1180
Langmuir 2000, 16, 1180-1184
Observation of Light Propagation in Single Layers of Composite Two-Dimensional Arrays Yoshie Yagi,† Sachiko I. Matsushita,† Donald A. Tryk,† Takao Koda,‡ and Akira Fujishima*,† Department of Applied Chemistry, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of Mathematical and Physical Science, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan Received May 14, 1999. In Final Form: August 9, 1999 Composite two-dimensional (2D) polystyrene (PSt) arrays, composed of three types of PSt particles, red fluorescent, green fluorescent, and nonfluorescent, were prepared from an aqueous suspension. These arrays form as a result of water evaporation and capillary forces. Using fluorescence microscopy and phase-contrast microscopy, we observed several interesting types of light propagation phenomena in single layers of the array, including anisotropic light propagation, an additive effect for light intensity, and a novel switch function. These characteristic light propagation phenomena indicate some of the possibilities of the 2D array as a photonic device.
Introduction Two-dimensionally ordered arrays can be prepared from monodisperse fine colloidal particles or protein molecules. These particles are packed in high-density, highly oriented layers over a wide surface area using capillary forces and water evaporation. This new type of nanoscale architecture1-3 has attracted interest mainly from the viewpoint of the orientation of protein molecules,4,5 from that of high-density optical memory devices,6,7 and from that of photocatalytic systems.8 We would also like to suggest that one of the attractive applications of the twodimensional (2D) array will be as a photonic material.7,9-11 These arrays may be regarded as 2D photonic crystals, characterized by a periodic change in the dielectric constant, with a period comparable to the wavelength of light.12-14 When we became interested in using these arrays as photonic materials, as a first step, we decided to examine the ways in which light propagates within the arrays. Because polystyrene (PSt) has a higher dielectric constant than air, if we can put fluorescein-based fluorescent PSt † ‡
University of Tokyo. Japan Women’s University.
(1) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311. (2) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (3) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706-709. (4) Nagayama, K.; Takeda, S.; Endo, S.; Yoshimura, H. Jpn. J. Appl. Phys. 1995, 34, 3947-3954. (5) Picard, G.; Nevernov, I.; Alliata, D.; Pazdernik, L. Langmuir 1997, 13, 264-276. (6) Micheletto, R.; Fukuda, H.; Ohtsu, M. Langmuir 1995, 11, 33333336. (7) Hayashi, S.; Kumamoto, Y.; Suzuki, T.; Hirai, T. J. Colloid Interface Sci. 1991, 144, 538-547. (8) Matsushita, S. I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Langmuir 1998, 14, 6441-6447. (9) Imhof, A.; Pine, D. J. Nature 1997, 389, 948-951. (10) Yablonovitch, E. J. Opt. Soc. Am. B 1993, 10, 283-295. (11) Foresi, J. S.; Villeneuve, P. R.; Ferrera, J.; Thoen, E. R.; Steinmeeyer, G.; Fan, S.; Joannopoulos, J. D.; Kimerling, L. C.; Smith, H. I.; Ippen, E. P. Nature 1997, 390, 143-145. (12) John, S. Phys. Rev. Lett. 1987, 58, 2486-2489. (13) Miguez, H.; Lopez, C.; Mesguer, F.; Blanco, A.; Vazquez, L.; Mayoral, R.; Ocana, M.; Fornes, V.; Mifsud, A. Appl. Phys. Lett. 1997, 71, 1148-1150. (14) Miyazaki, H.; Ohtaka, K. Phy. Rev. B 1998, 58, 6920-6937.
particles in the array,15 the emitted fluorescence light will propagate anisotropically and selectively within PSt particles in the array. The light scattered between the particles can be observed with a fluorescence microscope and can be used as a probe. The anisotropic propagation may be used as the basis for 2D photonic circuits. The unique feature of such composite arrays is that the nonfluorescent particles may act as an effective host medium, transporting the electromagnetic energy from one photoactive particle to another, at which a specific type of photon energy conversion process, e.g., fluorescence, can take place, much like the process occurring in a chlorophyll-based light harvesting system.16,17 Such an effect could also be an effective probe with which to characterize the propagation of light energy. In this paper, we report the preparation of composite 2D arrays from PSt particles whose diameter (1 µm) is of the same order of magnitude as the light wavelength and the observation of the manner in which the light propagates in single arrays layers using optical fluorescence microscopy and phase-contrast microscopy. Materials and Methods We prepared composite arrays from two types of fluorescent particles, i.e., red (λex/λem 541/612 nm; 1.01 ( 0.05 µm; Polymer Microspheres, Red Fluorescing, Duke Scientific Corp.) and green (λex/λem 458/ 540 nm; 0.973 ( 0.028 µm; Fluoresbrite carboxylate microspheres, Polysciences Inc.) with nonfluorescent particles (1.034 ( 0.020 µm; Particle-Size Standards, NIST Traceable, Duke Scientific Corp.) such that the fluorescent particles were present at a relatively low fraction, i.e., 1:1:400 (red-greennonfluorescent). Due to water evaporation and capillary forces,18 the particles that were suspended in water formed 2D arrays on a nonfluorescent glass substrate (Micro-Slide glass, Matsunami Co., Japan) (Figure 1). The speed of array formation was controlled at rates (15) Mastushita, S.; Miwa, T.; Fujishima, A. Langmuir 1997, 13, 2582-2584. (16) Fujimura, T.; Edamatsu, K.; Itoh, T. Optics Lett. 1997, 22, 1-4. (17) Fujimura, T.; Itoh, T.; Hayashibe, K.; Edamatsu, K.; Shimoyama, K.; Shimada, R.; Imada, A.; Koda, T.; Segawa, Y.; Chiba, N.; Muramatsu, H.; Ataka, T. Mater. Sci. Eng., B 1997, 48, 94-192. (18) Denkov, N. D.; Velev, O. D.; Krachevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183-3190.
10.1021/la9905867 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/17/1999
Light Propagation in Arrays
Langmuir, Vol. 16, No. 3, 2000 1181
Figure 1. Schematic diagram of Dimitrov cell used to fabricate the 2D arrays. The number of layers in the array was controlled by adjusting the distance between the cell and the substrate using the z-axis stage. During preparation of the array, the substrate is translated horizontally using the x-axis stage. from 0.5 to 2.5 µm/s by moving the substrate horizontally. The particle assembly process was observed using a CCD camera. Well-ordered 2D arrays with typical dimensions of 10 mm × 10 mm were prepared.15 The 2D arrays thus prepared were observed by use of phase-contrast microscopy and fluorescence microscopy (BX60-34-FLBD1, Olympus, Japan). The microscope light source was a Hg-Xe lamp (Olympus). For the fluorescence microscopy, an excitation filter (BP420-450, Olympus), which allows light of wavelengths from 420 to 450 nm to pass, was used. Also, an absorption filter (BA515F, Olympus), which allows light of wavelengths greater than 515 nm to pass, was used, and thus the fluorescence emission from both red- and green-fluorescing particles was detected. To measure the fuorescence light intensity, we used NIH Image software (http://thaigate.nacsis.ac.jp).
Results With three different types of particles, we were able to obtain composite 2D arrays exhibiting high-density, highly oriented hexagonal packing. We were able to obtain broad areas containing just a single monolayer of particles, typically on the order of 3-6 mm wide, with domains of near-perfect ordering on the order of 20-100 µm. There are other methods that have been developed to prepare 2D arrays,19,20 but the problem is that, with these methods, it is difficult to control the number of layers and to achieve a high degree of 2D ordering over a wide area. Such films typically have small domain sizes, and thus it would be difficult to observe the variety of different types of light propagation that have been observed in the present work plus that reported elsewhere. With the techniques developed in part in our laboratory,4,15 we can decrease the degrees of freedom of particle movement by making use of not only evaporation but also the translation of the glass substrate with respect to the supply of particle suspension. Essentially what this does is to deposit the particles row by row over a microscopic distance (up to several micrometers wide), so that we can obtain arrays with larger domains than those prepared merely by evaporation. Anisotropic Light Propagation. Figure 2a shows the phase-contrast microscopic image of a single-layer composite 2D array of 1-µm particles. The darker blue-gray spots are the shadows of the PSt particles. At this point, (19) Bogomolov, V. N.; Gaponenko, S. V.; Germanenko, I. N.; Kapitonov, A. M.; Petrov, E. P.; Gaponenko, N. V.; Prokofiev, A. V.; Ponyavina, A. N.; Silvanovich, N. I.; Samoilovich, S. V. Phy. Rev. E 1997, 55, 7619. (20) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266-273.
Figure 2. (a) Typical fluorescent microscopic image of a single layer in a composite 2D array and (b) phase-contrast microscopic image of the same area. The green fluorescence light intensity was measured along the row containing the two green fluorescent particles in (c). The top of the vertical axis corresponds to the maximum obtainable signal; the two largest peaks are off-scale.
we cannot determine which particles are fluorescent. This figure shows that the particles are highly oriented in a hexagonal packing arrangement. Figure 2b shows the fluorescence microscopic image of the same area as that in Figure 2a. Comparing these two figures in detail, we find that the two green fluorescent particles in Figure 2b are those particles marked with green tick marks in Figure 2a, and that the brighter spots emanating in six directions in Figure 2b are due to scattering of the propagated fluorescence light between the nonfluorescent PSt particles. Observing these brighter spots, we can see clearly which particles the light propagates through and that the green fluorescence light emitted from the two fluorescent particles propagates through the nonfluorescent particles in six directions outward from each fluorescent particle.16,17 Light Intensity Additive Effect. Using this anisotropic propagation effect, we were able to observe the addition of the fluorescence light intensities in the 2D arrays. In Figure 2b, two green fluorescent particles are situated along the same row, as shown in Figure 2a. On this row, we measured the intensity of the green fluorescence (Figure 2c). The vertical axis represents the fluorescence light intensity along the row and the hori-
1182
Langmuir, Vol. 16, No. 3, 2000
zontal line represents the particle position on the row. We compared the peak intensities at three points, i.e., peaks 1, 2, and 3, to check the additivity effect. As shown in Figure 2, these peaks are due to light being scattered at the interfaces between the particles, so that, if we compare the intensities, we can estimate the relative amounts of light propagating within the array particles. The distance between green fluorescent particle 1 and the point chosen for peak 1 is the same as that between particle 1 and the point chosen for peak 2, i.e., 6.0 µm. Moreover, the distance between green fluorescent particle 2 and the point chosen for peak 2 is also the same as that between particle 2 and the point chosen for peak 3, also 6.0 µm. Since the intensities for peaks 1 and 3 are essentially only due to the fluorescence of single particles, 1 and 2, respectively, it is predicted that the intensity for peak 2, which may include contributions from both particles 1 and 2, would be significantly greater. From Figure 2c, we can clearly see that the intensity for peak 2 is indeed greater than those for peaks 1 and 3. Path-Branching Phenomenon. In addition, we were able to observe an interesting light-path-branching phenomenon involving the combination of green and red fluorescent particles in the array. Figure 3a shows the fluorescence microscopic image of an area within a single layer of the composite array containing a single green fluorescent particle and two red fluorescent particles. Figure 3b shows the phase-contrast microscopic image of the same area as shown in Figure 3a. In Figure 3b, three particles are indicated with green or red tick marks. The green fluorescent particle and red fluorescent particle 1 lie along the same row (row 1) in the array, and red fluorescent particles 1 and 2 also lie along the same row (row 2). To monitor this phenomenon more precisely, we focused on red fluorescent particle 2, which was not on the same row with the green particle but on the same row with red particle 1 (row 2), so that red particle 2 could not be excited by the green fluorescence light propagated through nonfluroescent particles from the green particle, as shown in Figure 3b. Figure 3c represents the intensity of the red fluorescence light (vertical axis) along row 2, and the horizontal axis represents the positions of the PSt particles along row 2. In this figure, it can clearly be seen that the intensities of the red fluorescent particles themselves are extremely large and are off-scale at this sensitivity. In addition, the light scattered at the interface between the fluorescent particles and its nearest neighbors is also part of this overall intense peak. However, the light scattered at the interface between the nearest neighbor and next-nearest particle has a measurable intensity (peak 2). We can observe that the intensity of peak 2 on red particle 1 is larger than that on red particle 2. Discussion Anisotropic Light Propagation. Using phase-contrast microscopy, we were able to detect point defects and line defects in some areas of the arrays (not shown in Figure 1). At these defects, the light does not continue to propagate but is scattered completely. From this additional result, we can confirm that the light propagates selectively and anisotropically along rows of PSt particles that are in direct contact. This type of anisotropy has a simple optical explanation. Light can propagate easily in directions in which there is essentially a continuous medium of higher refractive index (nPSt ) 1.46; nair ) 1.00), along a straight line, and this
Yagi et al.
is only true along the three principal crystallographic axes. However, as we show in other work, it is not at all obvious that this is the case for arrays of particle of all sizes. In fact, we have found that anisotropic light propagation cannnot be observed in 2D arrays particles whose diameter is less than the light wavelength.21 Light Intensity Additive Effect. Figure 2c shows that the green fluorescence light emitted from fluorescent particles 1 and 2 propagates through the nonfluorescent particles along the row and is stronger at the midpoint of the row (peak 2) than peaks 1 and 3. In other words, we can say that light intensities propagating from different sources through nonfluorescent PSt particles strengthen each other where they meet in the array and are approximately additive. Path-Branching Phenomenon. In these experiments, we used an excitation filter that allows light of wavelengths from 460 to 490 nm to pass, so that both green and red fluorescent particles were excited. However, the relative efficiency of the excitation light for the red fluorescent particles should be significantly smaller than that for the green particles, because the wavelength of the excitation peak for the fluorescein dye in the red particles is 541 nm, which is not within the range passed by the excitation filter. According to the excitation spectrum, the relative intensities for this dye range from 19.8 to 46.5% over the wavelength range from 460 to 490 nm. Therefore, if the green fluorescence light propagates from the green fluorescent particle to red particle 1 through nonfluorescent particles along row 1, it is expected that the red particle 1 may be excited selectively. The efficiency for the excitation peak wavelength of the red dye by the green fluorescence light should be high, because the excitation peak for the red dye and the emission peak for the green dye are nearly identical (541 and 540 nm, respectively). We must also take into account light that is propagated through the glass substrate. This propagation pathway has two possible effects: (1) the destination particle could receive additional excitation light intensity, increasing its emission intensity, and (2) light scattered upward from the glass could be measured together with that at the interfaces between the particles, also leading to a spuriously high signal (see later). In this experiment, however, we note that the distance between red particle 1 and the green particle was nearly the same as that between red particle 2 and the green one, and thus, the green fluorescence should have propagated within the glass substrate to an approximately equal extent to both red particles. Thus, we can neglect this effect in this case. The peak 2 intensities can be compared for red particles 1 and 2. The peak 2 intensity associated with particle 1 is significantly larger than that associated with particle 2. This is evidence for the green fluorescence light propagating along row 1 to particle 1 and exciting it, resulting in additional intensity. This result indicates that the nonfluorescent PSt particles that are arranged in a straight line act as an effective medium for the propagation of the electromagnetic energy from the source (green fluorescent particle) to the receptor (red fluorescent particle), at which the green fluorescence light is converted to red. This result also demonstrates that the effective direction of the light propagation in an array can be changed at photoactive particles, but with a concurrent change in wavelength. We refer to this as a light-pathbranching phenomenon. (21) Matsushita, I. S.; Yagi, Y.; Miwa, T.; Tryk, D. A.; Koda, T.; Fujishima, A. Langmuir 1995, 15, 0000.
Light Propagation in Arrays
Langmuir, Vol. 16, No. 3, 2000 1183
Figure 3. Typical fluorescent microscopic image (a) of a single layer in a composite 2D array and phase-contrast microscopic image (b) of the same area of Figure 3a. We measured the red fluorescence light intensity along the row on which there are two red fluorescent particles. The top of the vertical axis corresponds to the maximum obtainable signal; the two largest peaks are off-scale.
Conclusions We have observed that light propagates selectively and anisotropically through PSt particles lying along rows
corresponding to the three principal axes of the hexagonal close-packed 2D lattice,16,17 with the result that light radiates in six directions from fluorescent particles. Using this anisotropic light propagation, we observed that light
1184
Langmuir, Vol. 16, No. 3, 2000
intensities propagating from different sources through nonfluorescent PSt particles strengthen each other where they meet in the array and are approximately additive. Moreover, we observed an interesting path-branching phenomenon in which propagating green fluorescence light is converted to red fluorescence light at a photoactive particle. These results may provide a basis for the future development of such composite 2D arrays as photonic devices, perhaps involving 2D photonic circuits, using these patterns of light propagation. The present results also provide a convenient methodology, involving the combination of highly ordered particle arrays and fluorescence microscopy, for further basic studies of both 2D and 3D photonic crystals.14
Yagi et al.
Acknowledgment. Thanks are due to Professor Hashimoto (Research Center for Advanced Science and Technology, University of Tokyo, Japan), Dr. Tetsuya Miwa (Japan Marine Science and Technology Center), and Dr. A. S. Dimitrov (L′OREAL Tsukuba Center, Japan) for their valuable discussions. The present work has been partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport and Culture of Japan, and by grants from the YKK R & D Center and from the Circle for the Promotion of Science and Engineering. LA9905867
