Length Scale of Correlated Dynamics in Ultra-thin Molecular Glasses

TL;DR

This study reveals that ultra-thin organic glass films below 30 nm exhibit liquid-like behavior and enhanced dynamics at temperatures below T_g, significantly affecting their morphological stability and properties.

Contribution

It demonstrates a sharp transition in dynamics and viscosity in ultra-thin organic glasses, highlighting long-range surface facilitation effects not previously characterized.

Findings

Liquid-like behavior below 30 nm thickness

Decreased activation barrier for molecular rearrangements

Large-scale morphological features and dewetting at low temperatures

Abstract

Physical vapor deposition (PVD) is widely used in manufacturing ultra-thin layers of amorphous organic solids. Here, we demonstrate that these films exhibit a sharp transition from glassy solid to liquid-like behavior with thickness below 30 nm. This liquid-like behavior persists even at temperatures well below the glass transition temperature, T$_{\mathrm{g}}$. The enhanced dynamics in these films can produce large scale morphological features during PVD and lead to a dewetting instability in films held at temperatures as low as T$_{\mathrm{g}}$-35 K. We measure the effective viscosity of organic glass films by monitoring the dewetting kinetics. These measurements combined with cooling rate-dependent T$_{\mathrm{g}}$ measurements show that the apparent activation barrier for rearrangement decreases sharply in films thinner than 30 nm. These observations suggest long-range facilitation of dynamics induced by the free surface, with dramatic effects on the properties of nano-scale amorphous materials.