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

Abstract

Molecular self-assembly is a key to wide-ranging nano-and micro-scale applications in numerous fields. Understanding its underlying molecular level science is therefore of prime importance. This study resolves theÅ-scale structure of the earliest and simplest self-assembled monolayer (SAM): octadecanol on amorphous-SiO 2 -terminated Si (001) substrate, and determines the structure's temperature evolution. At low temperatures lateral hexagonal order exists, with close-packed, surface-normal molecules. ∼ 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 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.

Surface Phases and Surface Freezing in an Ionic Liquid

“…The thinner-than-expected monolayer, ∼ 20Å only, may result from ordering of only the top parts of the chains, due to packing frustration caused by the areal mismatch between the headgroups and the chains. Similar mismatch-inhibited partial-length ordering has been observed in amorphous-SiO 2 -supported monolayers of alkyltrichlorosilanes64 and alkanols65 , while a nearly-matching sapphire substrate yields a full-length ordering for alkanols 66. Although here the alkyl tails' lower ends are not firmly anchored to solid substrate sites but rather to the cation headgroups, the headgroup size provides a hard-sphere closest approach for adjacent tails' lower ends, keeping them apart by more than the chain-preferred ∼ 4.8Å as firmly as if they were substrate-anchored.…”

Interactions of the Calcite {10.4} Surface with Organic Compounds: Structure and Behaviour at Mineral – Organic Interfaces

“…Haddad et al . 52 have characterised octadecanol on Si(001) by XRR and studied the structure of the alcohol monolayer as function of temperature by following the thickness of the monolayer which gets slightly thinner when raising the temperature above the alcohol melting point, indicating that all molecules are still roughly aligned with surface-normal. Similarly the structuring of a Langmuir self-assembled monolayers of 1-decanol and 1-dodecanol on a water surface has been studied by Berge et al .…”

Thermal and Chemical Stability of n -Hexadecanethiol Monolayers on Au(111) in O 2 Environments

“…The increase in capacitance in this temperature range is interpreted as decrease in the monolayer thickness according to the parallel plate capacitor formula. The decrease in thickness has been reported for alkyl monolayers on different substrates as a consequence of the disordering of the monolayer by an increased formation of gauche defects at high temperatures [39,40]. From 403 K, the monolayer decomposition is evident in the CV profiles and the increase in capacitance is more pronounced as shown in Fig.…”