Langmuir monolayers of co nanoparticles and their patterning by microcontact printing

Article abstract of DOI:10.1021/jp0443308

In this paper, we describe an easy and reliable method for the production of patterned monolayers of Co nanoparticles. A two-dimensional monolayer of Co nanoparticles is fabricated by spreading a nanoparticle solution over an air-water interface and then transferring it to a hydrophobic substrate by using the Langmuir-Blodgett (LB) method. Transmission electron microscopy (TEM) was used to show that, with increasing surface pressure, the Co nanoparticles become well-organized into a Langmuir monolayer with a hexagonal close-packed structure. By controlling the pH of the subphase, it was found that a monolayer of Co nanoparticles with long-range order could be obtained. Further, by transferring the Langmuir monolayer onto a poly-(dimethoxysilane) (PDMS) mold, the selective micropatterning of the Co nanoparticles could be achieved on a patterned electronic circuit. The electronic transport properties of the Co nanoparticles showed the ohmic I-V curve. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0443308

J. Phys. Chem. B 2005, 109, 13119-13123
13119
Langmuir Monolayers of Co Nanoparticles and Their Patterning by Microcontact Printing
Jong-Il Park,† Woo-Ram Lee,‡ Sung-Soo Bae,† Youn Joong Kim,§ Kyung-Hwa Yoo,‡
Jinwoo Cheon,*,‡ and Sehun Kim*,†
Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea,
Departments of Chemistry and Physics, Yonsei UniVersity, Seoul 120-749, Korea, and DiVision of
Nano-Material and EnVironmental Science, Korea Basic Science Institute, Daejeon 305-333, Korea
ReceiVed: December 13, 2004; In Final Form: April 18, 2005
In this paper, we describe an easy and reliable method for the production of patterned monolayers of Co
nanoparticles. A two-dimensional monolayer of Co nanoparticles is fabricated by spreading a nanoparticle
solution over an air-water interface and then transferring it to a hydrophobic substrate by using the Langmuir-
Blodgett (LB) method. Transmission electron microscopy (TEM) was used to show that, with increasing
surface pressure, the Co nanoparticles become well-organized into a Langmuir monolayer with a hexagonal
close-packed structure. By controlling the pH of the subphase, it was found that a monolayer of Co nanoparticles
with long-range order could be obtained. Further, by transferring the Langmuir monolayer onto a poly-
(dimethoxysilane) (PDMS) mold, the selective micropatterning of the Co nanoparticles could be achieved on
a patterned electronic circuit. The electronic transport properties of the Co nanoparticles showed the ohmic
I-V curve.
Introduction
However, highly ordered LB films and 2-D patterns of Co
nanoparticles have not yet been reported because of difficulties
in the synthesis of monodisperse Co nanoparticles.
In this paper, we describe a method for the systematic and
comprehensive fabrication of 2-D monolayers of Co nanopar-
ticles via the LB technique and of 2-D patterned monolayers
on various substrates, such as silicon wafers, using the micro-
contact printing method.
Convenient and effective organization of nanomaterials
(molecules, polymers, and nanoparticles) into one-, two-, and
three-dimensional (1-D, 2-D, and 3-D) structures is the key to
the realization of nanodevices.1-8 The most common method
of constructing 2- and 3-D nanoparticle structures involves
synthesizing the nanoparticles in solution by chemical methods
and then attaching them to various substrates using suitable
interactions such as van der Waals forces and electrostatic or
covalent bonds.9-11 In the case of Au nanoparticles, 1-D chains
and wires have been produced along nanofibrils derived from
the self-assembly of DNA and synthesized peptides.9,10,12 Three-
dimensinal crystal structures have been formed by a self-
assembly process with a ligand exchange reaction.13 During the
self-assembly process, 2-D monolayers are produced as a result
of interparticle interactions. To more effectively produce 2-D
monolayers of nanoparticles on solid substrates, many research-
ers have tried using various techniques, such as layer by layer
(LbL) deposition, the Langmuir-Blodgett method (LB), and
the spin-coating method.14-16 The LB technique is the most
promising method for producing well-organized 2-D monolayers
of surfactants, polymers, and nanoparticles, because it provides
fine control of the thickness and homogeneity of the monolayer
and of multilayers. For example, Heath et al., Markovich et al.,
and Xi et al. have reported the fabrication of LB films of Ag,
Fe3O4, and Fe2O3 nanoparticles in the presence of various
capping molecules such as fatty acids and alkanethiols.16-18
Furthermore, the formation of 2-D patterned monolayers of Au,
Fe2O3, and Pt@Fe2O3 nanoparticles by using a combination of
LB with microcontact printing (µ-CP) has been reported.19-22
Experimental Section
Materials. Octadecyltrichlorosilane (OD-TCS) and sodium
bis(2-ethylhexyl) sulfosuccinate (NaAOT) were purchased from
Aldrich Co. Tetradecanoic acid and all solvents were purchased
from Junsei Co. Dicobalt octacarbonyl was purchased from
Strem Inc. Deionized (DI) water (18 MΩ‚cm) was obtained
using a Millipore four-bowl purification system. PDMS elas-
tomer (Sylgard 184) was obtained from Dow Corning.
Preparation of Co Nanoparticles. Co nanoparticles were
prepared using the thermolysis of Co2(CO)8 in a refluxing
toluene solution containing tetradecanoic acid and NaAOT as
stabilizer.23 The Co nanoparticles solution was filtered through
a 0.45 µm nylon membrane filter to remove aggregated
nanoparticles and residual organic impurities.
Substrate Preparation. The Si(111) surface was cleaned by
dipping it in piranha solution (H2SO4:H2O2 ) 7:3) at 90 °C for
1 h followed by washing with deionized water. After cleaning,
the sample was sonicated in basic hydroxide solution (H2O:
H2O2:NH3 ) 5:1:1) for 30 min, then sonicated in acidic peroxide
solution (H2O:H2O2:HCl ) 6:1:1) for 30 min followed by
washing with deionized water, and kept in an oven at 120 °C
for 10 min. The slide glasses, oxidized Si(111) wafers, and mica
substrates were also cleaned by sonication in 2-propanol.
* To whom correspondence should be addressed. (S.K.) Phone: (+82)
42-869-2831. Fax: (+82) 42-869-2810. E-mail: sehun-kim@kaist.ac.kr.
(J.C.) Phone: (+82) 2-2123-5631. Fax: (+82) 2-364-7050. E-mail:
jcheon@yonsei.ac.kr.
The OD-terminated surface was produced by immersion in
a toluene solution (20 mL) containing OD-TCS (0.1 mL) at
room temperature (RT) for 10 min. After the reaction, each
† Korea Advanced Institute of Science and Technology.
‡ Yonsei University.
§ Korea Basic Science Institute.
10.1021/jp0443308 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/16/2005

13120 J. Phys. Chem. B, Vol. 109, No. 27, 2005
Park et al.
Figure 2. TEM images of LB films of Co nanoparticles at pH ) 11,
12, 13, and 14, respectively.
Figure 1. π-A isotherm graph of monodisperse Co nanoparticles (∼10
nm) and TEM images of monolayers of Co nanoparticles on carbon-
coated Cu grids fabricated with the LB method at surface pressures of
5, 15, 30, and 45 mN/m (A-D, respectively).
sample was washed with toluene and then used in the fabrication
of an LB film or in µ-CP.
LB Films. The NIMA 611 Langmuir trough was cleaned with
chloroform and hexane and then filled with deionized water.
The water surface was cleaned by pumping out the dust and
organic molecules on the water surface using a circulating
aspirator. A 2.2 × 10-6 M solution of Co nanoparticles in
toluene (10-50 µL) was spread over the water surface, and,
after waiting 10 min for the toluene to evaporate, a barrier
compression was applied at a constant rate of 10-50 mm/min.
At surface pressures in the range of 10-45 mN/m, Langmuir
films formed on the various substrates, namely, TEM grids, slide
glasses, and silicon wafers, as well as on mica and gold
substrates.
Figure 3. (A) TEM image and (C) SEM image of a 2-D monolayer
of Co nanoparticles fabricated with the LB technique under optimum
conditions. B and D are enlarged views of A and C, respectively.
Preparation of Micropatterned PDMS Stamps. PDMS
stamps with a line pattern (2 µm width, 1 µm spacing, and 500
nm depth) were replicated from a master silicon grating (TGZ03,
1 µm width, 2 µm spacing, and 500 nm depth) originally
fabricated for atomic force microscopy (AFM) tip characteriza-
tion.
Preparation of Patterned LB Films. The well-organized LB
films of Co nanoparticles were transferred onto the PDMS
stamps using the LB technique. The Co nanoparticle-covered
PDMS stamps were placed onto the OD-terminated Si surfaces
or silicon dioxide substrates containing E-beam lithographically
patterned gold electrodes and, after pressing by hand for 5 s,
were carefully removed from the substrate. A cross-bar pattern
was also produced by printing twice, with a 90° rotation of the
substrate between the two stampings.
Results and Discussion
After cleaning with chloroform and hexane, the LB trough
was filled with DI water, which was cleaned by pumping out
the dust and organic molecules on the water surface using a
circulating aspirator. When 30 µL of a 2.2 × 10-6 M Co
nanoparticle solution was spread over the water surface, the
hydrophobic Co nanoparticles floated on the water surface, after
evaporation of toluene for approximately 10 min. The barrier
compression of the nanoparticles at a constant speed of 10 mm/
min causes an increase in the surface pressure and results in
the close packing of the Co nanoparticles.
Figure 1 shows the surface pressure versus area (π-A)
isotherm for the Co nanoparticles on the air-water interface.
The Co nanoparticles on the water surface are present in three
different phases (gas, liquid, and solid); Interval A-B is the
gas phase, B-C is the liquid phase, and C-D is the solid phase.
The TEM images in Figure 1A-D show LB films on
amorphous carbon-coated Cu grids attached to slide glasses at
various surface pressures. At low pressures (∼5 mN/m), the
Instruments. The TEM images were obtained with an EM
912 omega (Zeiss, KBSI) instrument operating at 120 kV. The
deposited and patterned Langmuir films were analyzed with a
FE-SEM (XL30FEG, Philips) and an optical microscope
(Axio3.0, Zeiss).

Langmuir Monolayers of Co Nanoparticles
J. Phys. Chem. B, Vol. 109, No. 27, 2005 13121
of Co nanoparticles gradually forms, and the number density
of nanoparticles increases from 3.4 × 1011 to 6.2 × 1011
particles/cm2 in Figure 1B,C, respectively. The area per nano-
particle in Figure 1C is 160 nm2, which is near the close-packing
area of Co nanoparticles. In the highly compressed state (∼45
mN/m), the nanoparticles become densely packed with a density
of 6.4 × 1011 particles/cm2. On further increase of the pressure
to beyond the highly compressed state, folded layers are
observed to form. The highly compressed monolayer is slightly
contaminated by free fatty acid, which appears as the fuzzy
phase shown in Figure 1D.
To improve the quality of the Langmuir films, it was found
that further control of their conditioning was needed, such as
through the adjustment of the basicity of the subphase. If the
pH of the subphase is basic, the free fatty acid dissolves into
the subphase and the numbers of large gaps and voids between
the particles (due to areas occupied by free fatty acid) are
diminished. Therefore, the monolayers of Co nanoparticles have
a more closely packed structure in a strongly basic solution (1
mM NaOH, pH ) 11) than is achieved in DI water. The addition
of further sodium hydroxide produces more dissolution of free
fatty acid, but higher NaOH concentrations (e.g. 1 M NaOH,
pH ) 14) are not suitable for forming a monolayer of Co
nanoparticles because of contamination by NaOH and the
ionization of the Co nanoparticles by the base. By carefully
leaving a small amount of fatty acid in the Co nanoparticles
solution and optimizing the deposition conditions (particle
concentration, barrier rate, dipping rate, and varying the
substrate), we were able to obtain an almost close-packed 2-D
monolayer of Co nanoparticles (Figure 2).
Figure 4. Schematic views of (A) the formation of an LB film of Co
nanoparticles and (B) the microcontact printing of Co nanoparticles.
Co nanoparticles are randomly distributed and form only small
2-D aggregates during the evaporation of toluene (Figure 1A);
the number density of nanoparticles (counted from this TEM
image) is about 1.8 × 1011 particles/cm2, which is well-matched
with the surface area per particle (∼550 nm2). On compression
of the nanoparticles, a two-dimensionally organized monolayer
At optimum conditions (particle concentration ) 30 µL of
2.2 × 10-6 M solution, solvent ) toluene, pH of subphase )
11, barrier rate ) 10 mm/min, dipping rate ) 10 mm/min), the
Co nanoparticles were found to be well-organized with hex-
Figure 5. Patterns of microcontact printed LB films of nanoparticles: (A) optical microscope image, (B and C) SEM images of stripe patterns of
Co nanoparticles, (D) optical microscope images, and (E and F) SEM images, which are cross-line patterns.

13122 J. Phys. Chem. B, Vol. 109, No. 27, 2005
Park et al.
Figure 6. (A) Optical microscope image of the gold electrode and (B) FE-SEM image of LB films of Co nanoparticles microcontact-printed across
the gold electrode lines. C and D are enlarged views obtained with FE-SEM. E is the I-V curve of the LB films of Co nanoparticles microcontact-
printed across the gold electrode lines.
agonal close-packing at the air-water interface and could easily
be transferred with long-range ordering to a carbon-coated Cu
grid attached to a slide glass (Figure 3A,B).
a monolayer of Co nanoparticles, which was then imprinted on
an octadecyl-terminated Si wafer for ∼5 s.
Patterned monolayers of Co nanoparticles are not easy to see
with the naked eye, but are easily observed with an optical
microscope. Figure 5A shows a patterned monolayer of Co
nanoparticles on an octadecyl-terminated silicon wafer. A 2 µm
wide stripe pattern is visible with a black line in low-
magnification SEM images, which is composed of closely
packed Co nanoparticles in enlarged view (Figure 5B,C). The
cross-bar pattern shown in Figure 5E,F is produced by printing
on the stripe pattern of Co nanoparticles with another striped
line stamp coated with Fe3O4 nanoparticle. The striped lines of
Fe3O4 nanoparticles are across the horizontal lines of Co
nanoparticles.
This result was also obtained for other hydrophobic substrates,
such as for the gold surface shown in Figure 3C. In this SEM
image, the Co nanoparticles can be seen to be well-organized
with a hexagonal close-packing structure and are widely
distributed with a range greater than 10 µm. Some cracks in
the LB films occur during the dipping processes, because the
interaction between the particles is weaker than the dipping
force. However, most of the LB films of Co nanoparticles were
successfully transferred to the hydrophobic substrates (gold,
carbon-coated Cu grids, H-passivated silicon wafers, octadecyl-
terminated silicon wafers), in contrast to the nonuniform
monolayers formed on hydrophilic substrates (slide glasses,
mica, OH-terminated silicon wafers), because of the attractive
interaction between the Co nanoparticles and the hydrophobic
substrate.
The stripe pattern can also be stamped onto a silicon dioxide
substrate containing electron beam (E-beam) lithographically
patterned gold electrodes (Figure 6A). This substrate is also
terminated by the octadecyl functional group before µ-CP.
Figure 6B shows a SEM image of monolayer stripes of Co
nanoparticles printed across three sets of up-down gold
electrode lines. Each set of up-down gold electrodes is made
up of two parallel zigzag lines. The gap between the up electrode
and the down electrode is 200 nm, and the electrode line width
is 100 nm. As shown in Figure 6C, the gold electrodes have a
zigzag pattern, and the Co nanoparticles were successfully
printed onto this substrate across the gold electrodes. The
electrical measurements were carried out by passing an electric
A well-organized LB film of Co nanoparticles can be used
as the ink for a PDMS stamp. Figure 4 shows a schematic view
of the fabrication of a patterned monolayer of Co nanoparticles.
The PDMS stamps with a stripe pattern (2 µm width, 1 µm
spacing, and 500 nm depth) were replicated from a master
silicon grating (TGZ03) used in atomic force microscopy (AFM)
tip characterization. By vertical or horizontal dipping at a surface
pressure of 30 mN/m, the PDMS stamp could be covered with

Langmuir Monolayers of Co Nanoparticles
J. Phys. Chem. B, Vol. 109, No. 27, 2005 13123
current through the monolayer of nanoparticles between the two
electrodes. The patterned samples are electrically insulating
because of the long-chain organic capping groups and the
surface oxide layer.24 After annealing the substrate for 1 h under
reducing conditions (300 °C, 5% H2), ohmic transport was
observed in this pattern at room temperature instead of the
Coulomb blockade, as shown in Figure 6E.24 Multichannel
tunneling through the hexagonally packed nanoparticles is a
major contribution to the electron transport, contributing more
to the ohmic conductivity than single particle transport.25,26
Room-temperature resistance is about 120 kΩ, which is similar
to the value for annealed Co nanoparticles obtained by Black
et al.6 Low-temperature transport measurements were not carried
out in this study, but the resulting transport is expected to be
similar to Black’s results.
and (S2) TEM and FE-SEM images (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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In summary, the forced assembly of Co nanoparticles on an
air-water interface can be used to produce a wide range of
ordered Co nanoparticle films that under optimum conditions
can be easily transferred to hydrophobic substrates. The
hexagonal close-packing structures of the LB films were
controlled by variation of the particle concentration, the pH of
the subphase, the barrier rate, and the dipping rate. Further,
patterning of the Co nanoparticles was achieved by stamping a
Langmuir film coated onto a PDMS stamp; these patterns were
characterized by optical microscopy and FE-SEM. Furthermore,
we suggest that future nanodevices can be manufactured by
selectively imprinting Langmuir monolayers of nanoparticles
onto patterned gold electrodes.
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(24) As-prepared Co nanoparticles are oxidized during LB filming
procedure in basic solution and ambient condition but easily reduced by
annealing at 300 °C under H2 gas. (See Supporting Information.)
(25) Petit, C.; Cren, T.; Roditchev, D.; Sacks, W.; Klein, J.; Pileni, M.
P. AdV. Mater. 1999, 11, 1198.
(26) We have compared the SEM images before and after reduction at
300 °C under H2. After the reducing process, the interparticle distance was
shortened and the oxidized CoO layer and stabilizer of Co nanoparticles
were removed. The shortened interparticle distance and reduction of the
oxidized state are attributed to the conductance of Co nanoparticle films
(Figure S2 in Supporting Information).
Acknowledgment. We would like to thank KBSI for TEM
analyses. This research was supported by grants from KOSEF
through the Center for Nanotubes and Nanostructured Com-
posites, the Brain Korea 21 Project, KRF (Grant R02-2004-
000-10096-0), NCRC (Grant R15-2004-024-02002-0), the Na-
tional R&D Project for Nano Science and Technology, the
National R&D for Cancer Control (0320250-2), Korea Health
21R&D(0405-MN01-0604-0007),andAOARD(FA520904P0406).
Supporting Information Available: Two figures, showing
(S1) X-ray photoelectron spectra and X-ray absorption spectra