Objective aperture

In bright-field microscopy, a small objective aperture is used to block all diffracted beams and to pass only the transmitted (undiffracted) electron beam. In the... 

Figure 4. STEM images of Au particles on a MgO support, (a) Image taken with the small objective aperture used for microdiffraction (b) Image obtained with larger objective aperture showing better resolution.

Attempts to perform the MAED of these crystallites were unsuccessful because of the difficulty In observing the crystallites with the small objective apertures necessary to obtain reasonable MAED patterns, and the rapid mobility of these small crystallites In the stationary electron beam used In the MAED mode. Instrumental modifications are commercially available which might allow this measurement to be made. 

Sample B provided platinum crystallites that were analyzed by both EDS and MAED. MAED of several 3 nm crystallites shows a wide variation of orientations with respect to the electron beam, however, many of the patterns match (111) and (110) orientations. An example of the MAED patterns observed Is shown In Figure 2. The diffraction pattern was made with a 25 pm objective aperture at a camera length of 2 m. 

The gas reaction chamber and the objective aperture assembly occupied the gap between the upper and lower objective pole pieces, leading to a gas reservoir around the sample. Such ECELL systems were a major step forward in scientific capability, being used by Gai et al. (3,73-78), Doole et al. (79), Crozier et al. (80), and Goringe et al. (81) to characterize catalysis. Other developments for catalytic studies include an ex situ reaction chamber attached externally to the column of a TEM, for example, by Parkinson and White (82) and Colloso-Davila et al. (83). Reactions were carried out in the ex situ chamber (and not in situ), and the sample was cooled to room temperature and inserted into the column of the TEM (without exposure to the atmosphere) under vacuum. Baker et al. (84) used ETEM at gas pressures of a few mbar with limited resolution, and, in these experiments, representative higher gas pressures were not employed. 

However, the technique suffers the drawback that in real catalyst systems, particles are randomly distributed over the support and thus will not all be in the correct orientation for diffraction into the angular collection range subtended by the objective aperture, and that the smaller clusters diffract too weakly to be detected against the support. A further difficulty is that random superposition of atoms in amorphous support materials, such as charcoal or silica can give rise to "speckle" which may be easily confused with small catalyst clusters. (16,13). 

Figure 1. (a) Image formation in an electron microscope Ro - undiffracted beam O -objective aperture A as placed in (a) (b) Contrast transfer function of JEOL JEM 2010 200 kV electron microscope Scherzer underfocus 8 = -43.4 nm, a = 0.6 mrad, g = 5 nm ... 

Quantitative Processing. Plates or film with the diffraction patterns were scanned with a Joyce-Loebl microdensitometer. Radial (20) densitometric plots of the crystalline pattern (eventually three successive exposures of the crystalline pattern are analyzed) and of the corresponding amorphous pattern were recorded on the same curve. In this way, the plot of the amorphous pattern was used as a reference standard. The densitometric recording began with the optical density of the non-irradiated emulsion this allowed the evaluation and normalization of the optical density of the diffraction pattern. When the analytical slit passed through the image of the border of the 75 pm objective aperture, the densitometric curve showed a sudden density raise "A d". (Fig. 6) The plots of the amorphous and crystalline patterns were thus normalized to the same reference " A d". Crystallinity was determined on the normalized curves by measuring the areas "C + A" and "A" under the crystalline and amorphous plots respectively. 

Figure 5 is an example of a densitometric plot for PETP,. The shoulder corresponding to the "amorphous halo" of the destroyed pattern is clearly visible. The normalization procedure is illustrated s in that specific case, the amorphous pattern is underexposed in comparison with the crystalline pattern. Point A corresponds to the optical density dQO of the background of the slide (that is outside the image of the border of the objective aperture) point B corresponds to the "initial" density dQ of the crystalline pattern and to the "initial" density d Q of the amorphous pattern if the crystalline plot is taken as the reference, the amorphous plot density has to be multiplied by the normalization factore i.e. 

Darkfield (DF) imaging was performed with the tilted beam technique, the reflection selected by a 6 x 10 3 rad.objective aperture. 

Figure 15 Floating ion gun for low-energy depth profiling [62] (10), extraction aperture (11), extraction collimating aperture (12), objective aperture.

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