NANO LETTERS
Scalable Assembly Method of Vertically-Suspended and Stretched Carbon Nanotube Network Devices for Nanoscale Electro-Mechanical Sensing Components
2008 Vol. 8, No. 12 4483-4487
Byung Yang Lee,† Kwang Heo,‡ Jung Hoon Bak,† Sung Un Cho,† Seungeon Moon,§ Yun Daniel Park,† and Seunghun Hong*,†,‡ Department of Physics and Astronomy, Interdisciplinary Program in Nano-Science and Technology, Seoul National UniVersity, Seoul 151-747, Korea, and IT ConVergence Components Laboratory, Electronics and Telecommunication Research Institute, Daejeon 305-700, Korea Received August 10, 2008; Revised Manuscript Received September 22, 2008
ABSTRACT For the first time, vertically suspended and stretched carbon nanotube network junctions were fabricated in large quantity via the directed assembly strategy using only conventional microfabrication facilities. In this process, surface molecular patterns on the side-wall of the Al structures were utilized to guide the assembly and alignment of carbon nanotubes in the solution. We also performed extensive experimental (electrical and mechanical) analysis and theoretical simulation about the vertically suspended single-walled carbon nanotube network junctions. The junctions exhibited semiconductor-like conductance behavior. Furthermore, we demonstrated gas sensing and electromechanical sensing using these devices.
Carbon nanotubes (CNTs) have been drawing much attention due to their superb electrical and mechanical properties.1 In particular, suspended junctions comprised of CNTs and nanowires (NWs) have attracted early interest to study the intrinsic physical properties of individual CNTs/NWs by isolating them from any extrinsic substrate effects.2,3 Early works of suspended CNT devices were usually focused on laterally suspended structures. Lateral CNT devices were first fabricated on substrates, and then the substrate regions below the CNT channel were selectively etched to suspend the CNTs.4,5 On the other hand, vertical CNT devices have been demonstrated by first growing CNTs on predefined templates via the chemical vapor deposition (CVD) process and then adding electrode structures on them.6-12 However, the CVD process to selectively deposit vertically aligned CNT was not usually compatible with conventional CMOS processing, and it has still been extremely difficult, if not impossible, to mass produce vertically stretched CNT structures. Beyond * To whom correspondence should be addressed. E-mail: seunghun@ snu.ac.kr. † Department of Physics and Astronomy, Seoul National University. ‡ Interdisciplinary Program in Nano-Science and Technology, Seoul National University. § Electronics and Telecommunication Research Institute. 10.1021/nl802434s CCC: $40.75 Published on Web 11/08/2008
2008 American Chemical Society
the fundamental study of intrinsic CNT properties, vertical CNT architecture possesses many auspicious properties such as higher areal density and other application specific advantages. Herein, we report a method to mass-produce vertically suspended and stretched CNT junctions integrated with conventional semiconductor devices. In this method, we utilized the directed assembly method to build threedimensional vertically suspended CNT network devices.13-16 Furthermore, the residual stresses of the clamping electrical leads resulted in a stretched CNT-based electromechanical junction. The electromechanical properties of the fabricated vertically suspended CNT structures have been investigated under various temperature, background gas pressure, and mechanical stress conditions. The results indicate that the vertically suspended single-walled CNT (swCNT) structures could be utilized for nanoscale electromechanical sensing components. This is the first demonstration of large-scale assembly of vertically suspended CNT devices. Significantly, since this method involves only room-temperature processing steps and conventional microfabrication facilities, it is readily accessible to present device industries and should open up
Figure 1. Schematic diagram depicting the fabrication process. (a) Al sacrificial structure was formed on SiO2 substrate via the microfabrication process. (b) Hydrophobic layer of OTS molecules was patterned on the substrates using the photolithography method. (c) CNTs were selectively assembled only onto the hydrophilic bare SiO2 and Al regions by placing the molecule-patterned substrate in the CNT solution. (d) Electrode patterns were formed by photolithography followed by angled metal deposition. (e) Electrodes were formed by lift-off process. (f) The Al sacrificial structure was removed by wet-etching in base solution.
various new applications including nanoscale electromechanical sensors. Figure 1 illustrates the procedure to fabricate the vertically suspended CNT network junctions. The detailed procedure can be found in the Supporting Information. Briefly, Al sacrificial structures were first fabricated on SiO2 substrate via photolithography and lift-off processes (Figure 1a). Then, nonpolar octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) patterns were generated via photolithography, while leaving some regions of SiO2 and Al surfaces untreated (Figure 1b).14 When the patterned substrate was immersed in CNT solution (0.5 mg/ml in 1,2-dichlorobenzene), CNTs were selectively adsorbed onto the bare SiO2 and Al surfaces (Figure 1c and Figure S1 in Supporting Information). Then, source and drain electrodes were fabricated via photolithography, angled metal deposition (Figure 1d), and lift-off process (Figure 1e). Note that the angled metal deposition (75 nm Ni on 5 nm Ti) resulted in a shadowed region on one vertical surface of the Al structure where the CNTs remained uncovered by the deposited metals. Finally, the Al sacrificial structure was removed in base solution, and the sample was dried using the critical point drying process (Figure 1f). We would like to point out two important characteristics in our final device structures, which distinguish our results with previous works. First, the CNT channel is vertically suspended and stretched out by the electrode beam unlike previously reported laterally suspended and relaxed CNTs. 4484
Figure 2. Images of vertically suspended CNT network junctions. (a) Optical micrograph showing the 4 × 4 array of vertically suspended swCNT network junctions. (b) Scanning electron microscope (SEM) image of vertically suspended swCNT network junction. (c) SEM image of vertically suspended swCNT network stretched out by the residual stress in the electrode beam. (d) SEM image of a vertically suspended swCNT bundle between the source and drain electrodes.
Here, the electrode beam always tended to bend away from the surface due to the residual stress in the electrode beam (Figure S2 in Supporting Information).17,18 In the presence of CNTs, the outward electrode beam stress is in equilibrium with the CNT channel tension, and the structure showed in overall high stability due to this static mechanical equilibrium. Another interesting aspect of this structure is that the CNT/metal contact at the electrode beam side is exposed to air. This exposed contact can be advantageous for highly sensitive chemical sensing applications because the contact between CNTs and electrode was reported to be very sensitive to the chemical environments. Figure 2a shows the optical image of a 4 × 4 array chip of vertically suspended CNT junctions. Each junction is composed of a source (S) electrode, a drain (D) electrode, and the vertical CNT network channel. It should be noted that our process allowed one to build individually addressable devices with the final device dimension arbitrarily defined by photolithography. Furthermore, since we used only conventional microfabrication techniques, the method is easily scalable for wafer-scale fabrication. Figure 2b shows the scanning electron microscopy (SEM) image of a vertically suspended junction comprised of swCNTs. It shows the beam electrode (source) and the drain electrode connected by a CNT network channel in a vertical direction. The height, length, and width of the upper beam electrode were ∼600 nm, ∼2 µm, and ∼4 µm, respectively. By the same process, we could obtain vertically suspended double-walled CNT (dwCNT) network junctions, indicating the versatility of our method (Figure S3 in Supporting Information). Figure 2c shows a stretched swCNT network channel. Note that the CNT channel was stretched out between the source Nano Lett., Vol. 8, No. 12, 2008
Figure 3. Electrical properties and uniformity of vertically suspended swCNT network junctions. (a) I-V characteristics of a typical vertically suspended swCNT network junction. (b) Resistance distribution of a 4 × 4 array chip of vertically suspended swCNT network junctions. (c) Arrhenius plot of the resistance R versus temperature T of a vertically suspended swCNT network junction. The inset shows the linear-scale temperature dependence of conductance G. (d) Sensor response (∆G/G0) to NO2 gas at room temperature. The inset shows the time variance of resistance R when exposed to 0.1, 0.5, 0.5, 2, and 5 ppm concentration NO2 gas (arrows).
and drain electrodes, unlike previously reported laterally suspended CNTs which were usually relaxed and loosely hanging between the electrodes. The number density of CNTs in the network channel could be controlled by adjusting the CNT assembly time or the CNT solution concentration. For example, the number of connecting CNTs in the junction could be reduced by reducing the assembly time and solution concentration. To demonstrate such, a single vertically suspended CNT bundle connecting the source and drain electrodes was depicted (Figure 2d). Figure 3a shows a typical IV curve between the sourcedrain electrodes of a vertically suspended swCNT network junction. The junction exhibited symmetric and rather linear I-V characteristics at the low bias conditions. As expected from the vertical geometry, the device exhibited rather small gating effect compared with lateral swCNT devices when the back-gate bias voltage was swept over a large voltage ranges (Figure S4 in Supporting Information). The distribution of source-drain resistance exhibited a typical log-normal distribution of percolating conducting networks (Figure 3b).19 Here, ∼75% of the junctions exhibited resistance in the range of 1 to 10 MΩ, which is a bit larger than planar devices with Pd electrodes.14 In our CNT assembly process, CNTs were adsorbed up to a certain number density due to the “self-limiting” mechanism, which is advantageous in achieving rather uniform density of CNT networks.20 However, the variance in resistance can be enhanced by some other parameters such as composition of metallic-semiconducting CNTs and contact resistance between CNT and electrodes, which are often difficult to control precisely. Especially, we utilized Ti as the contact material due to its strong affinity Nano Lett., Vol. 8, No. 12, 2008
to SiO2 surface. Ti electrodes are known to have rather large variation in contact resistance due to the Schottky barrier formation at the contact between swCNTs and electrodes.14 The temperature dependence of the conductance of a typical vertically suspended swCNT network junction was presented in Figure 3c. Overall, the junction exhibited semiconducting behavior, where the conductance decreased at lower temperatures. Interestingly, the Arrhenius plot shows two activation energies. The high-temperature activation energy of 22.58 meV has the same order with the value 12 meV reported by Kulesza et al. for a buckypaper.21 On the other hand, the low temperature activation energy of 0.05 meV represents the asymptotic approach of the resistance to a limit value. The high-temperature activation energy of 22.58 meV can be attributed to the energy for overcoming the Schottky barrier at the CNT-electrode. In our case, since the average CNT length used for the device fabrication was 2∼3 µm, while the channel length being 600 nm, it is more likely that the majority of the CNTs in the channel directly connected the electrodes, and the contribution of CNT-CNT contact to the junction resistance is rather small. Similar behavior was observed on our planar swCNT network junction with channel width ∼2 µm and channel length ∼2 µm, where the swCNT network channel is also consisted of metallic and semiconducting paths (Figure S5 in Supporting Information).14 The semiconducting behavior of the channel can be utilized to build highly sensitive sensors to detect external chemical molecules. Figure 3d shows the response versus NO2 background gas concentration. Here, the sensor response was defined as ∆G/G0, where ∆G and G0 are the conductance change and the initial conductance of the junction, respectively. In this experiment, the resistance was monitored at room-temperature while gas with different NO2 concentration in N2 was flown successively and UV recovery was used to expedite the recovery time. The device shows rather linear sensing characteristics in the range of 0.1 to 5 ppm. The resistance was observed to decrease when exposed to NO2 gas.22-24 Notably, the device showed ppm-level sensitivity even at room temperature. This capability of room-temperature operation is advantageous for actual commercialization of the device, where the lack of a heating circuit can considerably save equipment power consumption. The electromechanical properties of the devices were measured by pressing the beam with an atomic force microscope (AFM) tip (Figure 4). First, the AFM tip was pressed against the hard substrate to obtain a force-distance curve of the tip itself. Then, the AFM tip was pressed on the center of the electrode beam to obtain its force-distance curve. Figure 4a shows the force versus the vertical displacement of the piezoelectric head Zpiezo comparing the cases when the AFM tip pressed the hard substrate and the electrode beam. The extending and retracting curves overlapped well, indicating that the electrode beam structure was not deformed during the force-distance experiment (Figure S6 in Supporting Information). Figure 4b shows the graph of force versus actual CNT retraction displacement D of the electrode beam. Here, the 4485
Figure 4. Electromechanical properties of vertically suspended swCNT network junctions. (a) Force-distance curves using an AFM tip on the SiO2 substrate (black) and on the electrode beam (blue and red). The overlapping extending (red) and retracting (blue) curves manifest the resilience of the electrode beam structure. The actual downward displacement δ of the beam at the contact point with the AFM tip can be obtained from the difference in cantilever displacement from both cases. (b) Graph of force versus CNT network retraction displacement D ∼ 2δ. The square data points represent the experimental data obtained from the results in panel a. The dashed line represents the force versus distance curve of only the electrode beam obtained by the finite element method simulation (see the Supporting Information). The inset diagrams and the vertical dotted line around D ∼ 190 nm show our model of CNT network bending, where the CNT network channel undergoes transition between vertical unstretching motion and lateral bending motion around D ∼ 190 nm. (c) Time-dependent modulation of the channel conductance G by the strain change of an initially stretched-out vertically suspended CNT network junction. After pressing the electrode beam, the AFM probe tip was kept still for 3 s before retracting, which accounts for the upper plateau on both data. The conductance (top) and deflection (bottom) data show the conduction modulation effect by pressing the electrode beam with an AFM tip. (d) Graph of conductance G versus compressive displacement D of the electrode beam when pressed with an AFM tip. The vertical dotted line at D ∼ 190 nm was drawn at the same position as in panel b. It shows strong correlation with mechanical behavior and the conductance modulation behavior.
actual downward displacement D of the electrode beam was obtained by calculating the Zpiezo value difference δ between the curves measured on the beam and the substrate as shown in Figure 4a, which corresponds to the actual beam displacement at the contact point with the AFM tip.25 Since the beam was pressed at the center of the beam, we can use D ∼ 2δ. It shows clear transition at D ∼ 190 nm. The dashed line in Figure 4b represents the elastic properties of the electrode beam estimated via the finite element method (Supporting Information). The results indicate that the electrode beam has linear behavior in this range. We propose it is related with the transition of elastic properties of vertically suspended swCNTs. Initially, the swCNT networks were stretched out due to the internal stress of the electrode beam. As the AFM tip began to press down the beam with a rather small displacement D < ∼190 nm, the swCNT networks began to 4486
relax and worked as a spring (left inset in Figure 4b). In this case, we can model the CNT/beam structure to be consisted of two elastic components, manifesting a total additive spring constant from the contribution of the CNT network and the electrode beam. However, as the AFM tip pressed down the beam with a large displacement D > ∼190 nm, the swCNT network was fully relaxed and began to bend laterally (right inset in Figure 4b). The elastic properties of the swCNT film can be obtained from the results in Figure 4b. First, the spring constant of the electrode beam was estimated from the data at a large displacement (D > 190 nm in Figure 4b) by assuming that the spring constant for the lateral bending of the thin swCNT monolayer film was negligible compared with that of the thick electrode beam. Then, the data with a small displacement (100 nm < D < 190 nm in Figure 4b) was modeled as a series of two springs comprised of the stretched swCNT film and the electrode beam, which allowed us to estimate the spring constant of the stretched swCNT film. Finally, the Young’s modulus of the stretched swCNT film was estimated to be 13.0 GPa using the apparent dimension of the film (W ∼ 3.94 µm, H ∼ 635 nm, and T ∼ 2.33 nm) measured from its SEM images. It should be noted that, unlike bulk material films, our swCNT film contained a large portion of empty volume in it, and we can expect the Young’s modulus value estimated by this method to be much smaller than that of a single swCNT. Previously, theoretical26 and experimental27 Young’s modulus values for a single swCNT were reported as ∼5.5 TPa and ∼1.25 TPa, respectively. On the other hand, the theoretical28 and experimental29 values for bulk swCNT films were reported as 0.9∼8.5 and 1.2 GPa, respectively. Our measured value was a bit larger than previous results for swCNT films, presumably because some swCNTs in the film were directly connecting between electrodes. The mechanical deformation of swCNT networks caused the conductance changes of the junctions. Figure 4c shows that the conductance of the swCNT network junction increased as the AFM tip pressed down the beam and relaxed the swCNT network. We could repeatedly modulate the conductance of the swCNT network junction by 12%. Previous reports showed that the conductance of individual swCNTs decreases under expansive stress, which was associated with significant local deformation of band structures in the individual swCNT.5 On the other hand, stretching swCNT network junction should weaken the CNT-electrode contact and connectivity of the networks, resulting in the reduced conductance of the junction. Both mechanisms indicate we can expect enhanced conductance as the stretched swCNT network in our device was relaxed. Figure 4d shows the graph of the conductance versus CNT retraction displacement D. As the AFM tip pressed down the beam, the conductance increased only up to D ∼ 190 nm and reached saturation. It should be noted that we could also observe a transition at D ∼ 190 nm in Figure 4b. Presumably, at small displacement (D < 190 nm), the stretched swCNT network was relaxed and its conductance was increased significantly. However, at a rather large Nano Lett., Vol. 8, No. 12, 2008
displacement (D > 190 nm), the swCNT network began to bend laterally, which did not change its conductance. In summary, we successfully fabricated vertically suspended CNT network junctions via the directed assembly method. The conductance of vertically suspended swCNT junctions showed semiconductor-like temperature dependence. In bending experiments, reduced tension on vertically suspended CNTs resulted in increased conductance. Our method is compatible with conventional microfabrication process, and it is fully scalable for large-scale fabrication. We expect these devices can be used for various possible applications such as gas sensor, pressure sensor, highfrequency resonators, and so forth. Acknowledgment. The work in this paper was supported by the NRL program (No. R0A-2004-000-10438-0). S.H. acknowledges the supports from the TND program of KOSEF and the System 2010 program of MKE. Y.D.P. was partly supported by Nano R & D program through the KOSEF funded by the Ministry of Education, Science, and Technology. Supporting Information Available: Supplementary figures, additional details on fabrication method, electrical characterization, and finite element method simulation. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (2) Bezryadin, A.; Lau, C. N.; Tinkham, M. Nature 2000, 404, 971. (3) Kasumov, A. Y.; Deblock, R.; Kociak, M.; Reulet, B.; Bouchiat, H.; Khodos, I. I.; Gorbatov, Y. B.; Volkov, V. T.; Journet, C.; Burghard, M. Science 1999, 284, 1508. (4) Sazonova, V.; Yaish, Y.; Ustunel, H.; Roundy, D.; Arias, T. A.; McEuen, P. L. Nature 2004, 431, 284. (5) Tombler, T. W.; Zhou, C. W.; Alexseyev, L.; Kong, J.; Dai, H. J.; Lei, L.; Jayanthi, C. S.; Tang, M. J.; Wu, S. Y. Nature 2000, 405, 769. (6) Awano, Y.; Sato, S.; Kondo, D.; Ohfuti, M.; Kawabata, A.; Nihei, M.; Yokoyama, N. Phys. Status Solidi A 2006, 203, 3611. (7) Horibe, M.; Nihei, M.; Kondo, D.; Kawabata, A.; Awano, Y. Jpn. J. Appl. Phys. 2004, 43, 6499.
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