Diffraction contrast

Image plane (single-stage magn i lied image) 

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Frank L, Muiierova i, Fauiian K and Bauer E 1999 The scanning iow-energy eiectron microscope first attainment of diffraction contrast in the scanning eiectron microscope Scanning 21 1-13... 

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. 

The diffraction patterns of isochiral clusters of tubes with different chiral angles in MWCNTs are superimposed in the composite pattern, the different chiral angles can be measured separately by diffraction contrast imaging [26]. 

The conventional hand of a particular isochiral cluster of tubes can be deduced from dark field diffraction contrast tilting experiments [26]. 

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. 

There are established criteria for obtaining b by using diffraction contrast (23). Briefly, the dislocation intensity (contrast) is mapped in several Bragg reflections (denoted by vector, g) by tilting the crystal to different reflections and determining the dot product of the vectors g and b (called the g b product analysis). 

The reflections include a particular g in which the dislocation is invisible (i.e., g b = 0 when b is normal to the reflecting plane). With these criteria in diffraction contrast, one can determine the character of the defect, e.g., screw (where b is parallel to the screw dislocation line or axis), edge (with b normal to the line), or partial (incomplete) dislocations. The dislocations are termed screw or edge, because in the former the displacement vector forms a helix and in the latter the circuit around the dislocation exhibits its most characteristic feature, the half-plane edge. By definition, a partial dislocation has a stacking fault on one side of it, and the fault is terminated by the dislocation (23-25). The nature of dislocations is important in understanding how defects form and grow at a catalyst surface, as well as their critical role in catalysis (3,4). 

These findings, coupled with the results of detailed diffraction contrast experiments (85,89), show that the defects are formed by glide shear the lattice is... 

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

Figure 4c shows a high angle annular detector image of the same area, where the angular collection range is now 80 to 400 mrad. Support diffraction contrast has been attenuated and Pt particle contrast is significantly enhanced compared with that... 

This parameter is important for the interpretation of diffraction contrast images and electron diffraction intensities. From (E29), let us take... 

TECHNIQUES OF HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY (HREM), TRANSMISSION EM (TEM) DIFFRACTION CONTRAST, AND ANALYTICAL EM (AEM)... 

Figure 4 (a) Schematic of the perfect crystal structure of 123. (b) Schematic of a twin, (c) Low magnification TEM (diffraction contrast) image of twins observed on (110) planes, (d) HREM structure image of 123 in [010] at a defocus of -70 nm. Atoms are white and individual atom columns are shown. The corresponding selected area... 

Classification of some important defect structures and diffraction contrast in catalysis... 

Figure 3.25. In situ catalysis (a) fresh VPO catalyst (b) dynamic real-time formation of atomic scale catalyst restructuring in butane after 2 min at 400 °C (c) enlarged image of (b) showing two sets of partial dislocations and (d) dynamic image of two sets of extended defects along symmetry-related (201) in (010) VPO after reduction in butane for several hours (diffraction contrast). The inset shows the defect nucleation near the surface. Careful defect analysis shows them to be formed by novel glide shear, (e) One set of the defects in high resolution (f) and (g) show diffraction contrast images of defects in 201 and 201. (After Gai et al, Science, 1995 and 1997 Acta Cryst. B 53 346.)...

The chemical stabilization of the structure is thus characterized by the absence of faults or discernible defect structures, confirmed by sample tilting experiments in diffraction contrast and by HRTEM. The corresponding electron diffraction in the (101) orientation, however, shows diffuse streaks (figure 3.38(b)). The streaks are mainly along the (111) and (010) directions. Some weak diffuse streaking along (101) is also observed. The streaks are indicative of considerable disorder in the structure. These results are consistent with the NMR data. 

An experimental diffraction contrast image of model catalysts of Ag/alumina, prepared by evaporation, is shown in figure 5.5(a) at 200 kV. The corresponding EDX spectra from and off the metal particle are shown in figures 5.5(b) and (c), respectively. An HRTEM image of Pt/alumina at 400 kV in figure 5.6. 

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