Photoelectron spectroscopy sampling depth

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

XPS spectra were recorded using unmonochromatized Mg K radiation (1253.6 eV), and an unmonochromatized He-resonance lamp was used for ultraviolet photoelectron spectroscopy (UPS). XPS spectra were taken with an analyzer resolution of 0.2 eV, and the net resolution measured as the full width at half-maximum (FWHM) of Au 4f(7/2) was 0.9 eV. The spectrometer is of our own construction and is, e.g., designed to provide optimum angle-dependent XPS or XPS(0) (12,l4). For high 5-values, the photoelectrons leave the sample surface near the grazing angel, and due to the limited escape depth of the electrons, this is a "surface sensitive" mode. In the "bulk sensitive" mode, for low 0-values, the photoelectrons exit near the surface normal, and hence more information from the "bulk" of the sample is obtained (15). 

Photoelectron spectroscopy of valence and core electrons in solids has been useful in the study of the surface properties of transition metals and other solid-phase materials. When photoelectron spectroscopy is performed on a solid sample, an additional step that must be considered is the escape of the resultant photoelectron from the bulk. The analysis can only be performed as deep as the electrons can escape from the bulk and then be detected. The escape depth is dependent upon the inelastic mean free path of the electrons, determined by electron-electron and electron-phonon collisions, which varies with photoelectron kinetic energy. The depth that can be probed is on the order of about 5-50 A, which makes this spectroscopy actually a surface-sensitive technique rather than a probe of the bulk properties of a material. Because photoelectron spectroscopy only probes such a thin layer, analysis of bulk materials, absorbed molecules, or thin films must be performed in ultrahigh vacuum (<10 torr) to prevent interference from contaminants that may adhere to the surface. 

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

Figure 7.38. XPS spectrum of an ionic liquid, [EMIM][Tf2N], detailing the C(ls) and N(ls) regions. Since there are no peaks from the Au substrate, the film thickness is hkely >10nm. Also shown (right) is the comparison between XPS, ultraviolet photoelectron spectroscopy (UPS, Hel = 21.2eV, Hell = 40.8 eV radiation), and metastable impact electron spectroscopy (MIES). Whereas XPS and UPS provide information from the first few monolayers of a sample, MIES is used for zero-depth (surface only) analysis, since the probe atoms are excited He atoms that interact with only the topmost layer of sample. Full interpretations for these spectra may be found in the original work Hofft, O. Bahr, S. Himmer-lich, M. Krischok, S. Schaefer, J. A. Kempter, V. Langmuir 2006, 22, 7120. Copyright 2006 American Chemical Society.

X-ray photoelectron spectroscopy is a surfece analytical (1 - 5 nm depth) technique. For the analyses presented here, it should be remembered that on the scale of the XPS analysis area (4 mm x 3 mm), the individual sample grains would be randomly oriented. The analyses will be averages of measurements from all possible directions of the grains, including those presenting the outer faces of the hexagonal structure to the analyser and others which are end-on, thus contributing information on elements and species within the open ends of the pore structure. 

Chemical characteristics and the oxidation state of elements in the nearsurface layer ( 5 nm) of a sample are recorded by photoelectrons that are produced by an X-ray beam. When this technique is combined with intermittent ion sputtering, data on depth distribution can be obtained. X-ray photoelectron spectroscopy goes beyond elemental analysis to provide chemical information such as distinguishing Si-Si from Si-O bonds. Elements from Li to U may be analysed with detection levels at 0.5% under high vacuum conditions. Raster scanning techniques produce images with a spatial resolution of 26 pm and depth profiles of 1 pm thick are possible (Mossotti et al., 1987 Wilson and Bums, 1987). 

The main factor in beam analysis that affects the reliability of the analytical information is the reproducibility of the surfaces. When using scanning electron microscopy (SEM), the apparati are connected to the computer, which makes it possible to obtain quite a bit of information about the sample, especially by X-ray and AES. However, the apparati cannot assure the same length for beam penetration on the surface, which means that the analytical information can be uncertain. Because the beam analysis is rapid, it requires very fast detectors, e.g., Ge/Li or Si/Li. The LA can be successfully used in surface analysis. An automated system has been constructed, laser-induced breakdown spectrometry (LIBS).213 This is an alternative to other surface techniques — secondary ion mas spectroscopy (SIMS), SEM, X-ray photoelectron spectroscopy (XPS) — and it increases the lateral and depth resolution. 

The transmission electron microscopy was done with a 100-kV accelerating potential (Hitachi 600). Powder samples were dispersed onto a carbon film on a Cu grid for TEM examination. The surface analysis techniques used, XPS and SIMS, were described earlier (7). X-ray photoelectron spectroscopy was done with a Du Pont 650 instrument and Mg K radiation (10 kV and 30 mA). The samples were held in a cup for XPS analysis. Secondary ion mass spectrometry and depth profiling was done with a modified 3M instrument that was equipped with an Extranuclear quadrupole mass spectrometer and used 2-kV Ne ions at a current density of 0.5 /zA/cm2. A low-energy electron flood gun was employed for charge compensation on these insulating samples. The secondary ions were detected at 90° from the primary ion direction. The powder was pressed into In foil for the SIMS work. 

A variety of techniques and apparatus have been developed to study and measure surface properties of pol5uners and other materials. Three of the most useful techniques for such measurements are electron spectroscopy for chemical analysis (ESCA) also known as x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and ion scattering spectroscopy (ISS). Table 10.8 shows a comparison of the sampling depth of traditional methods and the new techniques. These analyses can focus on a much shallower thickness of the surface and virtually yield analyses of the outermost layers of a polymer article. Some of these methods and examples of their application to fluoroplastics are discussed below. 

The interface chemistry of thin films can also be studied by using angle resolved photoelectron spectroscopy (5). As the angle between the sample normal and the entrance slit of the analyzer (9) is increased, the sampled depth is decreased by the cos 0. At grazing electron take-off... 

Equation (22) is particularly useful when a concentration gradient in depth exists. In this case, several spectra at different values of 9 are taken and the analysis is called angle resolved X-ray photoelectron spectroscopy. However, for a maximum efficiency, a flat surface (at an atomic level) is needed to avoid shade effects as shown by Fadley in his early works in the 1970s [44]. An additional problem exists the extraction of concentration profiles, cg(x), from Eq. (22) is an inverse problem the intensity as a function of the analysis angle is the Laplace transform of the composition depth profile of the sample [43] and does not have a unique solution. Several algorithms to solve the inversion problem were developed and tested [46]. They are all very unstable and sensitive to small statistical... 

X-ray photoelectron spectroscopy (XPS), or given its other name Electron Spectroscopy for Chemical Analysis (ESCA), uses X-rays to excite photoelectrons. The emitted electron signal is plotted as a spectrum of binding energies. The photon is absorbed by an atom, molecule or solid leading to ionization and the emission of a core electron. Analysis will reveal the composition from a depth of 2 20 atomic layers and the electronic state of the surface region of the sample. XPS has the ability to identify different chemical states resulting from compound formation, which are revealed by the photoelectron peak positions and shapes. 

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