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Self-assembled monolayers
(SAMs) of alkanethiols on gold are a
commonly used platform for nanotechnology owing to their ease of preparation
and high surface coverage. Unfortunately, the gold–sulfur bond
is oxidized at ambient conditions which alters the stability and structure
of the monolayer. We show using scanning tunneling microscopy and
X-ray photoelectron spectroscopy that decanethiolate molecules oxidize
into decanesulfonates that organize into a hitherto unknown striped
phase. Air-exposed SAMs oxidize, as can be determined by a shift of
the S 2p peak and the appearance of O 1s photoelectrons as part of
the decanethiol monolayer transforms into a lamellae-like decanesulfonate
structure when exposed to air. The herringbone structure of the Au(111)
surface is preserved, indicating that the interaction between the
molecules and the surface is rather weak as these findings are substantiated
by density functional theory calculations.
Polymer brush layers are responsive materials that swell in contact with good solvents and their vapors. We deposit drops of an almost completely wetting volatile oil onto an oleophilic polymer brush layer and follow the response of the system upon simultaneous exposure to both liquid and vapor. Interferometric imaging shows that a halo of partly swollen polymer brush layer forms ahead of the moving contact line. The swelling dynamics of this halo is controlled by a subtle balance of direct imbibition from the drop into the brush layer and vapor phase transport and can lead to very long-lived transient swelling profiles as well as nonequilibrium configurations involving thickness gradients in a stationary state. A gradient dynamics model based on a free energy functional with three coupled fields is developed and numerically solved. It describes experimental observations and reveals how local evaporation and condensation conspire to stabilize the inhomogeneous nonequilibrium stationary swelling profiles. A quantitative comparison of experiments and calculations provides access to the solvent diffusion coefficient within the brush layer. Overall, the results highlight the—presumably generally applicable—crucial role of vapor phase transport in dynamic wetting phenomena involving volatile liquids on swelling functional surfaces.
We have studied the
dynamic behavior of decanethiol and air-oxidized
decanethiol self-assembled monolayers (SAMs) on Au(111) using time-resolved
scanning tunneling microscopy at room temperature. The air-oxidized
decanethiols arrange in a lamellae-like structure leaving the herringbone
reconstruction of the Au(111) surface intact, indicating a rather
weak interaction between the molecules and the surface. Successive
STM images show that the air-oxidized molecules are structurally more
stable as compared to the nonoxidized decanethiol molecules. This
is further confirmed by performing current–time traces with
the feedback loop disabled at different locations and at different
molecular phases. Density function theory calculations reveal that
the diffusion barrier of the physisorbed oxidized decanethiol molecule
on Au(111) is about 100 meV higher than the diffusion barrier of a
chemisorbed Au-decanethiol complex on Au(111). A two-dimensional activity
map of individual current–time traces performed on the air-oxidized
decanethiol phase reveals that all the dynamic events take place within
the vacancy lines between the air-oxidized decanethiols. These results
reveal that the oxidation of thiols provides a pathway to produce
more robust and stable self-assembled monolayers at ambient conditions.
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