Ultra-high-vacuum

All experiments under vacuum conditions have been performed in a bakeable, stainless steel UHV chamber (analysis chamber in Fig. 3.1) at a base pressure below 2 X 10 mbar. For sample preparation, the semp is equipped with an e-gun and a MgO evaporator of in house design as well as an commercial sputter gun (EX03, Thermo Scientific, U.S.A.) and an evaporator (e-fiux electron beam evaporator, Tec-tra physical instruments, Germany) for Si (or Ti) evaporation. The introduction of gases is possible by means of a leak valve (VG-MD7) and an molecular beam doser (MBD), both attached to a gas manifold. For cluster deposition the gate valve towards the cluster source is opened and closed afterwards to keep the base pressure in the chamber low. 

Characterization and analysis are performed using the following surface science techniques temperature programmed desorption/reaction (TPD/TPR), pulsed molecular beam reactive scattering (pMBRS) (IRRAS), metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS) and auger electron spectroscopy (AES). First the experimental setup is briefly described, followed by the support preparation and characterization as well procedures utilized in this work. These descriptions include a concise introduction to the underlying physical principles of the applied techniques (including experimental details). 

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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. 

McFadden C F and Geiiman A J 1995 Ultra-high vacuum boundary lubrication of the Cu-Cu interface by 2,2,2-trifluoroethanol Langmuir 273-80... 

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. 

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. 

P. A. Redhead, J. P. Hobson, and E. V. Komelsen, The Physical Basis of Ultra High Vacuum, Chapman and Hall, London, 1968. 

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. 

The catalytic reaction of NO and CO on single crystal substrates, under ultra-high vacuum conditions, has been extensively studied. Neglecting N2O formation and CO desorption, the Langmuir-Hinshelwood mechanism of the NO + CO reaction can be described by the following sequence of steps [16,17] ... 

Rough vacuum Medium vacuum High vacuum Ultra-high vacuum... 

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. ... 

Rough vacuum Medium vacuum High vacuum Ultra high vacuum 760 torr to 1 torr 1 to 10-3 torr 10-3 to 10 torr 10 7 torr and below... 

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.

Ultra-High Vacuum Reactors. CVD reactions at extremely low pressures (i.e., 10 Torr) are being developed for the deposition of semiconductor materials, such as silicon-germanium and some optoelectronic materials. Advantages appear to be better control of the deposit structure and reduction of impurities. 

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... 

Ultra-high-vacuum reactive distillation following ultra-high- vacuum metallothermic (Al) reduction ... 

Atmospheric sensitivity renders the preparation of ultrapure samples difficult. Nevertheless, vacuum distillation ", ultra-high-vacuum reactive distillation " and crystal growth purification methods " are described zone-refining methods have been applied on a limited scale only - , presumably because of the high volatility of the metals and the unfavorable distribution coefficients. 

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