X-ray photoelectron spectroscopy

XPS is a powerful technique allowing estimation of the elemental and chemical composition of the upper 10 nm of a surface, and it is an effective tool to quantify the amount of protein immobilized or adsorbed during enzyme immobilization [16-21]. 

XPS finds utility in the analysis of electrocatalysts for fuel cells [22], and an extensive review on the application of XPS to biomaterials, particularly nanobiomaterials, was published by Baer and Engelhard in 2010 [1], 

The basic balance of energy equation relates binding energy E of an electron (which is specific to the element and its chemical environment) to kinetic energy (E ) and primary photon energy (hv), taking into account a work function of the instrument qi) (Equation 13.1)  

The sampling depth in XPS is determined by the following equation (Equation 13.2)  

Applications. XPS was used for the following purposes determination of elemental composition of nanocomposites, the effect of oxidation and reduction of carbon fibers by monitoring the 0/C ratio, 2- 35, 74 concentration of functional groups on the surface of carbon fibers, elemental composition of the surface of carbon fibers, the effect of surface coating on the surface composition of carbon fi- 

X-ray radiography of nickel-coated fibers, surface atoms of hydroxyapatite, coating analysis of silane treated hydroxyapatite, and composition of the failure area of an adhesive joint between rubber and metal. This review of applications shows that carbon fibers are the most frequently tested material by XPS. 

Testing procedure. The methods of testing used were fairly standard techniques of equipment operation. It was mentioned that the sampling depth of XPS ( 50-100 A) is sensitive enough to detect silane having a thickness of 5-10 monolayers. Standard methods. ASTM E 902 (instrument calibration). 

1 Hirschler M, Flame Retardants 96. Conference proceedings, London, 17th- 18th Jan.1996, 199-214. 

X-ray photoelectron spectroscopy (XPS) provides surface element analysis, including information on the oxidation states of the elements. It should be noted that XPS can be used in conjunction with sputtering the techniques, thus providing depth profiling of the composition of surface films [27, 28]. 

It is highly important to monitor and control the dissolution of transition metal cations to solution phase from the cathodes active mass. It is important to measure solutions in which electrodes were cycled and solutions in which pristine 

X-ray photoelectron spectroscopy (XPS), also referred to as electron spectroscopy for chemical analysis, is a surface characterization technique based on the photoelectron effect. XPS surveys the electron binding energy spectrum of a sample surface resulting in a plot of binding energy versus total electron count. Since the binding energy of electrons of different elements is different, XPS can be used to identify the different elements present on the surface and the composition ratio of each element. In theory, XPS can detect all elements. However, H and He are barely detected in practical situations [46]. 

XPS uses the photoelectronic effect to obtain binding energy information (Eqn (2.4)). When the ample surface is irradiated by X-rays, the X-rays knock out outer electrons on the surface. This process is an energy conservation process so the photonic 

Zwitterionic groups such as phosphoryl choline and sulfonyl betaine can form a hydrated layer to prevent protein adsorption [48-51]. Li and coworkers used urethane chemistry to synthesize a series of oligomeric polyurethanes with terminal 

Polyethylene glycol (PEG) is another well-known molecule used to reduce protein adsorption and/or platelet adhesion. Surface enrichment of a triblock oligomeric PEG containing additive from a polyurethane matrix was reported [54,55]. The authors used PEG as the active groups to suppress protein and platelet adhesion. The authors first synthesized a methylene diphenyl diisocyanate (MDI)-poly (tetramethylene oxide) (PTMO) 1000 prepolymer with a MW of approximately 4750 (PU4750), and then this prepolymer was terminally functionalized with mono amino-polyethylene oxide (PEG) with different MW (PEO550, 2000, or 5000, Table 2.3). This triblock copolymer was mixed with a polyurethane (MDI/ PTMO 1000/ethylene diamine (ED)) at different ratios in dimethylformamide (DMF) and cast into polymer films. The surface compositions of these films were evaluated by XPS. 

The degree of surface enrichment of PEO groups depends on not only the aging time but also the MW of the triblock copolymer. When the low MW copolymer 1 was blended with matrix PU, the surface of the blended polymer film had the same ether carbon content as that of copolymer 2 after just 3 days of aging 

X-ray photoelectron spectroscopy (XPS, commonly termed ESCA as an abbreviation for electron spectroscopy for chemical analysis) is eminently suited to the study of surfactant adsorption. The XPS method is highly sensitive to the surface composition and can characterize adsorbed surfactant layers without elaborate sample preparation. 

The work function (f depends on the sample and the spectrometer used for measuring photoelectron emission. 

The binding energies of the electrons are characteristic of the element and the environment of the atom in the molecule. Hence, XPS can characterize the composition and the chemical state of the near-surface region. 

The XPS spectra are strongly affected by the orientation of the sample, the source, and the spectrometer. Almost all (about 95%) of the signal emerges from the distance 3 A within the solid, where A is the inelastic mean free path of the electron, also called the attenuation length of the emerging electron. The sampling depth, d, of the subsurface analyzed by XPS is given by 

Although the method is considered to be nondestructive, sample damage and evaporative losses have been of concern [266]. The fluorine-to-carbon pho- 

Unlike the differences observed in the C(ls) XPS spectra, the 0(ls) XPS spectra for COj adsorbed on K/Ag(lll) surfaces with and without pre-dosed oxygen were similar to each ocher and were 

While the chemical composition of the glasses can be well calibrated by XPS, the XPS valence band spectra can provide useful Information on the chemical bonds in chalcogenide glasses. For example, Bergignat et al. showed that Ge 4s and Se 4s band shapes are very sensitive to the presence of Ge-Ge and Se-Se bonds [74]. The Ge 4s band at 5-10 eV and the Se 4s band at 11-15 eV generally become broader as the Ge and Se content increases in Ge fSci glasses. 

Acronyms XPS or XPES X-ray photoelectron spectroscopy, ESCA electron spectroscopy for chemical apphcations (originally analysis), PESIS photoelectron spectroscopy of inner shell, ARXPS angle resolved X-ray photoelectron spectroscopy. 

SRPES synchrotron radiation photoelectron spectroscopy. For photon energies less than 300 eV also SXPS soft X-ray photoelectron spectroscopy. 

Samples are exposed to monochromatic X-ray radiation (typically with an energy range of 12 keV but energies up to 10 keV may be used) and the characteristic energies of the emitted photoelectrons are measured to reveal information about the elemental composition, elemental distribution, and chemical bonding characteristics of the near surface region. 

The measurement is carried out in an UHV chamber. Monochromatic X-ray photons, typically from an Al-K (1486.6 eV) or Mg-K, (1253.6 eV) X-ray source, are shone at the sample to eject photoelectrons. These electrons have energies ranging from 0 eV up to almost the same energy as the incident photons but most are emitted at a few discrete energies that are characteristic of the elements present in the sample. The photoelectrons are collected by an electron energy analyser such as a hemispherical mirror analyser (HMA) to produce a spectmm of the number of electrons vs. their kinetic energy. Analysis of this spectmm provides quantitative information about the composition of the near surface region of the sample. 

Tilting the sample so that the analyser collects electrons that are emitted at a more grazing angle increases the surface sensitivity of the technique. By acquiring spectra at several different angles, which have different surface sensitivities, quantitative information about the depth distribution of the elements in the top few nanometers of the sample can be extracted, this is know as angle resolved XPS (ARXPS). 

In the technique of x-ray photoelectron spectroscopy we use an incident beam of monochromatic x-rays to irradiate the sample. The incident photon ejects an electron from the atom and imparts a kinetic energy to that electron. The energy required to remove the electron, Ej, plus the kinetic energy of the electron, must be equal to hv, the energy supplied by the photon. Thus 

The difference in the values between two neighboring elements ranges from 56 to 205 e V. It f ollows t hat we can easily distinguish one element from another by this technique. The binding energies depend slightly on the chemical environment. However, this variation is usually less than 10 eV and consequently does not hinder identification of the elements. 

7 Vacuum Techniques X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS), Auger Electron Spectroscopy 

Whilst a number of methods exist for the study of surfaces, three vacuum analysis methods have been most successfully applied to the characterization of polymer surfaces X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS). 

or as it is sometimes called electron spectroscopy for chemical analysis (ESCA), allows characterization of the elemental composition of the top 10-30 A of a material. Its disadvantage is that it is a high-vacuum technique and hence can only be used for the study of materials whose structure is not sensitive to the application of a vacuum. 

The incident X-ray beam has an energy given by hv, where v is the frequency. The X-rays are able to ionize an electron from the core K shell which is emitted with an energy given by 

Each element has a distinct set of binding energies that are characteristic of that atom (Table 9.3). 

FIG U RE 7.17 Schematic arrangement of (a) wavelength-dispersive and (b) energy-dispersive x-ray fluorescence spectrometers. (Reprinted from Karathanasis, A. D., and B. F. Hajek, Methods of Soil Analysis. Part 3. Chemical Methods, Ameriean Soeiety of Agronomy-Soil Science Society of Ameriea, Madison, Wisconsin, 1996. With permission from the Soil Science Society of America.) 

FIGURE 7.18 Wide-scan x-ray photoelectron spectrum of a podzolic soil from British Columbia, Canada, excited by Al radiation the upper band is the enlargement of the left 

Meier et al. [166] applied polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) to simultaneously monitor hquid phase and adsorbed species during benzyl alcohol selox over Pd(l 11). Anaerobic conditions favored CO surface accumulation via decarbonylation, while surface oxygen fadhtates CO oxidation, leaving no surface residues strongly adsorbed CO and oxygen poison the active surface sites. 

Friend and coworkers [167, 168] investigated ethanol oxidation on an oxygen-covered Au(l 11) surface using temperature-programmed desorption and vibrational spectroscopy. They demonstrated that the atomic oxygen concentration 

Miedziak, P.J., Dimitratos, N., Lopez-Sanchez, ).A., Dummer, 

De Winne, H., Corthals, S., Poelman, H., De Gryse, R., Meynen, 

Laszlo, P. (1991) in Comprehensive Organic Synthesis (eds B.M. Trost and 1. Fleming), Pergamon, Oxford, pp. 839-848. 

Consequently, XPS has developed from a large area analysis method to one which has some degree of spatial resolution (selected area analysis). There are essentially only two ways in which such an improvement can be obtained, operating the spectrometer in a microprobe mode, in which the X-ray 

Historically, the lack of imaging capabilities has hampered polymer characterisation by XPS. The combination of microscopy and spectroscopy has been the goal of a number of groups exploring photoelectron microscopy with X-ray or synchrotron radiation sources. The first real step towards imaging XPS (iXPS) was in 1988 (VG ESCAscope). The system allowed obtaining 2D spatial maps with a lateral resolution of 10 /xm. The second generation of this instrument achieved a spatial resolution of approximately 2 /txm [764,769]. 

Important and straightforward industrial applications of /xXPS with micro-focused X-ray sources are the determination of the diffusion characteristics of protective coatings or paint systems and the characterisation of multilayer packaging systems [770], Characterisation of such systems by vibrational spectroscopic techniques is often made difficult by the presence of inorganic particles. /xXPS has the spatial resolution needed for analysis of thin multilayer structures. 

There exists a need within polymer research, specifically with respect to coatings and adhesives, to attain molecular information at both microscopic 

Newer analytical tools such as iXPS and PA-FTIR can be used for surface mapping of polymers containing blooming additives [775]. These tools can be used to understand the mechanism of migration in addition to the rate of blooming. 

A companion field. Auger electron spectroscopy (AES), was developed simultaneously. AES does not provide chemical species information, only elemental analysis, as we will see. Since the electrMis ejected in these two techniques are of low energy and the probability of electron interaction with matter is very high, the electrons cannot esa ie from any significant depth in the sample. Typical esa ie depths for XPS and AES electrons range from 0.5 to 5 nm for materials. The phenomenon is therefore confined to a few atomic layers, combined or otherwise, which are at the surface of the sample, and provides a method of surface analysis. 

The kinetic energy of the escaping electron is designated as E. The binding energy of this electron is given by the equation 

The XPS spectrum is a plot of the number of emitted electrons per energy interval vs. their kinetic energy. The work function of the spectrometer can be measured and is constant for a given instrument, allowing the binding energies of the electrons to be determined. 

When excess electromagnetic energy is transferred to an electron that is in a further out shell, it is called an Auger electron. An analysis of these for chemical identification is known as Auger electron spectroscopy (AES). X-ray photoelectron spectroscopy (XPS) analyses electron emission of similar high energy [59,60]. XPS can be used to measure the chemical or electronic state of surface elements, detect chemical contamination or map chemical uniformity of biomedical implant surfaces. 

The preparation and characterization of vanadium oxide (V2O5) modified tungsten oxide (WO3) films by pulse laser deposition were recently examined [61]. Tungsten oxide coatings are used in many electro-optical applications including catalysts, solar cell windows, electrochromic and gasochromic devices, optical memory and 

XPS spectra of WO3 and samples WVl to WV5 which contain progressively higher levels 

Materials Characterization Yang Leng 2008 John Wiley Sons (Asia) Re Ltd 

Another feature in PES spectra is the so-called shake-up structures, appearing as weak satellites on the high binding energy side of the main line. The shake-up structure reflects the spectrum of the l-electron-2-hole states generated in connection with photoionization, and can give useful information about the valence 7i-electronic structure of a molecular ion. 

The PES measurements are performed with reference to the Eermi level of the photoelectron spectrometer, in solid specimens, as dealt with here, by the way the spectroscopy works. Thus, in cases when the Eermi level shifts due to some chemical modifications of the sample, i.e., in the intercalation of graphite or other layered compound [16] or in the doping of conjugated polymers [17], it is necessary to account for the change in the Eermi energy level before interpreting 

5 Electronic Structure of Surfaces and Interfaces in Conjugated Polymers 

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]. 

TABLE 4.3 Surface Characterization Metiiods and Information Gained 

Raman spectroscopy and surface-enhanced Raman spectroscopy 

Information on structural units, verification of chemical structure, binding states, H bonding and other noncovalent interactions, lateral resolution micrometer range (see Section 4.1) Information on structural units, verification of chemical structure, binding states, H bonding and other noncovalent interactions, lateral resolution 100 nm range (see Section 4.1) Presence of chromophores (see Section 4.1) Presence of fluorophores, lateral resolution in micrometer range 

Elemental analysis of the surface (topmost layer as well as down to 10 nm) 

Surface force microscopy Surface topology, homogeneity local differences 

Pena and co-workers [111] discuss factors affecting the adsorption of organophosphorus polymer stabilisers onto carbon black. 

For much more detailed descriptions of the techniques the interested reader should consult references [26-28]. 

The information from XPS is surface specific because the electrons that give rise to the useful peaks in the spectrum have emerged from the material elastically, i.e., without loss of energy. The inelastic mean free path X ) of electrons of energy in an inorganic solid is given, approximately, by Equation (3.11)  

Insulating samples charge up under X-ray bombardment due to secondary election emission. With conventional sources, this is counterbalanced by the flood of low-energy electrons emanating from the front face of the window separating the X-ray source from the analysis chamber. With monochromated sources this mechanism is not available and a discrete source of electrons needs to be provided from a low-energy flood gun . 

Bonding electrons are also photoemitted and these appear in the valence band between, say 0-30 eV BE. Emission from many closely spaced levels with different cross-sections gives rise to a complex spectrum, often rich in structure, which in principle contains more direct structural information than the core level peaks. The spectrum is rather low in intensity (typically only a few percent that of major core lines) but with higher power instruments it is routinely accessible. The fingerprint utility of the valence band is increasingly being augmented by full interpretations based on theoretical calculations. 

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Finally, similar effects can be seen in miscible polymer blends where the surface tension correlates with the enrichment of the lower-energy component at the surface as monitored by x-ray photoelectron spectroscopy [104],... 

XPS X-ray photoelectron spectroscopy [131-137] Monoenergetic x-rays eject electrons from various atomic levels the electron energy spectrum is measured Surface composition, oxidation state... 

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... 

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]. 

XPS is also often perfonned employing syncln-otron radiation as the excitation source [59]. This technique is sometimes called soft x-ray photoelectron spectroscopy (SXPS) to distinguish it from laboratory XPS. The use of syncluotron radiation has two major advantages (1) a much higher spectral resolution can be achieved and (2) the photon energy of the excitation can be adjusted which, in turn, allows for a particular electron kinetic energy to be selected. 

Powell C J, Jablonski A, Tilinin I S, Tanuma S and Penn D R 1999 Surface sensitivity of Auger-electron spectroscopy and x-ray photoelectron spectroscopy J. Eiectron Spec. Reiat. Phenom. 98-9 1... 

Powell C J 1994 Inelastic interactions of electrons with surfaces applications to Auger-electron spectroscopy and x-ray photoelectron spectroscopy Surf. Sc/. 299-300 34... 

X-ray photoelectron spectroscopy (XPS) is among the most frequently used surface chemical characterization teclmiques. Several excellent books on XPS are available [1, 2, 3, 4, 5, 6 and 7], XPS is based on the photoelectric effect an atom absorbs a photon of energy hv from an x-ray source next, a core or valence electron with bindmg energy is ejected with kinetic energy (figure Bl.25.1) ... 

XPS X-ray photoelectron spectroscopy Absorption of a photon by an atom, followed by the ejection of a core or valence electron with a characteristic binding energy. Composition, oxidation state, dispersion... 

Briggs D and Seah M P (eds) 1983 Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (New York Wiley)... 

Wagner C D, Riggs W M, Davis L E, Moulder J F and Muilenburg G E 1979 Handbook of X-ray Photoelectron Spectroscopy (Eden Prairie, MN Perkin Elmer)... 

McFeely and co-workers used soft x-ray photoelectron spectroscopy (SXPS) to measure the changes in binding energies of Si(2p) levels after slight exposure to fluorine atoms via dissociative chemisoriDtion of XeF2 [39]. Using synclirotron radiation at 130 eV as the source enabled extreme surface sensitivity. Since this level is split into a... 

Barr, T. L. (1994) Modern ESCA The Principles and Practice of X-ray Photoelectron Spectroscopy, CRC Press, Boca Raton, FL. 

Briggs, D. (Ed.) (1994) Practical Surface Analysis Auger and X-ray Photoelectron Spectroscopy, John Wiley, Chichester. 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (xps) and Auger electron spectroscopy (aes) are related techniques (19) that are initiated with the same fundamental event, the stimulated ejection of an electron from a surface. The fundamental aspects of these techniques will be discussed separately, but since the instmmental needs required to perform such methods are similar, xps and aes instmmentation will be discussed together. 

C. D. Wagner, W. M. Riggs, L. E. Davis, andj. F. Moulder, in G. E. Muilenberg, ed.. Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Prairie, Minn., 1979. 

X-rays provide an important suite of methods for nondestmctive quantitative spectrochemical analysis for elements of atomic number Z > 12. Spectroscopy iavolving x-ray absorption and emission (269—273) is discussed hereia. X-ray diffraction and electron spectroscopies such as Auger and electron spectroscopy for chemical analysis (esca) or x-ray photoelectron spectroscopy are discussed elsewhere (see X-raytechnology). 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

Near edge x-ray absorption fine stmcture spectroscopy (nexafs) and x-ray photoelectron spectroscopy (xps) have been used to study SAMs of OTS, octadecyltrimethoxysilane (OTMS), CH2(CH2) ySi(OCH2)3, and (17-aminoheptadecyl)-trimethoxysilane (AHTMS), H2N(CH2) ySi(OCH3)3 (149). A number of important observations have been reported. First, the chains in OTS SAMs are practicaUy perpendicular to the substrate surface (tilt angle... 

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