Recent methods of dark field TEM are being explored to extent the information and frequency of data capture in dynamic in-situ experiments on nanocatalyst systems under reaction conditions (1).    The broad need is for a faster frame rate than can conveniently be achieved with HAADF STEM imaging, starting from the similarly expressed goals for tomography of Bals et al (2) and gentle high resolution by Zhang et al (3).   Beam stop spiders described in (2,3) and other geometries including displaced aperture arcs stopping the central beam have been explored.    FIB fabricated custom devices have been introduced into a custom mechanism using regular 3mm apertures for easier customisation.   This is primarily inserted into the high contrast (lower) objective aperture position on our modified JEOL 2200FS with aberration correctors for both TEM image and STEM probe.   The preliminary results with the new applications are encouraging with 0.2nm lattice images recorded from larger (~10nm) particles (Fig.1) and sensitivity down to a few atoms (Fig.2) recorded using 1 second exposures, rather than requiring 10x that for direct STEM methods.    The aim is to be able to track individual migrating atoms and nanoparticles with sufficient frequency to have confidence for each one in their source and endpoint to better inform our understanding of key coarsening mechanisms (4) which lead to catalyst inefficiencies, including in environmental emission controls (5).    This requires analysis at sufficient frequency that the mean atom/nanoparticle movement is significantly less than the spacing between sites.

References
1. E D Boyes, M R Ward, L Lari and P L Gai, Ann Phys (Berlin), 525 (2013) 423

2. S Bals, G V Tendeloo and C Kisielowski, Adv Mater, 18 (2006) 892

3. C Zhang, Q Xua, P J Peters and H Zandbergen, Ultramicroscopy, 134 (2013) 200

4. P Wynblatt and N A Gjostein, Prog Solid State Ch, 9 (1975) 21

5. M R Ward, T Hyde, E D Boyes and P L Gai, Chem Cat Chem, 4 (2012) 1622

Acknowledgement

The AC ESTEM catalysis project at York is supported primarily by EPSRC (UK) strategic grant EP/J018058/1

Figures:

Fig.1: (a) Image of larger Au nanoparticle imaged with a restricted spider symmetrical central beam stop aperture and showing 0.2-0.24nm lattice resolution and (b) custom aperture shape used.

Fig.2: Image (a) of bright field and (b) dark field TEM of Pt islands of the same region recorded with round aperture positioned off-axis to exclude the 000 beam from the imaging process. Both images were acquired at the same magnification with 1 second exposure time.

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