In scanning transmission electron microscopy (STEM), differential phase contrast (DPC) imaging has been developed to visualize the local electromagnetic field distribution in materials at medium resolution [1, 2]. The electromagnetic field deflects the incident electron beam, and this deflection can be measured by taking the difference between signals detected in opposing detector segments. Recent rapid progress in high-sensitive segmented detectors has enabled DPC STEM imaging to be performed at atomic-resolution [3]. However, DPC STEM images are sensitive to thickness and defocus, because dynamical scattering strongly affects DPC imaging of crystals in zone axis orientations [4]. It thus remains a challenge to develop a practical imaging technique at atomic-resolution with the segmented detector.

    Fig. 1 shows images of SrTiO₃ simultaneously obtained by different segments on a new segmented annular all field detector (SAAF2) installed in an aberration-corrected STEM (JEOL JEM-300F, 300kV). The relative orientation of the detector and the crystal structure is shown in Fig. 2. The images were at a defocus value of -3.7 nm relative to the defocus condition giving maximum contrast in annular dark field (ADF) imaging. These 512×512 pixel images were recorded with a dwell time of 38 µs per pixel, so the total imaging time is about 10 seconds. Each segment image can be qualitatively interpreted by electron beam deflection due to electric field from nuclei, including from the light oxygen atomic columns, though dynamical effects should be taken into account. According to image simulations, the DPC image appearance is largely unchanged with sample thickness if a defocus value is selected to obtain the highest contrast DPC image. This suggests that DPC STEM imaging at atomic resolution with a proper defocus value may be a new robust imaging mode that enables visualization of atomic column positions, including for light elements. Furthermore, this new imaging mode may contain information on charge redistribution due to charge transfer or orbital hybridization.

    In addition, we have found that the detector is sensitive enough to allow both segmented annular dark field imaging and DPC STEM imaging of single atoms to be performed. The details will be discussed in the presentation.

References

[1] J. N. Chapman et al., Ultramicroscopy 3, 203 (1978).

[2] M. Lohr et al., Ultramicroscopy 117, 7 (2012).

[3] N. Shibata et. al., Nat. Phys. 8, 611 (2012).

[4] R. Close et. al., Ultramicroscopy 159, 124 (2015).

[5] This work was supported by PRESTO and SENTAN, JST, and the JSPS KAKENHI Grant number 26289234. A part of this work was supported by Grant-in-Aid for Scientific Research on Innovative Areas (25106003). A part of this work was conducted in the Research Hub for Advanced Nano Characterization, The University of Tokyo, under the support of “Nanotechnology Platform” (Project No.12024046) by MEXT, Japan. This research was supported under the Discovery Projects funding scheme of the Australian Research Council (Project No. DP110101570).

Figures:

Fig. 1. Simultaneous images obtained by detector segments 5-8, which are schematically illustrated in Fig. 2 (a).

Fig. 2. (a) Schematic of the segmented detector. Blue shadow shows the bright disc of the STEM probe. (b) Unit cell structure of SrTiO3 projected along the [100] direction. The orientations of the two illustrations are aligned relative to one another as they were in the experiment.

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