11.3 Templates Obtained Through the Self-assembly of Particles
389
to be collected at the surface must be charged, and the electrode surface charge
has to be of the opposite sign. Theoretical works showed [242, 243] that the loss
of the repulsion of the similarly charged particles has a combined background of
electrohydrodynamic and capillary forces. Hence, several densely packed particle
layers can be built up onto each other by electrophoresis. Due to the field-driven
nature of electrophoretic particulate template formation, it is particularly suitable
for non-planar substrate like microwires [244]. It is to be noted that electrophoretic
deposition of metallic particles is suitable for preparing self-assembled metal dot
surface structures, too [245].
Various colloidal template formation procedures are based on the self-assembly
of particles on the surface of a liquid [246, 247]. The so-called floating self-assembly
route [248] involves the formation of the arranged particulate multilayer on a liquid
surface followed by the solvent evaporation. Another similar method of particulate
template preparation is the transfer of a self-organized particle assembly from the
surface of a liquid onto a metal substrate (which can serve as electrode during the
electrochemical sample preparation). This technique, which is partly analogous to
the Langmuir–Blodgett technique of monolayer transfer from a liquid surface [249],
is particularly popular due to its relative simplicity. As the first step, a relatively
dense particle suspension is prepared on the surface of which the self-organization
of the particles can take place. The collector metal sheet is immersed into the solution
either vertically or in a tilted way so that it can serve as a platform for holding the
self-assembled surface layer (or multilayer) of particles as the solvent evaporates. For
facilitating the evaporation, the suspension usually contains some volatile component besides water (e.g., ethanol). With the appropriate adjustment of the particle
concentration and evaporation rate of the solvent, an intermittent surface structure,
exhibiting wire-like strips of assembled particles parallel to the liquid surface, can
also be synthesized [250].
For essentially all methods listed above, the particles have to be fixed before
using the holder as electrode since the self-assembled layer can be easily destroyed
by simply pouring a solution onto it. Either a mild heat treatment for partially melting
the contacting areas of the particles or a chemical cross-linking is required so that a
durable solid porous template can be obtained. The particulate template thus formed
is often termed as “opal template”, while the cavity system between the particles
(of the material filling this space) is of an inverse opal structure. An example for
the opal structure is shown in Fig. 11.12. It often occurs that the top particle layer
exhibits a nearly perfect ordering but the layers underneath have many growth defects
(vacancies, dislocations, etc.).
The composition of the particles used for opal template construction ranges from
polymers (latex, polystyrene /PS/, poly(methylmetacrylate) /PMMA/) through metal
salts to silica. Since the majority of the application of the filled opal structures is
related to its photonic properties, the lattice constant (and, thus, the diameter of the
spherical particles) is in the order of the wavelength of visible light. Although the
latter distance is about several hundreds of nanometres, the thickness of the deposit
in neck part of the inverse opal structure is often just a few tens of nanometres.
Nevertheless, the method of opal template preparation is uniform for any particle
389
to be collected at the surface must be charged, and the electrode surface charge
has to be of the opposite sign. Theoretical works showed [242, 243] that the loss
of the repulsion of the similarly charged particles has a combined background of
electrohydrodynamic and capillary forces. Hence, several densely packed particle
layers can be built up onto each other by electrophoresis. Due to the field-driven
nature of electrophoretic particulate template formation, it is particularly suitable
for non-planar substrate like microwires [244]. It is to be noted that electrophoretic
deposition of metallic particles is suitable for preparing self-assembled metal dot
surface structures, too [245].
Various colloidal template formation procedures are based on the self-assembly
of particles on the surface of a liquid [246, 247]. The so-called floating self-assembly
route [248] involves the formation of the arranged particulate multilayer on a liquid
surface followed by the solvent evaporation. Another similar method of particulate
template preparation is the transfer of a self-organized particle assembly from the
surface of a liquid onto a metal substrate (which can serve as electrode during the
electrochemical sample preparation). This technique, which is partly analogous to
the Langmuir–Blodgett technique of monolayer transfer from a liquid surface [249],
is particularly popular due to its relative simplicity. As the first step, a relatively
dense particle suspension is prepared on the surface of which the self-organization
of the particles can take place. The collector metal sheet is immersed into the solution
either vertically or in a tilted way so that it can serve as a platform for holding the
self-assembled surface layer (or multilayer) of particles as the solvent evaporates. For
facilitating the evaporation, the suspension usually contains some volatile component besides water (e.g., ethanol). With the appropriate adjustment of the particle
concentration and evaporation rate of the solvent, an intermittent surface structure,
exhibiting wire-like strips of assembled particles parallel to the liquid surface, can
also be synthesized [250].
For essentially all methods listed above, the particles have to be fixed before
using the holder as electrode since the self-assembled layer can be easily destroyed
by simply pouring a solution onto it. Either a mild heat treatment for partially melting
the contacting areas of the particles or a chemical cross-linking is required so that a
durable solid porous template can be obtained. The particulate template thus formed
is often termed as “opal template”, while the cavity system between the particles
(of the material filling this space) is of an inverse opal structure. An example for
the opal structure is shown in Fig. 11.12. It often occurs that the top particle layer
exhibits a nearly perfect ordering but the layers underneath have many growth defects
(vacancies, dislocations, etc.).
The composition of the particles used for opal template construction ranges from
polymers (latex, polystyrene /PS/, poly(methylmetacrylate) /PMMA/) through metal
salts to silica. Since the majority of the application of the filled opal structures is
related to its photonic properties, the lattice constant (and, thus, the diameter of the
spherical particles) is in the order of the wavelength of visible light. Although the
latter distance is about several hundreds of nanometres, the thickness of the deposit
in neck part of the inverse opal structure is often just a few tens of nanometres.
Nevertheless, the method of opal template preparation is uniform for any particle