NANO LETTERS
Simultaneous Deposition of Metallic Bundles of Single-walled Carbon Nanotubes Using Ac-dielectrophoresis
2003 Vol. 3, No. 8 1019-1023
R. Krupke,*,† F. Hennrich,† H. B. Weber,† M. M. Kappes,†,‡ and H. v. Lo1 hneysen§,| Forschungszentrum Karlsruhe, Institut fu¨r Nanotechnologie and Institut fu¨r Festko¨rperphysik, D-76021 Karlsruhe, Germany, and Institut fu¨r Physikalische Chemie and Physikalisches Institut, UniVersita¨t Karlsruhe, D-76128 Karlsruhe, Germany Received April 17, 2003; Revised Manuscript Received May 15, 2003
ABSTRACT A simple and scalable scheme based on alternating current (ac-) dielectrophoresis is used for the simultaneous and site-selective deposition of single bundles of single-walled carbon nanotubes (SWNTs) onto a large number of contacts requiring only one bond wire. This allows for a large number of transport measurements that show that the deposited bundles contain at least one metallic SWNT dominating the transport. With high-voltage pulses, metallic bundles are transformed into Schottky-barrier-type field-effect transistors with p-type, ambipolar, or n-type behavior. From a simple electromechanical model, we derive the fact that ac-dielectrophoresis selectively deposits bundles containing metallic tubes.
After more than 10 years of extensive research on carbon nanotubes, it is widely recognized that these unique 1-D nanostructures have considerable potential as building blocks in future nanoscale electronics.1-5 Yet, there are major difficulties that prevent the realization of carbon nanotubebased circuitry today. One issue is the separation of metallic from semiconducting SWNTs either by selective production or by after-growth separation schemes. Another important issue is the growth or assembly of SWNTs into a nanoscale structure. In this context, various methods have been developed for the site-selective deposition or growth of SWNTs on surfaces.6-10 Recently, dielectrophoresis has attracted much interest for positioning SWNTs. The alignment of SWNT bundles on a surface by using alternating electric fields was observed first by Yamamoto and coauthors.11 Since then, alternating current (ac)-dielectrophoresis has been used for the deposition and alignment of large numbers of SWNT bundles,12 for the assembly of small networks of carbon nanotubes,13 and for the positioning and contacting of small numbers of carbon nanotubes on submicrometer-scale electrodes,14 and recently it has been possible to deposit and contact a single bundle of SWNTs selectively by controlling the chemical bonds formed between the metal and the nanotube.15 * Corresponding author. E-mail:
[email protected]. † Forschungszentrum Karlsruhe, Institut fu ¨ r Nanotechnologie. ‡ Institut fu ¨ r Physikalische Chemie, Universita¨t Karlsruhe. § Physikalisches Institut, Universita ¨ t Karlsruhe. | Forschungszentrum Karlsruhe, Institut fu ¨ r Festko¨rperphysik. 10.1021/nl0342343 CCC: $25.00 Published on Web 07/09/2003
© 2003 American Chemical Society
In this paper, we will extend the controlled deposition of a single bundle of SWNTs to a simultaneous and controlled deposition of SWNTs onto multiple submicrometer electrode pairs by ac-dielectrophoresis. We will show a simple wiring scheme based on capacitive coupling between the substrate and the electrodes that limits the number of deposited tubes per contact and is independent of the number of contacts to be formed. Furthermore, we will argue that the metallic behavior of all contacts can be explained by the selectivity of the deposition method in favor of metallic SWNTs. Single-walled carbon nanotubes were grown by laser ablation, purified with nitric acid, and suspended in N,Ndimethylformamide (DMF).16 Ultraviolet to near-infrared absorbance measurements (UV-vis-NIR) of suspended SWNTs show three absorbance bands S1, S2, and M1 attributed to pairwise excitations between van-Hove singularities in the electronic density of states of semiconducting (S1, S2) and metallic (M1) SWNTs (Figure 1). For the acdielectrophoretic trapping experiments, the suspension was repeatedly sonicated and diluted to the extent that the suspension appeared colorless and transparent (SWNT concentration ≈ 10 ng/mL). Electrodes were prepared with standard e-beam lithography and were wired to the power supply as depicted in Figure 2. The power supply is a function generator with a maximum frequency f ) 30 MHz and an input impedance Z ) 500 Ω. Unwired electrodes are floating, and the substrate is grounded. After switching on the frequency generator to typically Vp-p ) 1 V at f ) 1 MHz, a drop of the nanotube suspension
Figure 1. UV-vis absorption spectra of SWNTs dispersed in DMF showing fully developed absorption bands, which indicates the absence of severe doping16. An asterisk (*) marks the absorption of H2O.17 The linear background was subtracted.
Figure 2. Schematic drawing of the wiring scheme (not to scale). The electrodes (yellow) are 20 nm thick, 80-150 nm wide, and have a gap of 400 nm on a p-type Si substrate (blue) with 600-nm thermally oxidized SiO2 (green). Ag has been used as the top-layer electrode material.
(∼10 µL) is applied to the chip with a pipet. After a delay of typically 1 min, the drop is blown gently off the surface with nitrogen gas. Finally, the generator is turned off, and the sample is characterized by scanning electron microscopy (SEM) or atomic force microscopy (AFM). A typical result of a simultaneous deposition of SWNTs onto an electrode array via ac-dielectrophoresis is shown in Figure 3. Eleven out of 16 contacts have been bridged by at least 1 bundle, which translates to a yield of approximately 70%, a typical number in our experiments under the given conditions. Counting the number of bundles deposited on each contact shows that six contacts are closed by a single bundle. At three contacts, we count two bundles, and at two contacts, we count three bundles. The numbers resemble the statistics from our experiments on individual contacts on Ag electrodes given in ref 15, where we have demonstrated the importance of matching the electrode material with the chemical functional groups of our carbon nanotubes to control the number of trapped bundles. In a previous publication, we anticipated that ac-dielectrophoresis would also work with individual tubes.15 Although these are rare species in SWNT/DMF suspensions, we were able to find a contact now bridged by what appears to be an individual SWNT (Figure 4). This conclusion can be drawn from AFM profiles across the tube that indicate the thickness of one tube. The wiring scheme that allows the simultaneous deposition of single bundles is based on capacitive coupling between 1020
the floating leads to the ground. Although the capacitive coupling in the vicinity of the solvent is difficult to calculate, we estimate from the geometry of our configuration a typical capacitance of C ≈ 100 pF. This translates at f ) 1 MHz to an impedance on the order of ZC ) 10 kΩ, coupling the floating electrodes to the grounded substrate. The potential difference between the driven and floating electrodes can be maintained up to f ) 20 MHz, where the capacitive coupling resistance becomes comparable with the impedance of our function generator. Performing the deposition at high frequency ensures that the potential of a floating electrode remains coupled to ground until an SWNT bundle forms a contact with a resistance on the order of ZC. As a result, the field is too weak to attract additional tubes. Because all floating electrodes are individually coupled to ground, the deposition of tubes occurs independently, at least on the characteristic length scales involved in our experiment. Note that this scalable scheme for the simultaneous deposition of single bundles of SWNTs requires only one bond wire regardless of the number of electrode pairs. This technique allows for a high throughput screening of SWNT contacts (e.g., by electrical transport measurements). To characterize the setup further, electrical transport measurements were performed for two configurations: (i) measuring the source-drain conductance GSD as a function of the gate voltage VG at room temperature (Figure 5); the gate voltage was applied to the Si substrate; and (ii) measuring the temperature dependence of the source-drain conductance GSD at VG ) 0 V (Figure 6); each electrode pair is separately measured using a source meter with one electrode (the drain) grounded. In our experiments, bundles deposited by ac-dielectrophoresis show only a weak dependence of G on the gate voltage (Figure 5a). This behavior is consistent with literature reports on metallic SWNTs,19 from which we deduce the existence of at least one metallic SWNT within our dielectrophoretically deposited bundles. Also, the temperature dependence of GSD, showing the power-law behavior G ∝ TR (Figure 6), is typical for our contacts formed by ac-dielectrophoresis and can be explained by tunneling into a Luttinger liquid provided that a metallic SWNT dominates the transport through the bundle. Deviations from the power law at low temperature have been reported in the literature and were attributed to the onset of a Coulomb blockade.20 Application of high-voltage pulses at zero gate voltage can change the gate dependence from metallic-like behavior to semiconducting-like behavior (Figure 5b). We assume that metallic SWNTs within a bundle are destroyed, whereafter the semiconducting SWNTs, which survived, dominate the transport.21 After the first high-voltage pulse, with typical currents of ISD > 30 µA, most metallic bundles transform into a p-type field-effect transistor (Figure 5c). Applying additional pulses, one can gradually transform the device from a p-type to an n-type transistor (Figure 5d).22 The described behavior is systematically observed. Because of the small change in VG*, at which the device switches from “on” to “off”, we explain the observed behavior in terms of Nano Lett., Vol. 3, No. 8, 2003
Figure 3. Scanning electron micrographs of single bundles of SWNTs deposited simultaneously onto an electrode array. Overview: The upper bond pads are driven by the frequency generator. The lower bond pads are all at the floating potential. Electrode pairs within areas A-C are separated from each other by 300, 100, and 10 µm, respectively18. Table 1. Probability for a Bundle of N SWNTs to Contain at Least One Metallic SWNT Given by the Sum over Binomial Distributions with a General Abundance for Metallic Tubes of 1/325 N P(N)[%]
Figure 4. One individual SWNT deposited by ac-dielectrophoresis onto two Ag electrodes. (Left) Scanning electron micrograph. (Right) Atomic force tapping-mode micrograph showing a mean thickness of d ) 1.2 nm in the inset.
Schottky barrier field-effect transistors, where the contacts, and hence the Schottky barriers, are gradually modified by the high-voltage pulses.23 From the rather small offset of VG*, we deduce the absence of severe doping of the SWNTs, meaning that the Fermi level is not shifted into the conductance or valence bands.24 This is consistent with the well-developed S1 absorption band in the UV-vis-NIR spectra (Figure 1) as compared to that of acid-treated SWNT films.16 It demonstrates that functional devices can be built from chemically purified SWNT material. Nano Lett., Vol. 3, No. 8, 2003
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The fact that we always detect metallic behavior of the bundles might be attributed to either (i) a selection of metallic bundles by our technique (which would be desirable) or (ii) simple combinatorics: the average bundle in our experiment contains N ≈ 7 tubes/bundle as estimated from typical bundle diameters of 3-4 nm. We then derive from a simple combinatorial calculation that there is a high probability (>90%) of at least one metallic tube being incorporated into a bundle (Table 1). Nonetheless, we believe that our method selects metallic rather than semiconducting bundles. This can be deduced from the following considerations of the electromechanical response of SWNTs. To describe the movement of suspended SWNTs in electric fields, it is important to differentiate between dielectrophoretic and electrophoretic forces.26,27 In our experiment, using the frequency f ) 1 MHz, electrophoretic forces due to charging28 vanish because of time averaging, and only the induced dipole moment of the SWNT,29 interacting with the inhomogeneous electric field, gives rise to translational movement along the electric field gradient. To consider only the simplest case, the time-averaged dielectrophoretic force for a dielectric sphere can be expressed as 1021
Figure 5. Source-drain conductance GSD vs gate voltage VG for various contacts formed by ac-dielectrophoresis. (a) Metallic behavior (no gate effect) of as-deposited bundles (VSD ) 0.1 V). (b) Transformation of a metallic bundle (upper, VSD ) 0.1 V) into an ambipolar SB-FET (lower, VSD ) 0.5 V) by high-voltage pulses. (c) p-type SB-FET (VSD ) 3 V). (d) n-type SB-FET (VSD ) 0.5 V). All measurement were taken at room temperature in vacuum (p ) 10-5 mbar).
calculated in ref 29 and was found to be inversely proportional to the square of their band gap EG, the energetic distance between the first pairwise vHs (S1): S = 1 +
Figure 6. Temperature dependence of the source-drain conductance GSD for various contacts formed by ac-dielectrophoresis. Partial fitting to a power law is marked by red dashes with the exponent R to the right.
p - m B FDEP ∝ m ∇E2 p + 2m rms
(1)
p and m are the dielectric constants of the particle and the solvent medium, respectively, and Erms is the average field strength27. Equation 1 is a valid approximation for our experiment because metallic and semiconducting SWNTs are ballistic conductors and insulators, respectively.30 The static dielectric constant for semiconducting SWNTs, S, has been 1022
( ) pωp 5.4EG
2
(2)
pωp ≈ 5 eV is the energy of the plasma oscillation along the nanotube axis.31 From tight-binding calculations with curvature-modified hopping parameters,32 we find for our tubes with a diameter d ) 1.1-1.4 nm the smallest possible band-gap values EG between 0.19γ0 - 0.24γ0, where γ0 is the tight-binding parameter. With γ0 ≈ 2.5-3.0 eV, we obtain a minimum band gap of EG ≈ 0.5 eV for the semiconducting tubes in our samples. Translating band gaps into dielectric constants according to eq 2, we obtain finite dielectric constants for semiconducting SWNTs with S < 5. For metallic SWNTs, owing to the mobile carriers, we expect a very large absolute value of the dielectric constant. In fact, it has been suggested that the polarizability of metallic SWNTs is infinitely large.29 This statement is also true for quasi-metallic SWNTs with energy gaps smaller than the thermal energy. Coming back to eq 1, we can calculate the sign of the dielectrophoretic force for SWNTs in DMF, with DMF ) 39 and assuming that (1 MHz) ) (0). As a conclusion, we derive a negative dielectrophoresis for semiconducting SWNTs, whereas for the metallic and Nano Lett., Vol. 3, No. 8, 2003
quasi-metallic SWNTs the dielectrophoresis is positive (attraction toward higher field strength). From these estimations, we conclude that via ac-dielectrophoresis we selectively deposit metallic SWNTs or bundles that contain at least one metallic SWNT. Experiments with individual SWNTs (not bundles) are in progress. In conclusion, we have demonstrated a simple scalable deposition scheme that allows the simultaneous deposition of single bundles of SWNTs onto a large number of contacts requiring only one bond wire. Transport measurements show that the deposited bundles contain at least one metallic SWNT dominating the transport. With high-voltage pulses, the metallic bundles can be transformed into Schottkybarrier-type field-effect transistors with p-type, ambipolar, or n-type behavior. Severe doping of the chemically cleaned samples can be excluded. We have derived that ac-dielectrophoresis should selectively deposit metallic SWNTs or metallic bundles because of the specific differences in the dielectric constants of metallic SWNTs, semiconducting SWNTs, and the solvent. Acknowledgment. We thank D. Beckmann and D. Secker for helpful discussions. References (1) McEuen, P. Phys. World 2000, 13, 31-36. (2) Yao, Z.; Kane, C. L.; Dekker, C. Phys. ReV. Lett. 2000, 84, 29412944. (3) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49-52. (4) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 2773-2775. (5) Wind, S. J.; Appenzeller, J.; Martel, R.; Derycke, V.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 3817-3819. (6) Gerdes, S.; Ondarcuhu, T.; Cholet, S.; Joachim, C. Europhys. Lett. 1999, 48, 292-298. (7) Ahlskog, M.; Seynaeve, E.; Vullers, R. J. M.; Van Haesendonck, C. J. Appl. Phys. 1999, 85, 8432-8435. (8) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125-129. (9) Krupke, R.; Malik, S.; Weber, H. B.; Hampe, O.; Kappes, M. M.; v. Lo¨hneysen, H. Nano Lett. 2002, 2, 1161-1164.
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(10) Kong, J.; Zhou, C.; Morpugo, A.; Soh, H. T.; Quate, C. F.; Marcus, C.; Dai, H. Appl. Phys. A 1999, 69, 305-308. (11) Yamamoto, K.; Akita, S.; Nakayama, Y. J. Phys. D: Appl. Phys. 1998, 31, L34-L36. (12) Chen, X. Q.; Saito, T.; Yamada, H.; Matsushige, K. Appl. Phys. Lett. 2001, 78, 3714-3716. (13) Diehl, M. R.; Yaliraki, S. N.; Beckmann, R. A.; Barahona, M.; Heath, J. R. Angew. Chem. 2002, 114, 363-366. (14) Nagahara, L. A.; Amlani, I.; Lewenstein, J.; Tsui, R. K. Appl. Phys. Lett. 2002, 80, 3826-3828. (15) Krupke, R.; Hennrich, F.; Weber, H. B.; Beckmann, D.; Hampe, O.; Malik, S.; Kappes, M. M.; v. Lo¨hneysen, H. Appl. Phys. A 2003, 76, 397-400. (16) Hennrich, F.; Wellmann, R.; Malik, S.; Lebedkin, S.; Kappes, M. M. Phys. Chem. Chem. Phys. 2003, 5, 178-183. (17) Krupke, R.; Hennrich, F.; Hampe, O.; Kappes, M. M. J. Phys. Chem. B 2003, 107, 5667-5669. (18) The spherical objects observed at some contacts have been identified as products of a reaction of DMF with water. (19) Bockrath, M.; Cobden, D. H.; Lu, J.; Rinzler, A. G.; Smalley, R. E.; Balents, L.; McEuen, P. L. Nature 1999, 397, 598-601. (20) Nygard, J.; Cobden, D. H.; Bockrath, M.; McEuen, P. L.; Lindelhof, P. E. Appl. Phys. A 1999, 69, 297-304. (21) Collins, P. G.; Arnold, M. S.; Avouris, P. Science 2001, 292, 706709. (22) Javey, A.; Wang, Q.; Ural, A.; Li, Y.; Dai, H. Nano Lett. 2002, 2, 929-932. (23) Martel, R.; Derycke, V.; Lavoie, C.; Appenzeller, J.; Chan, K. K.; Tersoff, J.; Avouris, Ph. Phys. ReV. Lett. 2001, 87, 256805. (24) Heinze, S.; Tersoff, J.; Martel, R.; Derycke, V.; Appenzeller, J.; Avouris, Ph. Phys. ReV. Lett. 2002, 89, 106801. (25) Besides armchair SWNTs, here we also regard small band-gap SWNTs as metallic tubes. The abundance of metallic tubes is then calculated from the possibility to roll up a sheet of graphite for a given diameter range, assuming chirality independent growth. (26) Pohl, H. A. Dielectrophoresis: The Behavior of Neutral Matter in Nonunivorm Electric Fields; Cambridge University Press: Cambridge, U.K., 1978. (27) Jones, T. B. Electromechanics of Particles; Cambridge University Press: Cambridge, U.K., 1995. (28) Gommans, H. H.; Alldredge, J. W.; Tashiro, H.; Park, J.; Magnuson, J.; Rinzler, A. G. J. Appl. Phys. 2000, 88, 2509-2514. (29) Benedict, L. X.; Louie, S. G.; Cohen, M. L. Phys. ReV. B 1995, 52, 8541-8549. (30) Bachthold, A.; Fuhrer, M. S.; Plyasunov, S.; Forero, M.; Anderson, E. H.; Zettl, A.; McEuen, P. L. Phys. ReV. Lett. 2000, 84, 60826085. (31) Pichler, T.; Knupfer, M.; Golden, M. S.; Fink, J.; Rinzler, A.; Smalley, R. E. Phys. ReV. Lett. 1998, 80, 4729-4732. (32) Ding, J. W.; Yan, X. H.; Cao, J. X. Phys. ReV. B 2002, 66, 73401.
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