Diffraction spots particles

Dark field images can be used to determine the shape of a small particle as shown by Yacaman et al. (6-7). When an electron beam enters on a small particle the presence of wedges will produce a splitting on the diffracted spots as shown by Gomez et al (8). A... 

For small particles supported on thin films of amorphous or microcrystalline materials it is not easy to determine whether there is any consistent correlation between the particle orientation and the orientation of the adjacent locally ordered region of the substrate. For some samples of Pt and Pd on gamma-alumina, for example, nanodiffraction shows that the support films have regions of local ordering of extent 2 to 5 nm. Patterns from the metal particles often contain spots from the alumina which appear to be consistently related to the metal diffraction spots. 

Note in this case the bidimensional resolution of the image both on the particle and on the support. The presence of 0.220 nm spots at 7 T in the DDP of the particle, Figure 4.17(b), allows to assign this image to a [110] zone axis orientation of metallic, fc.c., Rh. Likewise the DDP from the support. Figure 4.17(c), shows (1-11)-Ce ftTho20 .x (0.312 nm) and (002)-Ce. 8T, 202-, (0.277 nm) diffraction spots characteristic also of a [110] zone axis orientation of fluorite. Two major conclusions can be drawn from these results ... 

Metal particles do not grow with random orientations on the surface of the support but, instead, under well-defined crystallographic relationships. Note for example the alignment of the diffraction spots of ceria and Rh in the DDPs shown in Figures 4.17(b) and 4.17(c). As demonstrated further on, this is a consequence of an epitaxial relationship between the fluorite-type supports and the f.c.c. metal particles. 

An intensifying screen should not be used if it is important to record fine detail in the Laue spots, as in some studies of crystal distortion, since the presence of the screen will cause the spots to become more diffuse than they would ordinarily be. Each particle of the screen which is struck by x-rays emits light in all directions and therefore blackens the film outside the region blackened by the diffracted beam itself, as suggested in Fig. 5-5. This effect is aggravated by the fact that most x-ray film is double-coated, the two layers of emulsion being separated by an appreciable thickness of film base. Even when an intensifying screen is not used, double-coated film causes the size of a diffraction spot formed by an obliquely incident beam to... 

The analysis of the diffraction pattern of X-rays and neutrons was induced by atomic structures for separate particles, which can be used to smdy very small particles. The reflex angle AO increases with the particle size (A0 l/R, i.e., the Scherrer effect). The smaller particle sizes correspond to smaller numbers of lattice planes to give rise to an interference of the diffraction spot, whereas in large clusters diffraction rings have been observed. 

Qualitative SAED consists of observation of the pattern of diffraction spots obtained on the TEM viewing screen from a randomly oriented fiber or particle. Such a pattern indicates that the material is crystalline. Chrysotile fibrils, with their cylindrical form, will usually give the same characteristic pattern, corresponding to a 0.73 mn spacing for (0 0 2) planes, and a layer line repeat of 0.53 nm, as well as streaking of the (110) and (130) reflections. These observations and measurements can be made directly from the screen if the appropriate calibrated screen... 

The broadening of x-ray diffraction peaks provides a convenient method for measuring particle sizes below —0.1 xm (6-8). As the crystal size decreases, the width of the diffraction peak (or the size of the diffraction spot) increases. An approximate expression for the peak broadening is given by the Scherer equation ... 

Fig. 9.12. (a) Atomic force microscope image of the impression created on a Zr— 17.9Cu-14.6Ni-10Al-5Ti (atomic percent) bulk metallic glass alloy which was subjected to nanoindentation at a maximum load of 60 mN. Discontinuous shear bands encompass the indent, (b) SAD patterns showing diffraction spots which were produced by the formation of nanocrystalline particles at the indents and in the shear bands. The inset schematically shows six diffraction spots which were associated with the (111) plane of tetragonal Zr2Ni particles, (c) A small distance away from the indent only halo ring patterns characteristic of a fully amorphous structure are seen. Reproduced with permission from Kim et al. (2002). 

The arrangement of montmorillonite, hectorite, and nontronite is always turbostratic. Figure 6 shows an electron selected area diagram produced by a single thick particle of montmorillonite. The stacked layers show certain tendencies to preferential orientation, but the degree of disorientation is important (in the order of 10°). In certain montmorillonites, like those of Wyoming, the mutual orientation of the layers is absolutely arbitrary the diffraction spots are uniformly spread out on circles (Mering and Oberlin [1967]). 

As illustrated by Eig. 4.13, an electron microscope offers additional possibilities for analyzing the sample. Diffraction patterns (spots from a single-crystal particle and rings from a collection of randomly oriented particles) enable one to identify crystallographic phases as in XRD. Emitted X-rays are characteristic for an element and allow for a determination of the chemical composition of a selected part of the sample. This technique is referred to as energy-dispersive X-ray analysis (EDX). 

Diffraction patterns having relatively well-defined sharp spots can be obtained from small unit-cell crystals with an incident beam of diameter 10-158. Such patterns have been used in the study of the structures of small metal particles (22). For particles 10-20A diameter the electron beam can illuminate the whole of the particle... 

In a TEM with STEM attachment it is possible to ob-Q tain diffraction patterns from areas from 50 200 A. That allows in most cases to obtain patterns from individual particles. In order to study the crystal structure of the particle is more convinient to use a non-convergent beam 10 3 rad). This produces sharp spots and avoids interference effects such as the ones described by Roy et al. (9) that makes the interpretation of the data more complicated. Again in this case the operation conditions must be as clean as possible. 

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