Abstract
Imaging the complete atomic structure of materials, including light elements, with minimal beam-induced damage of the sample is a long-standing challenge in electron microscopy. Annular bright-field scanning transmission electron microscopy is often used to image elements with low atomic numbers, but due to its low efficiency and high sensitivity to precise imaging parameters it comes at the price of potentially significant beam damage. In this paper, we show that electron ptychography is a powerful technique to retrieve reconstructed phase images that provide the full structure of beam-sensitive materials containing light and heavy elements. Due to its much higher efficiency, we can reduce the beam currents used down to the subpicoampere range. Electron ptychography also allows residual lens aberrations to be corrected at the postprocessing stage, which avoids the need for fine-tuning of the probe that would result in further beam damage and provides aberration-free reconstructed phase images. We have used electron ptychography to obtain structural information from aberration-free reconstructed phase images in the technologically relevant lithium-rich transition metal oxides at different states of charge. We can unambiguously determine the position of the lithium and oxygen atomic columnswhile amorphization of the surface, formation of beam-induced surface reconstruction layers, or migration of transition metals to the alkali layers are drastically reduced.
Highlights
Imaging the complete atomic structure of materials, including light elements, with minimal beam-induced damage of the sample is a long-standing challenge in electron microscopy
Two important limitations are the challenge of imaging light elements when close to heavier ones, and the structural changes that can be caused by the electron beam irradiation
In many materials, such as the Li-rich battery cathode materials addressed here, both these problems occur, light elements often being susceptible to beam damage.[1,2]
Summary
Makes efficient use of the illuminating dose to form contrast in the image. Because of the intrinsically poor efficiency of ADFSTEM and ABF-STEM (of all the electrons that reach the detector plane, we only collect and integrate the signal over a small angular range), lowering the beam current will further worsen the signal-to-noise ratio. We show that electron ptychography in the STEM can be used to reconstruct aberration-corrected phase images with atomic resolution in pristine, charged and discharged particles even when beam currents in the sub-pA range are used. Both images contain the same structural information, revealing the position of the transition metals, the oxygen atoms in octahedral coordination around them, and the contrast in the alkali layer due to the presence of a reconstructed surface. This is extremely important to avoid misinterpretation of results because we can separate beaminduced structural arrangements from those produced by the charge/discharge cycles of the material
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