XPS applications

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

The application of AFM and other techniques has been discussed in general terms by several workers [350-353]. Other complementary techniques covered in these papers include FT-IR spectroscopy, Raman spectroscopy, NMR spectroscopy, surface analysis by spectroscopy, GC-MS, scanning tunnelling microscopy, electron crystallography, X-ray studies using synchrotron radiation, neutron scattering techniques, mixed crystal infrared spectroscopy, SIMS, and XPS. Applications of atomic force spectroscopy to the characterisation of the following polymers have been reported polythiophene [354], nitrile rubbers [355], perfluoro copolymers of cyclic polyisocyanurates of hexamethylene diisocyanate and isophorone diisocyanate [356], perfluorosulfonate [357], vinyl polymers... 

US National Institute of Standards and Technology (NIST) that can be utilized to obtain IMFP, EAL, and MED data for particular purposes and that are useful for other AES and XPS applications [7]. We then provide information on other data and software resources available for AES and XPS. 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

One of the more recent advances in XPS is the development of photoelectron microscopy [ ]. By either focusing the incident x-ray beam, or by using electrostatic lenses to image a small spot on the sample, spatially-resolved XPS has become feasible. The limits to the spatial resolution are currently of the order of 1 pm, but are expected to improve. This teclmique has many teclmological applications. For example, the chemical makeup of micromechanical and microelectronic devices can be monitored on the scale of the device dimensions. 

The example of B5H9 serves to show how the chemical shift may be used as an aid to determining the stmcture of a molecule and, in particular, in deciding between alternative stmctures. There are many examples in the literature of this kind of application which is reminiscent of the way in which the chemical shift in NMR spectroscopy may be employed. However there is one important difference in using the two kinds of chemical shift. In XPS there are no interactions affecting closely spaced lines in the spectmm, however close they may be. Figure 8.15 illustrates this for the C lx lines of thiophene. In NMR spectroscopy the spectmm becomes more complex, due to spin-spin interactions, when chemical shifts are similar. 

The book scries Electron Spectroscopy Theory, Techniques, and Applications, edited by C. R. Brundle and A D. Baker, published by Academic Press has a number of chapters in its 5 volumes which are useful for those wanting to learn about the analytical use of XPS In Volume 1, An Introduction to Electron Spectroscopy (Baker and Brundle) in Volume 2, Basic Concepts of XPS (Fadley) in Volume 3, Analytical. plications of XPS (Briggs) and in Volume 4, XPSfor the Investigation of Polymeric Materials (Dilks). 

Applications of ISS to polymer analysis can provide some extremely useful and unique information that cannot be obtained by other means. This makes it extremely complementary to use ISS with other techniques, such as XPS and static SIMS. Some particularly important applications include the analysis of oxidation or degradation of polymers, adhesive failures, delaminations, silicone contamination, discolorations, and contamination by both organic or inorganic materials within the very outer layers of a sample. XPS and static SIMS are extremely comple-mentar when used in these studies, although these contaminants often are undetected by XPS and too complex because of interferences in SIMS. The concentration, and especially the thickness, of these thin surfiice layers has been found to have profound affects on adhesion. Besides problems in adhesion, ISS has proven very useful in studies related to printing operations, which are extremely sensitive to surface chemistry in the very outer layers. 

Note The application breakdown for ABS and XPS is based on data for Western Europe for 1997. The data for SAN is based on data available in the early 1990s but still believed approximately correct. 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

XPS has been used in almost every area in which the properties of surfaces are important. The most prominent areas can be deduced from conferences on surface analysis, especially from ECASIA, which is held every two years. These areas are adhesion, biomaterials, catalysis, ceramics and glasses, corrosion, environmental problems, magnetic materials, metals, micro- and optoelectronics, nanomaterials, polymers and composite materials, superconductors, thin films and coatings, and tribology and wear. The contributions to these conferences are also representative of actual surface-analytical problems and studies [2.33 a,b]. A few examples from the areas mentioned above are given below more comprehensive discussions of the applications of XPS are given elsewhere [1.1,1.3-1.9, 2.34—2.39]. 

In a recent paper Pijpers et al. [2.42] have reviewed the application of XPS in the field of catalysis and polymers. Other recent applications of XPS to catalytic problems deal with the selective catalytic reduction of using Pt- and Co-loaded zeolites. Although the Al 2p line (Al from zeolite) and Pt 4/ line interfere strongly, the two oxidation states Pt and Pt " can be distinguished after careful curve-fitting [2.43]. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

In several applications depth profiles were recorded to check the extent of reaction of various materials with superconductors, or to analyze uniformity and concentration during preparation of superconductors. Such investigations have been performed by both XPS and AES. 

Like XPS, the application of AES has been very widespread, particularly in the earlier years of its existence more recently, the technique has been applied increasingly to those problem areas that need the high spatial resolution that AES can provide and XPS, currently, cannot. Because data acquisition in AES is faster than in XPS, it is also employed widely in routine quality control by surface analysis of random samples from production lines of for example, integrated circuits. In the semiconductor industry, in particular, SIMS is a competing method. Note that AES and XPS on the one hand and SIMS/SNMS on the other, both in depth-profiling mode, are complementary, the former gaining signal from the sputter-modified surface and the latter from the flux of sputtered particles. 

Together with XPS and AES, SSIMS ranks as one of the principal surface analytical techniques. Because its sensitivity for elements greatly exceeds that of the other two techniques and much chemical information is available, its use is rapidly expanding in many fields of application. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

The primary drawback to the application of XPS in adhesion science is associated with the limited spatial resolution of the technique. This can make it difficult to study processes that are highly localized, such as corrosion, or to accurately characterize certain types of failure surfaces where, for example, the locus of failure may pass back and forth between two phases. 

Many applications of XPS to problems in adhesion science have been reported in the literature. One interesting example is provided by the work of Tsai et al. on the use of XPS to investigate reactions between model rubber compound and plasma polymerized acetylene films that was discussed above [22,23], Consideration of that system permits some interesting comparisons to be made regarding the type of information that can be obtained from RAIR and XPS. 

Several UHV techniques which have been developed have not found such wide use in corrosion analysis, despite potential applicability. Ultraviolet photoelectron spectroscopy (UPS) is one of these, operating in a similar fashion to XPS (but using an ultraviolet excitation), and probing the valence electrons, rather than the core electrons of the atoms. Because the energies of the valence electrons are so very sensitive to the precise state of the atom, the technique is in principle very informative however exactly this high sensitivity renders the data difficult to interpret, particularly as a routine... 

Radioactivity The ability possessed by some natural and synthetic isotopes to undergo nuclear transformation to other isotopes, 513 applications, 516-518 biological effects, 528-529 bombardment reactions, 514-516 diagnostic uses, 516t discovery of, 517 modes of decay, 513-514 nuclear stability and, 29-30 rate of decay, 518-520,531q Radium, 521-522 Radon, 528 Ramsay, William, 190 Random polymer 613-614 Randomness factor, 452-453 Raoult s law A relation between the vapor pressure (P) of a component of a solution and that of the pure component (P°) at the same temperature P — XP°, where X is the mole fraction, 268... 

Table 8 shows results obtained from the application of various bulk and surface analysis methods to lithium metal at rest or after cyclization experiments, as well as at inert and carbon electrodes after cathodic polarization. The analytical methods include elemental analysis, X-ray photoelectron spectroscopy (XPS or ESCA), energy-dispersive analysis of X-rays (X-ray mi-... 

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