Electron diffraction contrast

Humphreys, C. J., Howie, A., Booker, G. R. (1967). Some electron diffraction contrast effects at planar defects in crystals. Phil. Mag., 15, 507-22. 

Tholen, A. R. (1970). On the ambiguity between moire fringes and the electron diffraction contrast from closely spaced dislocations, phys. slat, sol., (a)2, 537-50. 

In contrast to amorphous polymers, structural details of lamellae can be imaged using electron diffraction contrast in TEM. In this case, unstained uitrathin sections or solution-cast films are used. Electron diffraction patterns allow structural analysis at the level of the crystalline unit cell, and they can be used to detect the crystal orientations of a polymer. An example with a sheaf-like morphology of PE with low molecular weight is shown in Eig. 2.3 by different modes in TEM. 

H owever, there is electron diffraction contrast caused by nonuniform deformation along the circumference (not shown). The type 11 structure is a nanoring with its shell being a uniformly deformed single-crystal ribbon around the circumference (not shown). 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

Photoelectron spectra have confirmed the expected trends in the frontier orbitals.The tetrafiuoro derivative 12.12 (R = F) is prepared by treatment of C6F5SNSNSiMc3 with CsF in acetonitrile (Scheme 12.2). Several difiuoro- and trifiuoro-benzodithiadiazines have also been prepared by these methods.In contrast to 12.12 (R = H), which has an essentially planar structure in the solid state,the dithiadiazine ring in the tetrafiuoro derivative is somewhat twisted. In the gas phase, on the other hand, electron diffraction studies show that 12.12 (R = F) is planar whereas 12.12 (R = H) is non-planar. ... 

Sample H was evaluated In this manner, and showed higher H/Pd ratios than anticipated, 0.6 versus 0.3 for the crystallites observed. This result would suggest that the palladium crystallites are thin, thus explaining the Instability In the electron beam and lack of diffraction contrast. 

The most important information about the nanoparticles is the size, shape, and their distributions which crucially influence physical and chemical properties of nanoparticles. TEM is a powerful tool for the characterization of nanoparticles. TEM specimen is easily prepared by placing a drop of the solution of nanoparticles onto a carbon-coated copper microgrid, followed by natural evaporation of the solvent. Even with low magnification TEM one can distinguish the difference in contrast derived from the atomic weight and the lattice direction. Furthermore, selective area electron diffraction can provide information on the crystal structure of nanoparticles. 

On the basis of a number of physico-chemical methods (Mossbauer spectroscopy, electron diffraction, EXAFS) the iron cores of naturally occurring haemosiderins isolated from various iron-loaded animals and man (horse, reindeer, birds and human old age) were consistently shown to have ferrihydrite-like iron cores similar to those of ferritin (Ward et ah, 1992, 2000). In marked contrast, in the tissues of patients with two pathogenic iron-loading syndromes, genetic haemochromatosis and thalassaemia, the haemosiderins isolated had predominantly amorphous ferric oxide and goethite cores, respectively (Dickson etah, 1988 Mann etah, 1988 ... 

Fig. 5.16 (A) Bright-field TEM image and (B) element mapping carbon (brighter contrast corresponds to higher concentration of carbon) of ZnO synthesized in aqueous solution at 37 °C in pH 8 buffer for 4 h in the presence of 1.2 mgmL-1 of gelatin. The inset shows the electron diffraction pattern taken parallel to the platelet normal. (Reprinted with permission from [77], Copyright (2006) American Chemical Society).

They have many of the morphological and ultrastructural characteristics of disease filaments [11, 12] (Fig. 45-5). Assembly is a nucleation-dependent process that occurs through its amino-terminal repeats. The carboxy-terminal region, in contrast, is inhibitory. Assembly is accompanied by the transition from random coil to a [3-pleated sheet. By electron diffraction, a-synuclein filaments show a conformation characteristic of amyloid fibers. Under the conditions of these experiments, P- and y-synucleins failed to assemble, consistent with their absence from the filamentous lesions of the human diseases. When incubated with a-synuclein, P- and y-synucleins inhibit the fibrillation of a-synuclein, suggesting that they may indirectly influence the pathogenesis of Lewy body diseases and multiple system atrophy. 

Kaestner G. Many-heam electron diffraction related to electron microscope diffraction contrast, Akademie Verlag, Berlin, Germany, 1993. 

Reflected light differential interference contrast (DIC), 16 482 Reflection correction algorithms, 24 456 Reflection high-energy electron diffraction (RHEED), 24 74 Reflection indexing... 

The study of metals in biological systems requires techniques, some of them highly specific, some limited to certain aspects of the metal ion in question, some of more general applicability. Thus, Mossbauer spectroscopy in biological systems is restricted to iron-containing systems because the only element available with a Mossbauer nucleus is 57Fe. The EPR spectroscopic techniques will be of application only if the metal centre has an unpaired electron. In contrast, provided that crystals can be obtained, X-ray diffraction allows the determination of the 3-D structure of metalloproteins and their metal centres. 

In HREM images of inorganic crystals, phase information of structure factors is preserved. However, because of the effects of the contrast transfer function (CTF), the quality of the amplitudes is not very high and the resolution is relatively low. Electron diffraction is not affected by the CTF and extends to much higher resolution (often better than lA), but on the other hand no phase information is available. Thus, the best way of determining structures by electron crystallography is to combine HREM images with electron diffraction data. This was applied by Unwin and Henderson (1975) to determine and then compensate for the CTF in the study of the purple membrane. 

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