SIMS X-ray diffraction surface enhanced Raman spectroscopy (SERS) and scanning electrochemical tunneling microscopy... 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

STM has particularly great potential for in situ chemical studies. While our present knowledge of the atomic structure of catalyst surfaces is largely limited to those structures which are stable in ultra-high vacuum before and after reaction, STM may provide an insight into both adsorbate and catalyst surface structure in situ during the reaction. The following issues to be characterized by STM may be most relevant to characterization of catalysts and catalysis ... 

Pireaux, J.J., Chtaib, M., Delrue, J.P., Thiry, P.A., Liehr, M. and Caudino, R. (1984) Electron spectroscopic characterization of oxygen adsorption on gold surfaces I. Substrate impurity effects on molecular oxygen adsorption in ultra high vacuum. Surface Science, 141, 211-220. 

Abstract A review is provided on the contribution of modern surface-science studies to the understanding of the kinetics of DeNOx catalytic processes. A brief overview of the knowledge available on the adsorption of the nitrogen oxide reactants, with specific emphasis on NO, is provided first. A presentation of the measurements of NO, reduction kinetics carried out on well-characterized model system and on their implications on practical catalytic processes follows. Focus is placed on isothermal measurements using either molecular beams or atmospheric pressure environments. That discussion is then complemented with a review of the published research on the identification of the key reaction intermediates and on the determination of the nature of the active sites under realistic conditions. The link between surface-science studies and molecular computational modeling such as DFT calculations, and, more generally, the relevance of the studies performed under ultra-high vacuum to more realistic conditions, is also discussed. 

XPS and SIMS are both surface sensitive and surface-specific techniques that may be used to probe surface oxidation/degradation of polymers. They use X-rays and ions, respectively, to characterise surface species. Both XPS and SIMS are operated under ultra-high vacuum, UHV, conditions. 

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