High-resolution annular dark-field imaging in the scanning transmission electron microscope (ADF STEM) offers a powerful and readily interpretable mode for materials analysis at the atomic scale. However, like all serial (scanned) imaging techniques subtle disturbances in the instrument’s surroundings can lead to deleterious artefacts in the recorded data. Recent developments in non-rigid registration of multi-frame data allows a route to mitigate these scanning errors [1] and obtain accurate strain information in the STEM [2].

    Here we make use of these new multi-frame techniques, but go further and use experiment design to optimally fractionate an allotted total electron-budget to achieve maximum precision. For the conditions used this occurs at around 20-25 ADF frames. Using these optimised conditions we record data from two specimens as proof-of principle examples; a rod-like AlMgSi precipitate in an Al matrix and a Pt₃Co dealloyed nanoparticle.

    In the case of the Al precipitate, Figure 1, the wide field-of-view allows geometric phase analysis (GPA) to be used to analyse the strain field [3]. At a 0.6nm resolution we achieve a strain precision of 0.3%. The accuracy of these results were confirmed through comparison with DFT simulation.

    For the Pt₃Co nanoparticle GPA is no longer appropriate owing to its limited size and the need for atomic resolution detail. Here a real-space approach was used where atomic-column positions were compared to sites defined by a pair of base vectors. The offsets from these are shown in Figure 2. Here we see the last monolayer and a half exhibit an expanded lattice parameter consistent with a platinum enriched outer shell.

Acknowledgments

    The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative–I3) and NORTEM (Grant 197405) within the programme INFRASTRUCTURE of the Research Council of Norway (RCN). NORTEM was co-funded by the RCN and the project partners NTNU, UiO and SINTEF. AV was financially supported by Johnson Matthey. Computational resources were provided by the Notur consortium. The authors acknowledge Eva Mørtsell for providing the Al alloy specimen.

References

[1]         L. Jones, H. Yang, T. J. Pennycook, M. S. J. Marshall, S. Van Aert, N. D. Browning, M. R. Castell, and P. D. Nellist, Adv. Struct. Chem. Imaging 1, 8 (2015).

[2]         L. Jones, S. Wenner, M. Nord, P. H. Ninive, O. M. Løvvik, R. Holmestad, and P. D. Nellist, [in press]. (2016).

[3]         M. J. Hÿtch, E. Snoeck, and R. Kilaas, Ultramicroscopy 74, 131 (1998).

Figures:

Figure 1. a) Averaged non-rigid registered multi-frame ADF image of an AlMgSi precipitate in an Al matrix, and b) the strain field calculated by GPA.

Figure 2. ADF STEM image of a [110] oriented Pt3Co nanoparticle showing lateral displacements of columns from their ideal positions. Enlargement a) shows a {111} type surface. Arrows in a) and b) have been enlarged by a factor of 5 for clarity. Plot c) shows the lateral expansion of the surface lattice planes indicating a Pt rich surface.

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