monolayers: Laying by hand is superior to self-organization
Craftsmen tile walls or floors by hand; but how
can you get an ordered monolayer onto a substrate when the ¡°tiles¡± are
microscopically small instead of big and easy to handle? Previously,
self-organization processes have been the method of choice for this scale.
Korean researchers have now come to the realization that even such tiny
components can be arranged in a ¡°do-it-yourself¡± process. As they describe in
the journal Angewandte Chemie, their manually produced monolayers of
microcrystals are qualitatively superior to the self-organized
How small can components be such that they can still be glued
to a surface by hand? And conversely, how big can microscale components be such
that they can still be arranged by self-organization? Which method is best in
the size range where both techniques work? These questions have been
investigated by a team led by Kyung Byung Yoon at Sogang University in Seoul. To
find answers, they carried out experiments with zeolite crystals in a variety of
sizes. Zeolites are silicate minerals with applications in many technical
The powdered zeolite was applied by simply rubbing it on with a
finger (with wearing a latex glove). Alternatively, they were applied in
solution, and ultrasound was used to kick-start the self-organization process.
The ¡°glue¡± between the ¡°mini-tiles¡± and the substrate was the attraction between
oppositely charged groups of atoms, hydrogen bridge bonds, and chemical bonds
between reactive groups of atoms.
The experiments demonstrated that
self-organization only works for particles smaller than about 3 µm.
Hand-application works for crystals as small as 0.5 µm in diameter. In the
overlapping range (0.5 to 3 µm), hand application is preferable to
self-organization for quality: the packing is denser and the microcrystals are
oriented more regularly. Whereas self-organization produces individual crystals
grown at a 90 ° angle onto the monolayer, such ¡°parasites¡± are simply rubbed off
by hand. There are other ¡°handy¡± advantages of the manual process as well: it is
simpler, doesn¡¯t require a solvent or special equipment, runs more smoothly, and
allows for treatment of larger surfaces.