Sub-Angstrom resolution at medium accelerating voltages of 200-300 kV is routinely achieved in standard transmission electron microscopes by hardware correction of the spherical aberration of the imaging lenses [1, 2]. At such voltages, defective, very thin and/or light-element materials very often suffer from knock-on damage, demanding experiments at the lower voltage ends of the microscopes, which are by default 80kV resp. 60kV with a resolution limited to approximately 2Å. Often, however, the achievable resolution at these voltages is not sufficient to understand the electron beam-matter interactions. Consequently, there is a demand for instrumentation that allows atomic-resolution at much lower voltages [3]. Using a standard field-emission electron source, the resolution is strongly limited by the chromatic aberration of the imaging lens requesting hardware aberration corrector that corrects for both, spherical and chromatic aberrations of the objective lens [4, 5].

Here, we report results towards understanding voltage-dependent electron beam-sample interaction for single-layer MoS₂ and MoS₂-graphene heterostructures using our newly developed Cc/Cs – corrected Sub-Angstrom Low-Voltage Electron Microscope (SALVE) operating at voltages between 20-80kV [6]. Damage cross-sections are determined by directly counting the vacancies produced during the high-resolution TEM experiments obtained at defined voltages and with defined electron doses. As the vacancies are created by different damage mechanisms as knock-on damage, radiolysis, ionization, and chemical etching, we discuss our results in the light of these mechanisms, with the aim to separate their contributions. Similar 80kV Cs-corrected HRTEM studies have been performed earlier albeit with much lower resolution [7]: in this case the resolution is about half the resolution obtained at 30kV (Cc/Cs) (see for comparison Figure 1).  As the knock-on threshold energy for sulfur atoms in MoS₂ has been calculated to be about 90 keV [8], the measured increase in the damage rates from 80 to 30 kV is attributed to radiolysis and ionization effects because of increased scattering cross-sections [9].

References

[1] M. Haider, S. Uhlemann, E. Schwan, H. Rose, B. Kabius & K. Urban, Nature 392, 768-769 (1998)

[2] O.L. Krivanek, N. Dellby, M.F. Murfitt, M.F. Chisholm, T.J. Pennycook, K. Suenaga, V. Nicolosi; Ultramicroscopy 110(8), 935 – 945 (2010)

[3] P. Hartel, M. Linck, H. Müller, S. Uhlemann, J. Biskupek, U. Kaiser, M. Niestadt and M. Haider, Microscopy and Microanalysis July 2016

[4] M. Haider, P. Hartel, H. Müller, S. Uhlemann, & J. Zach, (2010) Microscopy and Microanalysis, 16(04), 393-408.

[5] U. Kaiser, J. Biskupek, J. Mayer, J. Leschner, L. Lechner, H. Rose, M. Stöger-Pollach, A. Khlobystov, P. Hartel, H. Müller, M. Haider, S. Eyhusen, G. Benner; Ultramicroscopy 111 (8), 1239 – 1246 (2011)

[6] www.salve-projcet.de

[7] G. Algara-Siller, S. Kurasch, M. Sedighi, O. Lehtinen, U. Kaiser, Appl. Phys. Lett. 103(20) , (2013)

[8] H.P. Komsa, J. Kotakoski, S. Kurasch, O. Lehtinen, U. Kaiser, A.V. Krasheninnikov, Phys. Rev. Lett. 109, 035503 (2012)

[9] The authors acknowledge funding from the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state Baden-Württemberg, Germany in the frame of the SALVE (Sub-Angstrom Low-Voltage) project

Figures:

Figure 1: Direct comparison of raw data high-resolution TEM images of a MoS2 monolayer recorded with (a) Cc/Cs – corrected SALVE/FEI machine at 30 kV and (b) Cs – corrected FEI Titan 80-300 at 80 kV. The images were recorded with bright atom contrast. In (a) single atoms are resolved so that the hexagonal structure of MoS2 can be observed while in (b) the structure can still be resolved but the single atom contrast is very poor (cf. magnificated areas in the lower left parts). Correspondingly, the FFT of (a) (right upper insert) shows stronger reflections and higher frequencies transfer up to 102pm than the FFT of (b). The scale bar corresponds to 4 nm.

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EMC Abstracts - https://emc-proceedings.com/abstract/quantitative-low-voltage-spherical-and-chromatic-aberration-corrected-high-resolution-tem-analysis-of-beam-specimen-interactions-in-single-layer-mos2-and-mos2graphene-heterostructures/