Assembly of Highly Aligned Carbon Nanotubes Using an Electro

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Assembly of Highly Aligned Carbon Nanotubes Using an Electro-Fluidic Assembly Process Zhimin Chai, Jungho Seo, Salman A. Abbasi, and Ahmed Busnaina ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06176 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Assembly of Highly Aligned Carbon Nanotubes Using an Electro-Fluidic Assembly Process Zhimin Chai,‡ Jungho Seo,‡ Salman A. Abbasi, Ahmed Busnaina* NSF Nanoscale Science and Engineering Center for High-Rate Nanomanufacturing (CHN), Northeastern University, Boston, Massachusetts 02115, United States *Corresponding Author: E-mail addresses: [email protected]

ABSTRACT

Carbon nanotubes (CNTs) are promising building blocks for emerging wearable electronics and sensors due to their outstanding electrical and mechanical properties. However, the practical applications of the CNTs face challenges of efficiently and precisely placing them at the desired location with controlled orientation and density. Here, we introduce an electro-fluidic assembly process to assemble highly aligned and densely packed CNTs selectively on a substrate with patterned wetted areas at a high rate. An electric field is applied during the electro-fluidic assembly process, which drives the CNTs close to the patterned regions and shortens the assembly time. Meanwhile, the electric field orientates the CNTs perpendicular to the substrate and anchors one end of the CNTs onto the substrate. When pulling the substrate out of the CNT suspension, the

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capillary force at the air-water-substrate interface stretches the free end of the CNTs and aligns the CNTs along the pulling direction. By adjusting two governing parameters, the direct current voltage and the pulling speed, we have demonstrated well aligned CNTs assembled in patterns with width from 1 µm to 100 µm and length from 20 µm to 120 µm at a rate 20 times higher than a fluidic assembly process. The aligned CNTs show improved electrical conductivity compared with the random networks and prove possibility for strain detection. Precise and reproducible control of the orientation and the placement of the CNTs opens up their practical application in the next generation electronics and sensors.

KEYWORDS: carbon nanotube, assembly, alignment, electric field, fluidic

Owing to outstanding electrical and mechanical properties, carbon nanotubes (CNTs) have been promising building blocks for electronics,1-3 supercapacitors4,

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and sensors.6-9 High flexibility

makes CNTs more promising for emerging bendable electronics such as wearable humaninteractive devices.10, 11 However, due to a high electrical heterogeneity and a low current-carrying capacity,12 an individual CNT faces a significant bottleneck in its practical application in functional devices. To achieve reproducibility in device fabrication, CNT films should be utilized because a large number of CNTs average the heterogeneity effect and provide good device-todevice uniformity as well as a high current output. To take advantage of their excellent intrinsic electrical properties, CNTs should be well aligned to enhance electron transportation and decrease inter-nanotube junctions.13 Aligned CNTs can be grown in situ by chemical vapor deposition (CVD) using directing gas flow,14 miscut crystalline substrates15 and patterned catalysts.16 However, CVD, a high


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temperature (500 to 900 °C) process, is incompatible with flexible substrates. Moreover, during the CVD process, CNTs with different diameters, lengths and chiralities are grown simultaneously, leading to high electrical heterogeneity and deteriorating device performance. An alternative method to the vapor-phase deposition is the solution-based post-synthesis assembly. The solutionbased assembly process has various advantages such as room temperature operation, no vacuum, low cost, ease of processing, and good compatibility with a variety of substrates. In addition, highly purified CNTs with controlled diameters, lengths and chiralities could be utilized.2, 17 Alignment of the solution-based assembly involves dispersing CNTs in suspensions and then assembling and aligning them by means of spin coating,18 Langmuir–Blodgett,19 microchannels,20,

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evaporation,22-25 electric field,26-28 magnetic field,29 chemical functionalization,30, 31 gas flow,32 mechanical shearing33, 34 and blown-bubble.35 Unlike the aforementioned alignment techniques, template guided fluidic assembly developed in our group relies on hydrophobic/hydrophilic property and confining channel geometry of patterned templates to direct assembly and achieve alignment.36-38 Even though our fluidic assembly method is able to provide uniform assembly at a wafer scale,39 the degree of alignment strongly depends on the relative size of the length of the nanotubes and the width of the channel. Only when the width of the channel is smaller than the length of the nanotubes, unidirectional alignment of CNTs can be accomplished. In many functional cases, uniaxial alignment in arbitrary patterns is needed. In addition, the fluidic assembly process is a diffusion-limited and time-consuming process. Hence, a way to significantly increase the assembly rate is required. Here, we present an electro-fluidic assembly method that utilizes a direct current (DC) electric field to speed up the assembly process while increasing the alignment. The applied electric field is capable of driving nanotubes close to the patterned substrate, which increases the local nanotube


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concentration and shortens the assembly time.40 Meanwhile, the electric field orientates the nanotubes perpendicular to the substrate and anchors one end of the nanotubes to the substrate. As the substrate is pulling up, the free end of the nanotubes is stretched by the capillary force, leaving behind well aligned nanotube arrays in the pulling direction. Using the electro-fluidic assembly process, we have demonstrated assembly of highly aligned CNTs in channels with arbitrary dimensions at a rate 20 times faster than the fluidic assembly process. The well aligned CNTs show improved electrical conductivity compared with random networks. The application of aligned CNTs in strain detection is explored. RESULTS AND DISCUSSION The template-guided fluidic assembly process has been employed to assemble and align CNTs.36-38 However, fluidic assembly is a time-consuming process which hampers its practical application in mass production. Moreover, the degree of nanotube alignment depends on the pattern geometry. In this section, we introduce an electro-fluidic assembly process that utilizes a DC electric field to speed up the assembly and enhance the alignment of the nanotubes. Patterned substrate preparation and the electro-fluidic assembly of the CNTs. Figure 1 presents a schematic illustration of the patterned substrate fabrication and the electro-fluidic assembly of the CNTs. Initially, a photoresist film was spun coated on a piranha cleaned silicon (Si) wafer with a 500 nm thermal oxide layer and then patterned via optical photolithography. Subsequently, a gold film was deposited on the patterned photoresist followed by a lift-off process to form two metal electrodes with spacings from 10 µm to 100 µm. One of the electrodes was used to apply an electric field for the electro-fluidic assembly and the other electrode was used to measure electrical properties of the assembled CNTs. Following this, a poly (methyl methacrylate) (PMMA) film was spun coated on the substrate and subsequently patterned using electron beam


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(E-beam) lithography to obtain channels 1 µm to 100 µm in width and 20 µm to 120 µm in length. Afterwards, the electro-fluidic assembly was performed by applying a DC electric field between the patterned substrate and a counter electrode in a CNT suspension. The DC electric field generated an electrophoretic force on the CNTs, attracting them to the patterned substrate. After assembly, the PMMA was striped by acetone, leaving behind well aligned CNTs.

Figure 1. Schematic illustration of the patterned substrate fabrication and the electro-fluidic assembly of the CNTs. Mechanism of electro-fluidic assembly of aligned CNTs. Prior to discussing the mechanism of electro-fluidic assembly, an overview of the fluidic assembly is given. Figure 2 (a) shows a schematic illustration of the fluidic assembly process. When the patterned substrate is immersed in the CNT suspension, a meniscus is formed at the three-phase (air-water-substrate) contact line. Water in the meniscus evaporates, which induces an upward convective flow to the contact line to compensate for the lost water. The convective flow carries CNTs to the contact line and results in assembly of CNTs on the substrate. The patterned substrate has different wetting areas, wherein the photoresist or PMMA defining the pattern is hydrophobic and the Si/SiO2 substrate cleaned by piranha solution is hydrophilic. CNTs are selectively assembled in the hydrophilic microchannels, leaving the hydrophobic area with no assembly. Typically, the assembled CNTs are aligned along the three-phase contact line.22, 41 Because of the hydrophobic/hydrophilic property of the patterned


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substrate, the contact line is not straight41 (Figure 2 (a), (b) and (c)), which results in varying orientation of the CNTs. The CNTs assembled at the edge of the channel are aligned along the channel and those in the center are aligned perpendicular to the channel (Figure 2 (b)). The overall orientation of the CNTs depends on the width of the channel. A narrower channel shortens the contact line lying in the channel, which makes it difficult to fit CNTs perpendicular to the channel and results in more CNTs aligning along the channel. When the width of channel is smaller than the length of the CNT (1.5 μm), almost all the CNTs are aligned along the channel (Figure 2 (c)). Besides the contact line confinement, another factor that contributes to the alignment of CNTs is the capillary force. For a narrow channel, one end of the CNTs anchors in the hydrophilic channel and the other end floats in the suspension. When pulling the substrate out of the suspension, the floating end of the nanotubes is stretched by the capillary force, which leads to aligning of the nanotubes along the pulling direction (parallel to the channel). Whereas, for the wide channel, both ends of the nanotubes anchor in the channel and the capillary force has almost no effect on the alignment.


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Figure 2. (a) Schematic illustration of the fluidic assembly of CNTs. For the fluidic assembly process, channel patterns were defined on the Si/SiO2 substrate directly without patterned gold electrodes. Typically, the assembled CNTs are aligned along the air-water-substrate contact line. Because the channel is hydrophilic and the photoresist is hydrophobic, the contact line is not straight, which makes the nanotube alignment pattern geometry dependent. When the width of the channel is larger than the length of the CNTs, the CNTs assembled at the edge of the channel are aligned along the channel and those in the center are aligned perpendicular to the channel (b). When the width of the channel is smaller than the length of the CNTs, all the CNTs are aligned along the channel (c). It should be mentioned that in (b) and (c) only the CNTs at the contact line


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are shown for clarity. (d) SEM images of the assembled CNTs (average length 1.5 μm) in channels with various widths. The CNTs are assembled at a pulling speed of 0.05 mm/min. As mentioned above, the alignment of the fluidic assembly process depends on the relative size of the length of the nanotube and the width of the channel. We introduce the electro-fluidic process (Figure 3 (a)) to make the alignment geometry independent. In the electro-fluidic process, a DC electric field is applied between the patterned substrate and a counter electrode, which polarizes the CNTs in the suspension42 and orients them perpendicular to the substrate (Figure 3 (c)).28, 43 Under the electric field, the polarized CNTs move towards the patterned region on the substrate where the electric field intensity is high (see the simulation result in Figure 5 (b)) and anchor one of their ends on the substrate. When pulling the substrate out of the suspension, the free end of the CNTs is stretched by the capillary force, which aligns the CNTs in the pulling direction. Similar to fluidic assembly, the hydrophobic/hydrophilic property of the patterned substrate is necessary to direct the assembly in the channel area. However, before assembled in the channel, the CNTs orient perpendicular to the substrate instead of along the contact line. The contact line confinement is not effective in this case. In addition, regardless of the channel width, the CNTs always have one free end floating in the suspension before they are assembled, which facilitates the capillary force to align the CNTs. These two reasons make the alignment of the electro-fluidic assembly process geometry independent. As seen in Figure 3 (d), the CNTs assembled in channels with width from 1 µm to 100 µm and length from 20 µm to 120 µm exhibit good unidirectional alignment. Besides the enhanced alignment, the electro-fluidic assembly shortens the assembly time significantly. The electric field drives nanotubes close to the patterned substrate, which increases the local nanotube concentration and also diffusion velocity of the CNTs.39 To achieve full coverage of the channel region, the pulling speed (same as the assembly speed) for the fluidic


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assembly should be lower than 0.05 mm/min, while the pulling speed for the electro-fluidic assembly can be as high as 50 mm/min.

Figure 3. (a) Schematic illustration of the electro-fluidic assembly of CNTs. The DC electric field applied between the patterned substrate and the counter electrode polarizes the CNTs and orients them perpendicular to the substrate (c). Meanwhile, the electric field drives the polarized CNTs towards the patterned substrate and anchors one of their ends on the substrate. When pulling the substrate out of the suspension, the capillary force stretches the free end of the CNTs and aligns them in the pulling direction. (d) SEM images of the aligned CNTs in channels with width from 1


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µm to 100 µm and length from 20 µm to 120 µm. The CNTs are assembled at a DC voltage of 2 V at a pulling speed of 5 mm/min. To evaluate the degree of alignment for the CNTs assembled via the fluidic and the electrofluidic processes, alignment angles of 30 randomly selected CNTs for each assembly method are measured (Figure 4 (a)). The CNTs with an average length of 1.5 μm are assembled in channels with a width of 3 μm (Figure 4 (b)). The fluidic assembly process provides CNTs with varying orientations, from parallel to the channel direction at the edge of the channel to perpendicular to the channel direction in the middle of the channel, while the electro-fluidic assembly process gives unidirectionally aligned CNTs along the channel direction, with the alignment angle within ± 25°.

Figure 4. Degree of alignment for the CNTs assembled via the fluidic and the electro-fluidic processes. The pulling speed for the fluidic assembly is 0.05 mm/min, and the DC voltage and the


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pulling speed for the electro-fluidic assembly are 2 V and 5 mm/min, respectively. (a) Schematic diagram of the nanotube alignment angle (θ) with respect to the channel direction (pulling direction). θ = 0° means aligning along the channel. The CNTs with an average length of 1.5 μm are assembled in channels with a width of 3 μm. The alignment angles of 30 randomly selected CNTs for each assembly method are plotted. (b) SEM images of the CNTs assembled by fluidic and electric-fluidic processes. Control of electro-fluidic assembly. The electro-fluidic assembly process can be controlled by the applied DC voltage and the pulling speed. To investigate these two parameters, CNTs are assembled in channels 3 µm wide and 40 µm long under various DC voltages and pulling speeds. The DC voltage controls the electro-fluidic assembly via a DC electric field. The electric field leads to an electrophoretic (EP) force on the negatively charged CNTs, which drives the CNTs towards the electrode of positive charges (patterned substrate). At the same time, the electric field polarizes the CNTs and induces a dipole moment along the longitudinal axis of the CNTs. The polarized CNTs experience a dielectrophoretic (DEP) force and move to the patterned substrate where the electric field strength is high (see the simulation result in Figure 5 (b)). A larger DC voltage tends to generate a higher electric field strength and larger EP and DEP forces, resulting in more assembly of CNTs. Figure 5 (a) shows SEM images of the assembled CNTs under various DC voltages at a pulling speed of 5 mm/min. At a low DC voltage of 1.5 V, the electric field strength cannot generate sufficient EP and DEP forces. Only a few CNTs are assembled in the channel, and the assembly happens around the top gold electrode where the electric field is applied. When increasing the DC voltage to 2 V, the EP and DEP forces increase and the whole channel is fully filled with CNTs. Further increasing the DC voltage to 2.5 V, the EP and DEP forces become too high. The substrate around the channel has over assembled CNTs as well.


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The electric field distribution between the patterned substrate and the counter electrode is simulated using the Flow-3D software. Figure 5 (b) shows the electric field strength contour around the channel under a DC voltage of 2 V. The electric field strength is high at the patterned gold electrode due to the effect of localized electric field (Figure S1 (a))44, 45 and higher at the goldPMMA interface and the edge of the gold electrode due to the fringing electric field (Figure S1 (b)).40 Moving away from the gold electrode, the electric field strength decays. The pulling speed controls the electro-fluidic assembly by affecting the exposure time of the channels to the CNT suspension. A higher pulling speed gives less exposure time, resulting in less assembly and thinner films. Figure 5 (c) shows the thickness of the assembled CNTs at various pulling speeds under a DC voltage of 2 V. The CNTs films obtained at pulling speeds of 1 mm/min for the electro-fluidic assembly and 0.05 mm/min for the fluidic assembly show similar average film thickness of 20 nm, which means the electro-fluidic assembly can speed up the fluidic assembly for 20 times. It should be mentioned that the thickness of the assembled CNTs decreases along the channel. At a pulling speed of 5 mm/min, the thickness of the assembled CNTs decreases from ~ 19 nm at one end of the channel to ~ 8 nm at the other end of the channel. The thickness variation may arise from voltage drop on the assembled CNTs. In our setup, the gold working electrode is not continuous in the channel. When the gold electrode is pulled out of the suspension, assembled CNTs works as the real working electrode for subsequent CNT assembly. Because the resistance of the assembled CNTs is at least 1000 times larger than that of the gold electrode, there would be a large voltage drop on the assembled CNTs, which decreases the actual voltage applied to the CNT suspension and thus leads to less assembly. To prove this, current between the working electrode and the counter electrode is measured. The measured current is ~ 50 µA at the end of the assembly, which results in a resistance of 40 kΩ. The resistance consists of the resistance of the


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working and counter electrodes, the resistance of the assembled CNTs and the resistance of the suspension. Because of high conductivity, the resistance of the gold working and counter electrodes is only a few ohms, which can be ignored. The resistance of the assembled CNTs at the pulling speed of 5 mm/min is ~ 5 kΩ (Figure 5 (e)). Considering a series circuit, there exists a 0.25 V voltage drop on the assembled CNTs. The voltage drop on the CNTs decreases the voltage applied to the CNT suspension, which results in less assembly of CNTs and decreased film thickness. When the gold working electrode is continuous in the channel, the problem of nonuniform film thickness along the channel could be avoided because the gold electrode always works as the real working electrode. However, in this case, the gold electrode is under the assembled CNTs. To make a functional device, the assembled CNTs should be transfer printed to another insulating substrate. SEM images of the CNTs assembled under a DC voltage of 2 V at various pulling speeds are shown in Figure S2. For each pulling speed, the alignment angles of 30 randomly selected CNTs are measured, and the alignment angle distribution is shown in Figure 5 (d). For all the pulling speeds investigated, the alignment angles are within ± 30°. The CNTs used in our paper are unsorted single walled carbon nanotubes (SWNTs) with 1/3 metallic nanotubes mixed with semiconducting ones. When the CNTs are densely packed, the metallic nanotubes form a percolating network that behaves like a conducting film.46 The aligned CNTs assembled at a pulling speed of 1 mm/min (average thickness 20 nm) have a conductivity of up to 140 kS/m, as seen in Figure 5 (e), which is higher than 11 kS/m47 reported previously.


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Figure 5. CNT assembly in channels 3 µm wide and 40 µm long under various DC voltages and pulling speeds. (a) SEM images of the CNTs assembled under various DC voltages at a pulling speed of 5 mm/min. (b) The electric field strength contour around the channel under a DC voltage of 2 V. (c) Thickness of the CNTs along the channel assembled at various pulling speeds at a DC voltage of 2 V. Only the CNTs between the two gold electrodes were measured. (d) Alignment angles of the CNTs assembled at various pulling speeds at a DC voltage of 2 V. (e) Resistance and conductivity of the CNTs assembled at various pulling speeds at a DC voltage of 2 V. Electrical characterization of the aligned CNTs. To demonstrate the superior electrical properties of the aligned CNTs, the resistance of the CNTs assembled by the fluidic and the electrofluidic methods is measured. The width, length and thickness of the CNT channels are 3 µm, 40 µm and 20 nm, respectively. 150 nm thick gold electrodes deposited by E-beam evaporation are utilized to probe the CNTs. However, contact resistance exists between the gold electrodes and the CNTs because of impurities at the gold-CNT interface.48 To get intrinsic electrical property of the CNTs, the contact resistance is excluded from the overall resistance employing the three electrodes measurement.48 The layout of the electrodes labeled A, B and C is illustrated in Figure 6 (a). The


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distance between two adjacent gold electrodes is 10 µm. Two-terminal resistance (RAB, RBC and RAC), which contains the resistance of the CNTs and the contact resistance, is measured first. Then, assuming the contact resistance between the CNTs and each electrode is approximately equal, the contact resistance is extracted from the overall resistance as 𝑅c =

𝑅AB + 𝑅BC ― 𝑅AC 2

(1)

where RAB = RSAB + 2Rc, RBC = RSBC + 2Rc and RAC = RSAC + 2Rc. RSAB, RSBC and RSAC are intrinsic resistances of the CNTs between each pair (A-B, B-C and A-C) of gold electrodes. All the measured and calculated resistances are listed in Table 1. The contact resistance between the gold electrode and the fluidic assembled CNTs is 0.91 kΩ, which is close to 1 kΩ reported previously,48 while the contact resistance between the gold electrode and the electric-fluidic assembled CNTs is 0.11 kΩ. Considering a channel width of 3 m, the width normalized contact resistances (RcW) are 2.73 kΩ m and 0.33 kΩ m, respectively. The normalized contact resistance is on the same order with that reported previously.49 Moreover, the CNTs assembled by the electro-fluidic method show a ~5 times lower resistance than those assembled by the fluidic method. The low resistance of the aligned CNTs can be attributed to the decreased inter-nanotube junctions and conduction path length.13

Figure 6. Electrical characterization of the assembled CNTs using the three electrodes method. (a) Layout of the three gold electrodes. (b) SEM image of the CNTs and the gold electrodes.


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Table 1. The measured and calculated resistances of the CNTs assembled by the fluidic and the electro-fluidic methods. Two-terminal resistance (kΩ)

Assembly methods

Resistance of the CNTs (kΩ)

Contact resistance (kΩ)

RAB

RBC

RAC

RSAB

RSBC

RSAC

Fluidic

7.25

6.56

12.00

5.45

4.76

10.21

0.91

Electro-fluidic

1.04

1.59

2.42

0.83

1.38

2.21

0.11

Aligned CNTs for strain detection. Flexible sensors8, 50 and electronics51 which are capable of being wrapped around non-planar surfaces have been essential components for emerging wearable devices. Carbon nanotubes are promising building materials for these devices because of their high flexibility. In this section, the use of aligned CNTs for strain detection is explored. The aligned CNTs are assembled in channels patterned on polyethylene naphthalate (PEN, DuPont Teijin Films) substrates using the electro-fluidic assembly process. The patterned substrates are prepared following the same process mentioned in the experimental section. The channels directing the assembly are 3 μm wide and 40 μm long. A DC voltage of 2 V and a pulling speed of 5 mm/min are used during the electro-fluidic assembly. After assembly, the PEN substrates are wrapped around objects with different radii (Figure S3) to generate various tensile strains in the CNTs (see Supporting Information pp 4 and 5): 𝜀=𝑅

𝑡2 𝑡 1― 2

(2)

where ε is the generated strain, t is the thickness of the PEN substrate (100 μm) and R1 is the diameter of the objects. At the meantime, the resistance changes corresponding to each strain are recorded, as shown in Figure 7 (b). The resistance of the CNTs increases with the applied strain, which can be attributed to decreased conductive junctions between the nanotubes.50 The almost


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linear response of the resistance change with the strain demonstrates the capability of using the aligned CNTs for strain detection. Gauge factor (GF), which is the slope of the relative resistance change with respect the applied strain, is calculated to be 2.7 for our CNT device. The GF value is close to that of CNT-based devices reported previously8, 52 and larger than that of commercial metal strain gauges (GF ~ 2).52

Figure 7. (a) Schematic illustration of the setup to generate tensile strain in the CNTs assembled on the PEN substrate. (b) Resistance change of the aligned CNTs assembled by the electric-fluidic method as a function of the applied strain. CONCLUSIONS In this paper, we present an electro-fluidic assembly process to assemble highly aligned and densely packed CNTs selectively on a substrate with patterned wetting areas at a high rate. By


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adjusting two governing parameters, the DC voltage and the pulling speed, well aligned CNTs are assembled in patterns with width from 1 µm to 100 µm and length from 20 µm to 120 µm at a rate 20 times higher than the fluidic assembly process. Higher DC voltage and lower pulling speed result in thicker CNTs. The aligned CNTs show decreased electrical resistivity compared with the random networks due to decreased inter-nanotube junctions and conduction path length. In addition, the aligned CNTs exhibit increased resistance to the applied strain, proving their possibility in strain detection. Precise and reproducible control of the orientation and placement of the CNTs opens up their practical application in emerging wearable electronics and sensors. EXPERIMENTAL METHODS Patterned substrate preparation. A Si wafer with a 500 nm thermal oxide layer was used as a substrate. The substrate was cleaned with piranha solution (sulfuric acid (98%) / hydrogen peroxide (30%), 2:1), rinsed with deionized (DI) water and then dried with high purity nitrogen. A 500 nm photoresist layer (S1805, Microchem Corp.) was spun coated on the substrate and then patterned via optical photolithography. Following this, a 150 nm gold film with a 5 nm titanium adhesion layer was deposited on the patterned substrate using E-beam evaporation and then lifted off in acetone to create two metal electrodes. A 500 nm PMMA (950 PMMA, MicroChem Corp.) film was then spun coated on the substrate at 5000 rpm and subsequently patterned using E-beam lithography to obtain channels 1 µm to 100 µm in width and 20 µm to 120 µm in length. Electro-fluidic assembly. The electro-fluidic assembly was performed by applying a DC electric field between the patterned substrate and a bare gold chip (5 nm titanium / 100 nm gold films on a Si wafer) in a water-based CNT suspension (0.046 wt%, Brewer Science CNTRENE C100). The majority of CNTs in the suspension are individual SWNTs with an average diameter of 1.25 nm and a length of 1.5 μm. Because of their short length, the SWNTs are not curved or


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coiled, which benefits the alignment of the SWNTs. The SWNTs are not sorted with 1/3 metallic nanotubes mixed with semiconducting ones, and the SWNTs are negatively charged in the suspension because of carboxylic acid groups. The patterned substrate works as an anode and the gold chip works as a cathode. Prior to assembly, the two electrodes with a distance of 5 mm were dipped into the suspension. To assemble CNTs, DC voltages from 1.5 V to 2.5 V generated by a DC power source (Keithley 2400) were applied between the two electrodes for 20 s and then the samples were pulled out of the suspension at a constant speed using a dip coater (KSV instruments). A number of pulling speeds from 1 mm/min to 50 mm/min were tried. After assembly, the PMMA was dissolved in acetone, leaving behind well aligned CNTs. Characterization of aligned CNTs. The microstructure and morphology of the assembled CNTs was imaged by a field emission scanning electron microscopy (SEM, Supra 25, Carl Zeiss) and an atomic force microscopy (AFM, NX 10, Park System). The electrical properties of the CNTs were measured using a semiconductor parameter analyzer (HP 4156C, Agilent Technologies, USA).

ACKNOWLEDGMENT The work is funded by the National Science Foundation Nanoscale Science and Engineering Center (NSEC) for High-rate Nanomanufacturing (NSF grant-EEC-0832785) and the Massachusetts Technology Collaborative. The experiments were conducted at the George J. Kostas Nanoscale Technology and Manufacturing Research Center at Northeastern University.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website. A schematic illustration of the localized electric field and the fringing electric field, SEM images of CNTs assembled under a DC voltage of 2 V at various pulling speeds, wrapping flexible CNT devices around objects with various radii to generate various tensile strains, the tensile strain calculation (PDF) AUTHOR INFORMATION Corresponding Author *E-mail addresses: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

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Assembly of Highly Aligned Carbon Nanotubes Using an Electro

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