Yanagisawa Susumu, Lee Kyuho, Morikawa Yoshitada
The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.
J Chem Phys. 2008 Jun 28;128(24):244704. doi: 10.1063/1.2940334.
We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq(3))Al interfaces by using density functional theoretical calculations, and clarified the origin of the interfacial dipole moment. We have examined various possible adsorption geometries of Alq(3) on Al surfaces and calculated the work function change induced by adsorption of Alq(3) on Al surfaces. We found that the stability depends crucially on the number of O-Al bonds formed at the interface, and Alq(3) tends to expose its O atoms to the Al substrate side and its N atoms to the vacuum side. Although the binding energies are influenced by the poor description of the van der Waals interaction by the density functionals used, the resulting bonding configurations are found to give correct binding energies when the van der Waals interaction is taken into account based on the recently proposed van der Waals density functional [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. This bonding configuration arranges molecular permanent dipoles of Alq(3) directed towards the vacuum, leading to the decrease of the surface work function. The calculated interface dipoles agree reasonably well with the experimental results and the origin of the interface dipole formation mainly comes from the alignment of the permanent dipoles of Alq(3). The HOMO levels of the Alq(3) molecules significantly depend on the orientation of the molecular permanent dipoles and the interfacial gap state observed by experiments is ascribed to the coexistence of the two orientations of the molecular dipole moments.
我们通过密度泛函理论计算研究了三（8-羟基喹啉）铝（Alq₃）与铝（Al）界面的原子几何结构和电子性质，并阐明了界面偶极矩的起源。我们研究了Alq₃在Al表面的各种可能吸附几何结构，并计算了Alq₃吸附在Al表面引起的功函数变化。我们发现稳定性关键取决于界面处形成的O-Al键的数量，并且Alq₃倾向于将其O原子暴露于Al衬底一侧，而将其N原子暴露于真空一侧。尽管结合能受到所用密度泛函对范德华相互作用描述不佳影响，但当基于最近提出的范德华密度泛函[Dion等人，《物理评论快报》92，246401（2004）]考虑范德华相互作用时，所得的键合构型能给出正确的结合能。这种键合构型使Alq₃的分子永久偶极指向真空，导致表面功函数降低。计算得到的界面偶极与实验结果相当吻合，界面偶极形成的起源主要来自Alq₃永久偶极的排列。Alq₃分子的最高占据分子轨道（HOMO）能级显著取决于分子永久偶极的取向，并且实验观察到的界面间隙态归因于分子偶极矩两种取向的共存。
