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Assembly of Microsized Colloidal Particles on Electrostatic Regions Patterned through Ion Beam Irradiation Hiroshi Fudouzi,* Mikihiko Kobayashi, and Norio Shinya Materials Engineering Laboratory, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan Received January 22, 2002. In Final Form: June 21, 2002 The fabrication of microstructures with micrometer and submicrometer scale patterns is of importance in wide fields of applications, such as electronics, photonics, magnetics, microactuators, and biochemical devices. In this paper, a novel assembly method of particles based on electrostatic force is proposed and subjected to examination. A positively electrified and stable pattern was formed on an insulating substrate by drawing with a focused ion beam. The substrate was immersed in a suspension in which microsized particles were dispersed in a nonpolar solvent. The particles were selectively deposited on the electrified pattern and were fixed on the substrate by heating. The developed method based on an electrophotographic technique enables fabricating microstructures assembled with micrometer particles.
Introduction The assembly of particles is one of the bottom-up approaches to fabricating a two-dimensional microstructure on a micrometer scale. The microstructure has attracted much attention because of its potential applications in the electric, photonic, and biochemical fields. For example, a novel biosensor was assembled with microsized particles that were coated with functional biomolecules.1 The technique of the patterned particles on a substrate plays a key role in fabricating the microstructure. Several methods have been proposed for patterned microstructures with particles using patterned SAM films,2-8 electrodes,9,10 and micromolds.11,12 Electrophotography, xerography, is one of the important technologies in offices, schools, and homes. The principal of electrophotography is patterned toner particles on the electrified images on paper. Recently, an electrophotographic technique was tried for fabricating patterned particles on a substrate. However, only a few studies have focused on patterned particles using the electrophotographic technique.13,14 One of the issues in the technique is the formation of a stable electrified pattern on an insulating substrate. An ion beam irradiation can form a stable electrified pattern on an insulator. The purpose of * To whom correspondence should be addressed. Tel: +81-29859-2450. Fax.: +81-298-59-2401. E-mail: FUDOUZI.Hiroshi@ nims.go.jp. (1) Velev, O. D.; Kaler, E.W. Langmuir 1999, 15, 3693. (2) Sato, T.; Hasko, D. G.; Ahmed, H. J. Vac. Sci. Technol., B 1997, 15, 45. (3) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. (4) Ha, K.; Lee, Y. J.; Jung, D. Y.; Lee, J. H.; Yoon, K. B. Adv. Mater. 2000, 12, 1614. (5) Aizenberg, J.; Braun, P. V.; Wiltzius, P. Phys. Rev. Lett. 2000, 84, 2997. (6) Masuda, Y.; Seo, W. S.; Koumoto, K. Jpn. J. Appl. Phys. 2000, 39, 4596. (7) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (8) Demers, L. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 3069. (9) Matsue, T.; Matsymoto, N.; Uchida, I. Electrochim. Acta 1997, 42, 3251. (10) Hayward, R. C.; Saville, D. A.; Aksay, A. Nature 2000, 404, 56. (11) Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1996, 8, 245. (12) Scho¨lzer, U. P.; Gauckler, L. J. Adv. Mater. 1999, 11, 630. (13) Fudouzi, H.; Kobayashi, M.; Egashira, M.; Shinya, N. Adv. Powder Technol. 1997, 8, 251. (14) Mesquida, P; Stemmer, A. Adv. Mater. 2001, 14, 1395.
this paper is the experimental demonstration of microstructures assembled with microsized particles using an electrified pattern drawn with a focused ion beam. Experimental Section Procedure. Figure 1 shows a schematic outline of the process for the two-dimensional microstructures assembled with microsized particles using electrification on a nonconductive substrate. The process consists of four steps: namely, drawing, dipping, rinsing, and heating. In the first step, an electrified pattern is drawn on the surface of an insulating substrate with a focused ion beam (FIB). The electrified pattern on the substrate was observed in situ with a scanning electron microscope (SEM) at 2.5 kV.15 In the second step, the substrate is immersed in a suspension in which particles are dispersed. Spherical silica particles (SiO2) were used as the model microsized particles in this experiment. In the third step, the substrate was dipped in a volatile solvent for rinsing the substrate and then dried in air. In the final step, the patterned particles were fixed on the substrate by heat treatment. Materials. Polycrystalline calcium titanate (CaTiO3, SD-100, Kyocera Co., Tokyo, Japan) was used as a nonconductive and high-dielectric substrate. The CaTiO3 substrate has a high permittivity (r ) 108) and a high resistivity (Fv > 1012 [Ω cm]). The CaTiO3 substrates were mechanically polished, and the average roughness of the substrates was less than 0.2 µm. The SiO2 particles and solvents were obtained from commercial sources and were used without further purification. The silica particles (SiO2, Hypresica, Ube-Nitto Kasei Co., Tokyo, Japan) are spherical and monodisperse with a 5.1 µm diameter. In this study, a perfluorocarbon liquid (Fluorinerte FC-40, Sumitomo 3M, Tokyo, Japan) was used as a solvent for suspending the particles. This liquid has a high resistivity and a low dielectric constant. The permittivity and the resistivity of the FC-40 are 1.89 [-] and 1014 [Ω cm], respectively. The SiO2 particles were dispersed in the FC-40 solvent by ultrasonic irradiation, and their concentration was adjusted to 5 × 106 [counts/mL]. After preparation of the electrified patterns, the substrates were immersed into the suspension at room temperature for periods from 30 s to a few minutes without ultrasonication. In the process of immersing the substrate in the suspension, we did not use ultrasonication to prevent aggregation of the microspheres in the suspension. This is because the ultrasonication would remove most of the microspheres deposited on the charged pattern on the substrate. The substrate was then rinsed with a volatile fluorocarbon liquid (Fluorinerte FC-72, Sumitomo 3M) for 30 s. (15) Fudouzi, H.; Egashira, M.; Shinya, N. J. Electrostatics 1997, 42, 43.
10.1021/la020072z CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002
Assembly of Microsized Colloidal Particles
Figure 1. Schematic illustration of the steps required for arrangement of microsized particles: (1) forming an electrified pattern on an insulating substrate by scanning a charged beam under vacuum; (2) dipping the substrate into a suspension in which the particles were dispersed; (3) rinsing the substrate with a volatile solvent and then drying the sample in air; (4) heating the substrate to bond the particles to the substrate. The boiling points of FC-40 and FC-72 are 155 and 56 °C, respectively. Apparatus. The formation and the observation of the electrified patterns were carried out using a dual-beam drawing system (SMI8000D, Seiko Instruments, Tokyo, Japan) at a pressure of 10-7 Torr. The positively electrified pattern was formed on the CaTiO3 substrate by irradiation with a Ga+ FIB at 30 kV. The electrified patterns were observed in situ with a scanning electron beam (SMI-8000D, SEM mode) at 2.5 kV as voltage contrast images.15 The electromigrations of the SiO2 particles in the FC40 solvent were investigated using a zeta potential meter with a nonpolar solvent cell (ELS-8000, Otsuka Densi Co., Tokyo, Japan). The surface potential of the charge on the substrate was measured in the atmosphere with a surface potential meter (model-344 with a probe 6000B-6C, Trek Co., New York). The probe was placed 1 mm above the center of the electrified pattern.
Results and Discussion A CaTiO3 substrate with the drawn pattern using the FIB at 30 kV was measured with a surface potential meter. The result showed that the pattern was electrified in positive polarity. The accelerated Ga+ ions penetrated the substrate and excited secondary electrons near the surface of the substrate. As a result, the irradiated area on the nonconductive substrate was electrified due to storage of the Ga+ ions and the escape of electrons. At the same time, the Ga+ ions were sputtered onto the surface of the substrate. However, the amount of dose ions was small
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Figure 2. SEM images of the electrified pattern and particles assembled on the pattern. (a) Voltage contrast image of the positively electrified pattern on a CaTiO3 substrate. The pattern was drawn with a focused Ga+ ion beam at 30 kV. The image was observed in situ with an SEM at 2.5 kV. (b) An enlarged part of (a). (c) The line profiling of the cross section of (b) indicated by the arrow.
enough in this experiment; therefore, damage to the surface was not serious relative to the surface flatness of the substrate. The stability of the charge of the electrified pattern is one of the most important factors in depositing microspheres from a nonpolar solvent. The surface potential at 2000 s was 90% of the initial surface potential in air. This result indicates that the charge of the electrified regions slowly decays on the CaTiO3 substrate. In contrast, the immersion time of the substrate was less than a few minutes in this experiment. Thus, the lifetime of the charge in the substrate is long enough compared with the immersion time of the substrate. We concluded that the electrified pattern drawn by the FIB is stable and that its lifetime is long enough for experiments where the pattern should attract particles onto the substrate. Figure 2 shows SEM images of the electrified pattern and its line profiling. Figure 2a shows a positively electrified pattern forming a local electric field, which will attract particles in the next step. The electrified pattern, six dotted lines of 100 µm in length with 5 µm spacing between the dots, was drawn with the FIB at 30 kV with a Ga+-ion dose of 3 × 1014 [ions/cm2]. The electrified pattern formed on the substrate was observed in situ using an
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Figure 4. Optical microscope images of the patterned SiO2 particles on the CaTiO3 substrates: (a) arranged particles forming a grid pattern with a spacing of 100 µm; (b) particles arrayed in orthogonal positions with a spacing of 50 µm.
Figure 3. (a) SEM image of the patterned SiO2 particles on the positively electrified pattern shown in Figure 2a; (b) SEM image of the patterned SiO2 particles after heat treatment at 1150 °C for a half-hour. The SiO2 particles were adhered strongly to the substrate.
SEM at 2.5 kV with a beam current of less than 5 pA. The drawn pattern on the substrate corresponds to the dark contrast image as shown in Figure 2a. In our previous study on the electrification, the dark contrast pattern is a positively electrified pattern formed on the substrate.15 The contrast image shows a visualization of the electrified pattern on the CaTiO3 substrate. Figure 2b shows an enlarged part of Figure 2a, and Figure 2c shows the line profiling of the cross section of Figure 2b indicated by the arrow. Periodic peaks exist, and the distance between the peaks is approximately 5 µm. This result indicates that the resolution of the charged dot is less than 5 µm. The spot size of the FIB was approximately 0.2 µm in the equipment. Consequently, the electrified dot was spread about 25 times larger than the irradiated area of the ion beam on the substrate. It is thought that the spreading of the electrified region results from the diffusion and conduction of charges on the substrate. Figure 3a shows an SEM image of the patterned SiO2 particles of 5.1 µm diameter on the CaTiO3 substrate. The image indicates that the SiO2 particles cover the electrified dots shown in Figure 2a. The SiO2 particles were selectively deposited onto the electrified pattern on the CaTiO3 substrate. However, the particles arranged on the substrate were easily removed from the substrate by ultrasonic irradiation in FC-72 solvent. To fix the patterned particles onto the substrate, the substrate was heated at 1150 °C for a half-hour. After heat treatment, no particles were removed from the substrate. From observation of high-magnitude SEM images, the adhered particles were sintered at the interface of the particles
and substrate. Figure 3b shows an SEM image of the SiO2 particles bonded to each other and adhering strongly to the CaTiO3 substrate. A two-dimensional microstructure was fabricated from the patterned microsized particles using the electrophotographic technique. One of the advantages of the developed technique is that it is easy to form an arbitrary particle pattern by changing the drawn pattern for the FIB scanning. Figure 4 demonstrates the other patterns assembled with the same SiO2 particles. A grid pattern with 100 µm spacing is shown in Figure 4a, and an orthogonal array with 50 µm spacing is shown in Figure 4b. The developed technique is a kind of solid free-forming process on a micrometer scale. Figure 5 shows the surface potential and the number of deposited particles as a function of the ion dose. In this new technique, the electric potential field of the electrified pattern strongly affects the amount of the deposited particles on the pattern. Figure 5a shows the relationship between the total charge of the Ga+ ion and the surface potential of the substrate. The surface potential of the electrified pattern on the substrate was measured with the surface potential meter. The surface potential linearly increases with the amount of the ion dose. The intensity of the electric potential field, E, can be controlled by the ion dose. It is difficult to determine the local electric potential field of the electrified pattern. We use the amount of the ion dose as the parameter of the local electric potential field of the electrified pattern. Figure 5b shows the relationship between the amount of ion dose on an electrified dot and the number of SiO2 particles adhering to the electrified dot. The electrified pattern consists of four dotted lines, 400 µm in length with 5 µm spacing, in parallel at a 100 µm distance on the CaTiO3 substrates. We then obtained four SiO2 particle lines on the electrified lines and counted the number of adhered SiO2 particles using an optical microscope. The number of adhered particles linearly increases with the amount of the ion dose of the patterns. The linear
Assembly of Microsized Colloidal Particles
Figure 5. Influence of the Ga+ ions on the surface potential of the electrification and the number of adhered particles: (a) relationship between the Ga+ ion dose and the surface potential of the substrate; (b) relationship between the Ga+ ion dose and the count of the particles adhered to the positively electrified pattern.
correlation means that the number of adhered particles can be controlled by the ion dose. The number of adhered particles linearly increased with the amount of the charge of the pattern. This result shows that the electric potential field formed by the ion dose affects the patterned particles on the substrate. The proposed electrophotographic technique can be used to control the deposition of the particles. The amount of adhered particles can be changed using the ion dose of the pattern. In addition, the slope of the line suggests an optimum value for one particle deposited on one electrified dot. The slope is 0.43 [count/pC] and its inverse value is 2.3 [pC/count]. Figure 6 shows two typical results of the influence of the intensity of dose ions on the electrified patterns. At the low dose shown in Figure 6a, the SiO2 particles are arrayed on the electrified line in series like a pearl chain. In this case, the dose ion was set to 2.3 [pC/dot] based on the results in Figure 5b. In contrast, Figure 6b shows the patterned SiO2 particles at a high dose of ions. The high electric potential field piled the particles up two or more layers on the electrified lines. These phenomena are illustrated in Figure 6c. In the left model, the electric field affects only one particle and then one particle is deposited on the charged area. The electric field increases with the dose ions of the charge. In contrast, many more particles will build up particle layers as illustrated in the right model. A possible explanation for the mechanism of the assembling particles is described as follows: Let us assume that a particle is placed in an insulating solvent near an
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Figure 6. Optical microscope images (a) and (b) correspond to low dose and high dose, respectively. The particle chain shown in the image in (a) was formed by drawing with a dose of Ga+ ions of 2.3 pC per dot. The illustration in (c) shows that the electric field affects the layer of the patterned particles.
electrified region on a substrate. An electrostatic force causes interaction between the particle and the electrified pattern. The electrostatic force, F, can be expressed by the following equation.16,17
F ) qE + 2πr310[(2 - 1)/(2 + 21)]∇E2
(1)
The first and second terms on the right side of the equation represent Coulomb’s force and the gradient force, respectively, where 1 is the relative dielectric constant of the solvent, 2 is the relative dielectric constant of the particle, r is the radius of the particle, E is the electric field caused by the electrified pattern, and q is the charge of the particle. The polarity of SiO2 in a perfluorocarbon liquid, FC-40, was obtained from a zeta potential meter. The electrophoretic mobility, u, of the SiO2 particle in the nonpolar solvent was 1.8 × 10-6 [cm2/(V s)]. The zeta potential, ζ, of the SiO2 particle was calculated from the following Hu¨ckel equation.18
ζ ) 1.5uη/10
(2)
where 1 is the relative dielectric constant of the solvent, 1.89 [-], 0 is the permittivity of a vacuum, 8.854 × 10-12 [F/m], and η is the viscosity of the solvent, 4.1 × 10-3 [Pa s]. By calculating using the above equation, we determined the ζ potential to be -66 mV. The average charge of the (16) Schaffert, R. M. Electrophotography; Focal Press: New York, 1965. (17) Jones, T. B. Electromechanics of particles; Cambridge University Press: New York, 1995. (18) Stotz, S. J. Colloid Interface Sci. 1978, 65, 118.
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SiO2 sphere, q, can be obtained from the following equation.19
q ) 4πrη10ζ
(3)
where r is the radius of the particle and its value is 2.5 × 10-6 m. From eq 3, the average charge of the SiO2 particle is -3.5 × 10-17 C in the nonpolar solvent. The charging mechanism of the particle in a nonpolar solvent such as FC-40 is different from that in an aqueous suspension. The small amount of water adsorbed on the surface of the SiO2 particle plays an important role in the charging of the particle. A SiO2 surface reacts with water to form a layer of silanol, Si-OH, groups. The negative charging of the SiO2 particle is due to the ionization of the hydrogen ion from the silanol groups. Coulomb’s force serves as an attraction force because the charge polarity of the SiO2 is negative and that of the electrified pattern is positive. In addition, the gradient force also plays a role in the attractive force because the relative dielectric constant of the particle (2 ) 4) is greater than that of the solvent (1 ) 1.89). The electrostatic force in eq 1 acts only as an attraction force under this experimental condition. The gradient force acts as an attractive force in a nonpolar solvent because the dielectric constant of a nonpolar solvent, 1 < 2, is smaller than that of the particle in most cases. The SiO2 particles charged in negative polarity were deposited on the negatively electrified pattern drawn by the electron beam.13 Here, Coulomb’s force, qE, acted as a repulsive force, and the gradient force was larger than Coulomb’s force. Therefore, the gradient force plays an important role in a nonpolar solvent acting as an attracting force in eq 1. Finally, we discuss applying the developed technique to nanometer scale particles. Nanosized particles have (19) Hiemenz, P. C.; Rajagopalan, R. Principles of colloid and surface chemistry; Marcel Dekker: New York, 1997.
been given much attention due to their useful functions. However, nanosized particles easily aggregate in a nonpolar solvent. Our future work will be directed toward the reduction of the patterns down to a nanometer scale using nanosized particles dispersed in a polar solvent, such as EtOH or H2O. By use of the polar solvent for the SiO2 particle, the gradient force becomes a repulsive force because the dielectric constant of the polar solvent, 1, is larger than that of the SiO2 particle, 2. To attract the particle on the electrified pattern, Coulomb’s force must be greater than the gradient force: qE > 2πr310[(2 1)/(2 + 21)]∇E2 in eq 1. Unlike the patterning particles in the nonpolar solvent reported in this paper, Coulomb’s force would play an important role in depositing particles on a substrate from a polar solvent. Conclusions In summary, this paper demonstrated a patterning process using microsized particles. Electrified patterns were first formed on nonconductive substrates by drawing with a focused ion beam. Microsized particles were then arranged on the substrates over the electrified pattern. The proposed method is a novel approach based on the electrophotographic technique for fabricating a microstructure assembled with particles. We believe that the electrostatic assembly can be applied to a wide variety of applications to fabricate patterned particles on a micrometer scale. Acknowledgment. The authors thank Dr. Hase and Dr. Miyazaki for their useful comments and discussions. This research was financially supported by the budget for Intelligent Materials Research from the Japanese government. LA020072Z
