Date of Award
Doctor of Philosophy (PhD)
Nanometer-sized thin films of small organic molecules are widely used in applications ranging from organic electronics and pharmaceuticals to coatings and nano-imprint lithography. Studies show that properties of these nanometer-sized thin films deviate strongly from their bulk counterparts, possibly due to enhanced surface dynamics and increased surface-to-volume ratio. Developing new techniques that can readily measure the surface dynamics of an organic glass can help understand such phenomena. In this thesis, I will first introduce a novel technique that uses tobacco mosaic virus as the probe particle to directly measure surface diffusion on molecular glasses. The surface diffusion is measured to be greatly enhanced compared to bulk counterpart. The surface diffusion is also investigated on ultrastable glasses and aged glasses with suppressed relaxation dynamics and ultrathin glasses with overall enhanced dynamics. The surface diffusion is found to stay fast and invariant on molecular glasses with varying bulk relaxation dynamics, suggesting that the surface diffusion is decoupled from bulk relaxation dynamics and is only a purely free surface motion. Further, I combine a morphological probe tracking the isothermal dewetting process in ultrathin molecular glasses with cooling-rate dependent glass transition temperature measurements to study the propagation length scale of the surface enhancement effect. Results show that organic glass films with thicknesses of 30 nm or less have dynamics significantly enhanced relative to bulk, induced by the free surface. Furthermore, there is a sharp glass to liquid transition observed around 30 nm, indicating long-range correlated dynamics in ultrathin molecular glasses. While these studies are important for a host of applications, they can also help elucidate the fundamentals of interfacial effects in thin film systems.
Zhang, Yue, "Enhanced Surface Dynamics And Propagation In Molecular Glasses" (2017). Publicly Accessible Penn Dissertations. 2666.