Electron Energy Loss Spectroscopy EELS

P(PO) [471] (Fig. 11-47). Extensive EELS studies have been carried out for P(Ac) s, P(DiAc) s, P(PP)-series, and P(Py) [166]. Salient results are cited. 

Early results on P(Ac) by Ritsko et al. and others [472] showed two excitations (Fig. 12-48L assigned to interband transitions between tt-tt, a-a transitions. The EELS spectrum of AsFs-doped (CH) shows greatly increased absorption in the gap. Ritsko et al. [473] have studied the EELS spectrum of Poly (diacetylene)-10L at various values of momentum transfer. Fig. 12-49 shows EELS data obtained for P(Py) under various conditions by Ritsko et al. [474]. 

Describe a series of simple analyses you would use for initial characterization of an unknown CP doped with an unknown dopant. 

Outline three methods for determination of DC conductivity ex situ and one method in situ, and one method for AC conductivity, and indicate under which circumstances they are applicable. 

Outline a simple technique, using IR, NMR and UV-Vis spectroscopies, that you might be able to use to determine if chain branching exists, and what type it is, in P(ANi) and P(o-toluidine). 

These and many other examples discussed in a review by Mestl and Srinivasan [47] show that Raman spectroscopy is indispensable in investigating the preparation of molybdenum catalysts. Several other applications of Raman spectroscopy in catalysis are discussed in a book by Stencel [48]. 

As noted in the introduction, vibrations in molecules can be excited by interaction with waves and with particles. In electron energy loss spectroscopy (EELS, sometimes HREELS for high resolution EELS) a beam of monochromatic, low energy electrons falls on the surface, where it excites lattice vibrations of the substrate, molecular vibrations of adsorbed species and even electronic transitions. An energy spectrum of the scattered electrons reveals how much energy the electrons have lost to vibrations, according to the formula  

The use of electrons requires that experiments are done in ultrahigh vacuum and preferably on the flat surfaces of single crystals or foils. 

The first EELS experiments were reported by Propst and Piper in 1967 and concerned the adsorption of H2, N2, CO, H20 on the (100) surface of tungsten [49]. Ibach studied the energy losses of electrons to phonons in ZnO surfaces around 1970 [50] and continued to develop the technique for studying adsorbates on metal surfaces [51,52]. In the 1980s EELS grew further into an extremely important and generally accepted tool in surface science. 

Where infrared and Raman spectroscopy are limited to vibrations in which a dipole moment or the molecular polarizability changes, EELS detects all vibrations. Two excitation mechanisms play a role in EELS dipole and impact scattering. 

A beam of electrons of known KE and incident to a specimen is scattered inelastically and loses part of the energy. By analyzing the energy with a spectroscope attached under a TEM or SEM will reveal information on the elemental composition and bonding state. The technique is particularly useful for the transition elements and elements of low atomic number such as Be, B, C, N and O. 

The other technique is HREELS (high resolution EELS) which utilises the inelastic scattering of low energy electrons in order to measure vibrational spectra of surface species. The use of low energy electrons ensures that it is a surface specific technique, and is often chosen for the study of most adsorbates on single crystal substrates. 

We will deal in the greater detail with the former application. 

Recent advances in instrumentation have made possible the study of Fe Z23 spectra via the technique of EELS and the related technique of energy-loss near-edge spectroscopy (ELNES). These methods have been used to study Ti, Mn, and Fe oxidations states in non-silicate systems by several workers, including Tafto and Krivanek (1982), Leapman et al. (1982), Often et al. (1985), Paterson and Krivanek 

It should be noted that for TEM at accelerating voltages of 100-400 keV the specimen thickness must be of the order of 10-100 nm which requires dedicated preparation techniques [2.173, 2.176, 2.178]. 

In electron-optical instruments, e.g. the scanning electron microscope (SEM), the electron-probe microanalyzer (EPMA), and the transmission electron microscope there is always a wealth of signals, caused by the interaction between the primary electrons and the target, which can be used for materials characterization via imaging, diffraction, and chemical analysis. The different interaction processes for an electron-transparent crystalline specimen inside a TEM are sketched in Eig. 2.31. 

In a closer view (Eig. 2.32), the following interactions occur on an atomic scale when a material is hit by electrons. Eirstly, in addition to elastic scattering, inelastic scatter- 

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Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

Vibrational spectroscopy provides detailed infonnation on both structure and dynamics of molecular species. Infrared (IR) and Raman spectroscopy are the most connnonly used methods, and will be covered in detail in this chapter. There exist other methods to obtain vibrational spectra, but those are somewhat more specialized and used less often. They are discussed in other chapters, and include inelastic neutron scattering (INS), helium atom scattering, electron energy loss spectroscopy (EELS), photoelectron spectroscopy, among others. 

Several structural characterisations of carbon nanotubes (CNTs) with the cylindrical graphite are reviewed from the viewpoint of transmission electron microscopy (TEM). Especially, electron energy loss spectroscopy (EELS) by using an energy-fdtered TEM is applied to reveal the dependence of fine structure of EELS on the diameter and the anisotropic features of CNTs. 

Knowledge of the stracture and bonding of molecnles to snrfaces has been obtained from such techniques as LEED, electron energy-loss spectroscopy (EELS), secondaiy-ion mass spectrometry (SIMS), infrared spectroscopy (IRS), Raman spectroscopy, and NMR spectrometiy. The scope of snch studies needs to be greatly expanded to include the effects of coadsorbates, promoters, and poisons. Greater emphasis should be given to developing new photon spectroscopies that would permit observation of adsorbed species in the presence of a gas... 

The vibrations of molecular bonds provide insight into bonding and stmcture. This information can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the stmcture of catalysts and the bonding of adsorbates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the stmcture of adsorbates present on catalyst and electrode surfaces. 

The reactions of ethylene, water, and methanol with coadsorbed oxygen on Pdf 100) were studied with temperature programmed reaction spectroscopy (TPRS) and high resolution electron energy loss spectroscopy (EELS). 

Firstly, the energy losses of the incident electrons which produce the inner shell excitations may be detected as peaks in electron energy loss spectroscopy (EELS). The elecrons transmitted by the specimen are dispersed in a magnetic field spectrometer and the peaks, due to K, L and other shell excitations giving energy losses in the range of 0-2000eV, may be detected and measured. 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

Figure 7. Electron energy loss spectroscopy (EELS) of a Cu/ZnO catalyst a) bright-field STEM image showing a 20nm copper oxide particle and a small 2nm Cu metal particle on ZnO, b) and c)...

The first two advantages listed above allow an optical method like transmission or reflection IR spectroscopy to be used for studies which would be impossible for a widely used competitive technique, electron energy loss spectroscopy (EELS). EELS must... 

Sexton BA. 1981. Identification of adsorbed species at metal-surfaces by electron-energy loss spectroscopy (EELS). Appl Phys A 26 1-18. 

The hybridization of carbon atoms is the major structural parameter controlling DLC film properties. Electron energy loss spectroscopy (EELS) has been extensively used to probe this structural feature [5. 6]. In a transmission electron microscope, a monoenergetic electron beam is impinged in a very thin sample, being the transmitted electrons analyzed in energy. Figure 27 shows a typical... 

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