Poly(N-vinyl-2-pyrrolidone) Inks

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J. Phys. Chem. C 2010, 114, 4659–4662

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Direct Inkjet Printing of Silver Nitrate/Poly(N-vinyl-2-pyrrolidone) Inks To Fabricate Silver Conductive Lines Jung-Tang Wu, Steve Lien-Chung Hsu,* Ming-Hsiu Tsai, and Weng-Sing Hwang Department of Materials Science and Engineering, Center for Micro/Nano Science and Technology, National Cheng-Kung UniVersity, Tainan, 701-01, Taiwan, R.O.C. ReceiVed: January 13, 2010

In this paper, a high molecular weight organic compound, poly(N-vinyl-2-pyrrolidone) (PVP), was added to silver nitrate to fabricate silver conductive lines and arrays by inkjet printing on flexible Kapton substrates. With the assistance of PVP, the dimension of conductive lines can be controlled more accurately. In addition, the morphological control and resolution of arrays and lines were further improved by using UV/O3 treatment of substrates and changing the substrate temperature. The silver nitrate/PVP inks can be reduced in ethylene glycol vapor to form silver conductive lines at low temperatures. By using a high concentration of silver nitrate/PVP ink, continuous and smooth silver conductive lines with a resistivity of 2.71 × 10-5 Ω cm have been produced. Their resistivity is close to the resistivity of bulk silver. 1. Introduction Recently, inkjet printing of functional materials has been of interest in a variety of fields such as displays, electronics, optics, and sensors due to the advantages of low production cost, low temperature process, and mass production.1–3 Several promising techniques of metallic materials have been proposed for inkjet printing. The first way is to control the metallic particle sizes of the ink, which have low sintering temperatures due to their small sizes.4–8 The second way is to use solutions of metalloorganic precursors, which can be converted to metal at low temperatures.9–12 The third way is to use solutions of metal compounds, such as silver nitrate solution, which can be converted to silver at high temperatures (above 400 °C).13,14 Silver-based inks are the most commonly used materials due to their low resistivity and reasonable prices. Several methods have been reported for preparing silver nanoparticle suspensions for ink applications, including chemical and physical methods.15–17 In previous literature, to prepare silver nanoparticles with the chemical reduction method, stabilizing agents such as poly(Nvinyl-2-pyrrolidone) (PVP), thiosalicylic acid, 1-nonanethiol, etc. are usually needed as the protecting agent.18–21 However, the formation of pores caused by the decomposition of the protecting agent during thermal treatment will decrease the conductivity of silver. The metallo-organic precursors approach also has the same problem. For inkjet printing of silver nanoparticle suspensions, the particles can be deposited inside the nozzle chamber, which ultimately results in clogging. To solve these problems, we have reported a novel approach, ethylene glycol vapor reduction, to fabricate conductive silver tracks directly from silver nitrate aqueous solution by inkjet printing.22 Direct ink writing offers a simple method of rapid manufacturing technology in printed electronic and optoelectronic devices. With a direct-write approach, patterns or structures can be placed on flexible substrates such as paper or plastics without the use of other conventional techniques. Particularly, using inkjet printing to fabricate conductive tracks (circuits) has been found to have a great potential in the electronic industry. In * To whom correspondence should be addressed. E-mail: lchsu@ mail.ncku.edu.tw.

addition, the inkjet printing process has several other advantages. For example, the pattern size is no longer limited, and inks can be used in the fabrication of three-dimensional structures. Although direct ink writing possesses many advantages, the morphology of silver conductive lines is difficult to control. Thus, the substrate treatment is a very critical issue for an inkjet printing process.23–26 In this study, we tried to add PVP as the additive to silver nitrate solution to limit the mobility of the silver nitrate ink for inkjet printing on a flexible plastic substrate. We found that adding PVP is a very effective way to improve morphological control, and renders a lower resistivity for the printed silver conductive lines. In addition, we also studied the relationship between the formation of silver conductive lines and the substrate surface conditions, which can be controlled by various substrate treatments, such as substrate heating and UV/ozone treatment. 2. Experimental Section 2.1. Materials. Silver nitrate (AgNO3) was obtained from Showa Chemical Co. Ethylene glycol was purchased from SigmaAldrich Co. Polyimide film (Kapton) was obtained from Du Pont Co. Poly(N-vinyl-2-pyrrolidone) (PVP) (molecular weight ∼10 000 g/mol) was obtained from ICN Biomedical Inc. 2.2. Preparation of Silver Conductive Lines and Arrays on a Polyimide Substrate. Silver nitrate was dissolved in deionized water in a beaker to prepare a solution. To this solution, PVP was added and with stirring for 30 min. A 2 cm × 2 cm Kapton film was cleaned with acetone and deionized water to remove the particles and organic contaminates on the surface. After cleaning, the film was treated with ozone by UVO-100 UV ozone from 0 to 30 min. The silver nitrate /PVP inks with different concentrations were printed by an inkjet printer onto Kapton substrates. The printer setup consisted of a drop-on-demand (DOD) piezoelectric inkjet nozzle, and the diameter of the orifice was 30 µm, as shown in our previous paper.27 Silver conductive lines and arrays were printed by using silver nitrate suspensions under different interspacing distances. The resulting lines and arrays were heated to 200 °C with ethylene glycol vapor inside a glass dish, and held at this

10.1021/jp100326k  2010 American Chemical Society Published on Web 02/22/2010

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Figure 1. TGA thermograms of the silver lines with different silver nitrate/PVP weight ratios prepared from ethylene glycol vapor reduction.

temperature for 1 h to convert to silver lines and arrays. The polyimide substrates were then cooled by natural convection to room temperature inside the glass dish. 2.3. Preparation of Silver Films on a Polyimide Substrate. The procedure of the preparation of silver films is similar to that of the silver conductive lines except that the silver nitrate/ PVP aqueous solution was dripped on the polyimide substrate but not printed by an inkjet printer 2.4. Characterization. The X-ray diffraction (XRD) experiment was conducted on a Rigaku D/MAX-IIIV X-ray diffractometer, using Ni-filtered Cu KR radiation with a scanning rate of 4° min-1 at 30 kV and 20 mA. The weight losses of the silver films were analyzed with a TA Instrument Thermogravimetric Analyzer (TGA) 2050 at a heating rate of 10 deg/min under air. The viscosities of the silver suspensions were measured with a Brookfield Viscometer DV-II+Pro with a UL/Y Adapter at 25 °C. The texture of silver lines after reduction was investigated by Zoom 125 optical microscopy (OM). A FTA-125 contact angle analyzer was used to measure the contact angles. The thickness of the silver conductive lines was measured with a KLA-Tencor/AS-IQ new Alpha-Step Profilometer. The resistivity of silver films was measured by the two-point probe method, using HP4145. The adhesion of the silver film to Kapton substrate was evaluated by a Scotch tape peeling test according to ASTM-3359B. 3. Results and Discussion 3.1. Silver Nitrate/PVP Ink Properties. In our previous study, we have prepared silver conductive lines via ethylene glycol vapor reduction for fabricating microinterconnects by inkjet printing.22 Although we were able to prepare silver conductive lines, the line width was relatively difficult to control. In this study, a high molecular weight organic compound, PVP (molecular weight ∼10 000 g/mol), was used as the additive to increase the ink’s viscosity and adhesion to the substrate. The amount of PVP needs to be controlled carefully due to the high decomposition temperature of PVP. The residue of PVP will affect the conductivity of silver lines. Figure 1 shows the TGA analysis of the PVP residue with different silver nitrate/PVP weight ratios prepared from ethylene glycol vapor reduction. It shows that the amount of PVP residue decreased with the increase of silver nitrate/PVP ratio. The residue of the organic additive can be calculated from the TGA curve. We can see that the inks with weight ratios of 20:1 and 30:1 have the smallest PVP residue when compared to the other silver inks. Thus, the silver nitrate/PVP ink with a weight ratio of 20:1 was used in this study.

Wu et al.

Figure 2. The viscosities of different concentrations of silver nitrate in the silver nitrate/PVP(20/1) solutions.

Figure 3. XRD diffraction pattern of the silver film prepared from ethylene glycol vapor reduction.

TABLE 1: Contact Angles of Silver Nitrate Solutions on Kapton with Different UV/O3 Treatment Time contact angles (deg) UV/O3 treatment time (min)

silver nitrate (1 M)

silver nitrate (5 M)

silver nitrate (10 M)

0 5 10 30

66 44 41 39

59 39 36 36

63 39 29 30

Figure 2 shows the viscosities of different silver nitrate concentrations of the silver nitrate/PVP (20/1) inks at room temperature. Clearly, the viscosity increased with the increasing silver nitrate concentration. The viscosities of the silver nitrate/ PVP inks are in the range of 1.55 to 11.3 cP, which are suitable for use in inkjet printing. The X-ray diffraction pattern of the prepared silver film, presented in Figure 3, shows the peak characteristics of metallic silver. The reflection peaks are indexed as the fcc (111), (200), (220), and (311) planes, indicating that silver is well crystallized. The tape peeling test shows that the silver film has the highest adhesion rating of 5B according to ASTM-3359B. 3.2. Surface Characterization of UV/O3-Treated Kapton Substrate. Kapton film has a high contact angle as shown in Table 1. To investigate the effectiveness of the Kapton film surface for inkjet printing, we tried to control the contact angle of the Kapton film by UV/O3 treatment, and to study the relationship between the surface treatment and inkjet printing parameters. With the increase of UV/O3 treatment time, the contact angles of the Kapton substrate decreased until 10 min (Table 1). After 10 min of UV/O3 treatment, the contact angles of Kapton substrate did not change obviously. The 10 min UV/ O3 treatment is critical for inducing the hydrophilic nature on the Kapton surface.

Fabrication of Silver Conductive Lines and Arrays

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Figure 4. Optical microscopic images of silver arrays printed on Kapton substrate at different UV/O3 treatment times.

To obtain smooth conductive patterns with high resolution by inkjet printing, various printing conditions, including the interspacing distance between dots, the UV/O3 treatment time, the composition of the silver inks, and substrate temperature, were adjusted. By using the silver nitrate/PVP inks (20:1), the driving parameters of the microdrop piezoelectric nozzle were easily optimized. The droplet size was about 30 µm in diameter when a nozzle with the interspacing distance of 200 µm between dots was used. Figure 4 shows the microscopic images of inkjet printed silver arrays on a Kapton substrate with different UV/ O3 treatment times with 1 M silver nitrate/PVP ink. We were also able to fabricate arrays of discrete dot patterns. The diameter of the dots was about 50-70 µm. When the UV/O3 treatment time increased, a corresponding decrease of the silver dot diameter was found. Since the contact angle did not change obviously, the silver dot diameter with the 30 min of UV/O3 treatment remained at 70 µm. Figure 5 presents the results of optical microscopic images of silver conductive lines inkjet printed on the Kapton substrate at room temperature as a function of interspacing distance between dots and UV/O3 treatment time. From the optical microscopic images, we can see the silver conductive lines with different interspacing distance between dots after UV/O3 treatment. Before UV/O3 treatment, the silver conductive lines formed with interspacing distance increasing to 40 µm between dots. At 30 µm of interspacing distance between dots, a series of regularly spaced bulges appeared in the printed lines. The phenomenon of bulges has been explained by Duineveld.29 Although we could prepare silver conductive lines before UV/ O3 treatment, we were unable to create well-defined straight lines. This is due to the contact angle of the Kapton substrate being too high. To solve this problem, we used UV/O3 treatment to decrease the contact angle. Compared with different UV/O3 treatment times at the same interspacing distance between dots, the width of the silver conductive lines increased from 50.8 µm to 62.5 µm as UV/O3 treatment time increased. Straight lines were obtained at 10 min of UV/O3 treatment, without any defects such as bulges or coffee-stain effects. This trend agreed well with the results of the contact angle analysis. After UV/ O3 treatment time increased to 30 min, a similar effect was observed. The width of silver conductive lines with 30 min of UV/O3 treatment remained at 62.5 µm. 3.3. Preparation of Silver Lines with Different Substrate Temperatures and Their Electrical Properties. From the experimental results of section 3.2, we found that the UV/O3

Figure 5. Optical microscopic images of silver lines inkjet printed on Kapton substrate at room temperature as a function of interspacing distance between dots and UV/O3 treatment time.

Figure 6. Optical microscopic images of silver lines inkjet printed on Kapton substrate with different silver nitrate concentrations and different substrate temperatures.

treatment is an effective way for creating fine line patterns. To solve the problem of bulge formation during printing, we further changed the substrate temperatures. Figure 6 shows optical microscopic images of patterns inkjet printed with interspacing distance of 40 µm between dots at different substrate temperatures and silver nitrate concentrations with 10 min of UV/O3 treatment. When the substrate temperature increased, the line width decreased from 66.5 to 52.5 µm. The line width decrease may be due to the increase of the evaporation rate of water at higher temperatures.25 However, the line width did not change when different silver nitrate concentrations were used. The formation of the bulges could be prevented when the temperature increased. Figure 7 shows the resistivities of silver conductive lines prepared by inkjet printing from different silver nitrate concentrations with and without PVP by ethylene glycol vapor

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Wu et al. conductive lines reduced by ethylene glycol vapor for 1 h at 200 °C was 2.708 × 10-5 Ω cm, which is relatively close to the resistivity of bulk silver. Acknowledgment. The financial support provided by the National Science Council through project NSC-95-2120-M-006003 and the Landmark Program of the NCKU Top University Project (A020) is greatly appreciated. References and Notes

Figure 7. Resistivities of silver conductive lines prepared by inkjet printing from different silver nitrate concentrations with and without PVP by ethylene glycol vapor reduction.

reduction. The thickness was measured by a KLA-Tencor/ASIQ new Alpha-Step Profilometer. The resistivity can be calculated by the equation7,24,30

F ) RA/L where the resistivity F of the conductive lines was calculated from the resistance R, the length L, and the cross sectional area A. Here, the effect of adding PVP can be easily noticed. For the silver nitrate inks without PVP, it could not form a continuous line at a low concentration. Compared to that, the silver nitrate/PVP inks could produce smooth lines at a low concentration. We anticipated that silver conductive lines prepared by silver nitrate/PVP inks could have a lower resistivity than silver conductive lines without PVP after ethylene glycol vapor reduction. With the increase of silver nitrate/PVP ink concentration, the resistivity of the lines decreased. They were 7.002 × 10-5, 4.679 × 10-5, and 2.708 × 10-5 Ω cm when the silver nitrate concentrations were 1, 5, and 10 M, respectively. The lowest resistivity of the silver lines was 2.708 × 10-5 Ω cm measured by the two-point probe method, which is about 10 times that of the resistivity of bulk silver (1.61 × 10-6 Ω cm). 4. Conclusions Using a high molecular weight organic compound, PVP, as the additive, we successfully prepared straight silver conductive lines by inkjet printing from silver nitrate/PVP inks on flexible Kapton substrates followed by ethylene glycol vapor reduction. Using various surface conditions by UV/O3 treatment and heating the substrate in addition to changing inkjet printing parameters, we could obtain continuous and smooth silver lines. The line width of the silver conductive lines was able to be controlled between 50.8 and 62.5 µm. The resistivity of the silver

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