HAADF mode

Figure 7.7 shows the STEM images obtained in HAADF mode. In this mode, the image formed is due to that part of the electron beam that is diffracted by... 

Alternatively, images can be formed with electrons scattered at high angles (HAADF mode, for High Angle Annular Dark Field mode). An annular detector is placed below the sample and the focused electron beam is scanned over the sample. Interestingly, the contrast is not altered by diffraction phenomena and depends both the thickness and the composition (25). 

Fig. 3.14 Schematic set-up of an electron microscope in the transmission (TEM) (a) and combined (TEM and STEM) (b) modes. Note how the dark field detector is off the central axis, as the dark field image is based on scattered electrons. The HAADF-STEM operation mode with bright— (on the central axis) and dark-field detectors (of the central axis) is shown in (c). Two different modes are available for dark field imaging, i.e. annular dark field (ADF) high-angle annular dark field (HAADF). Figures modified after [37]. J. W Niemantsverdriet Spectroscopy in Catalysis An Introduction, pages 182 and 187. 2007. Copyright Wiley-VCH Verlag GmbH Co. KGoA. Reproduced with permission...

Experimental details— Micrographs for this thesis have been recorded at three different locations TUM using TEM, LMU and CEN using both (BE) S/TEM and (HAADF)-STEM mode, with the latter method being preferential for Pt catalysts, due to Z-contrast. Assignments of the TEMs to the location and specimens as well as their instrumental specifications are stated in the appendix in Table A.2.2. Further a variety of different TEM samples were used, in order to mimic the support of the corresponding sample for actual reactivity measurements (e.g. INPS, nanorods, etc.) in the best possible manner the different types, supports and the used TEM location are indicated in Table 3.1 and further details in the appendix in Table A.2.3. 

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