Photoemission electron microscop

Figure 7. Soft X-ray microscopes, (a) conventional X-ray microscope (XM) (b) scanning transmission X-ray microscope (STXM) (c) Scanning Photoemission Microscope (SPEM) and (d) Photoemission Electron Microscope ( ), (taken from www.aLs.lbl.gov).

Having seen the power (and limitation) of nexafs spectroscopy in the preceding sections, one can readily envision the enhanced utility of nexafs spectroscopy as a characterization tool that would result from the addition of high spatial resolution capabilities. Since the spectroscopic sensitivity to specific moieties and functional groups can in many or even most cases be exceeded by ir, nmr, and Raman spectroscopies, nexafs microscopy will have to exceed the spatial resolution of these other spectroscopy techniques in order to be truly useful. To date, nexafs microscopy has surpassed a spatial resolution of 50 nm both in transmission to measure bulk properties (75-77) and in a reflection geometry to study surfaces (78,79). This level of spatial resolution is at least an order of magnitude better than what can be accomplished with complementary compositional analysis techniques. Future developments in nexafs microscopy might achieve a spatial resolution of a few nanometers (80,81). In addition, nexafs microscopy has exceptional surface sensitivity of about 10 nm, a sensitivity that could be improved to about 1 nm with photoemission electron microscopes (peem s) that incorporate a bandpass filter (80-82). 

Fig. 7. Schematic of a photoemission electron microscope (peem). Courtesy of S. Anders, ALS, and B. Tonner, University of Central Florida.

Figure 2.53 Chemical mappings of (a) W, (b) Mo and (c) S realized on the flat after the friction test with the mixture IF-WS2/IF-M0S2. These chemical mappings were realized with a synchrotron beam (photoemission electron microscope). Image (d) represents the ratio between the W and the Mo images...

Advancing such innovative, enabling instrumentation has been among the Department s core activities. Other instruments developed by the Department include a photoemission electron microscope (PEEM) with ultimate resolution attained by implementing corrections for both chromatic and spherical aberrations a micro calorimeter whose sensitivity is sufficient to measure temperature-dependent heats of adsorption on nano-particles with aggregate sizes down to about a hundred atoms and a photon STM, which adds chemical sensitivity through local excitation of a fluorescence signal by electrons from the tip. 

Figure 7.23 Ordering of adsorbates on a surface into islands gives rise to regions of different work function, which can be imaged because of the associated differences in photoelectron intensity. The principle forms the basis of photoemission electron microscopy (PEEM). The same principle underlies the imaging of single molecules in the field electron microscope (FEM) (see also Fig. 7.9).

In this section, we will present and discuss results from Sc2 C84, which is the most widely studied dimetallofullerene to date. Early scanning tunnelling microscopy [26] and transmission electron microscopic [27] investigations provided evidence in favour of the endohedral structure of this system, which was later confirmed by x-ray diffraction experiments utilising maximum entropy methods [28]. Before experimental data from this system were available, the Sc ions were predicted to be divalent from quantum chemical calculations [29]. Subsequent data from vibrational spectroscopy [30,31], core-level photoemission [32] and further theory [33] on this system were indeed interpreted in terms of divalent Sc ions. 

Block, J.H., Ehsasi, M., Gorodetskii, V., Karpowicz, A., and Berdau, M., Direct observation of surface mobility with microscopic techniques photoemission electron and field electron microscopy, in New Aspects of Spillover Effect in Catalysis Studies in Surface Science and Catalysis, Inui T., et al., Eds., Elsevier, Amsterdam, 1993, Vol. 77, pp. 189-194. 

SEM studies were performed on a PhiUps SEM 515 operating at 15 kV. Powdered samples were deposited on a grid with a holey carbon film before transferring to electron microscope. Photoemission spectra (XPS) were collected by a VSW Scientific Instrument spectrometer, equipped with a standard A1 Ka excitation source. The binding energy (BE) scale was calibrated by measuring C Is peak (BE = 285.1 eV). 

Thanks to the extensive literature on Aujj and the related smaller gold cluster compounds, plus some new results and reanalysis of older results to be presented here, it is now possible to paint a fairly consistent physical picture of the AU55 cluster system. To this end, the results of several microscopic techniques, such as Extended X-ray Absorption Fine Structure (EXAFS) [39,40,41], Mossbauer Effect Spectroscopy (MES) [24, 25, 42,43,44,45,46], Secondary Ion Mass Spectrometry (SIMS) [35, 36], Photoemission Spectroscopy (XPS and UPS) [47,48,49], nuclear magnetic resonance (NMR) [29, 50, 51], and electron spin resonance (ESR) [17, 52, 53, 54] will be combined with the results of several macroscopic techniques, such as Specific Heat (Cv) [25, 54, 55, 56,49], Differential Scanning Calorimetry (DSC) [57], Thermo-gravimetric Analysis (TGA) [58], UV-visible absorption spectroscopy [40, 57,17, 59, 60], AC and DC Electrical Conductivity [29,61,62, 63,30] and Magnetic Susceptibility [64, 53]. This is the first metal cluster system that has been subjected to such a comprehensive examination. 

The measurement of tunnelling spectra in a scanning tunnelling microscope offers the potential of measuring the local density of states at spatially defined sites whose topography can be established at an atomic scale by STM. This information is only available however at the price of losing the information about the k-dependence of electronic states that is available in photoemission and inverse photoemission. In particular STS offers the prospect of measuring local densities of states at defect sites whose real space atomic structure can be established by STM. 

To obtain the morphology information, including phase separation and crystalline, we can now use microscopic techniques, atomic force microscopy, transmission electron microscopy, electron tomography, variable-angle spectroscopic ellipsometry. X-ray photoemission spectroscopy, and grazing-incidence X-ray diffraction. The detailed information of this characterization methods can be found from the specific reference (Li et al., 2012 Huang et al., 2014). 

During the last 5 to 10 years there has been much interest in electron spectroscopies (Hiifner and Steiner 1982), since it was realized that these also suggest a very different picture of mixed valence Ce compounds. Valence photoemission spectroscopy (PES) studies showed that the f-spectrum of Ce has weight at (8p ) — 2eV below Ep (Platau and Karlsson 1978, Johansson et al. 1978). It was further found that core level X-ray photoemission spectroscopy (XPS) measurements were hard to understand unless A (Fuggle et al. 1980b) is much larger than previously assumed. This discrepancy between the interpretations of spectroscopic and thermodynamic data showed the need for a theoretical analysis, based on a microscopic model, of what kind of information can be extracted from different experiments. This was further emphasized when PES studies showed f-character in the spectrum both at —2eV and close to 8p = 0 (Martensson et al. 1982). This observation created a lively debate about how to interpret the PES spectra. 

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