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 Karl Franzens University Graz

Graz University of Technology 

Applications of low voltage STEM-EELS: from single atom spectroscopy in 2D materials to structure determination in complex oxides
Dr. Quentin Ramasse
SuperSTEM Laboratory, Daresbury, U.K.
17:15 - 18:15 Tuesday 07 January 2014 TUG P2

Modern aberration-corrected scanning transmission electron microscopes have been optimised to provide improved data collection ability and greater flexibility even at low acceleration voltages, and a wealth of complementary analytical signals is now available from a single experiment [1]. When combining Z-contrast and bright field STEM imaging, 2D chemical mapping together with advanced image analysis, it is possible to statistically determine chemical variations in complex oxide structures across a range of compositions, and to relate those to accurately measured atomic displacements. Similarly, the development of so-called 'gentle', dose-controlled STEM techniques has been particularly beneficial for the field of two-dimensional materials. By reducing the acceleration voltage to overcome knock-on damage limitations, many of these structures can be imaged directly at atomic resolution, revealing for instance the propensity of graphene to spontaneously 'heal' itself when perforated [2]. Having shown what atomic species are present and where single atom impurities or defects are located using spectroscopy, some fundamental questions remain: how exactly are these atoms bonded to one another and how do structural differences affect their electronic configuration? Answers to these questions can be provided one atom at a time by EELS fine structure analysis, which can distinguish unambiguously between bonding configurations [3]. 

[1] I. MacLaren et al., APL Materials. 1 (2013), 021102

[2] R. Zan et al., Nano Lett. 12 (2012), 3936. 

[3] Q.M. Ramasse et al., Nano Lett. 13 (2013), 4989.

Presentation: ramasse_graz.pdf