文摘
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 脜-scale structure of the earliest and simplest self-assembled monolayer (SAM), octadecanol on amorphous-SiO2-terminated Si(001) substrate, and determines the structure鈥檚 temperature evolution. At low temperatures lateral hexagonal order exists, with close-packed, surface-normal molecules. At 鈭?2 掳C above the alkanol鈥檚 bulk melting, a fully reversible disordering transition occurs to a novel 鈥渟tretched liquid鈥?phase, laterally disordered, but only 鈭?5% thinner SAM than in the crystalline phase. The SAM persists to 鈮?00 掳C. A thermodynamic model yields here a headgroup鈥搒ubstrate bond energy 鈭?0% lower than on crystalline sapphire, highlighting the importance of the substrate鈥檚 order, and near-epitaxy, for the SAM鈥檚 ordering and stability.
