Direct Observation of All Atomic Sites in Zeolite by a Novel Low-Dose STEM Technique—Optimum Bright-Field Imaging

Scanning transmission electron microscopy (STEM) enables us to directly observe atomic structures inside materials, which is powerful in unveiling their structure-property relationships. However, one of the most challenging problems in recent years of (S)TEM research is the atomic-resolution imaging of beam-sensitive materials, which have a low resistance to electron irradiation, such as battery materials, porous materials, and organic materials. The conventional STEM techniques such as annular dark-field and annular bright-field (ABF) imaging modes can robustly visualize atomic structures of electron-resistant materials using single annular detectors. For the beam-sensitive materials, however, there is room to improve the dose efficiency to minimize the irradiation damage because these imaging techniques use only a fraction of transmitted electrons for imaging. Recently, a high-speed segmented detector has been developed, which can record almost all the transmitted electrons by multiple detection channels [1]. Herein, we developed optimum bright-field (OBF) STEM using a segmented detector as a more dose-efficient imaging technique [2]. In the OBF method, a STEM image with the highest signal-to-noise ratio for a given detector is reconstructed via Fourier filters designed based on the phase contrast transfer function and the noise-evaluation theory [3]. OBF STEM has approximately two orders of magnitude higher dose efficiency than ABF STEM, and thus, OBF is promising for low-dose observation. Furthermore, by approximating this filtering process, OBF images can be reconstructed in real-time and displayed in sync with the probe scans as well as the conventional STEM imaging. The real-time display function enables operators to tune aberrations and field-of-view even under the low-dose condition. In this research, we applied the OBF STEM technique to the atomic-resolution imaging of zeolites, which are well-known beam-sensitive materials. The low-dose OBF observation successfully visualized all atomic sites, including even oxygen sites, inside the zeolitic framework. Furthermore, we also observed the twin defects in a zeolite and determined its atomic structure directly in combination with the first-principles calculations.<br/><br/>[1] N. Shibata et al., Journal of Electron Microscopy <b>59</b> (2010) 473-479.<br/>[2] K. Ooe et al., Ultramicroscopy <b>220 </b>(2021) 113133.<br/>[3] T. Seki et al., Ultramicroscopy <b>193 </b>(2018) 118-125.<br/>[4] The authors acknowledge funding from JSPS KAKENHI (Grant Number JP20H05659).