Ultrathin and Stable Aromatic Monolayers on Naturally Oxidized Aluminum: The Role of Type and Number of Anchoring Groups

Ultrathin aromatic self-assembled monolayers (SAMs) on aluminum oxide are attractive interfacial layers for organic electronics, yet their stability remains a key challenge. Here, we investigate the formation, structure, thermal stability, and chemical robustness of phenyl- and triptycene-based SAMs deposited on naturally oxidized aluminum (∼1 nm AlOx). The effects of anchoring group chemistry (carboxylic vs phosphonic acid) and bonding mode (monopodal vs tripodal) are systematically compared. X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy confirm the formation of densely packed, upright-oriented ∼1 nm thick monolayers irrespective of molecular architecture. Thermally programmed XPS reveals that phosphonic acid anchoring significantly increases desorption energies relative to carboxylic acid analogues, while tripodal binding provides an additional stability enhancement, yielding a high desorption energy of ∼1.9 eV. Exchange experiments demonstrate exceptional chemical robustness of tripodal phosphonic acid monolayers. These findings establish phosphonic acid anchoring combined with tripodal binding as an effective strategy for engineering ultrathin, highly stable aromatic interfaces on technologically relevant oxide surfaces.