- Email: [email protected]
Fabrication of highly conductive and flexible printed electronics by low temperature sintering reactive silver ink
Applied Surface Science 459 (2018) 249–256
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Fabrication of highly conductive and flexible printed electronics by low temperature sintering reactive silver ink ⁎
Yun Moua, Yuru Zhangb, Hao Chenga, Yang Penga, , Mingxiang Chena,c,
T
⁎
a
School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China c State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Reactive silver ink Low temperature sintering Highly conductive Flexible Printed silver film
To solve the problems of high sintering temperatures (∼200 °C), particle aggregation, nozzle clogging, and poor shelf life of silver nanoparticle inks, we prepared a transparent and stable reactive silver ink for fabricating printed electronics. The ink mainly consisted of ammonia and formic acid ligands, silver acetate, and hydroxyethyl cellulose (HEC) adhesive agent. The highly conductive and flexible silver films were fabricated by printing and low temperature sintering the reactive silver ink, and the effects of sintering temperature and sintering time on the electrical properties of the printed silver films were investigated. Consequently, the printed silver film sintered at 70 °C exhibits good electrical properties with a resistivity of 12.1 μΩ·cm, which is only seven times higher than that of bulk silver (1.65 μΩ·cm). Moreover, the silver film also displays excellent adhesive strength and mechanical flexibility in terms of bending and twisting.
1. Introduction Owing to their cost-effective and eco-friendly fabricating process and unique properties such as light-weight, bendable, and foldable, the printed electronics have attracted great attentions in various fields, such as flexible electrodes, low temperature bonding, solar cell arrays, radio frequency identification (RFID) tags, thin film transistors, and organic light emitting diodes (OLED) [1–7]. Conductive ink can be patterned by printing process to achieve robust tracks with high conductivity, which is a critical component for the printed electronics [8–10]. Currently, most conductive inks are based on silver nanoparticle inks because of their excellent electrical conductivity and good oxidation resistance [11–13]. Unfortunately, most of the reported silver nanoparticle inks require high sintering temperature above 200 °C to remove the organic stabilizers and sinter nanoparticles [14–17], which limits their practical applications on heat-sensitive flexible substrates. In order to decrease the sintering temperature, the chemical sintering of silver nanoparticle inks has been proposed to prepare the highly conductive silver tracks at room temperature [9,18,19]. However, the special sintering or destabilize agents and stabilizers are needed to achieve the coalescence and sintering of the nanoparticles, and the agglomeration of silver nanoparticles still is a severe issue to be solved, which results in clogging the nozzles and shortening the shelf life of the inks [20,21]. ⁎
Aiming to solve these problems, the particle-free silver reactive inks have received significant attentions because of their facile synthesis processes and low sintering temperature. More importantly, neither additional stabilizers nor reducing agents are required, which plays an important role in enhancing the performances of printed electronics [22,23]. At present, many researchers have synthesized different reactive silver inks to fabricate highly conductive silver tracks with the order of 10-5-10-6 Ω·cm [24–30]. However, most of the reported inks are less stable, and multi-component solvents in the inks have negative effects on the conductivity, which require relatively high sintering temperature (150–200 °C) to form highly conductive silver tracks. Although the lower sintering temperature and higher conductive silver tracks have been achieved by adjusting the ligands and silver precursors [31–34], the sintering process is too time-consuming (> 12 h) to be desirable for the actual production. Recently, the developed reactive metalorganic (ROM) inks can prepare silver tracks at room temperature by chemical reaction, which provide a lower thermal budget and faster route for fabricating the flexible printed electronics [35–37]. But this approach needs the reducing agents to trigger the chemical reaction, and the additional cleaning or curing process is essential to remove the residual organics. In addition, the advanced sintering technique (photonic and microwave-plasma) is a good alternative for decreasing sintering temperature and improve sintering efficiency [26,38,39], but thermal sintering is a preferred choice for fabricating conductive tracks
Corresponding authors at: School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China (M. Chen). E-mail addresses: [email protected] (Y. Peng), [email protected] (M. Chen).
https://doi.org/10.1016/j.apsusc.2018.07.187 Received 30 May 2018; Received in revised form 7 July 2018; Accepted 27 July 2018 Available online 29 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
morphological features of the printed silver films were observed by transmission electron microscope (TEM, FEI Tecnai G2 20 U-TWIN) and field-emission scanning electron microscope (SEM, FEI Nova Nano SEM 450), together with energy dispersive spectroscopy (EDS). Atomic force microscopy (AFM) was utilized to examine the 2D and 3D morphology of the printed silver films. The resistivity (ρ) of the printed silver films was calculated as ρ = Rs × W, where Rs is the sheet resistance and W is the thickness of the film. The sheet resistance and thickness of the film were measured by four-point probes resistivity measurement system (Probes Tech RTS-8) and step profiler (ET4000 Series), respectively. The resistance of the printed silver film was measured by the simple 2point I-V measurement method (Digital milliohmmeter, VC480C) during the peel-off and bending tests.
because of their low cost and simplicity. More importantly, the adhesion with substrate and printing stability is either not provided or discussed in most of the reported inks. Herein, we prepare a transparent and stable reactive silver ink, wherein ammonia and formic acid act as the ligands of silver acetate precursor. The highly conductive and bendable printed silver films on flexible substrates using the reactive ink were fabricated at 70 °C, and displayed good adhesion and stability. The effects of the sintering temperature and sintering time on the electrical properties of the printed silver films were systematically investigated, and the adhesive strength and mechanical flexibility of the printed silver films were discussed. 2. Materials and methods
3. Results and discussion
2.1. Materials
TG analysis was carried out to evaluate the thermal decomposition behaviors of the ink and silver acetate, as depicted in Fig. 2(a). The TG curve of the ink shows that the mass starts to decrease at room temperature, and the mass becomes constant at 130 °C. The weight loss of silver acetate starts at 225 °C and finishes at about 280 °C. The difference between the complete decomposition temperatures of silver acetate and the reactive silver ink is approximately 150 °C, which is attributed to the formation of silver ammonia complex and reduction of formic acid [40]. Besides, the remaining solid weights of the ink and silver acetate correspond to their silver weights, indicating that the organics are absolutely removed during the thermal sintering process. Moreover, when the reactive silver ink is hermetically stored in the refrigerator, it is stable and transparent, but the particles begin to form at room temperature with the evaporations of formic acid and ammonia. To analysis the crystal phases of the particles, XRD analysis was conducted after drying the ink at room temperature for 12 h, as shown in Fig. 2(b). Four diffraction peaks of the printed silver film are detected at 38.10°, 44.37°, 64.18°, and 74.16°, which represent the (1 1 1), (2 0 0), (2 2 0)and (3 1 1) planes of silver crystal. These characteristic peaks confirm that the printed silver film is face-centered cubic (FCC) silver phase without silver oxide or silver acetate. Therefore, TG and XRD analysis indicate that the reactive silver ink can be thermally decomposed to form silver particles at low temperature. To analyze the surface compositions of the printed silver film in depth, XPS analysis was performed after sintering the ink on PI substrate at 50 °C for 2 h. As shown in Fig. 3(a), carbon and oxygen elements are detected on the surface of the printed silver film, and may originate from silver acetate or HEC. Fig. 3(b) presents the peak fitting of Ag3d spectrum of the printed silver film. The characteristic Ag3d peaks at 367.5 eV and 373.6 eV are assigned to silver crystal, and no significant peaks of silver oxide or silver salt are separated, which demonstrates that the ink is completely thermal decomposed to silver at 50 °C. Fig. 3(c) shows the peak fitting of C1s spectrum of the printed silver film. The characteristic peaks appear at 284.8 eV and 286.6 eV, which correspond to the C-O group and C-C group of HEC, respectively. Fig. 3(d) shows the XPS spectra of O 1s peak of the printed silver film. The major peak is divided into two peaks at 531.2 eV and 533.0 eV, corresponding to the O1s peak of the adsorbed oxygen and C-O group of
Silver acetate (98%) and ammonia (25–28%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydroxyethyl cellulose-water solution (HEC, 4% in water) and Formic acid (88%) were purchased from Aladdin reagent Co. All chemical reagents were used without any further treatment. Polyimide (PI) substrates (thickness: 100 µm) were purchased from Toray Industries, Inc. 2.2. Preparation of reactive silver ink Fig. 1 illustrates the preparation process of reactive silver ink. Ammonia (2.0 g) and formic acid (0.36 g) were mixed at 5 °C in a glass vial. Subsequently, under magnetic stirring, silver acetate (1.0 g) was slowly dissolved in the mixing solution to prepare reactive silver ink, and the color of the solution changed from colorless to light brown. Finally, the obtained reactive silver ink was filtered by the filter syringe (200 nm), and stored at 3 °C in the refrigerator. Acting as the adhesive agent, HEC-water solution (0.6 g) was added to adjust the adhesion and wettability of the reactive silver ink. 2.3. Low temperature sintering process PI films were used as the flexible substrate in this work. To remove organic contaminations on the surfaces, the substrates were successively ultrasonic cleaned by in deionized water and ethanol for 10 min. Subsequently, the reactive silver ink was uniformly printed on the substrates by mask-printing method. The thickness of the mask was 50 μm. Finally, the printed reactive silver inks were sintered in the air from 50 °C to 100 °C for 0.5 h to 8 h. 2.4. Characterization and measurement The thermal behaviors of the reactive silver ink and silver acetate were investigated by thermogravimetric analyzer (TG, TG 209 F3 Tarsus) at a heating rate of 5 °C/min in the air. The crystal structures and surface compositions of the printed silver films were analyzed by Xray diffractometer (XRD, PANalytical PW3040/60) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi), respectively. The
Fig. 1. Schematic preparation process of the reactive silver ink. 250
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 2. (a) TG curves of the reactive silver ink and silver acetate, and (b) XRD pattern of the printed silver film at room temperature in the air.
HEC, respectively. Therefore, the printed silver film is made of silver particles and a small amount of HEC. The low electrical resistivity is a significant index for the conductive ink in various electronic devices. After sintering the reactive silver ink at different temperature for 2 h, the thickness of the printed silver films was measured using the step profiler, as shown in Fig. 4(a). The thicknesses of the edges of the printed silver films are higher about 1 μm than that of the central regions of the printed silver films, which may be induced by the hindrance of the mask. The thicknesses of the central regions are maintained at 2.5–3 μm, which suggests that the sintering temperature has slight effect on the thickness of the final silver film. Fig. 4(b) represents the change of the electrical resistivity of the printed silver films at different sintering temperature. The resistivity of the printed silver films decreases with increasing the sintering temperature. As the sintering temperature increases from 50 °C to 70 °C, the electrical resistivity of the printed film decreases from 46.2 μΩ·cm to 12.1 μΩ·cm. Whereas the sintering temperature further increases to 100 °C, the resistivity of the printed silver film slowly decreases to 9.3 μΩ·cm. It is remarkable that the electrical resistivity of the printed silver film
(70 °C) is only seven times higher than that of the bulk silver (1.65 μΩ·cm), meeting the requirements of the low temperature sintering and high conductivity. Fig. 4(c) presents XRD patterns of the printed silver films at different sintering temperatures. The four characteristic peaks are identified as silver crystal, and other characteristic peaks are not detected, which confirms that the reactive silver ink is completely decomposed to continuous silver layer in the air during the sintering process. Fig. 5 shows the microstructure evolution of the printed silver films at different temperature. The more uniform and compact printed silver films are obtained with increasing the sintering temperature. As shown in Fig. 5(a), silver particles with hundreds of nanometers are observed and have sintered with each other to form conductive films, and many nano-scale voids also are observed. With the sintering temperature further increases, smaller silver nanoparticles (about 50 nm) are observed in the printed silver films. As shown in the red dotted circles, these small nanoparticles can connect the adjacent large particles and the fill voids to form larger coarsening grains, which contribute to producing more compact microstructures. Meanwhile, these coarsening
Fig. 3. XPS spectra of the printed silver film at 50 °C. (a) Survey spectrum, (b) Ag3d spectrum, (c) C1s spectrum, and (d) O1s spectrum. 251
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 4. (a) Profile patterns, (b) Electrical resistivity, and XRD patterns of the printed silver films at different sintering temperatures.
silver ink, which is another important factor for achieving high conductivity. In addition, as the sintering time is more than 2 h, the resistivity is not significantly reduced. In view of sintering efficiency and conductivity, 70 °C and 2 h are the optimal sintering parameters to fabricate the highly conductive and flexible printed films. The cross section SEM micrograph of the printed silver films at 70 °C for 2 h is presented in Fig. 6(b). The adjacent particles connect with each other to form network structures with some pores, and no cracks in SEM micrograph are observed. The corresponding EDS spectrum demonstrates that the printed silver film is made of silver (93.7 wt%) and carbon (6.3 wt%) originating from HEC. Fig. 6(d) illustrates the surface and 3D AFM topography images of the printed silver film at 70 °C for 2 h, which reveals that the printed silver films are very smooth and dense without
grains provide more electron transport paths, playing a significant role in decreasing the electrical resistivity. To study the effect of sintering time on the electrical resistivity of the printed silver films, the sintering processes were performed at 70 °C in the air for different sintering times. The electrical resistivity of the printed silver film is 25.8, 20.2, 12.6, 12.3, and 10.3 μΩ·cm after sintering at 70 °C for 0.5, 1, 2, 4, and 8 h, respectively. The obtained results are plotted in Fig. 6(a), and corresponding SEM micrographs of the printed silver films also are presented. As seen in Fig. 6(a), as the sintering time increases, better conductivity of the printed silver film is obtained, and the SEM micrographs show more compact microstructures and larger coarsening grains. From this result, we confirm that, increasing sintering time promotes the sintering of the reactive
Fig. 5. SEM micrographs of the printed silver films at different sintering temperatures. 252
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 6. (a) Electrical resistivity of the printed silver films patterns at 70 °C against different sintering times, (b) Cross section SEM micrograph of the printed silver films at 70 °C for 2 h, (c) Corresponding EDS analysis of the printed silver films, and (d) Surface and 3D AFM topography images of the printed silver films at 70 °C for 2 h. Insets in Fig. 6(a) are corresponding SEM micrographs of the printed silver films. The scale bar in insets is 200 nm.
to surface diffusion and grain boundary diffusion [43,44]. Besides, the surfaces of nanoparticles are not coated by organic stabilizer, which contributes to reducing the sintering barriers and sintering temperature. Fig. 8(a) shows the result of the peel-off test of printed silver film at 70 °C for 2 h. It shows that the resistance of the printed silver film increases and trends to be stable with increasing adhesive times. After 100 times of the peel-off test, the relative resistance (R/R0) of printed silver film only increases to 1.24 times, and printed silver layer still adheres to the PI substrate and does not peel off, indicating the printed silver film has strong adhesion to the substrate. The reason is that the good adsorption between HEC and substrate is achieved by van der Waals forces, which plays a positive role in enhancing the adhesive strength [31,32]. Fig. 8(b) and (c) present the results of the bending test of printed silver film. The results show that printed film has highly mechanical bending properties without evident degradation of electrical performances. As the bending strain increases from 0.5% to 1.25%, the relative resistance (R/R0) of the printed silver film only increases from 1.01 to 1.25 times. After 1000 times of bending, the relative resistance (R/R0) of printed silver film remains below 1.3, 1.6, and 2.3 times at the bending strain of 0.5%, 0.63%, and 0.84%, respectively. It is believed that the highly mechanical flexibility of the printed silver film is achievable due to the densified microstructure and good adhesion between the printed silver film and PI substrate.
structural defects with a root mean square (RMS) roughness value of about 44.2 nm. The printed silver film was observed by TEM, as shown in Fig. 7. These particles with dozens to hundreds of nanometers connect with each other to form strong and conductive pathways. Fig. 7(b) shows the partial magnified TEM image of point 1. The adjacent silver nanoparticles have been fused with each other to form coarsening particles and sintering necks, and the surfaces of particles become smooth because of the nanoscale induced surface effect. In order to observe the interfaces of the coarsening particles, the detailed microstructures of point 2 and point 3 were further examined by high-resolution TEM method. The lattice spacing of 0.24 nm is assigned to (1 1 1) plane of silver crystal. The crystal planes with different directions and dislocations also are observed in these interfaces. Based on the above observations, we proposed a mechanism to explain the sintering behavior of the reactive silver ink. Firstly, silver nuclei are generated by the thermal decomposition of the reactive silver ink. These nuclei start to form small silver nanoparticles by reducing crystal interfacial energy and crystal–liquid interface [41,42]. The neck formation of adjacent particles in the sintering process is mainly induced by grain boundary diffusion and surface diffusion. Because of the high specific surface area and small size effect of nanoparticles, the surface activation energy and grain boundaries of nanoparticles are significantly higher than that of bulk material, which is more beneficial 253
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 7. TEM images and corresponding HR-TEM images of the printed silver film at 70 °C for 2 h.
film. The current values slightly decrease to 0.087 A and 0.088 A after the bending and twisting tests, respectively, which suggests that the changes of the resistances of the printed silver film are less than 0.12 Ω. In addition, the illumination intensity of LED remains constant without obvious degradation after the bending and twisting tests. These results indicate that the present ink possesses good mechanical performances and can be used in flexible printed electronic devices. Therefore, the prepared reactive silver ink provides a convenient and low temperature method for fabricating highly conductive and flexible printed
After the above adhesive tests and bending tests, the printed silver film was used as the conductive wire to light up a white light-emitting diode (WLED) during the bending and twisting tests, as shown in Fig. 9. WLED was fabricated by combing a blue LED chip (TR5050, Cree, USA) with the peak wavelength of 450 nm and transparent silicone embedded with yellow phosphor (YAG-05, Intematix, USA). The applied current and voltage were 0.3 V and 0.09 A, respectively, and the changes of the current were measured to evaluate the effect of bending and twisting tests on the electrical performance of the printed silver
Fig. 8. Relative resistance (R/R0) of the printed silver film at (a) different peel-off times, (b) different bending strains and (c) different bending times. R0 is the resistance of printed silver film before the tests, and R is the resistance of the printed silver film during the tests. The relative resistance (R/ R0) in Fig. 8(b) was measured after 100 times of bending tests. Insets in Fig. 8 are corresponding schematic diagram of tests.
254
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 9. Photographs of the WLED circuit with the printed silver film on PI substrate during the bending and twisting tests. Insets in Fig. 9 are corresponding current and voltage values during tests.
electronics.
(2014) 11084–11089. [3] S.E. Habas, H.A.S. Platt, M.F.A.M. van Hest, D.S. Ginley, Low-cost inorganic solar cells: from ink to printed device, Chem. Rev. 110 (2010) 6571–6594. [4] Y. Mou, Y. Peng, Y. Zhang, H. Cheng, M. Chen, Cu-Cu bonding enhancement at low temperature by using carboxylic acid surface-modified Cu nanoparticles, Mater. Lett. 227 (2018) 179–183. [5] K.E. Belsey, A.V.S. Parry, C.V. Rumens, M.A. Ziai, S.G. Yeates, J.C. Batchelor, S.J. Holder, Switchable disposable passive RFID vapour sensors from inkjet printed electronic components integrated with PDMS as a stimulus responsive material, J. Mater. Chem. C 5 (2017) 3167–3175. [6] H. Okimoto, T. Takenobu, K. Yanagi, Y. Miyata, H. Shimotani, H. Kataura, Y. Iwasa, Tunable carbon nanotube thin-film transistors produced exclusively via inkjet printing, Adv. Mater. 22 (2010) 3981–3986. [7] Z. Shu, E. Beckert, R. Eberhardt, A. Tünnermann, ITO-free, inkjet-printed transparent organic light-emitting diodes with a single inkjet-printed Al:ZnO:PEI interlayer for sensing applications, J. Mater. Chem. C 5 (2017) 11590–11597. [8] J. Perelaer, B.-J. de Gans, U.S. Schubert, Ink-jet printing and microwave sintering of conductive silver tracks, Adv. Mater. 18 (2006) 2101–2104. [9] M. Grouchko, A. Kamyshny, C.F. Mihailescu, D.F. Anghel, S. Magdassi, Conductive inks with a ‘built-in’ mechanism that enables sintering at room temperature, ACS Nano 5 (2011) 3354–3359. [10] W. Li, D. Hu, L. Li, C.-F. Li, J. Jiu, C. Chen, T. Ishina, T. Sugahara, K. Suganuma, Printable and flexible copper–silver alloy electrodes with high conductivity and ultrahigh oxidation resistance, ACS Appl. Mater. Interfaces 9 (2017) 24711–24721. [11] S. Wang, N. Liu, J. Tao, C. Yang, W. Liu, Y. Shi, Y. Wang, J. Su, L. Li, Y. Gao, Inkjet printing of conductive patterns and supercapacitors using a multi-walled carbon nanotube/Ag nanoparticle based ink, J. Mater. Chem. A 3 (2015) 2407–2413. [12] A. Kosmala, R. Wright, Q. Zhang, P. Kirby, Synthesis of silver nano particles and fabrication of aqueous Ag inks for inkjet printing, Mater. Chem. Phys. 129 (2011) 1075–1080. [13] J. Vaithilingam, M. Simonelli, E. Saleh, N. Senin, R.D. Wildman, R.J.M. Hague, R.K. Leach, C.J. Tuck, Combined inkjet printing and infrared sintering of silver nanoparticles using a swathe-by-swathe and layer-by-layer approach for 3-dimensional structures, ACS Appl. Material. Interfaces 9 (2017) 6560–6570. [14] K.-S. Chou, K.-C. Huang, H.-H. Lee, Fabrication and sintering effect on the morphologies and conductivity of nano-Ag particle films by the spin coating method, Nanotechnology 16 (2005) 779–784. [15] K.J. Lee, B.H. Jun, T.H. Kim, J. Joung, Direct synthesis and inkjetting of silver nanocrystals toward printed electronics, Nanotechnology 17 (2006) 2424–2428. [16] I.E. Stewart, M.J. Kim, B.J. Wiley, Effect of morphology on the electrical resistivity of silver nanostructure films, ACS Appl. Mater. Interfaces 9 (2017) 1870–1876. [17] H. Pan, S.H. Ko, C.P. Grigoropoulos, Thermal sintering of solution-deposited nanoparticle silver ink films characterized by spectroscopic ellipsometry, Appl. Phys. Lett. 93 (2008) 234104. [18] S. Magdassi, M. Grouchko, O. Berezin, A. Kamyshny, Triggering the sintering of silver nanoparticles at room temperature, ACS Nano 4 (2010) 1943–1948. [19] S.-S. Yoon, D.-Y. Khang, Room-temperature chemical welding and sintering of metallic nanostructures by capillary condensation, Nano Lett. 16 (2016) 3550–3556. [20] B.Y. Ahn, E.B. Duoss, M.J. Motala, X. Guo, S.-I. Park, Y. Xiong, J. Yoon, R.G. Nuzzo, J.A. Rogers, J.A. Lewis, Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes, Science 323 (2009) 1590–1593. [21] J. Li, C.M. Johnson, C. Buttay, W. Sabbah, S. Azzopardi, Bonding strength of multiple SiC die attachment prepared by sintering of Ag nanoparticles, J Mater. Process.
4. Conclusions In summary, we prepared a transparent and stable reactive silver ink, composed by ammonia and formic acid ligands, silver acetate, and HEC adhesive agent, for low temperature fabrication of highly conductive and flexible silver films. Attributed to the formation of silver ammonia complex and reduction of formic acid, the complete decomposition temperature of the ink was 150 °C lower than that of silver acetate. The electrical conductivity of printed silver films was enhanced by increasing sintering temperature and sintering time. As the sintering temperature of 70 °C and the sintering time of 2 h, the electrical resistivity of the printed silver film is 12.1 μΩ·cm, which is only seven times higher than that of bulk silver. Moreover, as the peel-off times of 100 and the bending times of 1000, the relative resistances (R/Ro) of the printed silver films still are below 1.25 and 2.4, respectively. The illumination intensity of LED also remains constant during bending and twisting tests. These results demonstrate the printed silver films not only possess high electrical conductivity, but also display good adhesive strength and mechanical flexibility. Therefore, the prepared reactive silver ink is a promising material for low temperature fabricating printed electronics on heat-sensitive substrate. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51275194, and 51775219), China Postdoctoral Science Foundation (2018M630852), Fundamental Research Funds for Central Universities (2016JCTD112 and 2017JYCXJJ006), and the Graduates' Innovation Fund, Huazhong University of Science and Technology (5003100033). The authors would like to thank Analytical and Testing Center of Huazhong University of Science and Technology for the support in TEM and SEM measurement. References [1] S. Hong, J. Yeo, G. Kim, D. Kim, H. Lee, J. Kwon, H. Lee, P. Lee, S.H. Ko, Nonvacuum, maskless fabrication of a flexible metal grid transparent conductor by low-temperature selective laser sintering of nanoparticle ink, ACS Nano 7 (2013) 5024–5031. [2] A. Shimoni, S. Azoubel, S. Magdassi, Inkjet printing of flexible high-performance carbon nanotube transparent conductive films by ‘coffee ring effect’, Nanoscale 6
255
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
[33] Y. Wu, Y. Li, B.S. Ong, A simple and efficient approach to a printable silver conductor for printed electronics, J. Am. Chem. Soc. 129 (2007) (2007) 1862–1863. [34] Q. Lei, J. He, B. Zhang, J. Chang, D. Li, Microscale electrohydrodynamic printing of conductive silver features based on in situ reactive inks, J. Mater. Chem. 6 (2018) 213–218. [35] J.J.P. Valeton, K. Hermans, C.W.M. Bastiaansen, D.J. Broer, J. Perelaer, U.S. Schubert, G.P. Crawford, P.J. Smith, Room temperature preparation of conductive silver features using spin-coating and inkjet printing, J. Mater. Chem. 20 (2010) 543–546. [36] K. Black, J. Singh, D. Mehta, S. Sung, C.J. Sutcliffe, P.R. Chalker, Silver ink formulations for sinter-free printing of conductive films, Sci. Rep. 6 (2016) 1. [37] J. Sun, Y. Guo, B. Cui, F. Chu, H. Li, Y. Li, M. He, D. Ding, R. Liu, L. Li, Y. Song, Inkjet printing bendable circuits based on an oil-water interface reaction, Appl. Surf. Sci. 445 (2018) 391–397. [38] K.R. Zope, D. Cormier, S.A. Williams, Reactive silver oxalate ink composition with enhanced curing conditions for flexible substrates, ACS Appl. Mater. Interfaces 10 (2018) 3830–3837. [39] M. Vaseem, S.-K. Lee, J.-G. Kim, Y.-B. Hahn, Silver-ethanolamine-formate complex based transparent and stable ink: Electrical assessment with microwave plasma vs thermal sintering, Chem. Eng. J. 306 (2016) 796–805. [40] W. Li, C.-F. Li, F. Lang, J. Jiu, M. Ueshima, H. Wang, Z.-Q. Liu, K. Suganuma, Selfcatalyzed copper–silver complex inks for low-cost fabrication of highly oxidationresistant and conductive copper–silver hybrid tracks at a low temperature below 100 °C, Nanoscale 10 (2018) 5254–5263. [41] J. Luo, Stabilization of nanoscale quasi-liquid interfacial films in inorganic materials: a review and critical assessment, Crit. Rev. Solid State 32 (2007) 67–109. [42] A. Muzikansky, P. Nanikashvili, J. Grinblat, D. Zitoun, Ag dewetting in Cu@Ag monodisperse core–shell nanoparticles, J. Phys. Chem. C 117 (2013) 3093–3100. [43] Z.Z. Fang, H. Wang, Densification and grain growth during sintering of nanosized particles, Int. Mater. Rev. 53 (2008) 326–352. [44] J.R. Greer, R.A. Street, Thermal cure effects on electrical performance of nanoparticle silver inks, Acta Mater. 55 (2007) 6345–6349.
Tech. 215 (2015) 299–308. [22] J.-T. Wu, S.L.-C. Hsu, M.-H. Tsai, W.-S. Hwang, Conductive silver patterns via ethylene glycol vapor reduction of ink-jet printed silver nitrate tracks on a polyimide substrate, Thin Solid Films 517 (2009) 5913–5917. [23] Y.-L. Tai, Z.-G. Yang, Z.-D. Li, A promising approach to conductive patterns with high efficiency for flexible electronics, Appl. Surf. Sci. 257 (2011) 7096–7100. [24] W. Yang, C. Liu, Z. Zhang, Y. Liu, S. Nie, One step synthesis of uniform organic silver ink drawing directly on paper substrates, J. Mater. Chem. 22 (2012) 23012. [25] C.-N. Chen, T.-Y. Dong, T.-C. Chang, M.-C. Chen, H.-L. Tsai, W.-S. Hwang, Solutionbased β-diketonate silver ink for direct printing of highly conductive features on a flexible substrate, J. Mater. Chem. C 1 (2013) 5161. [26] A.J. Kell, C. Paquet, O. Mozenson, I. Djavani-Tabrizi, B. Deore, X. Liu, G.P. Lopinski, R. James, K. Hettak, J. Shaker, A. Momciu, J. Ferrigno, O. Ferrand, J.X. Hu, S. Lafrenière, P.R.L. Malenfant, Versatile molecular silver ink platform for printed flexible electronics, ACS Appl. Mater. Interfaces 9 (2017) 17226–17237. [27] X. Nie, H. Wang, J. Zou, Inkjet printing of silver citrate conductive ink on PET substrate, Appl. Surf. Sci. 261 (2012) 554–560. [28] Y. Dong, X. Li, S. Liu, Q. Zhu, J.-G. Li, X. Sun, Facile synthesis of high silver content MOD ink by using silver oxalate precursor for inkjet printing applications, Thin Solid Films 589 (2015) 381–387. [29] M. Hu, X. Cai, Q. Guo, B. Bian, T. Zhang, J. Yang, Direct pen writing of adhesive particle-Free ultrahigh silver salt-Loaded composite ink for stretchable circuits, ACS Nano 10 (2015) 396–404. [30] W.J. Hyun, S. Lim, B.Y. Ahn, J.A. Lewis, C.D. Frisbie, L.F. Francis, Screen printing of highly loaded silver inks on plastic substrates using silicon stencils, ACS Appl. Mater. Interfaces 7 (2015) 12619–12624. [31] M. Vaseem, G. McKerricher, A. Shamim, Robust design of a particle-free silverorgano-complex ink with high conductivity and inkjet stability for flexible electronics, ACS Appl. Mater. Interfaces 8 (2015) 177–186. [32] K.S. Bhat, R. Ahmad, Y. Wang, Y.-B. Hahn, Low-temperature sintering of highly conductive silver ink for flexible electronics, J. Mater. Chem. C 4 (2016) 8522–8527.
256
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Fabrication of highly conductive and flexible printed electronics by low temperature sintering reactive silver ink ⁎
Yun Moua, Yuru Zhangb, Hao Chenga, Yang Penga, , Mingxiang Chena,c,
T
⁎
a
School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China c State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Reactive silver ink Low temperature sintering Highly conductive Flexible Printed silver film
To solve the problems of high sintering temperatures (∼200 °C), particle aggregation, nozzle clogging, and poor shelf life of silver nanoparticle inks, we prepared a transparent and stable reactive silver ink for fabricating printed electronics. The ink mainly consisted of ammonia and formic acid ligands, silver acetate, and hydroxyethyl cellulose (HEC) adhesive agent. The highly conductive and flexible silver films were fabricated by printing and low temperature sintering the reactive silver ink, and the effects of sintering temperature and sintering time on the electrical properties of the printed silver films were investigated. Consequently, the printed silver film sintered at 70 °C exhibits good electrical properties with a resistivity of 12.1 μΩ·cm, which is only seven times higher than that of bulk silver (1.65 μΩ·cm). Moreover, the silver film also displays excellent adhesive strength and mechanical flexibility in terms of bending and twisting.
1. Introduction Owing to their cost-effective and eco-friendly fabricating process and unique properties such as light-weight, bendable, and foldable, the printed electronics have attracted great attentions in various fields, such as flexible electrodes, low temperature bonding, solar cell arrays, radio frequency identification (RFID) tags, thin film transistors, and organic light emitting diodes (OLED) [1–7]. Conductive ink can be patterned by printing process to achieve robust tracks with high conductivity, which is a critical component for the printed electronics [8–10]. Currently, most conductive inks are based on silver nanoparticle inks because of their excellent electrical conductivity and good oxidation resistance [11–13]. Unfortunately, most of the reported silver nanoparticle inks require high sintering temperature above 200 °C to remove the organic stabilizers and sinter nanoparticles [14–17], which limits their practical applications on heat-sensitive flexible substrates. In order to decrease the sintering temperature, the chemical sintering of silver nanoparticle inks has been proposed to prepare the highly conductive silver tracks at room temperature [9,18,19]. However, the special sintering or destabilize agents and stabilizers are needed to achieve the coalescence and sintering of the nanoparticles, and the agglomeration of silver nanoparticles still is a severe issue to be solved, which results in clogging the nozzles and shortening the shelf life of the inks [20,21]. ⁎
Aiming to solve these problems, the particle-free silver reactive inks have received significant attentions because of their facile synthesis processes and low sintering temperature. More importantly, neither additional stabilizers nor reducing agents are required, which plays an important role in enhancing the performances of printed electronics [22,23]. At present, many researchers have synthesized different reactive silver inks to fabricate highly conductive silver tracks with the order of 10-5-10-6 Ω·cm [24–30]. However, most of the reported inks are less stable, and multi-component solvents in the inks have negative effects on the conductivity, which require relatively high sintering temperature (150–200 °C) to form highly conductive silver tracks. Although the lower sintering temperature and higher conductive silver tracks have been achieved by adjusting the ligands and silver precursors [31–34], the sintering process is too time-consuming (> 12 h) to be desirable for the actual production. Recently, the developed reactive metalorganic (ROM) inks can prepare silver tracks at room temperature by chemical reaction, which provide a lower thermal budget and faster route for fabricating the flexible printed electronics [35–37]. But this approach needs the reducing agents to trigger the chemical reaction, and the additional cleaning or curing process is essential to remove the residual organics. In addition, the advanced sintering technique (photonic and microwave-plasma) is a good alternative for decreasing sintering temperature and improve sintering efficiency [26,38,39], but thermal sintering is a preferred choice for fabricating conductive tracks
Corresponding authors at: School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China (M. Chen). E-mail addresses: [email protected] (Y. Peng), [email protected] (M. Chen).
https://doi.org/10.1016/j.apsusc.2018.07.187 Received 30 May 2018; Received in revised form 7 July 2018; Accepted 27 July 2018 Available online 29 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
morphological features of the printed silver films were observed by transmission electron microscope (TEM, FEI Tecnai G2 20 U-TWIN) and field-emission scanning electron microscope (SEM, FEI Nova Nano SEM 450), together with energy dispersive spectroscopy (EDS). Atomic force microscopy (AFM) was utilized to examine the 2D and 3D morphology of the printed silver films. The resistivity (ρ) of the printed silver films was calculated as ρ = Rs × W, where Rs is the sheet resistance and W is the thickness of the film. The sheet resistance and thickness of the film were measured by four-point probes resistivity measurement system (Probes Tech RTS-8) and step profiler (ET4000 Series), respectively. The resistance of the printed silver film was measured by the simple 2point I-V measurement method (Digital milliohmmeter, VC480C) during the peel-off and bending tests.
because of their low cost and simplicity. More importantly, the adhesion with substrate and printing stability is either not provided or discussed in most of the reported inks. Herein, we prepare a transparent and stable reactive silver ink, wherein ammonia and formic acid act as the ligands of silver acetate precursor. The highly conductive and bendable printed silver films on flexible substrates using the reactive ink were fabricated at 70 °C, and displayed good adhesion and stability. The effects of the sintering temperature and sintering time on the electrical properties of the printed silver films were systematically investigated, and the adhesive strength and mechanical flexibility of the printed silver films were discussed. 2. Materials and methods
3. Results and discussion
2.1. Materials
TG analysis was carried out to evaluate the thermal decomposition behaviors of the ink and silver acetate, as depicted in Fig. 2(a). The TG curve of the ink shows that the mass starts to decrease at room temperature, and the mass becomes constant at 130 °C. The weight loss of silver acetate starts at 225 °C and finishes at about 280 °C. The difference between the complete decomposition temperatures of silver acetate and the reactive silver ink is approximately 150 °C, which is attributed to the formation of silver ammonia complex and reduction of formic acid [40]. Besides, the remaining solid weights of the ink and silver acetate correspond to their silver weights, indicating that the organics are absolutely removed during the thermal sintering process. Moreover, when the reactive silver ink is hermetically stored in the refrigerator, it is stable and transparent, but the particles begin to form at room temperature with the evaporations of formic acid and ammonia. To analysis the crystal phases of the particles, XRD analysis was conducted after drying the ink at room temperature for 12 h, as shown in Fig. 2(b). Four diffraction peaks of the printed silver film are detected at 38.10°, 44.37°, 64.18°, and 74.16°, which represent the (1 1 1), (2 0 0), (2 2 0)and (3 1 1) planes of silver crystal. These characteristic peaks confirm that the printed silver film is face-centered cubic (FCC) silver phase without silver oxide or silver acetate. Therefore, TG and XRD analysis indicate that the reactive silver ink can be thermally decomposed to form silver particles at low temperature. To analyze the surface compositions of the printed silver film in depth, XPS analysis was performed after sintering the ink on PI substrate at 50 °C for 2 h. As shown in Fig. 3(a), carbon and oxygen elements are detected on the surface of the printed silver film, and may originate from silver acetate or HEC. Fig. 3(b) presents the peak fitting of Ag3d spectrum of the printed silver film. The characteristic Ag3d peaks at 367.5 eV and 373.6 eV are assigned to silver crystal, and no significant peaks of silver oxide or silver salt are separated, which demonstrates that the ink is completely thermal decomposed to silver at 50 °C. Fig. 3(c) shows the peak fitting of C1s spectrum of the printed silver film. The characteristic peaks appear at 284.8 eV and 286.6 eV, which correspond to the C-O group and C-C group of HEC, respectively. Fig. 3(d) shows the XPS spectra of O 1s peak of the printed silver film. The major peak is divided into two peaks at 531.2 eV and 533.0 eV, corresponding to the O1s peak of the adsorbed oxygen and C-O group of
Silver acetate (98%) and ammonia (25–28%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydroxyethyl cellulose-water solution (HEC, 4% in water) and Formic acid (88%) were purchased from Aladdin reagent Co. All chemical reagents were used without any further treatment. Polyimide (PI) substrates (thickness: 100 µm) were purchased from Toray Industries, Inc. 2.2. Preparation of reactive silver ink Fig. 1 illustrates the preparation process of reactive silver ink. Ammonia (2.0 g) and formic acid (0.36 g) were mixed at 5 °C in a glass vial. Subsequently, under magnetic stirring, silver acetate (1.0 g) was slowly dissolved in the mixing solution to prepare reactive silver ink, and the color of the solution changed from colorless to light brown. Finally, the obtained reactive silver ink was filtered by the filter syringe (200 nm), and stored at 3 °C in the refrigerator. Acting as the adhesive agent, HEC-water solution (0.6 g) was added to adjust the adhesion and wettability of the reactive silver ink. 2.3. Low temperature sintering process PI films were used as the flexible substrate in this work. To remove organic contaminations on the surfaces, the substrates were successively ultrasonic cleaned by in deionized water and ethanol for 10 min. Subsequently, the reactive silver ink was uniformly printed on the substrates by mask-printing method. The thickness of the mask was 50 μm. Finally, the printed reactive silver inks were sintered in the air from 50 °C to 100 °C for 0.5 h to 8 h. 2.4. Characterization and measurement The thermal behaviors of the reactive silver ink and silver acetate were investigated by thermogravimetric analyzer (TG, TG 209 F3 Tarsus) at a heating rate of 5 °C/min in the air. The crystal structures and surface compositions of the printed silver films were analyzed by Xray diffractometer (XRD, PANalytical PW3040/60) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi), respectively. The
Fig. 1. Schematic preparation process of the reactive silver ink. 250
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 2. (a) TG curves of the reactive silver ink and silver acetate, and (b) XRD pattern of the printed silver film at room temperature in the air.
HEC, respectively. Therefore, the printed silver film is made of silver particles and a small amount of HEC. The low electrical resistivity is a significant index for the conductive ink in various electronic devices. After sintering the reactive silver ink at different temperature for 2 h, the thickness of the printed silver films was measured using the step profiler, as shown in Fig. 4(a). The thicknesses of the edges of the printed silver films are higher about 1 μm than that of the central regions of the printed silver films, which may be induced by the hindrance of the mask. The thicknesses of the central regions are maintained at 2.5–3 μm, which suggests that the sintering temperature has slight effect on the thickness of the final silver film. Fig. 4(b) represents the change of the electrical resistivity of the printed silver films at different sintering temperature. The resistivity of the printed silver films decreases with increasing the sintering temperature. As the sintering temperature increases from 50 °C to 70 °C, the electrical resistivity of the printed film decreases from 46.2 μΩ·cm to 12.1 μΩ·cm. Whereas the sintering temperature further increases to 100 °C, the resistivity of the printed silver film slowly decreases to 9.3 μΩ·cm. It is remarkable that the electrical resistivity of the printed silver film
(70 °C) is only seven times higher than that of the bulk silver (1.65 μΩ·cm), meeting the requirements of the low temperature sintering and high conductivity. Fig. 4(c) presents XRD patterns of the printed silver films at different sintering temperatures. The four characteristic peaks are identified as silver crystal, and other characteristic peaks are not detected, which confirms that the reactive silver ink is completely decomposed to continuous silver layer in the air during the sintering process. Fig. 5 shows the microstructure evolution of the printed silver films at different temperature. The more uniform and compact printed silver films are obtained with increasing the sintering temperature. As shown in Fig. 5(a), silver particles with hundreds of nanometers are observed and have sintered with each other to form conductive films, and many nano-scale voids also are observed. With the sintering temperature further increases, smaller silver nanoparticles (about 50 nm) are observed in the printed silver films. As shown in the red dotted circles, these small nanoparticles can connect the adjacent large particles and the fill voids to form larger coarsening grains, which contribute to producing more compact microstructures. Meanwhile, these coarsening
Fig. 3. XPS spectra of the printed silver film at 50 °C. (a) Survey spectrum, (b) Ag3d spectrum, (c) C1s spectrum, and (d) O1s spectrum. 251
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 4. (a) Profile patterns, (b) Electrical resistivity, and XRD patterns of the printed silver films at different sintering temperatures.
silver ink, which is another important factor for achieving high conductivity. In addition, as the sintering time is more than 2 h, the resistivity is not significantly reduced. In view of sintering efficiency and conductivity, 70 °C and 2 h are the optimal sintering parameters to fabricate the highly conductive and flexible printed films. The cross section SEM micrograph of the printed silver films at 70 °C for 2 h is presented in Fig. 6(b). The adjacent particles connect with each other to form network structures with some pores, and no cracks in SEM micrograph are observed. The corresponding EDS spectrum demonstrates that the printed silver film is made of silver (93.7 wt%) and carbon (6.3 wt%) originating from HEC. Fig. 6(d) illustrates the surface and 3D AFM topography images of the printed silver film at 70 °C for 2 h, which reveals that the printed silver films are very smooth and dense without
grains provide more electron transport paths, playing a significant role in decreasing the electrical resistivity. To study the effect of sintering time on the electrical resistivity of the printed silver films, the sintering processes were performed at 70 °C in the air for different sintering times. The electrical resistivity of the printed silver film is 25.8, 20.2, 12.6, 12.3, and 10.3 μΩ·cm after sintering at 70 °C for 0.5, 1, 2, 4, and 8 h, respectively. The obtained results are plotted in Fig. 6(a), and corresponding SEM micrographs of the printed silver films also are presented. As seen in Fig. 6(a), as the sintering time increases, better conductivity of the printed silver film is obtained, and the SEM micrographs show more compact microstructures and larger coarsening grains. From this result, we confirm that, increasing sintering time promotes the sintering of the reactive
Fig. 5. SEM micrographs of the printed silver films at different sintering temperatures. 252
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 6. (a) Electrical resistivity of the printed silver films patterns at 70 °C against different sintering times, (b) Cross section SEM micrograph of the printed silver films at 70 °C for 2 h, (c) Corresponding EDS analysis of the printed silver films, and (d) Surface and 3D AFM topography images of the printed silver films at 70 °C for 2 h. Insets in Fig. 6(a) are corresponding SEM micrographs of the printed silver films. The scale bar in insets is 200 nm.
to surface diffusion and grain boundary diffusion [43,44]. Besides, the surfaces of nanoparticles are not coated by organic stabilizer, which contributes to reducing the sintering barriers and sintering temperature. Fig. 8(a) shows the result of the peel-off test of printed silver film at 70 °C for 2 h. It shows that the resistance of the printed silver film increases and trends to be stable with increasing adhesive times. After 100 times of the peel-off test, the relative resistance (R/R0) of printed silver film only increases to 1.24 times, and printed silver layer still adheres to the PI substrate and does not peel off, indicating the printed silver film has strong adhesion to the substrate. The reason is that the good adsorption between HEC and substrate is achieved by van der Waals forces, which plays a positive role in enhancing the adhesive strength [31,32]. Fig. 8(b) and (c) present the results of the bending test of printed silver film. The results show that printed film has highly mechanical bending properties without evident degradation of electrical performances. As the bending strain increases from 0.5% to 1.25%, the relative resistance (R/R0) of the printed silver film only increases from 1.01 to 1.25 times. After 1000 times of bending, the relative resistance (R/R0) of printed silver film remains below 1.3, 1.6, and 2.3 times at the bending strain of 0.5%, 0.63%, and 0.84%, respectively. It is believed that the highly mechanical flexibility of the printed silver film is achievable due to the densified microstructure and good adhesion between the printed silver film and PI substrate.
structural defects with a root mean square (RMS) roughness value of about 44.2 nm. The printed silver film was observed by TEM, as shown in Fig. 7. These particles with dozens to hundreds of nanometers connect with each other to form strong and conductive pathways. Fig. 7(b) shows the partial magnified TEM image of point 1. The adjacent silver nanoparticles have been fused with each other to form coarsening particles and sintering necks, and the surfaces of particles become smooth because of the nanoscale induced surface effect. In order to observe the interfaces of the coarsening particles, the detailed microstructures of point 2 and point 3 were further examined by high-resolution TEM method. The lattice spacing of 0.24 nm is assigned to (1 1 1) plane of silver crystal. The crystal planes with different directions and dislocations also are observed in these interfaces. Based on the above observations, we proposed a mechanism to explain the sintering behavior of the reactive silver ink. Firstly, silver nuclei are generated by the thermal decomposition of the reactive silver ink. These nuclei start to form small silver nanoparticles by reducing crystal interfacial energy and crystal–liquid interface [41,42]. The neck formation of adjacent particles in the sintering process is mainly induced by grain boundary diffusion and surface diffusion. Because of the high specific surface area and small size effect of nanoparticles, the surface activation energy and grain boundaries of nanoparticles are significantly higher than that of bulk material, which is more beneficial 253
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 7. TEM images and corresponding HR-TEM images of the printed silver film at 70 °C for 2 h.
film. The current values slightly decrease to 0.087 A and 0.088 A after the bending and twisting tests, respectively, which suggests that the changes of the resistances of the printed silver film are less than 0.12 Ω. In addition, the illumination intensity of LED remains constant without obvious degradation after the bending and twisting tests. These results indicate that the present ink possesses good mechanical performances and can be used in flexible printed electronic devices. Therefore, the prepared reactive silver ink provides a convenient and low temperature method for fabricating highly conductive and flexible printed
After the above adhesive tests and bending tests, the printed silver film was used as the conductive wire to light up a white light-emitting diode (WLED) during the bending and twisting tests, as shown in Fig. 9. WLED was fabricated by combing a blue LED chip (TR5050, Cree, USA) with the peak wavelength of 450 nm and transparent silicone embedded with yellow phosphor (YAG-05, Intematix, USA). The applied current and voltage were 0.3 V and 0.09 A, respectively, and the changes of the current were measured to evaluate the effect of bending and twisting tests on the electrical performance of the printed silver
Fig. 8. Relative resistance (R/R0) of the printed silver film at (a) different peel-off times, (b) different bending strains and (c) different bending times. R0 is the resistance of printed silver film before the tests, and R is the resistance of the printed silver film during the tests. The relative resistance (R/ R0) in Fig. 8(b) was measured after 100 times of bending tests. Insets in Fig. 8 are corresponding schematic diagram of tests.
254
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
Fig. 9. Photographs of the WLED circuit with the printed silver film on PI substrate during the bending and twisting tests. Insets in Fig. 9 are corresponding current and voltage values during tests.
electronics.
(2014) 11084–11089. [3] S.E. Habas, H.A.S. Platt, M.F.A.M. van Hest, D.S. Ginley, Low-cost inorganic solar cells: from ink to printed device, Chem. Rev. 110 (2010) 6571–6594. [4] Y. Mou, Y. Peng, Y. Zhang, H. Cheng, M. Chen, Cu-Cu bonding enhancement at low temperature by using carboxylic acid surface-modified Cu nanoparticles, Mater. Lett. 227 (2018) 179–183. [5] K.E. Belsey, A.V.S. Parry, C.V. Rumens, M.A. Ziai, S.G. Yeates, J.C. Batchelor, S.J. Holder, Switchable disposable passive RFID vapour sensors from inkjet printed electronic components integrated with PDMS as a stimulus responsive material, J. Mater. Chem. C 5 (2017) 3167–3175. [6] H. Okimoto, T. Takenobu, K. Yanagi, Y. Miyata, H. Shimotani, H. Kataura, Y. Iwasa, Tunable carbon nanotube thin-film transistors produced exclusively via inkjet printing, Adv. Mater. 22 (2010) 3981–3986. [7] Z. Shu, E. Beckert, R. Eberhardt, A. Tünnermann, ITO-free, inkjet-printed transparent organic light-emitting diodes with a single inkjet-printed Al:ZnO:PEI interlayer for sensing applications, J. Mater. Chem. C 5 (2017) 11590–11597. [8] J. Perelaer, B.-J. de Gans, U.S. Schubert, Ink-jet printing and microwave sintering of conductive silver tracks, Adv. Mater. 18 (2006) 2101–2104. [9] M. Grouchko, A. Kamyshny, C.F. Mihailescu, D.F. Anghel, S. Magdassi, Conductive inks with a ‘built-in’ mechanism that enables sintering at room temperature, ACS Nano 5 (2011) 3354–3359. [10] W. Li, D. Hu, L. Li, C.-F. Li, J. Jiu, C. Chen, T. Ishina, T. Sugahara, K. Suganuma, Printable and flexible copper–silver alloy electrodes with high conductivity and ultrahigh oxidation resistance, ACS Appl. Mater. Interfaces 9 (2017) 24711–24721. [11] S. Wang, N. Liu, J. Tao, C. Yang, W. Liu, Y. Shi, Y. Wang, J. Su, L. Li, Y. Gao, Inkjet printing of conductive patterns and supercapacitors using a multi-walled carbon nanotube/Ag nanoparticle based ink, J. Mater. Chem. A 3 (2015) 2407–2413. [12] A. Kosmala, R. Wright, Q. Zhang, P. Kirby, Synthesis of silver nano particles and fabrication of aqueous Ag inks for inkjet printing, Mater. Chem. Phys. 129 (2011) 1075–1080. [13] J. Vaithilingam, M. Simonelli, E. Saleh, N. Senin, R.D. Wildman, R.J.M. Hague, R.K. Leach, C.J. Tuck, Combined inkjet printing and infrared sintering of silver nanoparticles using a swathe-by-swathe and layer-by-layer approach for 3-dimensional structures, ACS Appl. Material. Interfaces 9 (2017) 6560–6570. [14] K.-S. Chou, K.-C. Huang, H.-H. Lee, Fabrication and sintering effect on the morphologies and conductivity of nano-Ag particle films by the spin coating method, Nanotechnology 16 (2005) 779–784. [15] K.J. Lee, B.H. Jun, T.H. Kim, J. Joung, Direct synthesis and inkjetting of silver nanocrystals toward printed electronics, Nanotechnology 17 (2006) 2424–2428. [16] I.E. Stewart, M.J. Kim, B.J. Wiley, Effect of morphology on the electrical resistivity of silver nanostructure films, ACS Appl. Mater. Interfaces 9 (2017) 1870–1876. [17] H. Pan, S.H. Ko, C.P. Grigoropoulos, Thermal sintering of solution-deposited nanoparticle silver ink films characterized by spectroscopic ellipsometry, Appl. Phys. Lett. 93 (2008) 234104. [18] S. Magdassi, M. Grouchko, O. Berezin, A. Kamyshny, Triggering the sintering of silver nanoparticles at room temperature, ACS Nano 4 (2010) 1943–1948. [19] S.-S. Yoon, D.-Y. Khang, Room-temperature chemical welding and sintering of metallic nanostructures by capillary condensation, Nano Lett. 16 (2016) 3550–3556. [20] B.Y. Ahn, E.B. Duoss, M.J. Motala, X. Guo, S.-I. Park, Y. Xiong, J. Yoon, R.G. Nuzzo, J.A. Rogers, J.A. Lewis, Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes, Science 323 (2009) 1590–1593. [21] J. Li, C.M. Johnson, C. Buttay, W. Sabbah, S. Azzopardi, Bonding strength of multiple SiC die attachment prepared by sintering of Ag nanoparticles, J Mater. Process.
4. Conclusions In summary, we prepared a transparent and stable reactive silver ink, composed by ammonia and formic acid ligands, silver acetate, and HEC adhesive agent, for low temperature fabrication of highly conductive and flexible silver films. Attributed to the formation of silver ammonia complex and reduction of formic acid, the complete decomposition temperature of the ink was 150 °C lower than that of silver acetate. The electrical conductivity of printed silver films was enhanced by increasing sintering temperature and sintering time. As the sintering temperature of 70 °C and the sintering time of 2 h, the electrical resistivity of the printed silver film is 12.1 μΩ·cm, which is only seven times higher than that of bulk silver. Moreover, as the peel-off times of 100 and the bending times of 1000, the relative resistances (R/Ro) of the printed silver films still are below 1.25 and 2.4, respectively. The illumination intensity of LED also remains constant during bending and twisting tests. These results demonstrate the printed silver films not only possess high electrical conductivity, but also display good adhesive strength and mechanical flexibility. Therefore, the prepared reactive silver ink is a promising material for low temperature fabricating printed electronics on heat-sensitive substrate. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51275194, and 51775219), China Postdoctoral Science Foundation (2018M630852), Fundamental Research Funds for Central Universities (2016JCTD112 and 2017JYCXJJ006), and the Graduates' Innovation Fund, Huazhong University of Science and Technology (5003100033). The authors would like to thank Analytical and Testing Center of Huazhong University of Science and Technology for the support in TEM and SEM measurement. References [1] S. Hong, J. Yeo, G. Kim, D. Kim, H. Lee, J. Kwon, H. Lee, P. Lee, S.H. Ko, Nonvacuum, maskless fabrication of a flexible metal grid transparent conductor by low-temperature selective laser sintering of nanoparticle ink, ACS Nano 7 (2013) 5024–5031. [2] A. Shimoni, S. Azoubel, S. Magdassi, Inkjet printing of flexible high-performance carbon nanotube transparent conductive films by ‘coffee ring effect’, Nanoscale 6
255
Applied Surface Science 459 (2018) 249–256
Y. Mou et al.
[33] Y. Wu, Y. Li, B.S. Ong, A simple and efficient approach to a printable silver conductor for printed electronics, J. Am. Chem. Soc. 129 (2007) (2007) 1862–1863. [34] Q. Lei, J. He, B. Zhang, J. Chang, D. Li, Microscale electrohydrodynamic printing of conductive silver features based on in situ reactive inks, J. Mater. Chem. 6 (2018) 213–218. [35] J.J.P. Valeton, K. Hermans, C.W.M. Bastiaansen, D.J. Broer, J. Perelaer, U.S. Schubert, G.P. Crawford, P.J. Smith, Room temperature preparation of conductive silver features using spin-coating and inkjet printing, J. Mater. Chem. 20 (2010) 543–546. [36] K. Black, J. Singh, D. Mehta, S. Sung, C.J. Sutcliffe, P.R. Chalker, Silver ink formulations for sinter-free printing of conductive films, Sci. Rep. 6 (2016) 1. [37] J. Sun, Y. Guo, B. Cui, F. Chu, H. Li, Y. Li, M. He, D. Ding, R. Liu, L. Li, Y. Song, Inkjet printing bendable circuits based on an oil-water interface reaction, Appl. Surf. Sci. 445 (2018) 391–397. [38] K.R. Zope, D. Cormier, S.A. Williams, Reactive silver oxalate ink composition with enhanced curing conditions for flexible substrates, ACS Appl. Mater. Interfaces 10 (2018) 3830–3837. [39] M. Vaseem, S.-K. Lee, J.-G. Kim, Y.-B. Hahn, Silver-ethanolamine-formate complex based transparent and stable ink: Electrical assessment with microwave plasma vs thermal sintering, Chem. Eng. J. 306 (2016) 796–805. [40] W. Li, C.-F. Li, F. Lang, J. Jiu, M. Ueshima, H. Wang, Z.-Q. Liu, K. Suganuma, Selfcatalyzed copper–silver complex inks for low-cost fabrication of highly oxidationresistant and conductive copper–silver hybrid tracks at a low temperature below 100 °C, Nanoscale 10 (2018) 5254–5263. [41] J. Luo, Stabilization of nanoscale quasi-liquid interfacial films in inorganic materials: a review and critical assessment, Crit. Rev. Solid State 32 (2007) 67–109. [42] A. Muzikansky, P. Nanikashvili, J. Grinblat, D. Zitoun, Ag dewetting in Cu@Ag monodisperse core–shell nanoparticles, J. Phys. Chem. C 117 (2013) 3093–3100. [43] Z.Z. Fang, H. Wang, Densification and grain growth during sintering of nanosized particles, Int. Mater. Rev. 53 (2008) 326–352. [44] J.R. Greer, R.A. Street, Thermal cure effects on electrical performance of nanoparticle silver inks, Acta Mater. 55 (2007) 6345–6349.
Tech. 215 (2015) 299–308. [22] J.-T. Wu, S.L.-C. Hsu, M.-H. Tsai, W.-S. Hwang, Conductive silver patterns via ethylene glycol vapor reduction of ink-jet printed silver nitrate tracks on a polyimide substrate, Thin Solid Films 517 (2009) 5913–5917. [23] Y.-L. Tai, Z.-G. Yang, Z.-D. Li, A promising approach to conductive patterns with high efficiency for flexible electronics, Appl. Surf. Sci. 257 (2011) 7096–7100. [24] W. Yang, C. Liu, Z. Zhang, Y. Liu, S. Nie, One step synthesis of uniform organic silver ink drawing directly on paper substrates, J. Mater. Chem. 22 (2012) 23012. [25] C.-N. Chen, T.-Y. Dong, T.-C. Chang, M.-C. Chen, H.-L. Tsai, W.-S. Hwang, Solutionbased β-diketonate silver ink for direct printing of highly conductive features on a flexible substrate, J. Mater. Chem. C 1 (2013) 5161. [26] A.J. Kell, C. Paquet, O. Mozenson, I. Djavani-Tabrizi, B. Deore, X. Liu, G.P. Lopinski, R. James, K. Hettak, J. Shaker, A. Momciu, J. Ferrigno, O. Ferrand, J.X. Hu, S. Lafrenière, P.R.L. Malenfant, Versatile molecular silver ink platform for printed flexible electronics, ACS Appl. Mater. Interfaces 9 (2017) 17226–17237. [27] X. Nie, H. Wang, J. Zou, Inkjet printing of silver citrate conductive ink on PET substrate, Appl. Surf. Sci. 261 (2012) 554–560. [28] Y. Dong, X. Li, S. Liu, Q. Zhu, J.-G. Li, X. Sun, Facile synthesis of high silver content MOD ink by using silver oxalate precursor for inkjet printing applications, Thin Solid Films 589 (2015) 381–387. [29] M. Hu, X. Cai, Q. Guo, B. Bian, T. Zhang, J. Yang, Direct pen writing of adhesive particle-Free ultrahigh silver salt-Loaded composite ink for stretchable circuits, ACS Nano 10 (2015) 396–404. [30] W.J. Hyun, S. Lim, B.Y. Ahn, J.A. Lewis, C.D. Frisbie, L.F. Francis, Screen printing of highly loaded silver inks on plastic substrates using silicon stencils, ACS Appl. Mater. Interfaces 7 (2015) 12619–12624. [31] M. Vaseem, G. McKerricher, A. Shamim, Robust design of a particle-free silverorgano-complex ink with high conductivity and inkjet stability for flexible electronics, ACS Appl. Mater. Interfaces 8 (2015) 177–186. [32] K.S. Bhat, R. Ahmad, Y. Wang, Y.-B. Hahn, Low-temperature sintering of highly conductive silver ink for flexible electronics, J. Mater. Chem. C 4 (2016) 8522–8527.
256