Applied Surface Science Synthesis

Applied Surface Science 258 (2012) 7384–7388
Contents lists available at SciVerse ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Synthesis of colourless silver precursor ink for printing conductive patterns on
silicon nitride substrates
Qijin Huang, Wenfeng Shen, Weijie Song ∗
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China
a r t i c l e
i n f o
Article history:
Received 21 February 2012
Received in revised form 5 April 2012
Accepted 5 April 2012
Available online 11 April 2012
Keywords:
Ink-jet printing
Silver precursor ink
Conductive silver patterns
a b s t r a c t
Silver precursor ink was synthesised by a simple and environmentally friendly method based on chemical
reduction. The stability, particle size, viscosity and surface tension of the ink were adjusted by adding
polyvinylpyrrolidone (PVP) and ethylene glycol (EG). The silver patterns were fabricated on the silicon nitride substrate and were characterised by means of X-ray diffraction (XRD), scanning electron
microscopy (SEM), and electrical measurements. The thickness of the sample printed three times was
approximately 0.66 ␮m, and it increased to 2.43 ␮m after 12 printings. The ink-jet-printed silver patterns
exhibited good conductivity when the samples were sintered at temperatures above 200 ◦ C. The resistivity value was observed to decrease to 3.1 ␮ cm after sintering at 500 ◦ C for 60 min, twice the value
of bulk silver (1.6 ␮ cm). The low resistivity of silver patterns suggests applications for ink-jet printing
of electronics devices.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Ink-jet printing is now widely used in many applications beyond
its conventional use in desktop publishing [1]. Its ability to deliver
a precise amount of material in a rapid, reproducible fashion to
pre-determined locations under computer control is a desirable
feature for such applications as producing conductive patterns for
electronic devices [2–4]. A major challenge in applying ink-jet processes for the preparation of conductive patterns is formulating
suitable ink. In the case of inks for metal conductive patterns, the
content of the ink must be adjusted to provide the required resolution while still providing good adhesion and the desired electronic
properties for conducting patterns [5–7].
There are two main types of ink that are used to obtain conducting patterns [8], which are nanoparticle (NP) ink and metal-organic
decomposition (MOD) ink. Most studies have focused on the ink-jet
printing of conductive patterns with silver nanoparticle ink because
of the desirable conductivity and anti-oxidation properties of the
silver patterns [9,6,10]. However, the silver nanoparticle ink is composed of a suspension of silver nanoparticles, which can deposit
inside the nozzle chamber and ultimately clog the nozzle, limiting
the application of this technology on a large scale [11,12]. Furthermore, silver nanoparticles are generally synthesised in an organic
medium that is primarily composed of hazardous organic solvents.
A variety of toxic wastes are generated throughout the complicated
∗ Corresponding author. Tel.: +86 574 87913375.
E-mail address: [email protected] (W. Song).
0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apsusc.2012.04.037
synthesis process, including the particle separation, washing and
dispersion steps. Therefore, a non-toxic and stable silver ink with
superior stability against aggregation is of great importance in this
technology.
In this work, we report the synthesis of a stable silver precursor
ink by a simple and environmentally friendly method. The ink is
colourless and transparent, similar to the “true solution”, so that
it can be printed directly onto the silicon nitride substrate with a
common colour ink-jet printer. The as-printed patterns deposited
with a range of printing cycles were thermally treated at various sintering temperatures and for sintering times. Their surface
microstructure and resistivity evolution were investigated. The
result shows that the lowest resistivity of sintered silver patterns is
twice the value of bulk silver. This approach involves a simple and
universal method for easily controlling the conductive pattern’s
shape, thickness and resistivity. The successful creation of this type
of silver precursor ink provides an easy technique for fabricating
electronic devices using ink-jet printing.
2. Experimental details
2.1. Materials
All reagents – AgNO3 , dextrose, ammonium hydroxide,
polyvinylpyrrolidone (PVP, Mw = 3000), ethanol, ethylene glycol
(EG), and 2-methoxyethanol (ME) – were analytical-grade reagents,
and they were purchased from Sinopharm Chemical Reagent Co.,
Ltd. Deionised water was used in all of the experiments.
Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388
7385
Fig. 1. (a) Optical image and (b) particle size distribution of silver precursor ink after one month of storage.
2.2. The preparation of silver precursor ink and silver conductive
patterns
3. Results and discussion
3.1. The physical properties of silver precursor ink
AgNO3 solution was prepared by dissolving silver nitrate
(0.05 M) in a mixed solvent (50 ml) of deionised water and ethanol
with a volume ratio of 1:3. NH4 OH was added to the AgNO3 solution drop by drop with stirring until a colourless and transparent
solution was formed. The pH value of the transparent solution was
approximately 9. Subsequently, 2-methoxyethanol and ethylene
glycol were incorporated into the transparent solution to adjust its
viscosity and surface tension. PVP (0.02 mM) was added as a filmforming agent. Then, 10 ml of dextrose solution (10 g of dextrose
dissolved in 100 ml of deionised water) was incorporated as a weak
reducing agent. After stirring for 10 min, the solution was filtered.
A colourless and transparent silver precursor ink was obtained, and
the final pH of the silver precursor was approximately 8.5.
2.3. Inkjet printing and heat treatment of silver patterns
The ink-jet printing technique used in this paper was similar
to the technique used in our previous work [13,14]. The modified
printer setup consisted of a drop-on-demand DOD ink-jet nozzle manufactured from Seiko Epson Corp. The modified printer
has a piezoelectric head with 90 openings of size about 28 ␮m,
and each droplet volume is of the order of 3 pl. The silver precursor ink was loaded into the cartridges and then printed on a
silicon nitride substrate using our modified ink-jet printer. If the
ink was printed onto a substrate more than once, the sample was
allowed to dry in air for 4 min between printings. When printing
was complete, the samples were kept in air at 50 ◦ C. After 4 h, the
samples were sintered at different temperatures in the range from
150 ◦ C to 500 ◦ C.
2.4. Characterisations
The crystal structure and chemical composition of the silver patterns were investigated by X-ray diffraction (XRD) using a Bruker
AXS D8 Advance diffractometer with Cu K␣, = 0.1542 nm. The
surface morphology of the samples was observed by a Hitachi S4800 field emission scanning electron microscope (FESEM) using
an operating voltage of 8 kV. The viscosity of the silver precursor ink was measured with a Brookfield Viscometer DV-II+
pro with a UL/Y adapter at 25 ◦ C. The resistivity was detected
by the 4-Point Sheet Resistance Test System, Lucas-Signatone
Pro4-4000.
To make ink-jet-printable ink, the properties of this ink, including its stability, particle size, viscosity and surface tension must be
formulated to fit the physical and rheological requirements of fluid
flow during the printing process. The stability of the silver precursor ink is a key factor in applications involving printing conductive
patterns using a common colour printer. To prevent the rapid precipitation of Ag, a low-molecular-weight organic compound, ME,
and a high-molecular-weight organic compound, PVP, were added
to the reaction media as excellent stabilising agents that can free
Ag+ ions gradually by forming coordination complex Ag+ ions. It
was observed that the silver precursor ink demonstrated good performance and stability over an extended period with cold-storage
at temperatures of 10 ◦ C and below. Fig. 1 shows the optical image
and particle size distribution of silver precursor ink after one month
of cold-storage. As shown in Fig. 1(a), it was observed that after
a month of storage, the ink remained colourless and transparent,
without any precipitates. From Fig. 1(b), it can be seen that the average particle size increased slowly from 250.2 nm to 581.1 nm after
a month of storage.
To form a well-shaped drop, the viscosity of the ink needs to be
adjusted to a suitable range. The surface tension plays an important
role in the interaction between the printer nozzle and the ink and
in the spreading of the pico-litre droplet over the substrate surface.
Inks possessing a surface tension on the order of 25–50 mN/m and a
Newtonian viscosity of 1–20 mPa s were shown to be most suitable
for ink-jet printing [11]. In this work, ME and EG were incorporated
into the reaction media to obtain suitable viscosity and surface
tension values for the ink-jet printing process. The physical properties of the silver precursor ink and the fluids used to prepare the
ink are summarised in Table 1. The silver precursor ink’s surface
tension and viscosity in our study were 34.5 mN/m and 3.2 mPa s,
respectively.
Table 1
Physical properties of fluids used to prepare the silver precursor ink.
DI water
Ethanol
Ethylene glycol
2-methoxyethanol
The ink
Surface tension
(mN m−1 )
Viscosity
(mPa s)
Boiling point
(◦ C)
72.8
22.3
48.5
27.2
34.5
1
1.2
19
1.6
3.2
100
78.4
198
125
–
7386
Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388
Fig. 2. Optical images of silver films sintered at (a) 50 ◦ C and (b) 350 ◦ C. The number of printing cycles was eight.
3.2. Morphology and microstructure of the silver patterns
The silver precursor ink was used to prepare electrically conductive thin patterns. Initially, films with a size of 15 mm × 15 mm were
printed eight times on a silicon nitride substrate. Fig. 2 shows the
optical images of the silver films sintered at (a) 50 ◦ C and (b) 350 ◦ C.
With heat treatment, the film changed colour from dark brown into
silvery white. The silver films was well adhered to the substrate
without cracks after sintering, and the nonconductive film became
conductive.
Fig. 3 shows the XRD patterns of the silver thin films that were
formed by printing precursor ink eight times on the silicon nitride
substrate and subsequently sintering the sample at 350 ◦ C for 5 min
in an ambient atmosphere. Peaks can be observed clearly at 38.2◦ ,
44.4◦ , and 64.5◦ , and they were attributed to the diffraction from the
(1 1 1), (2 0 0) and (2 1 1) crystalline planes of the face-centred structure of silver, respectively, according to the Silver Joint Committee
on Powder Diffraction Standards Database (File NO.87-0509). Fig. 3
also showed that there were no silver oxide peaks in the XRD pattern. It could be confirmed that the oxidation of silver does not occur
during the sintering process. Silicon peaks were also observed in
the XRD image due to the beam intensity, which was able to break
through the silver film.
The thickness of the silver thin film pattern could be controlled
by changing the number of printing cycles [15]. In this work, conductive silver thin films were prepared by printing silver precursor
ink patterns numerous times, and the films were sintered at the
temperature of 350 ◦ C for 5 min. The relationship between the
thickness and the number of printing cycles was studied. As seen
in Fig. 4, the thickness of the silver film increased linearly with
the number of printing cycles. The thickness of the sample after
three printing cycles was approximately 0.66 ␮m, and it increased
to 2.43 ␮m when the film was printed 12 times.
3.3. Electronic properties of the silver patterns
With the evaporation of the solvents during sintering, the silver clustered into small agglomerates with many voids, as shown
in Fig. 5(a). However, when the number of printings increased to
six and nine, as depicted in Fig. 5(b) and (c), respectively, the silver
nanoparticles grew; the voids diminished, and the solvents were
removed, which meant that the silver films became denser, thus,
resulting in better conductivity. The film’s thickness increased to
2.43 ␮m after twelve printing cycles. The microstructure observations from Fig. 5(d) clearly demonstrated that most of the voids
disappeared, and consequently, the electrical resistivity of the films
decreased. The silver pattern exhibits a relatively low resistivity (∼3.3 ␮ cm) after drying at 350 ◦ C for 5 min in air. With the
increase of drying time to 10 min and 30 min at 350 ◦ C, the resistivity of the silver films after twelve printing cycles decreased slowly.
They were 3.3 and 3.2 ␮ cm, respectively.
To investigate the effect of the sintering time on the silver patterns further, the sintering temperature was fixed at 500 ◦ C and
the number of printing cycles was set at twenty. Fig. 6 shows an
SEM image of the ink-jet-printed films as a function of the sintering
time. After the pattern was sintered for 10 min, the film consisted of
slightly sintered particles with a grain diameter of ∼0.4 ␮m as seen
in Fig. 6(a), and its resistivity reached a value of 4.3 ␮ cm. As the
(111)
2.8
·
2.6
·
2.4
·
*
*
(311)
(200)
*Si
·
2.2
Thickness(μm)
Intensity(arb.units)
Ag
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
30
40
50
60
2 Theta(degree)
Fig. 3. The X-ray diffraction analysis of the silver pattern obtained with 8 printing
cycles after heating at 350 ◦ C for 5 min.
2
4
6
8
10
12
Printing times(N)
Fig. 4. The relationship between the thickness of the films and the number of printing cycles after sintering at 350 ◦ C for 5 min.
Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388
7387
Fig. 5. SEM images showing the microstructure evolution of the silver patterns as a function of the number of printing cycles: (a) 3, (b) 6, (c) 9, and (d) 12. The samples were
sintered at 350 ◦ C for 5 min. The scale bar is 1 ␮m.
Fig. 6. An SEM image of ink-jet-printed Ag films sintered at 500 ◦ C for different lengths of time: (a) 10 min (b) 30 min and (c) 60 min. The number of printing cycles was 20.
The scale bar is 200 nm. (d) EDS result of ink-jet-printed Ag films sintered at 500 ◦ C for 60 min. The number of printing cycles was 20.
7388
Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388
and the number of printing cycles. When the silver films were
printed three and twelve times, their thicknesses were 0.66 ␮m
and 2.43 ␮m, respectively. After sintering at 500 ◦ C for 60 min, the
resistivity of silver conductive films deposited using 20 printing
cycles was 3.1 ␮ cm, which was twice as high as that of bulk silver. Moreover, this silver precursor ink promise enormous potential
for directly printing conductive features by a common colour inkjet printer, making ink-jet printing of low-cost electronics devices
a possibility.
25
Resistivity (μΩ·cm)
20
15
10
Acknowledgement
5
0
100
This work has been supported by Ningbo Natural Science Foundation (grant no. 2011A610009) and Zhejiang Provincial Natural
Science Foundation of China (grant no. Y12E020042).
200
300
Temperature (ºC )
400
500
Fig. 7. Resistivity variations of ink-jet-printed films as a function of sintering temperature. The number of printing cycles was 12. The sintering time was 1 h.
sintering time increased to 30 min, larger particles formed and the
porosity became less as seen in Fig. 6(b), resulting in good contact
and a lower resistivity (∼3.1 ␮ cm). Even when the film was sintered at 500 ◦ C for 60 min, as exhibited in Fig. 6(c), small voids were
still observed, which caused the film’s resistivity (∼3.1 ␮ cm) to
be identical to that obtained for 30 min of sintering. Further sintering does not bring about a significant decrease in the resistivity,
which is consistent with the percolation theory [16–18]. The key
aspects of this theory model are that microstructures consisting of
large grains are more conductive than microstructures consisting
of small grains and that less porosity are more favourable to conductivity [19]. Fig. 6(d) also shows the EDS results of silver patterns.
It can be seen that the weight ratio of C/Ag in the pattern sintered at
500 ◦ C for 60 min is approximately 0.044. This may be interpreted
as indicating that when the organic residues are almost completely
removed, the resistivity of the films are stable, even with increasing
sintering time.
The sintering temperature is an important factor that influences
the resistivity of the silver conductive pattern. Fig. 7 shows the
resistivity change of the silver pattern as a function of the sintering temperature from 150 ◦ C to 500 ◦ C for 1 h. When the film was
sintered at 150 ◦ C, the electrical resistivity had a higher value of
24.7 ␮ cm. By increasing the sintering temperature, the solvents
and the majority of capping molecules in the ink were gradually
removed, and the electrical resistivity decreased until it reached
a value of 3.3 ␮ cm at 350 ◦ C, which was two times higher than
that of bulk Ag (1.6 ␮ cm). The change of the resistivity was not
clear when the sintering temperature was increasing continuous.
The electrical resistivity of the silver film reached minimum values
of 3.1 ␮ cm at 500 ◦ C.
4. Conclusions
A stable and colourless silver precursor ink has been synthesised
for directly printing silver conductive patterns with a modified
colour ink jet printer. The pattern’s thickness and resistivity could
be controlled by adjusting the number of printing cycles. A linear relationship was observed between the silver films’ thickness
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