Ultrahigh Vacuum Materials

Technical Background ofVacuum Technology 3.1.3.1 Ultrahigh Vacuum Materials 

The vapor pressure of materials as well as the desorption rate of adsorbates increase with rising temperature. Therefore, once closed, UHV chambers are subjected to a bake-out procedure, that is, the whole vacuum system is heated to 150-250 °C for many hours while pumping. Likewise, filaments and the samples themselves have to be preheated to temperatures higher than in the following experiments in order to outgas unwanted contaminants (specific sample cleaning, see Section 3.1.4.1). Obviously, bake-out and outgassing temperatures put constraint on the materials that can be used for and in a UHV system. 

Depending on the materials and components involved, each individual UHV system will have its own maximal tolerable bake-out temperature, which will be determined by the most temperature-sensitive materials. Consequently, because of the exponential dependence of vapor pressure and desorption rate on temperature, the necessary bake-out duration will vary in practice from half a day up to several days. 

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Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

Several striking examples demonstrating the atomically precise control exercised by the STM have been reported. A "quantum corral" of Fe atoms has been fabricated by placing 48 atoms in a circle on a flat Cu(lll) surface at 4K (Fig. 4) (94). Both STM (under ultrahigh vacuum) and atomic force microscopy (AFM, under ambient conditions) have been employed to fabricate nanoscale magnetic mounds of Fe, Co, Ni, and CoCr on metal and insulator substrates (95). The AFM has also been used to deposit organic material, such as octadecanethiol onto the surface of mica (96). New appHcations of this type of nanofabrication ate being reported at an ever-faster rate (97—99). 

Single-crystal samples of conductors best other solid samples are suitable, including polycrystalline metals, polymeric materials, semiconductors, and insulators, ultrahigh vacuum compatible typically > 5 mm diameter, 1-3 mm thick... 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

Operationally, the electron techniques all require high-vacuum or, more likely, ultrahigh-vacuum environments, and the magnetic material of interest must be within a few atomic layers of the surface. MOKE analysis is not restricted by these constraints, although interesting samples may be. 

My principal objective in Section 10.4 has been to underline the necessity for a drastic enhancement of a crucial experimental technology, the production of ultrahigh vacuum, as a precondition for the emergence of a new branch of science, and this enhancement was surveyed in the preceding Section. It would not be appropriate in this book to present a detailed account of surface science as it has developed, so 1 shall restrict myself to a few comments. The field has been neatly subdivided among chemists, physicists and materials scientists it is an ideal specimen of the kind of study which has flourished under the conditions of the interdisciplinary materials laboratories described in Chapter 1. 

Electrocatalytic activity of supported metal particles has been investigated on surfaces prepared in an ultrahigh vacuum (UHV) molecular beam epitaxy system (DCA Instruments) modified to allow high throughput (parallel) synthesis of thin-film materials [Guerin and Hayden, 2006]. The system is shown in Fig. 16.1, and consisted of two physical vapor deposition (PVD) chambers, a sputtering chamber, and a surface characterization chamber (CC), all interconnected by a transfer chamber (TC). The entire system was maintained at UHV, with a base pressure of 10 °mbar. Sample access was achieved through a load lock, and samples could be transferred... 

Aniline. Aniline black is a well known polymer of aniline formed by electrophilic additionf3.41. Numerous investigators have formed poly(aniline) films by anodic deposition of Pt and other electrode materials. We have examined the interaction of aniline with clean Ni(lll) and Ni(100) surfaces in ultrahigh vacuum and found aniline to form an orientationally ordered, thermally stable polymer film. Electrochemically prepared poly(aniline) films also show the high degree of orientational ordering. 

X-ray photoelectron spectroscopy is indeed quite informative, but requires the use of expensive instrumentation. Also, the detection of photoelectrons requires the use of ultrahigh vacuum, and therefore can mostly be used for ex situ characterization of catalytic samples (although new designs are now available for in situ studies [146,147]). Finally, XPS probes the upper 10 to 100 A of the solid sample, and is only sensitive to the outer surfaces of the catalysts. This may yield misleading results when analyzing porous materials. 

Another difficulty arises from the chemical properties of the actinide metals. They are chemically reactive, rapidly corroded by moist air, pyrophoric, and, when in the molten state, dissolve common crucible materials. The radioactivity of short-lived isotopes of Am and Cm makes their long-term storage difficult small amounts can be stored successfully under ultrahigh vacuum. Large amounts of isotopes such 238pu with a Ti/2 of only 87.7 years are best stored under a pure inert... 

PVD reactors may use a solid, liquid, or vapor raw material in a variety of source configurations. The energy required to evaporate liquid or solid sources can be supplied in various ways. Resistive heating is common, induction heating of the source bottle is sometimes used, and electron beams are also employed. Molecular-beam-epitaxy (MBE) systems are PVD-type reactors that operate at ultrahigh vacuum. Very low growth rates are used ( 1 xm/h), and considerable attention is devoted to in situ material characterization to obtain high-purity epitaxial layers (2). 

Most solids are not crystalline on their surface. This is certainly true for amorphous solids. It is also true for most crystalline or polycrystalline solids because for many materials the molecular structure at the surface is different from the bulk structure. Many surfaces are for example oxidized under ambient conditions. A prominent example is aluminum which forms a hard oxide layer as soon as it is exposed to air. Even in an inert atmosphere or in ultrahigh vacuum (UHV) the surface molecules might form an amorphous layer on the crystalline bulk solid. 

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