1. Thin Solid Films 327–329 (1998) 854–856
Mesoscopic patterns of molecular aggregates on solid substrates
N. Maruyama*, T. Koito, J. Nishida, T. Sawadaishi, X. Cieren, K. Ijiro, O. Karthaus, M. Shimomura
Research Institute for Electronic Science, Hokkaido University, N12W6 Sapporo, Japan
Abstract
A two-dimensional micron-sized honeycomb structure was formed when a chloroform solution of an amphiphilic polymer was cast on
solid surfaces at high atmospheric humidity. This simple method is widely applicable for patterning of molecular aggregates on solid
surfaces. Mesoscopic patterns are demonstrated to be formed spontaneously from a variety of amphiphilic polyion complexes, amphiphilic
covalent polymers, and organic–inorganic hybrid materials. Size and structure of the patterns can be regulated by concentration, atmo-
spheric humidity, etc. © 1998 Elsevier Science S.A. All rights reserved
Keywords: Ordered pattern; Honeycomb structure; Cast film; Polyion complex; Convection; Hierarchic structure
1. Introduction Here, we want to extend the concept of mesoscopic pat-
terning to a wide variety of amphiphilic polymers, including
Self-assembly of molecules on the nano scale have been functional polymers, such as DNA/amphiphile complexes,
studied, and their mechanism and structure has been made saccharide-containing vinyl polymers (4), electrically con-
clear in many fields such as biology, chemistry, material ducting polythiophene complexes (3/5), or photoresponsive
technology, etc. [1]. For example, polyion complexes [2] azobenzene-containing complexes (1/8).
are known to form regular three-dimensional nanoscopic
structures. By introducing functional molecules or poly-
mers, cast films with various properties can be produced. 2. Experimental
Based on the functions, they can be used for molecular
recognition, charge separation and electron transfer, perm- The formulae of the compounds used are shown in Fig. 1.
selective membranes, sensors, or for optical applications Polystyrene sulfonate 1 (Mw = 60 000 g/mol, Tokuyama
[2]. Generally the films comprise a molecular-level layered Soda, Japan), poly(propylsulfonylthiophene) 3 (Showa
structure of the polyion complex. Van der Waals force and Denko, Japan), bis-hexadecyl-dimethylammonium bromide
Coulomb force in molecular assemblies are short-range 5 (Sogo Pharmaceutical, Japan), DNA (salmon testes, Wako,
forces acting on a molecular level, thus mesoscopic pattern- Japan), bis-dodecylsuccinate sulfonate 6 (Sogo Pharmaceu-
ing is not expected. On the other hand, surface tension, tical, Japan), bis-hexadecylphosphate 7 (Sogo Pharmaceuti-
convection, etc., forces that act on a larger length-scale, cal, Japan) and tetra-isopropyl-ortho-titanate 9 (Tokyo
can be used for constructing micron-sized structures. Kasei, Japan) were used without further purification. The
Francois at al. have already found that cast films of block
¸ synthesis of 2 [5] and 8 [6] is described in the literature.
copolymer prepared under high humidity consist of a Synthesis of 4 will be described elsewhere. The preparation
micron-sized honeycomb morphology [3]. They found this of polyion complexes follows the literature [2]. For pattern
phenomenon in carbon disulfide solutions of poly(styrene)– formation, solutions (ca. 0.5 ml) were cast on various sub-
poly(paraphenylene) block copolymers, and speculated that strates (glass, mica, silicon wafer, etc.) in a glove box at con-
this is a peculiar phenomenon for this combination of sol- trolled humidity at 25°C. The concentration of the complexes
vent and polymer. are given in mmol/l of repeat unit. The patterns were observed
by means of a fluorescence microscope (Olympus IX-70),
* Corresponding author. Tel.: +81 11 7062997; fax: +81 11 7064974; equipped with a CCD camera and a video recording system,
e-mail: maru@ae.hines.hokudai.ac.jp and by an atomic force microscope (Olympus NV-2500).
0040-6090/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved
PII S0040-6090 (98 )0 0777-9

2. N. Maruyama et al. / Thin Solid Films 327–329 (1998) 854–856 855
plex (Fig. 3a). Since the surface active polyion complex
reduces the surface tension between water and chloroform,
the water droplets are stabilized and do not fuse. The dro-
plets are dragged into the solution by convection or keep
floating on the solution surface (Fig. 3b-1). With shrinking
diameter of the evaporating chloroform solution, more
water droplets get close to the three-phase line and are hex-
agonally packed by capillary force generated at the solution
front (Fig. 3b-2) [7]. Then, the three-phase line (solution–
air–substrate) moves over the array of water droplets (Fig.
3b-3). The water droplets and some of the polymer between
them are left behind. Finally, the water evaporates, leading
to the observed honeycomb structure (Fig. 3b-4). A similar
mechanism was proposed by Francois et al. [3].
¸
As already shown in Fig. 2, the size of the honeycomb
structure varies between 1 and 5 mm for a complex of 1 and
5. Similar results were observed with the other compounds
used. The most important parameters to control the size of
the mesoscopic patterns are the relative humidity of the
atmosphere and the concentration of the polymer solution.
Fig. 1. Chemical formulae of the used compounds.
3. Results and discussion
Fig. 2a shows a fluorescence micrograph of a typical cast
film of a complex of 1 and 5 containing octadecyl rhoda-
mine B (0.5 mol% of 5) as a fluorescent probe. The polyion
complex cast film was found to have regular honeycomb
morphology with a size of ca. 1.5 mm per cell. A similar
honeycomb structure was also found to be formed from all
of the other compounds described above. The bright parts of
the picture stems from the rhodamine B fluorescence which
is dissolved in the polymeric complex. This means that the
polymer forms the honeycomb structure. The dark parts of
the micrograph are the voids of the honeycomb and do not
contain polymer. It is evident from atomic force microscopy
(AFM) that the pattern consists of an open honeycomb. Fig.
2b shows the AFM picture of a similar sample. One can see
that the honeycomb structure has a height of 200–300 nm.
Force mapping [4] with the AFM indicates that the bottom
of the voids is not covered by polymer, but exposes the bare
substrate.
We were able to visualize the formation of the pattern in
situ by an optical microscope. Fig. 3 is a schematic illustra-
tion of the mechanism for the formation of the honeycomb
structure: after placing a droplet of chloroform solution on Fig. 2. (a) Fluorescence micrograph of a cast film of polyion complex 1/5.
The polymer contains 0.5 mol% (per repeat unit of polymer) of octadecyl
the substrate, the chloroform starts to evaporate. This leads rhodamine-B as a fluorescent probe. [complex] = 1 mg/ml in chloroform,
to a cooling of the solution, and micron-sized water droplets 50% relative humidity. (b) AFM image of cast film of polyion complex 1/
condense onto the chloroform solution of the polyion com- 5. [complex] = 0.077 mg/ml, 50% relative humidity.

3. 856 N. Maruyama et al. / Thin Solid Films 327–329 (1998) 854–856
Fig. 4 shows the dependence of the mesoscopic pattern of a
complex of 3 and 5 on these two parameters. Regular pat-
terns with a size of the honeycomb of a few micrometers can
be formed with 1 mmol/l and 0.5 mmol/l of 3/5. As
expected, the rim of the honeycomb becomes thinner with
decreasing concentration. At concentrations as low as 0.1
mmol/l the amount of polymer is too low to stabilize the
water droplets and fusion of the droplets takes place,
destroying the pattern. Regular patterns were found between
50% and 80% relative humidity. Higher humidity leads to
larger honeycombs, because more water condenses into the
chloroform solution. This leads to larger water droplets and
thus to larger voids of the honeycomb. The smallest honey-
comb patterns, with a void size of 500 nm, were found in
mixtures of 7 and 9. Fig. 4. Concentration and humidity dependence of the honeycomb struc-
Even though all results presented in this paper were car- ture of a complex of 3 and 5. The concentrations are given in mmol of
repeat unit per liter.
ried out with glass as a substrate, other substrates can be used
as well. We found that the patterns can be formed not only on various hydrophilic substrates, such as glass, quartz, silicon
wafer, mica, or indium tin oxide, but also on hydrophobic
substrates, such as silanized glass. Furthermore, other sol-
vents than chloroform can be used, e.g. benzene. This find-
ing will broaden future applications of these patterns.
4. Conclusions
We have described the formation of ordered mesoscopic
honeycomb patterns by a simple solution casting process. A
wide variety of functional amphiphilic polymers can be
used. Together with the fact that the substrate is also vari-
able, this will broaden the applicability of these new pat-
terns in future.
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