Comparison of thermal decomposition and chemical

Thin Solid Films 636 (2017) 397–402

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Comparison of thermal decomposition and chemical reduction of particle-free silver ink for inkjet printing Yuan Gu a,⁎, Aide Wu b, John F. Federici a a b

Flexible Electronic Devices and Sensors Lab, Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102, USA Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USA

a r t i c l e

i n f o

Article history: Received 12 August 2016 Received in revised form 29 May 2017 Accepted 5 June 2017 Available online 28 June 2017 Keywords: Inkjet printing Chemical reduction Thermal decomposition

a b s t r a c t Two current major methods (thermal decomposition of silver organic salts and chemical reduction of silver salts) to formulate a particle-free silver ink are compared. X-ray diffraction (XRD) and scanning electronics microscopy (SEM) are used to investigate the printed silver patterns from conductive inks made from those two methods. Both processes are shown to make silver with high purity from XRD results. SEM images indicate that chemical reduction can make denser silver film than thermal decomposition. Thermogravimetric analysis and Differential thermal analysis indicates that the silver film is formed at very low temperature compared with thermal decomposition. This phenomenon may be caused by volume shrinkage of silver precursor decomposition after thermal sintering. The resistivity of printed silver patterns made by chemical reduction is also lower than the silver patterns made by thermal decomposition because of less porosity. Also, our results show that a polymer additive can render the silver film more uniform and easier to be sintered. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Increasing attention has been devoted to printable circuit boards, sensors and smart ID cards by using conductive/functional inks [1–6]. The principle of printable electronics is to deposit conductive/functional ink on flexible substrate by printing technologies (e.g. inkjet or screen printing) [7,8]. The most used substrate is Kapton (polyimide) because of its stable physical and chemical properties especially at relatively high temperature [9–11]. The printed patterns are then treated with sintering processes to obtain required electrical and mechanical properties. Various sintering processes like thermal, microwave, plasma and photonic sintering [12–15] have been studied. The key step of making printable devices is ink formulation. Commercially available conductive inks are usually metal nanoparticle (copper [16,17], platinum [18], nickel [19], silver [20–22]) dispersed in organic/ aqueous solvents. Among those materials, silver has high electrical conductivity, relatively low melting point and excellent chemical stability while sintered in air; those advantages make silver nanoparticle ink the

⁎ Corresponding author. E-mail address: [email protected] (Y. Gu).

http://dx.doi.org/10.1016/j.tsf.2017.06.010 0040-6090/© 2017 Elsevier B.V. All rights reserved.

most widely used for printing conductive devices. However, nanoparticle-based ink still requires high sintering temperature to join particles. Also, nanoparticle-based inks have limited printing resolution and reliability due to nozzle clogging issue during printing. Alternatively, particle-free silver inks have been developed to avoid nozzle clogging and achieve high resolution printing. Generally, silver salt complex solution is used as a particle free ink, after printing, drying and sintering, the silver complex is transformed into highly conductive silver thin film; the thermal treating temperature for particle-free ink is usually lower than the temperature required for nanoparticle-based ink. In particle-free silver ink, silver is produced by different chemical processes: (a) thermal decomposition of silver precursor like silver citrate [23,24], Ag(acetate)(ethanolamine)2 [25], silver oxalate [13] and other synthesized organometallic silver [26]; (b) reducing of silver complex [27–32] by external reductants. The former process usually occurs after solvent evaporation, and then the silver complex degrades to silver by disproportionation reaction during sintering. In contrast, the latter one occurs before solvent evaporation. Both methods can produce high quality silver conductive film. Unfortunately, no report compares the difference between those two chemical processes. In this article, different silver films synthesized by thermal decomposition and chemical reduction are compared. In order to eliminate the production diversity caused by silver sources, silver citrate is used as silver precursor for both ink formulations, which is able to

398

Y. Gu et al. / Thin Solid Films 636 (2017) 397–402

Fig. 1. SEM images of silver samples made by thermal decomposition: (a) without NaCMC and (b) without NaCMC. The insets are the cross section of the silver on Kapton. All the samples are sintered at 225 °C for 30 min.

decompose into silver and easily dissolved in water with the present of ammonia functional as complex agent shown in the follow equation [26,28]:

ð1Þ

2. Experimental procedure 2.1. Materials All of the chemical reagents used in the experiments are purchased from commercial sources with analytical purity and used without further purification. Silver citrate, ammonia, propanol alcohol, sodium carboxymethyl cellulose (NaCMC) and dimethylformamide (DMF) are purchased from Sigma Aldrich, USA. The formulated silver inks are deposited on Kapton film from Dupont Company, USA.

2.2. Synthesis of particle free silver ink

Dimethylformamide (DMF) is used as a mild reductant to reduce silver complex in aqueous solution by silver mirror reaction. Unlike normal Tollens reagents, no silver oxide intermediate produced during silver citrate dissolution which might react with ammonia to produce explosive silver azide [33]. Some research also report that a stable, nonexplosive silver organic solution can be obtained by avoiding using Tollens reagents [24,32]. On the other hand, silver citrate contains almost the same amount of silver element as silver nitrate in Tollens reagents (silver citrate: 63 wt%, silver nitrate: 64 wt%). The existence of hydrophilic\\OH groups on silver citrate is capable to increase the solubility of the silver complex by forming hydrogen bonds with water [25]. The influence of a sodium carboxymethyl cellulose (NaCMC) polymer additive is also investigated in this research. We analyze and demonstrate a formulation of particle-free silver ink which has excellent stability, printability and capability to produce silver with high electrical conductivity.

2.2.1. Thermal decomposable silver ink without NaCMC The ink solvent is made by mixing 1 g Deionized (DI) water and 1 g ammonia. After adding 1 g silver citrate to the solvent, the mixture is ultrasonicated for 15 min to produce silver ammonia complex solution. The final viscosity is adjusted by adding propanol alcohol to meet the requirement of printer (around 10 cp). The solution is pressed through a 0.5 μm syringe filter to eliminate large particles and other chemical remains. A clean and transparent particle free solution is obtained.

2.2.2. Thermal decomposable silver ink with NaCMC 0.15 wt% NaCMC solution is prepared by dissolving NaCMC in DI water at 50 °C and magnetic stirred for 30 min. 1 g NaCMC solution, 1 g ammonia and 1 g silver citrate is mixed, the mixture is ultrasonicated for 15 min to produce silver ammonia complex solution. The viscosity of silver ink is adjusted by adding propanol alcohol. The final NaCMC concentration is 0.05 wt%. NaCMC is used as polymer capping agent and film former [34].

Scheme 1. Of silver film made by (a) thermal decomposition and (b) chemical reduction.


399

resistivity of the patterns is measured by a four point probe (Jandel CYL-HM21, USA). Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA), (STA 449 F1 Jupiter from Netzsch, Inc. USA) are used to study the thermal behavior of different inks and chemical reaction processes. Measurement is carried out in air with a temperature rise rate of 5 K/min.

3. Results and discussion

Fig. 2. XRD patterns of silver made by thermal decomposition with/without NaCMC.

2.2.3. Chemical reductive silver ink with NaCMC The solvent for chemical reductive ink is prepared by dissolving NaCMC in ammonia, the concentration of NaCMC in ammonia is 0.15 wt%. Then 1 g NaCMC ammonia solution, 1 g DMF and 1 g silver citrate are mixed and ultrasonicated for 15 min to produce silver ammonia complex solution. The viscosity is adjusted by adding propanol alcohol. The final NaCMC concentration is 0.05 wt%. 2.3. Fabrication of conductive patterns by nanoparticle ink Conductive patterns are fabricated by printing the obtained inks on Kapton film at room temperature using a DMP-2800 single nozzle drop-on-demand (DOD) inkjet printer (Fujifilm, Japan). The substrate is pre-treated by propanol alcohol and DI water to remove all the organic contaminants on the surface. The printed patterns are dried on a hotplate in air at 100 °C for 30 min to evaporate the liquid solvent. The dried samples are then sintered at 225 °C for 30 min in air. The sintered films then cooled to room temperature and characterized. 2.4. Characterization X-ray diffraction (XRD) measurements are carried out on a Philips PW3040 X-Ray Diffractometer, 2θ ranges from 10° to 80° with CuK_α radiation (λ = 15.4 nm) with a step size of 0.02° and a time per step of 15 s. LEO 1530VP Field Emission Scanning Electron Microscope (SEM) is used to examine surface morphology of electrodes. The

Fig. 1 shows the SEM images of silver film made by thermal decomposition without NaCMC and thermal decomposition with NaCMC. The particles without NaCMC additive exhibit uneven structure with particle size ranges from 50 to 300 μm in Fig. 1(a). Not many “neck” formations discovered in this sample suggests poor sintering result; this could be explained by the large silver particles in the inhomogeneous structure which require higher sintering temperature: silver citrate particles precipitate from the ink solution tend to aggregate and join together to reduce the surface energy, grain growth are triggered due to the direct particle contact via elevated temperature [35] producing large silver citrate and degrade into large silver particles. In contrast, silver particles become more uniform after adding NaCMC from Fig. 1(b). The mean particle size is 150 μm and aggregate together forming second particle around 300 μm. NaCMC is found to be a very effective water soluble polymer stabilizer against aggregation. The negatively charged carboxyl radical (\\COO\\) group in NaCMC solution bind on the positive charged silver particles, the attached polymer layer provides a strong electrostatic or steric repulsions to cancel the attractive van der Waals to prevent agglomeration. “Neck” formations are discovered among silver particles after thermal sintering indicates small particle size and uniform structure benefits sintering effect. Though NaCMC additive modifies the morphology of the silver particles and benefits to the silver sintering, the silver film still exhibits significant porosity after sintering. The porosity is caused by the volume shrinkage during silver citrate transforms to silver particles in thermal sintering process (Scheme 1(a)). Generally, production of silver from silver citrate has two steps: (a) deposition of silver citrate on the substrate; (b) disproportionation reaction of silver citrate to produce silver particles. The volume of each silver particle from silver citrate disproportionation reaction can be calculated: the silver content is about 63 wt% based on the calculation from the silver citrate chemical formula (C6H5Ag3O7), and the density of the silver is 3 times greater than silver citrate salt, so the volume of a single silver particle is only 1/5 of the deposited silver citrate particle leaving 4/5 space. Insets in Fig.1 are the cross section of two samples. The thickness of both samples is less than 10 μm with visible cracks and voids marked by red dashes. A full bulk silver structure is impossible to achieve since the carbon oxides and water evaporation during the silver citrate decomposition displayed in Eq. (2). Based on the above analysis,

Fig. 3. SEM images of silver samples made by thermal decomposition: (a) with NaCMC and (b) chemical reduced by DMF with NaCMC. The insets are the cross section of the silver on Kapton. All the samples are sintered at 225 °C for 30 min.

400


Fig. 4. XRD patterns of silver made by thermal decomposition and chemical reduced by DMF, both have NaCMC additive.

porous structure is inevitable in silver film made by thermal decomposition ink. Ag3 C6 H5 O7 ¼ 3Ag↓ þ 6COx þ 2:5H2 O

ð2Þ

Fig. 2 shows the XRD patterns of silver after thermal sintering made by thermal decomposition of silver citrate with and without NaCMC additive marked by #1, and #2, respectively. All of them show at 2θ of 38.1°, 44.3°, 64.4°, 77.5°, and 81.5° index as (111), (200), (220), (311) and (222) (JCPDS file No. 04-0783) reflections of the Fm-3m space group. This means slight NaCMC additive has insignificant influence on the space structure of final silver. However, the intensity if XRD peaks drops dramatically after adding NaCMC comparing #1 and #2 patterns. The crystalline sizes and the width of XRD peaks have a reciprocal relationship according to Scherrer's equation which means that larger crystalline size give sharper signals [36,37]. NaCMC additive could be a barrier among particles (crystals) which has negative effect on the crystallization while thermal sintering. Polymer additive is usually required to be washed away to eliminate sintering barrier [38–40] before further ink formulation. However, washing away polymer additive is impossible in the one-step particle-free silver ink. Considering this point, polymer additive may poorer the performance of the printed silver trace. Fig. 3(b) shows the SEM morphologies of sintered silver patterns made by DMF reducing. Silver patterns synthesized by thermal decomposition also displayed in Fig. 3(a) as the comparison: volume shrinkage leads to the porous and discontinuous structure as discussed in Scheme

1(a), which lower down the conductivity of printed silver patterns. In contrast, silver film (Fig. 3(b)) made by chemical reduction are much denser than the one made by thermal decomposition in Fig. 3(a). The cross section of the silver made by the DMF reducing in Fig. 3(b) also exhibits more compact and density structure than the silver made by thermal decomposition in Fig. 3(a): not much peeling off or internal voids. This phenomenon can be explained by Scheme 1(b). The chemical reduction has two steps: (a) silver reduction in the DMF solution at low temperature drying process; (b) deposition of silver instead of silver citrate on the substrate which makes denser and more continuous structure after solvent evaporation. In DMF reducing process, there is no gas releasing and less volume shrinkage during the silver sintering compared with silver citrate thermal decomposition. Denser structure benefits to the thermal sintering because of the more solid diffusion paths among silver particles. Eventually enhances conductivity of silver patterns. Fig. 4 shows the XRD patterns of silver synthesized by thermal decomposition and DMF reduction. Both methods can produce high purity silver. Silver made by DMF reduction show sharper XRD peak suggest better crystallization. NaCMC has negatively effect on the silver crystallization during silver citrate decomposition as discussed above. Silver particles formed in DMF reducing process is a liquid state reaction, where silver particles have a better environment for nucleation and growth compared with solid state reaction. The well-formed silver particles are then covered by NaCMC and settle down on the substrate for the further sintering. No significant impurity detected indicates all chemical residues removed completely. Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) are carried out to study the thermal decomposition and chemical reduction processes. Fig. 5(a) indicates that thermal decomposition temperature of silver citrate powder is approximately 220 °C. DTA of silver citrate shows the mass loss is around 35% which corresponds to elements loss in citrate (C6H5O7)3. DTA and TGA results are similar to a previous report [23]. DTA in Fig. 5(b) shows that the chemical reduction of silver ions occurs at 95 °C which is even lower than the evaporation temperature of solvent, which means DMF can reduce silver ions into silver very effectively. TGA in Fig. 5(b) displays two mass loss steps: the first mass loss is from 30 to 95 °C, corresponding the formation of silver particles in water solvent; the second mass loss step between 95 and 160 °C attributed by the evaporation of water solvent. The mass loss in two steps is not exactly equal to the water and citrate content which might be associated with the dissolve of ammonia (formed from Eq. (3)) and thermal decomposition of carbamic acid in Eq. (4) [41]. The silver remaining is around ~ 25% which is close to the theoretical silver content 20%. HCONMe2 þ 2AgðNH3 Þ2 þ þ H2 O ¼ 2Ag↓ þ Me2 NCOOH þ 2Hþ þ 2NH3

ð3Þ

Fig. 5. Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) of (a) silver citrate powder and (b) silver citrate conductive ink reduced by DMF with NaCMC. All samples are tested in air with temperature rise rate: 5 K/min. In this figure, “exo” denotes an exothermic reaction.


Fig. 6. Inks performances of this work compared with other researches [13,23,25,29– 31,39,42–44].

Me2 NCOOH→CO2 þ Me2 NH

ð4Þ

Previously reported properties of silver patterns made by various silver inks are summarized in Fig. 6 for comparison. The sintering temperature ranges from 75 to 300 °C (only silver bulk is sintered at 961 °C). Silver particle-free ink requires lower sintering temperature compare with silver nanoparticle ink, and no nozzle clogging issues during the printing. Silver particle-free ink also skips silver nanoparticles synthesis process which usually requires strict chemical and physical conditions. In thermal decomposition inks, silver citrate is an excellent precursor to make silver particle-free ink due to its high silver content compare with other precursors (Silver neodecanoate (Ag(C10H19O2)): 39 wt%; AgO2C(CH2OCH2)3H: 38 wt%), the lower silver content gives the more

401

porous final products with poorer conductivity. The NaCMC additive contributes to unify the particle size; NaCMC is also a good film former contributes to the adhesion of silver patterns on polymer substrate. By comparison, chemical reducing is more efficiency to make higher conductive silver than other methods displayed in Fig. 6. The silver patterns fabricated from chemical reducing in this work have the lowest electrical resistivity; although some researches have more competitive sintering condition (lower sintering temperature and shorter sintering time). The resistivity of silver made by chemical reducing process is almost half compared to the silver made by thermal decomposition, which is contributed by less porosity. Fig. 7(a) shows “NJIT” metallic shiny patterns made by inkjet printing on flexible Kapton substrate, the mean width of patterns is 1 mm. No peeling off or cracking occurs while bending the patterns, which indicates a good flexibility of the printed patterns: the silver ink is sufficient to make circuits and sensors. The inset photograph show synthesized chemical reductive particle-free silver ink kept for 3 days at room temperature: the ink remains colorless and transparent without any precipitation suggests the good chemical stability. Certain temperature is necessary to trigger the silver mirror reaction which is the main reason of ink stability at room temperature for a long time. A tape test is performed to inspect the adhesion of synthesized silver patterns on Kapton substrate in Fig. 7(b). The printed patterns stay completely and smoothly after removal the tape suggests very strong adhesion between the silver patterns and Kapton substrate. Fig. 7(c) illustrates resolution of patterns made by silver ink is 40 μm. A small circuit board is also printed and shown in Fig. 7(d), the printed circuit exhibits fine appearance and has potential application in microelectronics, sensors and smart cards. 4. Summary and conclusion In summary, colorless particle-free silver inks formulated based on two major processes (thermal decomposition of silver salts and chemical reduction of silver complex) for inkjet printing are compared. Both inks produce silver without significant impurity. SEM images show that silver film made by chemical reduction exhibit a denser and more

Fig. 7. Images for (a) NJIT patterned silver thin flexible film made by inkjet printing. The inset is the particle-free silver ink (chemical reduction with NaCMC) kept at room temperature for 3 days; (b) tape test for printed silver thin film; (c) resolution test; (d) circuits printed by the particle free silver ink.

402


continuous structure than those made by thermal decomposition. The porous structure is caused by the volume shrinkage during silver precursor disproportionation reaction. Also, chemical reduction ink produces silver film at relative lower temperature (lower than 100 °C) compared to the thermal decomposition of silver citrate (above 200 °C). Our research also presents that NaCMC additive plays crucial role in the final particle size and uniformity. The chemical reduction ink with NaCMC additive has excellent stability and printability; the conductive silver film is obtained by thermal sintering the printed patterns at 225 °C for 30 min in air. No peeling off and cracking occurs when the printed silver film is bend with a large radius. The printed silver film also has low electrical resistivity (4 times the bulk silver) and survives the tape testing. These tests ascertain a highly satisfactory performance of chemical reduction silver ink and potential application on the fabrication of flexible electronics in future. Acknowledgements This work is partially supported by the US Army Armament Research, Development and Engineering Center (ARDEC) at Picatinny Arsenal. References [1] A.Y. Xiao, Q.K. Tong, A.C. Savoca, H. van Oosten, Conductive ink for through hole application, IEEE Trans. Compon. Packag. Technol. 24 (2001) 445–449. [2] R. Sangoi, C.G. Smith, M.D. Seymour, J.N. Venkataraman, D.M. Clark, M.L. Kleper, B.E. Kahn, Printing radio frequency identification (RFID) tag antennas using inks containing silver dispersions, J. Dispers. Sci. Technol. 25 (2005) 513–521. [3] S.Y.Y. Leung, D.C.C. Lam, Geometric and compaction dependence of printed polymer-based RFID tag antenna performance, IEEE Trans. Electron. Packag. Manuf. 31 (2008) 120–125. [4] M. Pudas, J. Hagberg, S. Leppävuori, Gravure offset printing of polymer inks for conductors, Prog. Org. Coat. 49 (2004) 324–335. [5] J. Fjelstad, Flexible circuit materials, Circuit World 34 (2008) 19–24. [6] J.H.G. Ng, M.P.Y. Desmulliez, M. Lamponi, B.G. Moffat, A. McCarthy, H. Suyal, A.C. Walker, K.A. Prior, D.P. Hand, A direct-writing approach to the micro-patterning of copper onto polyimide, Circuit World 35 (2009) 3–17. [7] F.C. Krebs, Polymer solar cell modules prepared using roll-to-roll methods: knifeover-edge coating, slot-die coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (2009) 465–475. [8] T.H. Van Osch, J. Perelaer, A.W. de Laat, U.S. Schubert, Inkjet printing of narrow conductive tracks on untreated polymeric substrates, Adv. Mater. 20 (2008) 343–345. [9] K. Akamatsu, S. Ikeda, H. Nawafune, Site-selective direct silver metallization on surface-modified polyimide layers, Langmuir 19 (2003) 10366–10371. [10] K. Akamatsu, S. Ikeda, H. Nawafune, H. Yanagimoto, Direct patterning of copper on polyimide using ion exchangeable surface templates generated by site-selective surface modification, J. Am. Chem. Soc. 126 (2004) 10822–10823. [11] Y. Li, Q. Lu, X. Qian, Z. Zhu, J. Yin, Preparation of surface bound silver nanoparticles on polyimide by surface modification method and its application on electroless metal deposition, Appl. Surf. Sci. 233 (2004) 299–306. [12] S. Norita, D. Kumaki, Y. Kobayashi, T. Sato, K. Fukuda, S. Tokito, Inkjet-printed copper electrodes using photonic sintering and their application to organic thin-film transistors, Org. Electron. 25 (2015) 131–134. [13] 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. [14] 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. [15] S. Wunscher, S. Stumpf, J. Perelaer, U.S. Schubert, Towards single-pass plasma sintering: temperature influence of atmospheric pressure plasma sintering of silver nanoparticle ink, J. Mater. Chem. C 2 (2014) 1642–1649. [16] Y. Kim, B. Lee, S. Yang, I. Byun, I. Jeong, S.M. Cho, Use of copper ink for fabricating conductive electrodes and RFID antenna tags by screen printing, Curr. Appl. Phys. 12 (2012) 473–478. [17] B. Lee, Y. Kim, S. Yang, I. Jeong, J. Moon, A low-cure-temperature copper nano ink for highly conductive printed electrodes, Curr. Appl. Phys. 9 (2009) e157–e160.

[18] K. Akamatsu, H. Shinkai, S. Ikeda, S. Adachi, H. Nawafune, S. Tomita, Controlling Interparticle spacing among metal nanoparticles through metal-catalyzed decomposition of surrounding polymer matrix, J. Am. Chem. Soc. 127 (2005) 7980–7981. [19] K. Akamatsu, K. Nakahashi, S. Ikeda, H. Nawafune, Fabrication and structural characterization of nanocomposites consisting of Ni nanoparticles dispersed in polyimide films, Eur. Phys. J. D 24 (2003) 377–380. [20] Y. Zhang, C. Lei, W.S. Kim, Design optimized membrane-based flexible paper accelerometer with silver nano ink, Appl. Phys. Lett. 103 (2013), 073304. [21] J.R. Greer, R.A. Street, Thermal cure effects on electrical performance of nanoparticle silver inks, Acta Mater. 55 (2007) 6345–6349. [22] S.K. Volkman, Y. Pei, D. Redinger, S. Yin, V. Subramanian, Ink-jetted Silver/Copper Conductors for Printed RFID Applications, MRS Proceedings, Cambridge Univ Press, 2004 (pp. I7. 8). [23] X. Nie, H. Wang, J. Zou, Inkjet printing of silver citrate conductive ink on PET substrate, Appl. Surf. Sci. 261 (2012) 554–560. [24] J.-j. Chen, J. Zhang, Y. Wang, Y.-l. Guo, Z.-s. Feng, A particle-free silver precursor ink useful for inkjet printing to fabricate highly conductive patterns, J. Mater. Chem. C 4 (2016) 10494–10499. [25] W.-d. Yang, C.-y. Liu, Z.-y. Zhang, Y. Liu, S.-d. Nie, One step synthesis of uniform organic silver ink drawing directly on paper substrates, J. Mater. Chem. 22 (2012) 23012–23016. [26] Z. Cai, X. Zeng, J. Liu, Laser direct writing of conductive silver film on polyimide surface from decomposition of organometallic ink, J. Electron. Mater. 40 (2011) 301–305. [27] S.-P. Chen, Z.-K. Kao, J.-L. Lin, Y.-C. Liao, Silver conductive features on flexible substrates from a thermally accelerated chain reaction at low sintering temperatures, ACS Appl. Mater. Interfaces 4 (2012) 7064–7068. [28] 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. [29] J.-T. Wu, S.L.-C. Hsu, M.-H. Tsai, W.-S. Hwang, Inkjet printing of low-temperature cured silver patterns by using AgNO3/1-Dimethylamino-2-propanol inks on polymer substrates, J. Phys. Chem. C 115 (2011) 10940–10945. [30] S.F. Jahn, T. Blaudeck, R.R. Baumann, A. Jakob, P. Ecorchard, T. Rüffer, H. Lang, P. Schmidt, Inkjet printing of conductive silver patterns by using the first aqueous particle-free MOD ink without additional stabilizing ligands, Chem. Mater. 22 (2010) 3067–3071. [31] A.L. Dearden, P.J. Smith, D.-Y. Shin, N. Reis, B. Derby, P. O'Brien, A low curing temperature silver ink for use in ink-jet printing and subsequent production of conductive tracks, Macromol. Rapid Commun. 26 (2005) 315–318. [32] S.B. Walker, J.A. Lewis, Reactive silver inks for patterning high-conductivity features at mild temperatures, J. Am. Chem. Soc. 134 (2012) 1419–1421. [33] E.S. Shanley, J.L. Ennis, The chemistry and free energy of formation of silver nitride, Ind. Eng. Chem. Res. 30 (1991) 2503–2506. [34] S. Magdassi, A. Bassa, Y. Vinetsky, A. Kamyshny, Silver nanoparticles as pigments for water-based ink-jet inks, Chem. Mater. 15 (2003) 2208–2217. [35] B. Ingham, T.H. Lim, C.J. Dotzler, A. Henning, M.F. Toney, R.D. Tilley, How nanoparticles coalesce: an in situ study of Au nanoparticle aggregation and grain growth, Chem. Mater. 23 (2011) 3312–3317. [36] A.L. Patterson, The Scherrer formula for X-ray particle size determination, Phys. Rev. 56 (1939) 978–982. [37] M. Goudarzi, N. Mir, M. Mousavi-Kamazani, S. Bagheri, M. Salavati-Niasari, Biosynthesis and characterization of silver nanoparticles prepared from two novel natural precursors by facile thermal decomposition methods, Sci. Report. 6 (2016). [38] D. Kim, S. Jeong, J. Moon, Synthesis of silver nanoparticles using the polyol process and the influence of precursor injection, Nanotechnology 17 (2006) 4019. [39] H.-H. Lee, K.-S. Chou, K.-C. Huang, Inkjet printing of nanosized silver colloids, Nanotechnology 16 (2005) 2436. [40] Y. Lee, J.-r. Choi, K.J. Lee, N.E. Stott, D. Kim, Large-scale synthesis of copper nanoparticles by chemically controlled reduction for applications of inkjet-printed electronics, Nanotechnology 19 (2008) 415604. [41] I. Pastoriza-Santos, L.M. Liz-Marzán, Reduction of silver nanoparticles in DMF formation of monolayers and stable colloids, Pure Appl. Chem. 72 (2000) 83–90. [42] S.B. Fuller, E.J. Wilhelm, J.M. Jacobson, Ink-jet printed nanoparticle microelectromechanical systems, J. Microelectromech. Syst. 11 (2002) 54–60. [43] J.B. Szczech, C.M. Megaridis, D.R. Gamota, J. Zhang, Fine-line conductor manufacturing using drop-on demand PZT printing technology, IEEE Trans. Electron. Packag. Manuf. 25 (2002) 26–33. [44] D. Kim, S. Jeong, S. Lee, B.K. Park, J. Moon, Organic thin film transistor using silver electrodes by the ink-jet printing technology, Thin Solid Films 515 (2007) 7692–7696.