High Stability of Ultrathin Vapor Deposited Molecular Glasses*

Previous studies demonstrate that a mobility gradient exists within organic glass films, where molecules at or close to the free surface are more mobile than those in the bulk. The stability of glasses formed via vapor deposition relies on the equilibration of as-deposited molecules at the surface. An ultrathin stable glass (SG) film (~50 nm) has a higher proportion of molecules affected by the free surface dynamics, therefore likely has enhanced stability compared to a bulk SG (>200 nm). Here, we show that ultrathin SG films of TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) have higher stability, indicated by a higher density increase and a lower fictive temperature than bulk SG films of TPD. Surprisingly, at optimal deposition conditions, density increases higher than the limiting super-cooled liquid states are achieved. Anisotropy indicated by synchrotron scattering measurements suggest a possibility that there exists an amorphous as-deposited state of the molecule, volumetrically denser than its supercooled liquid counterpart, that is more isotropic than bulk SG. The potential origins of this phenomenon will be discussed in this presentation.