Origin of Anisotropic Molecular Packing in Vapor-Deposited Alq3 Glasses

Anisotropic molecular packing is a key feature that makes glasses prepared by physical vapor deposition (PVD) unique materials, warranting a mechanistic understanding of how a PVD glass attains its structure. To this end, we use X-ray scattering and ellipsometry to characterize the structure of PVD glasses of tris(8-hydroxyquinoline) aluminum (Alq3), a molecule used in organic electronics, and compare our results to simulations of its supercooled liquid. X-ray scattering reveals a tendency for molecular layering in Alq3 glasses that depends upon the substrate temperature during deposition and the deposition rate. Simulations reveal that the Alq3 supercooled liquid, like liquid metals, exhibits surface layering. We propose that the layering in Alq3 glasses observed here as well as the previously reported bulk dipole orientation are inherited from the surface structure of the supercooled liquid. This work significantly advances our understanding of the mechanism governing the formation of anisotropic structure in PVD glasses.