Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy.

Timothy J Pennycook,Lewys Jones,Beatriz Mendoza-Sanchez,Valeria Nicolosi,Megan Canavan,João Coelho,Henrik Pettersson,Peter D Nellist

doi:10.1038/srep07555

Abstract

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science, and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and directly observe the atomic scale dynamics behind the transformation of Mn3O4 into MnO. The results demonstrate we now have the experimental capability to understand the complex atomic mechanisms involved in phase changes and solid state chemical reactions.

Highlights

Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy
1SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, Warrington WA4 4AD, United Kingdom, 2Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom, 3Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland, 4School of Physics, Trinity College Dublin, Dublin 2, Ireland, 5School of Chemistry, Trinity College Dublin, Dublin 2, Ireland. Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science, and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems
The results demonstrate we have the experimental capability to understand the complex atomic mechanisms involved in phase changes and solid state chemical reactions

Summary

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Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses It has made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. By recording a continuous series of rapidly scanned images in this region we provide the energy for the reaction to proceed while simultaneously recording the detailed motions of the atomic columns as the phase front advances. The B type columns increase in intensity until they reach the intensity of A type columns, indicating that enough additional Mn atoms have diffused into the column to fill the vacant sites This advances the phase front a single plane. The results illustrate how beam induced changes in materials can be informative, and pave the way towards exploring the complex energy landscapes of other dynamic systems with atomic resolution

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