Formation of stable glasses on soft substrates

Surface mediated stable glass (SG) formation upon physical vapor deposition allows access to near-ideal glassy states and tuning glass structures by adjusting the deposition substrate temperature and rate. Here, we demonstrate substrate rigidity as a new parameter to control glass stability and structure. We show that SGs of organic molecules (N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine) deposited on soft substrates (Polydimethylsiloxane) have higher refractive indices, more isotropic orientation, and dramatically suppressed layering structure compared to those deposited on Silicon. Measurements of the devitrification kinetics of these SGs show an additional moving transformation front initiated from the soft substrate, with faster initial velocity than the typically observed transformation front from the glass free surface. The fast transformation kinetics persist to a distance of ~80 - 100 nm from the soft interface, for SGs formed in the intermediate range of substrate temperatures. At lower substrate temperatures, SGs deposited on soft substrates exhibit higher kinetic stability than those deposited on rigid substrates. These results indicate increased range of mobility gradients on soft substrates which facilitates the glass kinetics, allowing access to lower energy states during vapor deposition.