J. Vacuum Sci. technol.B _Vol.19_No.6_2001_pp.2045-2049.pdf

Experimental conditions for a highly ordered monolayer of gold
nanoparticles fabricated by the Langmuir–Blodgett method
Shujuan Huang, a) Gen Tsutsui, Hiroyuki Sakaue, Shoso Shingubara,
and Takayuki Takahagi
Department of Electrical Engineering, Hiroshima University, Kagamiyama 1-4-1,
Higashi-Hiroshima 739-8527, Japan
Received 14 December 2000; accepted 20 August 2001
A highly ordered monolayer film of alkanethiol-encapsulated gold nanoparticles was fabricated on
a silicon substrate by using the Langmuir–Blodgett LB method. The effects on the particle order,
of the particle concentration and the type of solvent of the LB spreading suspension of encapsulated
gold particles, were studied. We found that a low particle concentration of 0.06–0.3 mg/mL in
chloroform is optimal for the fabrication of high quality gold particle monolayers. Since the
proposed method is not restricted to gold particles, it is believed to be a practical process for
fabricating quantum dot structures of various particle sizes and compositions. © 2001 American
Vacuum Society. DOI: 10.1116/1.1410943
I. INTRODUCTION
Constructing new nanostructured materials and devices
using nanoparticles has attracted much research interest for
their potential applications in electronics, optics, and magnetic
storage. 1–5 As known, only three parameters, particle
size, composition, and topology, determine electronic, magnetic,
and optical properties. Thus, much research has been
conducted regarding the fabrication, assembly, and utilization
of nanodot structures made of nanoparticles. In our previous
work, 6,7 we developed a method for fabricating a selforganized
planar ordered array of gold particles encapsulated
by alkanethiol. We have observed the Coulomb blockade
phenomena in the nanostructures consisting of 9-nm-diam
particles. 8 The fabrication method plays a significant role in
the study of the fabrication process and the characteristics of
nanodot structures. However, this method poses difficulty in
the fabrication of large-scale, well-controlled nanostructures.
In order to overcome this drawback, we developed a new
method for fabricating a large-scale monolayer of
alkanethiol-encapsulated gold particles using the LB method,
and studied mechanism governing the formation of this
monolayer. 9 In this article, we report the fabrication of a
highly ordered LB monolayer of gold particles encapsulated
by alkanethiol. Experimental conditions, such as the particle
concentration and the type of solvent of LB spreading suspension,
for fabricating a high-quality nanostructure are
studied and discussed in detail.
II. EXPERIMENT
The gold particles used in this work were synthesized by
using a mixture of trisodium citrate and tannic acid for the
reduction of chloroauric acid (HAuCl4). 10,11 It is reported
that gold particles synthesized by this method are homodispersed.
The size of the particles is determined by the quantity
of the tannic acid in the reducing solution. This method is
a Electronic mail: sjhuang@hiroshima-u.ac.jp
described in detail elsewhere. 10,11 In this work, we synthesized
gold colloids of 8.30.7 and 171.6 nm in diameter to
prepare the LB monolayer.
Because alkanethiols are water insoluble, the aqueous
gold colloidal solution is not suitable for preparing
alkanethiol-encapsulated gold particle solution in high
concentrations. 6 In addition, the preparation of the LB monolayer
of encapsulated gold particles requires the suspension
of encapsulated gold particles in a water-insoluble volatile
solvent such as chloroform or benzene. For these reasons, we
made use of a mediating solvent of ethanol, which is soluble
in water, chloroform, and benzene, to prepare the LB spreading
solution. The details of this method are described in the
following:
1 Suspending gold particles in ethanol. First, a gold colloidal
solution of 8.3 or 17 nm in diameter was dissociated
by centrifugation at a centrifugal force CF of 25 000 or
12 000 g for 30 min. After centrifugation, two phases of the
solutions, dark red, and transparent had formed. The supernatant
was removed as carefully and as completely as possible.
After the supernatant was removed, ethanol was infused
into the tube. This suspension of gold particles in ethanol
was centrifugalized for 30 min at CF25 000 g for 8.3 nm
particles and CF12 000 g for 17 nm particles. The centrifugation
produced a black spot of gold particle precipitate and
a supernatant of ethanol. After the supernatant was removed,
the precipitate of gold particles was dispersed in fresh ethanol
using an ultrasonic processor.
2 Preparing alkanethiol-encapsulated gold particle suspension
in ethanol. The suspension of alkanethiolencapsulated
gold particles in ethanol was prepared by mixing
the above-prepared gold particle suspension with
alkanethiol solution. 6,7 The typical experimental conditions
used in preparing encapsulated gold particles included mixing
20 mL of the gold particle suspension with 5 mL of 1
mM ethanol solution of dodecanethiol CH 3–CH 2 11–SH.
The mixed solution was kept at room temperature overnight.
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2046 Huang et al.: Experimental conditions for gold nanoparticles 2046
FIG. 1. Surface pressure–area per particle (-A) isotherm for 8.3-nm-diam
gold particles encapsulated by dodecanethiol.
The mixed solution was then centrifugalized for 30 min at
CF12 000 g for 8.3 nm particles and CF6000 g for 17 nm
particles in order to remove the unreacted dodecanethiol
molecules.
3 Suspending encapsulated gold particles in chloroform
or benzene. The above centrifugation caused the
dodecanethiol-encapsulated gold particles to precipitate at
the bottom of the tube, which could be dispersed in chloroform
or benzene using an ultrasonic processor. The concentration
of gold particles in the suspension was determined by
the amount of solvent infused.
The LB instrument used in this work was a KSV minitrough
manufactured by KSV Instruments. To spread the
sample, minute droplets 3 L of the encapsulated gold
particle suspension prepared above were very carefully cast
on the surface of the pure water in the LB trough at intervals
of 30 s. After the solvent evaporated, the hydrophobic
dodecanethiol-encapsulated gold particles remained on the
water’s surface. These particles were then compressed by
moving the barriers at a speed of 5 mm/min. The surface
pressure isotherm was recorded throughout the compression.
The gold particles were transferred by retracting a hydrogenterminated
silicon substrate, 12 which was immersed vertically
in the pure water in the trough before the particle suspension
was spread. The retracting speed was 0.5 mm/min.
The temperature of the pure water in the trough was about
24 °C.
In the present work, scanning electron microscopy SEM
observations of the transferred gold particles of 8.3 and 17
nm in diameter were performed on S-5000 and S-800 Hitachi,
respectively.
III. RESULTS AND DISCUSSION
A. LB monolayer of alkanethiol-encapsulated gold
particles
Figure 1 displays a smooth surface pressure–area per particle
(-A) isotherm for the 8.3-nm-diam dodecanethiolencapsulated
gold particle suspension with a particle concentration
of 0.3 mg/mL in chloroform, which provides rich
J. Vac. Sci. Technol. B, Vol. 19, No. 6, NovÕDec 2001
information on the formation of a stable monolayer of the
encapsulated gold particles. At the beginning of the LB compression,
the average distance between particles was quite
large, so that the surface pressure was very low. As the barrier
moved, the interparticle distance decreased and the surface
pressure increased slightly. From the point of about 140
nm 2 , the surface pressure apparently started to increase. This
was because the interparticle distance became very small so
that the interaction of the particles increased remarkably.
When the barrier compressed further, the surface pressure
increased abruptly and a densely packed monolayer of the
particles tended to form. We found that a surface pressure of
10 mN/m is the optimal condition for a good quality
monolayer of 8.3-nm-diam gold particles. At this point, the
area occupied per particle is about 91 nm 2 . According to our
previous study, 6
the estimated area per dodecanethiol-
encapsulated particle is about 78 nm 2 for a close-packed
monolayer. Thus, the particle coverage at this pressure is
about 85%. We have reported that the unoccupied area was
caused by the poor accommodation of the domains and the
arrangement defects of the particles. 9 When the gold particles
were compressed beyond 10 mN/m, some patches of
bilayer had formed and the monolayer collapsed.
The monolayer of the gold particles was deposited onto a
hydrogen-terminated silicon substrate over an area of 1 cm 2 .
SEM observations demonstrated that it was composed of
two-dimensional domains of ordered, close-packed gold particles.
The sizes of these domains varied, some of which
were more than 100 m 2 . Figure 2a shows the SEM image
of part of a domain of 8.3-nm-diam gold particles transferred
at 10 mN/m. The inset shows a high-magnification SEM
image. Figures 2b and 2c display fast Fourier transformation
FFT images. The high-magnification SEM image demonstrates
that a very high level of orderliness of the particle
arrangement had formed locally. Furthermore, its FFT image
Fig. 2c reveals clear first and second FFT orders, which
also indicates a highly ordered close-packed arrangement of
gold particles. However, at longer ranges, as shown in Fig.
2a, it can be seen that the gold particles have not formed a
perfect order. The particle arrangement looks like a polycrystal,
which has many ‘‘crystal grains.’’ Some particle vacancies
and dislocations were observed in SEM observations.
Correspondingly, the FFT image tends toward the shape of a
ring rather than hexagonal spots. This is due to arrangement
defects that are discussed in Sec. III B.
B. Effect of the spreading suspension’s particle
concentration on the formation of the
particle monolayer
In order to determine the most suitable conditions for the
fabrication of high quality monolayers of alkanethiolencapsulated
gold particles, experimental study was carried
out under different particle concentrations of the LB spreading
suspension from chloroform. Figure 3 shows the SEM
images of LB monolayers of 8.3-nm-diam gold particles fabricated
from particle concentrations of 0.06 mg/mL (1
10 13 /mL) and 0.6 mg/mL (110 14 /mL). Both monolayers

2047 Huang et al.: Experimental conditions for gold nanoparticles 2047
FIG. 2.a SEM image of a LB monolayer of dodecanethiol-encapsulated gold particles 8.3 nm in diameter. The inset shows a high-magnification SEM image,
b and c show the FFT images of the SEM image in a and the inset, respectively.
were transferred at 10 mN/m. Obviously, the encapsulated
gold particles formed in quite different arrangements.
In the case of a low particle concentration of 0.06 mg/mL,
the gold particle monolayer is similar to that made from a
medium concentration of 0.3 mg/mL, as shown in Fig. 2. The
films take on a crystal structure with some defects including
particle vacancies Fig. 4a, ‘‘edge dislocations’’ Fig.
4b, and grain boundaries. However, in the LB monolayer
made from a high particle concentration of 0.6 mg/mL, many
JVSTB-MicroelectronicsandNanometer Structures
narrow gaps or voids have formed between the ‘‘crystal
grains’’ in addition to particle vacancies and dislocations.
Moreover, the FFT image of the LB monolayer made from
the low particle concentration shows hexagonal spots,
whereas the FFT image of the monolayer made from the high
concentration is more like a ring rather than six clear spots.
This indicates that the degree of long-range order of the
monolayer made from the low concentration is better than
that made from the high concentration. In other words, a low

2048 Huang et al.: Experimental conditions for gold nanoparticles 2048
FIG. 3. SEM images of 8.3-nm-diam gold particle monolayers prepared
from LB spreading suspensions of a a low particle concentration of 0.06
mg/mL and b a high particle concentration of 0.6 mg/mL. The insets show
the corresponding FFT images.
particle concentration of 0.06–0.3 mg/mL is more suitable
than a high concentration of 0.6 mg/ml for the LB preparation
of gold particle monolayers.
We propose that the difference between the LB monolayers
made from the low- and high-particle-concentration suspensions
is caused by the different forming processes. In our
previous study of the forming mechanism of gold particle LB
monolayers, 9 we found that the ordered particle domains
were a result of a self-organization process induced by the
evaporation of the sample suspension, rather than the compression
of the barriers. More specifically, when a droplet of
the encapsulated gold suspension was cast onto the surface
of the water in a trough, the solvent’s water insolubility
FIG. 4. Defects in the LB monolayer of the encapsulated gold particles. a
A particle vacancy and b a dislocation indicated by the white line.
J. Vac. Sci. Technol. B, Vol. 19, No. 6, NovÕDec 2001
FIG. 5. Schematics of the formation of the defects in LB monolayers prepared
from a a low particle concentration and b a high particle concentration
of LB spreading suspensions.
caused it to spread into a thin layer. As the solvent evaporated
and became a thin layer with a thickness equal to the
particle size, the attractive force between particles induced
by the surface tension of the solvent increased significantly
and caused the particles to pack together into ordered arrays.
In the case of low particle concentration, when the sample
first began to spread, the particle density on the water’s surface
was quite low, so that only small domains of closepacked
particles had formed due to the surface tension of the
solvent. As the particle suspension was cast onto the water’s
surface in succession, the new incoming particles around
these initial domains were transported to them by the same
interaction as seen in the formation of the small domains.
Consequently, the domains of the ordered particles grew
larger by agglomeration. However, when a particle with a
much larger or smaller size was transported to the particle
array, as shown in Fig. 5a, the arrangement of the particles
would be changed, forming a defect in the particle arrangement.
Therefore, we can conclude that the main cause of the
defects in the particle arrangement in this case is the variation
of the particle sizes.
On the other hand, in the case of the high particle concentration,
as a droplet of the particle suspension was spread and
evaporated, many small particle domains had already formed
initially, as shown in Fig. 5b. Meanwhile, since the particle
concentration was quite high, some domains were quite close
to each other. As a result, the interaction between these domains,
caused by the solvent’s surface tension, was large
enough to drive them to coalesce together. Due to the poor
accommodation of the particle domains, many narrow gaps
between the ‘‘crystal grains’’ formed.
C. Effect of solvent on particle order
As known, the solvent of the particle suspension is another
important condition for fabricating a high-quality

2049 Huang et al.: Experimental conditions for gold nanoparticles 2049
FIG. 6. SEM images of 17-nm-diam gold particle monolayers prepared from
a chloroform suspension and b benzene suspension.
monolayer of alkanethiol-encapsulated gold particles. 6 This
is due to the dependence of the evaporation rate of the
sample, which plays a significant role in the formation of the
ordered particle domains, on the solvent. In this work, we
prepared dodecanethiol-encapsulated gold particle suspensions
from chloroform and from benzene, respectively. The
diameter of the gold colloidal particles is 17 nm. The particle
concentrations of these two suspensions were the same,
0.06 mg/ml. Figure 6 shows the LB monolayers prepared
from these two kinds of samples. It is noticed that the monolayer
prepared from the particle suspension in chloroform is
more ordered. The only arrangement defects were particle
vacancies and dislocations. However, the monolayer prepared
from the particle suspension in benzene reveals a
lesser ordering. Many gaps formed between particles, as
shown by arrow 1 in Fig. 6b. In addition, some particle
aggregations, as indicated by arrow 2, had also formed,
which demonstrates that the alkanethiol-encapsulated gold
particle suspension from benzene is not as stable as that from
chloroform.
JVSTB-MicroelectronicsandNanometer Structures
In regard to the gaps between particles, we consider that
they are due to benzene’s slower evaporation rate. As known,
benzene evaporates more slowly than does chloroform. In
the slower evaporation process of the gold particle spreading
suspension from benzene, on the one hand, the small particle
domains were first formed in the same way as the particle
suspension from chloroform. On the other hand, unlike the
fast evaporation of the suspension from chloroform, there
was enough time for these small domains to coalesce together
under the above-mentioned interaction, during benzene’s
slower evaporation. As a result, many gaps between
particles had formed due to the poorer accommodation of the
small particle domains.
IV. CONCLUSION
In this article, we reported the fabrication of a highly
ordered wafer-scale monolayer of alkanethiol-encapsulated
gold particles using the LB method. The effects of the particle
concentration and the solvent, from which the LB
spreading suspensions of the encapsulated gold particles
were made, were studied experimentally. The results demonstrate
that a low particle concentration of 0.06–0.3 mg/mL
resulted in a highly ordered monolayer of gold particles. In
addition, using chloroform as the solvent produced a more
stable particle suspension and a more ordered monolayer
than the use of benzene.
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