PolaRIS-3D - Polarization Retrieval Including Strain in Three Dimensions
Experimental Condensed Matter Physics
Final Report Abstract
Within materials physics, many basic and applied questions can only be answered by measuring and characterizing nanoscale order and disorder within crystals. When designing new materials – whether as ecologically friendly substitutes for existing materials or complex materials with entirely new properties for research or applications – the materials’ atomic structure directly reflects suitability. Thus, measuring deviations from "ideal" crystallinity is a critically important task for a wide range of materials, and the types and magnitudes of deviations can vary widely. For example, ferroelectric polarization arising from picometer-scale atomic displacements is organized in a domain structure, and the polarization magnitude is material-dependent. Transmission electron microscopy (TEM) combines sub-Angstrom spatial selectivity and versatility, and could answer these questions if suitable analysis algorithms existed that could take into account the strong beam-specimen interaction. The objective of this project was to apply a promising new algorithm to experimental TEM data and retrieve these variations. Before this project, the PRIMES algorithm (Parameter Retrieval Including Multiple Electron Scattering) was developed to fill this gap by providing two- and threedimensional domain structure retrieval from TEM data. However, the PRIMES algorithm had not been tested against experimental data – and a particular challenge is that the PRIMES algorithm needed extensive calibration against known data to evaluate the quality of existing specimen models. Thus, the PolaRIS-3D project was designed to compare experimental and simulated data by first evaluating the existing specimen models for both elastic and inelastic scattering. By evaluating how good the models were, the expected accuracy of the PRIMES algorithm could be determined, and, thus, it could be used on experimental data. The PolaRIS-3D project found that conventional isolated-atom elastic-scattering models (fast, but neglecting chemical bonding) were definitely unsuitable. Their replacement, density functional theory (DFT), is still an approximation, but TEM scattering models have a direct relation to the electronic structure problems that DFT has been particularly excellent at solving. Using different DFT codes showed that PRIMES works with this more advanced model, and the DFT models are a good match for experiment, and should be used in further TEM analysis. When comparing with experimental data, the PolaRIS-3D project also discovered that existing inelastic-scattering models, while a small percentage of the overall scattering, are still inaccurate enough to make direct experiment-simulation comparison difficult. New models for this would require angularly-resolved inelastic scattering measurements that only now can be performed efficiently and ideally (on e.g. a Nion HERMES S/TEM). To solve this during PolaRIS-3D with existing models, the most promising approach appears to be advanced pattern filtering that enables background reduction – however, the exact parameters of that pattern processing routine are highly dependent on the exact features of that input data. Future work in this area can emphasize two components. First, the development of new inelasticscattering models is of key importance for advanced quantitative electron microscopy, such as undertaken in PolaRIS-3D. Second, PolaRIS-3D showed the PRIMES algorithm to be worth additional research on new materials, since it provides direct retrieval of crystal parameters on the nanoscale and in bulk-like specimen geometries.
Publications
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Phys. Rev. B 97, 024112 (2018)
R. S. Pennington, C. Coll, S. Estradé, F. Peiró, and C. T. Koch
