Electrical and thermal properties of conducting microbead prepared by green electroless plating method using gold nanoparticles

Article abstract of DOI:10.1149/2.022112jes

We developed a green electroless plating method that enables saving of resources and reduction in emissions to the environment because it uses a self-assembling reaction between gold nanoparticles (AuNPs) and a nontoxic thiol binder. In this method, a uniform surface coating and thickness reflected by the diameter of AuNPs were obtained on micrometer-sized core plastic beads. Thermal analysis has revealed that AuNP adsorption is based on the chemical reaction and interaction among the AuNP, binder, and bead. Further, the AuNP-plated microbead has high physical, electrical, and thermal stabilities, which would enable its practical applications.

Full text of DOI:10.1149/2.022112jes

Journal of The Electrochemical Society, 158 (12) D689-D693 (2011)
D689
0013-4651/2011/158(12)/D689/5/$28.00 © The Electrochemical Society
Electrical and Thermal Properties of Conducting Microbead
Prepared by Green Electroless Plating Method Using Gold
Nanoparticles
Shiho Tokonami,a Shuji Shirai,b Itaru Ota,b Naoki Shibutani,b Yojiro Yamamoto,c
Hiroshi Shiigi,b,∗,z and Tsutomu Nagaokab,∗
aNanoscience and Nanotechnology Research Center and bDepartment of Applied Chemistry, Osaka Prefecture
University, Sakai 599-8570, Japan
cGreenChem. Inc., Izumi 594-1144, Japan
We developed a green electroless plating method that enables saving of resources and reduction in emissions to the environment
because it uses a self-assembling reaction between gold nanoparticles (AuNPs) and a nontoxic thiol binder. In this method, a
uniform surface coating and thickness reflected by the diameter of AuNPs were obtained on micrometer-sized core plastic beads.
Thermal analysis has revealed that AuNP adsorption is based on the chemical reaction and interaction among the AuNP, binder,
and bead. Further, the AuNP-plated microbead has high physical, electrical, and thermal stabilities, which would enable its practical
applications.
© 2011 The Electrochemical Society. [DOI: 10.1149/2.022112jes] All rights reserved.
Manuscript submitted July 22, 2011; revised manuscript received August 15, 2011. Published November 1, 2011.
Rapid progress and developments have been made in information
technology (IT) and its related fields; this has made it necessary to
miniaturize compact electronic devices as well as consumer electronic
products, such as mobile phones, personal digital assistants (PDAs),
and laptops through high-density packaging and wiring technologies.
Consequently, this has led to accelerated development in the metallic
thin film technologies over the last few decades. Conventional elec-
troless plating methods, which are well known and are commonly
employed to obtain such metal thin films, are cost efficient but they
involve environmentally-unfriendly processes and controlling these
multistep processes to achieve the best coating result poses inher-
ent difficulties. These methods utilize nongreen chemicals such as
cyanide, chromic acid, strong alkalis, and acids, which are conflicting
with the demand for lower emissions to the environment.1–3 Despite
the fact that as technological innovations have brought about conve-
nience and comfort to society, environmental degradation problems
have accelerated in recent years owing to the release of enormous
amounts of harmful resources and energies. Therefore, it is important
for researchers to consider environmental friendliness during research
and development of technologies.
Recently, inorganic nanoparticles (NPs) and their arrays have been
attracting the attention of researchers as important materials for nano-
electronics owing to their unique physical and chemical properties.4–7
Self-assembly technology can be effectively used to assemble well-
organized one- to three-dimensional structures, and interparticle con-
nections can be devised at a controllable single-particle level.8–10
Metal NPs are one of the most frequently studied inorganic materials
for application to various nanotechnology-oriented devices;11–13 their
distinct shapes and combinations, such as nanobarcodes, nanorods,
and nanowires, can impart unique functions to the devices.14,15 We
previously reported the electroconductivity of gold NP (AuNP) array
films, which consisted of AuNPs and thiol molecules as a binder.16–20
The films were prepared by a single-step procedure that uses a self-
assembly method for the deposition of a NP on a plastic substrate; we
showed that this method can dramatically reduce the release of waste
chemicals to the environment.
and the external circuit in a single installation procedure. This film
can be used with conducting microbeads to establish reliable electri-
cal connections. The beads have to be elastic to establish a reliable
contact between the bump of an active component, such as LCD and
large-scale integration, and an external bus line. Such beads have been
commercially available since 1980s and they can generally be metal-
lized with either one or more different layers, for example, a nickel
base layer with a gold layer on top. However, an increase in the metal-
lic layer thickness leads to decrease in the flexibility of the plated
layer and causes mechanical and electrical fractures. Apparently, the
crack formed on the metal layer often causes problems in the elec-
tronic conductions at the compressed ACF. Therefore, the thickness
of the metal layer on the beads is a key factor for the practical use.
Our plating technique to make efficient use of AuNPs has successfully
reduced the thickness of the plated layer roughly in halves comparing
with that of commercially available one, and given us good electrical
property of the bead for a practical use.
In this study, we conducted the heat-cycle and cyclic loading-
unloading tests to determine the electrical, thermal, and mechanical
stabilities of the developed beads, which are considered to be very
important for the practical use. Moreover, thermogravimetric analysis
has been carried out to clarify the formation mechanism of the AuNP
plated beads.
Experimental
Chemicals.— All the chemicals used were of reagent grade. Milli-
Q grade water (>18 Mꢀ cm), after ultraviolet sterilization, was
used throughout the experiment. Chloroauric acid and sodium cit-
rate, which were used to prepare the AuNPs, and decanethiol, which
was used as the binder, were purchased from Wako Pure Chemical
Industries, Ltd., Japan. Polyvinyl alcohol (grade MW 500), used as a
dispersing agent of AuNP-deposited beads, was obtained from Wako
Pure Chemical Industries Ltd., Japan. Acrylic resin beads with a mean
diameter ca. 6 μm were used as a substrate (M-11N, Hayakawa Rub-
ber Co., Ltd., Japan).
Recently, we reported a green electroless plating method for
preparing conducting microbeads using AuNPs, and successfully ap-
plied it to anisotropic conductive films (ACFs), which are adhesive
insulator films in which conductive particles are dispersed.21,22 The
ACFs have been used to construct many local connections between
the input/output connections of a liquid crystal display (LCD) panel
Preparation of AuNPs.— The AuNPs used here were prepared by
the reduction of aqueous HAuCl4 with sodium citrate, as follows. A
24 mL aliquot of 1 wt% HAuCl4 was added to 179 mL of 0.26 wt%
sodium citrate and stirred at 80◦C for 20 min. The AuNPs (0.57 gL−1
)
produced were stored at 5◦C. Characterization using a zeta-potential
& particle size analyzer (ELSZ-2Plus, Otsuka Electronics Co., Ltd.,
Japan) revealed that the AuNPs produced had a mean diameter of
28 4.0 nm.
∗
Electrochemical Society Active Member.
z E-mail: shii@chem.osakafu-u.ac.jp

D690
Journal of The Electrochemical Society, 158 (12) D689-D693 (2011)
series thermogravimetry-differential thermal analysis/mass spectrom-
(a)
Plastic microbeads
Binder
AuNP
etry (TG-DTA/MS) system under 99.9999% helium atmosphere. The
temperature was raised from 25◦C to 700◦C at a programmed heat-
ing rate of 5◦C min−1. Samples dried in a vacuum overnight were
transferred to a platinum (Pt) sample holder to measure the TG/MS
spectra.18,23–26
30 nm
AuNP
Electrical evaluation of Au-plated microbeads.— The electrical
resistance of a single conducting microbead was measured using a
digital multimeter (model 34970A, Agilent Technologies) in a con-
stant current mode (1 mA) using a laboratory-prepared probe setup
1
m
1st step
AuNP-deposition
in an air-conditioned room (25◦C
1◦C).21,22 The setup consisted
of a Pt plate (1 × 1 cm) that served as the common ground, and an
electrochemically polished tungsten (W) tip (5 μm in diameter) that
served as the movable electrode. During the measurements, a single
microbead was sandwiched between the probe tip and the Pt plate at
a compression rate of 20%. The resistance was measured for at least
10 plated beads to obtain an average value.
(b)
AuNP
50 nm
AuNP
Results and Discussion
1
m
Plastic
Plastic
Electrical stability of Au-plated Microbeads.— Figure 1 shows
the time course of the electrical resistance of the AuNP-plated (a) and
commercially available electroless-plated (b) microbeads, which were
kept in an incubator controlled at 85◦C (A) and –50◦C (B). The AuNP-
plated microbeads had an average electric resistance of approximately
0.8 ꢀ (with a variation of < 5%), which remained constant over
300 h at both higher and lower temperatures as shown in Figs. 1Aa and
Ba. In contrast, the electrical resistance of the conventional electroless-
plated microbeads was initially approximately 1.7 ꢀ with a variation
of 0.2 ꢀ ( 12%), and the readings were scattered over 30% during
the monitoring for 300 h (Figs. 1Ab and Bb).
30 nm
2nd step
AuNP-growth
(c)
50 nm
Au
Au
A heat-cycle test was carried out in detail to investigate the electri-
cal properties as well as environmental stability of the AuNP-plated
microbeads. The temperature profile of the test is shown in Fig. 1.
The AuNP-plated microbeads had an average electrical resistance
of approximately 0.8 ꢀ, which remained constant over 300 cycles
(Fig. 1Ca). In contrast, the electrical resistance of the conventional
electroless-plated microbeads was initially 1.7 ꢀ with a variation of
0.2 ꢀ and fluctuated over 0.5 ꢀ during the monitoring for 300
cycles (Fig. 1Cb). The AuNP-plated microbeads were found to have
higher thermal stability in the electrical property than the conventional
electroless-plated microbeads.
The mechanical properties of the AuNP-plated microbeads were
also investigated, as shown in Fig. 2. During the 10 load-unload tests,
a single bead was sandwiched at a compression rate of 40% between
the W tip and Pt plate. The resistance of the AuNP-plated microbeads
was 0.8 ꢀ at the first measurement, and it showed no change dur-
ing the repeated measurements. In contrast, the electrical resistance
of the conventional electroless-plated microbeads was 1.5 ꢀ at the
first measurement, and it increased with each subsequent measure-
ment; finally, the resistance was unmeasurable (over 100 Mꢀ) during
the 10th repeated measurement. As already mentioned, the AuNP-
plated microbeads were plated with a thin gold layer (ca. 50 nm),
(Scheme 1).21,22 The commercially available microbeads used for the
measurements obtained by the conventional electroless plating, had
a 100-nm thick conducting layer that consisted of a double metal-
lic layer, a 50-nm nickel (Ni) base layer and 50-nm gold (Au) top
layer.27–30 Commercially available conductive microbeads have Au/Ni
plated layers of approximately 100 nm thickness, whereas microbeads
subjected to AuNP plating can have a thinner plated layer (ca. 50 nm).
The increase in thickness of the metallic layer decreased the flexibility
of the plated layer on the commercial microbeads leading to a sub-
sidiary fracture and a corresponding broken conducting pathway. The
fracture occurs for two reasons: (1) the increase in the thickness of
the metallic layer and (2) the elastic modulus of nickel (1.8 × 10−10
Pa) being lower than that of gold (2.2 × 10−10 Pa). Therefore, the
electrical conductivity of the conventional electroless Au/Ni-plated
microbeads decreased owing to the subsidiary fracture of the plated
1
m
Plastic
50 nm
Scheme 1. Model illustration of the preparation procedure of a conducting
microbead. SEM images along with cross-sectional TEM images of an un-
coated microbead (a), a AuNP-deposited microbead (b), and a AuNP-plated
microbead (c).
Procedure for AuNP plating of microbeads.— Our plating method
consisted of two processes —-AuNP deposition and AuNP growth on
plastic beads, as shown in Scheme 1.21,22 The –SH group of the binder
molecules adsorbed onto the AuNP surface through chemical adsorp-
tion, whereas the alkyl chains bound with the plastic surface through
a hydrophobic-hydrophobic interaction (a and b). The NPs deposited
on the beads as follows: Microbeads (34 mg) and decanethiol (3 μg)
were added to 22.5 mL of the colloidal gold dispersion, and then the
mixture was stirred at room temperature for 1 day.21 After the beads
were filtered, they were washed with an ample amount of water and
then dried in a vacuum.
The AuNP-deposited beads were dispersed and stirred in an aque-
ous mixture comprising chloroauric acid and a reducing agent to
enlarge the AuNPs adsorbed on the beads by infilling the airspace
(grain boundary) and improving the electrical properties in the follow-
ing manner (AuNP-growth process). AuNP-deposited beads (10 mg)
were added to a 25 mL aqueous solution containing 50% ethanol, 18
nM polyvinyl alcohol, 0.18 mM sodium ascorbate as a reducing agent,
and 0.19 mM chloroauric acid. The mixture was stirred at 50◦C for 1
h. The beads were then filtered, thoroughly washed with water, and
dried in a vacuum (c). The surfaces of the microbeads were imaged
by field-emission scanning electron microscopy (FE-SEM S-4700,
Hitachi, Japan) at an applied voltage of 10 kV.
Thermal properties of Au-plated microbeads.— Thermogravimet-
ric analyses (TGA) were performed using a Bruker AXS 9600

Journal of The Electrochemical Society, 158 (12) D689-D693 (2011)
D691
3
3
2
1
0
A
B
b
a
2
1
b
a
Figure 1. Results of heat test at 85◦C (A) and
–50◦C (B), and heat-cycle test (C) to deter-
mine the electrical resistance of the conduct-
ing beads; AuNP-plated microbead (a) and
commercially available conducting microbead
(b). The inset shows the temperature profile of
the heat-cycle test.
0
3
0
50
100 150 200 250 300
Time / h
0
50
100 150 200 250 300
Time / h
C
Heat-cycle profile
30 min
2
1
0
b
a
85 oC
−50 oC
30 min 1 hour
0
50
100 150 200 250 300
Time / h
layer and disappeared during the measurements. These indicate that
the flexible AuNP-plated layer on the microbeads has high physical
and electrical stabilities than the conventionally plated ones.
TG curves of the uncoated (a), AuNP-deposited (b), and AuNP-
growth (c) microbeads exposed previously to the helium atmosphere
are shown in Fig. 3A. The total weight loss observed at 700◦C was
0
A
70
217
500
41.3%
c
50
450
62.6%
b
load-unload stress cycle
92.7%
a
100
W
load
0
200
400
600
Temperature / oC
bead
100
50
unload
70 oC
B
Pt
28
32
18
0
100
217 oC
450 oC
10
8
50
b
0
100
50
6
0
100
500 oC
100
4
50
0
2
0
a
20
40
60
m/z
80
Figure 3. TG/MS spectra (A) of the uncoated (a), AuNP-deposited (b), and
AuNP-plated (c) microbeads. Temperature was raised from 25◦C to 700◦C at a
programmed heating rate of 5◦C min−1. MS spectra (B) of the AuNP-deposited
microbead were measured at four different temperatures: 70◦C, 217◦C, 450◦C,
and 500◦C.
0
2
4
6
8
10
Cycle number / Times
Figure 2. Results of cyclic loading-unloading tests to measure the electrical
resistance of the conducting beads at 25◦C: AuNP-plated microbead (a) and
commercially available conducting microbead (b).

D692
Journal of The Electrochemical Society, 158 (12) D689-D693 (2011)
A
0
50
m/z 41
78
c
217
450
0.6
0.4
0.2
0
a
70
a
b
b
c
100
0
200
400
600
Temperature / oC
100
B
a
160
m/z 41
m/z 41
50
3.0
2.0
e
d
0
100
b
1.0
0
50
0
0
100
200
Temperature / oC
300
100
c
50
0
Figure 5. MS fragment intensity for the peak at m/z 41. Decanethiol (a),
decanethiol modified AuNP (b), AuNP-deposited microbead (c), uncoated
microbead (d), and decanethiol modified microbead (e). Temperature was
20
40
60
m/z
80
100
raised from 25◦C to 300◦C at a programmed heating rate of 5◦C min−1
.
Figure 4. TG/MS spectra (A) of decanethiol (a), uncoated microbead (b), and
AuNP (c). Temperature was raised from 25◦C to 700◦C at programmed heating
rate of 5◦C min−1. MS spectra (B) were measured at different temperatures:
70◦C (a), 450◦C (b), and 450◦C (c).
cept for the slight weight loss caused by the desorption of atmospheric
gases.
To investigate the composition of the AuNP-plated microbeads,
the intensity of the fragment peak at m/z 41, which is attributable to
the desorption of decanethiol, was plotted as a function of temperature
(see Fig. 5).23,31 A strong peak intensity was observed at 78◦C in the
curve of simple decanethiol (a). The decanethiol-modified AuNP (b)
and AuNP-deposited microbead (c) show a peak intensity at 217◦C,
whereas the decanethiol-modified microbead (e) has a broad peak at
160◦C. The peak intensities are attributed to the desorption of the
binder molecules from the surfaces of the AuNPs (217◦C) and the
microbead (160◦C) has been observed at a higher temperature than
that of simple decanethiol (78◦C). This means that decanethiol forms
a chemical bond and/or undergoes an interaction with the other com-
ponents, as shown in Scheme 2. The AuNP-plating is based on the
binding of the thiol group (–SH) of binder to the surface of AuNP (a)
and the hydrophobic-hydrophobic chemical interaction between the
alkyl chains of the binder and the surface of the microbead (b).22,32–34 It
is well known that the Au–S binding proceeded spontaneously.33,34 On
the other hand, we have reported that an increase in the surface rough-
ness of plastic substrate by adsorption of alkylthiol was confirmed in
the AFM observation.32 This implies that the alkyl chain of binder
adsorbs to the surface of substrate with the hydrophobic-hydrophobic
interaction, because binder and substrate have no functional group for
binding site with each other. In fact, it is hard to conclude the reaction
rates of the binder to Au and microbead, however, the binding force
of Au-S (a) is stronger than that of hydrophobic-hydrophobic inter-
action (b), according to the results by TG/MS analysis. The presence
of a binder between the AuNP and the microbead results in a strong
adhesion between the microbead and the AuNP-plated layer (c). Such
a strong adhesion lends excellent properties such as flexibility, con-
ductivity, and environmental stability to the AuNP-plated layer on the
microbead. Therefore, this method leads to the formation of a superior
92.7% for (a), 62.6% for (b), and 41.3% for (c). The slight weight loss
at 70◦C is attributed to the desorption of atmospheric gases, H2O (m/z
18), N2 (m/z 28), and O2 (m/z 32) as shown in Fig. 3B,18,23–26 these
gases are also included as a background for the entire temperature
region. Some fragment peaks of organic components (m/z 41) have
been observed at 217◦C in the TG curve (c). A large weight loss was
observed at about 450◦C, and many fragment peaks of the main chain
of organic polymers were observed owing to the degradation of the
bead core in all the microbeads. Residual components, accounting for
7.3% of the total components in the Pt holder after the TG analysis
are cold ashes of the uncoated microbeads in Fig. 3Aa. Therefore,
30.1% and 51.4% of gold as the metal component were estimated as
being the residues of the AuNP-deposited (b), and AuNP-growth (c)
microbeads, respectively. The TG measurements have been also car-
ried out for each constituent, i.e., decanethiol (a), uncoated microbead
(b), and AuNP (c), as shown in Fig. 4. The weight loss of decanethiol
was approximately 100% at less than 100◦C as shown in the TG curve
(a). Many fragment peaks attributed to the degradation of the core of
the microbead at 450◦C were observed in curve (b), corresponding to
those observed in Fig. 3. In contrast, slight weight loss was observed
in the case of AuNP (c). The degradation of the passivation layer con-
sisting of an organic citrate covering the AuNPs is negligible owing
to its small weight. Therefore, the weight loss at 217◦C is caused by
decanethiol, which is confirmed by the appearance of the peak at m/z
41 and some other fragments peaks (at m/z 55, 70, and 83) in Fig. 3B.
The weight loss was found to be due to only desorption of decanethiol
as a binder, because the microbead (b) and AuNP (c) did not undergo
any weight loss in the TG curves at the temperatures below 300◦C, ex-

Journal of The Electrochemical Society, 158 (12) D689-D693 (2011)
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Conclusion
The AuNP-plated microbead shows not only good electrical prop-
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tributed to the adhesion between the AuNPs and the microbead via
the binder, which is based on the Au–S bond between the thiol group
of the binder and the AuNPs, and the chemical interaction between
the alkyl chain of the binder and the microbead. Therefore, the pres-
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We thank Mr. Takahiro Fujita and Mr. Atsushi Mihashi for their
assistance in the electrical measurements. We gratefully acknowledge
the financial support provided by the Japan Society for the Promo-
tion of Science (JSPS) via a Grant-in-Aid for Young Scientists (A)
(21680041).