Novel method for room temperature sintering of Ag nanoparticle paste in air

Article abstract of DOI:10.1016/j.cplett.2007.05.033

We successfully developed a new method of sintering Ag nanoparticles protected by dispersant in an air atmosphere without heating. Ag nanoparticles on glass substrates were dipped in methanol for 10-7200 s to remove the dodecylamine dispersant. After 7200 s dipping, the removal of the dispersant became clear by surface analysis and Ag nanoparticles were densely sintered. The sintered Ag wire had excellent low resistivity of 7.3 × 10^-7 Ωm. Microstructural observations revealed that the Ag nanoparticles had agglomerated and coarsened with increased dipping time. Clear connections were found among the grown particles.

Full text of DOI:10.1016/j.cplett.2007.05.033

Chemical Physics Letters 441 (2007) 305–308
www.elsevier.com/locate/cplett
Novel method for room temperature sintering of Ag
nanoparticle paste in air
*
Daisuke Wakuda , Mariko Hatamura, Katsuaki Suganuma
The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Received 26 January 2007; in final form 8 May 2007
Available online 16 May 2007
Abstract
We successfully developed a new method of sintering Ag nanoparticles protected by dispersant in an air atmosphere without heating.
Ag nanoparticles on glass substrates were dipped in methanol for 10–7200 s to remove the dodecylamine dispersant. After 7200 s dip-
ping, the removal of the dispersant became clear by surface analysis and Ag nanoparticles were densely sintered. The sintered Ag wire
had excellent low resistivity of 7.3 · 10À7 Xm. Microstructural observations revealed that the Ag nanoparticles had agglomerated and
coarsened with increased dipping time. Clear connections were found among the grown particles.
ꢀ 2007 Elsevier B.V. All rights reserved.
1. Introduction
of excellent low resistivity, for example 10À8 Xm, can be
obtained. In the case of the most promising Ag nanoparticle
pastes, heating beyond 150 ꢁC is required to remove the dis-
persants from the Ag nanoparticles and to vaporize them
[2–5]. These heating treatments mean that the unique phe-
nomenon presented by Ag nanoparticles is not exploited
as much as it could be. The only reason that Ag nanoparti-
cles are unable to be sintered at low temperatures is due to
the presence of the dispersants and solvents. Furthermore, a
heating temperature beyond 150 ꢁC is too high for many
organic devices and printed circuit boards, which are gener-
ally unstable at these temperatures.
In the current study, we developed a novel room temper-
ature chemical method that removes the dispersant material
covering the Ag nanoparticles without requiring heating or
vacuum. The microstructural development and electrical
characteristics of the resulting wired circuits were examined.
We expect that the chemical method we present here for
Ag nanoparticle pastes will become a standard method
for nanoparticle wiring.
Recently, the sintering temperature descent phenomenon
of metallic nanoparticles has attracted a great deal of atten-
tion. Iwama et al. reported the room temperature sintering
of ultrafine powders of Au, Ag, Al and Cu in high vacuum
[1]. Thus, many metallic nanoparticles can be sintered, even
at room temperature, when their surface is clean and with-
out any oxide layer or protective organic dispersants. From
a practical point of view, the application of the sintering
temperature descent phenomenon to the wiring of circuits
combined with certain kinds of printing methods, such as
by ink-jet, has been studied. For excellent printability, a
suitable paste of the metallic nanoparticles is required. This
means that the nanoparticles must be covered by organic
dispersants to avoid aggregation and must be able to dis-
perse in solvents. Heating has always been required after
wiring metallic nanoparticle pastes to remove the organic
dispersants and solvents. When metallic nanoparticle pastes
are heated at 150–300 ꢁC, the dispersants can be effectively
removed, and the remaining active metallic nanoparticles
can then be successfully sintered. As a result, metal circuits
2. Experimental
*
Ag nitrate (AgNO3), 0.13 mol, and dodecylamine
(NH2C12H25), 0.26 mol, were dissolved in acetonitrile
Corresponding author. Fax: +81 6 6879 8522.
E-mail address: wakuda@eco.sanken.osaka-u.ac.jp (D. Wakuda).
0009-2614/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.05.033

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D. Wakuda et al. / Chemical Physics Letters 441 (2007) 305–308
(CH3CN), 7.0 · 10À4 m3, and the solution stirred at room
temperature for 10.8 · 103 s. The resulting white precipi-
tate was filtered from solution to leave an Ag complex
([Ag(NH2C12H25)2]NO3), 0.13 mol, after drying [6]. A mix-
ture of 20.0 g of the Ag complex, 30.0 g dodecylamine, and
5.0 · 10À7 m3 ethanol was mixed together and heated while
being stirred. The mixture surface took on a metallic blue
color at temperatures above 170 ꢁC. Next, the complex
was heated for 900 s with the temperature maintained
between 190 ꢁC and 200 ꢁC, and then the mixture was
allowed to cool naturally for 1200 s. The mixture was
washed with acetone (CH3COCH3) three times to remove
impurities, leaving a blue powder and a brown supernatant
solution. After the third washing, the supernatant solution
became colorless and transparent. The supernatant solu-
tion was removed and toluene (C6H5CH3), 2.0 · 10À5 m3
was added to the sediment. The sediment dispersed in tol-
uene and the solution took on a metallic blue color. The
solution was stirred for 600 s at room temperature and
was then filtered with glass fiber filter to remove particu-
late matter over 0.3 lm. A minute quantity of blue powder,
0.938 g, was present after vacuum concentration. Finally,
an Ag nanoparticle paste was produced by dispersing the
0.938 g of blue powder into 3.6 · 10À6 m3 (3.16 g) of
toluene.
sured 10 times for each sample, and the averages and stan-
dard deviations were plotted on a graph. For TEM
samples, the Ag nanoparticle paste diluted with toluene
was dropwise deposited onto Cu grids of ultrathin amor-
phous carbon films. The grids were then dipped in a meth-
anol bath for set periods of time and after natural drying
for 7200 s, observations were performed by transmis-
sion electron microscopy (TEM, JEOL LTD., JEOL-
JEM3000F) operated at 300 kV. To confirm the removal
of the dispersant, surface analysis was conducted. Sample
disks, 9 mm in diameter, were coated with Ag nanoparticle
paste using a spin coater at 600 rpm. The disks were then
dipped in methanol for 7200 s and analyzed by X-ray pho-
toelectron spectroscopy (XPS, VG Scientific, Microlab
mark III).
3. Results and discussion
Fig. 1 shows the microstructural changes of Ag nano-
particles as a result of methanol dipping. As shown in
Fig. 1a, which is the initial state of the paste, Ag nanopar-
ticles of average diameter 7 nm were observed packed in a
highly dense structure. After 3600 s dipping, as shown in
Fig. 1b, the Ag nanoparticles completely disappeared and
a high density Ag structure appeared.
The fabricated Ag nanoparticle paste was printed onto
glass substrates to give a circuit 50 mm in length, 300 lm
in width and 0.2 lm in thickness, as determined by laser
microscopy (Keyence Co. VK-9500) and field emission
scanning electron microscopy (FE-SEM, Hitachi High-
Tech. Co. S-5000). The wired glass substrates were dipped
in methanol for sintering at room temperature (23 ꢁC). To
study the influence of methanol, the dipping time was var-
ied from 10 s to 7200 s. After dipping, the substrates were
removed from the methanol bath and dried naturally for
7200 s at room temperature. Electrical resistivity measure-
ments were conducted using a four probe method, and the
microstructural observation was carried out by FE-SEM
after the drying process. The electrical resistivity was mea-
To clarify the microstructural changes after dipping,
TEM observations were carried out as shown in Fig. 2.
In the initial state, each particle can be clearly seen as being
independent from the others because of the presence of
dodecylamine as a dispersant, as shown in Fig. 2a. With
the passage of dipping time, the Ag nanoparticles grew,
as shown in Fig. 2b. Moreover, the coarsened particles
became connected with each other by forming metallic
bonds. It was surprising that the coalescing of the Ag nano-
particles started after only 30 s of dipping.
The results of the surface analysis study are shown in
Figs. 3 and 4. Comparing the surface spectra of the initial
sample and dipped sample, the intensity was normalized by
the peak intensity of Ag3d . Fig. 3 shows the spectrum of
5=2
Fig. 1. Microstructural changes in an Ag nanoparticle paste as observed by FE-SEM: (a) initial state, and (b) after dipping in methanol for 3600 s.

D. Wakuda et al. / Chemical Physics Letters 441 (2007) 305–308
307
Fig. 2. Microstructural changes in an Ag nanoparticle paste as observed by TEM: (a) initial state, and (b) coalescing of nanoparticles that starts after 30 s
of methanol dipping.
peak intensity of C1s, which comes from hydrocarbons
(contamination) and the alkyl group, clearly decreases after
methanol dipping. This decrease is attributed to the
removal of the alkyl group of dodecylamine, which is fur-
ther supported by the disappearance of the N1s peak.
Although there is a small peak after methanol dipping,
the peak can be thought of as contamination in the cham-
ber. These surface analyses prove that the dispersant was
effectively removed by our methanol dipping method.
Fig. 5 shows the change of electrical resistivity as a func-
tion of dipping time, which is expected to fall with dipping
time. The circuit obtained from this experiment possesses a
quite low resistivity, in the order of 10À6 Xm, after only
300 s of dipping. This decrease in resistivity corresponds
to the microstructural observations, which shows that the
Ag nanoparticle growth occurs in just a short space of time.
Fig. 3. Nitrogen (N1s) spectrum. The N1s peak disappears after methanol
dipping for 7200 s.
After 7200 s of dipping, the circuit finally acquired an
excellent resistivity of 7.3 · 10À7 Xm. This resistivity is
N1s. The chemical shift analysis reveals that the observed
peak is attributed to the amine moiety. From Fig. 3, the
peak of N1s can be seen to have disappeared after methanol
dipping for 7200 s. Fig. 4 shows the spectrum of C1s. The
nearly equal to the electrical resistivity, 5.5 · 10À7 Xm,
obtained by the same paste after heat treatment at 150 ꢁC
for 1800 s.
Fig. 4. Carbon (C1s) spectrum. The C1s peak intensity decreases after
methanol dipping.
Fig. 5. Electrical resistivity changes as a function of dipping time.

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D. Wakuda et al. / Chemical Physics Letters 441 (2007) 305–308
In summary, the above experimental results demon-
ticles without heating or vacuum. This method could
become a standard procedure for wiring Ag nanoparticle
pastes as it is both economic and rapid, while also being
environmentally friendly.
strate the mechanism of room temperature sintering of
Ag nanoparticle pastes. The initial Ag nanoparticles are
protected by dodecylamine in the paste. While there is still
debate about the mechanism of the bonding between Ag
nanoparticles and a dispersant [7,8], the nature of the
bonding seems relatively weak compared to strong chemi-
cal bonding such as covalent bonding. Dodecylamine mol-
ecules are found to dissolve easily during dipping in
methanol, a solvent which has high affinity with dodecyl-
amine. Generally, there is no chemical reaction between
dodecylamine and methanol at room temperature under a
regular atmospheric pressure. Once Ag nanoparticles are
freed from the dispersant, the bare Ag nanoparticles, which
possess active surfaces, can be sintered even at room tem-
perature. Finally, once the Ag nanoparticles are sintered,
the electrical resistance is observed to decrease.
Acknowledgements
This work was performed with the support of a Grant-
in-Aid for Science Research (A) of the Japan Society for
the Promotion of Science. The authors thank Dr. M. Inoue
and Dr. K.-S. Kim for constructive comments and
acknowledge the assistance of Mr. Takeshi Ishibashi for
the TEM observations.
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4. Conclusions
We demonstrated the successful development of a novel
sintering method for Ag nanoparticle pastes that is carried
out at room temperature in an air atmosphere. As the dip-
ping time increases, the Ag particles grow and become con-
nected with each other through the formation of metallic
bonds. The coarsening of the Ag nanoparticles was
observed after only 30 s. The fabricated circuits obtained
an excellent resistivity of 7.3 · 10À7 Xm after 7200 s dip-
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the dispersant and allows for the sintering of Ag nanopar-
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