[CCoE Notice] Seminar: Ultrafast TEM: Increasing Temporal Resolution by 10 Orders of Magnitude * CBB 108 * 10:30 am, Friday, March 8, 2019 * David Flannigan * University of Minnesota *

[Knudsen, Rachel W]

***** Seminar *****
Department of Electrical and Computer Engineering
Materials Engineering Program
Center for Integrated Bio and Nano Systems
  March 8, 2019
10:30 a.m., Room: CBB 108
Ultrafast TEM:  Increasing Temporal Resolution by 10 Orders of Magnitude
David Flannigan
University of Minnesota
Abstract: Improving the resolutions achievable with transmission electron microscopes (TEMs) has required sustained and substantial efforts spanning decades.  These efforts are now bearing significant fruit.  The DOE-funded TEAM effort at LBNL led to the successful demonstration of 0.5-Å spatial resolution, on the order of the Bohr radius.  Indeed, the technology capable of reaching such dimensions is now fairly ubiquitous, with aberration-corrected instruments capable of routinely generating 0.6-Å probe sizes being installed and used at many research universities, national labs, and companies.  Energy spreads of the incident beam have also been vastly improved in recent years through the development (among other things) of monochromated guns.  Distributions of the zero-loss electrons spanning ranges smaller than 10 meV have been achieved, representing a spread of only one part in 10 million.  What has been lacking, however, is a sustained and concerted effort to improve the severely-limited temporal resolutions of conventional TEMs.  In situ studies aiming to probe materials dynamics are restricted by detector capabilities, wherein readout rates of commercially-available CMOS chips and direct-electron detectors can reach only hundreds of microseconds at best.  A simple calculation of atomic speed-of-sound motions involved in phase transformations, as well as ballistic and diffusional transport processes and charge-carrier dynamics, illustrates the wealth of information that is missed with such temporally-restricted measurements.  Here, I will discuss how new paradigms are being developed to overcome detector limits and to push TEM temporal resolutions to the femtosecond regime, wherein the direct study of ultrafast materials processes becomes possible.  The main approach has been to couple a conventional TEM with a femtosecond pulsed laser and to use the photoelectric effect to generate discrete packets of photoelectrons in the gun region having durations on par with the laser-pulse duration.  A second triggering pulse is used to initiate dynamics in the specimen at a well-defined moment in time with respect to arrival of the probing electron packet.  Varying the time between the electron packet and triggering pulse enables reconstruction of the ultrafast response via otherwise typical imaging, diffraction, and spectroscopic TEM modalities.  Following a discussion of the technology and the conceptual approach, I will share some examples of the types of phenomena my group has studied before closing with a brief view of expanding the capabilities to study highly beam-sensitive materials and to mitigate radiation damage.  My objective is to illustrate how this and related approaches are leading to an improvement in TEM temporal resolution by 10 orders of magnitude, with a major goal being to cover a range spanning from femtoseconds to the detector limits without sacrificing the spatial and energy resolutions achievable with otherwise conventional methods.
Bio: David Flannigan is currently Associate Professor of Chemical Engineering and Materials Science at the University of Minnesota, where he studies materials dynamics using ultrafast electron scattering. He received his B.S. in Chemistry from the University of Minnesota in 2001 and his Ph.D. in Chemistry from the University of Illinois in 2006, after which he was a Postdoctoral Scholar in Ahmed Zewail’s group at Caltech. He is the recipient of an NSF CAREER Award, a DOE Early Career Award, and a Beckman Young Investigator Award. He is currently a McKnight Presidential Fellow and Director of Undergraduate Studies for Materials Science.
Contact Prof. Robles Hernandez fcrobles at Central.UH.EDU<mailto:fcrobles at Central.UH.EDU>  if you would like to meet with Dr. Flannigan.
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