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. 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. While these measurements are important for a host of applications, they can also help elucidate the fundamental mechanisms of glass transition phenomenon, a question that have attracted numerous theoretical and experimental studies in the past half century. We show that the sharp transition observed around 30 nm is inconsistent with two-layer models where the effect of interface propagates into the film over a constant thickness for films of all thicknesses. The results of these experiments are directly compared with some of the most well-known models of glass transition. Acknowledgement: NSF-Career award DMR-1350044, MRSEC: DMR-1120901

Long-range correlated dynamics in organic and inorganic glasses

254th National Meeting (2017)

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