1. Long-range correlated dynamics in ultra-thin molecular glass films
- Author
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Yue Zhang, Kareem Wahid, M. Li, Tianyi Liu, William Zhang, Zahra Fakhraai, Robert A. Riggleman, and Ethan Glor
- Subjects
Surface diffusion ,Length scale ,Materials science ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Condensed Matter::Soft Condensed Matter ,Viscosity ,Chemical physics ,Free surface ,Physical vapor deposition ,Dewetting ,Physical and Theoretical Chemistry ,Thin film ,0210 nano-technology ,Glass transition - Abstract
It has been previously shown that the free surface of molecular glasses has enhanced surface diffusion compared to the bulk. However, the degree by which the glass dynamics are affected by the free surface remains unexplored. Here, we measure enhanced dynamics in ultra-thin molecular glass films as a function of film thickness. We demonstrate that these films exhibit a sharp transition from glassy solid to liquid-like behavior when the thickness is reduced below 30 nm. This liquid-like behavior persists even at temperatures well below the glass transition temperature, Tg. The enhanced dynamics in these films can produce large scale morphological features during physical vapor deposition and lead to a dewetting instability in films held at temperatures as low as Tg − 35 K. The effective viscosity of these films are measured by monitoring the dewetting kinetics. These measurements combined with cooling-rate dependent Tg measurements show that the apparent activation barrier for rearrangement decreases sharply in films thinner than 30 nm. This sharp transition in the dynamics suggests that long-range correlated dynamics exists in these films such that the enhancement induced by the free surface can strongly affect the dynamics of the film over a length scale that is ten times larger than the size of the molecules.
- Published
- 2016