Towards High-throughput Low Dose Observation by OBF STEM

Scanning transmission electron microscopy (STEM) has been widely utilized with annular detectors such as annular dark field (ADF) and annular bright field (ABF).Recently, segmented and pixelated detectors have allowed for the detection of detailed electron scattering intensity in the diffraction plane.Utilizing this richer information, low-dose STEM imaging capabilities have been discussed [1,2].Optimum-bright field (OBF) STEM, a non-iterative imaging technique developed recently, uses a segmented or pixelated detector for low-dose observation [3].OBF images are reconstructed from multiple images recorded simultaneously with a segmented/pixelated detector (Fig. 1).The reconstruction process ensures that the reconstructed image achieves the maximum signal-to-noise ratio (SNR) under the weak phase object approximation (WPOA).As the reconstruction process is noniterative, OBF images can be displayed synchronously with the probe scan.OBF STEM is starting to be used as a dose-efficient imaging technique for beam-sensitive materials.Recently, we demonstrated low-dose atomic resolution observations of FAU-type zeolite using OBF STEM, successfully visualizing all atomic sites [4].Figure 2 compares the OBF image with integrated differential phase contrast (iDPC), annular bright field (ABF), and bright field (BF) imaging techniques.The OBF image displays the highest SNR, clearly visualizing both oxygen and silicon sites.Additionally, since the noise in the OBF image can be considered as white noise, its interpretation is straightforward.In contrast, noise in the iDPC image is intensified in the low-frequency region, leading to reduced image contrast.Although OBF STEM is a promising technique for low-dose observations, adjusting observation conditions such as aberrations and crystal orientations for beam-sensitive specimens remains challenging.However, OBF reconstruction can account for residual aberrations and crystal misorientation.We developed a method to estimate actual observation conditions, including residual aberrations, crystal misorientation, and specimen thickness, using data recorded by a pixelated detector.Under the conditions of a thick weak phase object approximation (tWPOA) [5,6], we formulated a mathematical expression that defines the relationship between the 4D dataset and observation condition parameters.We found that it is possible to estimate these parameters separately by fitting the dataset to the corresponding mathematical expressions.We tested this method on a low-dose observation of FAU-type zeolite, with the dataset intentionally recorded under suboptimal observation conditions.The OBF reconstructed image assuming ideal observation conditions appeared opaque due to residual aberrations and crystal misorientation.In contrast, the OBF image that took into account the estimated observation condition parameters accurately visualized all atomic sites of FAU-type zeolite.This method significantly enhances the efficiency of low-dose observations.Moreover, if real-time estimation becomes feasible, the dataset could be recorded under nearly ideal conditions by adjusting the electron microscope based on the estimated parameters [7].