STEM-Tomography for thick biological specimens
Abstract: STEM tomography offers several advantages even for biological specimens. The most important advantage is the ability to generate clear images for thick specimens. STEM is free from chromatic aberration, which is main obstructer to generate clear images for TEM. The second advantage is ‘dynamic focus’. Since TEM imaging uses a fixed focus, only a very narrow area can be optimally focused on for obtaining images of highly tilted specimens. Therefore, the introduction of a sufficiently large defocus is required in TEM tomography. However, the scanning beam can be focused flexibly in STEM. Therefore, the entire image area can be optimally focused on even in highly tilted specimens, and defocusing is not necessary. The third advantage is the ability to change the imaging mode. In STEM tomography, it is easy to switch between the bright field, dark field and HAADF modes; in contrast, it is very difficult to obtain stable dark-field images during data collection in TEM tomography. The fourth advantage is the linear contrasts of the images. It is not necessary to consider the contrast transfer function (CTF) of electron lenses in STEM imaging.
Key words: STEM (Scanning transmission electron microscopy), Electron tomography, Thick sections, Biological specimens