Anchoring and Bending of Pentacene on Aluminum

The adsorption of the first layer of organic molecules at the metal 12organic interface in electronic devices is critical as it influences the carrier injection across the interface and also forms the foundation for the growth of the subsequent layers, which in turn affects the overall performance of these devices. Pentacene (C22H14) has shown temperature-independent highest hole and electron mobility among the various organic small molecules. Aluminum is also a metal of potential interest for Si-free electronic devices. Pentacene coupled to aluminum could be used to fabricate devices like Schottky diodes and organic thin film transistors, and the interaction between pentacene and aluminum is also important in organic solar cells and memory devices. In our study of pentacene/Al(001), we combined ab initio simulations, based on density functional theory (DFT) corrected for the van der Waals interaction, with techniques to determine the structure of the system, such as reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) and core-level spectroscopies, namely X-ray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) [1]. We find a major change of the molecular backbone resulting into a peculiar V-shape bending, due to the direct anchoring of the two central carbons atop two Al atoms underneath. In the most stable adsorption configuration, pentacene is oriented with the long axis parallel to the substrate [110] direction, where such anchoring is favored by optimally matched interatomic distances. Remarkably, due to the generally low degree of order, we measure by STM a significant portion of molecules oriented along the [100] direction, which also display the same V-shape conformation, as driven by the bond of the central carbon atoms of pentacene to a pair of slightly displaced Al atoms. Even in absence of long-range order, the substrate thus favors a specific structural and electronic molecular conformation, which can be representative of a real interface layer of pentacene at an Al electrode. [1] J. Phys. Chem. C, 2015, 119 (7), pp 3624\u2013363