Bright field image weak-beam

Fig. 7.7. TEM images and SAD patterns (insets) of a poly crystalline ZnO film on silicon (111) PLD grown at 1 x 10 3mbar O2 and about 540°C (a) Bright field Si(lll) plane view observation, grain size is about 70 nm, (b) cross-section HRTEM lattice image with intermediate SiO layer, and (c) weak beam Si(110) TEM cross-section. The area from which the SAD patterns were taken are within the white circles. Reprinted with permission from [49]...

Microdiffraction.—Perhaps more important than SAD techniques, particularly in the context of catalyst research, microdiffraction allows the user to benefit from the small probe size generated in STEM in the structural analysis of small particles and localized areas in thin foils. If the small probe is stopped on a particle, then clearly a transmission diffraction pattern will be observable after the beam has traversed the sample, provided we have the means available for its display. In CTEM such a pattern will, of course, be formed by the imaging system in a manner identical to SAD, but in STEM the pattern must be scanned across the detector. This is accomplished by means of a set of post-specimen scan coils which once more scan the diffracted beams across the axial bright-field detector. Such a pattern is shown in Figure 13 where a beam of approximately 10 A FWHM was stopped on a small second-phase particle during the omega-phase transformation in a Zr-Nb alloy. The relatively poor definition of the reflection is a consequence of both the convergent nature in the probe (necessary in order to obtain the smallest probe sizes) and a S/N limited by the available current in the probe. Nevertheless, weak reflections with half-order indices are clearly visible between the main alloy reflections and it is therefore possible to attempt structural... 

Fig. 5.26 a Bright field TEM image of 14 (111) dislocations in MoSi2 + 2.5 at% Re alloys in compression at 1400 °C. Weak beam images of dislocations labeled x in (a) are shown in (b) and (c) respectively. The electron beam direction is 5-10° Ifom [100] [23]. With kind permission... 

The conditions for kinematic diffraction [160] are best approximated in the weak-beam method, which consists of making a dark-field image in a weakly excited diffraction spot. The dislocation image then consists of a narrow bright line on a darker background. 

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