Chiang Mai J. Sci. 2013; 40(6)
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Chiang Mai J. Sci. 2013; 40(6) : 985-993
http://epg.science.cmu.ac.th/ejournal/
Contributed Paper
Synthesis and Physical Properties of
Semi-Transparent Conductive Ag-Nanowire Network
Tula Jutarosaga*[a,b], Panita Chityuttakan [a,b], Wandee Onreabroy [a,b] and
Anuwat Hassadee [a]
[a] Department of Physics, Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok
10140, Thailand.
[b] Thin Film Technology Research Laboratory, ThEP Center, CHE, 328 Si Ayutthaya Rd., Ratchathewi,
Bangkok 10400, Thailand.
*Author for correspondence; e-mail: [email protected]
Received: 11 April 2012
Accepted: 8 August 2012
ABSTRACT
A synthesis method of semi-transparent conducting Ag-nanowire network on glass
substrates at room temperature using a simple oxidation-reduction reaction of Cu nanoparticles
and 0.1M AgNO3 solution was presented. The morphological, structural, electrical and optical
properties of the synthesized Ag nanowire networks were characterized using scanning electron
microscopy, X-ray diffraction, 4-point probe technique and UV-Vis spectrophotometry.
The synthesized Ag nanowires had FCC structure. The Ag-nanowire network exhibited the
semi-transparency up to 36% with the sheet resistance of about 104 Ω/sq. It was suggested
that the morphology of the Cu thin film may play an important role in controlling the Ag
nanowire density and morphology.
Keywords: Ag nanowire network, transparent conductive thin film, oxidation-reduction reaction
1. INTRODUCTION
Transparent conductive thin films have
been widely used in various applications such
as components in electronics displays or
photovoltaic cells. Indium tin oxide (ITO)
thin films, so far, are probably the most
conventional transparent conductive oxide
thin films to use in such applications because
their high transparency and the high electrical
conductivity. Other optional candidates to
do the jobs are being investigated, such as
graphene [1], other oxide thin films for
example doped-ZnO thin film [2], or
carbon-nanotube thin films [3]. Recently,
nanowire thin films have shown their
acceptable conductivity and transparency to
compete with the conventional ITO thin films.
In some cases, their cost effectiveness was
above the indium tin oxide films which the
materials and the required vacuum process are
expensive.
Ag nanostructure materials are attracted
to many researchers due to their unique
properties such as the optical, electrical
properties [4] and the antibacterial purpose
[5]. Especially, Ag nanowires show the
promising application for transparent
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conductive thin film. Various methods for
coating nanowires on the substrates were
presented such as roll-coating [6] and airspraying [7]. Liu and Yu showed that the
obtained Ag nanowire thin film using the rollcoating method provided the optical
transmission spectrum in the visible light
region (400nm - 700nm) approximately 74%
with the resistivity of 170 Ω/sq [6]. However,
as indicated, most reported techniques were
methods for transferring nanowires onto the
substrates. Groep et al. [8] showed the 2D
Ag nanowire network fabricated using
electron beam lithography with the nanowire
width varied from 45 nm to 100 nm. It was
found that the transmission dropped as Ag
nanowire width in the network increased.
One of the main causes of the drop was
attributed to the excitation of the localized
surface plasmon resonance (LSPR) of each
individual nanowire [8] as the nanowire size
increased. Therefore, in our case, we were
interested in fabricating self-assemble nanowire
network for the application of the transparent
conductive thin film in a single simple step
and being able to control the sizes of the
synthesized nanowires.
The reaction of Cu and AgNO3 is wellknown. However, not many research groups
have focused on direct synthesis of Ag
nanowire on transparent substrate. In our case,
the chosen reaction was an oxidation-reduction
reaction of Cu and AgNO3.
Cu(s) + 2AgNO3(aq)→2Ag(s) + Cu(NO3)2
(aq)
(1)
Ag acted as an oxidizing agent, causing the
copper to loose electrons. Cu ions displaced
Ag in the AgNO3, producing an aqueous
solution of Cu(NO3)2. The reduction reaction
happened when Ag ions gained electrons
from Cu and precipitated out as solid metal.
Chiang Mai J. Sci. 2013; 40(6)
By controlling the size and the arrangement
of Cu particles, we expected to control the
size and possibly control the density of Ag
nanowires.
MATERIALS AND METHODS
As indicated in the previous section, the
presence of Cu was important for the growth
of Ag crystal. It was expected that the size of
the seed particles may control the diameter
of the synthesized Ag crystal. Thin deposition
of Cu on substrate was then necessary to
obtain the particle sizes in the nanometer
range. Also, the film thickness could be easily
confirmed by the optical method. Therefore,
the chosen thin Cu films were then prepared
on glass substrates by a conventional thermal
evaporation. Two different evaporation
conditions provided two different thicknesses
with the difference in morphology. The
estimated thicknesses were about 35 nm and
45 nm. The 2.5 × 2.5 cm2 Cu films on glasses
were then soaked into 10 mL of 0.1M Ag
nitrate (AgNO3) solution at room temperature
at various times. Only data with soaking
times of 1-2 minutes and 15 hours were
presented. The films were then rinsed
with deionized water and dried. The
morphological, structural, electrical and optical
properties were then examined using a field
emission scanning electron microscope
(Hitachi-S4700), X-ray diffractometer, 4-point
probe (Signatone Pro4 S-302-4) and UV-Vis
spectrophotometer (Avantes AvaSpec). For
X-ray diffraction, the 2° glazing angle was
conducted using a Bruker AXS D8 Discover
using Cu Kα with the scan step of 0.05°/s.
The crystalline sizes (L) of Cu seed particles
and synthesized Ag-nanowire network were
then obtained using the following Scherrer
formula where B,the peak width, is a function
of θ ; K is the Scherrer constant; and λ is the
wavelength of Cu Kα (0.15 nm).
2.
Chiang Mai J. Sci. 2013; 40(6)
B
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(2)
The electrical properties of the films were
reported in term of sheet resistance. When
the film became thinner, the resistivity is a
strong function with film thickness. The sheet
resistance has the same unit as resistance.
However, it is specified as ohm/square
(Ω/sq) not to confuse with the resistance.
RESULTS AND DISCUSSION
Figure 1(a, b) show the scanning electron
micrographs of as-deposited 35-nm and
45-nm Cu thin film on glass substrate using
two different evaporation conditions of Cu
metal. Based on the simple observation, the
3.
35-nm Cu film showed the island-like
structure. Most particles are isolated. However,
in the case of 45-nm Cu film, the smaller Cu
particles were observed. We expected that the
Cu particles impinged and subsequently
formed continuous thin film. This would be
later confirmed by the electrical measurement
which we found that there was electrical
conductivity in the 45-nm Cu thin film, but
not in the 35-nm. The different in the Cu seed
sizes was possibly a result of the evaporation
conditions. However, we would not focus on
how to obtain this different morphology of
Cu thin film. We were more interested in
the effect of size of the Cu particles on
the morphological and structural of the
synthesized materials.
Figure 1. Scanning electron micrographs of (a) 35-nm Cu/glass and (b) 45-nm Cu/glass.
Therefore, the simple calculation based
on the correlation length concept [9] was
applied on the scan profile of the SEM
micrographs, instead of using AFM profile
image, in order to extract the correlation
length. As shown in figure 2(a) and 2(b),
the scan profiles were obtained from
SEM micrographs using Image J [10].
The roughness of the film was not directly
measured from the images. However,
the profile implied that the roughness of
35-nm Cu/glass was higher than 45-nm
Cu/glass. Note that all images were taken
at the similar depth of field setting. Our
analysis showed that the correlation length
in 35-nm Cu film and 45-nm Cu were
approximately 50 nm. The correlation
length is relatively similar to particle size of
the observed SEM images. However, in the
case of 35-nm Cu film, the correlation length
should be relatively larger than that of 45-nm
Cu, but the analysis showed a very similar
correlation length because, when looked
closely at the image, the Cu islands in
figure 1(a) consisted of smaller Cu particles.
Without the calculation, the observed
particle size might be misleading. The obtained
correlation length corresponded to the
observed particle size in the SEM
micrographs.
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Chiang Mai J. Sci. 2013; 40(6)
Figure 2. Scan profiles of the (a) 35-nm Cu/glass and (b) 45-nm Cu/glass.
Figure 3(a, b) show the synthesized
materials after soaking in 0.1M AgNO3
solution for 1 minute. The synthesized
structures in both cases were wire-like
structure. The density of the wire from
35-nm Cu/glass starting materials was
higher than that of 45-nm Cu/glass.
Also, figure 3(a) clearly show that the
observed wire formed network, while in
figure 3(b) the lower density wire with
sparse needle-like structures was observed
on the glass substrate. It is suggested that
the morphology of Cu nanoparticles
probably strongly affected the synthesized
products. The nanowire diameter of figure
3(a) was varied from approximately 40 nm
to about 450 nm with the average of 140 nm
± 80 nm.
Figure 3. Scanning electron micrographs of (a) wire-like nanostructure from 35-nm Cu/
glass and (b) that from 45-nm Cu/glass after soaking in 0.1M AgNO3 solution for 1 minute.
Figure 4(a, b) show the X-ray diffraction
patterns of as-deposited 35-nm and 45-nm
Cu thin films. The figure confirmed that
as-deposited films are Cu. The films are
face center cubic structure of Cu (JCPDS
file 04-0836). As expected, the high intense
peak for FCC, (111) plane, are observed in
both cases. In the case of FCC Cu, (111) plane
is at 43.297. Also, the thicker films showed
the higher intensity than the thinner one and
the (200) and (220) planes are presented
in the diffraction pattern of 45-nm Cu
thin film. Figure 4(c, d) show the X-ray
diffraction pattern of as-synthesized Ag
nanostructure from 35-nm and 45-nm Cu
thin films, respectively. Overall, the spectra
indicated the synthesized materials are FCC
Ag in both cases corresponding to the
JCPDS file no 89-3722. It is interesting to
point out that the (111) plane intensity of the
Chiang Mai J. Sci. 2013; 40(6)
figure 4(c) is stronger than that of figure 4(d)
while other peak heights are relatively similar.
This observation indicated that the Cu seed
thickness caused the preferred orientation
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of the synthesized materials. In addition,
in both as-deposited Cu thin film and
Ag-nanowire network, oxide had not
been observed.
Figure 4. X-ray diffraction spectra of (a) 35-nm and (b) 45-nm Cu thin film/glass substrates
and (c, d) Ag nanowires after the reaction between Ag nitrate and 35-nm and 45-nm Cu,
respectively.
Table 1 shows the calculated crystalline
size of Cu thin film and Ag-nanowire
network using Scherrer formula. For the Cu
thin film, the crystalline sizes are approximately
14 and 15 nm for 35-nm and 45-nm Cu thin
films, respectively. As shown in the table, after
processing 35-nm and 45-nm Cu thin films,
the crystalline sizes of the synthesized Ag
nanowires are approximately 24 nm and 22
nm, respectively. From equation 1, the reaction
of Cu(s) and AgNO 3 (aq) caused the
precipitation of Ag(s) and Cu(NO3)2(aq). At
the initial stage, the Ag crystalline size would
be controlled by the Cu crystalline size.
However, the crystal grew in both diameter
and length directions when the process
proceeded. The growth caused the increase
of the crystalline size in case of figure 3(a).
However, we observed less nanowire in figure
3(b) possibly because of the smaller size of
the starting Cu seeds and also possibly the
adhesion of the Cu film to the substrate. Unlike
the 35-nm Cu, 45-nm Cu films may compose
of layers of smaller Cu particle size. The
precipitate Ag particles possibly went from
the substrate back to the solution as colloid
during the reaction. Therefore, mostly needlelike Ag was observed as shown in figure 3(b).
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Chiang Mai J. Sci. 2013; 40(6)
Table 1. The crystalline sizess of Cu particles and Ag nanowires calculated from the X-ray
diffraction pattern using Scherrer formula.
Film condition
As-deposited 35-nm Cu/glass
As-deposited 45-nm Cu/glass
35-nm Cu/glass soaked in AgNO3 for 1 min
45-nm Cu/glass soaked in AgNO3 for 1 min
Table 2 shows the sheet resistance of the
as-deposited Cu/glass and Ag-nanowire thin
film. From the table, 35-nm as-deposited Cu/
glass film showed no electrical conductivity
which was possibly a result of the isolation
of Cu islands (figure 1(a)). In contrast, the sheet
resistance of 45-nm Cu/glass film was about
2.8±0.3 Ω/sq, indicated that the film are
continuous. After soaking in AgNO3 solution
for 1 min and 15 hours, the sheet resistance
changed. For the 35-nm Cu/glass substrate,
the sheet resistance became detectable.
As shown in figure 3(a), the connection among
Crystalline size (nm)
Ag
Cu
14
15
24
22
-
nanowires provided electrical conduction
paths. However, in the case of a sample from
45-nm Cu/glass, the sheet resistance could not
be measured. From figure 3(b), lower density
of the synthesized nanowires was observed
which provided no electrical conduction paths.
However, after soaking the solution for
15 hours, the sheet resistance could be
measured in both cases. The values are
approximately 38.5 and 104.2 Ω /sq,
respectively. It could be inferred that the
conduction paths were created after soaking
the sample in AgNO3 solution long enough.
Table 2. The sheet resistance of the as-deposited Cu/glass and Ag-nanowire network on
glass.
Film condition
As-deposited Cu/glass
Soaked in AgNO3 for 1 min
Soaked in AgNO3 for 15 hours
The resistivity of bulk Cu and bulk
Ag metal at 20°C is 1.69×10-6 Ω⋅cm and
1.59×10-6 Ω⋅cm [11], respectively. Based on
the values and the sheet resistance in table 2,
the calculated equivalent thickness of Cu thin
film with the sheet resistance of 2.8 Ω/sq
was 6 nm, while those of Ag thin films are
0.6 nm, 0.4 nm and 0.15 nm for the sheet
resistance of 25.5 Ω/sq, 38.5 Ω/sq and
104.2 Ω/sq, respectively. The equivalent
thickness of Cu thin film was a lot less
than our estimation from the evaporation
Sheet resistance (Ω/sq)
45 nm
35 nm
Cu/glass
Cu/glass
2.8 ± 0.3
NA
NA
25.5 ± 3.5
104.2 ± 19.0
38.5 ± 7.6
because the film was not completely
continuous. The observed high resistivity of
the film was normal for thin metallic film
with this similar structure [12]. Also,
the equivalent Ag thickness was in the sub
nanometer range possibly indicating that
the quality of the nanowire network was
inferior than the continuous film due to the
problem of nanowire contacts causing the
inferior electron transport. The further
improvement was then needed to be
investigated.
Chiang Mai J. Sci. 2013; 40(6)
Beside the electrical properties, in order
to use this network as transparent electrodes,
we were interested in the optical transmission.
The films were analyzed using UV-Vis
spectrophotometer to obtain the relationship
between the percent transmission (%T) and
the wavelength. The transmittance data were
taken and compared to the glass substrate.
Figure 5(a) and 5(b) show the optical
transmission spectra of 35-nm Cu/glass film
and their products as well as 45-nm Cu/glass
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film and their products, respectively. The
transmission of Cu thin film in figure 5(a) was
higher than that of figure 5(b) due to the
thickness effect. In both figures, the
transmittance (%T) of the Cu thin films
showed the similar characteristics. The percent
transmission rised as the incident wavelength
increased and then peaked at 580 nm,
following by the decrease of the transmission.
At the wavelength below 580 nm is where
the interband transition took place [13].
Figure 5. The optical transmission spectra of (a) 35-nm Cu/glass film and their products as
well as (b) 45-nm Cu/glass film and their products.
After the reaction, 35-nm Cu thin film
converted to Ag nanowires. The overall
transmission dropped. Due to the interband
absorption edge of Ag is at 4 eV, the
transmission increased and peaked at
320 nm. Then, the transmission decreased
sharply again as a result of the high reflectivity
of Ag in the visible wavelength [14]. In case
of 45-nm Cu thin film, after converting to
Ag nanowires, the transmission spectra
showed the similar characteristics to the
Ag nanowires from 35-nm Cu thin film.
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Chiang Mai J. Sci. 2013; 40(6)
However, the overall percent transmission
was higher due to the lack of the presence of
nanowire networks.
Table 3 shows the average percentage of
the transmission of the Cu thin film and Agnanowire films in the visible region (400 nm
to 700 nm). As expected thicker Cu films
transmitted lower percentage than the
thinner one. The 35-nm Cu film transmitted
approximately 41%, while the 45-nm Cu
film transmitted approximately 25%. In case
of 35-nm Cu film, after soaking in AgNO3
solution for 1 minute and 15 hours,
the transmittance reduced to 28% and
27%, respectively. The reduction of the
transmittance was possibly a result of the
formation nanowire network. In contrast,
the transmittance of samples for 45-nm Cu
seed increased from 25% to 46% after soaking
in AgNO3 solution for 1 minute. The increases
of the percent transmission corresponded
to the SEM micrographs in figure 3(b).
After soaking for 15 hours, the percent
transmission then reduced to 36%. We
started to see the shiny materials on the
substrate. Further investigation on the
effect of the particle size, the reaction
temperature, the addition of other additive
and the concentration of the reactants
are necessary to be studied in order to obtain
the high quality transparent Ag-nanowire
thin film.
Table 3. Average percent transmission (400 nm to 700 nm) of the as-deposited Cu/glass and
Ag-nanowire thin film.
Film condition
As-deposited Cu/glass
Soaked in AgNO3 for 1 min
Soaked in AgNO3 for 15 hours
Transmittance (%)
45 nm
35 nm
Cu/glass
Cu/glass
25
41
46
28
36
27
CONCLUSION
The Ag-nanowire thin films were
fabricated using a simple oxidation-reduction
reaction of Cu seed and AgNO3 solution at
room temperature. The preliminary semitransparent conducting nanowire films were
fabricated with the percent transmission of
36% and the sheet resistance of 104 Ω/sq.
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ACKNOWLEDGEMENTS
The work has been supported in part by
the funding from the Faculty of Science,
King Mongkut’s University of Technology
Thonburi. We would like to thank Dr. Chanwit
Chityuttakan for providing the Cu thin films.
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