Molecular orientation of vapor-deposited organic glasses at buried heterointerfaces

Ultra-stable glasses prepared with physical vapor deposition can exhibit a preferred molecular orientation that leads to birefringence. This phenomenon has been extensively characterized in bulk glass, yet there is little information on how this orientation is influenced near buried heterointerfaces despite their importance to material properties. Here, we use polarized resonant soft X-ray reflectivity (P-RSOXR) to simultaneously depth-profile chemical composition and molecular orientation in bilayers of vapor-deposited organic semiconductors. P-RSOXR allows us to characterize a compositionally mixed interface morphology at the nanometer length scale and provide insight into ultra-stable glass assembly during vapor deposition. Our results reveal parallels between the molecular orientation near a buried heterointerface and the bulk structure of co-deposited mixed glasses, likely due to a change in mobility at the interface caused by the underlying material. This work provides potential strategies for realizing control over interfacial properties, a critical step in governing emergent behavior for applications involving ultra-stable glasses.