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Highly conductive and stretchable Ag nanowire/carbon nanotube hybrid conductors

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Nanotechnology Nanotechnology 25 (2014) 285203 (7pp)


Highly conductive and stretchable Ag nanowire/carbon nanotube hybrid conductors Ju Yeon Woo1, Kyun Kyu Kim1, Jongsoo Lee, Ju Tae Kim and Chang-Soo Han2 School of Mechanical Engineering, zKorea University Anam-Dong, Seongbuk-Gu, Seoul 136-713, Korea E-mail: [email protected] Received 18 March 2014, revised 14 April 2014 Accepted for publication 20 May 2014 Published 27 June 2014 Abstract

Fabricating stretchable conductors through simple, cost-effective and scalable methods is a challenge. Here, we report on an approach used to develop nanowelded Ag nanowire/singlewalled carbon nanotube (AgNW/SWCNT) hybrid films to be used as high-performance stretchable conductors. Plasmonic welding, which was done at the junctions of AgNWs in order to form hybrid AgNW/SWCNT conductors on an Ecoflex substrate, enabled excellent electrical and mechanical stability under large tensile strains of over 480% without the need to pre-strain. Furthermore, we demonstrate highly stretchable circuits that are used to power LED arrays. The LED arrays are formed using the plasmonic-welded AgNW/SWCNT/Ecoflex hybrid material, which demonstrates suitability for interconnector applications in flexible electronics. Keywords: silver nanowire, carbon nanotube, plasmonic welding, transparent conductive film (Some figures may appear in colour only in the online journal) 1. Introduction

issues affecting AgNWs regarding their applications in stretchable electrodes. First, weak bonding at cross-junctions between the nanowires degrades the mechanical and electrical stability. Second, AgNWs tend to break after significant bending or stretching due to their rigidity. Third, the adhesion of AgNWs to a polymer matrix is restricted to physical bonding rather than chemical bonding, unless the surface of the AgNWs is chemically functionalized. As a result of these problems, the AgNWs in the polymer matrix can become physically isolated from the polymer, particularly following repeated strain [30]. Although much progress has been made in improving the performance of AgNW films through the development of various processing methods, including mechanical pressing [31, 32], thermal annealing [27, 33, 34], plasmonic welding [35, 36], incorporating additional conducting materials [37, 38] and layer transfer methods [23, 39], most reports have addressed only some of the above issues. Therefore, new methods should be developed to achieve highperformance stretchable conductors. Here, we describe approaches used to fabricate AgNW/ single-walled carbon nanotube (SWCNT) hybrid conductors embedded in a polymer matrix by using a plasmonic welding

Many future optoelectronic devices are expected to be soft, flexible, stretchable and curvilinear [1]. Among these characteristics, stretchability is particularly challenging and requires advances in technology, yet it is essential for the realization of stretchable circuits, flexible displays, wearable electronics and skin-like patchable sensors [2–12]. Considerable effort has been made in materials science to develop new stretchable and foldable conductors through the use of conducting polymers [13, 14], carbon nanotubes (CNTs) [15–17] serpentine-shaped graphene [18–20], wavy thin metals [21, 22] and random networks of metallic nanowires [23–26]. Recently, silver nanowires (AgNWs) have attracted a lot of research attention for their potential applications as stretchable conductors due to their easy and scalable manufacturing process, excellent electrical conductivity and high corrosion resistance [27–29]. However, there are a number of 1 2

The authors contributed equally. Author to whom any correspondence should be addressed.



© 2014 IOP Publishing Ltd Printed in the UK

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process. In the network, the SWCNTs form flexible conducting bridges between the AgNWs in the polymer matrix and drastically enhance the stretchability of the conductors and the stability of their conductivity. SWCNTs can assist with the adhesion between the AgNWs and the substrate, which greatly enhances their stretchability. We found that a nano-welding process in the hybrid AgNW/SWCNT material leads to improved bonding at wire-to-wire junctions and also to improved electrical and mechanical properties. As a result, our hybrid AgNW/SWCNT conductors exhibit stable conductance upon stretching; we recorded a tensile strain of more than 480%. We successfully fabricated a highly stretchable light-emitting diode (LED) circuit using our flexible hybrid conductors.

Vibracell™). The dispersed SWCNT solution was centrifuged at 10 000 RPM for 1 h to remove any agglomerates or impurities. The final concentration of the SWCNT was 20 μg mL–1. The AgNW and SWCNT dispersions were mixed to form the hybrid dispersion. By varying the mixing volume ratio of the AgNW to the SWCNT dispersions, we found the appropriate mixing conditions to obtain a uniform dispersion. The hybrid AgNW/SWCNT films were prepared via vacuum filtration (Electroc Aspirator VE-11, JEIO Tech) through cellulose ester membranes (0.2 μm, 47 mm, Advantec). Following filtration, the AgNW/SWCNT networks in the film were transferred to an Ecoflex 0010 substrate (Hyup Shin Co.) under uniform vacuum suction, and the membranes were peeled off. Plasmonic nanowelding was performed using unpolarised, broadband illumination from 21 Ushio tungsten halogen lamps positioned 45 mm from the sample and powered by a UP-3005D power supply, giving an illumination power density of approximately 30 W cm−2. To demonstrate the feasibility of our highly stretchable AgNW/SWCNT/Ecoflex hybrid conductors, we arranged a direct current (DC) circuit of two resistors (AgNW/SWCNT networked Ecoflex and a LED) and a voltage source. We used a 90 mm-long red LED bar (CarLED Co.) with six LEDs placed at the side of the flexible device.

2. Experimental details 2.1. Materials and methods

AgNWs were purchased from Nanopyxis Co. (Korea) and supplied as a suspension in deionised water. The AgNWs had an average diameter of 100 nm and a length of 50 μm, and the concentration of the AgNWs in the suspension was 5.0 mg mL–1. SWCNTs were purchased from Top Nanosys Co. (Korea), with a diameter of 1.5 nm and lengths in the range of 1–5 μm. SWCNTs (5 mg) were mixed for 30 min with 500 mg of sodium dodecyl sulfate (SDS, Sigma Aldrich) in 50 mL of deionised water using probe sonication (Sonics

Figure 1. (a) Schematic diagram illustrating the hybrid AgNW/SWCNT film fabrication process using vacuum filtration and transfer

methods. (b) Optical image of an AgNW/SWCNT film on an Ecoflex substrate. (c) SEM image of an AgNW/SWCNT film on a silicon wafer. (d) R/R0 as a function of strain for the AgNW films as a function of the AgNW content. 2

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20.5 NW + CNT 20.0

OFF state

Resistance (Ω)

19.5 19.0 18.5 18.0 17.5

ON state 17.0 16.5 0

Plasmonic welding









Time (s)



350 NW+CNT NW+CNT with welding

Average Strain (%)

300 250 200 150 100 50 0 100 : 0

80 : 20

60 : 40

40 : 60

20 : 80

0 : 100

Composition (NW : CNT) Figure 2. (a) Schematic diagram showing the plasmonic welding process. (b) The resistance of an AgNW/SWCNT film as a function of the illumination time. (c) Tilted cross-sectional SEM image of AgNW junctions before illumination and (d) after plasmonic welding on a silicon wafer. (e) Average strain of the hybrid AgNW/SWCNT conductor with and without welding as a function of the composition.

deionised water were collected on a mixed cellulose ester filter, thereby forming a uniform thin film of the AgNW/ SWCNT with a percolation network. The content of the AgNW dispersions and the mixing ratios of the AgNW and SWCNT dispersions were adjusted to control the AgNW/ SWCNT network density. The filtered AgNW/SWCNT films were then transferred to the target substrates by applying a uniform vacuum suction. Once peeled off of the filter paper, the hybrid AgNW/SWCNT thin film was bonded to the highly stretchable polymer substrate, forming a conductive and stretchable layer. Figure 1(b) shows a photograph of the prepared hybrid AgNW/SWCNT film, which shows no structural damage or residue on the Ecoflex substrate. To characterise the improvement in the stretchability of the hybrid conductor due to the incorporation of the SWCNTs, the hybrid AgNW/SWCNT film on the silicone was observed using FE-SEM, as shown in figure 1(c). The SEM micrograph reveals that the AgNW/SWCNT film consists of random networks of AgNWs without any significant bundling of wires or the agglomeration of SWCNTs entangled with AgNWs. In addition, the hybrid AgNW/SWCNT film clearly shows that the bundles of SWCNTs form bridges between the AgNWs and fill the spaces between the nanowires. These results suggest that the SWCNTs form conductive bridges that connect the AgNWs and can enhance the stretchability of the hybrid AgNW/SWCNT film. However, many AgNWs

2.2. Characterization

The resistance and current–voltage (I–V) curves of a 5 × 5 mm2 section of the conducting film were measured using a Keithley 2002 Multimeter via a two-probe method. Copper wires were attached to the film side at the two ends of the strips with silver paste for stable contact to the probe. Scanning electron microscope (SEM) images were acquired using a Hitachi 4700 field-emission SEM (FE-SEM) at a voltage of 5–10 kV and at a distance of 8–12 mm. Stretching cycles were performed at a speed of 2.5 mm s–1 using a linear servo actuator (with a stroke of 50 mm, a gearing ratio of 100:1 and a power 6 V supply, Firgelli Co.) that was controlled using a servo controller (Micro Maestro 6-channel controller, Pololu Co.).

3. Results and discussion Highly conductive and stretchable AgNW/SWCNT conductors were prepared by vacuum filtration and transfer methods onto a highly stretchable polymer substrate. The vacuum filtration and transfer method is simple and clean because it does not require filter dissolution or rinsing [26]. The fabrication process of the hybrid AgNW/SWCNT films is illustrated in figure 1(a). The AgNW/SWCNT mixtures in 3

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have weak adhesion to the substrate or to other AgNWs, which leads to poor mechanical stability in response to large strain. The SWCNTs, with diameters of several nanometres, help the AgNWs to bond onto the substrate and help provide good mechanical/electrical connections between the AgNWs and the substrate. Figure 1(d) shows R/R0 versus strain curves as a function of the AgNW content of the dispersions, where R0 is the original resistance of the sample, and R is the resistance of the strained sample. There were no noticeable differences in the R/R0 up to 60% strain; however, at strains greater than 60%, the R/R0 increased as the AgNW content of the dispersions increased. In general, the stretchability decreased rapidly when the AgNW density increased because the increased AgNW percolation in the networks reduced the mechanical stretchability of the individual AgNWs, which act as a thin metal film [26]. Based on these results, a maximum AgNW content of 100 μL was used in the dispersions for the remaining work reported here. The pristine AgNW/SWCNT conductors did not show good stretchability because of weak bonding at the junctions between the AgNWs, which were protected by the presence of surface ligands on the wires. However, a plasmonic nanowelding process can improve the electrical properties by allowing fusion between the AgNWs [35]. Figure 2(a) shows a schematic diagram of the plasmonic welding procedure, which was performed with the use of a tungsten/halogen lamp. The AgNW junctions naturally feature nanometre-scale gaps, and these small gaps enable extreme localisation of Joule heating due to the optical radiation coupling with the nanowire junctions. This results in good electrical and mechanical contact between the AgNWs, leading to improved mechanical compliance of the AgNW/SWCNT film and resulting in highly stretchable conductors. Moreover, strong bonding between the nanowire junctions and the polymer may be caused by localised heating during the plasmonic welding process. Figure 2(b) shows the resistance of an AgNW/SWCNT stretchable conductor as a function of the illumination time. The resistance of the hybrid film was initially 17 Ω sq−1; it was slowly increased to 19.8 Ω sq−1 and then plateaued. Further illumination did not result in a significant change in the resistance of the film, indicating the self-limited property of the plasmonic welding process [35]. Following termination of the illumination, the resistance slightly decreased and maintained a constant value of 18 Ω sq−1. Curiously, the plasmonic welding had very little effect on the resistance of the hybrid AgNW/SWCNT conductors, and the resistance remained almost constant following the plasmonic welding process. This small change in resistance is consistent with changes in the surface properties of the individual AgNWs. Figure 2(c) shows a tilted crosssectional SEM view of the AgNW junctions prior to plasmonic welding. The connections between the crossed AgNWs were mainly due to vacuum suction and Van der Waals interactions between the AgNWs. The AgNWs were stacked loosely, with spaces between them, and thus formed weak contacts. Figure 2(d) shows an SEM image of a plasmonicwelded AgNW/SWCNT film. Note the melting and fusion of the welded spot between the AgNWs, cross-linked at the

Figure 3. (a) The change in resistance as a function of strain curves for an AgNW film (black), a welded AgNW film (red), a hybrid AgNW/SWCNT film (blue) and a welded hybrid AgNW/SWCNT film (dark green) on Ecoflex substrates. Inset shows the change in resistance at a low-strain region. (b) Optical pictures and lowmagnification SEM micrographs of the deformation of the AgNW/ SWCNT film on an Ecoflex substrate during straining that corresponds to figure 3(a).

upper and lower positions that are in contact. This local plasmonic welding ensures both good electrical conductivity and mechanical compliance of the hybrid AgNW/SWCNT films under large mechanical strain. To investigate the stretchability of the AgNW/SWCNT films on the Ecoflex substrate, we compared films with and without the welding process. The average strain (the average strain means the strain value when the resistance increases tenfold in this study) of the films was measured as a function of the material composition, as shown in figure 2(e). In both cases, the average strain of the measured films gradually increased as the SWCNT content increased up to a ratio of AgNWs to SWCNTs of 1:4. However, when the SWCNT-only film was used, the average strain of the films rapidly dropped to almost zero; this was attributed to the embedded nano-sized SWCNTs on the Ecoflex substrate with the rough surface. Except for the SWCNT-only film, the high stretchability of the hybrid AgNW/SWCNT films with high SWCNT content was attributed to the decreased quantity of the AgNW networks and to their mechanical stability (see figure 1(c)) due to the presence of the bridging SWCNTs. Furthermore, the average strain of the plasmonic-welded hybrid AgNW/ 4

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SWCNT films was much greater than that of the pristine AgNW/SWCNT conductors, regardless of the SWCNT contents. We found that the hybrid AgNW/SWCNT films, formed by plasmonic welding, had optimal electrical and mechanical properties at a ratio of AgNWs to SWCNTs of 1:4. However, the relatively high loading of the SWCNTs slightly increased the resistance of the film. To assess their performance as a stretchable conductor, an AgNW film, a plasmonic-welded AgNW-only film, a hybrid AgNW/SWCNT film and a welded hybrid AgNW/ SWCNT film, all on Ecoflex substrates, were strained using a tensile testing stage while the electrical resistance was measured. The electrical resistance was measured under various tensile strains, as shown in figure 3(a). It can be seen from the figure that the R/R0 remained almost linear as a function of strain in all cases for tensile strains of less than 200%. A welded hybrid AgNW/SWCNT film structure was the most stable among all the films tested at this strain region, as shown in the inset of figure 3(a). However, beyond 200% tensile strain, the R/R0 of the AgNW film increased sharply due to the rupture of the film. The resistance of an AgNW film is dominated by the contacts between the nanowires [25]. When the AgNW film is stretched, the AgNWs may slide, and the resulting detachment of the nanowire contacts may lead to a significant increase in the resistance. The hybrid AgNW/ SWCNT conductor showed better performance compared with that of the AgNW-only film. The hybrid AgNW/ SWCNT conductor withstood strain of up to 260% without rupture. The increase in the resistance to strain can be explained by the presence of the bridging SWCNTs between the adjacent AgNWs in the network, which act as flexible

conducting interconnects. A welded AgNW film showed slightly better stability compared with the hybrid AgNW/ SWCNT film and withstood strain of up to 280%. This is attributed to improved junctions and mechanical compliance between the AgNWs. The hybrid plasmonic-welded AgNW/ SWCNT film could be strained up to 480% without a significant increase in the resistance. The hybrid welded AgNW/ SWCNT film was more robust and exhibited superior electrical and mechanical properties compared to the AgNW film, the welded AgNW film and the non-welded hybrid AgNW/ SWCNT film. The welded hybrid films could withstand large strains without pre-straining, a technique which is typically used to enhance strain resistance [26, 40]. This demonstrates that the synergetic effects of combining conductive interconnecting materials and plasmonic welding can be useful for high-performance stretchable electronics. The macroscopic surface morphology of the hybrid welded AgNW/SWCNT film, following the application of 480% tensile strain, is shown in figure 3(b). The film effectively accommodated the deformation by allowing the nanowires to align to the direction of the strain without a significant change in conductivity. Figure 4(a) shows a schematic diagram of the strain/ release test system. A remarkable strain-history-dependent feature of the real-time resistance was observed when the sample was subjected to repetitive straining cycles (300 cycles at 2.5 mm s−1). Figure 4(b) shows the R/R0 as a function of time during the measurements of a welded hybrid AgNW/SWCNT film with a strain of 100%. For the AgNWonly film, R/R0 increased considerably when the sample was strained and then partially recovered when the sample was

Figure 4. (a) Schematic diagram showing the strain–release system via a linear actuator at a speed of 2.5 mm s−1. (b) R/R0 during 300 strain/

release cycles for an AgNW film and a welded hybrid AgNW/SWCNT film with a strain of 100% and (c) with a strain of 160%. 5

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small increase in the electrical resistance when the welded hybrid AgNW/SWCNT film was strained up to 400%. There was only a negligible change in the electrical resistance, demonstrating excellent electrical and mechanical properties. The above indicates that our nanowelded hybrid AgNW/ SWCNT conductor is a potential resource for the fabrication of high performance stretchable electronics.

4. Conclusions We have described a method to fabricate highly stretchable conducting films by combining conductive interconnecting nanomaterials with a plasmonic welding process. The SWCNTs wrap around the AgNWs, resulting in an increased secondary conduction pathway, as well as improved mechanical support. The plasmonic welding provided good mechanical and electrical contacts between the AgNWs, thereby providing the mechanical compliance of the hybrid AgNW/SWCNT film. The welded hybrid AgNW/SWCNT film was robust, with superior electrical and mechanical properties under a large tensile strain of over 480% without applying any pre-strain. We successfully demonstrated the feasibility of the plasmonic-welded hybrid AgNW/SWCNT films for interconnect applications by fabricating stretchable LED circuits. We believe this approach is a practical method to realise soft and/or flexible electronic devices.

Figure 5. (a) Optical images of the LED integrated circuits connected to the welded hybrid AgNW/SWCNT film operating at 0%, 100%, 300% and 400% tensile strain. (b) Current as a function of voltage curves of the LED circuits with integrated welded hybrid AgNW/ SWCNT interconnects on an Ecoflex substrate as a function of the strain.

Acknowledgements Our research was financially supported by the CASE (Center for Advanced Soft Electronics) of MSIP in Korea.

released. This change in the R/R0 originates from the weak inter-wire junctions. The welded hybrid AgNW/SWCNT film exhibited almost constant R/R0 during strain/release cycles. Further strain/release cycles at strains in the range of 0–160% were carried out, as shown in figure 4(c). The AgNW film showed large fluctuations in R/R0, as well as irreversible changes in resistance, which correspond to the breakage of the weak junctions under repetitive strain cycles. Although the welded hybrid AgNW/SWCNT films also showed slight fluctuations in R/R0, they exhibited reversible mechanical and electrical changes after many straining cycles. We used a plasmonic-welded hybrid AgNW/SWCNT film on an Ecoflex substrate as the interconnects in order to illuminate a commercially available LED array, as shown in figure 5(a). We fabricated an array of six LEDs, which were connected at the side of the flexible substrate, and corresponding photo images were captured at an operating voltage of 8 V. Our stretchable conductor functioned well during cycles of tensile strain of up to 400% without showing any signs of irreversible degradation. The welded hybrid AgNW/ SWCNT film was strained, and the current–voltage characteristics of the integrated LED were examined, as shown in figure 5(b). The voltage was varied in the range of 1–10 V using a Keithley 2002 Multimeter. We observed only a very

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carbon nanotube hybrid conductors.

Fabricating stretchable conductors through simple, cost-effective and scalable methods is a challenge. Here, we report on an approach used to develop ...
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