SELF-ORGANIZED POLYMER MICRODOT -LINE AND HONEYCOMB STRUCTURES FOR OPTOELECTRONIC
APPLICATIONS
Olaf Karthaus, Keisuke Kai, Kenichi Kon, Kazuma Nakajima, Shouta Sasajima
Chitose Institute of Science and Technology, Hokkaido, Chitose, Japan
Ordering and patterning are basic principles for the construction of functional entities.
They are evident in natural as well as artificial systems. Here, will focus on selforganization and its use to prepare ordered and functional structures on the
micrometer scale. By using a dewetting process we were able to prepare
submicrometer-sized luminescent droplets of amorphous materials for organic light
emitting diodes, and single crystalline nanofibers of organic semicondcutors for
organic field effect transistors. By using solvent evaporation and resulting ‘breath
figures’ we are able to prepare microporous honeycomb films used as photocatalysts.
SELF-ORGANIZATION AND DISSIPATIVE STRUCTURES
It has to be noted that there is no clear-cut definition of self-organization, nor is there
a sharp boundary between self-organization and self-assembly [1]. Both, self
organization and self assembly, give rise to ordered one-, two-, or three-dimensional
structures on various length scales from a disordered precursor state without outside
guidance. One distinction between self assembly and self organization can be made
on the bases of their building blocks, their dynamics and energetics. Self-assembled
structures are made from preformed building blocks and the structures are static and
near the thermodynamic equilibrium. Self-organized structures on the other hand are
independent on the component shape, they are dynamic and far away from
equilibrium. In that sense, self assembly is a directed process in which the obtained
structure is stable and predictable. Interactions between the building blocks, atoms or
molecules, are short range. Self organization, on the other hand, is only present in faraway-from equilibrium states, and relies on the collective interaction of many parts in
a dynamic fashion.
It should be noted that in this definition also a system that ends up having a selfassembled structure may as well start far for equilibrium and may be dynamic until
the final structure has been reached.
Examples for Self assembly are snow crystals [2], Self-assembled monolayers [3],
blockcopolymer microphase separation [4], and micelles.
Examples for self organization are vesicles [5, 6], the Belouzov-Zhabotinsky reaction
[7, 8], and the biological cell. A cell is actually more than just a complex selforganized structure. In order to be functional, it needs preexisting information, passed
down form the previous generation through self-replication of hereditary material
(DNA or RNA).
It should be noted that the glassy state of amorphous polymers, metals, or other
inorganics is neither self-assembled, nor self-organized. Even though it is not an
equilibrium structure, it lacks the characteristic order and regular structure that is
inherent in self assembly or self organization systems.
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Even though the concept of self organization can be followed back to Descartes, who
theorized that the ordinary laws of nature tend to produce organization, the phrase
‘self-organization’ did not appear until 1947 [9]. Even after that, the concept lay
dormant with little progress until the 1980ies. Only since the last decade a significant
increase in scientific dissertations has been noted, reaching nearly 600 for the years
1991 to 2000.
Self-organization seems to be counterintuitive, since the order that is generated
challenges the paradigm of increasing disorder based on the second law of
thermodynamics. In statistical thermodynamics, entropy is the number of possible
microstates for a macroscopic state. Since in an ordered state, the number of possible
microstates is smaller than for a more disordered state, it follows that a self-organized
system has a lower entropy. However, the two need not to contradict each other: it is
possible to reduce the entropy in a part of a system while it increases in another. This
is valid even for isolated, closed systems. Furthermore, in an open system, the entropy
production can be transferred to the environment, so that here even the overall entropy
in the entire system can be reduced. This is the basis of the theory of dissipative
structures, developed by Ilya Prigogine. He noted that self-organization can only
occur far away from thermodynamic equilibrium [10]. Dissipative structures can
occur from the nanometer to megameter scale. In the following, one example of
dissipative structures, the tears of wine is explained in more detail, because it can be
used to produce nano- and micropatterns of organic materials. The surface motion of
wine has been described already in ancient times [11], and scientifically in a paper by
James Thompson, the brother of Lord Kelvin [12] in 1855. Recently, Neogi [13] and
Cazabat [14] reported on the formation of ordered structures in evaporating solutions
of two liquids. The meniscus of wine in a glass is drawn upward on the glass surface
and forms a thin film. Due to its lower boiling point, evaporation of alcohol leads to a
water-rich liquid surface that has a higher surface tension. This leads to a surface
tension gradient and a flow by which accumulated liquid forms “tears”, very often in
regular intervals along the edge of the liquid film. The tears-of-wine phenomenon can
be generalized to generally describe the flow near the surface and the edge of liquid
films driven by surface tension gradients that lead to ordered structures.
Fig. 1. Concept of dissipative structures.
Regular patterns can evolve, but by definition, dissipative structures disappear once
the thermodynamic equilibrium has been reached. When one wants to use dissipative
structures for patterning of materials, the dissipative structure has to be fixed. Then,
even thought the thermodynamic instability that led to and supported the pattern has
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ceased, the structure would remain. Here, polymers play an important role. Since
many polymers are amorphous, there is the possibility to “freeze” temporal patterns.
Furthermore, polymer solutions are non-linear in respect to viscosity and thus strong
effects are expected to be seen in evaporating polymer solutions. Since a
macromolecule is a nanoscale object, also conformational entropy will play a role in
nanoscale ordered structures of polymers (see figure 1).
Dynamics and Pattern Formation in Evaporating Polymer Solutions
When a polymer is dissolved in a volatile solvent, a surface cooling occurs
spontaneously by solvent evaporation. The heat of evaporation is enough to drive the
system out of equilibrium. For example, by placing a few µl of a dilute chloroform
solution of a fluorescent-labeled polymer on a substrate we could observe the
formation of Benard-like convection cells on a sub-millimeter scale [15], as can be
seen in figure 2.
Fig. 2. Top: Benard convection cells (indicated by the circle) and ‘tears of wine’
(indicated by the white arrows) in an evaporating fluorescence-labeled polymer
solution. Bottom: snap shops of an evaporating polystyrene solution on mica. The
time between both frames is about100 ms. The deposited droplets have a diameter of
3 µm.
In addition, a periodic intensity profile along the edge of the droplet can be seen.
Areas with strong fluorescence are separated by areas that do not fluoresce. This
indicates a periodic concentration profile, very similar to the tears-of-wine effect in
which surface tension gradients lead to periodic built-up of solution droplets. Similar
to the tears of wine, this concentration profile is dynamic. Monitoring the time course
revealed that the higher concentrated areas merge, and new areas of higher
concentration evolve between existing concentrated areas [16]. These dynamic
processes, convection cells and polymer-rich ‘finger’ structures, are due to solvent
evaporation. Consequently, the volume of the solution droplet is becoming constantly
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smaller, and thus the diameter of the solution droplets begins to decrease shortly after
the dynamic structures are formed. Hence, the dynamic structures, because they are
linked to the edge of the solution droplet, move across the substrate. In this process,
polymer may be deposited on the substrate, and droplet or line patterns are commonly
observed. Line patterns are formed when the polymer is continuously deposited at the
highly concentrated finger positions. Droplets are formed when the emerging line
exhibits another instability, the so-called Plateau-Rayleigh instability, in which the
line decompose into droplets, very much like in a dripping faucet [17].
Such a process occurs because the surface and interfacial energies of polymer and
substrate favor droplet formation, i.e. dewetting occurs. The dewetted polymer
solution may produce two-dimensionally ordered droplet arrays, because the droplet
formation occurs only at the edge of the evaporating solution. Typically, the droplets
have a diameter of 200 nm to 10 µm, and they have been used to immobilize organic
dyes [18] and inorganic nanoparticles in an orderly fashion on a substrate [19]. In
order to allow for more control during the evaporation, the solution can be placed in a
motor-controlled sliding [20] or rolling apparatus [21] that use capillary force to
confine the solution between two glass surfaces. The motor determines the speed at
which the solution edge is drawn over the substrate and is one of the main parameters
to control patterning. Droplet, stripe and ladder patterns have been observed.
APPLICATION OF DEWETTED STRUCTURES IN ORGANIC PHOTONICS
AND ELECTRONICS
Wetting and dewetting strongly depend on the surface energy of the substrate and the
interfacial energy of the polymer and the substrate. Thus dewetting on a patterned
substrate with areas of different surface energies gives rise to selective deposition of
the polymer on one surface functionalization only. Conducting polymers have been
demonstrated to selectively for dewetted structures on Au patterns on Si wafer [22].
Similarly, dewetting can be used to form micronsized gaps that can be used for the
self-organized assembly of electrodes for organic field effect transistors [23]. Besides
polymers, also other amorphous materials such as dendrimers or even small molecules
with a molar mass as low as 300 g/mol can be dewetted. The reason is that dissipative
structures in solution are independent of the molecular structure of the solute, as long
as the solute does not crystallize during pattern formation. Submicronsized droplets of
arylamines can be used as hole conductors in organic light emitting diodes [24].
Another, related organic device that is receiving increased attention recently is the
organic field effect transistor. Dewetting can be used for fabricating short-channel
polymer field effect transistors with conducting polymer employed as electrodes.
Source and drain contacts can be patterned by splitting a conducting polymer water
solution over a hydrophobic self-assembled monolayer [25], or by self-aligning inkjet printed gold contacts [26].
Besides forming the source-drain gap by dewetting, also patterning of the organic
semiconductor itself can be achieved by self-organization. Pentacene is a promising
material, and OFETs have been prepared by conventional vacuum evaporation
process [27]. Inevitable grain boundaries present in the resulting polycrystalline film
prepared by this method restrict the carrier mobility. Spin coating from hot
trichlorobenzene solution led to a preferred radial crystal orientation, but grain
boundaries were still present [28]. By using the roller apparatus to cast a hot
trichlorobenzene solution, we were able to produce unidirectionally oriented crystal
fibers wit a length of several hundred micrometer [29], as can be seen in figure 3.
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Fig. 3. Scanning electron micrograph of pentacene microfibers along the dewetting
direction.
As in the case of polymers, a concentration profile develops along the solution edge
during solvent evaporation and at areas with highest concentration, crystal seeds are
formed. Since the edge of the solution is dragged over the substrate, the crystal seed
can only grow in one direction – perpendicular to the solution edge. Thus ordered
arrays of unidirectionally aligned crystals are formed, when the receeding speed of the
edge matches the crystal growth rate. Even though the source-drain gap was only
partially covered by pentacene, charge mobilities of 0.1 cm2/Vs have been obtained.
BRATH FIGURES AND HONEYCOMB FILMS
The report by Francois et al. [30] already more than a decade ago that casting of
polymer solutions at high humidity leads to regular arrays of micrometer-sized holes
in a polymer film has led to vigorous research and numerous publications in the
emerging field of polymeric self-organized structures. Since the preparation technique
is simple and water droplet condensation is a common physical phenomenon that does
not depend on a chemical reaction whatsoever, many compounds (commodity
polymers, [31-33] block-copolymers, [30, 34-36] conjugated polymers, [37]
engineering plastics,[38] low-molar-mass organics,[39] nanoparticles, [40-42] metal
complexes, [43] etc) can be screened and assessed. Various aspects of the film
morphology (high surface area, light diffraction, [30, 44-45] super
hydrophobicity,[46] conductivity[41]) have been studied. Possible applications of the
microporous films include substrates for cell growths, [30, 47-49] bio-compatible
interfaces,[50] microstructured photocatalysts, [51] and molds for microlenses, [52]
just to name a few. Recently we have found that honeycomb films can be crosslinked,
either by using photoreactive cinnamoyl polymers [ ] or chemically reactive acid
anhydrides [ ] The tremendously increased stability towards organic solvents and
temperature will open new avenues for applications. As one example, we are
reporting the surface coating of crosslinked honeycombs with nanocrystalline TiO2
and the increased photocatalytic performance.
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Fig. 4. Scanning electron micrograph of a titanium dioxide-covered honeycomb film.
The pore diameter is 3 µm.
SUMMARY
Dissipative structures may form when a system is a non-equilibrium state that allows
the flow of energy and/or matter. The formed self-organized structures cover a wide
range of lengths scales and even though they are temporal by definition, it is possible
to fix them on solid substrates to produce nano- to micrometer-sized regular patterns.
By using functional organic materials, functional patterns can be prepared. This opens
up the possibility to use bottom-up self-organization in connection with top-down
patterning processes to produce functional devices for use in photonics and
electronics.
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