Atom‐Resolved STEM Imaging Using a Segmented Detector

Abstract

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 SrTiO 3 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.