Bradley Siwick

Université McGill (physique et chimie)
Structure and Dynamics with Ultrafast Electron Microscopes

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- microstructure[1]. 

First, I will briefly describe my group’s efforts to develop ultrafast electron diffraction using radio-frequency compressed electron pulses in the 100keV range[2], a system that approaches the capabilities of ~ $1B 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.  We have been able to identify and separate lattice structural changes from valence charge density redistribution in the material[3].  In doing so we uncovered a previously unreported, optically accessible metastable phase of vanadium dioxide that has the crystallographic symmetry of the monoclinic insulator, but electronic properties and structure like that of the rutile metal.  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 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 timescales[4,5], 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 (In Press, Oct. 24, 2014).  arXiv:1407.1304.

4. 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.  

5. 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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