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Webinar Agenda:

2.00-2.30pm

Dr. Takehito Seki (The University of Tokyo)

Title:  Ultra-high contrast imaging in scanning transmission electron microscopy

Abstract: Scanning transmission electron microscopy (STEM) has achieved the spatial resolution of 40.5 pm [1], and can directly visualize local atomic structures. In STEM, converged electron beam is scanned across specimens, and transmitted/scattered electrons at each raster are detected in an annular detector placed in the diffraction plane to form conventional STEM images. Annular dark field (ADF) and annular bright field (ABF) STEM have been widely utilized in material science fields, since ADF- and ABF-STEM images can be interpreted simply. However, the conventional STEM imaging techniques are less doseefficient than TEM, i.e., they need more electron dose to obtain high signal-to-noise ratio (SNR). Recently, STEM imaging modes using pixelated/segmented detectors have been intensively explored, and some of them appears to be promising because of their high doseefficiency. In conventional TEM, the contrast transfer function (CTF) has been used to evaluate dose efficiencies, since image contrast in TEM is directly related to the SNR. Although the CTF is also defined in STEM, SNR cannot be simply evaluated based on the CTF. This is because the background level is not directly related to the noise level. Thus, CTFs in STEM need to be normalized by the noise level to compare the dose-efficiencies across different STEM imaging modes [2]. Using this framework, we have explored high-SNR STEM imaging modes, and found a solution to maximize SNR for arbitrary detector segmentation [3]. We call this new imaging technique as optimum bright field (OBF) STEM. OBF for a pixelated detector is almost identical to the single-side-band (SSB) ptychography; namely, OBF is an extension of SSB ptychography to segmented detectors. OBF is further combined with integrated CTF [4], which consider specimen-thickness effect under the weak phase object approximation. It improves imaging characteristics for thick specimens. Fig. 1 show ABF and OBF images of LiCoO2, which were acquired at the same low dose condition. Very faint contrast at Li sites in ABF is significantly improved in OBF. The dose-efficiency of OBF has been improved by a factor of 70 compared to ABF. Details will be discussed in the presentation.

2.30-3.00pm

Dr. Daniele Pellicia (Instruments & Data Tools Pty Ltd)

Title: Ghost imaging as configurable detector

Abstract: In ghost imaging, a conventional pixel array detector is replaced by a varying structured illumination field and a single pixel sensor. Intensity correlation between the sensor counts and the known structured illumination allows the image of the scene to be reconstructed.