ARTICLE pubs.acs.org/JPCC
Inkjet Printing of Low-Temperature Cured Silver Patterns by Using AgNO3/1-Dimethylamino-2-propanol Inks on Polymer Substrates 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. ABSTRACT: 1-Dimethylamino-2-propanol (DP) was added to silver nitrate to fabricate silver conductive lines by inkjet printing at low sintering temperatures on flexible PET substrates. Using an optimal ratio of a mixed solvent (ethanol and ethylene glycol), the morphology of the pattern surface and the formation of coffee-rings could be controlled. X-ray diffraction (XRD) analysis indicated that the AgNO3/DP inks were converted to silver completely at low temperatures. Using the AgNO3/DP inks, continuous and smooth silver conductive lines with a resistivity of 1.37 ( 0.44 105 Ω cm were fabricated at 100 °C by an inkjet printing system. This resistivity was close to the resistivity of bulk silver.
1. INTRODUCTION Inkjet printing of functional materials has been widely investigated and used as a low-cost alternative in the production of organic thin-film transistors14 and radio frequency identification (RFID) tags.5 By means of the direct-write approach, patterns or structures can be obtained directly without using the conventional photolithography process. In particular, using inkjet printing to fabricate conductive tracks (circuits) has been shown to have great potential in the electronic industry.6 In recent literature, new silver-containing inks have been reported that can be converted at temperatures lower than 200 °C.710 Some materials can also be used for inks, such as conductive polymers11 and carbon nanotubes.12 However, these materials are characterized by poor electrical conductivity. Several methods have been proposed for the fabrication of silver conductive tracks using inkjet printing, including chemical reduction,13,14 thermal decomposition in organic solvents,15 photoreduction,16 silver oxide reduction,17 and microwave-assisted,18 and argon plasma-assisted methods.19 Recently, a number of silver-containing inks have been reported for inkjet printing.2022 However, they need relatively high sintering temperatures, typically above 200 °C, in order to render the printed features conductive.2325 The high sintering temperatures limit the application of the silver inks on flexible polymer substrates, such as poly(ethylene terephthalate) (PET), which has a low softening temperature at 150 °C.22 To solve this problem, metalorganic complexes in solution have been used to fabricate conductive silver tracks with a lowtemperature process.26,27 Although the inkjet printing system has several advantages, the morphology of silver conductive lines is difficult to control. Thus, the inkjet printing parameters and substrate treatment are very critical for the inkjet printing process.2835 r 2011 American Chemical Society
In this study, we report a new silver ink system by using 1-dimethylamino-2-propanol (DP) as both the protecting and reducing agent for AgNO3 to reduce the sintering temperatures and using ethylene glycol (EG) and ethanol (EA) mixed solvent to prevent the formation of coffee-rings.29,3638 The inkjet printing parameters and substrate treatment conditions were also studied.
2. EXPERIMENTAL PROCEDURE 2.1. Materials. Silver nitrate (AgNO3) was obtained from Showa Chemical Co. Ethylene glycol (EG) was purchased from Sigma-Aldrich Co. Ethanol (EA) was purchased from SigmaAldrich Co. Poly(ethylene terephthalate) films (PET) were obtained from Du Pont Co. 1-Dimethylamino-2-propanol (DP) was obtained from ICN Biomedical Inc. 2.2. Preparation of Silver Conductive Lines and Films on a PET Substrate. Silver nitrate was dissolved in a mixed solvent of ethanol and ethylene glycol. To this solution was added DP and the mixture stirred for 30 min. A 2 cm 2 cm PET film was cleaned with acetone and deionized water to remove the particles and organic contaminants on the surface. AgNO3/DP inks with different concentrations were printed by an inkjet printer onto the PET substrates. The printer setup consisted of a drop-ondemand (DOD) piezoelectric inkjet nozzle. A squeeze-mode piezoelectric printhead MJ-AT-01 manufactured by MicroFab Technologies Inc. The squeeze-mode piezoelectric printhead MJ-AT-01 was manufactured by MicroFab Technologies Inc. The diameter of the printhead nozzle orifice is 80 μm. The xy Received: November 19, 2010 Revised: May 1, 2011 Published: May 13, 2011 10940
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The Journal of Physical Chemistry C stage control area was 200 200 mm, and the xy accuracy was (15 μm. The repeatability was (5 μm, as shown in our previous paper.39 It is found that the regularity and repeatability of droplets formed are satisfactory when the frequency of droplet ejection is 300 Hz. The waveform was set to be 2 μs for trise, 5 μs for tdwell, 2 μs for tfall, 5 μs for techo, and 2 μs for tfinalrise. Silver conductive lines and patterns were printed using AgNO3/DP inks under different interspacing distances. The resulting lines were heated to 100 °C and held at this temperature for 60 min in order to convert AgNO3 lines to silver lines. The PET substrate was then cooled by natural convection to room temperature inside a glass dish. The procedure for the preparation of silver films was similar to that of silver conductive lines except that the AgNO3/DP inks were dripped onto PET substrate but not by an inkjet printer. 2.3. Characterization. An 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 deg min1 at 30 kV and 20 mA. The weight losses of the silver films were analyzed using a TA Instrument Thermogravimetric Analyzer (TGA) 2050 at a heating rate of 10 °C/ min under air. The viscosities of the ink solutions were measured with a Brookfield Viscometer DV-IIþPro with a UL/Y Adapter with a shear rate of 24.5 s1 at 25 °C. The texture of silver lines after reduction was investigated by Zoom 125 optical microscopy (OM). The 3 M AgNO3/DP ink was used for the contact angle measurement in this study. Contact angle measurements were performed using an FTA 125 Contact Angle Analyzer. The drop volume of the inks was 35 μL, and the contact angle was calculated using the FTA software. The thickness of the silver conductive lines was measured with a KLA-Tencor/AS-IQ new Alpha-Step Profilometer. The resistivity of the silver films was measured by the four-point probe method using Napson RT-7. The resistivity of the silver conductive lines was measured by a 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 In this study, the silver inks were prepared from a reduction of AgNO3 by 1-dimethylamino-2-propanol (DP) at room temperature. Two functions were expected with the DP molecule. One was related to the hydroxy group, which was used as a reducing agent. The other was the amine group, which could protect the Ag ion by a chemical interaction between Ag and NH2.4042 Because the nitrogen in DP has lone pair electrons which can be used as the ligand to dissociate lattices of silver nitrate and form silverorganic complexes, [AgN(CH2)3CHCH3CH2OH]þ, in an organic solvent, the silverorganic complexes are easy to become silver at low sintering temperatures.43 A mixture of EG and EA was used as a cosolvent for the silver inks to prevent the formation of coffee-rings, which may be due to the different evaporation rates, boiling points (Tb), and viscosities of these two solvents (EA: Tb = 78.4 °C, viscosity = 1.38 cP, and EG: Tb = 197 °C, viscosity = 13.7 cP). The cosolvent also has the reducing ability for the silver ion. Due to the high viscosity of EG, the morphological control can be effectively improved.29 In addition, particle segregation between the center and edge during evaporation can be eliminated
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Figure 1. Viscosity of solvents with different mixing ratios.
Figure 2. XRD diffraction patterns of silver tracks sintered at 100 °C for 60 min from 1 M AgNO3/DP inks with different solvent ratios.
due to the high boiling point of EG. In order to find the optimal mixing ratio of the cosolvent (DP, EG, and EA), different mixing ratios of cosolvents were prepared. With the content of EG decreased, the viscosity of the cosolvents decreased from 14.4 ( 0.1 cP to 1.73 ( 0.01 cP with a shear rate of 24.5 s1 at room temperature, as shown in Figure 1. When 1 M AgNO3 was added to the cosolvent, the viscosity of the solution increased from 4.99 ( 0.02 cP to 10.7 ( 0.1 cP with DP/EG/EA = 2:1:1 at a shear rate of 24.5 s1. The XRD patterns with different solvent ratios and sintered at 100 °C for 60 min on PET are shown in Figure 2. The results indicate that the silver ion can be transformed to silver crystal. The reflection peaks were indexed as fcc (111), (200), (220), and (311) planes. The intensities of the reflection peak increased with the increasing EG content. This is due to the reduction ability of EG.44,45 Because the boiling point of DP is 90 °C, we used 100 °C as the thermal treatment temperature. Form Figure 2, the silver ion can be transformed to silver crystal when the thermal treatment temperature is higher than 100 °C. We have found that the resistivities of the silver films were 9.09 ( 3.35 102 Ω cm, 5.32 ( 3.39 103 Ω cm, 1.02 ( 0.35 104 Ω cm, and 3.89 ( 0.93 106 Ω cm when the thermal treatment times were 15 min, 30 min, 45, and 60 min, respectively. After 60 min, the resistivity of the silver films 10941
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Table 1. Resistivity of the Silver Films Fabricated by SpinCoating from Different Solvent Ratios (DP/EG/EA) with 1 M AgNO3 Sintered at 100 °C for 60 min DP/EG/EA
2:1:1
4:1:3
8:1:7 0.1459
sheet resistivity (Ω)
0.0084
0.0397
thickness (μm)
4.62
4.32
2.38
resistivity (μ 3 Ω 3 cm)
3.89
17.23
34.73
Figure 4. The viscosity of different concentrations of AgNO3/DP inks with the solvent ratio of DP/EG/EA = 2:1:1.
Figure 3. XRD diffraction patterns of silver tracks sintered at different temperatures for 60 min from 1 M AgNO3/DP inks.
remained at the same level. To investigate the conductivity of the AgNO3/DP inks with different solvent ratios, they were spin-coated on the PET substrate and heat-treated at 100 °C for 60 min on a hot plate. The resistivity of the silver film was measured by the four-probe method, as depicted in Table 1. The resistivity was 3.89 ( 0.93 106 Ω cm, 1.72 ( 0.77 105 Ω cm, and 3.47 ( 1.89 105 Ω cm, respectively, when the ratios of the solvents (DP/EG/EA) were 2:1:1, 4:1:3 and 8:1:7, respectively. With the ratio of the solvents increased to 2: 1: 1, the resistivity decreased to 3.89 ( 0.93 106 Ω cm, which is approximately 3 times the resistivity of bulk silver. Due to this ratio (2:1:1) providing the highest intensity of the reflection peak and the lowest resistivity, this ratio was chosen in this study as the cosolvent ratio. Figure 3 shows the XRD images of the silver films sintered for 60 min on the PET substrate at different temperatures. It can be seen that the silver crystals have similar intensities at these temperatures. The viscosities of different concentrations of the AgNO3/DP inks at room temperature are shown in Figure 4. Apparently, the viscosity increased with the increasing silver nitrate concentration. The viscosities of the AgNO3/ DP inks were in the range of 7.99 ( 0.10 to 23.2 ( 0.2 cP at a shear rate of 24.5 s1. The adhesion of the silver film to the PET substrate has been measured by the tape test according to
Figure 5. Microscopic images of silver lines printed on PET substrates at room temperature: (a) Ldroplets = 140 μm (b) Ldroplets = 120 μm (c) Ldroplets = 100 μm (d) Ldroplets = 80 μm (e) Ldroplets = 60 μm (f) Ldroplets = 40 μm (Ldroplets = interspacing distance between dots).
ASTM-3359B. The adhesion rating was 4B, which means that for detachment of small flakes of the coating at the intersections of the cuts, a cross-cut area is not significantly affected by more than 5%. In order to obtain smooth conductive patterns with high resolution by inkjet printing, various printing conditions, including the interspacing distance between dots, the composition of the silver inks, and the substrate temperatures were adjusted. Using the AgNO3/DP inks (DP/EG/EA = 2:1:1), the driving parameters of the microdrop piezoelectric nozzle were easily optimized. The droplet size was about 80 μm in diameter when a nozzle with different interspacing distance between the dots was 10942
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Figure 6. Microscopic images of silver lines printed on PET substrates with different interspacing distance between dots at different substrate temperatures.
Figure 7. (a) Microscopic images of inkjet printed silver tracks sintered at 100 °C for 60 min. (b) SEM image. (c) Higher-resolution SEM.
Table 2. Resistivity, Contact Angle and Line Width of Silver Conductive Lines Fabricated by Inkjet Printing from Different Sintering Temperatures on Various Flexible Substrates thermal temperature Ag ink
substrate
(°C)
line width resistivity (u 3 Ω 3 cm)
bulk silver
961
AgDP
PET
100
silver(I) 2-[2-(2-methoxyethoxy)
PET
130
TEC-IJ-040, InkTec Co., Ltd., Korea AgNO3
polyethylene Kapton
130 200
12.6 27.1
Cabot Printing Electronics and Displays,
polyarylate
200
paper
100
1.61 13.7
contact angle (deg) 26.54
103.6
source this study
ref 25
20.2 29.29
197 52.5
ref 20 ref 28
7
31.8
40
ref 22
30
1300
ref 10
9.10
(μm)
ethoxy]acetate
Albuquerque, U.S.A. Ag carboxylate ink
used.34,35 Figure 5 shows the optical microscopic images of the inkjet printed silver conductive lines at room temperature on the
PET substrates using different interspacing distances between the dots. According to literature, the line width can be conjectured 10943
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The Journal of Physical Chemistry C by the following equation:46,47 3
w ¼ 2
d π 6p
θ cos θ 2 4 sin θ 4 sin θ
where the line width w was calculated from the contact angle θ and the interspacing distance between the dots p. From this equation, the line width was found to increase with decreasing p at the flexible substrate. This shows that it is possible to find the line width if θ and p are known, as shown in Figure 5. When the interspacing distance exceeded 100 μm, we only could observe circular patterns, as shown in a and b of Figure 5. With 100 μm of interspacing distance between dots, the silver conductive lines could be prepared with the line width at about 140 ( 5.1 μm. When the interspacing distance between dots decreased, the width of the silver conductive lines increased from 140 ( 5.1 μm to 204 ( 6.6 μm, as shown in Figure 5cf. Because the interspacing distance between dots decreases, the droplets between dots will have more overlap, and thus, the line widths will increase.4648 In order to investigate the influence of the substrate temperature on inkjet printing, we tried to use different substrate temperatures (30, 50, and 70 °C). Figure 6 shows the optical microscopic images of patterns printed with different substrate temperatures for each of the four interspacings. The line width decreased from 178 ( 6.4 μm to 137 ( 6.0 μm at 30 °C with the increase of interspacing distance between dots. A similar effect has been observed for the inks printed at either 50 or 70 °C.
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When the interspacing distance between dots was at 100 μm, the line width decreased from 137 ( 6.0 μm to 104 þ 5.0 μm with the increase of substrate temperature. The decrease of the line width may be due to the increasing evaporation rate of the solvent at higher temperatures.49 When the substrate temperature increased, the evaporation rate would rapidly increase. The intense evaporation at the interface of the substrate and the droplet produced a vapor recoiling force that pushes the interface into the liquid. When the interface was pulled toward the liquid, the contact angle would increase with the evaporation rate. Due to this phenomenon, the width of the silver conductive line decreased.50 The line widths obtained at room temperature were larger, compared to the other line widths obtained at higher temperatures. The optical microscopic images and SEM image of inkjet printed silver tracks sintered at 100 °C for 60 min as shown in Figure 7a,b. Figure 7c presents the higher-resolution SEM. The silver lines sintered at low temperatures could form smooth conductive lines. Table 2 shows the resistivity, contact angle, and line width of silver conductive lines obtained by inkjet printing from different sintering temperatures on various flexible substrates. The AgNO3/DP inks had a contact angle of 26.54° ( 2.3° on PET substrate without any substrate treatment. The optical microscopy image of the contact angle is shown in Figure 8. The surface tension of the ink is 30.89 mN/m for the 3 M AgNO3/DP ink. Due to the low contact angle, we were able to create welldefined straight lines with interspacing distance between dots at 100 μm. The thickness was measured by a KLA-Tencor/AS-IQ new Alpha-Step Profilometer. The resistivity can be calculated by the equation below17,21 F ¼ R 3 A=L
Figure 8. Optical microscopy image shows the contact angle for AgNO3/DP ink on the PET substrate.
where the resistivity F of the conductive lines was calculated from the resistance R, the length L, and the cross-sectional area A. After sintering, the average thickness of the lines was 323.8 nm, and the roughness was 45.2 nm. Both were measured by an Alpha-Step Profilometer. In this study, the resistivity of the silver conductive line was 1.37 ( 0.44 105 Ω cm at a low sintering temperature (100 °C) for 60 min. The resistivity of the silver conductive lines is comparable to the reported data (in Table 2). It is about 9 times the resistivity of bulk silver (1.61 106 Ω cm). The array patterns on the PET substrates are shown in Figure 9. On the left side, it can be seen that the “NCKU” image is displayed on the PET substrate by the inkjet printing system using AgNO3/DP inks (the ratio of solvent =2:1:1). On the right side, the “MSE” image can be seen clearly. We are also able to
Figure 9. Microscopic images of silver patterns printed on PET substrates at room temperature. 10944
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The Journal of Physical Chemistry C fabricate other patterns by the AgNO3/DP inks in the inkjet printing system.
4. CONCLUSIONS In summary, using DP as the protecting and reducing agent, we successfully prepared highly concentrated AgNO3/DP inks for inkjet printing. Using an optimal ratio of a mixed solvent (EG and EA), we were able to control the morphology of the pattern surface and inhibit the formation of coffee-rings. The AgNO3/ DP inks not only can reduce the silver sintering temperature but also can improve the pattern morphology and pattern resolution. In addition, we could fabricate continuous and smooth silver lines by controlling the inkjet printing parameters. The resistivity of the silver conductive lines sintered at 100 °C for 60 min was 1.73 ( 0.44 105 Ω cm, which is relatively close to the resistivity of bulk silver. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ886-6-2757575 ext. 62900. Fax: þ886-6-2346290. E-mail:
[email protected].
’ ACKNOWLEDGMENT The financial support provided by the National Science Council (Taiwan, ROC) through Project No. NSC 99-2120M-006-009 is greatly appreciated. ’ REFERENCES (1) Arias, A. C.; Daniel, J.; Krusor, B.; Ready, S.; Sholin, V.; Street, R. J. Soc. Inf. Disp. 2007, 15, 485. (2) Kim, D.; Lee, S. H.; Jeong, S.; Moonz, J. Electrochem. Solid State Lett. 2009, 12, 195. (3) Takenobu, T.; Miura1, N.; Lu, S. Y.; Okimoto, Asano1, H., T.; Shiraishi, M.; Iwasa, Y. Appl. Phys. Express 2009, 2, 025005. (4) Gamerith, S.; Klug, A.; Scheiber, H.; Scherf, U.; Moderegger, E.; List, E. J. W. Adv. Funct. Mater. 2007, 17, 3111. (5) Amin, R.; Li, Y.; Rushi, V.; Manos, M. T. IEEE Antennas Propag. Mag. 2009, 51, 13. (6) Ahn, B. Y.; Duoss, E. B.; Motala, M. J.; Guo, X.; Park, S. I.; Xiong, Y.; Yoon, J.; Nuzzo, R. G.; Rogers, J. A.; Lewis, J. A. Science 2009, 323, 1590. (7) Noguchi, Y.; Sekitani, T.; Yokota, T.; Someya, T. Appl. Phys. Lett. 2008, 93, 043303. (8) Nguyen, B. T.; Gautrot, J. E.; Nguyen, M. T.; Zhu, X. X. J. Mater. Chem. 2007, 17, 1725. (9) Woo, K.; Kim, D.; Kim, J. S.; Lim, S.; Moon, J. Langmuir 2009, 25, 429. (10) Suganuma, K.; Wakuda, D.; Hatamura, M.; Nogi, M. IEEE Nanotechnol. Mag. 2010, 4, 20. (11) Pudas, M.; Hagberg, J.; Lepp€avuori, S. Prog. Org. Coat. 2004, 49, 324. (12) Vaillancourt, J.; Zhang, H.; Vasinajindakaw, P.; Xia, H.; Lu, X.; Han, X.; Janzen, D. C.; Shih, W. S.; Jones, C. S.; Stroder, M.; Chen, M. Y.; Subbaraman, H.; Chen, R. T.; Berger, U.; Renn, M. Appl. Phys. Lett. 2008, 93, 243301. (13) Hsu, S. L. C.; Wu, R. T. Mater. Lett. 2007, 61, 3719. (14) Wu, R. T.; Hsu, S. L. C. Mater. Res. Bull. 2008, 43, 1276. (15) Shim, I. K.; Lee, Y. I.; Lee, K. J.; Joung, J. Mater. Chem. Phys. 2008, 110, 316. (16) Valeton, J. J. P.; Hermans, K.; Bastiaansen, C. W. M.; Broer, D. J.; Perelaer, J.; Schubert, U. S.; Crawforde, G. P.; Smith, P. J. J. Mater. Chem. 2010, 20, 543.
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