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 angstrom-scale structure of the earliest and simplest self-assembled monolayer (SAM), octadecanol on amorphous-SiO2-terminated Si(001) substrate, and determines the structures temperature evolution. At low temperatures lateral hexagonal order exists, with close-packed, surface-normal molecules. At similar to 12 degrees C above the alkanols bulk melting, a fully reversible disordering transition occurs to a novel stretched liquid phase, laterally disordered, but only similar to 15% thinner SAM than in the crystalline phase. The SAM persists to >= 100 degrees C. A thermodynamic model yields here a headgroup-substrate bond energy similar to 40% lower than on crystalline sapphire, highlighting the importance of the substrates order, and near-epitaxy, for the SAMs ordering and stability.