Fabrication of Conductive Copper Films on Flexible Polymer

Low-temperature Sintering of Composite Cu Ink in Air ... Keywords: conductive metal inks; flexible electronics; air-atmosphere sintering; conductivity...
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Fabrication of Conductive Copper Films on Flexible Polymer Substrates by Low-Temperature Sintering of Composite Cu Ink in Air Mai Kanzaki, Yuki Kawaguchi, and Hideya Kawasaki* Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan S Supporting Information *

ABSTRACT: The development of a thermal sintering method for Cu-based inks under an air atmosphere could greatly expand their application for printed electronics. However, it is well-known that Cu-based inks cannot produce conductive Cu films when sintered at low temperatures in air because Cu readily oxidizes under such conditions. In this study, we have successfully demonstrated air atmosphere sintering at low temperatures (less than 150 °C) via a simple hot plate heat treatment for producing conductive Cu films on flexible polymer substrates, using a novel Cu-based composite ink with sub-10 nm Cu nanoparticles protected with 1-amino2-propanol with micrometer-sized Cu particles and submicrometer-sized Cu particles; oxalic acid was also added to prevent the oxidation of the Cu during sintering. The Cu films showed a minimum resistivity of 5.5 × 10−5 Ω·cm when sintered in air at 150 °C for a very short period of 10 s. To the best of our knowledge, this is the first report of sintering of Cu-based inks in air at less than 150 °C. Another novel property of the present Cu-based composite ink is the lowest reported resistivity at 80 °C under N2 flow (5.3 × 10−5 Ω·cm at 80 °C and 8.4 × 10−6 Ω·cm at 120 °C). This fast, efficient, and inexpensive technology for thermal sintering in ambient air using composite inks could be a commercially viable method for fabricating printed electronics on flexible substrates. KEYWORDS: conductive metal inks, flexible electronics, air-atmosphere sintering, conductivity, nanoparticles

1. INTRODUCTION Conductive metal inks are widely used in the production of conductive films on flexible substrates for their application in printed/wearable electronics, whose latest advances have enabled fast, easy, and cost-effective flexible electronic devices.1−3 For printed electronics applications, metal inks should be able to be sintered at low temperature (less than 150 °C) to produce highly conductive films (with a resistance lower than 1 × 10−4 Ω·cm) on flexible substrates; common flexible substrates, such as poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), and paper, do not tolerate long treatments at temperatures more than 150 °C. Silver is the most common conductive ink material because of its high conductivity and excellent oxidation resistance.4−6 Compared to silver, copper is much more cost-effective (about 1% of the price) and has sufficiently high conductivity and less electromigration effects.1,2 However, copper is readily oxidized in air to form an insulating Cu oxide layer; the presence of Cu oxide not only increases the sintering temperature but also reduces the electrical conductivity of the sintered Cu film.1,2 For the past decade, researchers have been trying to achieve low-temperature sintering of Cu-based inks to yield conductive Cu films on flexible substrates, using microsized particles,7−9 nanosized particles,10−14 different precursors (e.g., metal− organic decomposition, MOD),15−18 and combination-type inks of MOD and particles.19−22 © 2017 American Chemical Society

In general, low-temperature thermal sintering of Cu-based inks on flexible substrates is known to produce conducting films under inert gas (e.g., N2), and reducing gas (H2, or vaporized formic acid), since Cu-based inks cannot produce conductive Cu films when sintered in air due to easy oxidation of copper under these conditions. However, the use of a reducing gas is often dangerous, and atmosphere control requires specific devices that increase the complexity of the system. A challenge is therefore the thermal sintering of Cu-based inks in air, which could allow their broader application for printed electronics with no use of special sintering process and instruments. One of the few reports on air atmosphere-thermal sintering of Cubased inks relates to the thermal decomposition of MOD inks. However, a relatively high heating temperature of 300 °C was required to produce highly conductive Cu films (∼10−5 Ω·cm) on glass substrates by strengthening the reduction ability of polyol solvents at high temperature.17 To solve these issues, flash light sintering methods have been developed to sinter Cu nanoparticles under air ambient conditions.23−28 Such lightmediated processes are very promising for air sintering of Cubased inks, but these approaches require expensive and specific equipment. Received: April 2, 2017 Accepted: June 2, 2017 Published: June 2, 2017 20852

DOI: 10.1021/acsami.7b04641 ACS Appl. Mater. Interfaces 2017, 9, 20852−20858

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Electrical resistivity of Cu films on polyimide substrates sintered at 100 °C for 60 min under a N2 flow produced from AmIP−Cu NPbased ink with the addition of 1 wt % of various antioxidant agents. (b) XRD patterns of the sintered Cu films.

antioxidant agents (at a concentration ∼1 wt %), including hydrazine, citric acid, ascorbic acid, formic acid, oxalic acid, malonic acid, and succinic acid. All samples were sintered at 100 °C under a N2 flow. Among the antioxidant agents examined here, the addition of oxalic acid into the AmIP−Cu NP-based ink showed the lowest electrical resistivity of the sintered Cu film (Figure 1a). The formation of copper oxide in the sintered Cu film was inhibited by the presence of oxalic acid, as demonstrated by the XRD pattern shown in Figure 1b. In the absence of oxalic acid, peaks from copper oxide and metallic copper (showing broad peaks due to the small crystallite size) were observed in the XRD pattern. Compared to succinic acid and malonic acid, the addition of oxalic acid was more effective in suppressing copper oxidation (Figure 1b). It is interesting that the oxalic acid, malonic acid, and succinic acid are clearly distinct in the antioxidant effect each other, although these compounds are similar dicarboxylic acid compounds containing two carboxyl groups. The effective suppression of copper oxide formation in the sintered Cu film may be attributed to the large dissociation constants of oxalic acids (i.e., pKa1 = 1.27 and pKa2 = 4.27 for oxalic acid, pKa1 = 2.85 and pKa2 = 5.05 for malonic acid, pKa1 = 4.21 and pKa2 = 5.41 for succinic acid). One possibility is that its carboxylic acids coordinate to the copper surface, resulting in the dissolution of surface copper oxide.31 Thus, we selected oxalic acid as the best antioxidant agent for addition to the AmIP−Cu NP-based ink to prevent the oxidation of the Cu NPs in air atmosphere thermal sintering. We added 1 wt % oxalic acid to the AmIP− Cu NP-based ink, since high contents of over 5 wt % hindered the decomposition of organic substances at temperatures below 150 °C, leaving residual organics that acted as insulators and blocked electron conduction. 2.3. Micro/Nano Cu Composite Ink. The sintered film produced from metal ink containing only Cu NPs had a high tendency to crack,32 resulting in a low conductivity. In this study, we added copper microflakes (∼3 μm, as shown in the SEM image in Figure S2a) to the AmIP−Cu NP-based ink, since a composite ink containing both micro- and nanoparticles has been shown to prevent cracking of sintered Cu films.33 The sub-10 nm Cu NPs can also act as fillers to bind the Cu microflakes at low temperatures, since the sub-10 nm Cu NPs

In this paper, we first demonstrate a simple hot plate heat treatment for producing sintered Cu films of 10−5 Ω·cm order on flexible polymer substrates in ambient air using a low temperature less than 150 °C and very short processing time less than 1 min. A novel Cu-based composite ink of sub-10 nm Cu nanoparticles protected with 1-amino-2-propanol (AmIP) (AmIP−Cu NPs) with submicrometer-sized was utilized; oxalic acid was also added to prevent the oxidation of the Cu during sintering. Moreover, this Cu-based composite ink resulted in films with the lowest reported resistivity at less than 120 °C under N2 flow (5 × 10−5 Ω·cm at 80 °C and 8 × 10−6 Ω·cm at 120 °C). The mechanisms behind the sintering behavior of the Cu-based composite ink are discussed in terms of the mixture of micro/nano Cu particles and the oxalic acid addition.

2. RESULTS AND DISCUSSION 2.1. Sub-10 nm AmIP−Cu Nanoparticle-Based Ink. The heating of Cu nanoparticle-based ink results in evaporation of the solvent and subsequent sintering of the Cu nanoparticles (Cu NPs) to create a continuously connected system and thus percolating paths for electron transfer. The conventional approach to sinter Cu NPs is thermal heating, since metal NPs are characterized by reduced melting temperatures with decreasing particle size (especially, for sub-10 nm particles).29,30 Recently, we reported the synthesis of sub-10 nm AmIP−Cu NPs of 3.5 ± 1.0 nm for low-temperature sintering of Cu NP-based ink at less than 150 °C under N2 flow where the reduction ability of the AmIP ligands was utilized.29 However, the sintering of the AmIP−Cu NP-based ink in air at 150 °C for 1 min produced nonconductive films due to the oxidation of copper (the corresponding XRD spectrum is shown in Figure S1). This indicates that the reduction power of the AmIP ligands alone is insufficient to suppress the oxidation of Cu in the AmIP−Cu NP-based film under air sintering. Hence, we decided to explore other antioxidants to achieve air atmosphere thermal sintering of the AmIP−Cu NP-based ink for production of conductive Cu films. 2.2. Suppression of Cu Oxide Formation by the Addition of Antioxidants. To examine the suppression of Cu oxide formation, we measured the electrical resistivity of Cu films formed from AmIP−Cu NPs with the addition of various 20853

DOI: 10.1021/acsami.7b04641 ACS Appl. Mater. Interfaces 2017, 9, 20852−20858

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Figure 2. TGA-DTA curves under air atmosphere. (a) AmIP−Cu NPs in the presence of 1 wt % oxalic acid and (b) composite Cu ink with a 3:1 wt % mixture of Cu microflakes to AmIP−Cu NPs with 1 wt % oxalic acid.

Figure 3. Analysis of Cu films on polyimide substrates produced from composite Cu ink with a 3:1 wt % mixture of Cu microflakes and AmIP−Cu NPs in the presence of 1 wt % oxalic acid. Electrical resistivity of Cu as a function of heating time at (a) 150 °C and (b) 100 °C. XRD spectra of the air-atmosphere sintered Cu films heated at (c) 150 °C for 20 s and (d) 200 °C for 20 s.

exhibit dramatic melting point reduction. We found that a composite ink with a ratio of 3:1 wt % microflake:sub-10 nm produced Cu films with the lowest resistance when sintered at 100 °C for 60 min under N2 flow (Figure S3). Thus, we investigated this composite Cu ink composition with the addition of 1 wt % oxalic acid (composite micro/nano Cu ink) for the air atmosphere thermal sintering at low temperature. 2.4. Sintering Behavior of the Composite Ink under Air Atmosphere. The sintering behavior of the composite micro/nano Cu ink was characterized using TGA-DTA under an air atmosphere. For comparison, TGA-DTA data for AmIP− Cu NP-based ink (without Cu microflakes) alone were also collected under the same sintering conditions. We observed two exothermic peaks (I and II) in the DTA curve of the AmIP−Cu NP-based ink (without Cu microflakes) (Figure 2a). The first exothermic peak (peak I) at around 100 °C was assigned to the sintering of the AmIP−Cu NPs, which was consistent with the decrease in the electrical resistivity at around 100 °C, as described later. The formation of necks

between particles involves surface diffusion of unstable surface atoms, which was observed as the exothermic peak in the thermal analysis.34 The second sharp exothermic peak (peak II) at around 160 °C was assigned to the oxidation of copper, since the weight increase from the oxidation was observed in the TG curve (Figure 2a). However, in the case of the composite micro/nano Cu ink, peak II was not observed (Figure 2b). A broad exothermic peak (peak II′), which was assigned to the oxidation of copper, was observed at a higher temperature (around 220 °C), and the weight increase due the oxidation was observed at more than 250 °C in the TG curve. This suggests that the mixing of Cu microflake with the AmIP−Cu NPs suppressed the oxidation of copper during sintering under air atmosphere at high temperature. It is not clear on that reason, but one possibility is that the sintered Cu NPs were able to fill the interstitial regions among Cu flakes and also deposited on Cu flakes. The enhanced sintering growth of Cu NPs in the presence of Cu flakes may suppress the oxidation of Cu film. From the TGA-DA data, we confirmed that the 20854

DOI: 10.1021/acsami.7b04641 ACS Appl. Mater. Interfaces 2017, 9, 20852−20858

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ACS Applied Materials & Interfaces thermal sintering of composite micro/nano Cu ink in air should be performed at temperatures less than 150 °C in order to avoid the oxidation of the Cu films. 2.5. Air-Atmosphere Sintering of the Composite Ink at Low Temperature. We measured the electrical resistivity of the Cu films on polyimide (PI) substrates, which were prepared from air atmosphere thermal sintering of the composite micro/nano Cu ink. Higher temperatures and longer sintering times promote the sintering of Cu NPs, but they enhance oxidation under air atmosphere. Under such kinetically competitive circumstances (i.e., thermal sintering vs copper oxidation), we must optimize the sintering temperatures and the heating times to produce highly conductive Cu films under air atmosphere. Figure 3a shows the electrical resistivity of the Cu films on PI substrates as a function of heating times between 10 s and 10 min at 150 °C. The lowest resistance Cu film was obtained for a short heating time of 20 s ((2.6 ± 0.5) × 10−4 Ω·cm). The XRD spectrum of this sample showed metallic copper without any diffraction peaks of Cu oxide (Figure 3c). However, increasing the heating time to 10 min resulted in a large increase in the resistance ((97 ± 45) × 10−4 Ω·cm) because of the oxidation of the Cu films, as observed by the XRD spectrum (Figure S4). In the case of the lower sintering temperature of 100 °C, a longer heating time of 10 min was needed to obtain the lowest resistance Cu film ((3.3 ± 2.5) × 10−3 Ω·cm) (Figure 3b). This was because the lower sintering temperatures slowed the sintering kinetics. Owing to the rapid oxidation of the Cu film sintered at 200 °C under air atmosphere (Figure 3d), a high resistance Cu film was obtained using a heating time of 20 s ((32 ± 18) × 10−4 Ω·cm). From the above results, the lowest resistance ((2.6 ± 0.5) × 10−4 Ω·cm) of the Cu films on PI substrates sintered under air atmosphere was achieved at 150 °C using a short heating time of 20 s. A similar resistance of (1.3 ± 0.2) × 10−4 Ω·cm was obtained for a Cu film on a PET substrate. Figure 4 shows SEM images of the Cu film produced from the composite micro/nano Cu ink at 150 °C for 20 s under an air atmosphere. The sintered AmIP−Cu NPs were randomly distributed with the Cu microflakes (Figure 4a), where only AmIP−Cu NPs are sintered, and the organic AmIP ligands should have decomposed. The Cu flakes were well connected via the sintered AmIP−Cu NPs, which ensured good electrical conductivity of the Cu film. However, we observed some cracking of the Cu film in higher magnification SEM images (as indicated by the arrows in Figure 4b). The packing density of the Cu flakes (several separate flakes are circled in Figure 4b) is relatively low. The cracking of the Cu film resulted in relatively high film resistance. In order to improve the electrical resistivity of sintered Cu films by increasing the packing density of Cu particles and decreasing the cracking, we used smaller Cu particles (∼0.3 μm; as shown in the SEM image in Figure S2b) in the composite inks of a 3:1 ratio (submicro:nano; wt %) with the addition of 1 wt % oxalic acid (composite submicro/nano Cu ink). We measured the electrical resistivity of Cu films on PI substrate prepared from the composite submicro/nano Cu ink at 150 °C under air atmosphere at heating times of 10 s−10 min, as shown in Figure S5. The lowest resistivity Cu film ((5.5 ± 1.7) × 10−5 Ω·cm) was obtained at a very short heating time of 10 s, which is 5 times lower resistivity (∼2.6 × 10−4 Ω·cm) of Cu film from the Cu microflakes. The XRD spectrum of the Cu film sintered in air

Figure 4. (a, b) SEM image of a Cu film formed from composite Cu ink with a 3:1 wt % mixture of Cu microflakes and AmIP−Cu NPs in the presence of 1 wt % oxalic acid by heating in an air atmosphere at 150 °C for 20 s. (c, d) SEM image of a Cu film formed from composite Cu ink with a 3:1 wt % mixture of submicrometer Cu particles and AmIP−Cu NPs in the presence of 1 wt % oxalic acid by heating in an air atmosphere at 150 °C for 20 s.

from the composite submicro/nano Cu ink showed the presence of metallic copper (Figure S6). Figures 4c and 4d show the SEM images of Cu films made from the composite submicro/nano Cu ink. Compared to the use of large Cu flakes, the packing density of Cu particles was improved, and less cracking was observed. The sintered Cu films from AmIP−Cu NPs filled the spaces between the Cu particles. Hence, the sintering of sub-10 nm Cu NPs connected the Cu particles, improving the electrical resistivity compared to films made from the Cu microflakes. To examine the distribution of submicrometer Cu particles or Cu flakes in the sintered Cu film, we analyzed SEM backscattered reflection images of a cross section of the sintered Cu films on PI substrates, as shown in Figure 5. Owing to the high crystallinity

Figure 5. SEM reflection images of a cross section of the sintered Cu films on polyimide substrates prepared (a) at 150 °C for 10 s under air atmosphere from composite Cu ink of the 3:1 mixture (wt %) of submicrometer Cu particles and AmIP−Cu NPs in the presence of 1 wt % oxalic acid and (b) at 150 °C for 20 s under air atmosphere from composite Cu ink of the 3:1 mixture (wt %) of Cu flakes and AmIP− Cu NPs in the presence of 1 wt % oxalic acid.

of the Cu particles or flakes, the SEM reflection images clearly show their distribution in the sintered Cu films. The improved packing density and reduced cracking are clearly seen in the sintered Cu film prepared from the composite submicron/nano Cu ink (Figure 5a) compared to that from the composite micro/nano Cu ink (Figure 5b). 20855

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ACS Applied Materials & Interfaces From the above result, we have demonstrated a simple hot plate heat treatment for the air atmosphere-sintering of highly conductive Cu films of 10−5 Ω·cm order on flexible polymer substrates in ambient air at a low temperature of 150 °C and for a very short processing time less than 1 min, as seen in Video S1 of the Supporting Information. 2.6. Sintering Mechanism. The possible sintering mechanism for the composite Cu-based inks is illustrated in Figure 6. The AmIP−Cu NPs were originally covered with an

Figure 7. (a) Electrical resistivity of Cu films from the composite ink with the 3:1 mixture (wt %) of submicrometer Cu particles and AmIP−Cu NPs in the presence of 1 wt % oxalic acid sintered at various temperatures for 60 min under N2 flow. (b, c) SEM images of the Cu film prepared at a sintering temperature of 80 °C.

The innovative behavior of such Cu composite inks developed in this study could be achieved not only by conventional thermal heating but also by other heating methods, such as electrical, microwave, plasma, laser, and flash lamp heating processes.38 This fast, efficient, and inexpensive thermal sintering technology under an air atmosphere using composite inks is an important processing development for achieving commercially viable printed electronics on flexible substrates. Possible applications include the fabrication of 3D interconnects, low-temperature joints, and RFID (radio-frequency identification) in air, in addition to flexible electronic devices for the future “internet of things” (IoT) society.39

Figure 6. Schematic diagram showing the sintering mechanism of the composite inks.

oxide shell,29 but we assumed that the surface copper oxide layer of Cu NPs was partially reduced to metallic copper by the combined reducing action of the AmIP ligands and the oxalic acid during sintering. The protective AmIP layer on the sub-10 nm Cu NPs is decomposed at elevated temperature of 150 °C, allowing the Cu NPs to coalesce; the sintered Cu NPs were able to fill the interstitial regions between Cu particles. In this way, the sub-10 nm Cu NPs can behave as a binder to form a unified film of Cu particles at low temperatures, thus reducing the electrical resistivity. Assuming that is the case of nano (∼3 nm)/submicro (∼300 nm) in the composite ink, we roughly estimate the number ratio of submicro vs nano NP = 1:106, using the size ratio of 1:100 and the mixing ratio of 1:3 (wt %). During the thermal heating process, the higher evaporation rate of propylene glycol compared to glycerol in the ink may generate capillary forces via meniscus formation from residual glycerol (with a high surface tension) between the Cu particles, resulting in densification of the Cu particles.35,36 Such a sintering mechanism should be further investigated by experimental studies in the future. 2.7. Low-Temperature Sintering of the Composite Ink under N2 Flow. We conducted low-temperature sintering (below 120 °C) of the composite submicro/nano Cu ink under N2 flow. The resistivity values of the Cu films at various sintering temperatures are shown in Figure 7a. Even at a very low temperature of 80 °C, we could obtain films with a low resistivity of the order of 5.3 × 10−5 Ω·cm. At 120 °C, the sintered Cu film showed a very low resistivity of 8.4 × 10−6 Ω· cm. To the best of our knowledge, this is the lowest reported resistivity (nearly 10−6 Ω·cm order) for such a Cu film prepared at 80 °C on a flexible substrate. SEM images of the sintered Cu films showed sintering of the AmIP−Cu NPs occurring, even at the very low temperature of 80 °C, where the AmIP−Cu NPs behaved as fillers to bind the Cu particles (Figure 7b,c). Conductive Cu films sintered at temperatures of 80 °C are enormously appealing for printed electronic applications on flexible plastic substrates, since glass transition temperatures (a criterion of heat resistance) of commodity plastics are less than 100 °C.37

3. CONCLUSION Composite Cu-based ink consisting of submicro/nano-Cu particles with oxalic acid as an antioxidant allowed sintering of the ink in air at temperatures less than 150 °C with a very short heating time (less than 1 min). Conductive Cu films with a resistance of 5.5 × 10−5 Ω·cm were fabricated at 150 °C under air thermal sintering. The total process for applying the Cu-based ink on plastic substrates (i.e., casting, drying, and sintering) takes less than 1 min. The mechanism behind the air sinterability of the Cu-based composite ink was explained in terms of two effects: (i) the ability of the oxalic acid to suppress the oxidation of copper and (ii) the mixing of the submicroand nanosized Cu particles which inhibited cracking of the Cu film and increased the packing density of Cu particles during the low-temperature sintering process. Another important finding was that the composite ink produced very low-resistivity (nearly down to 10−6 Ω·cm order) sintered Cu film at less than 100 °C under a N2 flow. 4. EXPERIMENTAL SECTION 4.1. Chemicals. All the chemicals were used as received without further purification. 1-Amino-2-propanol (AmIP, 98%), copper(II) acetate anhydrate (97.0%), propylene glycol (99.0%), ethylene glycol (99.5%), glycerol (99.0%), ethanol (99.5%), hydrazine monohydrate (98.0%), N,N-dimethylacetamide (98.0%), toluene (98.0%), and hexane (96.0%) were purchased from Wako Chemicals, Japan. The antioxidant materials, including oxalic acid (98%), malonic acid (98%), citric acid (98%), succinic acid (99.5%), formic acid (99%), and ascorbic acid (99%), were also purchased from Wako Chemicals, Japan. Micrometer-sized Cu flakes (Product No. Cu MA-C025-KP) were purchased from Mitsui Mining & Smelting Co., Ltd., Japan. The 20856

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size and the thickness of micrometer-sized Cu flakes are around 3 and 0.5 μm, respectively. Submicrometer Cu particles with average size of 0.3 μm (Product No. 0811DX, Cu nanopowder/nanoparticles, 99.5%, 300 nm) were purchased from SkySpring Nanomaterials, U.S.A. 4.2. Preparation of Inks. The solution synthesis and purification of AmIP−Cu NPs with an average size of 3.5 ± 1.0 nm were carried out according to partly modified methods described in our previous report.28 Typically, AmIP of 11.6 mL was added to ethylene glycol of 30 mL. Solid copper(II) acetate anhydrate of 2.73 g was added into the AmIP solution under an ultrasonic agitation (US-350S, D-SONIC), producing a blue solution due to the formation of an AmIP−Cu complex. This solution had a concentration of 300 mM Cu salt. Then, hydrazine monohydrate of 7.3 mL was at once added, under stirring at 1000 rpm and at room temperature (∼23 °C), to the blue solution. The color of the resulting solution rapidly changed from blue to a blackish deep red. It was then stirred at 1000 rpm at room temperature under an air atmosphere for about 24 h. After the above reaction, the AmIP−Cu NPs were precipitated by adding as prepared dispersion of Cu NPs (12.5 mL) into N,N-dimethylacetamide solution of 25 mL, and the resultant solution becomes turbid under gentle stirring at air. The precipitate was collected by centrifugation at 6000 rpm for 3 min, and the supernatant solution was removed. The precipitate was washed with N,N-dimethylacetamide of 7 mL again, then the precipitate was collected by centrifugation at 6000 rpm for 3 min, and the supernatant solution was removed. Next, the precipitate was washed with toluene (25 mL) and then hexane (25 mL). After the washing process, at once, the precipitate of the AmIP−Cu NPs was uniformly redispersed in a solvent mixture (800 μL) of propylene glycol and glycerol (3:1 vol %) by ball mill crushing with stainless-steel spheres (diameter of 5 mm) for 60 min. When hexane is separated as the supernatant in this process, we removed it. In addition, 1 wt % of the antioxidant material (oxalic acid, malonic acid, succinic acid, citric acid, formic acid, ascorbic acid, or hydrazine) was mixed with the Cu nanoink using a mixer (ARE 3 10, Thinky, Japan) for 10 min to produce the Cu nanoink (∼40 wt % Cu). The resulting AmIP−Cu NP-based nanoink was kept in a freezer before use, and it remained stable for at least two months; no change was observed in the resistivity of the Cu conductive film prepared from the Cu nanoink two months after it was produced. To produce the composite ink, the powder of micrometer-sized Cu flakes was mixed with the AmIP−Cu NP-based ink prepared in the above procedure using the mixer (ARE 3 10, Thinky, Japan) for 10 min. The resulting Cu nanoink was kept in a freezer before use. 4.3. Thermal Sintering Process. The doctor-blade method was used to coat the Cu-based inks onto polymer substrates. The blade was fixed at a height of 12 μm above the substrate surface, and the speed of the blade movement was about 10 mm/s. The thickness of each Cu film after heating was determined by a surface roughness measuring tool (SJ 310, Mitutoyo, Japan). The film thickness and the maximum height roughness (Rz) from composite submicro/nano Cu ink were ∼4 μm and ∼0.6 μm, respectively. Those from the composite micro/ nano Cu ink were ∼5 μm and ∼1.5 μm, respectively. The electrical resistivity of the Cu conductive films was analyzed using a four-point probe (Loresta AX MCP-T370, Mitsubishi Chemical Analytech Co., Japan). In the case of sintering under a N2 atmosphere, the coated Cu inks were then sintered at various temperatures for 60 min in an electric furnace (FT-6000, FuLL-TECH, Osaka, Japan) under a N2 gas flow of 1.1 L/min. In the case of air sintering, the coated Cu inks were sintered at various temperatures for different times (all less than 10 min) on a hot plate (C-MAG HP 4 Ikatherm, IKA JAPAN) in ambient air. 4.4. Characterization. Field-emission scanning electron microscope (FE-SEM; JSM-6700, JEOL, Japan) images were collected at an acceleration voltage of 5.0 kV. X-ray diffraction (XRD) patterns were obtained using a diffractometer (D2 Phaser, Bruker, Germany) with a Cu Kα radiation source (λ = 1.5406 Å). Thermogravimetric analysis (TGA) was performed using a TG analysis system (Thermo Plus EVO, Rigaku, Japan) at a heating rate of 10 °C/min under an air atomosphere.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04641. XRD pattern of a Cu film produced from the AmIP−Cu NP-based ink and sintered in air; electric resistivity of Cu films from composite Cu-based ink with various mixing ratios of micro- and nanosized particles; SEM images of Cu flakes and Cu particles; XRD patterns of Cu films produced from the composite Cu-based ink (PDF) Movie of sintering process (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.K.). ORCID

Hideya Kawasaki: 0000-0003-2713-2057 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grants 15H03520 and 15H03526) and also by the “Strategic Project to Support the Formation of Research Bases at Private Universities” with a matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology (MEXT, Grant S1311041), Japan.

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DOI: 10.1021/acsami.7b04641 ACS Appl. Mater. Interfaces 2017, 9, 20852−20858