- This event has passed.
When Less Really can be More….Optimized Sampling Schemes for High Resolution (S)TEM Observations of Beam Sensitive Materials and Processes
February 15, 2019 @ 11:00 am - 12:00 pm
Title: When Less Really can be More….Optimized Sampling Schemes for High Resolution (S)TEM Observations of Beam Sensitive Materials and Processes
Speaker: Nigel Browning
- Department of Mechanical, Materials and Aerospace Engineering and Department of Physics, University of Liverpool, Liverpool, L69 3GH
- Physical and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA
The last ten years have seen a paradigm change in scanning transmission electron microscopy with unprecedented improvement in the spatial resolution of images and spectra being realized by aberration correctors. Images/spectra can now be used routinely to observe and quantify the atomic scale structure, composition, chemistry, bonding, electron/phonon distribution and optical properties of nanostructures, interfaces and defects in many materials systems. However, quantitative and reproducible atomic resolution observations for some materials is actually harder with these new capabilities, as the increase in beam current with correctors also brings with it the potential for electron beam modification of the specimen during image acquisition. This has led to a widely used definition of samples that “work” for aberration corrected microscopy, and samples where microscopy can’t help – essentially disenfranchising a whole section of materials science from the benefits of electron microscopy. This beam effect is even more acute for the design of in-situ STEM observations, where the final desired outcome being investigated is a result of a series of complicated transients. If the electron beam changes these processes in any way, then the validity of the experiment can be called into question. The aim in developing and applying new methods in STEM is therefore now to focus on more efficient use of the dose that is supplied to the sample and to extract the most information from each image – reducing the beam effect and broadening microscopy applications to a wider range of samples and processes. Optimizing the dose/data content in non-traditional ways (i.e. not just simply lowering the beam current) involves two main strategies to achieve dose fractionation – reducing the number of pixels being sampled in STEM mode, or increasing the speed of the images in TEM mode. For the case of the STEM, inpainting methods allow a dose reduction of an order of magnitude or more, allowing data to be automatically recorded in a compressed form. For the TEM mode of operation, an increase in speed increases the number of images and means that compressive sensing and automated methods of tracking changes in the structure need to be developed so that only the important changes need to recorded. In this presentation, the basic approach to dose control using both conventional and unconventional sampling methods will be described. Results showing the use of in-situ liquid stages to study nanoscale dynamic processes involving electrochemical driving forces will be presented and the potential insights gained by increasing the image acquisition speed and/or decreasing the electron dose for future research projects will also be discussed.
This work was supported in part by the Chemical Imaging Initiative under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated by Battelle for the U.S. Department of Energy (DOE) under Contract DE-AC05-76RL01830. A portion of the research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. This work was also supported in part by the U.K. Faraday Institution.