Measuring Long-Range Correlated Dynamics in Amorphous Systems

Abstract: Nanometer-sized thin films of small organic molecules are widely used in applications ranging from organic photovoltaics and organic light emitting diodes, to protective coatings and high resolution nano-imprint lithography. Physical vapor deposition (PVD) is widely used in manufacturing ultra-thin layers of amorphous organic solids, with an underlying assumption that the properties of these layers are bulk-like. Here, we demonstrate that films of organic glass-formers with thicknesses of 30 nm or less have dynamics significantly enhanced relative to the bulk at temperatures well below the glass transition temperature, Tg. Furthermore, we show that a sharp glass to liquid transition exists when the thickness of the layer changes from 40nm down to 20nm. While these measurements are important for a host of applications, they can also help elucidate the fundamental mechanisms of glass transition temperature, a question that has attracted numerous theories in the past half century. Specifically, we are able to show that glassy systems have long-range correlated dynamics that can well exceed their inter-molecular interaction range.  

Bio:  Zahra Fakhraai received her B.Sc. and M.Sc. in physics from Sharif University of Technology in Iran. In 2003, she joined Jamie Forrest’s group in university of Waterloo and studied the dynamics of polymers in thin films and on their surfaces. She received her PhD in Physics from the University of Waterloo in 2007, for which she received the American Physical Society’s Padden award. From 2007 to 2008 Zahra worked in the Gilbert Walker’s group at the University of Toronto and studied the structure and chemical composition of block copolymers and protein aggregates using near-field infrared imaging. Subsequently, Zahra moved to Mark Ediger’s group at the University of Wisconsin-Madison (2009-2011) with an NSERC post-doctoral fellowship from the Canadian government.  She joined Penn Chemistry in 2011. 

Fakhraai group at Penn Chemistry explores structure and dynamics of materials at nanometer lengths scales. In 2014 Zahra received an NSF Career award to study the properties of organic glasses at interfaces and in nanometer lengths scale. These studies can help elucidate length scales of glass transition, and make ultra-stable glass films. Zahra is the recipient of 2015 Sloan fellowship in Chemistry and a co-PI of an NSF-PIRE grant for Research and Education in Active Coating Technologies (REACT) for the human habitat.

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