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2009, 113, 37–39 Published on Web 12/15/2008
Controlled Dielectrophoretic Assembly of Multiwalled Carbon Nanotubes Libao An,† Daw Don Cheam,‡ and Craig R. Friedrich*,† Department of Mechanical EngineeringsEngineering Mechanics, and Department of Electrical and Computer Engineering, Multi-Scale Technologies Institute, Michigan Technological UniVersity, 1400 Townsend DriVe, Houghton, Michigan 49931 ReceiVed: October 27, 2008; ReVised Manuscript ReceiVed: December 2, 2008
The authors report on a real-time control method for assembly of a single or small number of multiwalled carbon nanotubes (MWNTs) onto microelectrodes by dielectrophoresis (DEP). On the basis of an impedance model and a real-time gap impedance monitoring method to evaluate and identify the number of MWNTs spanning an electrode gap, it has been demonstrated that a real-time gap impedance signal can be used to control the DEP process of MWNTs. The goal is controlled assembly with a connecting resistance of less than a defined limit or with a defined number of MWNT connections. With a control strategy designed for the DEP assembly of a small number of MWNTs, predefined numbers of connections from 1-3 have been achieved across an electrode gap. Carbon nanotubes (CNTs) have been under study for a variety of potential applications over the past decade due to their excellent electrical, mechanical, and thermal properties.1-3 Among these applications, the high current-carrying capacity of metallic CNTs makes them suitable for electrical connections in nanodevices and circuits.4-6 Dielectrophoresis (DEP) has attracted much research as a manipulation method to assemble CNTs between a pair of electrical conductors or to integrate CNTs into a microelectronics system.7-9 The electrical contact properties of CNTs with metals are also under investigation.10 One aspect of automatically assembling CNTs by DEP is the difficulty in controlling the process in real time. Consequently, scanning electron microscopy (SEM) is needed for inspection, and the DEP results are not predictable or controllable by the process itself. This paper reports on a real-time control method for assembly of a single or a small number of multiwalled carbon nanotubes (MWNTs) onto microelectrodes by DEP. It follows previous work on real-time gap impedance monitoring of dielectrophoretic assembly of MWNTs.11 Impedance has been measured during DEP for the deposition of both a very large number12 and a small number11 of CNTs. In the former case of a very large number of trapped CNTs, conductance measurements were performed to quantify the bridging of CNTs, but the monitoring time lasted several hours, and the number of CNTs could not be measured. In the latter case, the authors of the present work developed an impedance model for DEP assembly of a small number of metallic CNTs. On the basis of the model, instantaneous gap impedance measurements were used to monitor the process and to identify the number of MWNTs spanning the electrode gap when the deposition of a small number was required. The model showed that determining the number of * To whom correspondence should be addressed. Tel: 1 906 487 1922. Fax: 1 906 487 2822. E-mail:
[email protected]. † Department of Mechanical EngineeringsEngineering Mechanics. ‡ Department of Electrical and Computer Engineering.
10.1021/jp809502x CCC: $40.75
assembled CNTs by measuring the gap resistance variation was the same as measuring the impedance variation if the electrode gap was bridged since the impedance change was dominated by the resistance change.11 The real-time impedance monitoring method provides an approach for deposition control. In the present work, the connecting resistance, or the number of connections across the gap, is measured. This signal is then used to switch off the electric field to stop the process after the required connecting resistance is obtained or the required number of connections is achieved. The novelty of this method is that it controls the deposition of a small number of MWNTs, one by one, down to a single MWNT over a short period of time. Therefore, it will be of value for applications such as CNT single-electron transistors, CNT interconnects, and biosensors using functionalized CNTs. In setting up experiments for controlled DEP, a precision LCR (inductance, capacitance, resistance) meter was used to generate the AC electric field for deposition while simultaneously measuring the instantaneous impedance of the electrode gap. A semiconductor characterization system (SCS) was used to control the LCR meter by C-programming to perform the DEP while recording the measurement results from the LCR meter. The current-voltage (I-V) characteristics of assembled MWNTs were measured using a two-terminal method with the SCS after DEP to investigate if the assembled resistance changed over a subsequent period of days. To achieve a connecting resistance of less than a certain required value, an impedance threshold was defined in software, and the control system monitored the gap impedance signal. When a particular connecting resistance and therefore an assembly of a corresponding number of MWNTs were reached, the DEP was permitted to continue for an adjustable time period (1-10 s) after the threshold was reached. Continuing DEP for several additional seconds helped ensure a stable last connection. For an assembly with a required small number of MWNT connections, a control strategy was designed in which the 2009 American Chemical Society
38 J. Phys. Chem. C, Vol. 113, No. 1, 2009
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Figure 1. DEP tests for assembly with a connecting resistance of less than a variety of defined limits. (a) Impedance signal when the threshold was 120 kΩ and the test voltage was U ) 1.5 Vp-p. (b) SEM inspection of (a). (c) I-V characteristics of assembled MWNTs in (b). (d) Impedance signal when the threshold was 60 kΩ and the test voltage was U ) 2 Vp-p. (e and f) SEM inspection of (d). (g) Impedance signal when the threshold was 40 kΩ and the test voltage was U ) 3 Vp-p. (h, i, and j) SEM inspection of (g).
Figure 2. DEP tests for assembly with a defined number of connections. (a) Impedance signal when the defined number of connections was one and the test voltage was U ) 2 Vp-p. (b) SEM inspection of (a). (c) Impedance signal when the defined number of connections was two and the test voltage was U ) 2 Vp-p. (d) SEM inspection of (c). (e) Impedance signal when the defined number of connections was three and the test voltage was U ) 3 Vp-p. (f, g, and h) SEM inspection of (e).
average impedance value of an adjustable number of impedance data points was compared with that of the same number of previous impedance data points to judge a sharp impedance drop indicating a connection or an additional connection. The acquisition rate was 3 Hz. The impedance model and DEP monitoring tests show that, if many MWNTs are deposited across the gap, it may not be possible to recognize the deposition of additional tubes due to insignificant changes of the impedance.11 MWNTs were grown by a plasma-enhanced chemical vapor deposition (PE-CVD) method. They were 0.5-4 µm long and about 50 nm in diameter. The MWNTs were dispersed in isopropyl alcohol (IPA) and sonicated for 2 h. Au/Cr (60/20 nm thick) electrodes with a tip separation of 2 µm were fabricated on silicon wafers by standard lithography and liftoff techniques. Besides multiparallel electrodes with 10 electrode pairs,11 a pair of offset parallel electrodes with a centerline spacing of 2 µm (Figure 2b) and a pair of perpendicular electrodes (Figure 2d) were also used. A 1 µL drop of MWNT/ IPA solution was introduced onto the electrode gaps through a micropipette. The AC electric field was switched on and automatically switched off after the preset impedance threshold was measured or the required number of connections was reached. After switch off, pure IPA was gently introduced onto the gap region to prevent further random deposition of MWNTs.
The magnitude of the applied voltage was selected between 1.5 and 3 V peak to peak (Vp-p), and the frequency was fixed at 500 kHz. The electric field was approximately 106 V/m; therefore, the DEP force was dominant over the Brownian motion of the MWNTs.13,14 The concentration of the MWNT/ IPA solution was 1 µg/mL. In Figure 1, the control method was tested to obtain assemblies with a connecting resistance less than a predefined limit. Previous tests showed that the gap impedance connected by a single MWNT varied between 25 and 120 kΩ.11 In Figure 1a, a threshold of 120 kΩ was set, and the DEP continued for 10 s after the impedance signal decreased to below the threshold. The electric field was switched on at time zero after dropping the MWNT/IPA solution. The threshold was reached at 57 s, and the impedance signal dropped to 45 kΩ. SEM inspection in Figure 1b shows that only one MWNT was deposited across the perpendicular electrode pair. Figure 1c shows the I-V characteristics of the assembled MWNT several days later, and the measured resistance was 82 kΩ. In Figure 1d, we set a threshold of 60 kΩ, and 2 of the 10 electrode pairs of the multiparallel electrode pattern were connected (Figure 1e and f). In Figure 1g, the threshold was set at 40 kΩ, and 3 of the 10 parallel electrodes were connected (Figure 1h-j). In Figure 2, assemblies were obtained with a specified small number of MWNT connections. First, the required number of
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J. Phys. Chem. C, Vol. 113, No. 1, 2009 39
TABLE 1: Final Impedances Measured during DEP and Resistances Measured after DEP number of connections
1
2
3
1
2
3
Z measured during DEP (kΩ) R measured after DEP (kΩ)
45 82
52 134
34 85
105 205
57 279
41 192
connections was set at one, and Figure 2a shows the gap impedance signal in which one sharp impedance drop to 105 kΩ was measured at 62 s. The DEP was continued for 10 additional seconds to stabilize the connection, and no further sharp impedance drops were measured. Figure 2b shows that one MWNT bundle was deposited across the gap between a pair of offset parallel electrodes. Figure 2c shows the gap impedance signal when the number of connections was two. Figure 2d shows that two MWNTs were assembled across the gap. Figure 2e shows the gap impedance signal when three connections were required on the multiparallel electrodes. SEM inspections of the connections are shown in Figure 2f-h. In all cases, the I-V characteristics of the assembled MWNTs were measured with sweeping voltages from -0.5 to 0.5 V. The final impedances measured during DEP and resistances measured several days later are listed in Table 1. It was found that the resistances of MWNTs measured several days after assembly were usually higher than the impedance values measured during DEP. One possible reason is that a tight contact between the MWNTs and the Au electrodes in IPA during DEP may have loosened after several days, resulting in an increase in contact resistance. This contrasts previous observations by Suehiro et al. that the resistance of assembled CNTs between Cr electrodes immediately after ethanol removal was lower than that before ethanol removal.12 On the basis of the assembly results, a connection across an electrode gap could be comprised of a single MWNT, or multiple MWNTs in a bundle or a chain. Our experiments showed that the DEP parameters, including the magnitude of the applied voltage, the time duration of the electric field, and specifically the concentration of the MWNT solution, played an important role in the deposition of MWNTs. In this work, low applied potentials (1.5-3 Vp-p) and MWNT concentration (1 µg/mL) were chosen to ensure a single connection or a very small number of connections. It has been reported that the total connecting resistance of an assembled CNT depends primarily on its contact resistance with the electrodes.10 This explains why the impedances of the electrode gap connected by a single MWNT varied between 25 and 120 kΩ. The impedance value can be even larger if the gap is connected by multiple MWNTs in a chain because of tube/tube contact resistance. In performing the controlled DEP, the measured impedance signals revealed more details of the process. In Figure 3a, an impedance signal is shown which changed continuously with time after a sharp drop. The SEM image in Figure 3b shows that several MWNTs tangled together and deposited across the gap. The impedance signal suggested that the tangled MWNTs
Figure 3. Continuous decrease of impedance signal and SEM inspection.
made gradual contact with the electrodes. Under an AC electric field, polarized particles exhibit interparticle chaining force,15 which may cause the tubes to attract each other during DEP. This reinforces the need for a low concentration of tubes which are well dispersed within the medium prior to deposition. In summary, a real-time control method for DEP assembly of MWNTs has been demonstrated. In this method, the measured electrode gap impedance indicating the DEP status was used as a feedback signal to control the deposition process. The method was able to automatically produce MWNT assemblies with a connecting resistance of less than a certain required limit or assemblies with a certain required number of MWNT connections. This could help automate assembly of metallic CNTs. The method might also be extended to other DEP applications such as for monitoring the response of nanotubes with different electrical behaviors and controlling the separation process of metallic and semiconducting single-walled CNTs. Acknowledgment. This research was performed in connection with Contract DAAD17-03-C-0115 with the U.S. Army Research Laboratory. References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (2) Terrones, M. Annu. ReV. Mater. Res. 2003, 33, 419–501. (3) Regan, B. C.; Alonl, S.; Ritchle, R. O.; Dahmen, U.; Zettl, A. Nature 2004, 428, 924–927. (4) Wei, B. Q.; Vajtai, R.; Ajayan, P. M. Appl. Phys. Lett. 2001, 79, 1172–1174. (5) Kreupl, F.; Graham, A. P.; Duesberg, G. S.; Steinhoegl, W.; Liebau, M.; Unger, E.; Hoenlein, W. Microelectron. Eng. 2002, 64, 399–408. (6) Li, J.; Ye, Q.; Cassell, A.; Ng, H. T.; Stevens, R.; Han, J.; Meyyappan, M. Appl. Phys. Lett. 2003, 82, 2491–2493. (7) Seo, H. W.; Han, C. S.; Choi, D. G.; Kim, K. S.; Lee, Y. H. Microelectron. Eng. 2005, 81, 83–89. (8) Chen, Z.; Hu, W.; Guo, J.; Saito, K. J. Vac. Sci. Technol. B 2004, 22, 776–780. (9) Li, J.; Zhang, Q.; Peng, N.; Zhu, Q. Appl. Phys. Lett. 2005, 86, 153116. (10) Andriotis, A. N.; Menon, M.; Froudakis, G. E. Appl. Phys. Lett. 2000, 76, 3890–3892. (11) An, L.; Friedrich, C. R. Appl. Phys. Lett. 2008, 92, 173103. (12) Suehiro, J.; Zhou, G.; Imakiire, H.; Ding, W.; Hara, M. Sens. Actuators, B 2005, 108, 398–403. (13) Kim, J. E.; Han, C. S. Nanotechnology 2005, 16, 2245–2250. (14) Mendes, M. J.; Schmidt, H. K.; Pasquali, M. J. Phys. Chem. B 2008, 112, 7467–7477. (15) Jones, T. B. Electromechanics of Particles; Cambridge University Press: Cambridge, U.K., 1995.
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