Visualization of Defect-Modulated Collective Excitations with Ultrafast Electron Microscopy
| Date/Time: | Monday, 22 Feb 2016 from 4:10 pm to 5:00 pm |
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| Location: | Physics 0003 |
| Phone: | 515-294-5441 |
| Channel: | College of Liberal Arts and Sciences |
| Actions: | Download iCal/vCal | Email Reminder |
Abstract
Increasing the spatial and energy resolutions of transmission electron microscopes (TEM) has been the focus of much research during the last two decades. Now, with aberration-correction modules and monochromated electron guns, features below 50 picometers and energies below 10 meV (80 cm-1) can be resolved. Remarkably, however, the temporal resolution of conventional TEMs has, until recently, been limited to milliseconds, largely due to reliance on compatible digital detectors. In the first half of this talk, I will introduce and describe current methods used to greatly improve the temporal resolution of TEMs such that timescales approaching 100 femtoseconds (fs) can now be accessed. Following this, I will share some recent results obtained in my lab by employing such methods, wherein we have directly visualized the emergence and propagation of discrete acoustic phonon modes in a variety of materials (e.g., WSe2, 1T-TaS2, and undoped, tetragonal LaFeAsO). From a series of time-resolved bright-field images and diffraction patterns (time steps ranging from 500 to 5,000 fs), we are able to extract such properties as frequency (GHz) and velocity (e.g., 5,500 m/s). Further, we are able to image the nanoscale points of origin, propagation directions, dispersion, interference, and decay of coherent waves across both pristine and defect-laden regions on the order of the phonon wavelengths (tens of nanometers). In addition to demonstrating the feasibility for greatly increasing TEM time resolution, the methods and results described here have broad-reaching implications for the study of the evolution of collective excitation phenomena in a variety of materials systems.
Bio
David Flannigan is currently a McKnight Land-Grant Assistant Professor of Chemical Engineering and Materials Science and a Graduate Faculty in Chemical Physics in the Department of Chemistry at the University of Minnesota. His research program focuses on the development and application of ultrafast electron microscopy for elucidation of nanoscale photoinduced structural dynamics and visualization of collective excitation phenomena. He received his B.S. in Chemistry from the University of Minnesota and a Ph.D. in Chemistry from the University of Illinois at Urbana-Champaign under the supervision of Prof. Kenneth S. Suslick. At Illinois, he studied the chemical processes and physical conditions generated during single-bubble acoustic cavitation. After receiving his Ph.D., he worked as a Postdoctoral Scholar in Chemistry in the labs of Prof. Ahmed H. Zewail at Caltech. While there, he worked on the development and application of ultrafast electron-based techniques, especially ultrafast electron microscopy.