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Structure and Dynamics with Ultrafast Electron Microscopes

Date Mi, 25.03.2015 - Mi, 25.03.2015
Time 14.00
Speaker Bradley J. Siwick, Center for the Physics of Materials, McGill University, Montreal, Canada
Location ETH Zurich, Hönggerberg Campus, HPF G6
Program Abstract: In this talk I will describe how combining ultrafast lasers and electron microscopes in novel ways makes it possible to directly ‘watch’ the time-evolving structure of matter, both at the level of atomic-scale structural rearrangements in the unit cell and at the level of a material’s nano- microstructure1.

First, I will briefly describe my group’s efforts to develop ultrafast electron diffraction using radio- frequency compressed electron pulses in the 100keV range2, a system that approaches the capabilities of xray free electron lasers for diffraction experiments. I will give two examples of the new kinds of information that can be gleaned from such experiments. In vanadium dioxide we have mapped the detailed reorganization of the unit cell during the much debated insulator-metal transition. In particular, we have been able to identify and separate lattice structural changes from valence charge density redistribution in the material3. In doing so we uncovered a previously unreported, optically accessible metastable phase/state of vanadium dioxide that has the crystallographic symmetry of the monoclinic insulator, but strongly modified electronic structure and properties that are more metal-like3. In graphite we have been able to directly observe the coherent and incoherent coupling of impulsive electronic excitation at 1.55 eV (800 nm) to optical and acoustic phonon modes directly from the perspective of the lattice degrees of freedom.

Second, I will show how dynamic transmission electron microscopy (DTEM) can be used to make direct, real space images of nano-microstructural evolution during laser-induced crystallization of amorphous semiconductors at unprecedented spatio-temporal resolution. This is a remarkably complex process that involves several distinct modes of crystal growth and the development of intricate microstructural patterns on the nanosecond to ten microsecond timescales4,5, all of which can be imaged directly with DTEM.

1. G. Sciani and R. J. D. Miller, Femtosecond electron diffraction: Heralding the era of atomically resolved dynamics, Rep. Prog. Phys. 71 (2011) 096101
2. R. P. Chatelain, V. Morrison, C. Godbout, and B. J. Siwick, Ultrafast electron diffraction with radio-frequency compressed electron pulses, Appl. Phys. Lett. 101 (2012) 081901.
3. V. Morrison, R. P. Chatelain, K. Tiwari, A. Hendaoui, M. Chakker and B. J. Siwick, A photoinduced metal-like phase of monoclinic vanadium dioxide revealed by ultrafast electron diffraction, Science 346 (2014) 445 – 448.
4. R. P. Chatelain, V. Morrison, Bart L. M. Klarenaar and B. J. Siwick, Coherent and incoherent electron-phonon coupling in graphite observed with radio-frequency compressed ultrafast electron diffraction, Phys. Rev. Lett. 113 (2014) 235502.
5. L. Nikolova, M. Stern, T. LaGrange, B. Reed, N. Browning, G. H. Campbell, J.-C. Kieffer, F. Rosei and B. J. Siwick, Complex crystallization dynamics in amorphous germanium studied with dynamic TEM. Phys. Rev. B 87 (2013) 064105.
6. M. Stern, L. Nikolova, J. MacLeod, B. Reed, G. H. Campbell, H. Ibrahim, F. Rosei, T. LaGrange, B. J Siwick, In situ investigation of explosive crystallization in a-Ge: Experimental determination of the interface response function using dynamic transmission electron microscopy, J. Appl. Phys. 116 (2014) 093512.

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Link Siwick Lab, McGill
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