A scalable fabrication of highly transparent and conductive thin films using fluorosurfactant-assisted single-walled carbon nanotube dispersions

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A scalable fabrication of highly transparent and conductive thin films using fluorosurfactant-assisted single-walled carbon nanotube dispersions Hyuck Jung, Jong Su Yu, Hawn Pyo Lee, Ji Min Kim, Jun Young Park, Dojin Kim

*

Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

A single-walled carbon nanotube (SWCNT)/sodium dodecylbenzenesulfonate (NaDDBS)

Received 13 July 2012

dispersion containing a fluorosurfactant was bar-coated in order to produce a highly trans-

Accepted 14 September 2012

parent and conductive thin film (TCF) for large-area application. The addition of a small

Available online 23 September 2012

amount of fluorosurfactant greatly reduced the surface tension of the CNT-dispersed solution, which produced a uniform film of CNTs by preventing agglomeration of CNTs during the drying process, and, furthermore, rendered bar-coating as the most practical large-area coating technique for a CNT solution. This particular fluorosurfactant addition maintained a CNT dispersion in the solution, which led to a dramatic improvement in the wettability of the CNT dispersion on the substrate towards high-performance TCF films. The thickness of the CNT films was controlled simply by adjusting the amount of CNTs in the solution. Moreover, the addition of a waterborne polymethyl methacrylate (PMMA) binder to the CNT dispersion improved the adhesion of the CNT films on a glass substrate.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Transparent conductive films (TCFs) are used for electrodes in flat-panel displays, touch-panels, electrostatic dissipation (ESD), electromagnetic interference (EMI), solar cells, and a variety of similar applied electro-optic devices. Indium tin oxide (ITO), the representative transparent and conductive substrate in current use, has limitations when applied to next-generation devices owing to insufficient flexibility, the scarcity and rising prices of the raw materials, and the high-temperature manufacturing process [1–3]. As a consequence, there has been a strong desire to develop TCF electrodes that employ cost-effective materials and processes. CNTs with unique electrical and optical properties, mechanical flexibility, and thermal stability have been considered as an ideal candidate for replacing existing transparent electrode materials [4–7], and recently CNTs were used as transparent electrodes in organic light-emitting diodes (OLEDs),

solar cells and touch-screen devices to replace existing metal oxide-based materials [8–10]. The feasibility of a solutionbased coating of CNTs for TCFs further motivated the use of CNTs on flexible polymer substrates, as well as on rigid glass substrates, with low cost, high mass, and a large surface area. Vacuum filtering, spin coating, dip coating, Langmuir– Blodgett and spray coating have been used for the production of CNT-based TCFs [11–15]. However, the majority of existing coating methods can only be used for a small area of substrate on the laboratory scale. On the other hand, wire-wound bar-coating has been widely used because it offers several advantages such as low cost, simplicity, and easy film-thickness control. The application of this coating technique to roll-to-roll coating, gravure, and slot-die coating systems makes it possible to cover a large surface area and achieve mass production [16]. For wire-wound bar-coating, controlling the wettability of the coating solution on the substrate mainly determines the quality of the coated films. The wettability of

* Corresponding author: Fax: +82 42 823 7648. E-mail address: [email protected] (D. Kim). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.09.027

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a solution on a substrate is mainly determined by the surface tensions of the solution and the substrate, and good wettability can be achieved by producing a substrate with a high surface energy and a solution with a low surface energy. If a solution does not wet the substrate well enough, a coffee-ring effect that occurs during the drying process step will cause surface defects, [17] because the wettability affects the evaporation rate of the solution and the flow of the particles within the solution. Oxygen-plasma treatment is a well-known method for improving the wettability of a solution on a substrate. However, with this method restoration of the surface wettability depends on the ambient temperature and humidity [18,19]. Recent reports have indicated that CNT-based TCFs can be produced by improving the solutions [20,21]. However, TCFs produced in this manner exhibit low optical and electrical performance because of additives in solution that are above the critical micelle concentration (CMC) and poor dispersion control of the CNTs in the solution. Furthermore, these techniques make no allowance for the adhesion between the substrate and the CNTs. In the present study, we investigated high-quality SWCNTbased TCFs printed by a bar-coating method using a SWCNT dispersion containing a co-surfactant. NaDDBS was used as a dispersion agent to prevent agglomeration between CNTs within the solution, which promoted a uniform dispersion. A fluorosurfactant was used as a wetting agent to improve the wettability of the CNT dispersion on the substrate. The wettability of the solution on the substrate was controlled by varying the addition of the NaDDBS and the fluorosurfactant. The wettability was measured by the surface morphology and the contact angles of the solution droplets on the substrates. The fluorosurfactant allows the CNTs in solution to spread evenly on the substrate and prevents the generation of surface defects after the bar-coating and drying processes, which helps produce a high-quality CNT film. The thickness of the CNT film can be controlled simply by varying the CNT concentration in the solution. The additional mixing of waterborne PMMA could produce a CNT/polymer hybrid film with superior adhesion to a substrate.

2.

Experimental

2.1.

Materials

SWCNT powder (AST-100F grade) was purchased from Hanwha Nanotech. This powder had been synthesized by the arc-discharge method, and was purified to a purity of 30 wt.% using a heat-treatment process. NaDDBS and gold chloride trihydrate (HAuCl4Æ3H2O) were purchased from Sigma–Aldrich. The fluorosurfactant (FC-4430) was purchased from the 3M Company. Water-borne PMMA binder (Mw = 300,000) synthesized by emulsion polymerization was purchased from Aekyung Chemical and was used with no additional purification.

2.2.

Fluorosurfactant-assisted SWCNT dispersion

A SWCNT powder (0.8 g) was added to 200 ml of aqueous solution dissolved with 1.2 g of NaDDBS. This solution was

dispersed using a horn-type ultrasonic processor (Sonics&Materials, VC 750) at 300 W for 5 min. This dispersion solution was then centrifuged at 25,000g for 30 min (Gyrozen, High-Speed Centrifuges 1736MGR). The majority of catalytic impurities and carbon particles contained in the SWCNTs were successfully removed by this ultrasonic and centrifuge treatment with no post-acid treatment processing (Fig. 1). The concentration of SWCNT in the SWCNT/NaDDBS solution after centrifugation was about 1.0 mg/ml. The concentration of SWCNTs in the solution was determined on the basis of absorption measurements [22]. SWCNT/NaDDBS solutions of different SWCNT concentrations ranging from 0.2 to 1.2 mg/ ml were prepared for the thickness control of the films. Fluorosurfactants (FS) that ranged from 0.05 to 0.5 vol.% were added to 50 ml of the SWCNT/NaDDBS solution (referred to here as SWCNT/NaDDBS/FS solutions). The mixtures were stirred for at least 30 min prior to coating.

2.3.

SWCNT/PMMA mixture solution

A SWCNT/PMMA mixture solution was prepared by adding different amounts of water-borne PMMA binder that ranged from 0.25–16 mg/ml to 50 ml of the SWCNT/NaDDBS/FS solution (CNTs concentration: 1.0 mg/ml). The mixtures were stirred for 5 min.

2.4.

SWCNT films and post-treatment

A wire-wound bar-coater (R.D.S. Webster Bar-coater, NO. 18) with the load wrapped with stainless steel wire was used to prepare the TCFs. Either hard or flexible substrate can be used depending on the field of application. In the present study, a soda-lime glass substrate (15 · 15 cm) was used with no pretreatment. Five ml of the SWCNT dispersed solution was dropped onto a glass substrate, and a wire-wound bar coater was pulled onto the upper part of the dispersion. The CNTcoated film was dried at 80 C for 5 min (or at 150 C with the addition of PMMA binder), then washed in deionized (DI) water to remove the residual surfactant. The resultant CNT film was post-treated in concentrated nitric acid followed by a 40 mM gold salt aqueous solution for 30 min. Subsequently, nitrogen gas was used to dry the post-treated CNT film.

2.5.

Characterization of the materials and devices

The du Nou¨y ring method (K100, KRUSS), [23] together with the use of a platinum-iridium ring, was used to measure the surface tension of the aqueous solutions with the addition of surfactants. A contact-angle analysis device equipped with a computer-controlled syringe (DSA100, KRUSS) was used to evaluate the changes in the contact angle of the solution with respect to the substrate at room temperature. The surface morphology of the film was observed using a field-emission scanning electron microscope (FE-SEM, JEOL-7000). A UV–vis spectrometer (SCINCO-3100) was used to analyze the concentration of CNTs in the solution as well as the transmittance of the film. The sheet resistance of the film was measured using a four-point probe (CMT-SR1000N, AIT).

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Fig. 1 – Raman spectrum of heat-treated SWCNTs and SWCNTs purified by centrifugation. The crystallinity measured by the ID/IG peak ratio of the SWCNTs was increased after centrifugation. The insert shows the TGA curves measured before and after purification. The residual impurity was reduced to 6% or lower after the purification. The morphology before and after the purification is depicted in (a) and (b), respectively.

3.

Results and discussion

Fig. 2 shows the surface tension of the aqueous SWCNT/NaDDBS/FS solutions with different concentrations of FS that ranged from 0.05 to 0.5 vol.%. The solutions were compared with the SWCNT dispersion in water and the SWCNT/NaDDBS solution. The morphology of the droplets showing the contact angles with respect to the glass substrates also is depicted in the inset. Since the balance between the surface tension forces at the substrate-liquid, substrate-air, and liquid–air interfaces determines the contact angle, the hemisphere shape of the water droplets for the SWCNT solution indicated a relatively large surface tension force between the solution and the substrate. When the NaDDBS surfactant was added to the water, or to the SWCNT/NaDDBS solution, the strong attractive forces among the water molecules in the solution were relaxed and led to a sharp reduction in the surface tension of the water from 72.5 to 38.2 mN/m, and in turn a reduction in the surface tension forces between the air-solution and the substrate-solution. This resulted in a change in the contact angle from 79.16 to 35.46 appearing as an improvement in the wettability of the solution on the substrate. However, when 0.1 vol.%, or more, of FS was added to the solution, the surface tension of the aqueous solution was further reduced to <20 mN/m. This result revealed a contact angle of 13.47, or lower, with a dramatic improvement in the wettabil-

Fig. 2 – The surface tensions of the SWCNT, SWCNT/ NaDDBS, and SWCNT/NaDDBS/FS solutions for varying concentrations of FS. The inset shows the morphology of the droplets on the glass substrate.

ity. This effect was attributed to the fact that NaDDBS and the fluorosurfactant have different functional groups. Hydrocarbon surfactants like NaDDBS comprise a hydrophobic tail group and a hydrophilic head group, and, therefore, they

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Fig. 3 – Deposition pattern after drying of (a) SWCNT/NaDDBS droplet, (b) SWCNT/NaDDBS/FS(0.05 vol.%) droplet, and (c) SWCNT/NaDDBS/FS(0.1 vol.%) droplet. Magnified optical images for the edges are shown below. mostly concentrate and arrange at the water/CNT interface; by contrast, fluorosurfactants have a fluorocarbon-based hydrophobic/oleophobic tail and a hydrophilic head, and tend to be concentrated and arranged at the water/air interface [24]. This means that fluorosurfactants are more effective than hydrocarbon surfactants in mitigating the imbalance of the surface tension forces at the solution-air interface. The wettability of a solution on a substrate also influences the flow of solute in the solution during the drying process, which in turn determines the final deposition pattern. Fig. 3 depicts the deposition patterns after the drying of SWCNT/ NaDDBS/FS droplets at different concentrations of FS. A droplet of SWCNT/NaDDBS with zero FS (Fig. 3a) exhibited a ring pattern showing a clear contrast between the central and edge regions because the CNTs were concentrated in the central region of the droplet leaving a huge non-uniform concentration of CNTs after drying. This non-uniform deposition pattern was formed by a combination of the coffee-ring effect and the Marangoni effect that occurred during the drying process. The formation of ring patterns, known as the coffee-ring effect, was due to the capillary flow towards the edge that was generated by the combined action of contact line pinning and an increase in the evaporation rate at the droplet edge, which happens when the solution does not wet the substrate sufficiently and globular water droplets form [17,25–27]. This capillary flow will carry the CNTs to the edges. However, as shown in Fig. 3a, the CNTs were concentrated in the middle regions in contrast to the normal coffee-ring phenomenon. This happened because a Marangoni flow to the center of the droplet, which is opposite to the outward capillary flow, was produced by the addition of the NaDDBS that set up of a surface tension gradient in the solution, which in turn carried the CNTs towards the middle of the droplet [28–30]. It was interesting to observe the repeated minor ring patterns that occurred on a 10 lm scale during the initial drying process, as shown by the magnified image in Fig. 3a. The morphology may depict an alternating change in the dominant flow due to competition between the capillary and Marangoni flows.

However, as shown by the millimeter scale over the droplet, the Marangoni flow overwhelmed the SWCNT/NaDDBS solution leaving CNT-expelled edges after drying. The deposition patterns changed when FS was added to the SWCNT/NaDDBS solution, as shown in Fig. 3b and c. The CNT dispersion droplets were larger due to a higher spreading-out rate on the substrate, and the CNT-dispersion uniformity in the pattern was improved. The concentration gradient along the minor coffee rings was lessened with a higher content of fluorosurfactant. As seen in Fig. 3c, the addition of more than 0.1 vol.% of FS showed a deposition pattern with little nonuniformity. Uniform water evaporation rates over a droplet surface are the direct cause for the dramatic enhancement of CNT dispersion on the glass substrates after drying. Unlike the hydrocarbon dispersants, functional groups of fluorosurfactants are concentrated at the solution/ air interface and reduce the surface energy of the solution. A solution with low surface energy spreads well over a substrate with a small contact angle leading to a uniform thickness of liquid. The evaporation rate at the edges is not much different from that in the middle of the droplet, thus reducing the driving force for the coffee-ring effect, and consequently the Marangoni effect as well [31]. Furthermore, the high thermal stability of fluorosurfactants due to the strong C–F bonds permits the maintenance of low surface energy in the solution during the drying process [23]. Fig. 4 shows the surface morphology of the SWCNT films bar-coated onto the glass substrates using the SWCNT/NaDDBS/FS solutions. As shown in Fig. 4a, the film coated with the SWCNT/NaDDBS solution without FS presented a surface morphology of irregular stripes, which were the concentration of the CNTs. The SWCNT/NaDDBS solution, having a relatively high surface tension, showed a relatively poor wetting on the substrate, and thus resulted in a flow of the solution similar to the occurrence shown in Fig. 3a. This flow of coating fluid and accompanying flow of CNTs on the substrate in a limited distance created the stripes shown on the CNTs. The addition of FS by 0.05 vol.% greatly improved the coating

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Fig. 4 – SEM image of the SWCNT films bar-coated using a solution of (a) SWCNT/NaDDBS, (b) SWCNT/NaDDBS/FS(0.05 vol.%), and (c) SWCNT/NaDDBS/FS(0.1 vol.%). The insert shows the film uniformity using the scale of the substrate size.

uniformity, as shown in Fig. 4b, but a partial aggregation of the CNTs persisted in the surface morphology. The SWCNT/ NaDDBS/FS(0.1 vol.%) solution showed a further enhancement of the uniform CNT networks owing to the better wettability of the solution on the substrate. The morphology was free of surface defects after the drying process, as shown in Fig. 4c. In general, the observation of all droplets coincided with the examples shown in Fig. 3. The thickness of the CNT film was controlled by changing the SWCNT concentration in the SWCNT/NaDDBS/ FS(0.1 vol.%) solution to a range of 0.2–1.2 mg/ml. The morphologies examined are summarized in Fig. 5. A higher concentration of CNTs in the aqueous solution naturally led to a higher density of the CNT network on the substrate. This increase in the density of the CNT network led to a change in the sheet resistance and transmittance as shown in Fig. 6. When the CNT films were treated with a gold solution, gold nano-particles were formed on the surfaces of the

CNT film owing to their spontaneous reduction (as shown by the insert of Fig. 5c). When the CNT concentration was changed from 0.2 to 1.2 mg/ml, the sheet resistance of the film changed from 858 to 182 ohm/sq, and the transmittance changed from 97% to 87%. The sheet resistance (Rs) and transmittance (T(k)), measured at a wavelength of 550 nm in TCFs, were related by.  2 188:5 rop ð1Þ TðkÞ ¼ 1 þ RS rdc with characteristic dc (rdc) and optical (rop) conductivity of the film. The properties of TCFs with different Rs and T values could be compared using the ratio rdc/rop as the figure-ofmerit [32]. As shown by the inset in Fig. 6, the rdc/rop value could be obtained from the slope of T1/2/(1  T1/2) versus the Rs plot, as derived from Eq. 1. The conductivity ratio value, rdc/rop = 14.3 ± 0.2, was comparable to that reported in the literature (rdc/rop = 9–15 as reported in Refs. [11–15]), but the

Fig. 5 – SEM images of the SWCNT films bar-coated onto a glass substrate at different SWCNT concentrations: (a) 0.2 mg/mL, (b) 0.4 mg/mL, (c) 0.8 mg/mL, and (d) 1.2 mg/mL. The insert in (c) shows the surface morphology after the treatment with goldions.

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Fig. 6 – Transmittance versus sheet resistance of the films bar-coated with different concentrations of SWCNTs ranging from 0.2–1.2 mg/ml. The inset is a curve, T1/2/(1  T1/2) versus Rs, as a modification of Eq. 1 to obtain the ratio rdc/rop.

value was higher than that obtained previously with SWCNTbased TCFs fabricated by the bar-coating method [21]. The improvement in rdc/rop values in our coating was attributed to the small surfactant/CNT ratio of 1.5 in the solutions. When the surfactant/CNT content ratio in the solution was high, micelle formation by the surfactant in the solution triggered the re-bundling of the CNTs. The bundling of the CNTs caused an increase in contact resistance between the CNTs [33–36]. Fig. 6 also shows that gold-ion treatment greatly enhanced the

electrical conductivity with a negligible change in the transmittance. The rdc/rop value of the film was approximately doubled to 31.5 ± 0.3 after the gold-ion treatment. The goldion treatment, which formed metal nanoparticles through the spontaneous reduction of metal ions on the CNT surface, yielded more stable electrical conductivity properties in air compared to acid treatment using HNO3 or SOCl2 [37,38]. Although CNT films of high transmittance and conductivity have been fabricated via optimization, the weak adhesion between the CNTs and the substrate limits their use in many fields. This problem has been solved by the development of a CNT/polymer composite film, which was fabricated by the addition of a polymer binder as an adhesion promoter to the SWCNT/NaDDBS/FS solutions. Varying contents of waterborne PMMA binder synthesized by emulsion polymerization [39] were mixed with SWCNT/NaDDBS/FS solutions for barcoating. Fig. 7a summarizes the adhesion and opto-electrical properties of the composite films. The rdc/rop values were measured after gold-ion treatment. The adhesion was measured by measuring the resistance of the as-prepared film (Ro) and the resistance was measured after a taping test of the film (R) [40]. As shown in the inset of Fig. 7 for the taping test, a piece of adhesive tape is applied firmly on the film surface and then peel off the tape (American Society for Testing and Materials ASTM D 3359: Test methods for measuring adhesion by tape test). If the film adhesion on the substrate was strong enough, taping on the film would not tear the film out of the substrate leading to a negligible change in the film resistance. Otherwise, the resistance was increased due to a detachment of the conducting CNT film. As shown in Fig. 7b–d, an increase in the polymer binder content in the

Fig. 7 – (a) rdc/rop ratio and adhesion property, as-measured by R/R0, of the SWCNT films as a function of the polymer/CNT ratio. SEM images (b)–(d) are the morphology observed with the PMMA/SWCNT composite films fabricated with a polymer/ CNT mixing ratio of (b) 0, (c) 2, and (d) 16. The insert of (a) shows the photo-image for taping test.

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solution enhanced the adhesion of the CNT film on the substrate, but this also increased the film resistance due to a degradation of the contacts among the CNTs caused by the intervention of the insulating polymer. Therefore, a gradual reduction in rdc/rop is evident in Fig. 7a. We observed that the adhesion of the CNT films was strong with a polymer/ CNT ratio of 2, which equates to a rdc/rop value of 19.74. Under these conditions, the transmittance was 80% and the sheet resistance was 80 ohm/sq.

4.

Summary

In summary, we developed a solution and process to fabricate uniform SWCNT films using a bar-coating method. Since barcoating is the practical method applicable to large-area rigid and flexible substrates when nanoparticle-dispersed solutions are used, we developed the solution for transparent conductive electrodes that are used for devices such as displays and solar cells. The addition of a small amount of fluorosurfactant as a wetting agent to the NaDDBS-based SWCNT dispersion dramatically improved the wettability of the solution on the glass substrates. The bar-coating of the solution produced a uniform distribution of the CNTs without macroscopic surface defects occurring during the drying process. The sheet resistance and transmittance of the SWCNT film fabricated as such revealed a high rdc/rop ratio of 14.3, which is a development from previous bar-coated CNT films. The rdc/rop value was further improved to 31.5 by gold-ion treatment, and furthermore, the adhesion of the CNT films was enhanced by the addition of a waterborne polymer to the SWCNT dispersion solution.

Acknowledgments This research was supported by the National Research Laboratory program and by a Grant from the Construction Technology Innovation Program funded by the Ministry of Land, Transport, and Maritime Affairs of Korea.

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