Effect of Substrate on the Stability of Ultra-thin Vapor Deposited Molecular Glasses

We have recently demonstrated the existence of long-range correlation between the free surface and substrate dynamics in ultrathin supported molecular glass films such that when the substrate interactions are changed from weakly interacting to neutral, the extent of T_g reduction and enhanced film dynamics is reduced and the stability against dewetting is increased for films as thick as 30 nm. The stability of vapor-deposited glasses is strongly influenced by the surface mobility and its gradients. Since the surface dynamics are affected by the substrate for ultra-thin films one would expect the stability to follow. Here, we show that ultra-thin films of TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) vapor-deposited on a neutral substrate have higher stability (higher density change and lower fictive temperatures) compared to those deposited on a weakly-interacting substrate. The density change is also found to be higher in ultrathin films (<50 nm) compared to the bulk films (>200 nm, steady state condition). These results suggest a possibility that film/substrate interactions could speed up effective aging rates, or deter dewetting during deposition, giving rise to more efficient amorphous packings.