Self-Assembled Monolayers Get Their Final Finish via
a Quasi-Langmuir–Blodgett Transfer
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Abstract
The growth of self-assembled
monolayers (SAMs) of octadecylphosphonic
acid (ODPA) molecules on α-Al<sub>2</sub>O<sub>3</sub>(0001)
and subsequent dewetting of the SAMs were studied with a combination
of in situ sum-frequency generation (SFG) and molecular dynamics (MD)
simulations. Although SAM growth after deposition times >8 h reduces
to nearly negligible values, the resultant ODPA SAMs in solution are
still not in a well-ordered state with the alkyl chains in all-trans
configurations. In fact, in situ SFG spectroscopy revealed a comparatively
high concentration of gauche defects of the SAM in the ODPA 2-propanol
solution even after a growth time of 16 h. Here, results of the MD
simulations strongly suggest that defects can be caused by ODPA molecules
which are not attached to the substrate but are incorporated into
the SAM layer with the polar headgroup oriented into the 2-propanol
solvent. This inverted adsorption geometry of additional ODPA molecules
blocks adsorption sites and thus stabilizes the SAM without improving
ordering to an extent that all molecules are in the all-trans configuration.
While persistent in solution, the observed defects can be healed out
when the SAMs are transferred from the solvent to a gas phase. During
this process, a quasi-Langmuir–Blodgett transfer of molecules
takes place which drives the SAM into a higher conformational state
and significantly improves its quality