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Copper Nanoparticle/Multiwalled Carbon Nanotube Composite Films with High Electrical Conductivity and Fatigue Resistance Fabricated via Flash Light Sintering Hyun-Jun Hwang,† Sung-Jun Joo,† and Hak-Sung Kim*,†,‡ †

Department of Mechanical Engineering, Hanyang University, Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea Institute of Nano Science and Technology, Hanyang University, Seoul, 133-791, Korea

‡

S Supporting Information *

ABSTRACT: In this work, multiwalled carbon nanotubes (MWNTs) were employed to improve the conductivity and fatigue resistance of flash light sintered copper nanoparticle (NP) ink films. The effect of CNT weight fraction on the flash light sintering and the fatigue characteristics of Cu NP/CNT composite films were investigated. The effect of carbon nanotube length was also studied with regard to enhancing the conductivity and fatigue resistance of flash light sintered Cu NP/CNT composite films. The flash light irradiation energy was optimized to obtain high conductivity Cu NP/CNT composite films. Cu NP/CNT composite films fabricated via optimized flash light irradiation had the lowest resistivity (7.86 μΩ·cm), which was only 4.6 times higher than that of bulk Cu films (1.68 μΩ·cm). It was also demonstrated that Cu NP/CNT composite films had better durability and environmental stability than those of Cu NPs only. KEYWORDS: printed electronics, Cu nanoparticle, multiwalled carbon nanotube, flash light, sintering

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INTRODUCTION Printed electronic techniques (e.g., gravure printing and inkjet printing) are a low-cost solution to the production of electronic devices, such as flexible displays, radio frequency identification (RFID) tags,1,2 and wearable electronics,3,4 since these techniques can replace the expensive and time-consuming photolithography technique.5 Conventionally, metal nanoparticle-based conductive inks have been generally used because of their low melting point and unique properties.5−7 Recently, copper nanoparticle inks have received the increased attention as a low-cost alternative to gold or silver nanoparticle inks for printed electronics.8 However, most copper nanoparticles can be easily oxidized, so that they cannot be sintered via thermal sintering method under ambient conditions. For these reasons, several approaches, such as the plasma9 and laser10 processes, have been developed to sinter copper nanoparticles without the oxidation. However, these approaches have constraints in production on a large scale, because of their low throughput, high complexity, and considerable environmental obstacles (e.g., vacuum conditions and chamber). Furthermore, a laser sintering process can cover only a small sintering area and requires a sophisticated 3D-gantry system to cover a large area. Therefore, a low temperature and rapid large area sintering technique is needed to realize mass production and flexible electronic devices on polymer or paper substrates. To overcome these limitations, we previously developed a flash light sintering method combined with PVP functionalized copper nanoparticle inks (Cu NP-inks).11−16 The flash light © XXXX American Chemical Society

sintering method can instantly reduce the copper oxide shell and sinter copper nanoparticles at room temperature while under ambient conditions in just a few milliseconds without damaging the substrate. Moreover, a large area of Cu NP-ink film can be sintered by flash light from a xenon lamp. Recently, with the boom of flexible electronics, mechanical flexibility has become another important metric for conductive film performance. Unfortunately, however, the sintered metallic NP films show fracture strain (εf) even with small elongations, causing a brittle failure derived from highly concentrated stress at the neck-like junction.17 Thus, it is difficult to employ the sintered metallic NP films in the applications of flexible electronics where they are subjected to repetitively bending conditions. Several approaches have been performed to enhance the fatigue resistance of the sintered metallic NPs thin films using various materials, such as carbon nanotubes (CNTs) and nanowires (NWs).18−20 CNTs are very promising materials for flexible electronics because of their unique properties, such as high intrinsic conductivity, solution processability, flexibility, and their potential for low-cost production.21−24 Metallic NWs are also one of the stronger candidates for fatigue resistance improvement, because of their intrinsic high conductivity and favorable reliability.25 However, the ease of oxidation under Received: August 31, 2015 Accepted: October 27, 2015

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DOI: 10.1021/acsami.5b08112 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematics of the flash light sintering system (a), and the shape change comparison of Cu NPs film only (b) and Cu NP/CNT composite film (c) after flash light irradiation. irradiation at room temperature under ambient conditions (Figure 1). The flash light sintering system was introduced in our previous work.13 For the optimization of the flash light sintering process, the irradiation conditions, such as irradiation energy density, pulse number, pulse duration, and pulse interval, was varied to minimize the resistivity of patterns. In our previous studies,13,14 it was found that sintering of Cu nanoparticles occurred when the irradiation energy density was higher than 7.5 J/cm2. It was also confirmed that single pulse produced better conductivity than multiple pulses, because the intensity of pulse decreased as the pulse number increasing to maintain the same total energy. Meanwhile, it was demonstrated that a single pulse with pulse duration of 10 ms could induce lower resistivity than any other conditions, without the reoxidation of Cu nanoparticles.13 For these reasons, to optimize the sintering of the Cu NP/CNT composite films, we varied the irradiation energy of the flash light from 7.5 to 17.5 J/ cm2; single pulse of white light with duration of 10 ms was used in this study. Meanwhile, the pulse voltage applied to xenon flash lamp was controlled to control the energy density. Subsequently, the energy density of the flash light was measured by a power meter (Nove II, People Laser Tech). Mechanical Fatigue Resistance Test. The mechanical fatigue characteristics of the sintered Cu NP/CNT composite films were investigated using a repeated tensile loading test (outer bending). The bending fatigue tester consisted of a fixed clamp to fasten the printed pattern on the flexible substrate and a motor-driven axial displacement to bend the Cu NP/CNT composite film. Fatigue tests were conducted at a frequency of 1 Hz, and the resistance changes were measured every 100 cycles until 1000 cycles (Figure 2). To generate three different bending radii (r = 7, 10, and 15 mm), the moving distances (dL) were controlled with the equation30,31

ambient conditions, poor adhesion to substrates, and selfaggregation of metallic (Ag and Cu) NWs has made it difficult to fabricate uniform NWs films on a large scale. For these reasons, in this study, multiwalled CNTs (MWNTs) were employed to improve the conductivity and fatigue resistance of the flash light sintered copper nanoink films (Figure 1). The effect of the weight fraction of the CNTs on the flash light sintering and the fatigue characteristics of the Cu NP/CNT composite films were investigated. The effect of the length of the carbon nanotubes on the electrical and mechanical performance was also studied. To increase conductivity and fatigue resistance, the effect of the flash light irradiation energy on the sintering of Cu NP/MWNT composite films was studied and is discussed, using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

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MATERIALS AND METHODS

Fabrication of Cu NP/CNT Composite Films. For the fabrication of Cu NP/CNT composite films, commercially available Cu nanoparticles (NPs) with oxide shells (30−50 nm in diameter, oxide thickness >2 nm; QSI Nano) and multiwalled CNTs (10−15 nm inner diameter, > 95% purity; Hanwha Nanotech) were used in this study. For better dispersion, CNTs were treated with nitric acid (16 M) for 3 h.26−29 Cu NP inks were prepared as in our previous work.13 Consecutively, CNTs were dispersed in the prepared Cu NPs ink using a three roll mill. To improve the electrical conductivity and the fatigue resistance, the weight fraction and length of CNTs were varied from 0.0 to 1.0 wt % and 20 to 200 μm, respectively. The Cu NP/CNT inks (solid contents: 60−65%) with a high viscosity (800−1000 centistokes) were printed on polyimide (PI) substrates using the doctor blade method. Flash Light Sintering of Cu NP/CNT Composite Films. The printed Cu NP/CNT composite films were sintered by flash light

r = L · [(dL· L−1) − {πhs 2 · (12L)−2 }]−0.5 B

(1)



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RESULTS AND DISCUSSION Effect of CNTs Weight Fraction. To sinter the Cu NP/ CNT composite films (length of CNTs = 20 μm), we varied

Figure 3. Resistivity of the flash light sintered Cu NP/CNT composite films with weight fraction from 0 to 1 wt %, as increasing the irradiation energy from 7.5 to 15 J/cm2. (The points inside the dashed box indicate damaged samples as shown in the inset image).

the irradiation energy of the flash light from 7.5 to 15 J/cm2; one pulse of white light with a duration of 10 ms was used. Figure 3 shows that the resistivity of the flash light sintered Cu NP/CNT films decreased as the energy density increased up to 12.5 J/cm2. In the unsintered Cu NP-ink film, the Cu NPs are surrounded by PVP binder, which leads to a blurry SEM image (Figure 4a). However, with the irradiation of flash light (irradiation energy = 12.5 J/cm2, pulse number = 1, pulse duration = 10 ms), the PVP binder was decomposed and evaporated allowing Cu NPs to be observed more clearly (Figure 4b). Simultaneously, the Cu NPs were sintered by flash light. The average grain size of Cu nanoparticles increased and the neck-like junctions among Cu NPs grew larger with flash light irradiation (Figure 4b), resulting in a decrease in the resistivity. These results are consistent with those reported in our previous studies.13−15 With light energy higher than 12.5 J/ cm2, the resistivity of the flash light sintered Cu NP/CNT films increased again except in the case of 1.0 wt % CNT. This is because the patterns were damaged due to excessive flash light irradiation energy (see inset in Figure 3). Meanwhile, it was also found that the resistivity of the sintered Cu NP/CNT composite films decreased as the weight fraction of the CNTs increased up to 0.5 wt %. This is because the electron mobility in the fabricated patterns increased as the CNT weight fraction increased, due to the novel well-defined tubular structure of the CNTs. CNTs have been used to increase electron mobility because of their molecular structure.32,33 Electrical transport inside the CNTs is affected by scattering resulted from defects and lattice vibrations, which result in resistance, similar to that in bulk materials. However, the 1D nature of the CNTs and their strong covalent bonding drastically affects these processes. Scattering by small angles is not allowed in a 1D material, only a forward and backward motion of the carriers. For this reason, CNTs have high electron mobility, allowing the Cu NP/CNT composite film to have a lower resistivity than that of the Cu NPs film only. Meanwhile, the electrical resistivity of a bulk powder is generally higher than that of the individual particles, since the

Figure 2. Mechanical fatigue resistance test. (a) Schematics of the bending fatigue test, (b) bending radius calculation, and (c) the relationship between the moving distance and bending radius (L = 4 cm, hs = 225 μm).

where L is the length of substrate (4 cm) and hs is the substrate thickness (225 μm). The relationship between the moving distance and the bending radius used in this study was shown in Figure 2c. Environmental Stability Test. The environmental stability tests were conducted under an ambient environment for one month and under a humid environment for 2 weeks. Room temperature (25 °C) and the relative humidity of 30% were used for an ambient environment, and the temperature of 85 °C and the relative humidity of 85% were used for a humid environment. To protect the samples from the external environment, the temperature and humidity were controlled and maintained in a chamber. Changes in the sheet resistance were measured and recorded. Characterization. The sheet resistances of the sintered Cu NP/ CNT composite films were measured by four-point probe station (Modusystems, Inc.). The line cross-sectional profiles of the Cu NP/ CNT composite films were measured using an alpha step (KLA Tencor AS500, Tencor instruments). The microstructure and surface of Cu NP/CNT composite films were examined by scanning electron microscopy (SEM, S4800 Hitachi). To confirm the reduction or oxidation of Cu nanoparticles with oxide shell, analysis of the crystal phase was performed by X-ray diffraction (XRD, D/MAX RINT 2000, CuK radiation). C


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Figure 4. SEM images (a−e) and XRD patterns (f) of the Cu NP/CNT composite films (CNT length = 20 μm) before and after sintering by flash light (irradiation energy = 12.5 J/cm2, pulse duration = 10 ms, pulse number = 1); (a) CNT 0.0 wt % (before sintering), (b) CNT 0.0 wt % (after sintering), (c) CNT 0.1 wt % (after sintering), (d) CNT 0.5 wt % (after sintering), and (e) CNT 1.0 wt % (after sintering).

Figure 5. Line cross-sectional profiles of Cu NP/CNT composite films before and after sintering by flash light (irradiation energy = 12.5J/cm2, pulse duration = 10 ms, pulse number = 1). (a) Cu NP films only, (b) Cu NP/CNT 0.1 wt % films, (c) Cu NP/CNT 0.5 wt % films, and (d) Cu NP/ CNT 1.0 wt % films.

D


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Figure 6. Outer bending fatigue test results of Cu NP/CNT composite films about bending radius of (a) r = 7 mm, (b) r = 10 mm, and (c) r = 15 mm. The SEM images of the fatigue-tested Cu NP/CNT composite films after 1000 cycles of bending fatigue test results of (d) 0.0, (e) 0.1, (f) 0.5, and (g) 1.0 wt % CNTs.

Cu2O) phase peaks decreased after flash light irradiation. This means that the grain size of the copper was reduced and the copper nanoparticles were not sintered well, compared to those of consisting of Cu NPs only. This result corresponded to that obtained from SEM images (Figure 4c, d). Therefore, it could be concluded that CNTs disturb the sintering of Cu nanoparticles, and simultaneously compensate for the poor conductivity of the small-grain-sized Cu NP/CNT composite films with their high electron mobility and by connecting Cu NPs, as mentioned above. Accordingly, it was observed that when the weight fraction of CNT was higher than 1.0 wt %, the resistivity increased again because of the excessive amount of CNTs in the Cu NP/CNT composite film, as shown in Figure 3. From these results, a weight fraction of 0.5 wt % for CNTs in the Cu NP/CNT composite films was used in this study. Meanwhile, to investigate the shape change of the Cu NP/ CNT composite films after flash light irradiation, the line crosssectional profiles of the patterns were measured from a to a′

interface between the particles offers extra resistance to charge transport. Even after sintering, the pores in the sintered NPs film induced an increase of resistance as shown in Figure 4b. Thus, the application of CNTs can increase the conductivity basically by connecting the copper nanoparticles and acting as additional electron transport channels. For these reasons, the resistivity could be decreased using the Cu NP/CNT nanocomposite film, as shown in Figure 3. Figures 4c−e show that CNTs with a length of 20 μm were well dispersed in the nanocomposite films. However, it was observed that the neck-like junctions among Cu NPs decreased as the proportion of CNTs increasing, as shown in Figures 4c− e. This may be because CNTs disturbed the sintering of copper nanoparticles during flash light irradiation. XRD patterns shown in Figure 4f demonstrated that pure copper phase peaks (43.2°, 50.4°, and 74.1°) decreased as the weight fraction of CNT increased, although the intensity of the pure copper phase peaks increased and the Cu oxide (copper(I) oxide, E


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surface roughness increased after flash light irradiation (see a red line in Figure 5a). This was because the pattern was damaged after the flash light sintering process (see an inset figure in Figure 5a) because oif the high intensity of the flash light pulse. However, it was noteworthy that the Cu NP/CNT nanocomposite patterns could withstand flash light irradiation and maintain their shapes even after flash light irradiation (Figure 5b−d). It seemed that CNTs enhanced the durability of the Cu NP/CNT nanocomposite films when exposed to flash light irradiation because of their outstanding mechanical strength and stiffness.34 This superior durability will be a considerable advantage in application of the flash light sintering technique. To investigate the fatigue resistance of the flash light sintered Cu NP/CNT composite films, outer bending tests were conducted with the bending radii of 7, 10, and 15 mm (Figure 6a−c). The resistance change (ΔR·R0−1) was determined by Figure 7. Resistivity of the flash light sintered Cu NP/CNT composite films with CNT length from 20 to 200 μm (CNT = 0.5 wt %), as increasing the irradiation energy from 7.5 to 17.5 J/cm2.

ΔR ·R 0−1 = (R − R 0) ·R 0−1

(2)

where R0 is the initial resistance, and R is the resistance measured after the bending test. The resistance was measured every 100 cycles. The results of ΔR·R0−1 in the bending tests (bending radius: 7 mm) for the Cu NP/CNT composites and Cu NPs only films were shown in Figure 6a. The ΔR·R0−1 value increased with increasing of the cycle number because of the

(see inset figures in Figure 5). It was observed that the unsintered Cu pattern has thickness of 25 μm with a smooth surface (see a black dotted line in Figure 5a). However, the average height of the Cu pattern decreased to 20 μm and the

Figure 8. Line cross-sectional profiles of Cu NP/CNT composite films before and after sintering by flash light (irradiation energy = 12.5 J/cm2, pulse duration = 10 ms, pulse number = 1). (a) Cu NP films only, (b) Cu NP/CNT 20 μm, (c) Cu NP/CNT 80 μm, and (d) Cu NP/CNT 200 μm films (CNT = 0.5 wt %). F


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Figure 9. Length effect of CNTs on the flash light sintering of Cu NP/CNT composite films. The SEM images (a−d) and XRD patterns (e) of the Cu NP/CNT composite films (CNT length = 20−200 μm, 0.5 wt %) when fully sintered by flash light. Schematics of the flash light sintered Cu NP/ CNT composite films according to the length of CNTs. (f) 20 μm CNT-based composite film with relatively short electron pathway. (g) 200 μm CNT-based composite film with longer electron pathway.

crack initiation and propagation, which occurred in the film owing to the repeated tensile loading. Meanwhile, it was observed that the ΔR·R0−1 values decreased as the weight fraction of the CNTs increased. The ΔR·R0−1 value of the 0.5 wt % CNTs case was 7.67 after 1000 cycles, which is 84% of the 0 wt % CNT case (9.65). The ΔR·R0−1 values of the 0.1 and 1 wt % CNT cases were 94% and 73% of the Cu NPs case, respectively. For an in-depth study on this phenomenon, SEM analysis was carried out after the bending test of 1000 cycles (Figure 6d−g). In the case of CNT 0 wt %, a large crack derived from the crack propagation was observed in the film (Figure 6d). However, after 1000 cycles of the outer bending test, Cu NP/CNT composite films had smaller cracks than those consisting of Cu NP films only. As the weight fraction of the CNTs increasing, this phenomenon was more clearly observed over the entire Cu NP/CNT composite films (Figure 6d−g). Figure 6g shows that the crack size in the Cu films decreased, since the CNTs prevented neck-like junctions from being broken and retarded crack propagation even after 1000 cycles of the outer bending test. Also, CNTs connected the cracked Cu NPs and acted as a channel for electrons.35−37 Thus, the Cu NP/CNT composite films could have better fatigue resistance than films consisting of Cu NPs only. Meanwhile, in the cases of 10 and 15 mm bending radii, the ΔR·R0−1 values after 1000 cycles of outer bending decreased with increasing the weight fraction of the CNTs. These results corresponded to the result of the 7 mm bending radius case, verifying the role of CNTs in retarding the crack propagation among Cu NPs. It was also demonstrated that CNTs enhance

the fracture strength of the sintered Cu films by distributing the stress concentrated at neck-like junction in the porous structure. Thus, it was concluded that randomly dispersed CNTs, buried among copper nanoparticles, can improve the fatigue resistance of Cu NP/CNT composite films under tensile loading. Consequently, Cu NP/CNT composite films with 0.5 wt % CNT had outstanding stretchability and flexibility, while sustaining the electrical conductivity despite of even the most highly porous structure. Effect of Length of the CNTs. To investigate the effect of CNT length in the Cu NP/CNT composite films, the length of the CNTs was varied from 20 to 200 μm, using the optimized weight fraction (0.5 wt %). Even when 15 J/cm2 flash light was used for irradiation, the Cu NP/CNT composite films with CNTs of 80 and 200 μm were not damaged, while those of the 20-μm CNT case were damaged and the resistivity was increased (Figure 7). As shown in Figure 8, it was demonstrated that longer CNT-based composite patterns could maintain their cross-sectional shape even after flash light irradiation. It seemed that longer CNTs could sustain Cu NPs in their original shape more effectively, acting as a net skeleton among the Cu NPs during flash light irradiation (see Figure 1c). Meanwhile, it was also revealed that Cu NP/CNT composite films with longer CNTs shows lower resistivity than the case of relatively short CNTs when fully sintered (Figures 7). As shown in the SEM images (Figures 9b−d) of flash light sintered Cu NP/CNT composite films, it was observed that the neckG


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Figure 10. Outer bending fatigue test results of Cu NP/CNT composite films about bending radius of (a) r = 7 mm, (b) r = 10 mm, and (c) r = 15 mm. The SEM images of the fatigue-tested Cu NP/CNT composite films after 1000 cycles of bending fatigue test results of (d) 0, (e) 20, (f) 80, and (g) 200 μm of CNTs.

resistivity of the films could be decreased by applying longer CNTs to Cu NP/CNT composites (Figure 7). As shown in Figure 9d, the optimized conductive Cu NP/ CNT composite films (CNT length = 200 μm, 0.5 wt %) had a smooth surface similar to that of the bulk phase, and pores were hardly observed. Finally, highly conductive Cu NP/CNT films with a resistivity of 7.86 μΩ·cm were successfully achieved by using flash light sintering; this resistivity is only 4.6-fold higher than that of the bulk copper (1.68 μΩ·cm). Figure 10 shows the results of outer bending fatigue test for the Cu NP/CNT composite films with different CNT lengths. The ΔR·R0−1 values after 1000 cycles of outer bending decreased as the length of the CNTs increasing, for all cases of 7-, 10-, and 15 mm bending radii. This phenomenon is mainly associated with the grain size of the sintered Cu NP/ CNT films, since the ductility of the Cu material is proportional to the grain size of the films. Note that the nanocrystalline copper with a grain size under 25 nm has a ductility of lower than 2%, while the bulk copper has much higher ductility, up to

like junctions among Cu NPs grew larger and the pores size decreased when increasing the CNT length. Also, the XRD patterns shown in Figure 9e demonstrated that copper oxide peaks decreased and the pure copper phase peaks increased as the length of the CNTs increased. These findings are consistent with the SEM images. It was previously demonstrated that a number of CNTs disturb the sintering of copper nanoparticles, as mentioned above. When weight fraction of the CNTs is maintained, composite films with longer CNTs will have a smaller number of CNTs. Smaller numbers of longer CNTs may prevent interruption of the sintering of copper nanoparticles as much as large numbers of the shorter CNTs (Figure 9f, g). Accordingly, the composite films with CNTs longer than 80 μm have a large grain boundary of copper nanoparticles (Figures 9c, d), similar to that of the only Cu NP case (Figures 9a). Furthermore, CNTs with a length of 200 μm could provide inherently longer conductive pathways than those of the 20 μm CNTs, as shown in Figure 9f, g. For these reasons, the H


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Figure 11. Environment stability of Cu NP film only and Cu NP/CNT composite film. (a) The environment stability test under ambient condition (25 °C and RH 30%) for a month. (b) The environment stability test under humid condition (85 °C and RH 85%) for 2 weeks. XRD patterns of Cu NP film only and Cu NP/CNT composite film after a month under ambient condition (c) and after 2 weeks under humid condition (d).

films showed a slight increase in the copper oxide peak (20.5%) and a slight decline in the pure Cu phase peak (−4.9%) (see blue and green lines in Figure 11c), resulting in a smaller increase of the sheet resistance than observed in the case of only Cu NP films about 30 days after the sintering (Figure 11a). This may be a result of differences in the porosity of the Cu/CNT film as a function of the CNT length. As shown in the SEM image in Figure 9a, the only Cu NP film sintered by the flash light still had a few pores, enabling copper nanoparticles to be easily oxidized. Thus, the large surface area of the porous Cu film with NPs caused the oxidation of copper as time increased. Meanwhile, the Cu NP/CNT composite film had a smaller surface area owing to its smooth surface and very few pores (Figure 9d). For these reasons, the Cu NP/CNT composite film was less oxidized compared to that consisting of Cu NPs only. Also the CNT themselves might act as a protective layer against the oxidation of the Cu films. Meanwhile, the environmental stability test under a harsh condition at 85 °C and in RH 85% was also conducted for 2 weeks (Figure 11b). In the first few days, the sheet resistance dramatically increased by around 40% for Cu NPs films only, while only 20% increasing was shown in the case of Cu NP/ CNT composite films. Comparing the results of ambient condition-test and humid condition-test, the rapid increase of sheet resistance may be ascribed to higher humidity and temperature. The sheet resistance gradually rose with time and reached saturation after 7 days. It was also found that the CNT-

70% elongation.38,39 For these reasons, the 20-μm CNT/Cu films could not sustain repeated tensile conditions as much as the 200-μm CNT/Cu films, because of the stress highly concentrated at small neck-like junctions (please see Figure 9a, b) causing a brittle fracture of the films. Meanwhile, in the 200μm CNT/Cu film case, the grain size was larger (Figure 9d), thereby resulting in much greater ductility and better durability under repeated bending conditions. Another reason for this phenomena might be that the longer CNTs could retard crack propagation among Cu NPs more effectively than in the case of the shorter CNTs. SEM images of the Cu/CNT films after 1000 cycles of bending fatigue (Figure 10d−g) demonstrated that cracks among Cu NPs could be retarded more effectively as the length of the CNTs increased. To investigate the environmental stability of the Cu NP/ CNT composite film, the flash light sintered films were placed under an ambient environment (25 °C and RH 30%) for one month (Figure 11a) and under a humid condition (85 °C and RH 85%) for 2 weeks (Figure 11b). Changes in the sheet resistance were measured and recorded as shown in Figure 11. It was found that the sheet resistance of flash light sintered films increased as time passed. This is because reduced and sintered Cu nanoparticles were oxidized again by reaction with oxygen in air. XRD patterns of the Cu films demonstrated that copper oxide (Cu2O, copper(I) oxide) peak (36.4°) increased about 30%, and the pure Cu phase peak (43.2°) decreased significantly (−19.6%) 30 days after the sintering (see black and red lines in Figure 11c). However, CNT-based composite I


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ACS Applied Materials & Interfaces based composite films showed a smaller increase of the sheet resistance than observed in the case of only Cu NP films about 14 days after the sintering. This result corresponded to that of the environmental stability test under an ambient condition (Figure 11a). Therefore, it was demonstrated that Cu NP/ CNT composite film had a better environmental stability than that of Cu NPs only in both of ambient and humid conditions, although there was a little difference in the increasing tendency of sheet resistance.

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CONCLUSION In this study, multiwalled carbon nanotubes (MWNTs) were applied to a Cu NP ink for highly conductive films using the flash light sintering method. The conductivity and fatigue resistance of the Cu NP/CNT composite films increased by using CNTs of 0.5 wt %. Meanwhile, the effects of CNT length on the flash light sintering process of Cu nanoink were also studied. It is noteworthy that, using CNT-based composite films (CNT length = 200 μm, 0.5 wt %) and the flash light sintering process (flash light = 15 J/cm2, 1 pulse, 10 ms) developed in this study, highly conductive Cu NP/CNT composite films were successfully produced at room temperature and under ambient conditions in a matter of milliseconds. These films showed a resistivity of 7.86 μΩ·cm, which is only 4.67 times higher than that of the bulk copper (1.68 μΩ·cm). Therefore, it is expected that the newly developed photonic sintering technique of Cu NP/CNT composite films would be a strong alternative to realize in situ sintering in electronics printed at room temperature.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08112. The resistivity of the flash light sintered Cu NP/CNT composite films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012029029). This work was also supported by the Technology Innovation Program (or the Industrial Strategic Technology Development Program, 10048913, Development of cheap nanoink, which is sintered in air for smart devices) funded by the Ministry of Trade, Industry, & Energy (MI, Korea).

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