Fabrication and sint..

INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY
Nanotechnology 16 (2005) 779–784 doi:10.1088/0957-4484/16/6/027
Fabrication and sintering effect on the
morphologies and conductivity of nano-Ag
particle films by the spin coating method
Kan-Sen Chou 1 ,Kuo-Cheng Huang and Hsien-Hsuen Lee
Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013,
Taiwan
E-mail: kschou@che.nthu.edu.tw
Received 12 November 2004, in final form 3 March 2005
Published 5 April 2005
Online at stacks.iop.org/Nano/16/779
Abstract
By adjusting the rotating speed and colloidal concentration in the spin
coating of silver nanoparticles onto a glass substrate, the coating density
could be varied from 1.5 × 10 −6 to 1.1 × 10 −4 gcm −2 ,orinotherwords
from a dispersed distribution to multi-layered close packed films. The
coating density was found to be proportional to the volume fraction of silver
colloids and inversely to the rotating speed to the 0.442 power. The
multi-layered films were further investigated for sintering behaviour. Our
results indicated that the film started to exhibit a noticeable sintering effect
at about 100 ◦ C, and at the same time it became electrically conductive. As
the treatment temperature increased further, the film resistivity reached a
minimum value at 250 ◦ Cand finally became insulating again when the
temperature was over 400 ◦ C due to silver coalescence into large particles
andbreaking up of the conductive paths. Smaller silver nanoparticles,
however, would exhibit a similar phenomenon but at lower temperatures.
1. Introduction
In recent years, nanotechnology has risen in importance in
various fields due to the special characteristics of nanomaterials
or nanostructures. For example, metal nanoparticles can
be used as the catalysts to grow nanowires. Transitionmetal
nanoparticles are also useful in fabricating highdensity
magnetic recording media [1]. Besides, nanoparticles
possessing surface plasma resonance can also be used as
biosensors [2]. In the application of nanoparticles, one of
the fundamental issues is how to distribute nanoparticles
uniformly on the substrate with precise control of density [1].
For carbon nanotube growth, nickel nanoparticles are used as
the catalysts and a certain distance is needed to prevent the
shielding effect between these grown carbon nanotubes [3]
while maintaining sufficient density. However, in other
applications such as high-density magnetic recording media,
close packing or even a multi-layered structure is required [4].
There are several methods proposed for dispersing
nanoparticles on a substrate. In many studies, the self-
1 Author to whom any correspondence should be addressed.
assembly technique was employed utilizing the chemical
interaction between nanoparticles and the substrate surface.
For example, organic molecules containing thiol (–SH) or
amine (–NH2) can be adsorbed on the gold surface and
form a well organized self-assembly monolayer on the
substrate [5]. Another popular method is lithography, which
is able to produce a well ordered pattern and accurately
controlled density. However, this method requires a lot
of processing procedures and is relatively complicated and
expensive. Therefore, a simpler and cheaper method by the
traditional spin coating has recently been proposed [1, 4–
7] as an alternative. By this technique, different coating
densities could be achieved by controlling the rotating speed
and colloidal concentration. Nevertheless, in the literature
very few systematic studies on the effects of the processing
(coating) parameters of nanoparticles had been reported. In
this work, due to our ability to obtain well dispersed nano-silver
colloids in high concentrations,thecoating behaviour of nanosilver
particles are therefore investigated over an extensive
range of Ag concentrations and rotating speeds to obtain Ag
coatings having different densities. Subsequently, an empirical
equation was determined to correlate coating density with Ag
0957-4484/05/060779+06$30.00 © 2005 IOP Publishing Ltd Printed in the UK 779

K-S Chou et al
concentration and rotating speed. In addition, the sintering
of coated Ag colloids and its electrical resistivity are also
investigated as a function of heating temperature.
2. Experimental details
2.1. Synthesis of Ag nanoparticles
Ag nanoparticles were synthesized by the chemical reduction
method; silver ion was reduced by formaldehyde under the
presence of protecting agent, polyvinyl pyrrolidone (PVP;
MW = 50 000, Acros Organics, USA) [8, 9]. The silver
colloid was then washed to remove excess protecting agent
and then re-dispersed in de-ionized water to obtain the working
suspension. Details of experimental procedure can be found
elsewhere [8]. The actual silver content in the suspension
was determined by first dissolving it with nitric acid and
then by atomic absorption spectroscopy (AAS; SpectraAA-30,
Varian, USA) after appropriate dilution. Quantities of residual
PVP were estimated from results of thermal gravimetric
analysis and morphology of silver colloids was observed by
transmission electron microscopy (TEM, TECNAI 20, Philips,
The Netherlands).
2.2. Spin coating process
Aglassslide (cut to 3 cm × 3cm,thickness 1–1.2 mm,
FEA company, Taiwan) was first immersed in HNO3 solution
(0.01 N, EP grade, Union Chemical, Taiwan) and then moved
into acetone (EP grade, Union Chemical, Taiwan) in order to
remove the particles and organic contaminants on the surface.
After cleaning, the slide was air-dried at room temperature.
In the coating process, 0.15 ml Ag colloid of a chosen
concentration was placed on the slide and dispersed uniformly
to the whole surface by a tip. Then the slide was rotated by the
spinner (PM-490, Synrex, Taiwan) at different speeds for 30 s.
The coated slide was dried in the air at room temperature. By
adjusting the Ag colloid concentration, the Ag deposition on
theslidecould be controlled from a monolayer well dispersed
state, to monolayer close packing and finally to multi-layered
structures.
2.3. Coating density analysis
Twomethods were utilized to determine the coating density
of silver, namely the AAS method and image analysis of
microscopic photos. For the AAS method, Ag nanoparticles
deposited on the glass slide were dissolved by HNO3 solution
(0.1 N, 10 ml) and subsequently analysed by atomic absorption
spectroscopy (AAS) to determine the Ag quantity. The coating
density could then be calculated by dividing the Ag quantity by
the slide area. This number represented an overall average. As
for the image analysis method, software (Image J, NIH, USA)
was used to determine the number and diameter distribution
of coated Ag nanoparticles within a few selected areas, and
thus coating density could be directly estimated from the SEM
photograph. This number on the other hand represented a very
small local region and was applied to dispersed distributions
only. Results from both methods were compared to confirm
the validity of the data and also to express the uniformity of
the coating.
780
Figure 1. TEM image of silver nanoparticles.
The thickness of multi-layered Ag coating was also
measured by the microscopy (SEM, S-4700I, Hitachi, Japan).
The roughness of the surface was determined by scanning
probe microscopy (SPM, NanoScope E, Digital Instruments,
USA).
2.4. Sintering and resistivity measurements
Ag coated glass slides (as a film of 82 nm thickness), using
Ag suspension at 2.42 vol% (20.7 wt%) and rotating speed of
6000 rpm, were heat-treated in air at various temperatures. The
sample was heated to the desired temperature in 30 min and
kept at that temperature for another 30 min. Then the slide was
naturally cooled in the oven until the temperature was below
50 ◦ C. The surface morphology of the slide was observed
by SEM and the resistivity was estimated by the four-probe
method (C4S-54/5S, Cascade Microtech, USA).
3. Results and discussion
3.1. Ag nanoparticles characteristics
The Ag nanoparticles used for the coating study were shown
in figure 1. The average size was about 46 nm, but it also
contained a few large particles of about 100 nm. The silver
suspension exhibited yellow colour and showed a UV–vis
absorption peak at 410 nm. The residual PVP quantity was
estimated around 0.05 g/g Agforthis batch of sample.
3.2. Coating morphology
Figure 2 shows the Ag deposition from dispersed distribution
to multi-layered close packing obtained by varying Ag colloid
concentration at a constant rotating speed (6000 rpm). At
low Agcolloid concentrations, the nanoparticles were well
dispersed and separated. When the concentration was
increased, the nanoparticles became gradually aggregated in
the micro-scale but still remained roughly uniform on the
macro-scale. Conceivably, under this situation, the liquid film
might break into small droplets during the evaporation stage

Fabrication and sintering effect on the morphologies and conductivity of nano-Ag particle films by the spin coating method
0.15 vol.% 0.52 vol.%
0.70 vol.% 1.33 vol.%
1.85 vol.% 2.42 vol.%
Figure 2. SEM photographs of Ag nanoparticle coating of different concentrations at 6000 rpm for 30 s.
and hence caused the aggregation of particles within that small
region. Finally, when the concentration increased further, the
nano-silver particles packed to a multi-layered structure with
ashiny surface, suggesting a relatively smooth surface. A
representative SPM image is shown in figure 3(a), and the
statistical mean roughness of the surface is about 5.5 nm, while
the maximum height within this area is about 47.5 nm. This
result is consistent with the SEM observations. For a multilayered
coating, its coating density could also be determined
from the thickness data by assuming close packing of particles,
which was in general agreement with results from the AAS
method.
3.3. Parameters affecting the coating density
Next shown in figure 4 is the comparison of results from
both AAS and image analysis for low-coating-density cases,
showingreasonable agreement. It therefore suggested that the
coating obtained in thiswork was uniform. As a result, the
data from the micro-scale (image analysis) agreed well with
those from the macro-scale (AAS method).
Shown in figure 5 are the correlations between coating
density and Ag colloid concentration at different rotating
speeds. In theory, the coating density is proportional to the
Ag colloid concentration and the thickness of the Ag colloid
liquid film during the spinning procedure. As for the film
thickness, it is mainly decided by two factors: liquid viscosity
and rotating speed. In our case, the viscosity did not change
much with silver concentration and hence could be taken as
constant. Therefore, the coating density could be controlled
by varying Ag colloid concentration and rotating speed in our
experiments, as indicated in figure 5.
Because the coating density was mainly affected by the Ag
colloid concentration and the rotating speed, the relationship
781

K-S Chou et al
Figure 3. (a) SPM image of the surface of Ag coating. The mean
roughness of the surface is 5.5 nm. (b) Side-view of Ag coating
(coating condition: Ag concentration 2.42 vol% and rotating speed
6000 rpm). The top view of this coating is shown in figure 2.
Image analysis
1e-5
8e-6
6e-6
4e-6
2e-6
0
Coating density correlation
R 2 = 0.80
0 1e-6 2e-6 3e-6
AAS method
4e-6 5e-6 6e-6
Figure 4. Correlations between AAS method and image analysis for
the determination of coating density (under low-coating-density
conditions).
could be represented as
D = αC β ω γ
where D (g cm −2 )isthecoating density, C is the Ag colloid
concentration (expressed asavolumefraction), and ω (rpm)
is the rotating speed, with α, β and γ being the empirical
constants. Since our results in figure 5 exhibited a linear
782
(a)
(b)
(1)
Coating density (g/cm 2 )
1.4e-4
1.2e-4
1.0e-4
8.0e-5
6.0e-5
4.0e-5
2.0e-5
0.0
-2.0e-5
6000 rpm R 2 = 0.98
4500 rpm R 2 = 0.99
3000 rpm R 2 = 0.99
1500 rpm R 2 = 0.99
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Ag colloid concentration (vol.%)
Figure 5. Coating density as a function of Ag colloid concentration
at different rotating speeds.
relationship to Ag concentration, and hence β was taken as
unity. The final result was as follows (with R 2 = 0.97):
D = 0.1265 × Cω −0.442 . (2)
From spin-coating theory, if not considering the effect
of evaporation, the thickness of liquid film is inversely
proportional to the rotating speed, i.e., γ =−1[10]. However,
when the evaporation is considered, the value of γ would be
close to −0.5 depending on the volatility and concentration
of solvent [11]. In this study, γ was approximately −0.44,
suggesting that the evaporation must be important here.
3.4. Sintering of the coated Ag nanoparticles
The coated Ag nanoparticles (with a thickness of about 82 nm)
were heated in an oven at different temperatures, and the
resulting morphologies are shown in figure 6. It can be noted
that Ag nanoparticles began to exhibit the sintering effect at
100 ◦ Corprobably even below this temperature, because the
deposited particles started to exhibit conductivity after 70 ◦ C
treatment. As the temperature increased, the Ag nanoparticles
became coalesced together into a porous film due to the high
mobility of silver atoms in these nanoparticles. When the
temperature increased more, the voids of the porous film
grew, and eventually, above 400 ◦ C, the silver film shrank
into numerous large Ag particles and the conducting path was
broken. Corresponding data for electrical resistivity are shown
in figure 7.
Before the heat treatment, the resistivity of deposited
Ag particles was extremely large and could be considered
as an insulated material. Presumably, at this stage, the Ag
particles were still covered with a layer of the protective agent
(PVP molecules) and were barely in contact with each other.
When the heating temperature was 70 ◦ C, which might be
sufficient for PVP molecules to move away from the particle
surface to coalescence together and separate from the particles,
the silver particles then touched each other and conducting
electricity. Further heating would enlarge the contact area
between particles and thus lower the resistivity. Due to the
high activity of nanoparticles, the film resistivity reached a
minimum value of 2.4 × 10 −5 cm after 250 ◦ Ctreatment as
sintering progressed. Further increase of heating temperature

Fabrication and sintering effect on the morphologies and conductivity of nano-Ag particle films by the spin coating method
(a) 100 o C (b) 200 o C
(c) 250 o C (d) 300 o C
(e) 350 o C (f) 400 o C
Figure 6. SEM morphologies after heat treatment at different temperatures for 30 min: (a) 100 ◦ C; (b) 200 ◦ C; (c) 250 ◦ C; (d) 300 ◦ C;
(e) 350 ◦ C; (f) 400 ◦ C.
would continue the densification process of sintering by
decreasing the surface area and thus increasing particle size.
At the same time, the voids within the film also combined
into large pores. Consequently, the conductive silver networks
were gradually broken, and the resistivity started to increase.
At last, all the silver particles sintered into larger and separated
ones, and the film became insulating again.
Also included in figure 7 are the results from another batch
of nano-silver particles for comparison. The average size of
thesecond-batch particles is about 37 nm (versus 46 nm of the
first batch); besides, the distribution is narrower in the second
batch. As a result, the film reached a minimum resistivity after
200 ◦ Cheat treatment and lost conductivity after 300 ◦ C. Both
temperatures were lower than the previous case, indicating the
size effect of these silver particles.
In view of the very low resistivities developed in these
films and easy handling of the suspension, there is a good
chance to successfully apply the inkjet printing technique to
these silver nanoparticles for the manufacture of conducting
lines. Also, the low-temperature treatment would enable
the use of alternative polymeric substrates. However, more
research is needed for their development.
4. Conclusions
In this work, systematic studies on the effect of Ag colloid
concentration and the rotating speed on the coating density
were made. It varied from a dispersed distribution to a
crowded distribution and then to multi-layered close packing
structures. An empirical correlation was established as D =
0.1265 Cω −0.442 .Onecould also estimate particle-to-particle
distance utilizing this equation for dispersed distribution
situations. By heating an 82 nm coated film to above 70 ◦ C,
it started to exhibit electrical conductivity and reached a
783

K-S Chou et al
Resistivity (Ω-cm)
2.8e-5
2.6e-5
2.4e-5
2.2e-5
2.0e-5
1.8e-5
1.6e-5
1.4e-5
1.2e-5
1.0e-5
8.0e-6
46.2 nm
36.8 nm
Temperature ( o 6.0e-6
50 100 150 200 250 300 350 400
C)
Figure 7. Electrical resistivity of the coated Ag films after heat
treatment at different temperatures (two different batches of silver
nanoparticles).
minimum value of 2.4 × 10 −5 cm after 250 ◦ C, 30 min
treatment. This film, however, would lose conductivity if the
treatment temperature were over 400 ◦ C, due to the growth of
both silver particles and pores and the eventual breaking up
of conducting lines. A smaller size and narrower distribution
of the starting colloids would shift these temperatures to even
lower values.
784
Acknowledgment
The authors wish to thank Chun-Shan Institute of Science
and Technology for financial support via project number
BD93020P.
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