Vapor-Deposited Glass Structure Determined by Deposition Rate-Substrate Temperature Superposition Principle.

We show that deposition rate substantially affects the anisotropic structure of thin glassy films produced by physical vapor deposition. Itraconazole, a glass-forming liquid crystal, was deposited at rates spanning 3 orders of magnitude over a 25 K range of substrate temperatures, and structure was characterized by ellipsometry and X-ray scattering. Both the molecular orientation and the spacing of the smectic layers obey deposition rate-substrate temperature superposition, such that lowering the deposition rate is equivalent to raising the substrate temperature. We identify two different surface relaxations that are responsible for structural order in the vapor-deposited glasses and find that the process controlling molecular orientation is accelerated by more than 3 orders of magnitude at the surface relative to the bulk. The identification of distinct surface processes responsible for anisotropic structural features in vapor-deposited glasses will enable more precise control over the structure of glassy materials used in organic electronics.