Exploiting the full potential of the advanced two-hexapole corrector for STEM exemplified at 60kV

• Published in: Ultramicroscopy 2022-03, Vol.233, p.113440-113440, Article 113440
• Main Authors: Sagawa, Ryusuke, Yasuhara, Akira, Hashiguchi, Hiroki, Naganuma, Tomoyuki, Tanba, Shinichi, Ishikawa, Takaki, Riedel, Thomas, Hartel, Peter, Linck, Martin, Uhlemann, Stephan, Müller, Heiko, Sawada, Hidetaka
• Format: Article
• Language: English
• Subjects: Aberration corrector DCOR/ASCOR; Low-voltage observation; Maximized resolution; Optimized aberrations; Ronchigram's flat area; Scanning transmission electron microscope
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2021.113440

•Deep sub-Angstrom STEM imaging with CEOS ASCOR in JEOL NEOARM at 60kV.•Analytical optimum for chromatic- and diffraction-limited probe angles.•Residual six-fold astigmatism is negligible and plays no role at all.•Maximized flat area in Ronchigram compromises resolution. Optimize your probe!•Simple guideline to balance sixth-order by fourth-order three-lobe aberration. Ultimate resolution in scanning transmission electron microscopy (STEM) with state-of-the-art aberration correctors requires careful tuning of the experimental parameters. The optimum aperture semi-angle depends on the chosen high tension, the chromatic aberration and the energy width of the source as well as on potentially limiting intrinsic residual aberrations. In this paper we derive simple expressions and criteria for choosing the aperture semi-angle and for counterbalancing the intrinsic sixth-order three-lobe aberration of two-hexapole aberration correctors by means of the fourth-order three-lobe aberration. It is noteworthy that for such an optimally adjusted electron probe the so-called flat area of the Ronchigram is explicitly not maximized. The above considerations are validated by experiments with a CEOS ASCOR in a C-FEG-equipped JEOL NEOARM operated at 60 kV. Sub-Angstrom resolution is demonstrated for a Si[112] single crystal as well as for a single-layered MoS2 crystalline film. Lattice reflections of 73 pm for silicon and 93 pm for molybdenum disulfide are visible in the Fourier transform of the images, respectively. Moreover, single sulfur vacancies can be clearly identified in the MoS2.