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Photoluminescence Modification in 3D-Ordered Films of Fluorescent Microspheres Yuanzhi Li, Toyoki Kunitake,* Shigenori Fujikawa, and Kazunari Ozasa Frontier Research System (FRS), The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Woko-shi, Saitama, 351-0198, Japan ReceiVed March 2, 2007. In Final Form: May 30, 2007 3D-ordered latex films made of fluorescent microspheres were fabricated by sonication-assisted casting. Angledependent changes of photoluminescent (PL) emission were observed for the as-prepared fluorescent latex films with diameter of 200 nm. The PL emission at 483 and 512 nm was enhanced when they were near the edge of the stop band, and the suppression of PL emission was observed at 483 and 512 nm when they were located in the stop band. Resonance enhancement of PL emission was observed at 512 nm for the latex film with latex diameter of 1011 nm, in which the fluorescent sphere acted as both active emitting source and resonance microcavity.
Introduction The control of spontaneous emission (SE) has received much attention because of its many important applications in lowthreshold lasers, high-efficiency light emitters, and single-photon sources.1,2 There is a growing interest in the study of periodic dielectric structures (photonic crystals), and the inhibition of light wave propagation in photonic crystals provides a way to control spontaneous emission.3-5 Among the effects of photonic crystals on the radiation property are interesting phenomena such as nonexponential decay of spontaneous emission near the band edge and strong inhibition of emission and enormous enhancement of radiation.6-11 Experimental studies in relation to photonic crystals include the emission property of adsorbed dye molecules in opal or inverse opal,12-16 incorporation of dye molecules in PMMA beads,17 and embedding emitting sources inside photonic crystals.18-19 The enhancement, suppression, and attenuation of spontaneous emission are observed in all or certain directions with respect to the normal vector of photonic crystal. Recently, * Corresponding author. Tel: +81-48-467-9601. Fax: +81-48-464-6391. E-mail:
[email protected]. (1) Bechger, L.; Lodahl, P.; Vos, W. L. J. Phys. Chem. B 2005, 109, 9980. (2) Yablonovitch, E.; Gmitter, T. J.; Leung, K. M. Phys. ReV. Lett. 1991, 67, 2295. (3) Yablonovitch, E. J. Opt. Soc. Am. B 1993, 10, 283. (4) Yamasaki, T.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 1957. (5) Megens, M.; Wijnhoven, J. E. G. J.; Lagendijk, A.; Vos, W. L. J. Opt. Soc. Am. B 1999, 16, 1403. (6) Zhu, S. Y.; Li, G. X.; Yang, Y. P.; Li, F. L. Eur. Phys. Lett. 2003, 62, 210. (7) Sprik, R.; Van Tiggelen, B. A.; Legendijk, A. Eur. Phys. Lett. 1996, 35, 265. (8) Asatryan, A. A.; Busch, K.; McPhedran, R. C.; Botten, L. C.; Martijn de Stedrke, C.; Nicorovici, N. A. Phys. ReV. E 2001, 63, 046612. (9) Lousse, V.; Vigneron, J. P.; Bouju, X.; Vigoureux, J. M. Phys. ReV. B 2001, 64, 201104. (10) Vats, N.; John, S.; Busch, K. Phys. ReV. A 2002, 65, 043808. (11) John, S.; Quang, T. Phys. ReV. A 1994, 50, 1764. (12) Petrov, E. P.; Bogomolov, V. N.; Kalosha, I. I.; Gaponenko, S. V. Phys. ReV. Lett. 1998, 81, 77. (13) Koenderink, A. F.; Bechger, L.; Schriemer, H. P.; Lagendijk, A.; Vos, W. L. Phys. ReV. Lett. 2002, 14, 143903. (14) Romanov, S. G.; Maka, T.; Sotomayor, T. C. M; Muller, M.; Zentel, R. Appl. Phys. Lett. 1999, 75, 1057. (15) Yoshino, K.; Lee, S. B.; Tatsuhara, S.; Kawagishi, Y.; Ozaki, M. Appl. Phys. Lett. 1998, 73, 3056. (16) Schroden, R. C.; Al-Daous, M.; Stein, A. Chem. Mater. 2001, 13, 2945. (17) Muller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Sotomayor Torres, C. M. Chem. Mater. 2000, 12, 2508. (18) Ogawa, S.; Imada, M.; Yoshimoto, S.; Okano, M.; Noda, S. Science 2004, 305, 227. (19) Bulu, I.; Caglayan, H.; Ozbay, E. Phys. ReV. B 2003, 67, 205103.
Ryu et al.20 and Zelsmann et al.21 reported that the best method of achieving high efficiencies of spontaneous emission was to etch 2D-photonic crystal directly in the emitting-active layer. Another approach to tailor spontaneous emission can be achieved by placing an emitter in microcavities that support resonant modes with a high quality factor and a small mode volume.22,23 By this approach, the spontaneous on-resonance emission of an emitter with a cavity mode is enhanced, while off-resonance emission is suppressed.22 Using, for example, semiconductor microdisks or pillar microcavities, both onresonance enhancement and off-resonance SE suppression were demonstrated.24-28 Recently, photonic crystal defect microcavities were reported to exhibit higher SE enhancement.1,29-31 Up to now, there has been no report on the angle-dependent modification and on-resonance enhancement of spontaneous emission in photonic crystals made of emitting materials. In a previous work, we developed a facile method of sonicationassisted casting to fabricate large 3D latex films.32 We applied this method to fabricate functionalized 3D latex films in this study. Here, we describe fabrication of 3D-ordered latex film made of fluorescent microspheres and observed a strong angledependent modification of photoluminescent (PL) spectra and resonance enhancement of PL emission. (20) Ryu, H. Y.; Lee, Y. H.; Sellin, R. L.; Bimberg, D. Appl. Phys. Lett. 2001, 79, 3573. (21) Zelsmann, M.; Picard, E.; Charvolin, T.; Hadji, E.; Heitzmann, M.; Dalzotto, B.; Nier, M. E.; Seassal, C.; Rojo-Romeo, P.; Letartre, X. Appl. Phys. Lett. 2003, 83, 2542. (22) Purcell, E. M. Phys. ReV. 1946, 69, 681. (23) Andreani, L. C.; Panzarini, G.; Gerard, J. M. Phys. ReV. B 1999, 60, 13276. (24) (a) Gerard, J. M.; Sermage, B.; Gayral, B.; Legrand, B.; Costard, E.; Thierry-Mieg, V. Phys. ReV. Lett. 1998, 81, 1110. (b) Gerard, J. M.; Gayral, B. Physica E 2001, 9, 131. (c) Gayral B.; Gerard, J. M. Physica E. 2000, 7, 641. (25) Bayer, M.; Reinecke, T. L.; Weidner, F.; Larionov, A.; McDonald, A.; Forchel, A. Phys. ReV. Lett. 2001, 86, 3168. (26) Kiraz, A.; Michler, P.; Becher, C.; Gayral, B.; Imamoglu, A.; Zhang, L.; Hu, E.; Schoenfeld, W. V.; Petroff, P. M. Appl. Phys. Lett. 2001, 78, 3932. (27) Solomon, G. S.; Pelton, M.; Yamamoto, Y. Phys. ReV. Lett. 2001, 86, 3903. (28) Fang, W.; Xu, J. Y.; Yamilov, A.; Cao, H.; Ma, Y.; Ho, S. T.; Solomon, G. S. Opt. Lett. 2002, 27, 948. (29) Happ, T. D.; Tartakovskii, I. I.; Kulakovskii, V. D.; Reithmaier, J. P.; Kamp, M.; Forchel, A. Phys. ReV. B 2002, 66, 041303. (30) Painter, O.; Lee, R. K.; Scherer, A.; Yariv, A.; O’Brien, J. D.; Dapkus, P. D.; Kim, I. Science 1999, 284, 1819. (31) Ryu, H. Y.; Kim, S. H.; Park, H. G.; Hwang, J. K.; Lee, Y. H.; Kim, J. S. Appl. Phys. Lett. 2002, 80, 3883. (32) Li, Y. Z.; Kunitake, T.; Fujikawa, S. Colloids Surf. A. 2006, 275, 209.
10.1021/la700610p CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
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Scheme 1
Experimental Section Materials. Glass slides are products from Iwaki Glass. Reagentquality sulfuric acid (95%) and hydrogen peroxide (30%) were bought from Wako Pure Chemical. The following monodispersed fluorescent polystyrene microspheres (Fluoresbrite plain YG microsphere) with different diameters were bought from Polysciences: latex A (2.72% solid latex, diameter 0.200 µm, standard deviation 0.012 µm), latex B (2.50% solid latex, diameter 0.474 µm, standard deviation 0.012 µm), latex C (2.60% solid latex, diameter 0.742 µm, standard deviation 0.025 µm), and latex D (2.60% solid latex, diameter 1.011 µm, standard deviation 0.023 µm). The latex is modified with fluorescein isothiocyanate as shown in Scheme 1. Fabrication of 3D-Ordered Fluorescent Latex Films. Glass slides (1.9 × 1.3 cm) used for the experiment were first treated with a mixture of 30% hydrogen peroxide and 70% sulfuric acid for 8 h, washed with deionized water, and finally dried by flushing nitrogen gas. 3D-ordered fluorescent latex films were fabricated by our recently developed method of sonication-assisted casting.32 Forty microliters of latex dispersions was dropped on the glass slide and spread uniformly on the whole area of the slide with the tip of a pipet, and then the ultrasonic apparatus was turn on. Thin latex films were formed on the glass slide after solvent water evaporated at ambient temperature. Nonphotonic crystal reference samples were prepared by immersing the obtained latex films in the vapor of toluene to destruct the 3D-ordered structure so that the resulting reference film has the same amount of the dye per unit area as 3D-ordered fluorescent latex films. Thus, 1 mL of toluene was placed in a small glass bottle. The bottle was saturated with toluene vapor at ambient temperature for 10 min. Then, after the 3D-ordered film on glass slides (1.9 × 1.3 cm) was kept in this atmosphere for 30 min, it became a flat thin film without pores, because the 3D-ordered structure collapsed due to dissolving of the latex spheres in toluene. Characterization. Scanning electron microscopy (SEM) images were obtained by using a Hitachi S-5200 instrument. Absorption spectra were recorded on a UV3100 UV-vis-NIR spectrophotometer. The sample was mounted on a rotation stage. The angle between the surface normal and detecting light was changed from 0° to 30° by rotating the stage. The surface normal is perpendicular to the (111) plane of the fcc lattice, which is parallel to the surface of glass slide. Photoluminescence spectra were recorded at room temperature on SP-6500 spectrometer using 440 nm excitation light. The pump beam is focused on the sample surface at an incidence angle, θp. In order to acquire emission spectra as a function of detection angle, θe, relative to the surface normal, the sample was mounted on a rotation stage. The refractive index of the fluorescent latex was measured on a spectroscopic ellipsometer (EC-400, J. A. Woollan Co.). To prepare the sample for measuring refractive index, 0.5 mL of latex D was centrifuged, and then the dry latex was dispersed in 0.7 mL of a mixture of toluene and acetone (v/v 2.5/1). A fluorescent thin film was prepared by spin coating on a Si wafer.
Results and Discussion Fabrication of Fluorescent Latex Films. 3D-ordered fluorescent latex films were fabricated by sonication-assisted casting.32 Figure 1 shows a top-view SEM image of a fluorescent latex
Figure 1. Top-view SEM image of a 3D-ordered fluorescent latex film with a latex diameter of 200 nm.
film with a latex diameter of 200 nm. It has a closely packed, hexagonal array. Figure 2 shows corresponding cross-section SEM images of the latex films with different latex diameters from 200 to 1011 nm. As can be seen from Figure 2a, when the diameter of latex microspheres is 200 nm, the obtained latex film has a thickness of 3.2 µm, with 16 latex layers. Close stacking of the microspheres along the direction perpendicular to substrate is related to the formation of a face-centered cubic (fcc) structure with the (111) face parallel to the substrate surface. When the latex diameter increases from 474 to 742 to 1011 nm, similar fcc structures were obtained (Figure 2b-d). The number of the latex layer with latex diameters of 474, 742, and 1011 nm is 11, 11, and 8, respectively. Their film thickness is 5.2, 8.2, and 8.1 µm, respectively. Effect of Stop Band on PL Emission. In our pervious work,32 we showed that the stop band of 3D latex film, which is determined by the Bragg formula, is dependent on the latex diameter. To investigate the influence of photonic stop band on photoluminescence (PL) spectra of the as-prepared 3D-ordered latex film, the 3D-ordered fluorescent latex film with latex diameter of 200 nm was chosen for which the stop band overlaps with the PL emission band. Figure 3 shows the absorption and PL spectra of the 3D-ordered latex film with latex diameter of 200 nm and the corresponding reference film at different detection angles. The shapes of PL spectra of the reference sample are independent of incident angle. As can be seen from Figure 3a, the reference film has two PL emission peaks at 512 and 483 nm, when measured at a detection angle of 30°. The intensity ratio of the two PL emission peaks (I483/I512) is 1.19. In contrast, the 3D-ordered latex film with latex diameter of 200 nm has a different PL spectra pattern, in which the peak at 512 nm is enhanced and the peak at 483 nm is suppressed. The intensity ratio of the two PL emission peaks, I483/I512, is altered from 1.19 to 0.92 upon formation of the ordered latex film. From the corresponding absorption spectra measured at the same detection angle of 30° (Figure 3b), a stop band appears at 475 nm with latex diameter of 200 nm. Obviously, the PL emission peak centered at 483 nm is located in the stop band region, but one centered at 512 nm is near the red edge of this stop band. The PL shape difference between the ordered latex film and the reference film is attributed to the band edge effect of the stop band. 6,7,19,33,34 Spontaneous emission is (33) (a) Dowling, J. P.; Scalora, M.; Bloemer, M. J.; Bowden, C. M. J. Appl. Phys. 1994, 75, 1896. (b) Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6307. (c) Halaoui, L. I.; Abraams, N. M.; Mallouk, T. E. J. Phys. Chem. B. 2005, 109, 6334.
3D-Ordered Films of Fluorescent Microspheres
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Figure 2. Cross-section SEM image of 3D-ordered fluorescent latex films with different diameters: (a) 200 nm, (b) 474 nm, (c) 742 nm, and (d) 1011 nm.
Figure 3. PL spectra (a) and absorption spectra (b) of 3D-ordered fluorescent latex film with different diameter measured at different detection angles.
determined by local density of state (LDOS), which is proportional to the amplitude of the electric field where the emission source (34) Vlasov, Yu. A.; Luterova, K.; Pelant, I.; Honerlage, B. Appl. Phys. Lett. 1997, 71, 1616.
is located and is inversely proportional to group velocity of mode. Near the edge of the stop band there exist low group velocities of light along with high electronic-field intensity, which result in enhancement of spontaneous emission.6,7,19,33,34 The emission band at 483 nm is suppressed due to its location in the stop band region, and the emission band at 512 nm is enhanced due to its location at the edge of the stop band. For the ordered latex films with latex diameter of 200 nm, angle-dependent modification of PL emission was observed. With decreasing detection angles from 30° to 20°, its stop band shifts to a higher wavelength (495 nm). The PL emissions at 483 and 512 nm are located in the stop band region and are suppressed. The shape of the PL spectra measured at detection angle 20° is similar to that of the reference thin film (Figure 3a). Further, the decrease in the detection angle to 10° results in a shift of the stop band from 495 to 508 nm (Figure 3b). Thus, the PL emission at 483 nm at the detection angle of 10° moves to the blue edge of this stop band, but PL emission at 512 nm is still located in the stop band region, leading to its suppression. The intensity ratio of the two PL emissions (I483/I512) at the detection angle of 10° increases to 1.71 from 1.19 for the reference film. Effect of Multiple Scattering on PL Emission. It is also seen from Figure 3a that the PL emission intensity of the 3D-ordered latex film is greatly enhanced relative to that of the reference film, without regard to the detection angle. For example, the PL emission intensity at 483 and 512 nm (measured at the normal detection angle of 30°) of the 3D-ordered latex film with latex diameter of 200 nm is 6.1 and 7.9 times higher than that of the reference film, respectively. Moreover, this enhancement is independent of the size of fluorescent latex spheres. For example, the PL emission intensity at 483 and 512 nm (measured at the normal detection angle of 30°) of the 3D-ordered latex film with latex diameter of 474 nm is 4.8 and 5.1 times higher than that of the reference film (Figure 3a), respectively. Obviously, this effect could not be reasonably explained by the above-mentioned edge effect of the stop band. Although the ordered latex film and
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Figure 5. (a) The refractive index (n) of the fluorescent latex as a function of wavelength and (b) normalized rate R/A, which is obtained by a simulation based on the theory of Kien et al.,37 for an atom in the center of a fluorescent sphere with latex diameter of 1011 nm as a function of wavelength.
Figure 4. PL spectra (a) and absorption spectrum (b) measured at the detection angle of 30° of ordered latex films with latex diameters of 1011 and 742 nm.
the reference film consist of the same amount of PL active material per unit area, the thickness of the former is 1.35 times larger than that of the latter because of better ordering. This means that the ordered latex film has a longer optical path length than the reference flat film. In addition, multiple scattering becomes significant when light wavelength is comparable to the periodicity of photonic crystal. The multiple scattering leads to a much longer optical path length for both the excitation light and PL emission in the ordered latex film.34,35 The increased excitation path length can lead to a large enhancement of PL emission intensity. Resonance Modification of PL Emission in Dielectric Sphere. The influence of resonance modification on PL emission was investigated by changing the latex size. Figure 4 shows absorption spectra and the corresponding PL spectra of the ordered latex film with latex diameter of 1011 and 742 nm and the reference film at the detection angle of 30°. The ordered latex film with latex diameter of 742 nm has a PL spectrum pattern similar to that of the reference thin film, but a different PL spectrum pattern with enhanced peak at 512 nm was obtained with the latex diameter of 1011 nm. The intensity ratio of the two emission peaks at 483 and 512 nm (I483/I512) decreases to 0.802 from 1.19 of the reference film. As shown in Figure 4b, the two latex films with latex diameters of 1011 and 742 nm has first-order stop bands centered at 2280 and 1600 nm, respectively, which is far away from the region of PL emission (483-512 nm). Bechger et al.1 investigated the influence of the second(35) Usami, A.; Ozaki, H. J. Phys. Chem. B. 2005, 109, 2591.
order Bragg stop band on emission of rhodamine 6G laser dye (R6G) and observed that the emission spectrum of R6G modified by the second-order Bragg stop band is completely different from the modified emission spectra of R6G in polystyrene opals with a first-order Bragg stop band. In the second-order Bragg diffraction, a series of attenuation at several wavelengths were seen, as opposed to the clear single stop band in the first-order Bragg diffraction. They pointed out the importance of investigating the higher order Bragg diffraction. In our case, there exist higher order Bragg stop bands in the region of the PL emission, but their PL spectra do not have a series of attenuation at several wavelengths, as expected from the effect of the second-order and higher order Bragg diffraction. Benner et al.36 observed structure resonance in the fluorescence spectra from a dye-impregnated single polystyrene microsphere. They stated that the resonant peak corresponds to the natural modes of oscillation of a dielectric sphere. Kien et al.37 made a theoretical investigation into spontaneous emission from an atom inside a dielectric sphere and obtained the following spontaneous emission rate of an atom that is placed at the center of a dielectric sphere:
R ) f 2A/U(z) Here the function U(z) is given by
U(z) ) [K + B cos(21/2z) + C sin(21/2z)]/(23z6) with (36) Benner, R. E.; Barber, P. W.; Owen, J. F.; Chang, R. K. Phys. ReV. Lett. 1980, 44, 475. (37) Kien, F. L.; Quang, N. H.; Hakuta, K. Opt. Commun. 2000, 178, 151.
3D-Ordered Films of Fluorescent Microspheres
K ) 2( + 1)z6 - ( - 1)2[z4 - ( + 1)z2 - 1] B ) ( - 1)[2z6 - ( +3)z4 +( - 1)2z2 - ( - 1)] C ) -2( - 1)1/2z(z4 - z2+ - 1) f ) 3 /(2 + 1) where z ) k0a ) 2πa/λ is size parameter, λ is wavelength, a is the diameter of dielectric sphere, is the dielectric constant, A is the rate of the spontaneous emission of the atom in free space. They predicted that the normalized rate R/A oscillated with small amplitudes when k0a increased. In order to make a simulation according to the theory for the fluorescent sphere with different diameter, the refractive index (n) of the fluorescent latex as a function of wavelength was measured with a spectroscopic ellipsometer (Figure 5a). The dielectric constant of the fluorescent latex is calculated according to ) n2.38 The normalized rate R/A oscillated when λ increased. For the fluorescent sphere with diameter of 1011 nm, there is resonance enhancement at λ ) 512.5 nm (Figure 5b), which is in good agreement with the experimental observation in Figure 4. It should be noted that the experimentally obtained PL enhancement factor is much lower than the theoretically expected enhancement. This is probably attributed to the complicated structure in the present fluorescent sphere compared with the ideal atom model in the simulation. (38) Brandrup, J; Immergut, E. H.; Grulke, E. A. Polymer Handbook; John Wiley Sons, Inc.: New York, 1999.
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In the case of the fluorescent sphere with diameter of 742 nm, the λ of resonance enhancement shifted to 543.5 nm, and the fluorescent spheres only have weak PL emission. Therefore, the PL spectrum pattern was similar to the reference thin film. We also made a simulation for the latex diameter of 200 nm. A resonance enhancement appeared at 464 nm, where the fluorescent spheres only have weak PL emission. Therefore, the observed modification of PL spectrum for the latex diameter of 200 nm is not caused by the resonance enhancement in the microsphere but by the edge effect of the stop band. In reported works, onresonance enhancement of spontaneous emission was demonstrated by using semiconductor microdisks or pillar microcavities.24-28 Happ et al.,29 Painter et al.,30 and Ryu et al.31 obtained higher on-resonance SE enhancement by fabricating a photonic crystal defect microcavity. In this work, the resonance effect on PL emission was obtained by controlling the diameter of fluorescent microspheres.
Conclusion In summary, we described fabrication of 3D-ordered films of fluorescent latex spheres by sonication-assisted casting. The photoluminescence of these films were apparently modified by the photonic crystal effect of the ordered latex and by the resonance effect of the microsphere as microcavity. These effects can be altered with the latex diameter, which acts as an effective means to tailor the emission mode of emitting materials. LA700610P
