Size Effects and Extraordinary Stability of Ultrathin Vapor Deposited Glassy Films of Toluene

Abstract

We report the first experimental evidence of size effects in the glass transition of thin films of an organic molecule grown from the vapor phase. In as-deposited films grown at 90 K (0.80Tg), both the fictive temperature, Tf, and the onset of the glass transition, Ton, decrease with thickness. The thinnest layers (∼4 nm) exhibit the highest thermodynamic and lowest kinetic stability. Films refrozen at 2000 K/s after being heated to the liquid state during a previous scan demonstrate no size effects. The width of the glass transition for both as-deposited and refrozen films is independent of the film thickness down to 4 nm. Our heat capacity data suggest that ultrathin vapor-deposited glasses transform into liquid by a faster dynamic influenced by the outer film surface.