Micro-area electron diffraction

Characterization methods. The 100 kV Vacuum Generator HB-5 STEM was used to mlcroanalyze samples. The HB-5 has a KEVEX SI(LI) energy dispersive X-ray spectrometer (EDS) and micro area electron diffraction (MAED) capabilities In conjunction with simultaneous bright and dark field Imaging capabilities. A more detailed explanation of the Instrumental operation can be obtained In a publication by C. Lyman(12). 

Professor Tadakoro s recent book(l ) illustrates the considerable advantages and benefits to be gained by coupling infra-red spectroscopy with fibre x-ray diffraction. The increasing availability of Fourier transform infra-red spectrometers allows the same thick samples, suitable for x-ray work, to be used in the spectrometer thus ensuring that both sets of information emanate from the same structure. The delightful selected area electron diffraction patterns obtained from polysaccharides by Dr. Chanzy (2), which exhibit such remarkable resolution and definition, indicate the importance and value of the modern application of electron micro-... 

Some studies reported enhancements in photoactivity in the presence of a small amount of rutile phase [122-124]. Even a mechanical mixture of anatase and rutile showed much higher photoactivity for naphthalene oxidation than either pnre anatase or rutile powders [123,124]. The P25 powder is produced from TiCl4 in a flow reactor [122]. Based on a detailed investigation by x-ray diffraction (XRD) and micro-Raman spectroscopy, the rutile (formed directly in the flame) was fonnd to be covered by anatase [122,124]. However, another study based on transmission electron microscopy (TEM) with selected-area electron diffraction reported the presence of separate particles of anatase and rutile in P25 [125,126]. Diffuse reflectance spectra of P25 could be reproduced by a mechanical mixture of anatase and rutile powders, and particles of pure rutile phase were isolated from P25 upon HF treatment. Photoactivity for the decomposition of 4-chlorophenol in water was compared on four commercial photocatalysts, applying criteria of (a) initial rate of pollutant disappearance, (b) amount of intermediate products formed, and (c) time necessary to achieve total mineralization [127]. Based on criterion (c), P25 was concluded to be the most efficient photocatalyst even though it contains 20% rutile and has a moderate BET surface area (ca. 50 m /g). It was also reported to have a higher photoactivity than catalyst All in the degradation of reactive black 5 (an azo-dye) [128]. 

Figure 14.25 FUgh resolution image ( 110 / ) and selected area electron diffraction pattern ( 110 /) recorded from Smo.zoCeo.aoO/ go sintered body. Dashed line area means micro-domain with ordered structure. (Reprinted with permission from 74 Copyright (2002) Elsevier Ltd.)...

Selected-area electron diffraction was carried out with the Philips EM-420 for the larger crystal which may contain several tens of chains and for simultaneous analysis of a large number of single-chain crystals. Micro-beam electron diffraction was performed on isolated single-chain crystal. For the latter, the electron beam was concentrated to a spot with a diameter of 30-40 nm, to cover no more than a single-chain ciystal. 

Macro refers to macroscopic, large area TEM experiments, micro to microscopic, same area, sequential TKM ejqieriments, while ED refers to analytical TEM using electron diffraction MOss refers to Mdssbauer spectroscopy. 

X-ray or electron diffraction allows Identification of crystalline species by the long-used Hannawalt-Dow-ASTM-JCPDS system(4). Small particles can be removed for analysis in a small rotating specimen X-ray powder camera, or by extraction replication and selected area diffraction in a Transmission Electron Microscope (5). For those specimens where a residue of reactant or corrosion product is too adherent, the material may be removed for analysis by micro-bulldozing (with a microhardness Indentor), micro-jack hammering (with a needle attached to a small piezoelectric crystal on a pencil-like rod), and micro-boring (with a precision controlled dental drill)(5). 

For all compositions, the polyacetylene domains are crystalline as is revealed by X-ray diffraction. This is further confirmed by electron diffraction from thin microtomed sections of blends with polyacetylene compositions of 40% or higher. Micro-toming of samples with less than 40% polyacetylene without sample cooling is difficult due to the rubbery nature of the composite. The polycrystalline nature of the polyacetylene domains is established from the observation that even selected area aperture of a few hundred angstroms produces Debye ring patterns. 

The results of recent electron diffraction studies of selected areas of single kaolin crystals have thrown considerable doubt on the usefulness of such symbols. These studies are revealing that zones with different stacking patterns exist within minute kaolin crystals and that micro-twinning of crystals is common. Under these circumstances, the adoption of any current system of polymorphic symbols represents an oversimplification of the true structure. A much more complex system of symbols will be necessary, but its development must await the publication of many more detailed experimental results. 

WDS is another technique utilized for elemental chemical analysis in reverse engineering. EDXA and WDS are usually used in conjunction with SEM, or an electron probe microanalyzer (EPMA). EPMA is a nondestructive elemental analysis technique, similar to SEM but with a more focused analysis area. It works by rastering a micro volume of the sample with an electron beam typical of an energy level of 5 to 30 keV. It then collects the induced X-ray photons emitted by the various elemental species and quantitatively analyzes the spectrum with precise accuracy, up to ppm. In contrast to EDXA, WDS analyzes the electron diffraction patterns based on Bragg s law and has a much finer spectral resolution and better accuracy. WDS also avoids the problems associated with artifacts in EDXA, such as the false peaks and the background noise from the amplifiers. The noise intensity that appears in... 

EDSA of thin polycrystalline films has several advantages First of all the availability of a wide beam (100-400 pm in diameter) which irradiates a large area with a large amount of micro-crystals of different orientations [1, 2]. This results into a special t5q)e of diffraction patterns (DP) (see Fig.l). Thus it is possible to extract from a single DP a full 3D data set of structure amplitudes. That allows one to perform a detailed structure analysis with good resolution for determining structure parameters, reconstruction of ESP and electron density. 

Any regions of the sample under the incident beam (usually a few pm diameter) exhibiting crystallinity, will diffract electrons away from the central spot, forming a diffraction pattern observable at the back focal plane of the objective lens. Like X-ray diffraction, this can provide identification of crystalline phase, orientation, and lattice parameters. In micro-diffraction, the incident beam is focused down to sub-micron areas, but this focusing degrades the diffraction pattern. 

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