Application of Silver Nitrate Solution and Inkjet Printing in the

Article pubs.acs.org/JPCC

Application of Silver Nitrate Solution and Inkjet Printing in the Fabrication of Microstructural Patterns on Glass Substrates Hung-Ju Chang, Ming-Hsiu Tsai, Weng-Sing Hwang,* Jung-Tang Wu, Steve Lien-Chung Hsu, and Hsin-Hung Chou Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701-01, Taiwan, R.O.C. ABSTRACT: Aqueous silver nitrate inks of 1, 5, and 10 mol/liter (M) were employed in a piezoelectric inkjet printing apparatus to construct microstructural patterns. On the basis of droplet observation, single droplets of silver nitrate inks were ejected at a suitable pulse amplitude of waveform. The characteristics of drying processes for silver nitrate solutions were investigated after measuring the contact angle on the prepared glass substrates. Because of the crystallization of silver nitrate, the decrease of local surface tension was induced with decreasing solute concentration. In addition, local surface tension was possibly reduced by exothermal reaction during the formation of crystal solid. The actual printing qualities of array and line patterns were then analyzed by variation of dot interval and substrate temperature. The pure silver patterns were reduced by ethylene glycol vapor at 250 °C for 10 min. Unsteady spreading features of printed array and line patterns on hydrophilic glass are found to be due to silver nitrate crystal during droplet drying, which is dominated by solute concentration and substrate temperature. Optimal printing quality with a continuous morphology of fine lines was generated successfully with a dot interval of 40 μm and an average line width of 90 μm at substrate temperature of 90 °C.

1. INTRODUCTION Over the past decade, the microelectronics industry has been pressed to raise their manufacturing efficiency and refine the developments of elemental electronics components.1 Piezoelectric inkjet printing method offers subtractive processing and a maskless, low temperature process of microfabrication for hard and flexible substrates.2−6 Ink materials are deposited only at the desired position with high precision; hence, this solution based method is less material wastage.7−15 Particularly, it is feasible for products to be manufactured using an inkjet printing process, including electronic device, sensors, and displays.16−23 Conductive ink materials for inkjet printing techniques have been researched for several years. In general, bulk metal was employed as conductive material, whereas it exhibited high melting point and required a specific apparatus. The other candidates of metallic nanoparticle suspensions and precursors have been developed recently and appropriately develop the low thermal loading processing. The suspensions have consisted of nanoparticles covered by a protective molecular layer on the surface and then dispersed in proper solvents.24 However, high sintering temperature of these particles was required with adequate reduction gas due to the decomposition temperature of the protective molecular layer. Most reviewed literatures show poor conductivity due to the formation of cracks and voids dispersed in the printed patterns after sintering.25,26 The present inks use metallic compounds, which are directly dissolved into solvents. The conductive matters were simply generated after heat treatment of 250 °C on a hot plate using a reductive atmosphere for 10 min.27 Controlling the feature size plays a vital role in the printing quality of inkjet printing. Reviewing the literature of © 2012 American Chemical Society

Schiaffino et al., the four kinds of regimes noted during the droplets impacting on the substrate are spreading, receding, and rebound situations until the droplet toward its static regime.28 Recently, Lim et al. investigated spreading and evaporation of an inkjet-printed picoliter droplet on a heated substrate.29 Moreover, Xue et al. have successfully fabricated silver source/ drain electrodes for low-cost polymer thin film transistors using an inkjet printing method.30 It demonstrated that the silver nitrate based solution was a potential conductive ink material for developing a low temperature manufacturing process. Although silver nitrate inks mixed with some additive have been used to fabricate conductive microstructures on hard/ flexible substrates, the detail mechanics and characteristics of printing features have not been well investigated.31,32 The objective of this study is to develop a simple and low temperature manufacturing process for microelectronics applications using and inkjet printing technique. In order to fabricate optimum patterns, the single droplet behavior is chosen as the most desirable condition to printing array and line patterns. The dot interval and substrate temperature are variables in control of the printing quality of patterns.

2. EXPERIMENTAL METHOD 2.1. Experimental Apparatus. The apparatus and procedure for observing droplet formation processes have been reported previously and are recounted here.33,34 The printer Received: October 3, 2011 Revised: January 29, 2012 Published: February 13, 2012 4612

dx.doi.org/10.1021/jp209523w | J. Phys. Chem. C 2012, 116, 4612−4620

The Journal of Physical Chemistry C

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setup consisted of four components, including a fitting for piezoelectric print head and reservoir, waveform generator, observation system, and two moving stages. A print head with 30 μm nozzle diameter was fixed on a fitting, which was mounted on a vertical moving axis. A Teflon tube was connected between print head and ink reservoir to provide sufficient ink continuously. The waveform generator delivered the driving pulses to drive the print head and flash a strobe light emitting diode (LED) light of the horizontal observation system. Meanwhile, the individual droplet images were captured by a charge-coupled device (CCD) camera. During the printing process, the gap between print head and substrate on the planar moving stage was adjusted to 5 mm, and the patterns were printed on a computer-controlled gantry system capable of movement accurate to ±5 μm. 2.2. Preparation of Silver Nitrate Inks and Glass Substrates. Silver nitrate of 1.7 g was directly dissolved in deionized water of 10 mL to prepare the silver nitrate ink of 1 mol/liter (M). The other inks adding silver nitrate for 5 and 10 M were 8.5 and 17.0 g, respectively. The viscosity of the silver nitrate inks was measured by a viscometer; Brookfield DV-II+Pro. The surface tension and density had been reported by Goard.35 Table 1 summarizes the viscosity, surface tension,

Figure 1. Schematic diagram of a typical bipolar pulse waveform.

printed on the prepared glass substrates and then dried under ambient conditions. Finally, the conductive silver patterns were reduced in ethylene glycol vapor on a preheated hot plate at 250 °C for 10 min.27 The morphology and feature sizes of patterns were analyzed by OM (optical microscopy, OPTEM). 2.4. Analysis of Equilibrium Contact Angle. Figure 2 illustrates the contact angle measurement combined with the

Table 1. Properties of the AgNO3 Solutions Employed in This Study and DI Water at 30 °C35 ink property

1M

5M

10 M

DI water

viscosity (mPa·s) surface tension (mN·m−1) specific gravity (g·cm−3)

1.66 74.37 1.129

2.12 77.69 1.667

3.22 81.79 2.028

0.80 71.40 1.00

and specific gravity for silver nitrate inks with various concentrations, and those properties were also compared with deionized (DI) water. Glass substrates (microscope cover glasses, Menzel-Glasser, Germany) were cleaned by an ultrasonic bath of isopropyl alcohol for 30 min followed by a DI water rinse. After cleaning, the substrates were exposed to the ultraviolet (UV)−ozone (O3) jet cleaning treatment for 30 min to remove organic contamination on the substrate surface. The high surface energy of hydrophilic substrates was then prepared, and subsequently, printing procedures were conducted. 2.3. Experimental Conditions. A waveform was used to deform the piezoelectric device and caused volume deformation of the capillary surrounded by the piezoelectric device. According to our previous studies, the bipolar pulse was set as 2 μs for trise, 5 μs for tdewell, 2 μs for tfall, 5 μs for techo, and 2 μs for tfinalrise as shown in Figure 1.33,34 Therefore, the ejection of a single droplet behavior for different ink materials was formed within suitable pulse amplitudes. In order to capture good quality images of droplet evolution, the frequency of waveform was maintained at 500 Hz. A negative pressure about 0.29 psi of the reservoir was regulated to prevent ink from leaking from the nozzle of the print head. Array patterns were printed with a dot interval of 500 μm. The straight lines were constructed with the dot spacing of 10, 20, 30, and 40 μm. All patterns were printed with the stage velocity of 10 mm·s−1. The gap between print head and substrate was 5 mm, which was equal to the distance of the droplet observing setup. Furthermore, the critical factor of substrate temperatures was heated to 30 °C, 50 °C, and 90 °C. The patterns were

Figure 2. Schematic illustration of using the piezoelectric printing method to the measure contact angle of inks with a substrate.

piezoelectric printing technique. A static contact angle was obtained after depositing 15 000 droplets at one fixed position on the hydrophilic glass. The volume of the accumulated droplet was easily controlled at approximately 0.1−0.2 μL, which provided sufficient evaporation time to observe the drying dynamics of the silver nitrate droplet. The images of contact angle and drying evolution were recorded by the horizontal and vertical CCD camera, respectively.

3. RESULTS AND DISCUSSION In general, the fluids with viscosity of 1−10 mPa·s and surface tension above 35 mN·m−1 are suitable to serve as the inkjet inks.36 Table 1 shows the fluid properties of silver nitrate inks employed in this study and DI water at 30 °C. As the concentration of inks increases from 1 to 10 M, the fluid viscosity was varied from 1.66 to 3.22 cP, and the surface tension was between 74.37 and 81.79 mN·m−1. Meanwhile, the specific gravity ranged from 1.129 to 2.028 g·cm−3. Basically, three kinds of droplet behaviors had been investigated under various pulse amplitudes of bipolar waveform using the inkjet printing method.33,34 For actual applications, a single droplet generated and ejected for each cycle of the waveform is the most desirable condition of the DOD (drop on demand) inkjet printing process. The droplet diameters are typically in the range 4613



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of 10−100 μm with impact velocity of 2−5 m·s−1 to ensure sufficient ballistic accuracy.37 In order to obtain stable printing quality, the maximum droplet velocity of the single droplet formation was adapted as the printed condition to achieve precision features on the substrates. In this work, the effects of printing conditions such as ink concentration, dot spacing, and substrate temperature were studied to control the printing quality of various patterns. 3.1. Effects of Ink Concentration on Single Droplet Formation. Previous studies had demonstrated that nanometallic suspensions of 20−50 wt % could establish a high conductivity microstructure using an inkjet printing technique.38 Therefore, a high concentration of silver nitrate solution should be required. Fortunately, a large amount of silver nitrate of 2.16 kg could be dissolved into DI water of 1 L at 20 °C, and the concentration is equivalent to 10 M. Recently, Wu and coworks have fabricated high conductive lines on a polyimide substrate using mixture precursor inks. In order to control printing quality effectively, pure silver nitrate solutions without adding any additive were dissolved into DI water as ink materials. The ink concentrations of 1, 5, and 10 M were equal to 15, 45, and 65 wt %, respectively. For the design of usable inkjet inks, the prepared silver nitrate inks are suitable to employ the fluid viscosity within 1−10 mPa·s and surface tension above 35 mN·m−1.39 Among different rheological properties of ink, the influence of printing quality will be discussed. For the dilute ink of 1 M, microdroplets were continuously ejected at the minimum pulse amplitude of ±21 V, below which the piezoelectric print head could not produce any droplets. In appropriate operation pulse amplitudes between ±21 and ±24 V, single droplet formation was available. The overall droplet jetted process exhibits a similar droplet behavior without satellite or undesirable droplets within this range as shown in Figure 3, where the pulse voltage of waveform is ±24 V. Initially, an extruded liquid column was formed during 20− 38 μs. Following this, the liquid column was separated from the orifice of the print head at 40 μs. The break-off liquid column then began to contract itself and adjust its morphology as well as undergo several oscillations. Eventually, one single spherical droplet with a diameter of 24 μm was formed for each pulse cycle. Under lower pulse amplitudes between ±21 V and ±23 V, the small droplet diameters were ranged in 20− 23 μm with a velocity of 1.4−2.2 m/s. Although big droplets were produced at the higher pulse amplitude of ±24 V, its droplet volume grew less than 1% compared with that at ± 21 V. In addition, the higher kinetic was beneficial to ensure sufficient ballistic accuracy since the droplet velocity at ±24 V was increased to 2.6 m·s−1. For 5 and 10 M inks, the upper pulse amplitude of single droplet formation was extended to ±27 V. Overall printing progress presented the liquid column break-up, contraction, and oscillation as in Figure 3. The pinch-off time of the liquid column was around 40 μs, except 10 M ink is late to 42 μs as shown in Figure 4. The droplet diameter of 5 M ink was 28 μm with an average velocity of 2.6 m·s−1. While the concentration was increased to 10 M, the droplet diameter increased to 29 μm with a velocity of 2.9 m·s−1. Fromm used the Z number grouping of fluid properties, which includes viscosity as well as surface tension to provide a dimensionless analysis of the mechanics of droplet formation in DOD print heads: Z = (Dnozρσ)1/2/ μ. In the relationship for Z, Dnoz is the nozzle diameter. Fromm predicted that droplet formation in the DOD system was only possible for Z > 2 and that for a given pressure pulse the droplet

Figure 3. Single droplet evolution for 1 M AgNO3 ink at the pulse voltage of ±24 V.

volume increases with increasing value of Z.40 In this study, the equivalent Z numbers for 5 and 10 M inks were approximately 29 and 22, respectively. Nevertheless, this result does not correspond to the droplet observation at the same waveform. Actually, the ink flowing on the nozzle plate of the inkjet print head is observed during 70−450 μs in Figure 4. Because the nozzle orifice is open to the atmosphere, the exposed printing ink evaporates during the printing process. Therefore, the concentration of the residue silver nitrate layer around the orifice increases gradually and even starts to form the silver nitrate crystalline. This issue may change the characteristics of the droplet behavior in terms of velocity, volume, and misdirection. In order to resolve this unstable possibility for silver nitrate inks, generating single droplets with the highest velocity under maximum pulse amplitude is the priority option. 3.2. Effects of Dot Interval and Substrate Temperatures on Microstructural Patterns. Conductive lines had been successfully fabricated on polymer substrates using silver nitrate based inks.27,30,41 In order to avoid the influence of blending or deteriorating problems happening in polymer substrates at high temperature ambient, this study adopted glass slides as substrates. Sequences of single droplets were ejected to generate array and line patterns on the hydrophilic glass with various dot spacing and substrate temperatures. The quality of printed patterns after reduction treatment was investigated as below. 3.2.1. Effects on Array Patterns. Lim et al. had studied the evaporation of an inkjet printed picoliter droplet on a heated substrate.29 Their results show that the saturated vapor 4614



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decreases sharply. In the case of water, even at the substrate temperature of 100 °C, the evaporation does not affect the spreading history. Therefore, the major factor of dot feature at higher substrate temperature is controlled by the ink properties and drying behavior. Figure 5 shows the array patterns printed with silver nitrate solutions of various concentrations at various substrate temperatures of 30, 50, and 90 °C. For 5 M ink, the continuous surface of dot morphology was obtained easily and was of better quality than that of others. For diluted ink of 1 M, the coverage of dots consisted of many fractals. For extreme concentration ink of 10 M, large dots with diameters of about 200 μm and of irregular shape were generated. All printed array patterns of silver nitrate inks show low quality with undesirable dot shape and nonuniform dot size except at the substrate temperature of 90 °C. Higher evaporation produced at this high substrate temperature enables full circle coverage. At substrate temperature of 30 °C, many small fractals were dispersed on the coverage for 1 M ink. This poor quality was believed to be the result of the drying feature of tiny droplets with receding phenomenon. The continuous surface of dot morphology was obtained with increasing ink concentration to 5 and 10 M. Their dot diameters were increased to 95 and 170 μm, respectively. The dot diameters were shown to increase as ink concentration increased. Generally, small coverage with large contact angle was formed at higher surface tension of fluid. However, this relationship is the opposite of our experimental results of various ink concentrations. Sivakumar et al. used the Weber number We = ρU2D/σ, which is a dimensionless group of fluid properties and droplet behavior including density, surface tension, and droplet diameter as well as velocity to provide an analysis of the mechanics of droplet deposition on rough substrates.42 In the relationship for We, ρ is liquid density (g·cm−3), σ is liquid surface tension (mN·m−1), and U and D are droplet velocity (m·s−1) and diameter (μm) before impact on the substrate, respectively. The increased tendency of the Weber number for silver nitrate inks ranged from 2.26 to 6.05. It shows that the effect of surface tension is weakened and that of inertia

Figure 4. Single droplet evolution for 10 M AgNO3 ink at the pulse voltage of ±27 V.

concentration increases significantly as the temperature of the droplet increases, and as a result, the evaporation time

Figure 5. Array patterns for 1, 5, and 10 M inks printed on the hydrophilic glass substrates at 30, 50, and 90 °C. 4615



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Figure 6. Line patterns for 1, 5, and 10 M inks printed on the hydrophilic glass substrates with various dot spacing of 10, 20, 30, and 40 μm at 30 °C. The magnified images for parts of g and h are inserted in the upper right corners.

related to the spacing of adjacent droplets and contact angle by the following equation:

is strengthened during droplet impaction for high concentration ink. At a substrate temperature of 90 °C, the concise relationship varied with ink concentration can be firmly confirmed from 65 to 220 μm. According to the literature, the dot diameter can be estimated by the following equation:

w2 =

πD3 6p θ 4 sin 2 θ

⎛ ⎞1/3 b 4 sin3 θ ⎟⎟ = ⎜⎜ D ⎝ (1 − cos θ)2 (2 + cos θ) ⎠

−

cos θ 4 sin θ

where the line width w was calculated from the droplet diameter D, the dot spacing p, and the contact angle θ.44 When the contact angle of the liquid with the substrate is measured and the diameter of the droplet and dot spacing are known, the line width can be calculated. From this equation, the line width of 10 M is about 60 μm printed with dot spacing of 40 μm, which is approximately one-fifth times of the printed line width in Figure 6l. It indicates that some mechanisms are formed during the dying process using high concentration inks. For 5 M ink, overlap droplets deposited alongthe linear direction are easily contracted into big budges connected with a nonuniform line. This undesirable situation can be modified at an appropriate dot interval of 40 μm. While the dilute ink of 1 M cannot provide enough solute, empty lines are filled with many small particles, and the width of lines are reduced with increasing dot interval. The period of 1−4 ms between two droplet ejections was a function of various dot intervals from 10 to 40 μm with a constant velocity of moving stage at 10 mm·s−1. Therefore, long periods between two droplets increases the time duration of droplet evaporation, which corresponds to obtaining better printing quality at a dot interval of 40 μm. Substrate temperature is one of the important factors to control inkjet printed features. Lee et al. have investigated a silver nanosuspension of 30 wt % printed on a hydrophobic surface of silicon substrates treated by UV−O3 exposure.45 Their results show that a high evaporation rate with heating substrate temperature can produce fine lines. Figure 7 shows line patterns printed with dot spacing of 40 μm at different

where b is the dot diameter on the substrate, and θ is the droplet contact angle on the substrate.43 The contact angle of silver inks has been estimated in detail in section 3.3 of this article. When the contact angle of the liquid with the substrate is measured and the diameter of the droplet is known, the width of a printed droplet on the substrate can be calculated. From this equation, the dot diameters of inks should be below 100 μm. However, the dot diameter of 10 M is higher than this value that proposes the drying behavior of high ink concentration resulted in the dot diameter of 170 μm. 3.2.2. Effects on Line Patterns. In order to construct coherent lines on the hydrophilic glass, the individual droplets have to be printed in such a way that consecutive droplets partially overlap. Reviewing the literature, Duineveld investigated the influence of dot intervals on the sequence droplet constituted patterns.43 It suggests that certain problems of linear structure such as breaking into individual droplets, scalloping, and bulging can be resolved with small dot spacing. Figure 6 shows the line patterns printed with dot intervals between 10 and 40 μm onto the hydrophilic glass substrate at 30 °C. For 10 M ink, the distorted line can be improved as the dot interval increases, whereas the spreading phenomenon during the formation of silver nitrate crystal cannot be eliminated. According to the literature, the line width can be 4616



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Figure 7. Line patterns for 1, 5, and 10 M inks printed with dot spacing of 40 μm at different substrate temperatures of 30, 50, and 90 °C. The magnified images for parts of d, e, and f are inserted in the upper right corners.

volumes of about 0.1−0.2 μL, the use of the accumulated droplets was attempted to simulate the drying evolution of a tiny droplet effect on the quality of printed patterns. This drying process of silver nitrate inks was described on the printing quality of patterns using the inkjet printing technique. Figure 8a shows the evaporation evolution of 1 M ink during droplet drying. A schematic illustration is depicted to assist in understanding the overall drying process of the colorless silver nitrate droplet as described below. The decrease of droplet volume with a receding contact line on the hydrophilic glass was observed between 30 and 90 s. This receding phenomenon induced increasing of surface tension gradually with increasing ink concentration during droplet evaporation. At 120 s, this colorless droplet shrank and balled into a semispherical droplet to achieve minimum surface energy. During 120−180 s, the droplet appeared gray, while the silver nitrate crystal was formed. Until 400 s, final coverage area was expanded slightly, whereas it was still smaller than the initial droplet. Figure 8b shows the evolution of 5 M ink drying on hydrophilic glass with a small expanding behavior. During 30−60 s, the silver nitrate crystallization is rapidly distributed around the entire droplet. After 90 s, the fraction of coverage area extends out. Finally, it can occupy a larger area than initial droplet. This radial distention on hydrophilic glass might be caused by the crystallization of silver nitrate during droplet drying. Figure 8c shows the drying evolution with obvious spreading phenomenon for 10 M ink. During 30−60 s, a semispherical droplet was formed on the substrate. At 90 s, the silver nitrate crystallization was distributed along the bottom of the entire droplet. During 90−120 s, the silver nitrate preferred to crystallize at the bottom and near the contact line of the droplet. Therefore, the droplet was divided into an upper part of colorless solution and bottom layer of silver nitrate precipitation. Between 180 and 800 s, the crystalline compound extended the coverage region surrounding the dried droplet continuously.

substrate temperatures. According to these images, the narrow line width was ranged in 90−110 μm with 5 M ink. The line width and printing quality for 10 M ink cannot be decreased and improved by varying substrate temperature. However, high substrate temperature was controllable to limit the development of overlap droplets for dilute ink of 1 M within the line width about 90 μm, which exhibited a continuous surface of line morphology. It is believed to be a result of the spreading phenomenon of crystal growth limited by fast evaporation. 3.3. Drying Behavior of Silver Nitrate Inks. A specific coffee ring structure near the edge of the printed patterns not only appeared for nanosuspension inks but also happened on precursor inks.41,46 Much evidence proposed that such an issue was caused by the surface tension gradient due to the edge region having a higher evaporation rate than the central area.46,47 In this study, the ring structure is also shown for 10 M ink, but the actual reasons are still unclear. According to the observation images of drying behavior and printed patterns, the drying behavior of silver nitrate crystallization was the possible mechanism to form the wide ring feature of patterns. All of the following printing experiments were then adopting single droplet conditions at maximum pulse amplitude. First, the equilibrium contact angle near 4° was obtained by depositing 15 000 droplets of DI water onto a hydrophilic surface of UV− O3 treated glass substrates at 30 °C. The 15 000 droplets were used to enhance the behavior of ink and let the images of drying behavior be observed easily. For the 1 M silver nitrate ink, the static contact angle of 11° was bigger than that of DI water by direct measurements. As the additive of silver nitrate increases to 5 and 10 M, the contact angles become 13° and 28°, respectively. This trend corresponds to the surface tension from 74.37 to 81.79 mN·m−1 as the solute concentration increases. After measuring the contact angle, accumulated droplets enlarge the actual mechanisms of tiny droplets on hydrophilic glass. Although three inks were deposited under various 4617



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Figure 8. Drying evolutions for silver nitrate inks on the hydrophilic glass of 30 °C are shown in experimental images and schematic illustrations of the optical images. (a) 1 M; (b) 5 M; (c) 10 M.

The peripheral crystal ring then continues to grow and forms an annular stain of interconnected but faceted crystals at the ends. For the observation in this study, the silver nitrate droplets of 0.1−0.2 μL for various concentrations are performed differently. The drying process of silver nitrate inks shows that the surface tension of an entire droplet is distributed uniformly before the crystallization of silver nitrate occurs. Therefore, the influence of the droplet shape was dominated by the surface tension of inks. As the concentration approaches a saturated solution above 5 M, a static droplet shape was almost constant initially. Because of the small interface produced by higher surface tension for 10 M ink, a low evaporation rate was exhibited at coverage of 1.0 mm2 with a static contact angle of 28°. A semispherical droplet was kept on the same shape and the transferring time of crystallization was triggered at 90 s. In contrast, a higher evaporation rate was produced by the large coverage of 1.7 mm2 for 5 M ink with a contact angle of 13°. In Figure 8b between 30 and 60 s, the flat droplet was still of constant shape, and immediately, the uniform crystal was formed. However, a similar flat droplet for dilute ink of 1 M was covered about 2.0 mm2 with low contact angle of 11°. In spite of a higher evaporation rate at the early stage, the droplet volume was not only reduced

Meanwhile, the upper part was still maintained under the bottom layer. Until 1200 s, the saturated upper part was consuming all the water, and the crystal extended to maximum area. It was verified by the above-mentioned distention behavior of silver nitrate crystallization. Similar spreading propensity of droplets for typical salt solutions on the hydrophilic substrates was also studied by Shahidzadeh-Bonn et al.48 It turns out that the crystallization starts at the edge of a droplet, at the contact line of the droplet, followed by the apparition of some crystals at the liquid−air interface, meanwhile decreasing the solute concentration and surface tension at that same region. Another possible reason is speculated to be caused by the exothermal reaction during silver nitrate crystallization, which increases local temperature near the silver nitrate crystal as well as lowering the local surface tension. Both reasons, in turn, lead to the growth of dendrite patterns on the hydrophilic surface in what appears to be a precursor wetting film extending out from the droplet edge. Previously, Takhistov and Chang reported the complex stain morphologies of salt solutions using one single droplet volume of 20 μL.49 Their results show that the radii of droplets remain constant, but the thickness decreases linearly as such big droplets continuously evaporate on the hydrophilic glass. 4618



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but also followed with dramatic receding of the contact line. Once a silver nitrate crystal was formed, this receding phenomenon was ceased immediately, and then, the area started to extend at 180 s. This slight expanding coverage was also formed for 5 M ink because of its large interface with air. Particularly, the extended coverage results in four times larger than origin droplet for 10 M ink. Because of the lower evaporation rate of the small interface, the entire drying evolution required nearly 20 min, including the precipitation of silver nitrate crystal at the bottom interface and droplet edge until water was completely consumed. Moreover, a high amount of solute was precipitated from the evaporating droplet and expanded on an axial growth direction. 3.4. Optimum Printing Quality. The optimum conditions of patterns include fine size, smooth edge, and continuous surface morphology. From the drying progress of silver nitrate inks on hydrophilic glass, a big droplet about 0.2 μL with 5 M shows that the initial coverage is similar to final results without influence of the receding phenomenon of droplet evaporation and axial growth of silver nitrate crystallization. The small dots with the diameter of 100 μm were generated uniformly at a substrate temperature of 90 °C. Moreover, a fine line width of 90 μm was deposited with a dot interval of 40 μm. However, a higher operation substrate temperature might encounter some problems. Since the print head was exposed to the high temperature environment, solvent in the nozzle would vaporize more rapidly and result in fouling and clogging. This particular issue would easily happen in the printing process of 10 M ink. In spite of the substrate temperature of 90 °C, the performance of 5 M ink is still able to print well-defined patterns.

4. CONCLUSIONS In this study, drying of silver nitrate droplets of various concentrations on hydrophilic glass substrate shows different characteristics that affect the printing quality of patterns. Optimal array and line patterns were generated at the appropriate dot spacing and substrate temperature. The following conclusions could be drawn. (1) Good quality of metallic microstructures was obtained using silver nitrate ink of 5 M. (2) The fractal feature of dot and empty lines and receding phenomenon of the droplet before crystallization of silver nitrate were observed for a dilute ink concentration of 1 M. (3) The decentralization of patterns was observed with 10 M ink due to the large amount of silver nitrate provided and the distention behavior of silver nitrate crystallization.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +886-6-2757575, ext. 62928. Fax: +886-6-234-4393. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the National Science Council (NSC 100-2120-M-006-006). A part of the present work was also supported by the Research Center for Energy Technology and Strategy, National Cheng Kung University in Taiwan, which is gratefully acknowledged (D100-23003).

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REFERENCES

(1) Roy, S. J. Phys. D: Appl. Phys. 2007, 40, R413. 4619



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Application of Silver Nitrate Solution and Inkjet Printing in the

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