Room-Temperature Joining of Silver Nanoparticles Using Potassium

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Room Temperature Joining of Silver Nanoparticles Using Potassium Chloride Solution for Flexible Electrode Application Peng Peng, Lihang Li, Wei Guo, Zhuang Hui, Jian Fu, Chao Jin, Yangai Liu, and Ying Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10601 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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The Journal of Physical Chemistry

Room Temperature Joining of Silver Nanoparticles Using Potassium Chloride Solution for Flexible Electrode Application Peng Peng1,2, Lihang Li1, Wei Guo1*, Zhuang Hui1,3, Jian Fu1, Chao Jin4, Yangai Liu3, Ying Zhu1 1

School of Mechanical Engineering and Automation, Beihang University, Beijing

100191, China 2

International Research Institute for Multidisciplinary Science, Beihang University,

Beijing 100191, China 3

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid

Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China 4

Department of Systems Design Engineering, University of Waterloo, Waterloo,

Ontario N2L 3G1, Canada

Email: [email protected] (W.G.)

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

ABSTRACT: Flexible electrodes have broad applications in microelectronics. In this work, flexible electrodes were fabricated through chemically induced joining of silver nanoparticle at room temperature. After joining, the resistance of the silver track decreased 8 orders of magnitude. The joining parameters were systematically investigated including the concentration of chemical solution, sintering time and drying temperature. This chemical method achieved the same effect at room temperature as compared to the ones using thermal sintering. The changes of microstructure and surface composition of silver nanoparticles were characterized to better understand the nanojoining process at room temperature.


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INTRODUCTION Flexible electrodes have attracted great interests due to their potentials in wearable electronics and human-machine interactivity, such as health monitoring, disease diagnostics, and smart robots

1-3

, ranging from sensors for monitoring

temperature, pressure, oxygen concentration of blood, and electrophysiological activities to actuators for providing therapeutic heat and healing factors 2. A typical way to fabricate flexible electrodes is to deposit conductive materials onto flexible substrates. Direct writing is found to be simple, rapid and low-cost compared to other available methods (e.g. spin coating, chemical vapor deposition) for deposition

4, 5

. It

is desirable that these flexible electrodes could be highly conductive to make sensors be sensitive. Due to the high conductivity of Ag, many works have demonstrated Ag NPs or nanowires (NWs) are the promising candidates for conductive materials

4, 6, 7

.

Meanwhile it has been found that Ag NPs or NWs have a better chemical stability than copper or aluminum NPs

8, 9

. Creating metallic bonds between those Ag

nanomaterials (NPs, NWs, etc.) in electrodes is a key process to enhance their conductivity

10, 11

. Usually, thermal sintering can be adopted because of its feasibility

in mass production. It was found that the interconnection and conductivity could be increased at an elevating sintering temperature

5, 12

. However, those used plastic

substrates for flexible electrodes usually cannot tolerate high temperature during the thermal sintering process. To reduce the joining temperature, various joining processes have been attempted, such as laser-induced sintering or pressure-assisted sintering

6, 13, 14

. Although these methods could be completed at low temperatures,

accompanying localized heating or pressing may damage the plastic substrate or Ag electrodes. Therefore, low temperature or even room temperature processing techniques are in demand. Recently, room-temperature joining of metal NPs or NWs has attracted considerable attention

15, 16

. Among these, some chemical solutions, hydrogen

peroxide for example, are proved to be able to complete joining of metal nanomaterials

15

. It can ensure less damage to the plastic substrates while a good

conductivity can be achieved. This chemically induced joining of Ag NPs which can ACS Paragon Plus Environment


be operated at room temperature has been reported using chloride ion based chemicals, such as PDAC (poly(diallyldimethylammo-nium chloride)) 7, or chloride salts (NaCl, MgCl2, and CaCl2 ) 16, 17. However, the mechanism of the chemically induced joining of Ag NPs by chloride ions and joining process are still unclear. In this study, Ag NPs ink was prepared and printed onto a polycarbonate (PC) substrate. The resistance of the Ag track showed a remarkable decline after treating with KCl solution at room temperature. Flexible pressure sensor based on the fabricated Ag tracks was demonstrated. To evaluate chemically induced joining process, resistance under various operational conditions including ion concentration, sintering time and drying temperature and the corresponding microstructure and surface chemical characteristics (SEM, TEM and XPS) were examined respectively. Further, the room temperature joining mechanism was also discussed and validated.

EXPERIMENT Materials and Ag NPs Synthesis: Analytical reagents including silver nitrate (AgNO3), PVP (K30, Mw≈40000 g·mol-1, ethylene glycol (EG), ethanol, polydimethylsiloxane (PDMS), sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl2)) have been used in the experiments. All reagents were purchased from Aladdin Industrial Corporation. Ag NPs were synthesized using the modified procedure from literature 18. In general, 5.1 g of silver nitrate (AgNO3) was dissolved in 30ml deionized (DI) water. Meanwhile, 12 g glucose (C6H12O6) and 10 g PVP (polyvinyl pyrrolidone) was dissolved in 200ml deionized water. Then 2 g sodium hydroxide was first added into glucose solution (mixture). After adjusting pH between 8~9, the mixed solution was heated up to 90 °C for 10 mins. At this temperature the AgNO3 solution was dropped into the mixed glucose solution with stirring for 30 mins, and then naturally cooled. Finally, the mixture (nearly 250 ml) was ultrasonic and centrifuged to 5 ml as Ag NPs ink for further usages. Direct writing and room temperature nanojoining: Figure 1a illustrates the direct writing system using the prepared Ag NPs ink. The Polycarbonate (PC) substrate installed on a programmable 2-axis platform was selected as the substrate for direct ACS Paragon Plus Environment

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writing and nanojoining. During the direct writing process, the syringe filled Ag NPs ink 4 was set up to move at the speed of 0.02 ml/min on the PC substrate. After direct writing, PC substrate with printed Ag tracks was soaked in chlorides solution to reduce its resistance. To quantitatively investigate the impacts of operational processes on joining process, various operational parameters were systematically investigated. Specifically, experiments under 6 levels of chloride concentrations ranging from 0.2-1.2 mol/dm3, 7 sintering time under 3 different temperatures (30, 70 and 110 oC) were conducted. After the soaking process, the treated Ag track was covered by PDMS using spin coating for protection. Characterization: The microstructure and morphology were characterized by field emission scanning electron microscopy (FESEM, Merlin compact, Germany) and transmission electron microscope (TEM, JEOL 2100F, Japan). X-ray photoelectron spectroscopy (XPS, Thermo Fisher, ESCALAB 250Xi America) was used to characterize the surface chemistry of Ag NP without/with sintering. Electrical measurement of Ag NP electrodes was carried by Desktop Digital Multimeter (FLUKE, 8808A, America). The bending tests were conducted on a 2-axis platform. One side of the substrate was fixed on the platform while another side was fixed on the desktop. As the platform moved repeatedly for a settled distance, the substrate with Ag track would be repeatedly bent to the same degree. The chemical tests were completed by spraying water spray on Ag tracks with/without PDMS using a compression atomizer. The Ag electrodes were made by attaching wires at both ends of the Ag track through conductive adhesive and covering the PDMS film with a spin coating method (1000 rpm for 30s).

RESULTS AND DISCUSSION Characterized by SEM (Figure 1b), it was found that the as-synthesized Ag NPs have the diameter 50 nm. They were mostly spherical without significant interconnection. This Ag NP ink was printed onto PC substrate at room temperature using the direct writing system as introduced before. After printing, the Ag NPs tracks without chemical treatment were first examined using SEM (Figure 1c). It was found ACS Paragon Plus Environment


that the resistance of the Ag NP track was above 10 MΩ/cm. The size of Ag NP was un-changed, which suggested that Ag NPs were not joined together during the printing process. In contrast, after treating with 0.2 mol/dm3 KCl solution for 6 mins, Ag NPs formed a network structure and the size of NPs increased to 140 nm (Figure 1d). These changes suggest that the Ag NPs were joined together after exposure to KCl solution. Surprisingly, the resistance of the Ag tracks reduced to 4.43±2.65 Ω/cm, 8 orders of magnitude lower than the ones before treatment. This remarkable decrease of resistance of the Ag track was largely due to the interconnections of Ag NPs 7. Before chemically induced joining, the Ag NPs were mostly isolated from each other with less interconnection, resulting in high resistance. However, the NPs were joined together resulting in increased diameters and networks structure via interconnections, thus the much less electrical resistance of Ag track. Since the printed electrodes are porous, the chloride ions can get into the inside structure and initiate joining in solution. Therefore, this chemically induced joining can reach quite deep, and the significant reduction of resistance can also prove it. To improve the mechanical and chemical performances of as-fabricated flexible electrodes, a polydimethylsiloxane (PDMS) film laminated on chemically treated Ag tracks functioned as a passive layer to enhance the stability of the electrodes. This is because flexible electrodes are usually required to resist to bending or chemical corrosion to ensure their stability when used in wearable electronics. Figure 2a shows the images of the flexible electrodes fabricated with Ag NPs. Figure 2b is the SEM image of the Ag track after 2500 bending cycles without PDMS passive layer. It is found that the Ag track appeared some cracks with a width of about 0.35 µm after bending due to frequent deformation. The direction of the crack was perpendicular to the direction of bending. The white arrow pointing to some minor cracks (Figure 2b) indicates that the main cracks might propagate along this direction as bending continues. The formation of these cracks could be results of increased resistance after bending. To quantify the performance of Ag track, average resistance R/R0 was plotted as a function of bending cycles (Figure 2c) with and without PDMS layer where R/R0 ACS Paragon Plus Environment

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represents the ratio of the measured electrical resistance after multiple bending cycles to the original one. It is found that as the bending cycles increased, the R/R0 values increased on both conditions (with and without PDMS), but the growth rate was different. After 2500 bending cycles, the ratio of Ag track with PDMS coverage (R/R0=1.5) was much lower than the one without PDMS (R/R0=2.7). These results suggest that PDMS film on the electrodes have the ability to prevent crack formation and/or propagation in the Ag track, consistent with the reported literature 3. To further evaluate the chemical stability of Ag track on flexible electrodes at in humid environment, the prepared Ag track were blown with atomized water for several hours using a compression atomizer. Figure 2d displays the average resistance R/R0 with and without PDMS layers as a function of time of atomization. Within an hour atomizing, the R/R0 values both increased but the difference between them was not minor. After one hour of atomization, the R/R0 values without PDMS increased significantly to 1.21 while the one with PDMS remained unchanged at around 1.02. These two tests suggest that that PDMS film on the electrodes have the ability to prevent crack formation and/or propagation in the Ag track and also able to protect the Ag track from chemical corrosion. To demonstrate the functionality of the as-printed, chemically treated and PDMS laminated flexible electrodes afore mentioned, the proposed method was used to fabricate a vibration sensor for demonstration purpose herein. Figure 2e shows the resistance variation of the Ag track (about 7 cm long) under slight pressure change where Figure 2f demonstrated the electrical response when the wearer was saying “ah” with a flexible electrode attached on the throat. The resistance was measured under two conditions: the pressure was applied on the up-surface and down-surface. Here, up-surface refers to Ag track on the top surface of substrate and another is down-surface, as shown in inset images of Figure 2e. The resistance increased when the pressure was on down-surface but decreased on up-surface. When the pressure was applied on the up-surface, the substrate bended slightly upward resulting in a compressive stress in Ag track. This increased the contacting possibility of Ag NPs and contributed to the decrease of resistance. In contrast, when the pressure was ACS Paragon Plus Environment


applied on the down-surface, the Ag track was under tensile stress, causing the increase of resistance because of a less contacting possibility of Ag NPs. It should be noticed that the resistance varied linearly with applied small loads within limits on both two conditions. If the applied bending is greater than the limits, the change of resistance should not be linear anymore. The sensitivity (S=∆R/∆m) was calculated to be 3.2×10-3 Ω/g when the pressure applied on the up-surface and 4.8×10-3 Ω/g in another case. These tests provide the potential for the fabrication of wearable devices. To improve the conductivity of the flexible Ag NPs electrodes, different operational parameters including the concentration of solution, sintering time and drying temperature were investigated respectively, aiming to identify the optimal joining parameters. The chloride ions such as NaCl, CaCl2 and KCl solutions have been applied to sinter the Ag NPs because these solutions could remove the PVP from Ag surface 17, 19. Preliminary results suggested that the conductivity of Ag tracks using KCl solution (12.8 Ω/cm) are the lowest as compared to the ones using NaCl (48.6 Ω/cm) and CaCl2 (132.7 Ω/cm). This might because the electronegativity of K is the lowest among as compared to Na and Ca

20

. Accordingly, KCl solution has been

selected to sinter the Ag NPs in the present study. Figure 3a describes the impact of solution concentration on the resistance of Ag tracks. It can be seen that there is a non-linear non-monotonic relationship between the KCl concentration and resistance. When KCl concentration increased from 0.2 to 1.2 mol/dm3, the resistance of Ag tracks first decreased to a minimum value of 4.0 Ω/cm at 0.8 mol/dm3, and then an increase of resistance was observed from 0.8-1.2 mol/dm3. The concentration of KCl positively influences the particle size and neck size. As shown in Figure 3b, when the Ag track was treated by 0.2 mol/dm3 KCl solution, the particle size and neck size both increased rapidly from 71 to 140 nm and 0 to 110 nm, respectively. Then, with the solution concentrate further increasing, the particle size and the neck size both had a slow increment. The particle size for 0.2 mol/dm3 (in Figure 3c) was measured as 123-153 nm, and this size increased to 160-208 nm for 0.8 mol/dm3 (in Figure 3d). Moreover, the neck size increased from 80 to 120 nm as shown in Figure 3c-d. It can be seen that most of Ag NPs were joined ACS Paragon Plus Environment

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together leading to increasing of measured particle size and the formation of network as highlighted in boxes of Figure 3d. These changes could increase the probability of particle bonding thus resulting in the improvement of the conductivity 7. Figure 4a describes the impact of sintering time. The result indicated that the increase of sintering time would contribute to the reduction of resistance, similar to the effect of solution concentration (Figure 3a). It is found that sintering time negatively contributes to the resistance of Ag track. A longer sintering time could lower the measured resistance. In contrast, concentration of KCl has limited impacts on the reduction of resistance. When the concentration of KCl was too high (above 0.8 mol/dm3), the resistance increased instead (Figure 3a). This might because chloride ions have an influence both on the reduction of resistance and the oxidation of Ag simultaneously. The high concentration of chloride ion makes the oxidation of Ag a dominate change compared to the reduction of resistance.

20

The particle size

and the neck size also increased with the increase of sintering time as shown in Figure 4b. Figure 4a and 4b compare the resistance, the particle size and the neck size of Ag track using current method as compared to the ones after thermal sintering at 300 oC with 5 mins. The resistance of Ag track treated by 0.8 mol/dm3 KCl solution was very close to the resistance of thermal sintering at 300 oC (1.0±0.3 Ω/cm). Meanwhile, the particle size and the neck size for chemical sintering were both higher than that of thermal sintered. Comparing this chemically induced joining and thermal sintering at 300 oC, it indicates that KCl solution induced joining at room temperature could achieve the same effect of thermal sintering at 300 oC. Verified by SEM images, the particle size measured as 91-125 nm for 3 min treatment (in Figure 4c) increased to 140-152 nm for 9 min treatment (in Figure 4d). The neck size increased from 78 to 118 nm in the meanwhile. It is concluded that the aforementioned chemical sintering could join Ag NPs at room temperature and reduce the resistance very similar to that of thermal sintering and laser sintering 21-23 without physical and chemical impacts on the substrate. The drying temperature of the Ag track after chemical treatment was also ACS Paragon Plus Environment


investigated to further explore the optimal operational conditions. Before drying process, the Ag track was washed with deionized water to remove residual KCl. The resistance of Ag track dried at room temperature (about 30 oC) was 14.0±4.5 Ω/cm, while it was 7.0±0.6 Ω/cm at 70 oC and 6.3±0.7 Ω/cm at 110 oC. The drying time remained unchanged for 30 mins. The resistance of Ag track dried at the temperature above 70 oC was slightly lower than the one dried at room temperature, meaning proper elevation of drying temperature also has a positive effect on the improvement of conductivity. This insignificant improvement of conductivity was related to further thermal sintering of Ag NPs. However, it was not significant (2 times improvement) as compared to the chemical sintering process (8 orders improvement) suggesting that the chemically induced joining of Ag NPs at room temperature is comparable with high temperature sintering (also has been demonstrated above with 300 oC thermal sintering as shown in Figure 4a-b). Therefore, the optimal drying temperature could be selected according to the property of used substrates. Since the coefficient of thermal expansion (CTE) has quite significant effect at the interface of two dissimilar materials, the Ag electrode might fall off from PC substrate during the drying process. However, no detachment of Ag electrodes was found in this work due to the porous feature of Ag electrodes. This kind of structure could reduce the thermal stress and absorb deformation energy caused by CTE mismatch. Meanwhile, the PC substrate has low CTE at the temperature lower than 180 oC. Therefore, CTE mismatch has little effect on this porous Ag/ PC interface. Summarizing the discussion above, 70 oC was the appropriate drying temperature. The change of surface composition of Ag NPs was analyzed by XPS as shown in Figure 5. The peaks of Ag3d in Figure 5a were identified as Ag3d5/2 at 367.5 eV and Ag3d3/2 at 373.5 eV

24, 25

belong to Ag element

19, 25

. These two peaks both existed

before and after chemically induced joining. Ag3d5/2 at 367.9 eV and Ag3d3/2 at 373.9 eV were identified as Ag2O

26, 27

, and these two peaks only existed after chemical

treatment. O1s peak at 531.2 eV in Figure 5b was identified as C=O bond from PVP 28, 29

and peak at 532.3 eV was from C-O- bond 28-30 for pristine ink. After joining peak at

530.8 eV appeared and referred to Ag2O 31, no C-O bond was found but C=O bond still ACS Paragon Plus Environment

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existed. Combined with Ag and O patterns (Figure 5a, b), it can be concluded that Ag and PVP were bonded through C-O-Ag bond during the preparation process resulting in a thin film of PVP presented on the surface of Ag NPs

32

. During the process of

chemically induced joining, C-O- bond became C=O bond and then the thin film of PVP detached from Ag surface 33, 34, leading to a close contact or agglomeration of Ag NPs to let the inter-diffusion of Ag atoms occur to facilitate joining at room temperature. It is worth mention that some of Ag NPs could be oxidized turning Ag0 into Ag2O

35

(Figure 5a, b) at room temperature once the protective PVP layer was detached because the KCl aqueous solution could resolve some oxygen. This might be more obvious when KCl concentration was high or a long treating time was used, leading an increase of resistance, see Figure 3a and 4a. It can also be proved by C1s pattern that C-O- bond only existed before joining as shown in Figure 5c. It is interesting but also promising that both joined and un-treated samples illustrated aliphatic carbon at 284.4-284.8 eV binding energy 28 indicating that the detached PVP or those organic components on the surface of Ag NPs from synthesis process are still existing in the electrodes, which is very unlike the thermal or laser sintering caused decomposition or burn out of organics 5, 36. The peaks of N1s pattern were identified as C-N bond of PVP at 399.5 eV 28 and AgNO3 at 406.6 eV 31. The peak of C-N bond both existed before and after joining, indicating that C-N group remain unchanged during joining process which further confirmed the structural stability of PVP molecule after chemically induced joining. The peak of AgNO3 only existed before joining, because silver inks might still contain residual AgNO3 that was not reduced completely during the synthesis process. It was dissolved in the solution during joining process. Meanwhile, some of Ag NPs could be oxidized and Ag0 was turned into Ag2O 35. Therefore, the process of this chemically induced joining destroyed the C-O-Ag group of Ag NP surface, and turned C-O- into C=O so that the PVP film around the particle no longer prevented silver particles from joining

20, 33

at room

temperature. However, unlike the decomposition of PVP during thermal or laser sintering, most of PVP would remain in the electrodes suggesting its protection would ACS Paragon Plus Environment


be still functional even the joining process was completed. Figure 6a and 6b are the TEM images of the pristine Ag NPs. The distances of lattice fringes of all the particles in Figure 6b are generally 2.09 Å, corresponding (200) plane of Ag. During the chemical treatment with 0.8 mol/dm3 KCl solution for 8 min, the Cl ions could break C-O-Ag bond between Ag NPs and PVP as mentioned above (Figure 5b, 5c). This is because of the stronger interactions between Ag and Cl- ions to detach PVP away from Ag surface to stimulate inter-diffusion and joining 19, 32 causing the interconnection of particles, which contributed to the increase of particle size as shown in the TEM observation of Figure 6c and 6d. The growth of particle size was also shown in the statistic results of Figure 3, 4. The dashed lines in Figure 6c and 6d represent the interface of two NPs. With the help of such lines, the original shape of Ag NPs interconnected together could be identified. According to the lattice orientation of the joined particle (Figure 6c-d), the interface indicated that the Ag lattices could line up to form a clear crystalline interface with less mis-orientation, where the distances are 2.09 Å (200 planes) and 2.35 Å (111 planes) 11, 37. The angle between two planes was also marked in the image (Figure 6c-d). When the particles were joined by the same plane (111) of Ag (pointed by blue arrows), the lattice line of them is almost parallel and the interface was well matched. Certainly, although the interface was formed by interconnection with same planes (111), they could also generate misorientation with 23° or 5° angles (Figure 6c-d). However, when the interface joined between (111) and (200), see Figure 6d, the lattices were not well matched and obvious stacking faults existed as indicated by yellow arrow indicated. With such generated stacking faults, the particles could compensate for differences in lattice spacing and be joined at certain angles to form a more stable interface. The angle between the plane (200) and (111) was measured as 135° at this interface with stacking faults deviated from the theoretical value of 125.26° in a single crystal of FCC Ag 37, 38 suggesting that the misorientation along this interface is about 10°. Therefore, it can be concluded that this room temperature joining process could produce prefect interface with well-matched lattices like single crystal or less misoriented lattices like low-angle grain boundaries than joining processes involved heat. ACS Paragon Plus Environment

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CONCLUSION We demonstrated a novel and cost-effective method to fabricate flexible electrodes using Ag NP ink at room-temperature with chemically induced joining. The printed and chemical treated flexible sensor can effectively measure vibration, and can resist mechanical and chemical damage after covering with PDMS film. After treating with KCl solution, the resistance of Ag track decreased 8 orders of magnitude as compared to the one of un-treated and become comparable with thermal sintering at 300 oC. The mechanism of chemical sintering was different from that of thermal sintering and laser sintering. PVP was detached from the surface of Ag NPs by chlorine ions, but it still existed in electrodes as confirmed by XPS analysis. After joining the Ag NPs were joined together through lattice matching or forming low angle grain boundaries at the interface. This chemically induced joining of Ag NPs at room temperature provides an alternative routine for fabricating flexible electrodes or other heat sensitive components in a much cost effective and feasible manner in practice.

ACKNOWLEDGEMENTS We acknowledge funding from the National Key R&D Program of China (2017YFB1104901) and the National Natural Science Foundation of China (Grant No.51605019, 51675030).



REFERENCES 1. Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C. K.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.; Bao, Z. Skin-Like Pressure and Strain Sensors Based on Transparent Elastic Films of Carbon Nanotubes. Nature Nanotechnology 2011, 6, 788-792. 2. Han, S.; Kim, M. K.; Wang, B.; Wie, D. S.; Wang, S.; Lee, C. H. Mechanically Reinforced Skin-Electronics with Networked Nanocomposite Elastomer. Adv. Mater. 2016, 28, 10257-10265. 3. Wang, Q.; Jian, M.; Wang, C.; Zhang, Y. Carbonized Silk Nanofiber Membrane for Transparent and Sensitive Electronic Skin. Adv. Funct. Mater. 2017, 27, 1605657. 4. Li, R.; Hu, A.; Zhang, T.; Oakes, K. D. Direct Writing on Paper of Foldable Capacitive Touch Pads with Silver Nanowire Inks. ACS Applied Materials & Interfaces 2014, 6, 21721-21729. 5. Ghosale, A.; Shankar, R.; Ganesan, V.; Shrivas, K. Direct-Writing of Paper Based Conductive Track Using Silver Nano-Ink for Electroanalytical Application. Electrochim. Acta 2016, 209, 511-520. 6. Huang, G.; Xiao, H.; Fu, S. Paper-Based Silver-Nanowire Electronic Circuits with Outstanding Electrical Conductivity and Extreme Bending Stability. Nanoscale 2014, 6, 8495-8502. 7. Magdassi, S.; Grouchko, M.; Berezin, O.; Kamyshny, A. Triggering the Sintering of Silver Nanoparticles at Room Temperature. ACS Nano 2010, 4, 1943-1948. 8. Yang, C.; Wong, C. P.; Yuen, M. M. F. Printed Electrically Conductive Composites: Conductive Filler Designs and Surface Engineering. J. Mater. Chem. C 2013, 1, 4052-4069. 9. Shankar, R.; Groven, L.; Amert, A. K.; Whites, K. W.; Kellar, J. J. Non-Aqueous Synthesis of Silver Nanoparticles Using Tin Acetate as a Reducing Agent for the Conductive Ink Formulation in Printed Electronics. J. Mater. Chem. 2011, 21, 10871-10877. 10. Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; Mcgehee, M. D.; Brongersma, M. L. Self-Limited Plasmonic Welding of Silver Nanowire Junctions. Nature Materials 2012, 11, 241-249. 11. Peng, P.; Hu, A.; Huang, H.; Gerlich, A. P.; Zhao, B.; Zhou, Y. N. Room-Temperature Pressureless Bonding with Silver Nanowire Paste: Towards Organic Electronic and Heat-Sensitive Functional Devices Packaging†. J. Mater. Chem. 2012, 22, 12997-13001. 12. Guo, W.; Zeng, Z.; Zhang, X.; Peng, P.; Tang, S. Low-Temperature Sintering Bonding Using Silver Nanoparticle Paste for Electronics Packaging. Journal of Nanomaterials 2015, 2015, 10. 13. Zhou, W.; Bai, S.; Ma, Y.; Ma, D.; Hou, T.; Shi, X.; Hu, A. Laser Direct Writing of Silver Metal Electrodes on Transparent Flexible Substrates with High Bonding Strength. ACS Applied Materials & Interfaces 2016, 8, 24887-24892. 14. Madaria, A. R.; Kumar, A.; Ishikawa, F.; Zhou, C. Uniform, Highly Conductive, and Patterned Transparent Films of a Percolating Silver Nanowire Network on Rigid and Flexible Substrates Using a Dry Transfer Technique. Nano Research 2010, 3, 564-573.


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15. Yoon, S.; Khang, D. Room-Temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation. Nano Lett. 2016, 16, 3550-3556. 16. Long, Y.; Wu, J.; Wang, H.; Zhang, X.; Zhao, N.; Xu, J. Rapid Sintering of Silver Nanoparticles in an Electrolyte Solution at Room Temperature and Its Application to Fabricate Conductive Silver Films Using Polydopamine as Adhesive Layers. J. Mater. Chem. 2011, 21, 4875-4881. 17. Layani, M.; Grouchko, M.; Shemesh, S.; Magdassi, S. Conductive Patterns on Plastic Substrates by Sequential Inkjet Printing of Silver Nanoparticles and Electrolyte Sintering Solutions. J. Mater. Chem. 2012, 22, 14349-14352. 18. Lee, P. C.; Meisel, D. Adsorption and Surface-Enhanced Raman of Dyes on Silver and Gold Sols. J. Phys. Chem. 1982, 86, 3391-3395. 19. Layani, M.; Gruchko, M.; Milo, O.; Balberg, I.; Azulay, D.; Magdassi, S. Transparent Conductive Coatings by Printing Coffee Ring Arrays Obtained at Room Temperature. ACS Nano 2009, 3, 3537-3542. 20. Zhuang, H.; Yangai, L.; Wei, G.; Lihang, L.; Nan, M.; Chao, J.; Ying, Z.; Peng, P. Chemical Sintering of Direct-Written Silver Nanowire Flexible Electrodes Under Room Temperature. Nanotechnology 2017, 28, 285703. 21. Wakuda, D.; Kim, K.-S.; Suganuma, K. Room Temperature Sintering of Ag Nanoparticles by Drying Solvent. Scripta Mater. 2008, 59, 649-652. 22. Peng, P.; Hu, A.; Zhou, Y. Laser Sintering of Silver Nanoparticle Thin Films: Microstructure and Optical Properties. Appl. Phys. A 2012, 108, 685-691. 23. Makrygianni, M.; Kalpyris, I.; Boutopoulos, C.; Zergioti, I. Laser Induced Forward Transfer of Ag Nanoparticles Ink Deposition and Characterization. Appl. Surf. Sci. 2014, 297, 40-44. 24. Hoflund, G. B.; Weaver, J. F.; Epling, W. S., Ag Foil by Xps. Surf. Sci. Spectra 1994, 3, 151-156. 25. Tselesh, A. S. Anodic Behaviour of Tin in Citrate Solutions: The Ir and Xps Study on the Composition of the Passive Layer. Thin Solid Films 2008, 516, 6253-6260. 26. Lindgren, B.; Nilsson, T.; Husebye, S.; Mikalsen, O.; Leander, K.; Swahn, C. Preparation of Carboxylic Acids from Aldehydes (Including Hydroxylated Benzaldehydes) by Oxidation with Chlorite. Acta Chem. Scand. 1973, 27, 888-890. 27. Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892-7895. 28. Terzyk, A. P.; Rychlicki, G. The Influence of Activated Carbon Surface Chemical Composition on the Adsorption of Acetaminophen (Paracetamol) in Vitro. Colloids Surf. A: Physicochemical and Engineering Aspects 2000, 163, 135-150. 29. Lu, S.; Yu, J.; Cheng, Y.; Wang, Q.; Barras, A.; Xu, W.; Szunerits, S.; Cornu, D.; Boukherroub, R. Preparation of Silver Nanoparticles/Polydopamine Functionalized Polyacrylonitrile Fiber Paper and Its Catalytic Activity for the Reduction 4-Nitrophenol. Appl. Surf. Sci. 2017, 411, 163-169. 30. Hu, A.; Guo, J.; Alarifi, H.; Patane, G.; Zhou, Y.; Compagnini, G.; Xu, C. X. Low Temperature Sintering of Ag Nanoparticles for Flexible Electronics Packaging. Appl. Phys. Lett. 2010, 97, 153117. 31. Kaushik, V. K. Xps Core Level Spectra and Auger Parameters for Some Silver



Compounds. J. Electron. Spectrosc. Rel. Phenom. 1991, 56, 273-277. 32. Yan, J.; Zou, G.; Hu, A.; Zhou, Y. N. Preparation of Pvp Coated Cu Nps and the Application for Low-Temperature Bonding. J. Mater. Chem. 2011, 21, 15981-15986. 33. Gao, Y.; Jiang, P.; Liu, D. F.; Yuan, H. J.; Yan, X.; Zhou, Z. P.; Wang, J. X.; Song, L.; Liu, L.; Zhou, W. Evidence for the Monolayer Assembly of Poly(Vinylpyrrolidone) on the Surfaces of Silver Nanowires. J. Phys. Chem. B 2004, 108, 12877-12881. 34. Wang, H.; Qiao, X.; Chen, J.; Wang, X.; Ding, S. Mechanisms of Pvp in the Preparation of Silver Nanoparticles. Mater. Chem. Phys. 2005, 94, 449-453. 35. Xu, Y.; Xu, H.; Yan, J.; Li, H.; Huang, L.; Xia, J.; Yin, S.; Shu, H. A Plasmonic Photocatalyst of Ag/Agbr Nanoparticles Coupled with G-C3n4 with Enhanced Visible-Light Photocatalytic Ability. Colloid Surf. A: Physicochemical and Engineering Aspects 2013, 436, 474-483. 36. Paeng, D.; Yeo, J.; Lee, D.; Moon, S.; Grigoropoulos, C. P. Laser Wavelength Effect on Laser-Induced Photo-Thermal Sintering of Silver Nanoparticles. Appl. Phys. A 2015, 120, 1229-1240. 37. Peng, P.; Hu, A.; Gerlich, A. P.; Liu, Y.; Zhou, Y. N. Self-Generated Local Heating Induced Nanojoining for Room Temperature Pressureless Flexible Electronic Packaging. Scientific Report 2015, 5, 9282. 38. Peng, P.; Liu, L.; Gerlich, A. P.; Hu, A.; Zhou, Y. N. Self-Oriented Nanojoining of Silver Nanowires Via Surface Selective Activation. Part. Part. Syst. Char. 2013, 30, 420-426.


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Figure 1. (a)The schematic of the direct writing set-up. The SEM image of silver nanoparticle in (b) origin silver ink, (c) after direct writing on PC substrate, and (d) after chemical treatment with 0.2 mol/dm3 KCl solution for 6 mins.



Figure 2. (a) Digital photo of flexible electrodes under bending, inset: the flexible electrode covered with PDMS layer. (b) The SEM image of the Ag track after bending test without PDMS layer. (c) The change of R/R0 for flexible electrodes with and without PDMS layer in bending test. (d) The change of R/R0 for flexible electrodes in humid environment. (e) The change of resistance of flexible electrodes under slight loading. (f) Recognition of a sound signal by changing vibration to current variation when saying “ah” with a flexible electrode on the throat.


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Figure 3. The change of (a) resistance, (b) particle size and neck size as a function of solution concentration of KCl with 6 mins treating time. The SEM micrographs of Ag tracks treated by (c) 0.2 mol/dm3 and (d) 0.8 mol/dm3 KCl solution.



Figure 4. The change of (a) resistance and (b) particle size and neck size as a function of sintering time with 0.8 mol/dm3 KCl solution.The SEM micrographs of Ag tracks treated by 0.8 mol/dm3 KCl with (c) 3 min and (d) 9 min.


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Figure 5. XPS patterns of (a) Ag3d (b) O1s (c) C1s (d) N1s for Ag electrodes before and after chemically induced joining.



Figure 6. TEM image of Ag NPs (a) (b) before and (c) (d) after chemical joining


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Room-Temperature Joining of Silver Nanoparticles Using Potassium

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