Surface Mediated Formation of Stable Glasses

It is undertowed that enhanced surface mobility mediates stable glass (SG) formation upon physical vapor deposition (PVD). The constraints imposed on the free surface also result in unique structures, such as the wetting of a chemical motif or the preferred orientation of elongated molecules. These structures can be immobilized throughout a PVD glass, which can be varied with substrate deposition temperature and rate. Here we investigate the thermodynamic and kinetic stability of SGs produced by two structurally similar molecules with different equilibrium shapes and molecular level flexibility. We demonstrate that molecular orientation is not necessary to produce SGs, and instead is an indication of reduced surface mobility and stability. Given sufficient equilibration time, with a thick enough mobile surface layer, PVD can generate isotropic SGs that resemble their extrapolated supercooled liquid state. As such, variation of orientational anisotropy can indirectly inform of the thickness of the mobile layer during deposition. Surprisingly, while more flexible molecules can produce kinetically improved stable glasses at high substrate temperatures, their improved packing is detrimental to maintaining enhanced mobility at low temperatures, which reduces their ability to form SGs.