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Laser direct writing of silver metal electrodes on transparent flexible substrates with high bonding strength Weiping Zhou, Shi Bai, Ying Ma, Delong Ma, Tingxiu Hou, Xiaomin Shi, and Anming Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07696 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 29, 2016

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ACS Applied Materials & Interfaces

Laser direct writing of silver metal electrodes on transparent flexible substrates with high bonding strength Weiping Zhou,† Shi Bai,† Ying Ma,† Delong Ma,† Tingxiu Hou,† Xiaomin Shi,† Anming Hu,†, ‡,* †

Institute of Laser Engineering, Beijing University of Technology, 100 Pingle Yuan, Chaoyang

District, Beijing 100124, China ‡

Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee,

Knoxville, 1512 Middle Drive, Knoxville, TN 37996, USA KEYWORDS: laser direct writing, silver metal electrodes, flexible electronics, flexible display, polyvinyl pyrrolidone, silver citrate

ABSTRACT: We demonstrate a novel approach to rapidly fabricate conductive silver electrodes on transparent flexible substrates with high bonding strength by laser direct writing. A new type of silver ink composed of silver nitrate, sodium citrate, and polyvinyl pyrrolidone (PVP) was prepared in this work. The role of PVP was elucidated for improving the quality of silver electrodes. Silver nanoparticles and sintered microstructures were simultaneously synthesized

ACS Paragon Plus Environment

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and patterned on substrate using a focused 405 nm continuous wave laser. The writing was completed through the transparent flexible substrate with a programmed 2D scanning sample stage. Silver electrodes fabricated by this approach exhibit a remarkable bonding strength, which can withstand a 3M tape test for at least fifty times. After a 1500 times bending test the resistance only increased 5.2%. With laser induced in-situ synthesis, sintering and simultaneous patterning of silver nanoparticles, this technology is promising for the facile fabrication of conducting electronic devices on flexible substrates.

INTRODUCTION Over the past decades, silicon-based devices are dominant in the microelectronics industry. However, to develop portable and wearable electronics, there are increasing interests to fabricate electronic devices based on flexible substrates. 1,2 These flexible electronics devices are also driven by emerging markets of consumable electronics and Internet of Things (IOT). Recent studies on flexible, stretchable, and wearable electronics have demonstrated great potential in a wide range of application, such as batteries, 3 sensors, 4 , 5 lighting, 6 displays, 7 , 8 robotics and automation,9 electronic skins for robotics and prosthetics.10,11 Dramatic progress has been made in integrating inorganic nano materials on flexible substrates through different printing methods to prepare flexible electronics. Wang et al. 12 synthesized a UV-curable resin mixing with nanosized silver (Ag) particles grown on modified multi-walled carbon nanotubes. These particles were subsequently patterned for conductive grating by a method of nanoimprint lithography with the help of a polydimethylsiloxane (PDMS) stamp on a polyethylene terephthalate (PET) substrate. An inkjet printing method13 can be used to deposit conductive inks on different flexible substrates with a large area substrates, such as PET, paper, and other


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polymer substrates, due to its lost-cost characteristics. Screen printing 14 is also an economic, simple printing technique for flexible electronics fabrication. Besides printing techniques, the methods of solution-flow-assisted assembly,15 and Langmuir–Blodgett16 were also developed to integrate nanowires on flexible substrates for flexible electronics. But these aforementioned techniques are difficult to simultaneously meet the requirements of low-cost, mask-less, highresolution, and rapid fabrication. The laser direct writing (LDW) technique appears to be one of the promising approaches to address these issues17. LDW is also competitive by considering the compatibility with a wide range of materials, the adaption of surface morphology and surface chemistry with a high resolution down to a sub-micron dimension, i.e. the optical diffraction limit, as well as the minimized heat effect to the substrate. 18 , 19 Compared to conventional approaches including synthesis and patterning, LDW provides high flexibility via non-contact and maskless fabrication processes, which significantly reduce the fabrication cost. 20 More importantly, LDW can achieve a one-step fabrication by combining local processing with patterning, resulting in a significantly enhanced manufacturing efficiency. Despite various progresses have been made so far, nanoparticle ink is still need to be prepared first and then the laser was used to sinter metal nanoparticle ink on flexible substrate.21,22,23 The poor mechanical performance such as the limited adhesive of conducting circuits to the substrate and strong conductive

dependence

of

bending

prevent

LDW-based

flexible

electronics

from

commercialization. In this study, we demonstrate a novel approach to rapidly fabricate conductive silver electrodes on transparent flexible substrates with high bonding strength by laser direct writing. A new type of silver ink composed of silver nitrate, sodium citrate, and polyvinyl pyrrolidone (PVP) was developed in this work. Silver nanoparticles and sintered microstructures were simultaneously


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synthesized, patterned and sintered on substrate at one-step writing using a continuous wave laser. The silver electrodes fabricated by this approach exhibited a remarkable bonding strength. It can withstand 3M tape test, an ASTM class: 5B. Development of this approach represents a crucial step of LDW method for the fabrication of high quality flexible electronics. EXPERIMENTAL SECTION Silver ink preparation. A new type of silver ink was prepared by the following procedures. Silver nitrate (AgNO3), sodium citrate, and Polyvinyl pyrrolidone (PVP, MW=40,000) were all purchased from Tianjin Fu Chen Chemical Reagents Factory (Tianjin, People's Republic of China). All these reagents, in analytical grade, were used without further purification. 3.0 g sodium citrate and 250 mg PVP were dissolved in 100 mL deionized water. 5.2 g AgNO3 was dissolved in 80 mL deionized water. Then the AgNO3 solution was slowly poured into the mixed sodium citrate and PVP solution under magnetic stirring. After one hour stirring, the silver ink was obtained. The viscosity of the ink was 69.50 mPa.s measured by a rheometer. Substrate preparation and silver ink coating. Polycarbonate (PC) film was chosen as the flexible substrate in this study. The PC film with thickness of 0.3 mm was washed by alcohol under ultrasonication. Then the film was treated by oxygen plasma to increase its hydrophilicity and improve adhesion of the silver to the substrate. The surface of the PC film was treated by reaction ion etching for 2.5 minutes under the condition of O2 20 L/min, 200 W. After these processing, the silver ink was coated on the PC film by a simple scraper coating technology. Then the sample was dried in oven at a temperature of fifty degrees at ambient atmosphere. Laser direct synthesis and pattering process. A continuous wave laser with a wavelength of 405 nm (Beijing Laserwave Optoelectronics Technology Co., Ltd) was used in this study. A beam expander and a microscope objective were used to focus the laser beam on the surface of


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the PC substrate. The laser beam was focused into a spot with a size less than two hundred microns. Schematic of the experimental setup for LDW was shown in Figure S1. Silver electrodes were written by focusing laser beam to the interface between PC film and silver ink layer through the transparent PC film. After the scanning process was completed, the PC surface was washed with deionized water to remove the sodium ions and unreactive silver ion film. Figure S2 shows the energy dispersive spectrometer (EDS) patterns measured before and after washing, which demonstrates that the sodium ions were almost washed away. RESULTS AND DISCUSSION In this study, AgNO3 was mixed with sodium citrate at a molar ratio of 3:1 at room temperature. The main component of the prepared ink was thus silver citrate. Ag ion ink was synthesized via an ion exchange method at room temperature according to the following reaction mechanism (Scheme 1):24

Scheme 1. Reaction of the AgNO3 and sodium citrate. In the process of preparation of silver ink, a small amount of PVP was added in it. PVP as a protective agent plays an important role in controlling superfine silver particle size and size distribution by reducing silver nitrate.25,26 In our study, another purpose of adding PVP was to increase the hydrophilicity of silver ink. Figure 1(a1, b1) shows the contact angles of the silver ink without PVP and with PVP on the surface of a same PC film. The silver ink does not wet PC substrate very well and spread out without PVP. Besides, PVP led to improve the quality of laser direct patterning process.27 Figure 1(a2-3, b2-3) shows images of silver electrodes with the same


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processing conditions except without and with PVP. Without the addition of PVP, some parts of the pattern are washed away. While with the addition of PVP the adhesion of the pattern is significantly improved, and the patterns have a better thickness uniformity. Cheng 27 et al. investigated the effect of PVP-coated Ag nanoparticles (NPs) using a laser direct patterning process by considering a photothermal effect. Ag NPs were synthesized first. PVP was mixed with Ag NPs as ink. A UV laser was used to direct patterning of Ag NPs conductive patterns on a polyimide substrate. The effect of Ag NPs with different PVP contents before and after laser direct patterning was elucidated. They found that the PVP/Ag weight ratio will affect the particle size distribution. In their research, they found that the higher PVP content, the better surface uniformity. The smooth boundary of Ag NPs conductive lines is achieved with the PVP/Ag weight ratio ranging from 2 to 8 wt%. The adhesion between Ag NPs and PI substrate also increases with the PVP content of Ag NPs. Our result verifies similar roles of PVP in a LDW process with 405 nm laser. In our research the weight ratios of PVP/AgNO3 was 5 wt%. Combined with good control of the laser parameters we got flexible electrode with high performance.


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Figure 1. Contact angle of the silver ink without PVP (a1) and with PVP (b1) on the surface of a same PC film. Photos of the silver electrodes before washing process without PVP (a2) and with PVP (b2), and after washing process without PVP (a3) and with PVP (b3), which other conditions were the same. Based on previous studies,25-27 the protection mechanism of PVP is divided into three stages. At first, the PVP-silver ions complex is constructed via a coordinative bonding. Secondly, silver reduction and nucleation are conducted with the complex, which tends to produce small silver particles. Thirdly, a steric effect of PVP is induced via a physical and chemical bonding, which inhibits particle-particle contact. The steric effect is remarkable during the grain growth step and also during washing process.25,26 The steric effect arises from the long polyvinyl chain of PVP on the surface of silver particles. It is difficult to remove the PVP from the surface of the silver particles because of the chemical bonding between the PVP and silver particles. So PVP is helpful for improving the adhesion between the silver particles and the polymer substrate because PVP is also easy to bond to both polymer substrate and silver nanoparticles. In this research an innovative way is unveiled to improve the adhesion between the silver particles and the polymer substrate. Silver electrodes were written by focusing the laser beam to the interface between the PC film and silver ink layer through the transparent PC film, as shown in Figure S1. Figure 2 shows SEM images of a PC film using the laser direct writing with the laser power of 1.0 W on the silver ink layer from the front side and to the interface between PC film and silver ink layer after removing the surface silver electrodes. It can be seen that different damage degrees of the PC film at two writing methods. The laser beam causes more microscale damage of PC films when writing through the PC film on the interface between PC film and silver ink layer. In this case, microscale damage is generated because of the thermal effect during


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the laser reducing and sintering the silver nanoparticles on the PC surface. This rough surface, however, can improve the adhesion of silver electrodes to the substrate. Meanwhile, there is no damage on the other side of the PC film, which is evident by a smooth surface, as shown in SEM images in Figure S3.

Figure 2. SEM images of the PC film using the laser direct writing with the laser power of 1.0W on the silver ink layer from the front (a) and to the interface between PC film and silver ink layer from the backside (b) after removing away the silver electrodes. To optimize laser processing, a 405 nm continuous wave laser was used as a photonic reduction and sintering tool to fabricate silver pattern by comparison to an 808 nm continuous wave laser. The absorption spectra of the silver ink without PVP and with PVP are shown in Figure 3. It can be seen that the addition of PVP can produce strong ultraviolet absorption at the same silver ion concentration. Thus the 405 nm laser is better absorbed. This is consistent with the reported results.25 So for processing this silver ink, 405 nm laser is more suitable for LDW than a long wavelength laser at 808 nm.


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No PVP With PVP

3.0 2.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.5 1.0 0.5 0.0 200

300

400

500

600

700

800

900

Wavelength (nm)

Figure 3. Absorption spectra of the silver ink without PVP (black curve) and with PVP (red curve) at the same silver ion concentration To investigate the effects of the processing parameters of LDW, silver patterns were written at different laser powers and scanning speeds. Figure 4 shows SEM images of silver patterns fabricated at different laser processing conditions. Figure 4(a1-a3) are the images of silver electrodes written with various laser powers, but at the same speed of 3 mm/s. In Figure 4(a1), most of the silver patterns fabricated with a laser power of 0.5 W are washed away. The silver citrate ink is nonconductive. The laser writing track changes into grey and becomes conductive after the irradiation. The inset SEM image in Figure 4(a1) shows that the silver nanoparticles are just reduced from the silver ink but not sintered. Therefore, the writing track can be washed away although the metal silver is reduced from the silver citrate. XRD results are shown in Figure 5. XRD patterns of silver electrode show that there are four diffraction peaks, which agree well with the (111), (200), (220), and (311) diffraction of face centered cubic silver. The space group is Fm3തm (JCPDS File 04-0783)28. To elucidate the laser reduction and sintering mechanism, Figure S4 shows SEM images of the silver ink annealed at different conditions. Apparently, more particles are reduced and larger silver nanoparticles are thermally sintered as the


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temperature rises up. At 250℃, the reduced nanoparticles were sintered to a network structure. On the other hand, as shown in Figure S4(f), it is notable that a UV photoreduction can only reduce silver nanoparticles from silver ink. There is no sintering under UV illumination with 100 W for 1 hour. This means that the reduction and sintering of silver nanoparticles are two different procedures. High temperature is necessary for simultaneous synthesis and sintering of silver nanoparticles.29,38 Increasing the power to 1.0W, the conductive line has a better surface uniformity and smooth boundaries. The reduced nanoparticles are also sintered to a network structure as shown in the Figure 4(a2). As the power is continuously increased to 1.5W, the surface uniformity and boundaries of the silver conductive line are even better than that with 1.0W, but the PC substrate is deformed. This phenomenon also appears at 1.0 W under the low speed of 1.5 mm/s, as shown in Figure 4(b1). But if the speed is too fast which up to 3.0 mm/s, the writing effect is not good either. Such as the surface was not uniform and the boundaries were not smooth, as shown in Figure 4(b3).

Figure 4. SEM images of the silver patterns written at different laser powers (a1: 0.5W, a2: 1.0W, a3: 1.5W) but at same speed of 3.0 mm/s, and different scanning speeds (b1: 1.5 mm/s, b2: 3.0 mm/s, b3: 4.5 mm/s) but with the same laser power of 1.0W. Inset is a corresponding high resolution SEM image.


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Figure 5. XRD patterns of the silver patterns written at (a) different laser powers, and (b) scanning speeds The nucleation-growth mechanism has been studied to produce metal particles with citrate reduction.30,31 Particles grow through the transfer of atoms or molecules from smaller particles to larger particles. This effect can be also seen from Figure S4(b-d) under thermal sintering. As the temperature rises, more and larger silver nanoparticles reduced from silver ink. Moreover, as shown in Figure S5, diffraction peak intensities of silver nanoparticles increase. Mass transfer between two nanoparticles during growth can be contributed to differences of the chemical potentials µ, where the µ varies as the particle radius of curvature.32 This mass transfer can be thermally facilitated. 33 ,38 This explains the simultaneous reduction and sintering of silver nanoparticles at a higher laser power shown in Figure 4. On the other hand, PVP can increase the probability of nucleus formation and photochemical process. 34,35 PVP macromolecules likely adopt a pseudo-random coil configuration, and take part in some form of association with the metal atoms. N and O atoms from PVP polar groups have a strong affinity to metallic silver particles. This allows PVP to decrease the surface energy of silver particles and keep them away from aggregation. But in a LDW process, the photothermal effect of nanoparticles4,27 can overcome the PVP protection and sintered silver nanoparticles into larger conducting grains or network, similar to a thermal annealing.


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Veeco surface profiler was used to accurately detect the surface morphology of the silver patterns which were written with different laser powers and scanning speeds. The results are shown in Figure 6 and Table 1. From these results it can be seen that as the gradual increase of the laser power and speed, the width of the silver conductive line increases gradually. The height of the electrodes is not changed if the same speed is used, but with the decrease of writing speed, the height increased gradually. Due to the thermal effect, the line width is roughly equal to the spot size. It is practical to write a narrower line with a shorter laser wavelength or by a twophoton absorption effect with ultrafast pulsed laser36. The further work on this direction is under way. The conductivity is also presented in the Table 1.

Figure 6. Two dimensional surface profiles of silver patterns written at different laser powers (a1: 0.5W, a2: 1.0W, a3: 1.5W), scanning speeds (b1: 1.5 mm/s, b2: 3.0 mm/s, b3: 4.5 mm/s)

Table 1. Width, height and conductivity of the silver lines written at different laser powers and scanning speeds. Parameter

Width (µm)

Height (µm)

Conductivity (S/m)

0.5w (3 mm/s)

199

23.1

--

1.0w (3 mm/s)

404

23.2

2.39×105


12

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1.5w (3 mm/s)

609

23.0

1.45×106

1.5 mm/s (1.0W)

520

35.2

2.04×106

3.0 mm/s (1.0W)

418

23.1

2.39×105

4.5 mm/s (1.0W)

218

17.9

4.65×104

Based on the aforementioned results, we fabricate silver electrode on PC substrate with the laser power of 1.0 W and a writing speed of 3.0 mm/s. The mechanical and electrical stability of the silver pattern fabricated by LDW on PC substrate are examined under adhesive tape test and cyclic bending deformation. The adhesion strength was assessed by a 3M-tape test. The tape was pressed down strongly on the surface and then peeled off. Figure 7(a) shows that silver patterns are not peeled off from PC substrate with the tape. It means that the adhesion strength passed ASTM class: 5B. Figure S6 shows the photos after different test times using 3M-tape. It can withstand at least fifty test times. This shows an excellent adherence of silver patterns to PC film for various practical applications. According to the equation of resistivity: ρ=RS/L, we can roughly calculate resistivity of silver electrode is about 4.18 µΩm, better than a value of 6.67 µΩm of silver nanoparticles film on polydimethylsiloxane substrate annealed in the convection oven at 140 degrees for 20 minutes.37 The conductivity arises from nanoparticle sintering but the resistivity was still limited by a porous structure and grain boundaries.38 We have tested the transmission and haze of the PC film used in this research and the silver electrodes on the PC film at the visible light band. The transmission of the PC film is about 89.7% and the silver electrode is 17.5%, as shown in Figure S7. The haze of the PC film and the silver electrodes are 1.8% and 49%, respectively. The bending test of the flexible substrate was created by moving one end while the other end was fixed with the frequency of 1 Hz. The deformation yielded a radius of curvature r = 7 mm at the maximum bending, as shown in Figure 7(b). The resistance


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increases in the initial 500 bending cycles. After that the change of the resistance becomes slow and gradually saturated. The experiment shows that through 1500 bending cycles, the change of resistance is only 5.2% of its original value. Figure 7(c, d) show SEM images of the silver nanoparticles electrode before and after 1500 bending cycles testing. There are some micro cracks appeared in the silver electrode. These microcracks may cause the resistance increase. Without the addition of PVP in silver ink, the electrodes are badly damaged after only one time bending test, as shown in Figure S8.

Figure 7. Adhesion (a) and bending (b) test results of silver patterns on PC film by LDW. SEM images of the silver nanoparticles electrode before (c) and after (d) 1500 bending cycles testing. Inside is a high resolution SEM image. Figure 8 shows an application of flexible LDW electrodes. A light-emitting diode (LED) array was connected to the silver electrodes on PC substrate using silver paste. Figure 8 (a) and (b) show LED glows under both the relaxing and twisting states of the flexible substrate.


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Figure 8. Flexible electrodes fabricated by the DLW process for flexible display, (a) LED glows under the normal state, (b) LED glows perfectly after twisting the substrate CONCLUSION We developed a novel approach to rapidly fabricate conductive silver electrodes on transparent flexible substrate with high bonding strength by laser direct writing. A new silver ink composed of silver nitrate, sodium citrate, and polyvinyl pyrrolidone (PVP) was developed in this work. The purpose of adding PVP into silver ink is to increase the hydrophilicity of the ink and also improve the adhesion of the pattern with PC substrate by LDW. Silver nanoparticles and sintered microstructures were directly synthesized and simultaneously patterned on substrate using a continuous wave laser. The resistivity is about 4.18µΩm. The silver electrodes fabricated by this approach exhibit the remarkable bonding strength. It can withstand at least fifty times 3M tape test, which passed ASTM class: 5B. From the bending test result, after 1500 times test the resistance only increased 5.2%. These results suggest that the silver pattern exhibit an excellent performance and suitable for various practical applications. These advantages show this technology to be a promising process for the fabrication of next generation electronic devices on flexible substrates.


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ASSOCIATED CONTENT Supporting Information Schematic of the experimental setup for LDW, EDS patterns of the samples, SEM images of backside of the film, SEM images and XRD patterns of the silver ink treated at different conditions, optical transmission of the samples, and photos and SEM images of silver electrode without PVP after bending. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Anming Hu. Address: Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, 1512 Middle Drive, Knoxville, TN 37996, USA. E-mail: [email protected]; [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The work is supported by National Natural Science Foundation of P. R. China (51575016) and a strategic research grant (KZ20141000500, B-type) of Beijing Natural Science Foundation, P. R. China and a Joint Development Research and Development (JDRD) program (U013960010) between University of Tennessee Knoxville and Oak Ridge National Lab (ORNL), USA.


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Laser-Direct Writing of Silver Metal Electrodes on ... - ACS Publications

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