EEL spectra

For the parallel recording of EEL spectra in STEM, linear arrays of semiconductor detectors are used. Such detectors convert the incident electrons mto photons, using additional fluorescent coatings or scintillators in the very same way as the TEM detectors described above. 

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

In addition to qualitative analysis of nearly all the elements of the periodic table, EEL spectra also enable determination of the concentration of a single element which is part of the transmitted volume and hence gives rise to a corresponding ionization edge. As in all comparable spectroscopic techniques, for quantification the net edge signal, which is related to the number N of excited atoms, must be extracted from the raw data measured. The net intensity 4 of the feth ionization shell of an individual element is directly connected to this number, N, multiplied by the partial cross-section of ionization ) and the intensity Iq of the incident electron beam, i.e. ... 

Fig. 2.44. Chemical-bond mapping (a) comparison of EEL spectra recorded from matrix and SiC-fibre with the window for energy filtering shown (b) map of oxidic-bound Si.

Fig. 4. EEL spectra of (a) graphite and (b) diamond. These carbon allotropes represent different spectra sp bonding especially exhibits 7c -excitation peak lower than the o -excitation peaks (modified from ref. 16).

Fig, 6. EEL spectra of bundle of four SWCNTs, MWCNT and graphite in the energy ranges (a) from 0 to 45 eV (plasmon region) and (b) from 280 to 300 eV (carbon K-edge) (modified from ref. 14). 

Dravid et al. examined anisotropy in the electronic structures of CNTs from the viewpoint of momentum-transfer resolved EELS, in addition to the conventional TEM observation of CNTs, cross-seetional TEM and precise analysis by TED [5]. Comparison of the EEL spectra of CNTs with those of graphite shows lower jc peak than that of graphite in the low-loss region (plasmon loss), as shown in Fig. 7(a). It indicates a loss of valence electrons and a change in band gap due to the curved nature of the graphitic sheets. 

Fig. 7. (a) Low-loss EEL spectra of CNT and graphite and carbon core-loss EEL spectra of graphite and tubes in (b) normal geometry (the electron beam normal to the c-axis) and in (c) parallel geometry (the electron beam parallel to the c-axis of graphite and perpendicular to the tube axis) (modified from ref. 5). 

Left Fig. 8. Schematic illustration of angular-resolved EEL spectra for CNT with anisotropic structure. 

Right Fig. 9. EEL spectra of an MWCNT obtained from the locations at 000, intermediate and 002 reflexions in the reciprocal space (modified from ref. 16). 

EELS spectra showing the distinction between copper atom environments on the basis of the near edge fine structure of the CuL edge,... 

The Fine Structure Before and After Each Edge. ELNES is the term use to describe the energy-loss near edge structure, and this can be quite different for an element in different compounds. For example the shape of the aluminium L edges are quite different in EELS spectra from metallic aluminium and aluminium oxide, so that the chemical form of a given element may be indentified from these small variations in intensity after the edge. 

EELS spectra can also be employed to map the distribution of selected elements present in a sample, in the same way that X-ray elemental can be exhibited in analytical TEM experiments. 

Figure 9.13 EELS spectra of CO adsorbed on Pt(111) promoted with 9% potassium for increasing CO coverages. The influence of the promoter is most clearly seen in the spectra at low CO coverages. The CO stretch frequencies of CO adsorbed on clean Pt(lll) are about 2120 and 1875 cm 1 (from Pirug and Bonzel [441).

Shown in Figure 6-A are EELS spectra of the entire series of pyridine carboxylic acids and diacids adsorbed at Pt(lll) from acidic solutions at negative electrode potential. Under these conditions all of the meta and para pyridine carboxylic acids and diacids exhibit prominent 0-H vibrations (OH/CH peak ratio near unity). In contrast, at positive potentials only the para-carboxylic acids display pronounced 0-H vibrations, Figure 6-B. All of the 0-H vibrations are absent under alkaline conditions, Figure 6-C. This situation is illustrated by the reactions of adsorbed 3,4-pyridine dicarboxylic acid ... 

Figure 6. EELS spectra of pyridine carboxylic acids adsorbed at Pt(lll). Experimental conditions (A and B) adsorption from 1 mM NA in 10 mM KF at pH 3, followed by rinsing with 2 mM HF (pH 3) (C) adsorption from 10 mM KF (pH 3), followed by rinsing with 0.1 mM KOH (pH 10) other conditions as in Figure 4. A. Adsorption at -0.2 V vs. Ag/AgCl (pH 3). Continued on next page.

An EELS spectrum of PYR adsorbed at Pt(lll) from aqueous solution is shown in Figure 8. Also shown is the mid-IR spectrum of liquid PYR (18). The EELS spectrum of adsorbed PYR is essentially the envelope of the IR spectrum of liquid PYR, with the exception of a peak at 416 cm- attributable at least in part to the Pt-N bond. Assignments of the EELS peaks based upon accepted IR assignments (19) are given in Table 2. There is also a close correspondence between the EELS spectrum of PYR adsorbed from aqueous solution and the EELS spectra reported for PYR adsorbed at Pt single-crystal surfaces from vacuum (20). 

Electrons which are channelled along a flat surface in this way are particularly effective in exciting surface states of the crystal. Since they spend only a small fraction of their time "inside the crystal they are not greatly attenuated by bulk scattering processes and their EELS spectra do not show bulk excitation peaks. They are therefore effective probes for studying surface excitations. 

---