Exploring the Anisotropic Structure of Vapor Deposited Stable Glasses

Glasses prepared through Physical Vapor Deposition (PVD) can adopt preferred molecular orientations and structural layering that depend on the deposition conditions, molecular shape [1], and film thickness [2]. Here, we explore the anisostropic orientation and layering of PVD glasses of isomeric molecules - 9-(3,5-Di(naphthalen-1-yl)phenyl)anthracene (α,α-A) and 9-(3,5-Di(naphthalen-1-yl)phenyl)phenanthracene (α,α-Phen) as a function of film thickness. While these molecules have a similar Tg, they have differences in their intra-molecular rotation barriers, resulting in variations in their equilibrium shape [1]. Spectroscopic Ellipsometry and Grazing Incidence Wide Angle X-ray Scattering measurements indicate that similar to bulk, orientational anisotropy, and layering is present in thin films of α,α-A and α,α-Phen, particularly at low deposition temperatures. A strong layering feature is observed in both glasses regardless of the molecular shape, leading to stronger anisotropy in films with thicknesses of 30-50 nm. We discuss how this effect may be related to a low-temperature structural transition in these systems.