XPS spectra

Fig. XVIII-7. Br(3p) XPS spectra of a 2.2-L dose of DBr on Pt(l 11) before (a) and (b) after UV irradiation. The 182.8-eV peak is due to DBr and the 181.5 peak, to atomic Br. On irradiation the latter peak increases relative to the former [38]. (Reprinted with permission from American Chemical Society, copyright 1992.)...

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

Figure Bl.25.2 shows the XPS spectra of two organoplatinum complexes which contain different amounts of chlorine. The spectrum shows the peaks of all elements expected from the compounds, the Pt 4f and 4d doublets (the 4f doublet is iimesolved due to the low energy resolution employed for broad energy range scans). Cl 2p and Cl 2s, N Is and C Is. Flowever, the C Is caimot be taken as characteristic for the complex only. All surfaces that have not been cleaned by sputtermg or oxidation in the XPS spectrometer contain carbon. The reason is that adsorbed hydrocarbons from the atmosphere give the optimum lowering of the surface free energy and hence, all surfaces are covered by hydrocarbon fragments [9].

Figure Bl.25.3. Pt 4f XPS spectra of platinum metal (top) and of the two organoplatinmn compounds (a) and (b), middle and bottom respectively, shown in figure B 1.25.2, illustrating that the Pt 4f bmding energy reflects the oxidation state of platinum (from [9]).

This is demonstrated by the XPS spectra in figure B 1.25.5(a) which show the Si 2p spectra of a silicon crystal with a thin (native) oxide layer, measured under take-off angles of 0° and 60° [12]. When the take-off angle is... 

Figure Bl.25.5. (a) XPS spectra at take-off angles of 0° and 60° as measured from the surface nonnal from a silicon crystal with a thin layer of Si02 on top. The relative intensity of the oxide signal increases significantly at higher take-off angles, illustrating that the surface sensitivity of XPS increases, (b) Plot of...

Aiiger peaks also appear in XPS spectra. In this case, the x-ray ionized atom relaxes by emitting an electron with a specific kinetic energy E. One should bear in mind that in XPS the intensity is plotted against the bindmg energy, so one uses ( Bl.25.1) to convert to kinetic energy. 

Xps spectra also bear a relationship between photoelectron intensity and number of surface atoms sampled (19,27). Quantitation of these data can be achieved with a precision to within ca 20%. For a homogeneous sample analy2ed in a fixed geometry, the relationship between xps intensity and number of atoms is given by... 

Fig. 2.13. Schematic diagram ofthe type of information obtainable from XPS spectra from an Fe-Cr alloy with oxide film underneath a contaminant film [2.57],...

Fig. 2.14. The C Is XPS spectra recorded by Chin-An Chang et al. [2.68] from perfluoroalkoxy polymer (PFA). (a) before deposition, (b) deposition of copper, (c) deposition of chromium,...

Fig. 2.15. Comparison of the Cu 2p3 2 and satellite XPS spectra from several copper compounds with the spectrum from the superconducting oxide YBajCujOj [2.76],...

There are several considerations that go into selecting an X-ray line to excite XPS spectra. Included are the energy of the X-rays and the width of the line. If the energy is too low, the number of photoelectron lines that will be excited will be too small for general use. If the line width is too large, the resolution in the XPS spectrum will also be too small. Therefore, it is useful to consider the processes involved in X-ray generation. 

Since the K radiation emitted from a magnesium anode consists of the intense Kbinding energies by 8.0 and 10.2 eV [20]. The satellites excited by the Kas and K<,6 lines are usually too weak to be observed. Similar features can be observed in XPS spectra excited using Ko, X-rays from aluminum. 

It can be seen that varying the take-off angle will change the depth of analysis. Thus, a depth profile can also be constructed by obtaining XPS spectra as a function of take-off angle. 

Fig. 19. High-resolution (A) C(ls) and (B) 0(ls) XPS spectra of a plasma polymerized acetylene film with a thickness of 75 nm that was deposited onto a polished steel substrate. Reproduced by permission of John Wiley and Sons from Ref. [22].

Fig. 21. High-resolution XPS spectra of PMMA spin-coated onto (a) silicon and (b,c) iron. The C(ls) spectnim of PMMA on silicon was explained by four components but an additional component was needed to explain the C(ls) spectrum of PMMA on iron. Reproduced by permission of Gordon and Breach Science Publishers from Ref. [24].

Fig. 30. C(ls) XPS spectra of high-density polyethylene before and after flame treatment. Reproduced by permission of VNU Science Press B.V. from Ref. [8].

Fig. 32. High resolution C(ls) XPS spectra obtained from the bulk (A) polyamic acid and (B) polyimide of PMDA/4-BDAF. Reproduced by permission of the American Chemical Society from Ref. [39].

Fig.. 86. Photograph of the metal initiation zone of a lap joint preptired from hot-dipped galvanized. steel substrates showing the six locations where small-area XPS spectra were acquired. Reproduced by permission of John Wiley and Sons from Ref 411.

For the paramagnetic case the expre.ssion for the photo current in Eq. (2) can be simplified to a concentration weighted sum over the products of the K-resolved partial density of states (DOS) ri (F) and a corresponding matrix element that smoothly varies with energy [13]. This simple interpretation of the XPS-spectra essentially also holds for the more complex spin-resolved ca e in the presence of spin-orbit coupling as studied here. 

Figure 2 Theoretical spin-resolved VB-XPS spectra of Coo.6Pto.4 for left (LCP) and right (RCP) circularly polarized radiation and photon energy hu 1253.6 eV.

The UPS spectra (not shown here) recorded upon Al deposition onto the conjugated thiophene systems shows only small visible changes in the positions of the peaks in the spectra [84]. The main effect is a rapid decrease in intensity, which indicates that a metallic overlayer is formed since the cross-sections for the Al(3p) or Al(3s) are much lower than for the C(2p) or S(3p) orbitals. This is consistent with the Al(2p) XPS spectra discussed above. 

Lithium carbonate and hydrocarbon were identified in XPS spectra of graphite electrodes after the first cycle in LiPF6/EC-DMC electrolyte [104]. Electrochemical QCMB experiments in LiAsF6/EC-DEC solution [99] clearly indicated the formation of a surface film at about 1.5 V vs. (Li/Li+). However the values of mass accumulation per mole of electrons transferred (m.p.e), calculated for the surface species, were smaller than those of the expected surface compounds (mainly (CF OCC Li ). This was attributed to the low stability of the SEI and its partial dissolution. 

Figure 5.35. Effect of electrochemical O2- pumping on the O Is spectrum of Pt/YSZ6 (A-C). XPS spectra at 400°C (A) AUWR=0, 1=0 (B) AUWr=1-2 V, 1=40 pA (C) 0 Is difference spectrum.6 Reprinted with permission from the American Chemical Society.

Figure 5.36. Effect of electrochemical O2 pumping on the Zr 3dj XPS spectra of Pt/YSZ at 400°C (a) Zr 3d5/2 spectrum shift from AUWr=0 (solid curve) to AUwr=1. 2 V (dashed curve) (b) effect of overpotential AUv/r on the binding energy, Eb) and kinetic energy, (AEk--AEb) shifts of Zr 3dS/2 (filled circles, working electrode grounded) and Pt 4f7/2 (open circle, reference electrode grounded).6 Reprinted with permission from the American Chemical Society.

The technique of SPEM allows one to obtain XPS spectra from extremely small (-0.02 pm2) surface areas and thus one can study O Is spectra obtained from small (-0.02 pm2) spots on the Pt surface.67... 

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