Composition Dictates Molecular Orientation at the Heterointerfaces of Vapor-Deposited Glasses

Physical vapor deposition (PVD) can prepare organic glasseswitha preferred molecular orientation. The relationships between depositionconditions and orientation have been extensively investigated in thefilm bulk. The role of interfaces on the structure is less well understoodand remains a key knowledge gap, as the interfacial region can governglass stability and optoelectronic properties. Robust experimentalcharacterization has remained elusive due to complexities in interrogatingmolecular organization in amorphous, organic materials. Polarizedsoft X-rays are sensitive to both the composition and the orientationof transition dipole moments in the film, making them uniquely suitedto probe molecular orientation in amorphous soft matter. Here, weutilize polarized resonant soft X-ray reflectivity (P-RSoXR) to simultaneouslydepth profile the composition and molecular orientation of a bilayerprepared through the physical vapor deposition of 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph)on a film of aluminum-tris(8-hydroxyquinoline) (Alq3). The bulk orientationof the DSA-Ph layer is controlled by varying deposition conditions.Utilizing P-RSoXR to depth profile the films enables determinationof both the bulk orientation of DSA-Ph and the orientation near theAlq3 interface. At the Alq3 surface, DSA-Ph always lies with its longaxis parallel to the interface, before transitioning into the bulkorientation. This is likely due to the lower mobility and higher glasstransition of Alq3, as the first several monolayers of DSA-Ph depositedon Alq3 appear to behave as a blend. We further show how orientationat the interface correlates with the bulk behavior of a codepositedglass of similar blend composition, demonstrating a straightforwardapproach to predicting molecular orientation at heterointerfaces.This work provides key insights into how molecules orient during vapordeposition and offers methods to predict this property, a criticalstep toward controlling interfacial behavior in soft matter.