The Role of Surface Diffusion in Stable Glass Formation

Physical vapor deposition (PVD) produces glasses with exceptional stability and unique properties (higher density, anisotropy etc.). The current understanding is that the free surface, with its enhanced dynamics compared to the bulk, allows access to these near-equilibrium configurations during PVD. To this end, surface diffusion measurements on molecular glasses have been performed and showed it to be enhanced and decoupled from bulk relaxation dynamics. In this study, we measure the surface diffusion of three structurally very similar organic molecular glasses below their glass transition temperature (T_g) using tobacco mosaic virus as a probe of their surface evolution. Vapor deposited glasses of these compounds are found to have very similar increase in density when deposited at 0.85Tg. However, in spite of their ability to form stable glasses,only one of these molecules shows enhanced surface diffusion below T_g, while others are found to have no surface diffusion within our ability to measure even at temperatures close to T_g (T_g - 4K). Our results suggest that lateral surface diffusion may not be a good indicator of the enhanced relaxation dynamics required to produce stable glasses and other types of measurements may be necessary.