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Three-Dimensional Arrangements of Polystyrene Latex Particles with a Hyperbolic Quadruple Electrode System Masahiko Abe,*,†,‡ Masanori Orita,† Hiroyuki Yamazaki,† Shinya Tsukamoto,† Yuki Teshima,† Toshio Sakai,† Takahiro Ohkubo,† Nobuyuki Momozawa,†,‡ and Hideki Sakai†,‡ Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, and Institute of Colloid and Interface Science, Tokyo University of Science, 1-3, Kagurazaka, Shinjuku, Tokyo 162-0825, Japan Received December 29, 2003. In Final Form: April 8, 2004 An attempt was made to arrange polystyrene latex particles (2, 5, and 10 µm in diameter) dispersed in aqueous media making use of their dielectrophoresis and electrophoresis with a hyperbolic quadruple electrode system. Application of a high-frequency ac field enabled the particles to arrange themselves between the electrodes forming a particle monolayer due to the negative dielectrophoretic force. Simultaneous application of high-frequency ac and dc fields caused the particles to gather in the region surrounded by the electrodes to form particle multilayers. Appropriate choice of the way of applying an electric field thus allowed the reversible control of particle arrangements (monolayer, multilayer, dispersion). Reapplication of an ac field to the particle layers produced highly dense particle multilayers.
1. Introduction The technology to form the precise assembly of colloids has attracted much attention because the colloidal crystals can be applied to important devices such as photonic crystals. Establishment of the technique to prepare threedimensional photonic crystals that can trap light tightly within them is needed to construct microphotonic devices with light propagation properties that are characteristic of the crystals.1-5 Preparation of three-dimensional photonic crystals has so far been attempted using various techniques. Those crystals, which have photonic band gaps in the wavelength region from microwave to milliwave, are mechanically prepared by making holes in dielectrics with a drill.1 Threedimensional microprocessing with a half-wavelength accuracy is essential, however, to prepare such photonic crystals that have photonic band gaps in the region of visible light. Although various methods using semiconductor processing techniques have been reported for the preparation of photonic crystals,6,7 the use of these methods is intrinsically difficult for preparing three-dimensional photonic crystals that extend over several tens of periods because the methods are based on the two-dimensional processing consisting of thin film formation and etching. Then, many methods have recently been developed to prepare three-dimensional periodical structures of a micrometer order by three-dimensionally arranging and * To whom correspondence should be addressed. Tel and Fax: +81-471-24-8650. E-mail:
[email protected]. † Faculty of Science and Technology. ‡ Institute of Colloid and Interface Science. (1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (2) Yablonovitch, E.; Gmitter, T. J.; Leung, K. M. Phys. Rev. Lett. 1991, 67, 2295. (3) Joannoopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1955. (4) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143. (5) Lin, S. Y.; Fleming, J. G.; Hetherington, D. L.; Smith, B. K.; Biswas, R.; Ho, K. M.; Sigalas, M. M.; Zubrzycki, W.; Kurtz, S. R.; Bur, J. Nature 1998, 394, 251. (6) Cheng, C. C.; Scherer, A.; Arbet-Engels, V.; Yablonovitch, E. J. Vac. Sci. Technol., B 1996, 14, 4110. (7) Noda, S.; Yamamoto, N.; Sasaki, A. Jpn. J. Appl. Phys. 1996, 35, L909.
Figure 1. Plate-plate and plate-needle electrode systems: (a) plate-plate electrode system; (b) plate-needle electrode system.
packing small (polystyrene or silica) particles of microand submicrometer orders. Most of these methods need, however, much time, high technique, and special apparatus and are lacking in simplicity since they adopt a technique of manipulating particles one by one.8,9 Some methods using an electric field for particle arrangement have also been reported, in which electrophoresis (for twodimensional arrangement) or dipole-dipole interaction (for one-dimensional pearl chainlike arrangement) is used.10,11 Recently, many reports have dealt with two- or three-dimensionally layered particle arrangement in an electric field.12-19 Trau et al., for instance, showed the (8) Saito, S.; Miyazaki, H.; Sato, T. Proc. IEEE Int. Conf. Rob. Autom. 1999, 2189. (9) Miyazaki, H.; Saito, T. Adv. Rob. 1997, 11, 169. (10) Junno, T.; Anand, S.; Deppert, K.; Motelius, L.; Samuelson, L. Appl. Phys. Lett. 1995, 66, 3295. (11) Pohl, H. A. Methods Cell Sep. 1997, 1, 67. (12) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (13) Wijnhoven, J. E. G. J.; Vos, W. L. Proc. IEEE Int. workshop Micro Electro Mech. Syst. 1996, 318.
10.1021/la036465v CCC: $27.50 © 2004 American Chemical Society Published on Web 05/14/2004
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Figure 2. Hyperbolic quadruple electrode system: (a) whole system and a design drawing of enlarged central part; (b) enlarged region between the electrodes and the relationship between field strength and the distance from the system center (A, B, and C correspond to the respective positions in the enlarged region).
Figure 3. Schematic illustration of the system used in this work.
possibility of two- or three-dimensional colloidal crystals at the electrode surface in a dc or ac field, respectively. The colloidal crystal with electrophoretic deposition can allow rapid production and a relatively larger crystal compared with that obtained by other methods. However, if we wish to control the size of a two- or three-dimensional colloidal crystal, we have to apply other methods such as a quadruple electrode system. The geometry of the quadruple electrode system can limit the cross section of a colloidal crystal, and therefore, we can form small colloidal crystals. We then examined ways of three-dimensionally arranging polystyrene latex particles using dielectrophoresis (14) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (15) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; IKhayrullin, I.; Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897. (16) Mukaiyama, T.; Takeda, K.; Miyazaki, H.; Jimba, Y.; KuwataGonokami, M. Phys. Rev. Lett. 1999, 82, 4623. (17) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Ostrander, J. W.; Ragoisha, G. A. Chem. Mater. 2000, 12, 2721. (18) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. Adv. Mater. 2003, 15, 1025. (19) Golding, R. K.; Lewis, P. C.; Kumacheva, E. Langmuir 2004, 20, 1414.
and electrophoresis with a hyperbolic quadruple electrode system20,21 that generates inhomogeneous electric fields. 2. Experimental Section Polystyrene latex samples (Duke Scientific Corp., particle diameters 2.013 ( 0.025, 4.988 ( 0.035, and 10.15 ( 0.06 µm) were used. Each of the samples was diluted with distilled water for injection (Otsuka Pharmaceutical Co.) to yield an electrolytefree latex dispersion. The ζ potential of the latex particles was measured by the laser Doppler method with a particle sizing system (NICOMP380ZLS) to give -64 mV. Three types of electrode systems with different shapes (plateplate (Figure 1a), plate-needle (Figure 1b), and hyperbolic quadruple (Figure 2a) electrode systems) were used. The plateplate electrode system was for observing the electrophoretic behavior of the latex particles while the plate-needle electrode system was used to observe the dielectrophoretic behavior of the particles since the electrode system generates inhomogeneous electric fields where the field strength decreases from the needle electrode to the plate electrode. A hyperbolic quadruple microelectrode system (Figure 2b) was used to control particle arrangement because it generates electric fields that decrease toward the center of the region surrounded by the four electrodes. Application of electric field to this electrode system was performed by giving the same polarity to opposing two electrodes on the
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Figure 4. ac (1.5 Vrms) frequency dependence of dielectrophoretic behavior of polystyrene latex particles (5.0 µm) in plate-needle electrode system. Frequency: (a) 1 kHz; (b) 1 MHz. diagonal line. The electrodes used were made of soda glass with thin metal layers evaporated on it by the lift-off etching method (lower layer, ∼10 nm thick chrome; upper layer, ∼200 or 500 nm thick ruthenium). An aliquot of the polystyrene latex dispersion was dropped into the hyperbolic quadruple electrode system. Complete sedimentation and dispersion of the latex particles were then checked. Application of electric field was conducted using a function synthesizer (model 1915, NF Electric Instruments). An ac field (100 Hz-1 MHz, 1.0-2.5 Vrms) was applied first to the system for 15 min. Then, the ac field and a dc field (1.5 V) were applied simultaneously. The direction of the dc field was altered every 30 s for 5 min. Finally, the dc field was removed and the ac field alone was applied for 5 min. The particle behavior was observed under a Nomarski-type differential interference microscope in each of the applied electric fields while monitoring with a CCD camera (CS900, Olympus), and digital particle images were recorded using a video cassette recorder (WV-DR7, Sony) (Figure 3). The dielectrophoretic force, FDEP, given by eq 1 acts on a polarizable particle in an inhomogeneous electric field. The particle migrates in the direction of weaker (negative DEP) or stronger (positive DEP) field when FDEP is negative or positive22
FDEP ) 2πr3m Re[Ke]∇|Erms|2
(1)
where r is particle diameter, Re[ ] is the real part, and Erms is the field strength. Ke is the effective polarizability (ClausiusMossoti coefficient) given by eq 2
p* - m* Ke ) p* + m*
(2)
where p* and m* are the complex relative permittivities of the
Figure 5. Behavior of polystyrene latex particles (5.0 µm) in applied ac field (1 MHz, 1.5 Vrms) in hyperbolic quadruple electrode system (200 nm electrode thickness): (a) before field application; (b) 10 s after field application; (c) 15 min after field application. particle and the solvent, respectively. The complex relative permittivity is defined by eq 3
* ) - j(σ/ω)
(3)
where j is x(-1), is the relative permittivity, σ is the conductivity, and ω is the angular frequency. All parameters except Re[Ke] are always positive in eq 1 and the sign of Re[Ke] determines that of FDEP. In general, particles move in the direction of stronger field when Re[Ke] is positive (20) Watarai, H.; Sakamoto, T.; Tsukahara, S. Langmuir 1997, 13, 2417. (21) Tsukahara, S.; Sakamoto, T.; Watarai, H. Langmuir 2000, 16, 3886. (22) Pohl, H. A. Dielectrophoresis; Cambridge University Press: Cambridge, 1978.
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Figure 6. Behavior of polystyrene latex particles (5 µm) in an ac field (1 kHz, 1.5 Vrms) applied to the particle arrangement in Figure 5c. while they migrate toward the center of the electrode system where the field is weaker if Re[Ke] is negative. Hence, we can arrange the structure of a colloidal crystal by applying an electric field. Especially, we can form a small and well-ordered crystal of colloid particles by using a quadruple system which consists of two pairs of electrodes.
3. Results and Discussion 3.1. ac Field Frequency Dependence of Dieletrophoretic Particle Behavior. The direction of dielectrophoresis is known to depend on the frequency of the ac field.22 The effect of ac frequency was then examined on the dielectrophoretic behavior of polystyrene latex particles (5.0 µm diameter) using the plate-needle electrode system (Figure 1a) that generates an inhomogeneous electric field. The particles moved toward the needle electrode on the stronger field side at 1 kHz (Figure 4a), whereas they migrated to the plate electrode on the weaker field side at 1 MHz (Figure 4b). This demonstrates that the particles move dielectrophoretically in the direction of stronger field at low frequencies while they migrate in the direction of weaker field at high frequencies. The inversion of the direction of particle dielectrophoresis in an ac field suggests that the sign of Re[Ke] changes at a frequency between 1 kHz and 1MHz. 3.2. Control of Particle Arrangement with a Hyperbolic Quadruple Electrode System. The migration behavior of polystyrene latex particles (5.0 µm diameter) in an electric field was observed at frequencies from 1 kHz to 1 MHz using the hyperbolic quadruple electrode system (200 nm ruthenium thickness) that generates an inhomogeneous electric field as does the plate-needle electrode system. The initially dispersed latex particles (Figure 5a) moved to the center of the electrode system where the field is weaker upon applying an ac field in the frequency range around 1 MHz (Figure 5b). The particles were found to arrange themselves forming a particle monolayer between the electrodes in 15 min after field application (Figure 5c). The particles in the monolayer (Figure 5c) migrated to the electrode on the stronger field side, and the particle arrangement started to break when an ac field at 1 kHz was applied to the system (Figure 6). This finding is in accordance with that obtained with the plate-needle electrode system. That is, the particles migrated in the direction of stronger (positive DEP) or weaker (negative DEP) field at low or high frequencies. Application of a dc field (1.5 V) to the particle monolayer formed by an ac field (1 MHz, 1.5 Vrms) caused the particles
Figure 7. Behavior of polystyrene latex particles (5.0 µm) in ac (1 kHz, 1.5 Vrms) and dc (1.5 V) fields simultaneously applied to the particle arrangement in Figure 4c: (a) immediately after field application (upper right and lower left: cathodes); (b) 15 s after field application (upper right and lower left: cathodes); (c) 5 min after field application.
to move toward the center of the electrode system and gather in the region between the electrodes as shown in parts a and b of Figure 7. Changes in the direction of the dc field every 15 s produced further particle gathering between the electrodes to form particle multilayers (Figure 7c). This would arise from the expulsion of negatively charged latex particles due to the electrostatic repulsive force acting between the cathodes and the particles. It is noteworthy that the negatively charged particles migrated to the center of the electrode system, instead of moving toward the anodes. Negatively charged polystyrene latex particles migrate electrophoretically to the anode when a dc field alone is applied. Actually, the latex particles migrated to the anode when a dc field alone is applied to the plate-needle electrode system (Figure 8). Simultaneous application of dc and ac fields always generates,
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Figure 8. Behavior of polystyrene latex particles (5 µm) in a dc field (2.0 V) in a plate-plate electrode system: (a) before field application; (b) 3 min after field application.
however, a dielectrophoretic force directing to the center of the electrode system. The dielectrophoretic force would have exceeded the force causing the latex particles to move toward the anodes. Although simultaneous application of the dc and ac fields caused the latex particles to gather in the region between the electrodes to form multilayers, the particle packing in the layer was rather loose and random (Figure 7c). Then, an attempt was made to arrange the particles densely and orderly in the layer by applying again the ac field at 1 MHz. The attempt was successful, and the particles were packed densely as shown in Figure 9a. Note here that the particles are densely accumulated around the center of the electrode system where the gradient of electric field is small. This indicates that the applied field produces no electrophoretic migration of the particles located near the center of the electrode system while the force directing to the system center (negative dielectrophoretic force) acts on the particles located apart from the center. The particles in upper layers enter into the spaces between those in lower layers to give denser packing. In fact, several closest packed particle layers are piled up as seen in Figure 9b. 3.3. Factors Affecting Particle Migration in a Hyperbolic Electrode System. The effects of ac voltage, particle diameter, and electrode thickness were examined on the migration and arrangement of polystyrene latex particles. Particle arrangement control was performed in the same way as that used before. Figures 10 and 11b show the effect of the thickness of ruthenium electrode on particle arrangement. While the thick (500 nm) and thin (200 nm) electrodes gave particle arrangements similar to each other, the rate of particle migration toward the center of the electrode system was
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Figure 9. Behavior of polystyrene latex particles (5.0 µm) in ac field (1 kHz, 1.5 Vrms) 3 min after its application to the particle arrangement in (c): (a) whole view; (b) enlarged view of (A).
Figure 10. Effect of rethenium electrode thickness (particle diameter, 5.0 µm; electrode thickness, 200 nm; ac field, 1 MHz, 2.5 Vrms).
higher for the former than the latter. Moreover, optical microscopic observations of the final particle arrangement while moving the focal point vertically revealed that the thick electrode produces particle multilayers more densely than the thin electrode. This would suggest that the thick electrode exerts the electric field on the particles more widely than the thin electrode. Figure 11 shows the arrangements of polystyrene latex particles (5 µm) at different applied ac voltages (1.0 and 2.5 Vrms). Only one particle layer was formed when the applied ac voltage was 1.0 Vrms, due probably to the weak dielectrophoretic force generated (Figure 11a). This would be ascribed to the fact that the dielectrophoretic force is proportional to the gradient of voltage according to eq 1.
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Figure 12. Effect of particle size on particle migration (rethenium electrode thickness, 500 nm; ac field, 1 MHz, 2.5 Vrms). Particle sizes were (a) 5.0 µm and (b) 10.1 µm.
system (Figure 12a), due presumably to the weak dielectrophoretic force because the force is proportional to particle diameter according to eq 1. In contrast, the largest particles were found to form a monolayer because they sedimented into lower layers due to their weight even though they moved into the central region in the applied dc field. It is concluded, therefore, that the middle-sized (5.0 µm) particles are appropriate for forming multilayers in the conditions used in the present work. 4. Conclusion Figure 11. Effect of ac field on particle migration (particle diameter, 5.0 µm; rethenium electrode thickness, 500 nm): (a) ac field (1 MHz, 1.0 Vrms); (b) ac field (1 MHz, 2.5 Vrms); (c) 15 min after ac field (1 MHz, 2.5 Vrms) application.
Application of an ac voltage of 2.5 Vrms caused the particles to arrange themselves forming multilayers (Figure 11b). The dielectrophoretic force at this voltage is presumably strong enough to enable the particles to form multilayers in 15 min without applying dc voltage (Figure 11c). 3.4. Particle Size. Particle arrangement control was attempted using polystyrene latex particles with diameters of 2.0, 5.0, and 10.1 µm. Figures 12a, 10b, and 12b show the results for the particles with diameters of 2.0, 5.0, and 10.1 µm, respectively. The smallest particles were found to hardly gather in the central region of the electrode
As have been mentioned so far, the arrangement control of polystyrene latex particles could be possible from dispersion to monolayer, multilayer, and three-dimensional arrangements only through electric field manipulation. Controls of particle arrangement were also possible by suitably choosing ac frequency and voltage, particle size, and electrode thickness. For instance, an application of a low-frequency (100 kHz) ac field to the arranged particles caused the arrangement to disrupt and a stoppage of applying field enabled the arranged particles to disperse. The only procedure needed in these cases was to manipulate the field to be applied. Because of this, the present method of arranging particles is much simpler than those developed for orderly accumulating particles (for example, one by one particle manipulation). LA036465V
