NANO LETTERS
Aligned Carbon Nanotube−Polymer Hybrid Architectures for Diverse Flexible Electronic Applications
2006 Vol. 6, No. 3 413-418
Yung Joon Jung,*,†,⊥ Swastik Kar,*,‡,⊥ Saikat Talapatra,‡ Caterina Soldano,§ Gunaranjan Viswanathan,‡ Xuesong Li,‡ Zhaoling Yao,‡ Fung Suong Ou,‡ Aditya Avadhanula,| Robert Vajtai,‡ Seamus Curran,# Omkaram Nalamasu,‡ and Pulickel M. Ajayan‡ Department of Mechanical and Industrial Engineering, Northeastern UniVersity, Boston, Massachusetts 02115, Department of Materials Science and Engineering and Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, and Departments of Chemical Engineering and Physics, New Mexico State UniVersity, Las Cruces, New Mexico 88001 Received November 13, 2005; Revised Manuscript Received January 2, 2006
ABSTRACT We present the fabrication and electrical characterization of a flexible hybrid composite structure using aligned multiwall carbon nanotube arrays in a poly(dimethylsiloxane) (PDMS) matrix. Using lithographically patterned nanotube arrays, one can make these structures at any length scale from submicrometer levels to bulk quantities. The PDMS matrix undergoes excellent conformal filling within the dense nanotube network, giving rise to extremely flexible conducting structures with unique electromechanical properties. We demonstrate its robustness against high stress conditions, under which the composite is found to retain its conducting nature. We also demonstrate that these structures can be utilized directly as flexible field-emission devices. Our devices show some of the best field-enhancement factors and turn-on electric fields reported so far.
Carbon nanotube-polymer composites have been researched extensively for many applications requiring the combination of unique electronic, optical, and/or mechanical properties of carbon nanotubes and polymer materials.1-7 However, to build integrated carbon nanotube-polymer-based systems, it is necessary for one to have a state-of-the art ability of incorporating organized nanotube architectures8,9 in selected polymer matrixes as well as engineer the interfaces between the two constituents. Here we present a direct and effective method for fabricating flexible carbon nanotube-polymer composites by incorporating aligned and patterned multiwalled carbon nanotubes (MWNT) into a soft poly(dimethylsiloxane) (PDMS) matrix. The MWNT-PDMS composite systems produced by our method have three remarkable features. First, a nanotube architecture/network of any shape and dimension can be transferred completely into the PDMS matrix without * Corresponding authors. E-mail:
[email protected];
[email protected]. † Northeastern University. ‡ Department of Materials Science and Engineering, Rensselaer Polytechnic Institute. § Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute. | Department of Chemical Engineering, New Mexico State University. ⊥ These authors contributed equally. # Department of Physics, New Mexico State University. 10.1021/nl052238x CCC: $33.50 Published on Web 02/01/2006
© 2006 American Chemical Society
disturbing the nanotube alignment. Second, the resulting MWNT-PDMS composite film is flexible and remains conducting under extreme tensile and compressive strains. Third, these composites can be used as strain gauges, tactile10,11 and gas sensors,12 and field-emission devices.13-18 In particular, these hybrid structures can be used directly as flexible field-emitters. The effective suppression of mutual screening,13-15 when the nanotubes are embedded in the polymer matrix, gives the devices a very impressive fieldemission behavior. With large field-enhancement factors13,14,16,17 (β ≈ 104) and extremely low (sub-1 V/µm) turnon fields, these devices can operate easily at high current densities (>1 mA/cm2) at low voltages, making them suitable for applications in electrically and mechanically stable flexible display devices and sensors.19 In addition, these patterned, aligned nanotube arrays in a flexible medium open up new applications in flexible nanoelectronics,20 switches,21 displays,22,23 and biocatalytic systems.24 We have fabricated these composite structures by impregnating vertically aligned arrays of MWNTs into a transparent PDMS matrix. Our approach for building the nanotubePDMS composite system is shown schematically in Figure 1a. Vertically aligned MWNT architectures are grown on
Figure 1. (a) Schematics illustrating the fabrication steps of the aligned MWNT-PDMS array structures. SEM images showing the MWNT architectures before and after PDMS infiltration in two different scales and shapes: arrays of nanotube pillars (500 µm diameter and 100 µm height) (b) before and (c) after infiltration. (d) A top view of nanotube walls (7 µm line width, 7 µm distance between walls, 100 µm height) before PDMS polymerization. (e) A cross-sectional SEM image of the same nanotube walls after infiltration. The inset is a highmagnification image showing that the nanotube pattern and alignment remain intact after infiltration.
prepatterned SiO2/Si substrates using thermal chemical vapor deposition (CVD) of Ferrocene and Xylene at 800 °C.8 A PDMS prepolymer solution, a viscous mixture of base/curing agent (weight ratio of 10:1), is poured over the aligned nanotube structures on the substrate. Any excessive PDMS solution is removed carefully to obtain the optimum thickness for a nanotube-PDMS composite film. The PDMS is then thermally cured at 100 °C for 1 h and subsequently, selfstanding nanotube-PDMS composite films are peeled off from the Si substrate carefully. 414
Figure 1b is a tilted SEM image of an array of cylindrical pillars of selectively grown aligned MWNT structures (diameter ≈ 500 µm). Figure 1c shows the surface morphology of these nanotube pillars after PDMS infiltration and subsequent polymerization. We were also able to transfer smaller (a few micrometers scale) and more densely distributed MWNT architectures into the PDMS matrix. These structures precisely retain their original alignment, shape, and size inside the resulting composite matrix, even after the polymerization process and the subsequent peeling off from Nano Lett., Vol. 6, No. 3, 2006
Figure 2. Optical pictures demonstrating (a) the versatile geometry (aligned nanotube structures (black) embedded inside a transparent PDMS matrix) and (b-d) flexibility of the MWNT-PDMS composite systems. Note that both nanotube length and PDMS film thickness can be controlled by changing the carbon nanotube growth time during CVD and by adjusting the amount of removed PDMS before the curing process.
the substrate (Figure 1d and e). Figure 1e also shows clearly that in our method PDMS undergoes excellent conformal filling and is infiltrated into the spaces between individual nanotubes and building blocks effectively. It is very interesting to note the complete lack of any visible distortion or defects (bubbles, etc.) in the resulting structure. It is assumed that the good wettability of PDMS on MWNT25 and a few tens of nanometers distance between nanotubes enable the PDMS to infiltrate effectively into the gaps available in the nanotube forest and form the relatively defect-free composite film. The composite films detached from the substrate are extremely flexible and can be deformed easily into various geometries without destroying the nanotube assemblies. In Figure 2a, a MWNT network (in a mesh structure) is shown embedded in a relatively thick polymer matrix by using a two-step polymerization process. A series of optical images (Figure 2b-d) shows the flexibility of a thinner detached composite film. The electrical resistance of the nanotube-PDMS composite structures was tested against physical deformations because electromechanically stable structures of such composites are extremely important for usage in flexible electronic applications. The composites were found to be resilient under a variety of structural deformations and were able to sustain their conducting nature over large percentages of strain. Systematic measurements of resistance were performed as a function of tensile and compressive strains, where both the deformation and measurements were done Nano Lett., Vol. 6, No. 3, 2006
in a direction perpendicular to the alignment of the nanotubes, as shown with schematics in Figure 3. In all of these experiments, a linear shape of the composite was chosen, with sample lengths of 1-2 cm, width of 1-2 mm, and height ∼100 µm. For compression tests, the linear samples were embedded in a larger block (∼1-2 cm3) of PDMS, and the whole block was compressed to enable uniform deformation along the length of the composite. Titanium wires were embedded into the composite matrix while it was being cured in order to obtain robust electrical contacts. The zero-strain lateral resistivity of the composite material varied (from sample to sample) between 1 and 10 Ω‚cm. The resistance was found to increase monotonically for both tensile as well as compressive strains. After the first strain cycle, the zero-strain resistance undergoes a small increase (∼10-15%). In subsequent cycles, there is negligible change in the zero-strain configuration. Upon stretching, the resistance grows linearly beyond a small strain (∼2.5%), returning to its initial value when released. Figure 3a shows the behavior in a typical device. The inset summarizes the zero strain condition before and after each strain cycle. Upon compression, the normalized resistance is found to increase following a power-law dependence on the strain, as shown for a typical device in Figure 3b. It also shows (inset) that the device can detect small changes in pressure easily (∼1000 N/m2). The aligned nanotube forest is a lateral network of conducting fibers, which are connected to each other, giving rise to the conducting path. It appears from the relatively high value of the resistivity that the connectivity is only 415
Figure 3. (a) Typical variation of the normalized composite resistance ∆R/R0 in a device under an applied tensile strain. The resistance scales linearly beyond a small strain (∼2.5%). The inset shows the change in zero-strain resistance before (open markers) and after (filled markers) each strain cycle, conducted over a period of 6 days. There is an irreversible increase of ∆R/R0 ≈ 15% after the first strain cycle and becomes constant for subsequent strain cycles. (b) Normalized resistance as a function of compression. The values are displayed as a “log-log” plot showing clear power-law dependence. The inset shows the sensitivity of resistance to the applied pressure in the same device. In both a and b, the schematics show the direction of the nanotube alignment (short black vertical lines) and the corresponding direction of strain (blue arrows).
partial. At the nanoscale level, both tensile and compressive strains can cause this connectivity to get disturbed and is possibly the reason that the resistance increases under both kinds of strain. The actual functional forms of the resistance change due to strain (both tensile and compressional) are attributed to the changing contact area between neighboring filaments in an aligned network of nanotubes, accompanied by a complex dynamic self-adjustment of their position and shape. This hypothesis is supported by SEM images (not shown), but its involved analysis lies outside the purview of this work. The systematic and reproducible dependence of resistance on strain reiterates the extreme structural integrity at the individual nanotube scale in these composites. It also underlines the possibility of utilizing these structures directly 416
as strain and pressure/touch sensors. Furthermore, its ability to retain its conducting nature under strain is extremely useful for various flexible electronic requirements, in particular, as a flexible cathode of an integrated field-emission device. Owing to their extremely high aspect ratio and favorable electrical conductance, carbon nanotubes are excellent field emitters.18 In the past, field emission from carbon nanotubes in various architectures, with both single-walled and multiwalled nanotubes, has been studied extensively. The inherent advantages of our architecture make it a natural choice for testing field-emission properties. We have chosen patterned MWNT-PDMS composites of a cylindrical shape with a diameter of 500 µm to make our field-emission devices. By suitably adjusting the quantity of PDMS used while fabricating the composite, we could make films with very few exposed nanotubes on the top surface. The bottom surface of the pattern, where the ends of the nanotubes were completely exposed, was coated with Ti/Au and fixed to a metal electrode using a conducting silver paint to form a composite cathode. A metal anode with an adjustable separation distance was positioned parallel to the top surface of the MWNT-PDMS composite. As a process of preconditioning, the gap between the electrodes was adjusted in a way such that by running high currents any existing long and entangled masses of nanotubes were burnt off and short and isolated nanotubes were retained on the top surface of the film. The spacing was then fixed at ∼50 µm, and fieldemission measurements were performed under a vacuum of ∼5 × 10-4 Torr. Field emission occurs when the effective electric field around a nanotube tip is large enough to overcome the work function of the nanotube (typically 5 eV for carbon nanotubes14). The emitted current follows a very well-known mechanism, called the Fowler-Nordheim mechanism, in which the current density is approximately related to the effective field through the equation, JFN ) (e3F2/8πhφ)‚ exp[-(8πx(2m)/3he)(φ3/2/F)]. Here, F is the effective electric field seen by the emitting region, and φ is the work function of the nanotube. If the separation distance is d, then the field-enhancement factor, β, is the ratio between the effective field and the applied field, β ) F/(V/d), where V is the applied voltage across the device electrodes. If the effective surface area of emission is R, then the measured current is given by I ) RJFN. In that case, the expression can be written as ln(I/V2) ) ln C1 - C2/V, where, C1 ) (e3/ 8πhd2φ)β2R and C2 ) (8πdφ3/2x(2m)/3he)(1/β). A plot of ln(I/V2) against 1/V is popularly known as the FowlerNordheim (FN) plot. Figure 4 shows the FN plots for two such devices, with the insets showing the emission current for applied voltage in each device. It can be seen that the emission process follows the FN equation rather well for nearly the entire range of applied voltages, with slight deviations at the lowest and highest bias values. The deviation at lower voltages comes from the instrument insensitivity, and the deviation at the highest values comes from an enhanced field current, either due to the participation of a second nanotube in the emission process or due to an increase in the tip temperature, Nano Lett., Vol. 6, No. 3, 2006
Figure 4. Fowler-Nordheim plot of field emission in two devices. The inset shows the emission current for applied voltages in the two corresponding devices. Table 1. Device Properties Obtained from the Fowler-Nordheim Plotsa device
enhancement Eto comments on factor, β (V/µm) current density
1 (present work) 2 (present work) CNT on carbon cloth14 patterned MWNT26
8000
0.87
19100
0.5
18800
0.2
5000
0.6
SWNT/silica27
15000
0.6
SWNT films28
10000
1.5
CNT film/borosilicate29 glass
8435
2.1
1 mA/cm2 @ 2.16 V/µm 1 mA/cm2 @ 0.76 V/µm 1 mA/cm2 @ 0.4 V/µm 1 mA/cm2 @ ∼0.7 V/µm 1 mA/cm2 @ 1.8 V/µm 10 mA/cm2 @ 3.9 V/µm >400 µA/cm2 @ 6 V/µm
a For comparison, we have added the best results from a few past works. Some of the values have been extrapolated from data provided in the references.
as reported previously26. From the slope of the FN plots, we estimated a field-enhancement factor, β, of ∼8000 for device 1 and a better value of ∼19100 for device 2. The turn-on fields, Eto (in our case, when the detectable change in field current was ∼1 nA), were also calculated for the two devices, and the values are listed in Table 1. For comparison, we have included some of the best values obtained by other groups in the past. It is evident from our data that the devices exhibited excellent field emission. A nominal current density of 1 mA/cm2 (considering the entire top surface area of the pillar) was achieved easily at threshold fields of 2.16 and 0.76 V/µm for devices 1 and 2, respectively, which can be reduced greatly by patterning smaller-diameter pillars. Studies16 have shown that the presence of other nanotubes near the one emitting (and which are at a similar potential) screens the enhancement factor, β, drastically if the separation of the nanotubes is less than the length of the emitting nanotube (which is often the case in mats or aligned arrays). Nano Lett., Vol. 6, No. 3, 2006
In most devices constructed from nanotube mats/arrays, the field-emission current density at threshold is about 1 mA/ cm2. Considering that a single nanotube can give about ∼1 nA at turn-on, only about 106 nanotubes/cm2 are required to obtain this emission current density. The rest do not contribute and actually screen down the field enhancement. In effect, their presence simply makes the emission process less efficient. Hence, it is desirable to electrically isolate the emitting nanotubes from its neighbors by a dielectric material (insulator) that prevents the screening and allows just a few nanotubes to remain exposed at the surface so that a more efficient field emission is achieved. In our devices, the effective surface areas, R ) 0.96 × 10-18 m2 and 2.76 × 10-18 m2 for devices 1 and 2, respectively, indicate that the emission takes place from a very small region of the tips of individual nanotubes. SEM images (not shown) of our functional devices show that the very few tips that are exposed above the PDMS surface are 2-3 µm long and are separated by distances of similar or larger lengths. This fact substantially decreases mutual screening of the electric fields and gives rise to the large field-enhancement factors and hence low turn-on fields. The inherent nature of this architecture also prevents any “pulling out” of nanotubes from the cathode during high-field operations, adding to the durability of the devices. In conclusion, we have fabricated patterned, aligned MWNT-PDMS architectures in various geometries and sizes. These composites remain electrically conducting under large strain and show promising potential as strain and gas sensors and flexible field-emission devices. For any such device, sub-1 V/µm turn-on fields and threshold fields of a few volts per micrometer are extremely desirable. Although very few architectures can even come close to delivering these numbers, none until now have been reported to meet these demanding criteria remaining stable, flexible, and transferable. These structures and their properties have immediate and immense implications for the development of versatile, low-cost, and portable electronic and electromechanical devices for diverse applications. Acknowledgment. We acknowledge the financial support received from RPI, the Interconnect Focus Center New York at RPI, and the NSF Nanoscale Science and Engineering Center. Y.J.J. acknowledges the support from NSF Center for High Rate Nanomanufacturing and College of Engineering at Northeastern University. S.C. acknowledges support from AFOSR grant no. 106427. References (1) Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Topics in Applied Physics 80; Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P., Eds.; Springer: New York, 2001. (2) Calvert, P. Nature 1999, 399, 210. (3) Lahiff, E.; Ryu, C. Y.; Curran, C.; Minett, A. I.; Blau, W. J.; Ajayan, P. M. Nano Lett. 2003, 3, 1333. (4) Ahir, S. V.; Terentjev, E. M. Nat. Mater. 2005, 4, 491. (5) Suhr, J.; Koratkar, N.; Keblinski, P.; Ajayan P. M. Nat. Mater. 2005, 4, 134. (6) Koerner, H.; Price, G.; Pearce, N. A.; Alexander, M.; Vaia R. A. Nat. Mater. 2004, 3, 115. (7) Hinds, B. J.; Chopra, N.; Rantell, T.; Andrews, R.; Gavalas, V.; Bachas, L. G. Science 2004, 303, 62. 417
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Nano Lett., Vol. 6, No. 3, 2006
