Enhancement of resolution and precision of TEM imaging by scanning illumination

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In transmission electron microscopy (TEM), mainly two imaging modes are differentiated: Conventional TEM (CTEM), which uses a plane coherent electron wave to illuminate the specimen, and scanning TEM (STEM), where the electron beam is scanned over the specimen and scattered electrons are registered by a detector. CTEM has the advantage of direct imaging which is not influenced by errors of positioning the electron beam. In contrast, as STEM is an incoherent imaging mode, the interpretation of images is easier and it provides a better spatial resolution.We were able to demonstrate in an article published in Phys. Rev. Lett. by experiment and simulation that the new imaging mode ISTEM (imaging STEM), which applies scanning illumination in the CTEM imaging mode, combines advantages of CTEM and STEM. It provides better spatial resolution than CTEM, interpretation of contrast patterns is easier and there is no influence of errors in positioning the electron beam. However, the advantages of ISTEM are accompanied by the disadvantage of a more difficult practicability. A first goal of the project is to improve the applicability of ISTEM by writing dedicated scripts to operate and adjust the microscope as well as reducing the contamination of the sample. In addition, advantages of ISTEM imaging that have only been shown by simulation so far will be demonstrated experimentally. These advantages include the higher precision of measuring the positions of atom columns, as well as the outstanding possibilities to image light atom columns in direct vicinity of heavy atom columns. In a recent publication of Prof. L. Allen et al. in Australia it was suggested to apply ISTEM for elemental analysis by energy filtered imaging with reduced artifacts. This approach will be tested in cooperation with the ER-C Jülich.The advantages of ISTEM imaging will be demonstrated using technologically relevant material systems that are connected with highly topical problems. The improved precision of the measurement of atom column positions will be applied to measure polarization domains in only a few nanometers thick ferroelectric tunnel barriers which are potential candidates for application in memory devices. Particularly, we plan to investigate the switching behavior of these barriers using in-situ applied electric fields. In addition, the possibilities of ISTEM to image light atom columns will be applied to measure tilts of oxygen-octahedra across the interfaces between different perovskite materials. Further applications are the measurement of spontaneous wrinkling of lattice planes in Te nanowires which can be applied in NO2 sensors, as well as the investigation of lattice strain close to the surface of nanoporous gold, which influences the catalytic activity of this material system.