Professor Zahra Fakhraai of the University of Pennsylvania will present
"Long-range Correlated Dynamics in Glassy Materials and its Role in Producing Stable Glasses"
on September 27, 2017 at 4:10 PM in Neville Hall, Room 3.
Nanometer-sized thin films of small organic molecules are widely use 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. In this presentation, I demonstrate that films of organic glass-formers with thicknesses of 30 nm or less have dynamics significantly enhanced relative to the bulk dynamics at temperatures well below the glass transition temperature, Tg. I show that a sharp glass to liquid transition exists when the thickness of the layer is changed from 40 nm down to 20 nm. This significant change in the glass dynamics is due to the enhanced mobility at the air/glass interface and the length scale over which the effects of this perturbation can propagate due correlated dynamics in the bulk glass. As such, we show that these glassy systems have long-range correlated dynamics over length scales of about then times the size of the molecules, well exceeding their inter-molecular interaction range. We discuss the effect of variety of factors such as substrate interactions and molecular construct on the observed length scale. We show that this length scale is roughly the same in a wide range of organic systems, while it changes significantly when an inorganic glass such as amorphous selenium is studied. These results can also help elucidate fundamental mechanisms of glass transition phenomenon, a question that have attracted numerous theoretical and experimental studies in the past half century. Furthermore, we show that the enhanced surface dynamics along with long-range correlation in the dynamics can allow formation of stable glasses through the PVD process, enhancing our ability to engineer properties of glasses over a large range of properties.