388
11 Templated Systems
11.3 Templates Obtained Through the Self-assembly
of Particles
11.3.1 Template Preparation from Uniform Solid Particles
The self-assembly of uniform colloidal particles on a flat surface, often termed
as “nanosphere lithography”, has long been considered as an economic alternative of conventional lithographic surface patterning. When this technique was first
mentioned, it was used for non-lithographic mask for substrate etching [233].
Although the self-assembly of uniform spherical particles is indeed a low-cost method
that can easily be implemented in any laboratory, it is less versatile than lithography
since the pattern obtained is always hexagonal (or, in other words, 111-oriented fcc)
if periodicity can be achieved, as opposed to a custom-defined lithographic pattern.
Nevertheless, it is sufficient or even desired for many applications, especially for
photonic crystals [234]. Even though some publications report dense particulate
layers with no obvious regularity, it was commonly observed that a regular periodic
system can be achieved at the length scales of several hundreds of micrometres along
the surface. A wide range of preparation methods for three-dimensionally ordered
macroporous (3DOM) solids has been developed [235] shortly after the suitable
monodisperse particles became commercially available, although the monodispersed
particles are often home-prepared even nowadays.
The simplest method to prepare colloidal templates is the sedimentation of the
colloidal particles onto to surface of the metal to be used as electrode in the forthcoming electrodeposition step [236, 237]. Sedimentation is a suitable template
formation tool for neutral particles. Sedimentation is a time-consuming process that
requires a template formation period longer than a day. However, unlike some other
methods detailed below, the thickness of the particulate template is limited by the
amount of the particles only. The latter feature of the sedimentation process makes
it possible to electrodeposit porous structures of a thickness up to a few tens of
micrometres [236]. Nevertheless, the organization level of the sedimented particulate
multilayers is usually smaller than that available with other methods.
Dip coating followed by solvent evaporation is an alternative route with a smaller
time necessity [238]. Another simple mechanical process, spin coating can also be
applied after a careful adjustment of the substrate rotation speed and the suspension
concentration in order to obtain ordered surface structures, by which monolayer
coverage with particles is also available [239]. A compact assembly of templateforming particles can also be obtained by filtering them through a filter of sufficiently
small pore size. An advanced method of filtering is the collection of the dispersed
particles directly on the cathode of a thin-layer flow-through electrochemical cell
[240] in which a side wall is porous in order to release the excess solvent. The latter
method is suitable for the synthesis of particulate templates up to 100 μm.
Electrophoretic pattern formation on electrode surfaces from colloidal solutions
[241] can provide a nearly defect-free surface coverage on a large lateral scale-up to
about 20 particle layers. Since electrophoresis is a field-driven process, the particles
11 Templated Systems
11.3 Templates Obtained Through the Self-assembly
of Particles
11.3.1 Template Preparation from Uniform Solid Particles
The self-assembly of uniform colloidal particles on a flat surface, often termed
as “nanosphere lithography”, has long been considered as an economic alternative of conventional lithographic surface patterning. When this technique was first
mentioned, it was used for non-lithographic mask for substrate etching [233].
Although the self-assembly of uniform spherical particles is indeed a low-cost method
that can easily be implemented in any laboratory, it is less versatile than lithography
since the pattern obtained is always hexagonal (or, in other words, 111-oriented fcc)
if periodicity can be achieved, as opposed to a custom-defined lithographic pattern.
Nevertheless, it is sufficient or even desired for many applications, especially for
photonic crystals [234]. Even though some publications report dense particulate
layers with no obvious regularity, it was commonly observed that a regular periodic
system can be achieved at the length scales of several hundreds of micrometres along
the surface. A wide range of preparation methods for three-dimensionally ordered
macroporous (3DOM) solids has been developed [235] shortly after the suitable
monodisperse particles became commercially available, although the monodispersed
particles are often home-prepared even nowadays.
The simplest method to prepare colloidal templates is the sedimentation of the
colloidal particles onto to surface of the metal to be used as electrode in the forthcoming electrodeposition step [236, 237]. Sedimentation is a suitable template
formation tool for neutral particles. Sedimentation is a time-consuming process that
requires a template formation period longer than a day. However, unlike some other
methods detailed below, the thickness of the particulate template is limited by the
amount of the particles only. The latter feature of the sedimentation process makes
it possible to electrodeposit porous structures of a thickness up to a few tens of
micrometres [236]. Nevertheless, the organization level of the sedimented particulate
multilayers is usually smaller than that available with other methods.
Dip coating followed by solvent evaporation is an alternative route with a smaller
time necessity [238]. Another simple mechanical process, spin coating can also be
applied after a careful adjustment of the substrate rotation speed and the suspension
concentration in order to obtain ordered surface structures, by which monolayer
coverage with particles is also available [239]. A compact assembly of templateforming particles can also be obtained by filtering them through a filter of sufficiently
small pore size. An advanced method of filtering is the collection of the dispersed
particles directly on the cathode of a thin-layer flow-through electrochemical cell
[240] in which a side wall is porous in order to release the excess solvent. The latter
method is suitable for the synthesis of particulate templates up to 100 μm.
Electrophoretic pattern formation on electrode surfaces from colloidal solutions
[241] can provide a nearly defect-free surface coverage on a large lateral scale-up to
about 20 particle layers. Since electrophoresis is a field-driven process, the particles