Highly Anisotropic Adhesive Film Made from Upside-Down, Flat, and

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Highly Anisotropic Adhesive Film Made from UpsideDown, Flat and Uniform Vertically Aligned CNTs Sanghyun Hong, Troy Lundstrom, Ranajay Ghosh, Hamed Abdi, Ji Hao, Sun Kyoung Jeoung, Paul Su, Jonghwan Suhr, Ashkan Vaziri, Nader Jalili, and Yung Joon Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10395 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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ACS Applied Materials & Interfaces

Highly Anisotropic Adhesive Film Made from Upside-Down, Flat and Uniform Vertically Aligned CNTs Sanghyun Hong†, Troy Lundstrom†, Ranajay Ghosh†‡, Hamed Abdi†, Ji Hao†, Sun Kyoung Jeoung§, Paul SuⅡ, Jonghwan SuhrⅡ, Ashkan Vaziri†, Nader Jalili†, and Yung Joon Jung†* †

Department of Mechanical and Industrial Engineering, College of Engineering, Northeastern

University, Boston MA 02115, USA ‡

Department of Mechanical & Aerospace Engineering, University of Central Florida, Orlando,

FL 32816, USA §

Korea Automotive Technology Institute, Cheonan-Si 330-912, Korea

Ⅱ

FM Global, Norwood, MA 02062, USA

⊥

Department of Polymer Science & Engineering, Department of Energy Science,

Sungkyunkwan University, Suwon 440-746, Korea. KEYWORDS anisotropic adhesion, high static friction, low adhesion, vertically aligned carbon nanotubes, transfer process

1 Environment ACS Paragon Plus


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ABSTRACT We have created a multifunctional dry adhesive film with the transferred vertically aligned carbon nanotubes (VA-CNTs). This unique VA-CNT film has been fabricated by employing multi-step transfer processes, converting flat and uniform bottom of VA-CNTs, grown on atomically flat silicon wafer substrates, into the top surface of an adhesive layer. Unlike asgrown VA-CNTs that have a non-uniform surface, randomly entangled CNT arrays, and weak interface between CNTs and substrates, this transferred VA-CNT film shows an extremely high static friction coefficient (COF) of up to 60, and a shear adhesion force 30 times higher (12N/cm2) than the as-grown VA-CNTs under very small preloading of 0.2N/cm2. Moreover near zero normal adhesion force has been observed with 20mN/cm2 preloading and maximum of 100µm displacement in a piezo scanner, demonstrating ideal properties for an artificial gecko foot. Using this unique structural feature and anisotropic adhesion properties, we also demonstrate effective removal and assembly of nanoparticles into organized micrometer-scale circular and line patterns by a single brushing of this flat and uniform VA-CNTs film.

Introduction Carbon nanotubes (CNTs) are considered among the stiffest materials in the world with singlewalled CNTs exhibiting Young’s modulus of up to 1TPa in axial loading, well beyond the performance envelopes of traditional metals and alloys.1-3 The principal source of this strength, which stems from the high cohesive energy of C-C bonds on the CNT surface, also results in surprisingly poor stiffness in bending loads.4 The upshot of this non-traditional behavior is the simultaneous stiffness and exceptional flexibility of CNTs depending on the loading direction. This highly anisotropic behavior of CNTs is amplified when they are aligned together vertically,


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resulting in VA-CNTs which can exhibit out of plane effective Young’s modulus as small as 100kPa.5,

6

This low value of modulus puts them firmly in the property envelopes of soft

viscoelastic materials according to the well-known Dahlquist’s criterion,7, 8 and thus makes them suitable for applications as pressure sensitive adhesives (PSA). However, the inherent fibrillar structure of VA-CNTs systems, resembling the gecko feet structure well known to simultaneously exhibit strong shear binding and weak normal adhesion, holds tremendous promise as transient adhesives which can be applied and reapplied repeatedly, unlike traditional PSAs. Particularly in the nanometer scale, the remarkable adhesion force by Van der Waals force is up to 200 times higher than the gecko setae.9 Table 1 compared the adhesion forces using VACNTs with different preloading and unloading directions. Qu et al.10 fabricated vertically aligned multi-walled CNTs (VA-MWCNTs) based adhesive films with entangled tops, and found substantial difference between normal and shear binding forces. Particularly, these VAMWCNTs films, applied with a preload of ~120N/cm2 exhibited high shear adhesion forces with a strong dependence on the length of the CNTs, which increased substantially with increasing CNT length. This high shear adhesion force is attributed to two components; the interaction between CNTs and a target substrate and the length dependent bending moment along the CNT caused by the shear loading.11 In contrast, the normal adhesion force showed only weak dependence on the length, in which case higher normal adhesion force of 29 N/cm2 was observed with the same preloading.12 Ge et al.13 also reported VA-MWCNTs based synthetic gecko tapes that mimicked the setae array structure of gecko feet with aligned and patterned MWCNTs. The shear strengths measured after a preloading of 25-50N/cm2 were better than gecko feet and also exhibited a marked dependence on CNT height. Through Scanning electron microscope (SEM) images, the authors deduced that cohesive binding between the CNT and the substrate is the



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primary mechanism for the high performance of these tapes. However high preloading force, 50N/cm2 often resulted in severe the deformation of CNT film, and limits the repeatable use.14

Materials Gecko (Tokay) VAMWCNTs

Adhesion force (N/cm2) 10 ~100 ~20 29

Preloading force (N/cm2) 0.2 (50g/230mm2) ~120

Unloading Ref. direction shear shear normal normal

1 10

~120 12 VA-SWCNTs VA50 25-50 shear 13 MWCNTs ~12 0.2 shear VA-SWCNTs (Our work) near zero 0.02 normal Table 1. Comparison of adhesion forces with different VA-CNTs and conditions. In this paper, we demonstrate the design and fabrication of high performance and multifunctional dry adhesive film using the transferred VA-CNTs structured by converting relatively flat bottom of VA-CNTs, directly grown on atomically flat silicon wafer substrates, into the top surface of adhesive layer. This unique surface of VA-CNTs maximizes contact between nanotubes and substrate with a small loading, leading to a static COF of 40-60 which is ~30 times higher than that of as-grown VA-CNTs, while exhibiting a shear adhesion force comparable to gecko feet, and a near-zero normal adhesion strength under extremely low preloading condition.. By harnessing this highly anisotropic adhesion property along with a flat and uniform surface morphology and nanoscale high density-aligned nanotube structure, we were also able to clean and assemble nanoparticles into various highly organized micro-patterns with a simple one time brushing. This contrasting directional anisotropy as well as the multifunctional behavior in macro-, micro-, and even nano-scale dimensions can prove to be important for a number of industrial applications such as an end effector for a reusable tape13, a robot hand15, MEMs16, a


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biomedical device17, and also in applications requiring increasing the mobility in space18 where high static friction and lower adhesion properties with a small preloading are required. Results and discussion In order to fabricate high performance and multifunctional dry adhesive films, high-density VACNTs were synthesized on a polished silicon oxide wafer by a thermal chemical vapor deposition (CVD) method. Most of the CNTs grown are single walled CNTs (SWCNTs) and a few double walled CNTs (DWNTs), which was confirmed by high-resolution Transmission electron microscope (HR-TEM) and Raman spectroscopy (Figure S1).19, 20 During the initial period of growth, the CNT network had no long-range order and grew randomly. Under this initial ‘randomly ordered crust layer’, well-aligned CNTs began to form due to their high packing density nucleation and growth, particularly when the thickness was over 10µm.21 As shown in Figure 1, 2a and S1, typical as-grown VA-SWCNTs film exhibits an intrinsic concave shape at the free end, especially when the aligned SWCNTs are continuous and without cracks on the surface.22 Both the inhomogeneity arising from the initial crust layer underneath and the overall concave shape of the VA-CNTs films often result in irregular contacts with the surface, thereby concentrating the contacting surfaces to a limited proportion of the total CNTs film area. This would lead to both undesirable friction properties of the film and high susceptibility of the CNT to damage, rendering the film useless.



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Figure 1. Transfer process of VA-CNTs film

To overcome these challenges, we employed a simple but highly effective transfer process which inverts the VA-CNTs film, thereby exposing the better aligned, flat and uniform lower end of the CNTs. The entire transfer process is illustrated in Figure 1. First, the continuous Cr layer 1µm thick was deposited on the top surface of as-grown VA-CNTs film, where CNTs were randomly formed into a crust layer (Figure 2a-b), using a DC sputter coater. This Cr layer was infused into a CNT crust to take hold of the top surface of entangled VA-CNTs films.23 In order to proceed with the transfer, we also prepared a receiving substrate which consisted of a flat polished Si wafer of 380µm thickness covered with a drop-casted layer of epoxy resin. The as-grown VACNTs film was then inverted and a top Cr infused crust layer was placed inside of an epoxy resin on the receiving substrate, followed by curing at 100˚C for 2 minutes for better mechanical integrity of the upside-downed VA-CNT film. The role of the Cr layer is critical, as in its absence up to 80% of volume of the VA-CNTs can be densified by capillary action of the epoxy resin, resulting in a loss of their elasticity.24 Therefore, the metal interlayer acted as the separator that prevented any capillary action as well as a binder for an entangled CNT crust layer. Finally,


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the SiO2 wafer substrates used for nanotube growth was removed from the inverted VA-CNT film, resulting in flat, uniform, and highly aligned surface of VA-CNTs, as shown in Figure 2cd.25 The transfer process used to apply for exposing the bottom soft CNTs.26 However, we obtained both the bottom soft CNTs and the overall flatness in a large scale (Figure 2c). We confirmed the well-developed multi-layer of VA-CNTs/Cr layer/epoxy adhesive in Figure 2e.

Figure 2. SEM images of (a) As-grown VA-CNTs film in a cross-sectional view, and (b) high magnification images of top area. ((b-1-2) detail morphology images of (b)) (c) Flat and uniform surface of VA-CNTs after the transfer in a tilted view. High magnification SEM images of transferred VA-CNTs; (d, d-1) top CNTs, and (e) bottom interface between VA-CNTs and substrate.

The frictional property of our transferred VA-CNTs film was tested first through its (static) COF. To determine the COF of the VA-CNTs, we selected an atomically flat 3” Si wafer as a target substrate, 380µm in thickness and 20g in weight. This wafer was balanced over three 6mm×6mm VA-CNTs films which were affixed to another flat plate using a commercially available glue.



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The plate was then tilted until the supported wafer began sliding (or breaking of contact), as shown schematically in Figure 3a. The COF was calculated using a balance of forces on the wafer at the moment of sliding. Furthermore, if the Coulomb’s friction law is assumed, the COF can be calculated as the tangent of the inclination angle.27 We used three different types of friction films of identical area made of as-grown VA-CNTs, transferred VA-CNTs and PDMS, which is widely used for adhesion purposes, as a control. Each type of VA-CNTs films had four specimens of differing heights between 100µm and 500µm. The height of VA-CNTs film was controlled by controlling CNT growth time during CVD process (Figure S1d). In order to make a uniformly flat PDMS surface, PDMS solution was cured on the polished Si wafer. The inclination angle of impending sliding for both the as-grown and transferred VA-CNTs films was measured multiple times for consistency.


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Figure 3. COF measurements data. (a) Schematic for the angle COF, and the optical photo of the inclined plane; three VA-CNTs films on it, and Si wafer on transferred VA-CNTs films (N1=N2=N3=a normal preloading at the each pad=1/3 of total Si wafer weight of 20g (W/3)). COF of (b) as-grown samples and (c) transferred samples with CNT different heights. (■Coefficient of friction, ▲-Breaking angle) The tangent of breaking angle (ߠ) matches with COF (ߤ ൌ ‫)ߠ݊ܽݐ‬. (d) Comparison of static COF between PDMS and VA-CNTs films.



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Figure 3b-c shows the change in COF of as-grown and transferred VA-CNTs films respectively for different CNT heights. Our results indicate a positive correlation between the COF and the height of the CNTs, although the improvement was more pronounced and consistent for transferred surfer-flat VA-CNTs films. More strikingly, the transferred VA-CNTs films with the longest CNT height of 500µm showed extremely high static COF up to 60, which is about 30 times higher than the as-grown VA-CNTs and close to the value exhibited by the PDMS pads as shown in Figure 3d which shows the average COF of all measurements at the best condition. The calculated maximum shear adhesion force of converted VA-CNTs film was 12N/cm2, which is equivalent or higher than natural gecko feet. It is also very important to note that the weight of the wafer produced a normal pressure (preloading for initiating contacts between nanotubes and substrate for shear adhesion) of about 0.2N/cm2 on the film, which is significantly smaller than other reported values.5, 6, 10-13 Furthermore, we also found with repeated measurements that in contrast to the transferred VA-CNTs films, as-grown VA-CNTs films were easily broken and detached from the growth substrate (Figure S2). This poorer overall performance of as-grown VA-CNTs could be attributed to the localized force on CNTs due to the previously discussed concave shape of the end surface and the weak interface between CNTs and growth substrate.


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Figure 4. Adhesion force measurement. (a) Schematic of experiments, (b) comparison of adhesion force between PDMS and transferred VA-CNTs film with pulling speed of 2.25µm/s. (c) Dynamic adhesion (or breaking) force measurements of PDMS film with different pulling speeds, and (d) relationship between pulling speeds and adhesion force of the PDMS film, except



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the fastest pulling speed* (e) Dynamic adhesion force measurement with the transferred VACNTs film with different pulling speeds. *To look at the effect on adhesion force for the most extreme displacement scenario possible a step function was applied to the piezoelectric stage. Piezo actuators have response times on the order of several micro seconds. For this experiment, the actuator ideally traveled 90 µm in 0.01 seconds yielding a velocity of 9000 µm/sec.

In addition to friction properties, we also explored the normal adhesive behavior of transferred VA-CNTs, using PDMS as a control. In Figure 4a, the motion of the target substrate to the normal direction is controlled by the piezoelectric stage affording a small preloading in mN level, and up to 100µm of displacement. The as-grown VA-CNTs films were found to be too fragile for this test and easily disintegrated. In order to compare the adhesive performance of transferred VA-CNT films with PDMS, the PDMS sample was preloaded to a maximum compression force equal to that of the transferred VA-CNTs samples, and a subsequent pull-off rate of 2.25µm/sec was used. This was compared to the results from the slowest VA-CNTs pulloff measurement, where the adhesion force was plotted versus piezoelectric stage displacement or time. As shown in Figure 4b, the transferred VA-CNTs film showed near-zero normal adhesion force, while perceptible adhesive forces (approximately 0.88mN, average of multiple measurement (Figure S3d)) were clearly observed for the PDMS sample. Furthermore, a linear increase in adhesion force with increasing pulling speed was clearly observed for the PDMS sample, Figure 4c and d. This linearity conforms to similar linearity observed in the case of tacky


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films28. On the other hand, the transferred VA-CNTs film exhibited near-zero adhesion force for all pull off speeds (Figure 4e).

Figure 5. Nano-particle patterning by a simple brushing. (a) A fluorescent microscope image and (b and c) SEM images of 200nm diameter nano-particles uniformly dispersed on a patterned substrate. (d) A fluorescent microscope image and (e and f) SEM images of nano-particles in the pattern after a simple brushing using a transferred VA-CNTs film. (g and h) SEM images of the VA-CNTs film surface after a single brushing. In addition to frictional properties, the unique structural features of the VA-CNT films such as a flat surface and a nano-sized (10-15nm) gap between the aligned CNTs could also be used to manipulate and remove nanoparticles over flat surfaces with very high accuracy through a



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brushing action. We found that the strong adhesion force between high density CNTs and nanoparticles is higher than the adhesion force between nanoparticles and the substrate. This strong adhesion force between CNTs and nanoparticles would be useful for removing nanoparticles and even constructing micro-patterned nanoparticle assemblies, which is imperative for building nanoparticle based functional devices. To test the efficacy, a sweeping of the brush on nanoparticles dispersed onto flat and patterned substrates (Figure 5, S4) was performed. To this end, fluorescence silica nanoparticles with 200nm diameter were initially sprayed evenly at a thickness of about one particle layer (confirmed using SEM images) over the patterned SiO2 (400nm)/Si substrate where 10µm diameter of cylindrical cavities were spaced at 20µm and of depth 400nm, enough to hold only about two nanoparticles vertically (Figure 5a-c). Figure 5d-f shows a striking example of arrays of organized micro-assemblies of silica nanoparticles achieved by a single sweeping of a transferred VA-CNTs brush. During the brushing action, the transferred VA-CNTs maximized contacts with the fluorescence nanoparticles on the flat surface and moved in shear direction, leading to the highly effective removal of nanoparticles due to the higher static friction force between CNTs and nanoparticles than that of nanoparticles and the substrate (Figure 5d-f). However silica nanoparticles sitting on the bottom of micro-cavities remained in place as the VA-CNT arrays did not touch nanoparticles inside of 400 nm thick cylindrical micro-patterns as shown in Figure 5f (compare with Figure 5c). SEM micrographs (Figure 5g-h) show that during brushing with VA-CNTs film, the removed nanoparticles were attached to the ends of aligned CNTs, clearly demonstrating strong static friction force between CNTs and nanoparticles. We also tested the effectiveness of this unique dry particle cleaning and patterning technique for various sizes of nanoparticles (10500nm diameter). It was observed that nanoparticles with diameters smaller than the depth of


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cavities (400nm in this our pattern) could be removed effectively while leaving nanoparticles inside of micro cavities undisturbed. However nanoparticles with larger diameters (500nm and above) than the depth of cavities were moved along the brushing direction by the CNTs, as larger nanoparticles protruded out of the cavities. (Figure S4) Conclusions In summary, we are able to fabricate highly anisotropic multifunctional adhesive film by creating upside-downed, flat and uniform VA-CNTs films. This adhesive film exhibits a pronounced contrast between frictional and normal adhesive behavior. The measured shear adhesion force is substantial (12N/cm2) while showing near-zero normal adhesion force under astonishingly small preloading of 0.2N/cm2 demonstrating ideal properties of artificial gecko foot. By using these unique nanostructures and adhesion properties, we were also able to demonstrate the selective removal and formation of micro-assemblies of nanoparticles. These highly anisotropic and unique, multiple functions of reported CNT films have immediate and immense implications for the development of multifunctional platforms for robot hands, reusable tapes, end effectors, brushes for cleaning nano/micro scale contaminations, organized assembly of nanoparticles, and MEMs, among other applications. Experimental details VA-CNTs sample synthesized by a thermal CVD using anhydrous ethanol (Sigma-Aldrich, 99.95%) as a carbon source. 100nm SiO2 layered (100) Si wafer (p-doped, Universitywafer) were used as a growth substrate. Aluminum thin film was used as a buffer layer and was prepared by using an Argon sputter coater for 90 seconds with a rotation at the power of 1.5kW. This buffer layer was annealed for 3 days in the air. As a catalyst metal, Cobalt was deposited by



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e-beam evaporator for 3 to 5 seconds with 1Ȧ/s deposition rate. The prepared growth substrate was carried by the alumina boat and placed in the middle of horizontal quartz tube. The reaction chamber was heated to 850°C under 150sccm (sccm denotes cubic centimeter per minute at STP) of 5% hydrogen balance argon gas flow. The pressure was fixed at 700torr by a needle valve with a rotary pump. The ethanol source was induced by bubbling of 5% H2 balanced Ar carrier gas. The length of the carbon nanotube forest of around 500µm was achieved in 30-45 minutes. It was proportional to the reaction time until the Co catalyst was deactivated for certain reasons; Oswald ripening and poisoning by deposited amorphous carbon on the catalyst surface (Figure S1d,e). HR-TEM was operating at 200kV (2010F, Jeol, Figure S1f). Raman spectrum of VACNTs film was conducted by using a Raman spectroscope (LabRAM HR-800 with 532nm YG laser, Horiba, Figure S1g). For the transfer process, we deposited 1.2µm of Cr over the VACNTs surface by using MRC 8867. We were used the power of 500W, the base-pressure of 3×10-7torr, and the Ar flow rate of 15sccm for 10 minutes. The deposited metal penetrated VACNTs several hundreds of nanometer, and it is confirmed by SEM. The epoxy resin (Epon-812, SPI Supplies) was applied on Si wafer at the hot plate temperature of 100˚C for 2 minutes. The growth substrates were removed after curing of epoxy resin. In order to measure the surface roughness of our transferred VA-CNTs film, we conducted AFM characterization (Figure S5). Three 6mm×6mm sized both as-grown and transferred VA-CNT pads were placed on the incline. Each pad supported the same normal force, and the normal force was only the mass of Si wafer, 20g. Each normal force is labeled as N1, N2, and N3 in Figure 3a, and all normal force on each pad will be equal as 1/3 of total weight of Si wafer (W/3). Humidity of the measurement condition was about 40% of RH. The angle of the incline was measured by an accelerometer. We


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increased the tilted angle slowly, and stopped it when the Si wafer started sliding. We measured the angle 5 times for each conditions. For the adhesion force measurements, a labeled image of the experimental setup is shown in Figure S3a-c. The adhesion force measurement setup is composed of a manual XYZ, an active XY piezoelectric stage, a composite load cell for measuring contact forces in both the X (shear) and Y (compression/adhesion directions), a support structure for holding the target substrate and a base plate to which all major components are mounted. To ensure the good contact between the target substrate and adhesion pads, we applied a rotary compliance at the mount. Although there is some potential for a minute moment to be imparted to the sample-holding element of the load cell, the sample holder is mounted in such a way as to be compliant in the rotary direction, but rigid in the transverse direction (normal). In this way, strain measurement error due to concentrated moments at the load cell tip were reduced by several orders of magnitude relative to the dominant, concentrated point load. The validity of this measurement technique was verified during calibration. The load cell is composed of two, beam-style, full-bridge load cells mounted at right angles to one another to simultaneously measure compression/adhesion and shear forces. The signals are amplified by a 2-channel instrumentation amplifier and fed into a dSpace® data acquisition system. In addition, both the load cells and instrumentation amplifier are supplied with 10 V from a low noise power supply and the piezoelectric stage is controlled by a Physik Instrumente® piezoelectric amplifier/controller using an output signal from the dSpace® system. To measure the adhesion force for all samples, the following compression/pull-off procedure (Table S1) was used for the various pull-off rate (>2.25µm/sec): For the brush experiments, the patterned silicon oxide wafer was prepared by a photo lithography process. The circular pattern was 10µm in a diameter, and the strip pattern was 4.5 µm in a



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width. The thickness of the oxide was 400nm, and it etched by HF solution. Fluorescence silica nanoparticles were dispersed in ethanol, and were spread out on a patterned silicon oxide wafer by a spay gun. VA-CNTs films were fixed using a stand, and a stage was controlled x- and y-axis motor. First, VA-CNTs films were made flush with the substrate. Then we moved the stage using the motor.


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Supporting Information. Experimental details of materials, a piezoelectric stage setup, failure images of as-grown VACNTs, adhesion force measurement data with different pulling speeds, and nanoparticles brushing results when we used different size and patterns. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT Y.J.J. acknowledges the financial support from the National Science Foundation-DMEREF (1434824), Technology Innovation Program (10050481) funded by the Ministry of Trade, Industry & Energy (MI, Korea). A.V. and H.A. acknowledges the financial support from National Science Foundation-CMMI (1149750). We thanks to A. Keklak and H. Jeong at Northeastern University for the discussion.



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Table of Contents Graphic


Highly Anisotropic Adhesive Film Made from Upside-Down, Flat, and

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