Order and Melting in Self-Assembled Alkanol Monolayers on Amorphous SiO₂

Molecular self-assembly is a key to wide-ranging nano- and microscale applications in numerous fields. Understanding its underlying molecular level science is therefore of prime importance. This study resolves the Å-scale structure of the earliest and simplest self-assembled monolayer (SAM), octadecanol on amorphous-SiO₂-terminated Si(001) substrate, and determines the structure's temperature evolution. At low temperatures lateral hexagonal order exists, with close-packed, surface-normal molecules. At ∼12 °C above the alkanol's bulk melting, a fully reversible disordering transition occurs to a novel "stretched liquid" phase, laterally disordered, but only ∼15% thinner SAM than in the crystalline phase. The SAM persists to ≥100 °C. A thermodynamic model yields here a headgroup-substrate bond energy ∼40% lower than on crystalline sapphire, highlighting the importance of the substrate's order, and near-epitaxy, for the SAM's ordering and stability.