Anchoring and Bending of Pentacene on Aluminum (001)

We study the structural, electronic, and spectroscopic properties of pentacene adsorbed on Al(001) surface, combining density functional theory (DFT) methods including van der Waals interactions with X-ray photoemission (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). We find a major change of the molecular backbone resulting in 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.