Atomic-scale imaging of ultrafast materials dynamics
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Introduction Our understanding of materials and their structures is typically derived from information obtained from static, equilibrium states. An excellent example of this is the extraordinary advances made in the structural characterization of materials using electron and x-ray scattering methods during the last 100 years. These methods have led to our current understanding of the atomic-level arrangement of materials and of the nature of defects, surfaces, buried interfaces, and nanoscale morphologies. Such static pictures form only a small part of the materials story. The atoms in materials are in constant motion due to thermal energy or quantum-mechanical effects, and the transformation from one phase or structure to another inherently involves the movement of those atoms (or molecules) to new thermodynamically (meta)stable positions. Advances in applied materials science and engineering rely heavily on in situ understanding of the processing, performance, and transformation of materials under dynamic, nonequilibrium conditions. As such, a great deal of effort has been devoted to extending static electron and x-ray scattering methods into the time domain. For example, rapid progress has been made in the commercial development of cameras for electron microscopes that are capable of read-out rates in the
hundreds of frames per second. The high spatial resolutions can be coupled with up to millisecond time resolutions to study transient atomic-scale materials phenomena, such as the change in faceting of nanocrystals in a precursor solution and the structural transformation of defects.1,2 The response of materials to perturbation and excitation originates at the atomic scale. Owing to the associated spatial dimensions, such motions occur on short time scales (femtoseconds [fs] to picoseconds [ps]; 10–15 to 10–12 seconds, respectively). A rough estimate of these time scales can be obtained by scaling the relevant lengths involved by an effective speed of sound, such that materials with, for example, one nanometer dimensions of interest will give rise to dynamics occurring on hundreds of femtosecond time scales.3 In order to obtain a complete understanding of materials dynamics, development of experimental tools capable of probing such ultrafast responses on the appropriate spatiotemporal scales— down to angstroms and femtoseconds—is important. Scattering methods employing brief pulses of fast electrons and x-ray photons are especially well suited for studying ultrafast materials dynamics.4,5 Because the scattering of these particles is affected by the positions and configurations of atoms in the material, a comprehensive picture of the structural
David J. Flannigan, Department of Chemical Engineering and Materials Science, University of Minnesota, USA; [email protected] Aaron M. Lindenberg, Department of Materials Science and Engineering, Department of Photon Science, Stanford University, USA; [email protected] doi:10.1557/mrs.2018.146
• VOLUME • JULY © 2018 Materials Research Society MRS BULLETINCore 43use, 2
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