Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706.
Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO 80309.
Proc Natl Acad Sci U S A. 2021 Oct 19;118(42). doi: 10.1073/pnas.2111988118.
Glasses prepared by physical vapor deposition (PVD) are anisotropic, and the average molecular orientation can be varied significantly by controlling the deposition conditions. While previous work has characterized the average structure of thick PVD glasses, most experiments are not sensitive to the structure near an underlying substrate or interface. Given the profound influence of the substrate on the growth of crystalline or liquid crystalline materials, an underlying substrate might be expected to substantially alter the structure of a PVD glass, and this near-interface structure is important for the function of organic electronic devices prepared by PVD, such as organic light-emitting diodes. To study molecular packing near buried organic-organic interfaces, we prepare superlattice structures (stacks of 5- or 10-nm layers) of organic semiconductors, Alq3 (Tris-(8-hydroxyquinoline)aluminum) and DSA-Ph (1,4-di-[4-(N,-diphenyl)amino]styrylbenzene), using PVD. Superlattice structures significantly increase the fraction of the films near buried interfaces, thereby allowing for quantitative characterization of interfacial packing. Remarkably, both X-ray scattering and spectroscopic ellipsometry indicate that the substrate exerts a negligible influence on PVD glass structure. Thus, the surface equilibration mechanism previously advanced for thick films can successfully describe PVD glass structure even within the first monolayer of deposition on an organic substrate.
通过物理气相沉积（PVD）制备的玻璃具有各向异性，通过控制沉积条件可以显著改变平均分子取向。虽然之前的工作已经对厚 PVD 玻璃的平均结构进行了表征，但大多数实验对接近基底或界面的结构不敏感。鉴于基底对结晶或液晶材料生长的深远影响，基底有望显著改变 PVD 玻璃的结构，而这种近界面结构对于通过 PVD 制备的有机电子器件的功能至关重要，例如有机发光二极管。为了研究埋入有机-有机界面附近的分子堆积，我们使用 PVD 制备了有机半导体 Alq3（三（8-羟基喹啉）铝）和 DSA-Ph（1,4-二-[4-（N，-二苯基）氨基]苯乙烯基苯）的超晶格结构（5nm 或 10nm 层的堆叠）。超晶格结构显著增加了埋入界面附近的薄膜分数，从而能够对界面堆积进行定量表征。值得注意的是，X 射线散射和光谱椭圆偏振都表明基底对 PVD 玻璃结构几乎没有影响。因此，先前为厚膜提出的表面平衡机制即使在有机基底上的沉积的第一个单层内也可以成功描述 PVD 玻璃结构。
