Three-Dimensional Printing of Highly Conductive Carbon Nanotube

Aug 26, 2016 - ... and nozzle clogging, we design a polyvinylpyrrolidone-wrapped MWNT ink with uniform dispersion and appropriate rheological properti...
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Three-Dimensional Printing of Highly Conductive Carbon Nanotube Microarchitectures with Fluid Ink Jung Hyun Kim,† Sanghyeon Lee,†,# Muhammad Wajahat,†,‡ Hwakyung Jeong,† Won Suk Chang,†,# Hee Jin Jeong,† Jong-Ryul Yang,§ Ji Tae Kim,∥ and Seung Kwon Seol*,†,‡ †

Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea ‡ Electrical Functional Material Engineering, Korea University of Science and Technology (UST), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea § Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan-si, Gyeongsangbuk-do 38541, Republic of Korea ∥ Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China # Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Republic of Korea S Supporting Information *

ABSTRACT: Moving printed electronics to three dimensions essentially requires advanced additive manufacturing techniques yielding multifunctionality materials and high spatial resolution. Here, we report the meniscus-guided 3D printing of highly conductive multiwall carbon nanotube (MWNT) microarchitectures that exploit rapid solidification of a fluid ink meniscus formed by pulling a micronozzle. To achieve high-quality printing with continuous ink flow through a confined nozzle geometry, that is, without agglomeration and nozzle clogging, we design a polyvinylpyrrolidone-wrapped MWNT ink with uniform dispersion and appropriate rheological properties. The developed technique can produce various desired 3D microstructures, with a high MWNT concentration of up to 75 wt % being obtained via post-thermal treatment. Successful demonstrations of electronic components such as sensing transducers, emitters, and radio frequency inductors are also described herein. We expect that the technique presented in this study will facilitate selection of diverse materials in 3D printing and enhance the freedom of integration for advanced conceptual devices. KEYWORDS: 3D printing, 3D-printed electronics, meniscus-guided printing, CNT microarchitecture, fluid ink two-dimensional or low aspect ratio structures on flat substrates only, whereas three-dimensional patterning on nonflat substrates is essential for advanced applications. Three-dimensional printing (also known as additive manufacturing) is widely regarded as a revolution in manufacturing technology, with significant promise for electronic applications; this field is known as 3D-printed electronics.15−19 However, it is difficult to obtain functional 3D structures for electronics, although we can easily produce plastic or metallic 3D objects with coarse resolution via various commercial 3D-printing

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rinted electronics technology allows electronic and photonic devices to be printed using graphic printing techniques, such as inkjet, gravure, or screen printing, with conducting or semiconducting inks.1−10 In contrast to multistaged, expensive, and wasteful photolithography for typical rigid electronics, printing techniques offer a rapid and cheap means of printing electrical circuits on diverse flexible substrates. Furthermore, the advances in functional nanomaterials, which are included in the ink, have led to the rapid development of printed electronics in recent years. To date, printed electronics have introduced excellent opportunities for the creation of advanced products, which are developed by adding electronic functions to flexible substrates composed of materials such as plastics, rubber, or paper.11−14 However, conventional graphic printing methods are limited to the patterning of © 2016 American Chemical Society

Received: July 18, 2016 Accepted: August 26, 2016 Published: August 26, 2016 8879

DOI: 10.1021/acsnano.6b04771 ACS Nano 2016, 10, 8879−8887

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Figure 1. (a) Viscosity as a function of shear rate and (b) storage and loss moduli as functions of ink shear stress. (a, Inset) Histogram comparing viscosities of CNT inks with 7 wt % A-MWNT and different PVP concentrations (0, 10, 17, 25 wt %). The viscosity at a 10 s−1 shear rate increases gradually from 12.64 to 88.38 mPa·s. (c) Schematic diagram of 3D-printing process based on meniscus-guided printing. A 3D square MWNT microarchitecture (FE-SEM image) is printed by horizontally pulling a micronozzle filled with CNT ink (PVP-wrapped A-MWNT suspended in water), with water evaporating from the ink meniscus during the nozzle pulling.

methods, such as stereolithography (SLA),20 fused deposition modeling (FDM),21 or selective laser sintering (SLS).22 Finding the appropriate printing approach for the achievement of functional 3D structures with high spatial resolution remains one of the major challenges in the field of 3D-printed electronics. In this context, recent works have reported the production of functional 3D structures with micrometer-scale resolution using printing approaches involving inks. For example, Lewis et al. developed an extrusion-based 3D-printing approach using highviscosity inks for 3D-printed electronics.17,23 This technique requires ink with specific rheological (having transitions between solid- and fluid-like behaviors with respect to the applied shear stress) and viscoelastic properties in order to facilitate extrusion of the ink through a nozzle under applied pressure.17,23−27 If the ink viscosity is increased, a higher applied pressure is required during the extrusion step. After extrusion, the ink filament rigidity must quickly increase in order to facilitate shape retention. With the use of tailored silver nanoparticles and lithium metal oxide nanoparticle inks, these researchers have successfully printed stretchable spanning microelectrodes23 and lithium-ion microbatteries.17 Carbon nanotubes (CNTs)28 are an attractive nanomaterial for electronics because they exhibit good thermal and electrical properties, mechanical strength, chemical stability, and the potential for production at low cost and can be formulated into ink form and printed as conducting or semiconducting 2D layers in a device structure.29−33 With regard to 3D printing to achieve functional 3D structures, however, CNTs are primarily employed as additives to improve the conductivity of the host polymer materials used to retain the 3D shape. As the CNT loading in the printed structures is increased, the product properties are enhanced. Several strategies for the fabrication of CNT/polymer (CP) nanocomposite-based 3D structures have emerged. Advanced SLA

based on a two-photon absorption technique is prominent, being a powerful procedure for the manufacture of 3D nanocomposite structures with spatial resolution down to a few hundred nanometers.34 However, the low conductivity of these printed structures limits the application of this technique to functional electronics. Other research efforts have involved the fabrication of conductive CP 3D structures with resolutions of hundreds of micrometers using FDM and ultraviolet (UV)-assisted and extrusion-based 3D-printing methods. Therriault and co-workers reported the printing of CP 3D microcoils with a conductivity of ∼10−4 S m−1 via UV-assisted 3D printing, which was accomplished using UV-curable polyurethane nanocomposite ink containing 0.5 wt % CNTs and 5 wt % fumed silica.35 CP 3D microstructures with increased conductivity have also been fabricated using extrusion-based 3D printing.36 In that case, 10 wt % CNTs was dispersed homogeneously in thermoplastic polymer (PLA) using a high-volatility solvent, forming a CP 3D structure with a conductivity of ∼100 S m−1. Recently, 3DXTech proposed a commercial CP composite filament with 109 Ω surface resistivity for the 3D printing of CNT-enhanced polymer structures using a conventional FDM 3D printer. However, despite persistent efforts, all of the abovementioned approaches continue to suffer from difficulties in increasing the spatial resolutions and conductivities of 3D-printed CNT structures. In this work, we demonstrate a 3D-printing strategy to produce highly conductive 3D multiwall CNT (MWNT) microarchitectures with a high MWNT concentration of ∼75 wt %. Our approach is based on the precise additive deposition of MWNTs via meniscus-guided printing37−40 with CNT aqueous ink having fluid-like behavior at room temperature, which is composed of MWNTs and polyvinylpyrrolidone (PVP). The PVP is subsequently removed via thermal treatment. The CNT fluid ink is supplied continuously through a 8880

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diameter (Dn) = 12 μm) without applying any pressure. Sufficient stress was applied by the nozzle pulling, enabling PVP-wrapped A-MWNT flow in the nozzle. When the nozzle made contact with the substrate in the initial printing stage, an ink meniscus was created outside its opening. The meniscus height was decided by a gap (Gns) between the nozzle and the substrate. As the micronozzle was pulled horizontally with a 75 μm s−1 pulling rate (υh), the lateral layers were solidified in accordance with the square movement of the nozzle via rapid evaporation of water from the ink’s meniscus. This led to the formation of a 3D hollow microarchitecture. It is reasonable that the accelerated water molecule evaporation was induced by the very high surface area to volume ratio achieved in the microsized meniscus, leading to a significant increase in the area exposed to air. In this manner, it is possible to construct complex 3D architectures in a layer-by-layer process, using a continuous supply of ink with a fluid-like behavior. Figure 2a shows the CNT fluid ink printability as a function of the PVP concentration. Two-dimensional lines were drawn at

micronozzle by pulling the nozzle, with no applied pressure. During the pulling process, 3D structures are formed and retained as a result of rapid solvent (water) evaporation from the ink meniscus. We successfully produce a variety of 3D MWNT microstructures: a pillar array, a freestanding bridge, variously shaped walls, and a zigzag structure consisting of 2D lines and 3D bridges on a curved substrate. We describe three applications of the MWNT structures printed with this methodology, that is, a gas sensor transducer, a point emitter, and a radio frequency (RF) inductor.

RESULTS AND DISCUSSION Rheological Properties of CNT Inks for MeniscusGuided Printing. First, we developed a printable CNT aqueous ink for use with the 3D-printing technique based on meniscusguided printing. Ink with the appropriate rheological properties for reliable flow through the micronozzle was obtained via acid treatment of MWNTs and the addition of PVP as a stabilizer and a rheology modifier. During CNT ink preparation, it is critical to uniformly disperse the CNTs, which tend to agglomerate in an aqueous solution via van der Waals interactions. Acid treatment of MWNTs can generally improve their dispersion in a solution because the introduction of functional groups, such as carboxylic acid (−COOH) and hydroxyl (−OH), on the MWNT surface tends to enhance the ionic character of the nanotubes (Figure S1, Supporting Information).41 The pure acid-treated MWNT (A-MWNT) ink (7 wt % A-MWNT) developed in this study exhibited a viscosity of 12.64 mPa·s (at 10 s−1 shear rate), which is an order of magnitude higher than that of water. The ink also exhibited a clear shear thinning behavior, occurring from the rearrangement of MWNTs under shear stress (Figure 1a). However, the A-MWNT suspension did not possess the required rheological behavior for 3D-printable ink, which would allow continuous flow through the micronozzle and printed structure shape retention. Thus, PVP was added to the A-MWNT suspension to improve the CNT ink printability (Figure S2, Supporting Information). PVP is a hydrophilic watersoluble polymer; here, it was noncovalently grafted onto the CNT surface through wrapping. In general, the PVP improved the steric stabilization of the CNT via the resultant presence of hydrophilic chains on the CNT surface.42 As the PVP content increased, the ink viscosity, which exhibited shear thinning behavior, also gradually increased. The viscosity range of the developed ink was slightly higher than that of typical inkjet inks (2−30 mPa·s). The storage (G′) and loss (G″) moduli of inks with different PVP concentrations are shown in Figure 1b. Although the G′ of the inks increased by an order of magnitude when the PVP concentration was increased to 25 wt %, all inks exhibited fluid-like behavior (G′ is lower than G″) as a function of the shear stress. In extrusion-based 3D printing, which is most representative method employing functional ink, low-viscosity fluid ink is inappropriate for the printing of 3D structures.17,23−27 This is because the ink immediately spreads upon exiting the nozzle under an applied pressure and lacks self-supportability. Here, we successfully printed 3D MWNT microstructures using the developed CNT fluid ink by employing meniscus-guided printing. Figure 1c comprises a schematic diagram and field emission scanning electron microscopy (FE-SEM) images showing the 3D printing of a 3D MWNT square microarchitecture with a hollow feature. The layer-by-layer printed hollow structure was obtained by horizontally pulling the ink-filled (7 wt % A-MWNT, 17 wt % PVP) micronozzle (nozzle inner

Figure 2. (a) FE-SEM images of printed 2D line patterns. Lines with uniform features are drawn for 17 and 25 wt % PVP concentrations. All scale bars are 20 μm. After thermal treatment at 450 °C for 1 h in vacuum, the MWNTs embedded in the PVP are exposed via PVP removal. (b) TGA curves for CNT inks with A-MWNT (7 wt %) and with PVP-wrapped A-MWNT (7 wt % A-MWNT, 17 wt % PVP). Inset: Magnification of gray region in the TGA curve.

room temperature on platinum (Pt)-coated silicon (Si) substrate (Dn = 20 μm, υh = 75 μm s−1, Gns = 2 μm). Line patterns with widths (Ws) of 24 μm and thickness of 2 μm were obtained uniformly for PVP content values of 17 and 25 wt %, using a 8881

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ACS Nano continuous ink flow through the micronozzle with no applied pressure on the nozzle rear side. However, the ink flowed unstably at less than 17 wt % PVP, yielding discrete and nonuniform line patterns (Movie S1, Supporting Information). The improved printability with increased PVP concentration can be explained by the inducement of a smooth MWNT flow without blockage in the nozzle, which is due to improved dispersion stability and increased ink viscosity. Ink with 17 wt % PVP content was selected as being optimal for printing, as this ink possessed adequate rheological properties and the minimum PVP concentration for printability. Note that it is critical to reduce the PVP concentration necessary for the printing process because the PVP must be removed in order to improve the electrical properties of the printed structure. After the printing, thermal treatment was conducted at 450 °C for 1 h in vacuum in order to remove the PVP via decomposition. We investigated the weight changes of pure A-MWNT (7 wt % A-MWNT) and PVP-wrapped A-MWNT (7 wt % A-MWNT and 17 wt % PVP) inks using thermogravimetric analysis (TGA), using the same heating procedure as that of the thermal treatment (Figure 2b). The temperature, which was increased from 25 to 450 °C at a 10 °C/min heating rate, was maintained at 450 °C for 1 h. The ink weight loss at approximately 100 °C was associated with water evaporation, yielding 7 wt % A-MWNT and 24 wt % PVP-wrapped A-MWNT. The A-MWNT and PVP content values were in good agreement with those of the designed inks. At a constant temperature of 450 °C for 1 h, 2 wt % loss of A-MWNT was apparent, resulting from the elimination of organic moieties such as −COOH and −OH functionalities. On the other hand, the PVP-wrapped A-MWNT content was reduced from 24 to 6.6 wt % under the same heating conditions because of PVP decomposition and elimination of the functional groups. This indicated that the thermal-treated final structure had 75 wt % MWNT and 25 wt % PVP solid content. In the enlarged FE-SEM images shown in Figure 2a, it is clearly apparent that the MWNTs embedded in the PVP were exposed after thermal treatment, due to removal of the PVP in the printed lines. We also studied the water stability of the printed 2D pattern before and after thermal treatment (Figure S3, Supporting Information). For the water stability test, deionized (DI) water microdroplets (yellow dotted circles of 150 μm in diameter) were dropped on the printed patterns by precisely controlling the micronozzle. The as-printed pattern was destroyed immediately after the water was dropped, forming a large ring due to the coffee-ring effect. Interestingly, the thermally treated pattern was preserved with no deformation. That is, the printed pattern had enhanced hydrophobicity due to removal of the hydrophilic PVP and reduction of the functional groups on the A-MWNT during the thermal treatment. Three-Dimensional Printing of CNT Microarchitectures. Several different 3D MWNT microarchitectures were successfully fabricated using different Dn values, which ranged from 8 to 30 μm (Figures 3−5). The printed 3D objects maintained their shapes, exhibiting a total volumetric shrinkage of ∼30% after thermal treatment was performed at 450 °C for 1 h in vacuum. Figure 3a shows an array of freestanding MWNT pillars with 9 μm diameter (Ds) consisting of 121 pillars with a 30 μm interdistance. Each pillar was produced via vertical pulling (vertical pulling rate, υv = 20 μm s−1) of the micronozzle (Movie S2, Supporting Information). A MWNT microbridge was also fabricated in three steps: contact, vertical pulling,

Figure 3. (a) FE-SEM image of an array of 121 freestanding MWNT pillars (Ds = 9 μm). (b) FE-SEM image of a suspended junction pattern consisting of MWNT bridges (Ds = 9 μm) and straight lines (Ws = 13 μm). Labels 1−3 denote the printing sequence. (c) I−V characteristics of the word “KERI” fabricated using three bridges (Ds = 9 μm) and lines (Ws = 13 μm). The applied voltage ranged from −1.0 to +1.0 V in 20 mV steps. FE-SEM and optical images show the printed “KERI” on a polyimide substrate and an LED lamp activated using the structure as an electrical interconnect. (d) FE-SEM images of a concatenated MWNT bridge-line structure on curved glass substrate with an 8.4 mm curvature radius.

and attachment. A suspended junction pattern composed of MWNT bridges (Ds = 9 μm) and straight lines (Ws = 13 μm) was printed by assembling three concatenated MWNT bridge-line structures (Figure 3b and Movie S3, Supporting Information). In order to test the stability of the printed bridge, the bridge was pushed toward the substrate using a precisely controlled micronozzle. When the central point of the bridge made contact with the substrate, the nozzle was moved upward and the original shape was restored (Movie S4, Supporting Information). The FE-SEM images at the top of Figure 3c show the word “KERI”, with the letters being fabricated using three bridges (Ds = 9 μm) and lines (Ws = 13 μm). The letters with 5 mm interconnecting lines between two silver electrodes were printed 8882

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ACS Nano sequentially in a left-to-right direction on polyimide (PI) substrate. The linear current−voltage (I−V) curve in the right plot of Figure 3c shows the presence of an ohmic contact between the MWNT pattern and the silver electrodes. The MWNT letters had an electrical conductivity of ∼2540 S m−1 at room temperature. They could be used to activate a lightemitting diode (LED) lamp, as shown in the photograph at the bottom of Figure 3c (and Movie S5, Supporting Information). The performance of the MWNT bridge-line structure as a stretchable interconnect was tested for a bridge with Ds = 9 μm, which was printed between PI substrates with a variable gap (Lg). A strain was applied by increasing Lg, and the corresponding resistance variation of the bridge-line structure was measured. The resistance showed a negligible variation for an applied strain (ΔLg/Lg0) up to 105%. Under the applied ΔLg/Lg0, the MWNT bridge shape changed in a manner that avoided the generation of substantial strain on the bridge itself (Figure S4, Supporting Information). Another interesting architecture is the concatenated MWNT bridge-line structure printed on curved glass substrate with an 8.4 mm curvature radius shown in Figure 3d. For 3D-printed electronics, it is important to build functional 3D architectures on a nonflat substrate. The details observed at higher magnification (Figure 3d, bottom) indicate a zigzag structure consisting of MWNT bridges with Ds = 9 μm at the peak curvature region of the substrate. In addition to bridge and vertical pillars, complex 3D objects can be printed in a layer-by-layer manner by overlaying using repetitive horizontal pulling (υh = 75 μm s−1) of the micronozzle. For layer-by-layer deposition, the nozzle was alternately pulled rightward and leftward, with precise vertical movement (∼1.2 μm) of the nozzle between each pull (Movie S6, Supporting Information). Figure 4a shows a FE-SEM image of an alphanumeric structure, “3D”, which was printed by adjusting the nozzle movement along the x- and y-axes during the MWNT stacking process on the z-axis. These alphanumeric structures consist of flat and round walls with a thickness (t) of 10 μm and height (h) of 24 μm. Printing with a stepwise MWNT deposition yielded a four-stair structure with orthogonal features. Each stair (with t = 10 μm) had h ∼ 12 μm, which was achieved using 12 print repetitions (Figure 4b). Figure 4c presents a 3D crossshaped structure with an elliptical hollow feature consisting of round walls with t = 10 μm and h = 40 μm. The round-wall curvature could be adjusted via accurate control of the nozzle. A slight protrusion of the cross center compared to the h of the other parts was formed because of the repeated printing at the intersection part of the cross. Functional 3D MWNT microarchitectures fabricated using the described 3D-printing technique may be used as core components in a wide range of different devices. For example, the performance of a 3D MWNT microbridge as a transducer in a gas sensor was examined. This device was assembled by printing a bridge with Ds = 5 μm between two platinum electrodes separated by a 10 μm insulating gap (Figure 5a). The transducer performance was characterized by measuring its electrical resistance upon exposure to ammonia (NH3) at room temperature. Gas sensing essentially depends on electron transfer reactions, which are redox reactions. The MWNT is dominated by the presence of positively charged carriers (holes), whereas the NH3 is primarily an electron donor.43 Electron transfer from the adsorbed NH3 to the MWNT yields a decrease in the hole concentration, resulting in an increase of the electrical resistance of the MWNT bridge. Figure 5b shows the response of the

Figure 4. FE-SEM images of 3D wall architectures with different shapes. (a) “3D” shaped wall (thickness (t) = 10 μm and height (h) = 24 μm). (b) Four-stair structure with orthogonal feature (t = 10 μm, h = 48 μm). (c) Three-dimensional cross-shaped structure with elliptical hollow feature (t = 10 μm, h = 40 μm). All scale bars are 20 μm.

transducer constructed using the MWNT bridge to variation of the NH3 concentration in the 10−70 ppm range. Although the sensitivity of the gas sensor examined in this work remained low, the response (ΔR) was dependent on the NH3 concentration, and the resistance changes were essentially observed to be reversible at room temperature, except for a relatively low upward drift in the initial resistance, R0. We also demonstrated the use of a 3D MWNT wall as a point emitter. The wall (t = 6.5 μm, h = 30 μm, and length (Ls) = 70 μm) was fabricated on the ITO glass substrate (Figure 5c). The current density−electric field (J−E) characteristic of the MWNT wall is shown in Figure 5d. To remove the emission noise and instability, electrical aging consisting of several steps (I−V sweeping and high-voltage annealing) was conducted until the emission currents exhibited negligible fluctuations. The 3D MWNT wall demonstrated a turn-on field (1.72 V/μm at 10 μA/cm2), which was lower than those reported for CNT field emitters.44,45 Figure 5e shows the Fowler−Nordheim (F−N) plot of the 3D MWNT wall. From F−N theory, the geometrical field enhancement factor of the emitters is inversely proportional to the slope of the F−N plot (β = −BΦ1.5S−1, where B is a constant, Φ is the work function, and S is the slope). The calculated 8883

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Figure 5. (a) FE-SEM image of MWNT bridge (Ds = 5 μm) printed on Pt electrodes separated by a 10 μm insulating gap. (b) Resistance change (response) of sensor assembled using a single MWNT bridge over time, during exposure to 10−70 ppm of NH3 at room temperature. (c) FE-SEM image of a MWNT wall (t = 6.5 μm, L = 70 μm, h = 30 μm). (d) J−E characteristics of a MWNT wall. (e) F−N plot of a MWNT wall. (e, Inset) Optical image showing luminescent appearance on the phosphor substrate. (f) FE-SEM image of printed RF inductor consisting of 2D transmission lines (line width = 35 μm and line spacing = 100 μm) and 3D bridge-type interconnections (Ds = 18 μm) on a glass substrate. (g,h) Inductance, Q factor, and alternating current (ac) resistance of printed inductor as functions of frequency (Hz).

field enhancement factor of the 3D MWNT wall was 6204, assuming that the work function was 5.0 eV (equal to that of graphite) for the 3D MWNT wall. An emission image of the 3D MWNT wall is shown in the inset of Figure 5e (and Figure S5, Supporting Information), where the applied electric field and emission current density are 2.36 V/μm and 0.2 mA/cm2, respectively. One can clearly observe a bright electron emission spot from the 3D MWNT wall. These high-field emission characteristics of the 3D MWNT wall are likely due to the uniform length, high density, and strong interaction of the CNT emitters with the substrate. A further interesting application is the fabrication of an RF inductor consisting of 2D transmission lines (line width = 35 μm, line spacing = 100 μm) and 3D bridge-type interconnections (Ds = 18 μm) on a glass substrate (Figure 5f). The quality (Q) factor of the inductor at the operating frequency (ω), which is defined as the ratio of its reactance to its resistance, is a typical value indicating the device performance that can be expressed as

Q=

ωL R

ranging from a few tens of ohms to a few kiloohms at the examined operating frequencies, are 100 times higher than those of RF inductors fabricated on the Si substrate. Because the skin effect is a dominant factor determining the high-frequency resistance, a high-Q inductor can be fabricated using an additional process to improve the surface conductivity of the nanostructures or pasting materials.47 These results show that a printed RF inductor can be easily designed to meet certain electrical application specifications.

CONCLUSIONS We have shown that the fabrication of highly conductive MWNT microstructures that almost fully comprise MWNTs (∼75 wt %) can be realized at room temperature via meniscus-guided 3D printing with CNT fluid ink, which is composed of PVP-wrapped MWNTs, and by subsequent removal of the PVP via thermal treatment. The key factor for the successful 3D printing of MWNT structures is modification of the MWNT suspension to have uniform dispersion and appropriate rheological properties for flow through a confined nozzle geometry. In our approach, the CNT fluid ink is supplied continuously through the micronozzle, simply by pulling the nozzle; there is no pressure application. During the pulling process, 3D structures are formed and retained as a result of rapid water evaporation from the ink meniscus. Successful 3D printing of layer-by-layer architectures using a fluid ink of ∼100 mPa·s viscosity is of great importance for expansion of 3D-printable inks and available materials in this field. In general, fluid ink with low viscosity is regarded as being

(1)

where L is the effective inductance in Henrys and R is the effective resistance in ohms. The Q factor of the 3D-printed inductor in Figure 5g is approximately 2 in the frequency region, and the maximum operating frequency of the inductor is 1.5 GHz because of the increased effect of the parasitic capacitance over the frequency. The measured Q factor is similar to those reported in previous studies on RF inductors, although the inductance is higher in this case.46 The AC resistances shown in Figure 5h, 8884

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placed in a homemade chamber with inlet and outlet pipes for gas flow. Mass flow controllers (ATOVAC AFC500) were used to regulate the nitrogen (N; 99.999% purity) and ammonia gas flow. The NH3 concentration was varied between 10 and 70 ppm by adjusting the NH3/ N rate. Pure N was used to purge the system from any residual NH3 and allowed the sensor resistance to return to the baseline condition (R0); however, a relatively low upward drift did occur. The sensor response was defined as ΔR, where R0 was the resistance of the bridge before exposure to NH3 and ΔR was the change in resistance induced by exposure to NH3. The electrical resistance of the sensor was measured under a bias voltage of 1 V using a semiconductor characterization system (Keithley 4200-SCS). Electron field emission studies of the printed 3D MWNT wall were performed in a diode-type system with a dc bias inside a vacuum chamber, where the base pressure was 5 × 10−7 Torr. A 3D MWNT wall as a point emitter was printed on ITO glass and 500 μm thick glass and phosphor-coated glass substrates were used as the spacer and anode, respectively. For the RF inductor application, the device under test was fabricated by assembling a printed circuit board (PCB) and glass substrate (1 × 1 mm2) with a printed MWNT inductor. The transmission line on the PCB was connected to the signal pattern printed on the glass substrate using silver paste. The inductance and quality (Q) factor of the printed inductor were characterized by measuring the two-port scattering (S) parameter with a vector network analyzer (E5071C, Agilent Tech.).

unfit for 3D printing because the ink spreads immediately upon exiting the nozzle and lacks self-support capacity. The spatial resolutions (a few micrometers) and conductivities (∼2540 S m−1) of the printed MWNT structures obtained in this study, which were achieved using simple strategies, render our method fully competitive with the state-of-the-art results obtained via SLA, FDM, and the extrusion-based 3D-printing method. Further, the 3D-printed MWNT microstructures obtained in this study exhibit ideal properties for the production of components for electrical devices, such as sensing transducers, point emitters, and RF inductors. We are currently conducting a full characterization of the printing process in order to increase the conductivities of the printed structures and to reduce the printed structure size to nanometer scale using a stretched meniscus of fluid ink by modulation of the pulling process. For example, variations of the ink formulation and the effects of the nozzle opening size and pulling rate are being investigated. We believe that this approach is quite effective for the highresolution 3D printing of functional CNT structures for 3Dprinted electronics applications.

METHODS

ASSOCIATED CONTENT

Preparation and Characterization of CNT Ink. The MWNT and PVP (molecular weight (Mw) = 10 000) used in this study were purchased from Iljin Nanotech (Korea) and Aldrich (USA), respectively. For the MWNT acid treatment, the MWNTs were suspended in a mixture of condensed H2SO4 and HNO3 (3:1 volume ratio) and heated under reflux at 70 °C for 24 h. The acid-treated MWNTs (A-MWNTs) were washed several times with deionized water until the pH of the rinsing water was neutral and then dried in a vacuum oven at 100 °C for 24 h. CNT inks were prepared by mixing A-MWNT (7 wt %) and different PVP concentrations (7, 10, 17, 25 wt %) in water. The ink rheological properties were characterized using a rheometer (MCR102, Anton Paar) equipped with a cone-and-plate geometry. A strain sweep was conducted from 10 to 102 s−1 in order to measure the ink viscosity at varying shear rate. A stress sweep at a constant frequency of 1 Hz was also performed in order to record the variations in the storage and loss moduli as functions of the sweep stress. The MWNT and PVP mass loss was estimated via TGA (TA Instruments, Q600, USA). The samples were heated to 450 °C in alumina crucibles at a 10 °C/min heating rate. They were then maintained at 450 °C for 1 h in an argon atmosphere (flow speed = 100 mL/min). Three-Dimensional Printing. Three-dimensional MWNT microarchitectures were fabricated at the meniscus formed at the tip of a micronozzle filled with an aqueous ink comprising A-MWNT (7 wt %) and PVP (17 wt %). Glass micronozzles with opening diameters of approximately 8 and 30 μm were obtained using a nozzle puller (P-97, Sutter Instrument). The CNT ink was introduced at the back of the micronozzle and drawn to the front tip by capillary forces with no applied pressure. The micronozzle position and pulling rate during the 3D printing were accurately controlled using three-axis stepping motors with 250 nm positioning accuracy. The growth process was observed in situ using a high-resolution monitoring system consisting of an optical lens (200×) and a charge-coupled device camera (Spot RT Xplore) (see Figure S6, Supporting Information). Platinum-coated Si wafers, polyimide, and curved glass were used as substrates for the MWNT structures. The as-printed MWNT structures were heated at 450 °C for 1 h in vacuum. MWNT Architecture Characterization. The microscopic characteristics of the MWNT architectures were analyzed using FE-SEM (HITACHI S-4800). The current−voltage (I−V) characteristics of the individual MWNT structures were measured using a two-probe method with a Keithley 2612A instrument at room temperature. A sensor produced by printing MWNT microbridges on two Pt-coated Si electrodes with a 10 μm gap was tested at room temperature. Lead wires were attached to the electrodes with silver paste and used to obtain an electrical connection with the measurement system. The sensor was

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04771. FTIR spectra of MWNTs, MWNT dispersion, water stability of printed pattern, stretchable behavior of printed MWNT microbridge, emission of 3D MWNT wall, experimental 3D printing apparatus (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI) Movie S5 (AVI) Movie S6 (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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