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Fluid Flow-Assisted Dielectrophoretic Assembly of Nanowires Kieseok Oh,† Jae-Hyun Chung,*,† James J. Riley,† Yaling Liu,‡ and Wing Kam Liu§ UniVersity of Washington, Department of Mechanical Engineering, Campus Box 352600, Seattle, Washington 98195-2600, Department of Mechanical and Aerospace Engineering, UniVersity of Texas at Arlington, 500 West First Street, Arlington, Texas 76019, and Department of Mechanical Engineering, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3111 ReceiVed June 13, 2007. In Final Form: August 17, 2007 The dielectrophoretic assembly of silicon carbide (SiC) nanowires in a microfluidic flow is shown to enhance the orientation and deposition yield of nanowires. The fluid flow delivers and orients the nanowires in the vicinity of a gap, and they are attracted and deposited by a dielectrophoretic force. Depending upon their lengths, the nanowires are selectively attracted to the gap because the dielectrophoretic force is largest when the lengths are comparable to the gap size. Precise control over the fluid flow and dielectrophoresis shows various interesting phenomena such as landing, shifting, and uniform spacing of nanowires during the assembly process. As a result, the precise control enables the selective positioning of nanowires only at the gap where the fluid direction is consistent with the electric field orientation.
Introduction Various nanowire- or nanotube-based devices have been developed in the past decade (e.g., field-effect transistors,1-3 logic circuits,4-6 and electromechanical switches7,8). To fabricate such devices, a variety of methods have been proposed for assembling nanowires or nanotubes on a substrate.9-12 Among the methods, electric field-induced methods have been frequently attempted to assemble nanowires (or nanotubes).13-16 When an inhomogeneous electric field is applied in a medium including nanowires, an electric dipole on the nanowires is induced, which generates a dipole moment. Dielectrophoresis induced by the dipole moment then attracts and orients the * Corresponding author. E-mail:
[email protected]. † University of Washington. ‡ University of Texas at Arlington. § Northwestern University. (1) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Appl. Phys. Lett. 1998, 73, 2447-2449. (2) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66-69. (3) Zheng, G. F.; Lu, W.; Jin, S.; Lieber, C. M. AdV. Mater. 2004, 16, 18901893. (4) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317-1320. (5) Javey, A.; Wang, Q.; Ural, A.; Li, Y. M.; Dai, H. J. Nano Lett. 2002, 2, 929-932. (6) Chen, Z. H.; Appenzeller, J.; Lin, Y. M.; Sippel-Oakley, J.; Rinzler, A. G.; Tang, J. Y.; Wind, S. J.; Solomon, P. M.; Avouris, P. Science 2006, 311, 1735-1735. (7) Cha, S. N.; Jang, J. E.; Choi, Y.; Amaratunga, G. A. J.; Kang, D. J.; Hasko, D. G.; Jung, J. E.; Kim, J. M. Appl. Phys. Lett. 2005, 86. (8) Kaul, A. B.; Wong, E. W.; Epp, L.; Hunt, B. D. Nano Lett. 2006, 6, 942-947. (9) Franklin, N. R.; Wang, Q.; Tombler, T. W.; Javey, A.; Shim, M.; Dai, H. J. Appl. Phys. Lett. 2002, 81, 913-915. (10) Lewenstein, J. C.; Burgin, T. P.; Ribayrol, A.; Nagahara, L. A.; Tsui, R. K. Nano Lett. 2002, 2, 443-446. (11) Rao, S. G.; Huang, L.; Setyawan, W.; Hong, S. H. Nature 2003, 425, 36-37. (12) Huang, Y.; Duan, X. F.; Wei, Q. Q.; Lieber, C. M. Science 2001, 291, 630-633. (13) Yamamoto, K.; Akita, S.; Nakayama, Y. J. Phys. D: Appl. Phys. 1998, 31, L34-L36. (14) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 1399-1401. (15) Englander, O.; Christensen, D.; Kim, J.; Lin, L. W.; Morris, S. J. S. Nano Lett. 2005, 5, 705-708. (16) Vijayaraghavan, A.; Blatt, S.; Weissenberger, D.; Oron-Carl, M.; Hennrich, F.; Gerthsen, D.; Hahn, H.; Krupke, R. Nano Lett. 2007, 7, 1556-1560.
nanowires to electrodes.17 Because a dielectrophoretic force is generated for all of the polarized nanowires in the vicinity of electrodes, the dielectrophoretic assembly may have the potential for the parallel assembly of nanowires if the behavior of the individual nanowires is controlled. A composite electric-fieldguided assembly method combining ac and dc has demonstrated the potential to deposit individual multiwalled carbon nanotubes (MWCNTs) as an array form.18 A floating-potential method has achieved an alignment of individual SWCNTs between two specified electrodes.19,20 The magnitude of the dielectrophoretic force is proportional to the volume of the nanowires because the polarizable lengths of nanowires determine the amplitude of the dipole moment.21 The dielectrophoretic force ranging from 10-12 N (pN) to 10-9 N (nN) attracts only the nanowires in the vicinity of electrodes (e.g., in the range of tens of micrometers). If nanowires are not present in the range dominated by dielectrophoresis, then a gap (or electrodes) in the dielectrophoresis region will remain empty without nanowires. To improve the deposition yield, long-range transport delivering nanowires to the dielectrophoresis region is required. In the case of the composite field,18 the electroosmotic flow generated by a dc field could continuously transport MWCNTs into the vicinity of the electrodes, which might contribute to a high assembly yield of 90%. To understand the electric field-induced assembly process, the immersed electrokinetic finite element method (IEFEM) was recently developed.22-29 The modeling method provided the (17) Lee, Y. C.; Parviz, B. A.; Chiou, J. A.; Chen, S. C. IEEE Trans. AdV. Packag. 2003, 26, 217-226. (18) Chung, J. H.; Lee, K. H.; Lee, J. H.; Ruoff, R. S. Langmuir 2004, 20, 3011-3017. (19) Krupke, R.; Hennrich, F.; Weber, H. B.; Kappes, M. M.; von Lohneysen, H. Nano Lett. 2003, 3, 1019-1023. (20) Dong, L. F.; Chirayos, V.; Bush, J.; Jiao, J.; Dubin, V. M.; Chebian, R. V.; Ono, Y.; Conley, J. F.; Ulrich, B. D. J. Phys. Chem. B 2005, 109, 1314813153. (21) Jones, T. B. Electromechanics of Particles; Cambridge University Press: New York, 1995. (22) Liu, Y. L.; Liu, W. K.; Belytschko, T.; Patankar, N. A.; To, A. C.; Kopacz, A.; Chung, J. H. Int. J. Numer. Methods Eng. 2007, 71, 379-405. (23) Liu, W. K.; Karpov, E. G.; Zhang, S.; Park, H. S. Comput. Methods Appl. Mech. Eng. 2004, 193, 1529-1578. (24) Zhang, L.; Gerstenberger, A.; Wang, X. D.; Liu, W. K. Comput. Methods Appl. Mech. Eng. 2004, 193, 2051-2067.
10.1021/la701755s CCC: $37.00 © 2007 American Chemical Society Published on Web 10/13/2007
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Figure 1. Drag forces exerting on nanowires: (a) cylinder oriented parallel to the flow direction and (b) cylinder orthogonal to the flow direction.
fundamental framework for a comprehensive understanding of the deposition mechanism, which is expected to contribute to the improvement of the assembly yield significantly. On the other hand, a shear force induced by fluid flow was successfully applied to assemble nanowires.12 A higher flow rate could achieve a higher orientation of deposited nanowires as a result of the larger shear force.12 The deposition yield can also be improved by a laminar flow that can continuously transport nanowires to electrodes. Compared to the dc field in the composite field, the fluid flow-induced assembly can avoid the electrophoretic deposition of unwanted particles. This article presents a fluid flow-assisted dielectrophoretic assembly method to assemble individual silicon carbide (SiC) nanowires across electrodes. The method combines an ac electric field and shear flow in a microfluidic channel for the nanowire assembly. The uniform shear flow transports the individual nanowires into the vicinity of the electrodes, which can compensate the small magnitude of the dielectrophoretic force induced by an ac field. The approach verifies our previous prediction26 that the nanowires can be sorted on the basis of their lengths by a dielectrophoretic force. In addition, the precise manipulation over the fluid flow and the electric field demonstrates the feasibility of assembling the individual nanowires in a controlled fashion. The assembly mechanism and results are analyzed by the recently developed IEFEM method.
Simulation To understand the underlying mechanism of the assembly process, the dielectrophoretic force acting on nanowires was analyzed by the IEFEM method.26 In the IEFEM, a complex formulation is used to determine the electric field. For an ac field, an arbitrary potential oscillating at frequency ω is defined as φ(x, t) ) {Re}[φ˜ (x)ejωt], where φ˜ is a complex potential phasor. Using the frequency-dependent complex permittivities ˜ ) + (σ/jω) expressed as the complex combination of the conductivity (σ), permittivity (), and angular frequency of the electric field, the governing equations for the electric field are
∇‚(˜ ∇φ˜ ) ) 0 E ˜ ) -∇φ˜
(1) (2)
(25) Liu, Y. L. Ph.D. Dissertation, Northwestern University, 2006. (26) Liu, Y. L.; Chung, J. H.; Liu, W. K.; Ruoff, R. S. J. Phys. Chem. B 2006, 110, 14098-14106. (27) Liu, Y. L.; Liu, W. K.; Belytschko, T.; Patankar, N. A.; To, A. C.; Kopacz, A.; Chung, J. H. Int. J. Numer. Methods Eng. 2007, 71, 379-405. (28) Liu, W. K.; Karpov, E. G.; Park, H. S. Nano Mechanics and Materials: Theory, Multiple Scale Analysis, and Applications; Springer: Berlin, 2005. (29) Liu, W. K.; Liu, Y. L.; Farrell, D.; Zhang, L.; Wang, X.; Fukui, Y.; Patankar, N.; Zhang, Y.; Bajaj, C.; Lee, J.; Hong, J.; X.;, C.; Hsu, H. Comput. Methods Appl. Mech. Eng. 2006, 195, 1722-1749.
n‚||˜ E ˜ || ) 0 on Γs
(3)
where Γs is the surface of the particle and n is the unit outward normal vector of the surface. By defining the Maxwell stress tensor as σM ) EE - 1/2E‚ Eδ, the electric force, Fe, on an object can be calculated through a surface integral of the Maxwell stress tensor, Fe ) ∫Γs (σM‚n) dA, or as a volume integral, Fe ) ∫Ωs∇‚σM dV using the divergence theorem. Such electric force is included in the fluid momentum equation
Ff v3 f ) ∇‚σf + Ffg + ∇‚σM
(4)
where vf is the fluid velocity, Ff is the fluid density, σf is the fluid Cauchy stress tensor, and σM is the time-averaged Maxwell stress. The electric field is coupled to the fluid/solid motion through σM. The details are described in a previous paper.26 Shear flow can orient a rod-shaped particle to be parallel lengthwise to the flow direction. Let us assume that an infinitely long cylinder lies in a plane parallel to the electrode surface. When the axial direction of the cylinder is parallel to the flow direction (Figure 1a), the drag coefficient per unit length is given as30
c| )
2πη h cosh-1 r
(5)
where h is the distance from the cylinder axis to the wall, η is the fluid viscosity, and r is the radius of the cylinder. Here the drag coefficient per unit length is defined in terms of the drag force fdrag, c|LV, where L and V are the length and velocity of the cylinder, respectively. When the axial direction of the cylinder is perpendicular to the flow direction (Figure 1b), the drag coefficient is c⊥ ) 2c|.30 The drag force acting on the nanowires is minimized when the orientation of the nanowires is parallel to the direction of fluid flow. The nanowires oriented in the direction of fluid flow experience a smaller drag force than those aligned perpendicular to the direction of fluid flow. The flow velocity can also determine the deposition yield of nanowires. In our simulation, nanowires positioned at an initial height (16 µm) from the electrodes approach a gap (15 µm) in laminar flow as shown in Figure 2a. When the nanowires approach the gap, they are attracted to the electrodes by dielectrophoresis. For successful deposition, the nanowire should land on the gap before it passes the gap. At high flow speed (maximum flow speed 150 µm/s with a shear rate of 5 s-1), the nanowire passes the gap in Figure 2b. At the lower shear rate of 2.5 s-1 (maximum flow speed 75 µm/s), two nanowires span the gap, and three nanowires span the gap at the lowest shear rate of 1.25 s-1 (maximum flow speed 37.5 µm/s). Thus, proper fluid flow (30) Howard, J. Mechanics of Motor Proteins and the Cytoskeleton; Sinauer Associates: Sunderland, MA, 2001.
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Figure 3. Layout of the device. A Cr/Au layer is deposited on a SiO2 layer for electrodes. The PDMS fluidic channel is bonded to the wafer. The thickness of the SiO2 layer is 110 nm. The thickness of the Cr/Au layer is 10 nm/200 nm.
Figure 2. Nanowire assembly according to different flow speed. The modeling volume is 100 × 20 × 30 µm3 (length × height × width). (a) Initial conditions for nanowires. An electric field (0.5 V/µm) is applied between two electrodes (gap size, 15 µm), and a laminar flow having different shear rates is generated in the fluid channel. The nanowires (400 nm in diameter) are 20, 15, and 30 µm in length from the top nanowire, and their initial orientation angles are 10, 30, and 60° according to the flow direction, respectively. The right panel is a side view showing the initial height of the nanowires (16 µm). The electrical properties of the nanowires and the medium are the same as those in ref 26. (b) A nanowire passes the gap at a shear rate of 5 s-1. (c) Two nanowires span the gap, whereas one is on the right side of the right electrode at the shear rate of 2.5 s-1. (d) All three nanowires span the gap at a shear rate of 1.25 s-1. See the movie files in Supporting Information.
speed is important to increase the deposition yield. As a result, the fluid flow will influence both the orientation and the deposition yield of the nanowires. Experimental Section Figure 3 shows the configuration of the fabricated microfluidic device for the assembly of nanowires. An oxide layer (110 nm) was thermally grown on a silicon (Si) wafer. Gold (Au) electrodes (gap sizes of 5, 15, 50 µm) were patterned on the oxide layer by optical lithography. To fabricate microfluidic channels, a silicon mold was fabricated by optical lithography and etched by a deep reactive ion etching process (DRIE), which resulted in a 40 µm high mesa structure in the Si wafer. Using the mold, a polydimethylsiloxane (PDMS) channel structure was fabricated, aligned, and bonded onto the silicon substrate having the gold electrodes. The stamp-and-stick bonding technique was used for the final assembly of the device.31 The cross section of the fabricated fluidic channel was 500 µm × 40 µm. Regarding the assembly, SiC nanowires (Advanced Composite Materials Corporation, Greer, SC) were used in this experiment. The nanowires were visible under a light microscope because the nanowires were 1-100 µm in length and 300-500 nm in diameter. To disperse SiC nanowires in a solvent, ethanol and dimethyl formamide (DMF) were prepared. Because the density of DMF (0.949 g/cm3) is higher than that of ethanol (0.789 g/cm3), the (31) Satyanarayana, S.; Karnik, R. N.; Majumdar, A. J. Microelectromech. Syst. 2005, 14, 392-399.
nanowires in DMF sank in the fluidic channel more slowly than those in ethanol. For the suspension in ethanol, various concentrations of the nanowires were prepared: 250, 25, 2.5, and 0.25 µg/mL. The solutions suspending the nanowires were sonicated in an ultrasonic bath (model 1510, Branson Ultrasonic Corp.) for 30 min and additionally for 5 min just prior to assembly. To investigate the suspensions in ethanol, a 2 µL drop of the different concentrations was dried onto an oxide layer of the Si wafer at room temperature. After the drop dried in air, a light microscope and a scanning electron microscope (SEM; FEI Sirion SEM) were used to investigate the nanowires. In the observation, nanowires at concentrations of 2.5 and 0.25 µg/mL were found to be individually present, whereas the bundled nanowires were found at higher concentrations of 25 and 250 µg/mL. Thus, the suspension having the 2.5 µg/mL concentration in ethanol was used in this experiment. The concentration in DMF was also determined in a similar way and was found to be 5.0 µg/mL. To investigate the efficiency of the dielectrophoretic assembly in a microfluidic device, assembly was performed under two different conditions; one was in air without a fluidic device (open deposition), and the other was in a microfluidic device (channel deposition). Because the fluid flow was generated in a microfluidic channel, the latter process was performed in a uniform laminar flow whereas the former process was in a random flow. Random flow was generated by the conversion of potential energy into kinetic energy in the dropping of the solution and the evaporation of the solution in air. Note that most of the previous nanowire assembly results18,20 have been achieved using the open deposition format. For the open and channel deposition experiments, the same ac electric potential (10.6Vpp) at a frequency of 5 MHz was applied to all of the assembly processes. Note that 10.6Vpp was the measured value at the electrodes. Because of the capacitance formed between the electrodes and the Si substrate, the applied voltage (20Vpp is the maximum value of the function generator, Agilent 33220A) was decreased to 10.6Vpp. Because the applied voltage was the same for the different gap sizes, the electric field amplitudes were 2.12Vpp/ µm at 5 µm gaps, 0.71Vpp/µm at 15 µm gaps, and 0.21Vpp/µm at 50 µm gaps. The applied frequency was chosen to be 5 MHz because electrokinetic flow was not generated at frequencies higher than 1 MHz. Therefore, the electrokinetic flow did not interfere with the fluid flow generated by capillary reaction and the syringe pump. To evaluate the statistics of the deposition pattern and yield, each experiment was performed three times under the same experimental conditions. For each gap (gap sizes of 5, 15, and 50 µm), the three experimental results were averaged to compare the open and channel depositions in terms of deposition yields and patterns. The monitoring time was 1 min, which was counted as soon as the solution was placed in the channel and gap. The 1 min time span was determined to compare the experimental results of the open and channel depositions.
Dielectrophoretic Assembly of Nanowires In the open deposition, a 9 µL ethanol suspension was gently dropped onto electrodes under the application of an ac field. The assembly process was monitored by a light microscope. The solution was spread on electrodes and evaporated in approximately 1 min in air. For the channel deposition, the same amount of ethanol was placed in the inlet of the fluidic device, and it was transported to the electrodes by capillary action. The velocity of nanowires near the electrodes without an electric field was measured to be in the range of 4 to 6 mm/s. Precise control over the fluid flow and the electric field was attempted in order to demonstrate the feasibility of the uniform alignment of nanowires. For this experiment, the SiC nanowires were dispersed in DMF. As mentioned above, the concentration of SiC nanowires was 5 µg/mL. At a lower velocity (e.g., 300 µm/s), the nanowires in ethanol sank to the bottom prior to the arrival on electrodes as a result of the relatively high density of SiC nanowires (3.16 g/cm3). The nanowires in DMF were successfully delivered to electrodes under the lower velocity of fluid flow. A syringe pump (Pump 11 pico plus, Harvard Apparatus) was used to control the velocity of the fluid flow. In the assembly, 50-µm-wide gaps were used. The ac potential for the gaps was manipulated to be between 0.6Vpp and 10.6Vpp. With a potential of less than 0.6Vpp (5 MHz), most nanowires were not attracted to the gap. At a higher amplitude (0.75Vpp at 5 MHz), the dielectrophoretic force attracted the nanowires but did not strongly hold the nanowires to the electrodes, which provided flexibility to adjust the position and alignment of the nanowires by fluid flow. The velocity of the fluid flow was manipulated in the range of 50 to 300 µm/s by the syringe pump. In this velocity range, the orientation and spacing of the nanowires in the fluid could be controlled. The competition of both forces could provide a periodic patterning of the nanowires. Note that the fluid velocity was estimated by measuring the speed of the nanowires in the fluid channel. To determine whether a light microscope had enough capability to observe the nanowire motion involved in the assembly, the images from the optical microscope were compared to those from SEM images. Figure 4 shows the images from a light microscope (Figure 4a) and SEM (Figure 4b). The light microscope image was captured in the presence of the ethanol solution, whereas that from SEM was imaged after drying ethanol in air. As shown in the Figures, the number, length, and orientation of nanowires were almost identical for both images, though the images from the SEM were evidently clearer. Thus, the video recorded images from the light microscope demonstrated a good enough resolution to show the dynamic movement of nanowires during the assembly. It should be noted that nanowire bundles could be present in the assembly in spite of the low concentrations. The bundles were speculated to originate from the synthesis of the nanowires because the nanowires appeared to be chemically connected in the SEM investigation. The bundles might not be individually separated in the sonication step. The bundled nanowires were not considered in the statistical analysis because the number, orientation, and position of the bundles could not be identified.
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Figure 4. (a) Video recorded image under an optical microscope of deposited SiC nanowires across the 15 µm gap in the PDMS fluidic channel; ethanol is present in the channel. (b) SEM image of the same electrode pair after the drying of ethanol.
Results and Discussion Dielectrophoretic Assembly: Open Deposition vs Channel Deposition. Deposition Yields and Patterns. When the ethanol suspension was dropped in the inlet of the microfluidic device, the nanowires were transported through the microfluidic channel from right to left by capillary action (Figure 5a). For convenience of description, the area between the electrodes is described as the “gap”, and the other area near the edge of electrodes is the “side areas” as shown in Figure 5a. When constant voltage (10.6Vpp) was applied to 5-, 15-, and 50-µm-wide gaps, SiC nanowires were attracted to electrodes. In the case of the 5- and 15-µm-wide gaps, the nanowires were deposited across electrodes (Figure 5b,c), whereas most of the nanowires in the 50-µm-wide gap were attracted to an edge of the right electrode as a result of the smaller lengths of the nanowires compared to the gap size.
Figure 5. Video recorded images of SiC nanowires on electrodes in the fluidic channels. The channels are filled with ethanol. The applied voltage is 10.6Vpp, and the frequency is 5 MHz for each experiment. (a) The gap and the side areas are described with the flow direction. (b) 5 µm gap electrodes. θ is the orientation angle. (c) 15 µm gap electrodes. (d) 50 µm gap electrodes. (Scale bar, 50 µm).
The preference of nanowires for the right edge of the electrodes was ascribed to the flow direction. The nanowires in the vicinity of electrodes were trapped by the right electrode rather than the
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Figure 6. Length distribution of SiC nanowires of the source and deposited nanowires on 5, 15, 50 µm gap electrodes after deposition. (a) Normalized number (Nn) of nanowires on each gap. (b) The ratio Nn/Nn-s of the normalized number (Nn) on each gap to the normalized number of the source (Nn-s). 10.6Vpp was applied to all of the electrodes at 5 MHz. The average nanowire length and standard deviation of the source were 10.8 and 10.4 µm, respectively. After the assembly, the average nanowire length was changed to 8.3 µm on the 5 µm gap, 13.9 µm on the 15 µm gap, and 17.1 µm on the 50 µm gap. Note that the average length on the 50 µm gap was not significantly increased as a result of the small portion (2%) of source nanowires larger than 35 µm. Table 1. Average Number with Standard Deviation of SiC Nanowires on the Gap and the Side Area of the Electrodes in Open and Channel Depositiona gap
a
side areas
gap size
5 µm
15 µm
50 µm
5 µm
15 µm
50 µm
open deposition: number of nanowires channel deposition: number of nanowires
6.0 ( 5.3
0.7 ( 0.6
3.3 ( 3.1
13.3 ( 1.5
3.0 ( 3.5
12.7 ( 6.0
16.7 ( 3.5
12.0 ( 1.7
12.7 ( 0.6
7.0 ( 3.6
6.0 ( 3.0
7.7 ( 2.1
Deposition time, 1 min. Each values is the average of three consecutive experiments.
left one as a result of the flow direction. It was also interestingly observed that most nanowires were attracted to the gap rather than the side areas. These results occurred because the flow direction in the gap was the same as the electric field orientation, whereas the flow direction was orthogonal to the e-field orientation in the side areas. In the open deposition, the random movement of SiC nanowires was observed upon the placement of ethanol. Because of the random directional flow, most of the nanowires were deposited in the side areas rather than the gap. As a result, the average number of deposited nanowires in the gap was about 4 times smaller than that in the channel deposition.
Table 1 shows the statistical results comparing the channel deposition with the open deposition. As shown in the Table, most of the nanowires in the channel deposition were deposited in the gap rather than the side areas. Regardless of the gap sizes, over 12 nanowires were deposited in the gap in the channel deposition. On the contrary, the result of the open deposition showed that most nanowires were deposited in the side areas rather than the gap. Due to the randomly directed flow in the open deposition, most of the nanowires were attracted to the side areas rather than the gap. By the increase of the deposition time (currently 1 min), the number of deposited nanowires will be greater in the channel deposition. The nanowire number in the
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Figure 7. Orientation angle distribution of the SiC nanowires deposited on 5, 15, 50 µm gap electrodes after 1 min of deposition. The voltages applied to the electrodes are 10.6Vpp, and the ac frequencies are set to 5 MHz. The average orientation angles are 5.8, 2.4, and 3.5° for 5, 15, 50 µm gap sizes.
Figure 9. SiC nanowire in DMF moves on an edge of the electrode. The applied voltage is 750mVpp, and the ac frequency is 5 MHz. The approaching speed of the nanowire in the solution is 300 µm/s. The nanowire of interest is colored black for a clear view. Time is (a) 0, (b) 1.1, and (c) 16.7 s.
Figure 8. SiC nanowire in DMF approaches the gap, and the nanowire is located with one end at the edge. The nanowire is aligned with the direction of the fluid flow. The voltage of the electrodes is 750mVpp, and the ac frequency is 5 MHz. The speed of the SiC nanowire in the solution is 300 µm/s. The nanowire of interest is colored black for a clear view. Time is (a) 0, (b) 1, and (c) 2 s.
open deposition, however, will not increase because the solution dries in approximately 1 min. Although the deposition yield in the channel deposition is 4 times that in the open deposition, only a small number of nanowires compared to the nanowire number in the solution is deposited on the basis of our estimation. Using the nanowire
density (3.16 g/cm3), the average diameter (400 nm), and the average length (10.8 µm), the estimated number of nanowires in the 9 µL solution is approximately 5000, considering the 2.5 µg/mL nanowire concentration. Assuming 10 nanowires spanning electrodes for 1 min, only 0.2% nanowires are assembled in the gap. To improve the deposition yield, the fluid flow speed should be decreased as shown in Figure 1, and the electric field should be increased. Additionally, the cross section of the channel should be decreased to confine the transporting path of nanowires. Sorting on the Basis Nanowire Length. More interesting phenomena were observed in the channel deposition. The lengths of the deposited nanowires were dependent on the gap size; the nanowires, whose lengths were comparable to the gap size, were selectively deposited across electrodes. In other words, nanowires much smaller than or larger than the gap size were not attracted to the gap because of the lower dielectrophoretic force. Figure 6a shows the normalized numbers of original nanowires and deposited nanowires at the 5, 15, and 50 µm gaps according to the lengths. As the gap size increases, the length of the deposited nanowires increases. Figure 6b shows the ratio of the normalized number of deposited nanowires to that of the original source for each length. As shown in the Figure, the dominant length of the deposited nanowires in the 5 µm gap is less than or equal to 15 µm, whereas that in the 15 µm gap is in the range of 15 to 35 µm. In the 50 µm gap, the dominant length is in the range of 35 to 40 µm. It clearly demonstrates that the deposited nanowires can be sorted on the basis of their lengths relative to the gap
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Figure 10. Shift of nanowires (ac voltage, 750mVpp at a frequency of 5 MHz; fluid velocity, 300 µm/s). (a) At 0 s, a short SiC nanowire is deposited on the corner of an electrode, and a long SiC nanowire is approaching the nanowire. (b) At 2 s, the short nanowire is being shifted from the corner. (c) The shifting is continued and completed at 5 s. The final spacing between the nanowires is 16 µm. The nanowires of interest are colored black for a clear view.
Figure 11. Deposited SiC nanowires are aligned with uniform spacing. The average distance between neighboring nanowires is 5.4 µm. Few SiC nanowires are shown on the edges of the side areas. The voltage of the electrodes is 750mVpp, and the frequency is 5 MHz. The nanowires of interest are colored black for a clear view. The nanowires indicated by the red arrows are not individual ones when they approach the gap and are not considered for the spacing calculation. Nanowires in the red rectangle could not be calculated for the distance because the image is obscure.
sizes. This result supports our previous prediction26 that the dielectrophoretic force varies according to the ratio of the gap size to the nanowire length. The dielectrophoretic force is reduced
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when the gap size is much smaller than a nanowire length because the major polarizable direction is changed from the axial direction of the nanowire to the radial direction. In such a situation, the effective polarizable axis of the nanowire is the short axis (diameter) rather than the long axis (length) because the smaller gap cannot effectively polarize the larger nanowires in the longitudinal direction. This dielectrophoretic force also decreases when a gap size is much larger than a nanowire length because the electric field gradient around the nanowire is reduced at a larger gap. The maximum dielectrophoretic force is found when the gap size is 0.8 times the nanowire length.26 This experimental result verifies the fact that the length of the deposited nanowires is determined by the gap size in dielectrophoretic assembly. This sorting principle can be used for other nanowires and biomolecules that are dominated by positive dielectrophoresis. Orientations of Nanowires. The orientation of the nanowires deposited in the gap area was also investigated. In the open deposition, few nanowires in the gap were deposited in the gap, and the orientation could not be evaluated. In the channel deposition, 97% of the nanowires at 15-µm-wide gaps were aligned in the range of 10°, which shows a much higher degree of orientation compared to that for the nanowires at the 5- and 50-µm-wide gaps (Figure 7). For 5- and 50-µm-wide gaps, 84 and 88% of the nanowires were oriented within 10°, respectively. The higher orientation at the 15-µm-wide gap is dependent on dielectrophoretic torque. According to the computational results,26 the largest torque is obtained when the ratio (λ) of gap length to nanowire length is in the range of 0.5 to 2.0. When λ is larger than 2.0, the torque is decreased by the reduced gradient of the electric field. In case λ is smaller than 0.5, the polarizable, effective length of the nanowires is changed from the long axis of the nanowire to the short axis (i.e., the radial direction), which results in a negligible or negative torque. To generate the largest torque, the λ ratio should be in the range of 0.5 to 2.0. The corresponding length of the nanowires is 7.5 to 30 µm at the 15 µm gap. It should also be noted that the average length (10.4 µm) of the SiC nanowires (2.5µg/mL in ethanol) is within this range. Regarding the length of the nanowires deposited in the gaps (Figure 6a), approximately 78% of nanowires in the 15-µm-wide gap fall within the range of 0.5 < λ < 2.0. In the case of 5 µm gaps, 74% of the nanowires are in the range of 0.5 < λ < 2.0 whereas 91% of nanowires in the 50 µm gaps are in the range of λ > 2.0. Although the 91% nanowires in the 50 µm gaps are out of the range of 0.5 < λ < 2.0, higher orientation is observed for the 50 µm gaps than that for the 5 µm gaps. The lack of orientation angle at the 5 µm gaps is ascribed to the negative or negligible dielectrophoretic torque when λ < 0.5.26 Compared to the 5 µm gaps, a small but positive amplitude of the dielectrophoretic torque is expected for the 50 µm gaps when λ > 2.0. This is the reason that the orientation is better for the 50 µm gaps than for the 5 µm gaps. Considering the dielectrophoretic torque dependency on the approaching angle of the nanowires and the effect of shear flow, the experimental results are consistent with the theoretical predictions. From the experimental results, it is suggested that a gap size should be designed by considering the average length of nanowires to achieve a high degree of nanowire orientation. Controlled Assembly. In the controlled assembly of nanowires, a syringe pump was employed to manipulate the fluid velocity. To achieve uniform patterning in the assembly of nanowires, the applied ac potential was 750mVpp, as mentioned above. Because the dielectrophoretic force is proportional to the gradient of the squared electric field, the lowered amplitude reduces both
Dielectrophoretic Assembly of Nanowires
Langmuir, Vol. 23, No. 23, 2007 11939
Figure 12. Shifting simulation of two nanowires. Fluid flow of 100 µm/s is generated from right to left as indicated by the arrow. The nanowires are assumed to shift only along the electrode edge once deposited as a result of the dielectrophoretic force. The flow deformation generates repulsive flow to have spacing between the nanowires. The magnitude and direction of the nanowire speed are shown by the colored arrows. The repelling speed decreases as the distance between the nanowires increases. The spacing between the nanowires in the simulation is found to be proportional to the flow speed and inversely proportional to the square of the electric field strength. Time is (a) 0, (b) 0.7, (c) 1.4, and (d) 2.0 s.
the attractive force for the nanowires near the gap and the holding force for the nanowires already deposited. The lowered attractive force again decreases the approaching velocity. The lowered approaching velocity provides a longer time for the nanowires to be oriented by the fluid flow and the electric field. The lowered dielectrophoretic force can also weakly hold the deposited nanowires such that the position and orientation of the deposited nanowires can be adjusted by fluid flow after the deposition. The position and orientation of nanowires can be adjusted because a permanent, specific binding force due to dielectrophoresis is not present between the deposited nanowires and electrodes. To achieve permanent, stable electrical contact of the deposited SiC nanowires, additional metal layers should be patterned.26 During the assembly, the flow velocity was manipulated in the range of 50 to 300 µm/s. In this velocity range, the attracted and deposited nanowires were not removed by the flow. Furthermore, the nanowires could be realigned and repositioned by the combination of the flow-induced drag force and the dielectrophoretic force. In this experiment, the nanowires suspended in DMF were used. Landing. When the amplitudes of the ac potential and fluid flow were lowered to 750mVpp and 300 µm/s, respectively, the
attracted nanowires gently landed on electrodes with a high degree of orientation in the flow direction (Figure 8). In the Figure, a nanowire approached the electrode edge having the highest electric field gradient due to dielectrophoresis. The nanowire anchoring on the edge rotated and aligned on the gap. If the applied electric potential were larger, then the nanowire could be deposited with less orientation because the dielectrophoretic force holding the nanowire to the electrodes would be stronger. The stronger force could limit the flexibility for adjusting the nanowire orientation. At the given electric potential and flow velocity, the position and orientation of most of the nanowires were successfully adjusted during assembly. Shifting. The nanowires attracted to the side areas of a gap were finally transported along the edge of electrodes to the gap (Figure 9). Because the dielectrophoretic force holding the SiC nanowire on the electrodes was controlled to gently release but not to lose it, the nanowire was transported to the gap while maintaining the orientation and position of the nanowire. During transportation, the orientation of the nanowire was parallel to the direction of the electric field and was dominated by the dielectrophoretic torque, which was orthogonal to the flow direction. The drag force due to the flow transported the nanowire
11940 Langmuir, Vol. 23, No. 23, 2007
to the gap while maintaining the orientation along the electric field. Because most nanowires attracted to the side area were transported to the gap area, nanowires could not be found in the side areas in the controlled assembly. This means that nanowires can be selectively positioned at the electrodes where the flow direction is consistent with the electric field orientation. The repositioning of nanowires after deposition can be crucial for the uniform assembly of nanowires. If nanowires can be aligned on parallel electrodes with uniform spacing, then this can be extended to align individual nanowires on an array of gaps with high yield.18 Figure 10 shows the repositioning process of the already positioned nanowire on electrodes in the presence of a nanowire. When the long nanowire in the circle approached the prepositioned short nanowire indicated by an arrow, the prepositioned one was shifted to maintain the spacing. The shifting resulted from the repulsive flow generated by the fluid flow and also the distorted electric field16 around the deposited. Spacing. The shift due to the interaction of nanowires has been observed for most of the nanowires at given experimental conditions, even though the distance of the movement varied. As a result, the deposited nanowires appeared to have approximately uniform spacing (Figure 11). The average spacing of the nanowires in Figure 11 was 5.4 µm. This periodic spacing was repeatedly observed for the given experimental conditions. Recently, similar experimental results for the shifting and spacing during silicon nanowire assembly were observed by Hamers et al.32 The spacing between the nanowires can be determined by the flow speed and the electric field strength. A simulation was performed using the IEFEM method. In the simulation, nanowires were modeled as a moving solid body composed of a Lagrangian mesh having 1536 nodes and 6253 elements, whereas the NavierStocks equations were solved in the fluid domain by a fixed Eulerian mesh having 29 525 nodes and 157 079 elements. The initial spacing of the nanowires was assumed to be 5 µm, and a flow of 100 µm/s was generated from right to left. Because of the pressure and viscous stresses generated by the flow between the two nanowires, they were repelled from each other. From the simulation shown in Figure 12, the repelling velocity of the nanowires is 0.04 times the flow speed (100 µm/s). The repelling velocity is a computed value when the nanowire spacing is half of the final spacing at constant flow speed. During this repelling process, the nanowires are still in contact and experience friction with the electrode surface. According to the experimental observation, nanowires shift along the electrode edge once deposited. The final spacing is determined by the balance between the frictional force and the repelling force due to the pressure and viscous stresses generated. It should be noted that the frictional force is proportional to the dielectrophoretic force holding the nanowires to the electrode. In a series of simulations, nanowires were placed in fluids with different flow velocities. Figure 13 shows that the final spacing is proportional to the flow speed. The spacing is also (32) Hamers, R. J.; Beck, J. D.; Eriksson, M. A.; Li, B.; Marcus, M. S.; Shang, L.; Simmons, J.; Streifer, J. A. Nanotechnology 2006, 17, S280-S286.
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Figure 13. Final spacing between nanowires at various flow speeds. The spacing is normalized by 5 µm, and the flow velocity is normalized by 100 µm/s. For each simulation, the flow speed is constant.
determined by the magnitude of the frictional force that is proportional to the dielectrophoretic force. Because the dielectrophoretic force is proportional to the gradient of the squared electric field, the spacing between the neighboring nanowires can be controlled by the flow speed and electric field strength. A similar spacing phenomenon may occur for the smaller nanowires of sub-100-nm diameter. The uniform spacing of nanotubes has been observed.18 Similar flow cell formation under an electric field has been reported for the assembly of spherical particles having a 16 nm diameter.33
Conclusions The fluid flow-assisted dielectrophoretic assembly method can deposit a larger number of SiC nanowires in the gap than random flow deposition. The nanowires were selectively deposited on the basis of their lengths in accordance with gap sizes. In the controlled assembly, the orientation and spacing of neighboring nanowires could be manipulated by the controlled electric field and fluid flow. According to the simulation results, the repulsive force due to the flow between the nanowires contributed to the uniform spacing of nanowires. As a result, the controlled assembly allowed the selective positioning of nanowires with uniform spacing in the gap area where the electric field orientation was the same as the fluid flow direction. Acknowledgment. K.O., J.-H.C., and J.J.R. acknowledge the support of the National Science Foundation (NSF) (grant CMMI 0624597). The support of W.K.L. by the NSF, Office of Naval Research (ONR), and the NSF Summer Institute on Nano Mechanics and Materials is gratefully acknowledged. Supporting Information Available: Movie files. This material is available free of charge via the Internet at http://pubs.acs.org. LA701755S (33) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706-709.
