XPS, core-level spectra

Figure 5-19. N(ls) XPS core level spectra of emeraldine base adsorbed on ITO. The top most spectrum corresponds to ultra-thin Him (in the mono layer regime) while the bottom spectrum corresponds to thick film.

Figure 8. Valence band XPS (a) and UPS (b) spectra of silver islands on native oxide covered Si(l 0 0) during bombardment with 1 keV Ar" ions. Substrate related contributions are removed. Numbers at each spectra stand for the Ag/Si ratio determined from the appropriate XPS core level spectra. The uppermost curve is the spectrum of polycrystalline bulk Ag. (Reprinted from Ref [146], 1998, with permission from Elsevier.)...

Salaneck [3] presented in the mid-1980s a fine review on the application of photoelectron spectroscopic techniques to the study of electroactive polymers. However, a substantial number of new and significant XPS studies have since appeared, in conjunction with the ever increasing research on new families of electroactive polymers. This is particularly true for aniline polymers and some polyheterocycles. Thus, updating XPS work on electroactive polymers appears to be appropriate. This review will focus mainly on XPS core-level spectra, with some references made to valence band spectra. First, a brief description of the basic principles of XPS is presented for readers who are less familiar with this technique. Next, the type and level of information that its application provides for the elucidation of the intrinsic structure, the CT interaction, and the stability and degradation behavior of each family of electroactive polymers are presented and discussed in detail. Finally, some future directions for the applications of... 

In this review, some of the electroactive polymers most commonly studied during the past one and a half decades have been selected to illustrate the type and level of information obtainable from XPS core-level spectra. It concerns (a) the intrinsic structure, (b) the CT interaction, and (c) the stability and degradation behavior. The review is meant to be comprehensive, although emphasis has been placed on some specific issues related to these three basic physicochemical properties. For example, the chemical nature of the nitrogens in PPY and PAN has been critically compared on the basis of XPS data. Some of the major discrepancies in the XPS literature of electroactive polymers have also been examined. In most cases, preference has been given to results for which proper justification and careful comparison with available data are possible. Finally, some future trends in the application of XPS and other more surface sensitive techniques to the study of highly reactive conjugated polymer surfaces have been mentioned. 

Figure 1. Cls XPS core-level spectra of (a) PMDA-ODA starting material, (b) potassium polyamate, (c) polyamic acid and (d) re-cured polyimide. The takeoff angle of electrons was 35° from the sample surface.

Figure 2. C Is XPS core level spectra of benzene on copper.

Radiation damage might occur during the study of polymers. In our investigations, only the chlorine-containing compounds were found to present a color change, but no loss of chlorine could be detected on the XPS core level spectra. 

Fluoro-substituted Polymers. The fluoropolymers were between the first to be studied by the XPS technique because the substitution of F atom(s) in the -CH.-CH - unit induced very large modifications in the XPS core level spectra (shifts up to 8eV) that were easy to detect and interpret. The XPS valence band spectra of similar compounds, namely poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVF2), poly(trifluoroethylene) (PVF3), and poly(tetrafluoroethylene) (PTFE) (26, 27, 28) are also expected to reflect the induction of such strong electronic effects at the valence molecular level. 

Head-to-Head and Head-to-Tail Linkages. Errors in the linkages between successive monomeric units of the poljrmers are possible (and always statistically present). The effect of head-to-head (HH) and head-to-tail (HT) bonds in the XPS core levels spectra of substituted polymers have been computed and found at the limit of the sensitivity of the technique (e.g. ). The control of these linkages during the synthesis is difficult and the number of polymers that can be prepared with 100% of HH or HT linkages is small (43). 

Spectra. This can be done, for example, by combining the XPS core-level spectra with UPS valence band spectra, as described in more detail in the next section, or by referencing to model compound [13]. 

Figure 5-11. XPS core level spectra recorded during successive removals of the aluminum layer on PPV (adapted from [55]).

Fig. 4.7. XPS core-level spectra of Zr 3d levels of Ni64Zr36 after exposure to 80 L 02 at 420 K [4.66, 67]. Reference spectra of Zr02 (obtained after exposure to 1000 L 02) and clean metallic alloy (0 L 02) are shown for comparison. Zr 3dJ/2 core-level positions of Zr and Zr02 are indicated by vertical lines...

Figure 4.21 shows the XPS core level spectra of the Fe 2p and Zr 3d electrons measured for the stable active catalyst. The outer surface is covered with iron oxide (in hematite-like forms) and zirconia exists as non-stoichio-metric Zr02-x. In the iron 2p spectrum, a contribution of metallic iron is visible indicating that the surface oxide film is thin within the information depth of XPS (ca. 2.5 nm). It has been suggested that the surface oxide stabilizes the iron... 

Fig. 4.21. XPS core-level spectra of final stable active catalyst derived by in situ activation of glassy Fe91Zr9 [4.44]. Times in minutes refer to Ar sputtering (scanning ion source at 5 kV with 10 mA emission at 2 x 10 6 Torr). The dotted lines denote centroid positions of elemental Fe (2p-3/2) and Zr (3d-5/2)...

The pioneering work of Clark [32], Dilks [33] and Briggs [34] on the application of the XPS technique to polymers, coupled with non-empirical molecular orbital calculations, demonstrated the indisputable capabilities of the technique in elucidating many important physicochemical properties of the polymer surface and interface. Table 3.1 summarizes the level of information that can be derived from the principal features of typical XPS core-level spectra of polymers. From the characteristic BEs of the photoelectrons, the elements or chemical species involved can be identified. XPS peaks are usually named according to the photoemitting levels, eg, Cls, Nls, C12p, Br3d,... 

Table 3.1. Informaiion associaied wiih ihe main features of ihe XPS core-level spectra of polymers...

Figure 3.20. Nls and Br3d XPS core-level spectra (a = 75°) for three LM-bromine complexes with surface Br ratios of. (a) and (b) 0.15 (c) and (d) 0.55 and (e) and (f) 1.0. (Reprinted with permission from ref 90, Gordon and Breach Sci. Pub. S.A.)...

Changes in the XPS core level spectra are partly discussed in section 2.2.2. For more detailed information the reader is referred to the literature cited therein. 

Fig. 22.11 XPS core level spectra of tested CCM with 10 pg/cm of Ru (a) and Ir (b). Working electrode side Ru 3p and Ir 4f, peaks (upper spectra). Counter electrode side (lower spectra) indicating that no Ru nor Ir peaks are detected in the same BE region on the counter electrode (uncoated Pt-NSTF)...

FIGURE 4.2 (a) Cu 2p XPS core level spectra of arrayed ZnO NR Cn NP nanocomposites prepared with different copper decoration concentrations from 0.5 to 3mM 7=normahzed intensity, (b) Cu K-edge XANES spectra of arrayed ZnO NR Cn NP nanocomposites and btrlk reference sample copper foil A = normalized absorbance. Reprinted with permission from Ref. [62]. John WUey Sons. 

Fig. 13. La 3d XPS core level spectra of La-Ni compounds (Fuggle et al. 1983a,b).

Figure 4 XPS core-level spectra for the NTS and NTS ooH monolayers. The take-off angle of photoelectrons was 10 ...

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