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Manipulation of the Spontaneous Emission in Mesoporous Synthetic Opals Impregnated with Fluorescent Guests Yuri Yamada,* Hisashi Yamada, Tadashi Nakamura, and Kazuhisa Yano Toyota Central Research & Development Laboratories., Inc., Nagakute, Aichi 480-1192, Japan Received June 2, 2009. Revised Manuscript Received July 8, 2009 The spontaneous emission of light from light-emitting materials adsorbed within the ordered pores of monodispersed mesoporous silica spheres (MMSS) has been investigated. By taking advantage of the ordered starburst pores of MMSS, we can provide a simple strategy for fabricating synthetic opals consisting of homogeneous individual building blocks in which fluorescent guests are uniformly and stably impregnated. In this study, tris(8-hydroxyquinolinato)aluminum(III) (Alq3) and Rhodamine B (Rh B) are selected as the fluorescent guests. The former has a wider emission band than the reflection spectrum of MMSS synthetic opals, whereas the emission band of the latter is considerably narrower than the reflection spectrum of the opals. The spontaneous emissions of these functionalized synthetic opals are clearly influenced by the stop band governed by the Bragg equation. In the case of the Alq3-MMSS conjugate, the shape of the Alq3 emission spectrum varies in accordance with the shift in the stop band. The emission of the Rh B-MMSS conjugate is noticeably narrowed, and its intensity is enhanced when the excitation intensity is increased. These results are well explained by an inhibition of spontaneous emission caused by a reduction in the density of optical states within the stop band. The results of this study indicate that MMSS synthetic opals are promising for use in novel optical applications in which the spontaneous emission can be manipulated.

1. Introduction Photonic crystals have attracted considerable interest since the concept behind them was first independently proposed by Yablonovitch1 and John.2 They are spatially periodic structures in which the refractive index is modulated. This periodicity in the refractive index may lead to the formation of a photonic band gap (PBG). The existence of a PBG inhibits the propagation of light within a particular wavelength range that satisfies the Bragg condition. In particular, three-dimensional (3D) photonic crystals are one of the most promising candidates for fabricating nextgeneration photonic devices.3,4 Among the various applications of 3D photonic structures that have been proposed, manipulation of the spontaneous emission from light-emitting materials embedded in photonic crystals is especially attractive. According to Fermi’s golden rule, the rate of spontaneous emission in the weak oscillator-field coupling regime depends on the density of optical electromagnetic modes (or density of optical states: DOS)5 around an atom or molecule. Spontaneous emission in photonic crystals with a complete PBG will be totally inhibited because DOS is zero within the PBG frequency level. Up until now, only theoretical calculations have succeeded in demonstrating a PBG totally inhibits spontaneous emission.6 However, many experiments have been conducted on the effects of photonic structures with an incomplete PBG (known as a photonic stop band).7,8 Differing from photonic crystals with a complete PBG, photonic *Corresponding author. E-mail: [email protected].

(1) Yablonovich, E. Phys. Rev. Lett. 1987, 58, 2059. (2) John, S. Phys. Rev. Lett. 1987, 58, 2486. (3) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (4) Meng, X.; Al-Salman, R.; Zhao, J.; Borissenko, N.; Li, Y.; Endres, F. Angew. Chem., Int. Ed. 2009, 48, 2703. (5) Liu, X. D.; Wang, Y. Q.; Cheng, B, Y.; Zhang, D. Z. Phys. Rev. E 2003, 68, 036610. (6) Ho, K. M.; Chan, C. T.; Soukoulis, C. M. Phys. Rev. Lett. 1990, 65, 3152. (7) Shkunov, M. N.; Vardeny, Z. V.; DeLong, M. C.; Polson, R. C.; Zakhidov, A. A.; Baughman, R. H. Adv. Funct. Mater. 2002, 12, 21. (8) Li, J.; Jia, B.; Zhou, G.; Gu, M. Appl. Phys. Lett. 2007, 91, 254101.

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stop band prohibits the light only for certain directions because the DOS is reduced and does not entirely vanish.9 However, even incomplete photonic stop band exerts a significant influence on the emission properties of light-emitting materials depending on the spectral position and width of their emission spectra relative to the stop band. Therefore, there has been a growing interest in the use of photonic stop band to demonstrate the suppression, enhancement, and attenuation of spontaneous emission of the light-emitting materials.10-15 Various kinds of active media (including organic dyes, quantum dots, lanthanides, and rare-earth atoms) have been used for investigating stimulated emissions from photonic structures with stop bands (e.g., synthetic opals).16-18 Organic dyes are commonly used due to their high fluorescence quantum yields and easy handling. Directional fluorescence spectra of Rhodamine 6G in opals and inverse opals have been studied in the stop band, and an enhancement in the emission intensity on the blue side of the spectrum for a given direction of emission has been reported.19 (9) Gaponenko, S. V.; Germanenko, I. N.; Kapitonov, A. M.; Petrov, E. P. Phys. Rev. E 1997, 55, 7619. (10) Petrov, E. P.; Bogomolov, V. N.; Kalosha, I. I.; Gaponenko, S. V. Phys. Rev. Lett. 1999, 83, 5402. (11) Aloshyna, M.; Sivakumar, S.; Venkataramanan, M.; Brolo, A. G.; Veggel, V. J. Phys. Chem. C 2007, 111, 4047. (12) Thijissen, M. S.; Sprik, R.; Wijnhoven, J. E. G. J.; Megens, M.; Narayanan, T.; Lagendijk, A.; Vos, W. L. Phys. Rev. Lett. 1999, 83, 2730. (13) Ventura, M. J.; Gu, M. Adv. Mater. 2008, 20, 1329. (14) Blanco, A.; Lopez, C.; Mayoral, R.; Miguez, H.; Meseguer, F.; Mifsud, A.; Herrero, J. Appl. Phys. Lett. 1998, 73, 1781. (15) Yoshino, K.; Tatsuhara, S.; Kawagishi, Y.; Ozaki, M.; Zakhidov, A. A.; Vardeny, Z. V. Appl. Phys. Lett. 1999, 74, 2590. (16) Yang, Z.; Zhou, J.; Huang, X.; Yang, G.; Xie, Q.; Sun, L.; Li, B.; Li, L. Chem. Phys. Lett. 2008, 455, 55. (17) Withnall, R.; Martinez-Rubio, M. I.; Fern, G. R.; Ireland, T. G.; Silver, J. J. Opt. A: Pure Appl. Opt. 2003, 5, S81. (18) Gaponenko, S. V.; Bogomolov, V. N.; Petrov, E.; Kapitonov, A.; Yarotsky, D. A.; Kalosha, I. I.; Eychmueller, A. A.; Rogach, A. L.; McGilp, J.; Woggon, U.; Gindele, F. J. Lightwave Technol. 1999, 17, 2128. (19) Bechger, L.; Lodahl, P.; Vos, W. L. J. Phys. Chem. B 2005, 109, 9980.

Published on Web 07/30/2009

DOI: 10.1021/la901959f

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In many previous studies, organic dyes were introduced into photonic structures by simple infiltration or homogeneous embedding.20,21 However, infiltration may cause the initial photonic structure to collapse, thus reducing the efficiency of controlled emission. Moreover, there are several drawbacks of using dyes as light-emitting materials in photonic structures, the most critical being their poor stability and their tendency to aggregate. Organic dyes readily aggregate at high concentrations, almost completely suppressing fluorescence. Additionally, it is difficult to control the location of the embedded dyes by simple infiltration techniques. Barth et al. found that dye molecules are preferentially absorbed on the surface of polystyrene beads.22 Therefore, it will be more effective to incorporate these active species within the building blocks of synthetic opals prior to the fabrication. Several researchers use commercially available dyed colloidal microspheres to overcome this problem,23,24 or fluorophore-modified colloidal microspheres are newly synthesized.25,26 Some of them have succeeded in fabricating three dimensionally photonic structures and have demonstrated the effect of the band gap on spontaneous emission from these structures. However, despite the huge range of organic dyes that are available, very limited variation of them can be a candidate to be attached to the building blocks and act as a light-emitting source. A sandwichlike structure has been proposed for investigating the effect of a stop band on emission, and a few groups have succeeded in demonstrating laser emission.27,28 Although this strategy is very attractive, it is a little complicated compared with using building blocks embedded with fluorescent dyes. Mesoporous materials are considered to be highly suitable hosts for being impregnated by various kinds of guests, including organic dyes. The photophysics and photochemistry of molecules adsorbed in the restricted cavities of mesoporous materials have been studied. Avnir et al. have shown that the photostability of dye molecules embedded in silica glass is much higher than that of dyes in aqueous solution.29 Yamashita et al. have investigated the fluorescence properties of Rhodamine B adsorbed on silica, zeolite, and mesoporous molecular sieves, and they found that a high dispersion of Rhodamine B can be realized only when mesoporous molecular sieves are used as supports.30 Recently, we synthesized highly monodisperse mesoporous silica spheres (MMSS) that contained ordered starburst (radially aligned) mesopores with hexagonal periodicity.31 The radially aligned and highly ordered mesopores of MMSS offer high accessibility to guest molecules, which results in high performance in catalytic reaction.32 The high accessibility to guest molecules enable us to fabricate nanocomposite spheres with various kinds of guests incorporated, not only organic dyes but (20) Nikolaev, I. S.; Lodahl, P.; Vos, W. L. J. Phys. Chem. C 2008, 112, 7250. (21) Murai, S.; Fujita, K.; Nakanishi, K.; Hirao, K. J. Non-Cryst. Solids 2004, 345-346, 438. (22) Barth, M.; Gruber, A.; Cichos, F. Phys. Rev. A 2005, 72, 085129. (23) Li, Y. Z.; Kunitake, T.; Fujikawa, S.; Ozasa, K. Langmuir 2007, 23, 9109. (24) Nair, R. V.; Vijaya, R.; Kuroda, K.; Sakoda, K. J. Appl. Phys. 2007, 102, 123106. (25) Bosma, G.; Pathmamanoharan, C.; de Hoog, E. H. A.; Kegel, W. K.; van Blaaderen, A.; Lekkerkerker, H. N. W. J. Colloid Interface Sci. 2002, 245, 292. (26) Baert, K.; Song, K.; Vallee, R. A. L.; Van der Auweraer, M.; Clays, K. J. Appl. Phys. 2006, 100, 123112. (27) Furumi, S.; Fudouzi, H.; Miyazaki, H. T.; Sakka, Y. Adv. Mater. 2007, 19, 2067. (28) Jin, F.; Li, C.-F.; Dong, X.-Z.; Chen, W.-Q.; Duan, X.-M. Appl. Phys. Lett. 2006, 89, 241101. (29) Avnir, D. Acc. Chem. Res. 1995, 28, 328. (30) Yamashita, H.; Tanaka, A.; Nishimura, M.; Koyano, K.; Tatsumi, T.; Anpo, M. Stud. Surf. Sci. Catal. 1998, 117, 551. (31) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (32) Suzuki, T. M.; Nakamura, T.; Sudo, E.; Akimoto, Y.; Yano, K. Microporous Mesoporous Mater. 2006, 93, 190.

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also a magnetic oxide, gold, and a conducting polymer.33-35 In the case of gold and iron oxide as guest molecules, TEM images have given us assurance that these nanoparticles were distributed uniformly inside the mesopores of MMSS. We have also succeeded in obtaining highly monodispersed nanoporous carbon spheres by adaptation of nanocasting procedure.36 In this case, not only macroscopical morphology but also internal structure of MMSS was well retained, showing that the resulting carbon spheres possess a novel starburst structure which inversely replicates the radially aligned mesopores of MMSS. From these previous reports, the guest molecules are thought to be uniformly impregnated all over the MMSS. Furthermore, the resultant nanocomposite spheres are monodisperse and have smooth surfaces; they can be used as building blocks to form a solid colloidal crystal film with a stop band.34,35,37 In light of these experimental findings, MMSS impregnated with fluorescent dyes are expected to be attractive building blocks for producing photonic structures to investigate the stop band effect. Here, we introduce new building blocks embedded with fluorescent guests and reveal the effect of the stop band on the spontaneous emission from synthetic opals fabricated from them. The focus of this paper is principally on using MMSS impregnated with fluorescent dyes as building blocks, enabling a simple and one-pot strategy for fabricating synthetic opals. The adsorption properties and stability of the guest molecules are also investigated in detail.

2. Experimental Section Synthesis. Tetramethoxysilane (TMOS, Tokyo Kasei), cetyltrimethylammonium chloride (CTMACl, Tokyo Kasei), 1 M sodium hydroxide (Wako Inc.), methanol (Wako Inc.), ethanol (Wako Inc.), chloroform (Wako Inc.), and concentrated hydrochloric acid (Wako Inc.) were used without further purification. Highly monodispersed mesoporous silica spheres were prepared from TMOS and CTMACl in accordance with the method given in our previous report.31 For example, 7.04 g of CTMACl and 6.84 g of 1 M sodium hydroxide were dissolved in 800 g of a methanol-water (50:50 w/w) solution. 5.28 g of TMOS was then added to the solution with vigorous stirring at 298 K. The clear solution turned opaque several minutes after the addition of the silica source and produced a white precipitate. After 8 h of continuous stirring, the mixture was aged overnight. The precipitate was then filtered, washed three times with distilled water, and dried at 318 K for 72 h. The powder was calcined in air at 823 K for 6 h to remove the surfactant. In this study, we used MMSS with diameters of 265, 320, and 564 nm (see Supporting Information). Encapsulation of Light-Emitting Materials. N-[9-(2-Carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidine]-N-ethylethanaminium chloride (Rhodamine B, abbreviated as Rh B, Indeco) and tris(8-hydroxyquinolato)aluminum(III) (Alq3, Tokyo Kasei) were chosen as the light-emitting materials (fluorophores). Rh B was mainly used in this study. MMSS with diameters of 320 and 564 nm were chosen to form Rh B-MMSS conjugates, which were prepared as follows. 0.2 g of MMSS was added to 4 mL of Rh B ethanol solution (1.25-5 mM). The suspension was then shaken overnight at ambient temperature to reach adsorption equilibrium. The Rh B-MMSS conjugate was collected by centrifugation and dried at room temperature. The amount of Rh B adsorbed into the pores of the MMSS was determined spectrophotometrically by measuring the absorbance of the supernatant solutions (33) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2006, 16, 2417. (34) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2007, 17, 3726. (35) Kelly, T. L.; Yamada, Y.; Che, S. P. Y.; Yano, K.; Wolf, M. O. Adv. Mater. 2008, 20, 2616. (36) Yamada, Y.; Nakamura, T.; Yano, K. Chem. Lett. 2008, 37, 378. (37) Yamada, Y.; Nakamura, T.; Ishii, M.; Yano, K. Langmuir 2006, 22, 2444.

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Article Table 1. Structural Summary of the Colloidal Crystals Consisting of the Fluorophore-MMSS Conjugates

sample

diameter of MMSS (nm)

fluorophore occluded

amount of adsorbed fluorophore (mg/MMSS-100 mg)

1 2 3

265 265 564

Alq3 Rh B Rh B

18.9 0.20 0.20

after centrifugation at a wavelength of 545 nm, which is the characteristic wavelength of the absorption band of Rh B. In the case of the Alq3-MMSS conjugates, Alq3 was dissolved in chloroform (1.25-40 mM) and adsorbed by the pores of the MMSS with a diameter of 265 nm by the same procedure as described above.

Fabrication of Colloidal Crystals Consisting of Fluorophore-MMSS Conjugates. To obtain a well-ordered colloidal crystal, an aqueous dispersion of the fluorophore-MMSS conjugates (1.0 wt %) was prepared. After continuous sonication for more than 3 h, the conjugates were further purified by centrifugation/redispersion cycles with water. A sufficiently dispersed solution was fed into a fluidic cell, which consisted of two flat glass substrates with two spacers sandwiched between them.38 The glass substrates were cleaned with hydrochloric acid and rinsed with distilled water, successively treated in a UV/ozone cleaner (Nippon Laser) for 20 min. Double-stick tape (Nitto Denko Corp., No. 5601, 30 μm thick) was used as the spacer. The colloidal crystal film was formed between the glass substrates by the gradual evaporation of water from the opening of the cell at room temperature. The properties of the obtained colloidal crystals are summarized in Table 1. Characterization. The absorption spectra were measured on a Shimadzu (Kyoto, Japan) spectrophotometer MPS-2400. Scanning electron microscope (SEM) images were taken with a S-3600N (Hitachi High-Technologies Corp.) at an acceleration voltage of 10 kV. The surfaces of the samples were coated with gold prior to the observations. Nitrogen adsorption isotherms were measured at 77 K with a Quantachrome Autosorb-1. The specific surface areas were calculated by the BET (i.e., “Brunauer-Emmett-Teller”) method with adsorption data ranging from P/P0 = 0.05 to 0.15, and pore size distribution curves were analyzed by the BJH (i.e., “Barrett-Joyner-Halenda”) method. Photoluminescence spectra were collected using a Jasco FP-6500 spectrometer. The incident angle of light is set 60° to the sample (at normal incidence, θ = 0°), where the detection angle is 30° with the sample surface. In order to study the effect of the photonic stop band on the emission characteristics, we should consider that the stop band overlaps sufficiently with the emission spectrum. For this aim, we have conducted two kinds of angle-resolved spectra measurement. Angle-resolved reflection spectra measurement was conducted using a multichannel spectrometer (Soma Optics, Fastevert S-2650, S-2710) to map the photonic stop band at different angles of incidence.38 These spectra were measured by changing the angle of incidence θ between the beam and the normal of the sample surface by collecting the light scattered in the Bragg configuration. The peak wavelength of the light scattered was determined by fitting each reflection to the Gaussian curve. The wavelength of light diffracted from (111) planes of a face-centered-cubic (fcc) colloidal crystal (or a synthetic opal) is given by the Bragg equation (eq 1) λpeak ¼ 1:633dðneff 2 -sin2 θÞ1=2

ð1Þ

where λpeak is the wavelength of the diffracted light (Bragg stop band), d is the mean diameter of the spheres, neff is the mean refractive index of the crystalline lattice, and θ is the angle between the incident light and the normal to the (111) planes (at normal (38) Ishii, M.; Nakamura, H.; Nakano, H.; Tsukigase, A.; Harada, M. Langmuir 2005, 21, 5367.

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Figure 1. Schematic drawing of the experimental arrangement used in the emission studies. The top view of the θ stage showing that the angle-resolved emission can be collected by setting the detector at arbitrary angles. incidence, θ = 0°). This equation represents that the wavelength of the Bragg stop band can be changed with the incident angle θ when d and neff are constant values. We have also measured angle-resolved emission spectra to investigate the directional effect of the stop band on the spontaneous emission. Figure 1 illustrates the experimental setup we used to investigate the emission characteristics of the MMSS synthetic opals with adsorbed light-emitting materials. It consists of two stages. The sample (i.e., the synthetic opal) is set on the sample stage, which is surrounded by a circular θ stage. This θ stage can be rotated to any angle, and it is attached to an arm that is connected to the optical fiber. Thus, by setting the arm of the θ stage at different angles with respect to the direction of the incident laser beam, angle-resolved emission spectra can be collected. Because the sample position and the excitation laser direction remain fixed throughout the measurements, emission spectra measured with the detector at different angles can be directly compared. When measuring the emission from the colloidal crystals of the Rh B-MMSS conjugates, the second harmonic light (532 nm) from a pulsed Nd: YAG laser beam was used for excitation. The laser had a pulse duration of 3 ns and a repetition frequency of 10 Hz. The excitation pulse energy was controlled by using neutral-density filters and an aperture. The laser beam was incident normal to the surface of the colloidal crystals and had a spot size of about 1 mm. When the spot size was confined, the laser was focused on the sample with a lens of focal length 150 mm to give a spot size of 100 μm. The emission from the sample was collected and focused onto the entrance of the optical fiber connected to a spectrometer (Ocean Optics, HR4000). In the case of the Alq3-MMSS conjugate, the third harmonic light (355 nm) of an Nd:YAG laser was selected for excitation. These two kinds of angle-resolved spectra measurement enabled us to determine whether the stop band position overlaps with the emission profiles of the light-emitting materials.

3. Results and Discussion 3.1. Alq3-MMSS Conjugate. Adsorption. Alq3 is an organic light-emitting material that has been extensively studied. Because the luminescence of Alq3 is not as strong as that of a laser dye, there is no possibility to observe the enhancement of the DOI: 10.1021/la901959f

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Figure 2. Adsorption isotherm of Alq3 into the pores of MMSS. Sample 1 was fabricated from MMSS whose pores were filled with Alq3 (18.9 mg/100 mg of MMSS), as indicated by the arrow.

emission to cause the laser action. However, it is readily occluded into the mesopores of silica.39 In addition, the emission band of Alq3 is sufficiently wide that the effect of the stop band should be easily observable. We first investigated the relationship between the stop band of the colloidal crystal fabricated from the Alq3MMSS conjugate and the emission spectrum of Alq3 encapsulated in the mesopores of the MMSS. Figure 2 shows the adsorption isotherm of Alq3 onto the MMSS (265 nm), which is classified as a Langmuir-type adsorption, revealing strong adsorbent-adsorbate interactions. The resultant powder is bright yellow. The nitrogen adsorption isotherm of MMSS is type IV with a capillary condensation step at P/P0 = 0.3 (see Supporting Information). In the case of the Alq3-MMSS conjugate, the relative pressure at which capillary condensation occurs shifts to lower pressure and its nitrogen uptake is less than that of the MMSS. The BJH pore size and the BET surface area are calculated to be respectively 25 A˚ and 1100 m2 g-1 for the MMSS, whereas these values decrease to 20 A˚ and 400 m2 g-1 for the Alq3-MMSS conjugate. On the basis of these adsorption properties, we can conclude that Alq3 was effectively adsorbed into the mesopores of the MMSS. It should be emphasized that due to the ordered starburst pores of the MMSS, Alq3 was thought to be embedded inside the MMSS very homogeneously as was convinced from our previous reports.32-36 Thus, using MMSS as hosts is an effective method for attaining good dispersion of the guest molecules. Employing MMSS impregnated with fluorescent molecules is a simple strategy for fabricating synthetic opals consisting of homogeneous single building blocks. Emission Study. The Alq3-MMSS conjugate containing 18.9 mg of Alq3 (indicated by the arrow in Figure 2) was selected to fabricate a colloidal crystal (denoted as sample 1). Figure 3 shows the angle-resolved reflection spectra and the photoluminescence spectrum obtained by exciting sample 1 with light from a Xe lamp. The photoluminescence spectrum of sample 1 has a broad band centered at about 500 nm derived from the Alq3. The reflection spectra characterized by the optical stop band of sample 1 is somewhat narrower than the photoluminescence spectrum. The reflectance peak is at about 500 nm at an angle of incidence of 10°, and it shifts to shorter wavelengths when the angle of incidence is increased, in accordance with Bragg’s law (see Experimental Section). Since the reflectance peak shifts in the emission band of the Alq3 when the angle of incidence is varied, it is reasonable to expect that the emission profile will change when the detection angle is altered. This prediction can be clarified by conducting an angle-resolved emission study using the experimental setup depicted in Figure 1. (39) Tagaya, M.; Ogawa, M. Chem. Lett. 2006, 35, 1436.

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Figure 3. Angle-resolved reflection spectra and fluorescence spectrum of sample 1 excited with 355 nm light from a Xe lamp.

Figure 4. Detection angle dependence of emission spectra obtained from sample 1 excited with Nd:YAG laser at 355 nm.

The emission spectra obtained at six different detector angles with an excitation energy of 1200 μJ/pulse are shown in Figure 4. An obvious change in the emission spectra is observable. The remarkable dip in the broad emission band (indicated by the broken arrow) shifts to shorter wavelengths with an increase in the detection angle accompanied by the optical stop band. This result indicates that the emission spectra are affected by the presence of the stop band; specifically, spontaneous emission is suppressed within the stop band. To the best of our knowledge, this is the first report of suppression of spontaneous emission from a fluorophore embedded in ordered mesopores of colloidal spheres. 3.2. Rh B-MMSS Conjugates. We have shown that the emission spectra from a fluorophore embedded in mesopores have a substantial dip, which is clear evidence of the stop band effect. As the next step, Rh B with high fluorescence yield was impregnated into MMSS (320 nm, 564 nm) to investigate in detail the influence of the stop band upon the fluorescence properties. Rh B is one of the most readily available and useful organic laser dyes. It has been frequently studied due to its unique properties, such as the dependence of its fluorescence yield on concentration.40 Adsorption. The adsorption measurement results for MMSS with a diameter of 320 nm are shown as representative. Unlike the adsorption behavior of Alq3, the amount of adsorbed Rh B onto the MMSS increases in proportion to the equilibrium concentration of the supernatant solutions for the three points plotted in Figure 5. It is assumed that he difference in the adsorption isotherms for Alq3 and Rh B is mainly derived from solvophobic (40) Hinckley, D. A.; Seybold, D. G. Spectrochim. Acta 1989, 44, 1053.

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Figure 5. Adsorption isotherm of Rh B in the pores of MMSS. The amount of Rh B adsorbed in the mesopores of MMSS (100 mg) is (a) 0.2, (b) 0.42, and (c) 0.84 mg.

Figure 6. Time-resolved absorbance for Rh B-MMSS conjugates dispersed in chloroform (filled circles) and dye dissolved in chloroform (open circles).

interaction.41 When the solubility of the organic molecules to the solvent is low, the hydrophobic interaction between the surface of the silica and the organic molecules is induced, which gives a Langmuir type adsorption isotherm (like Alq3), causing a high amount of adsorption. On the contrary, in cases where guest molecules are easily soluble to the solvent, solvophobic interaction is not provoked and efficient incorporation of guest molecules will not be fulfilled (like Rhodamine B in this study). With an increase in the amount of Rh B adsorbed in the pores of the MMSS, the fluorescence intensity decreases and a bathochromic shift is observed (see Figure S3). This result indicates that the mutual interaction between Rh B molecules increases causing aggregation to occur with an increase in the amount of adsorbed Rh B. The fluorescence yield of Rh B decreases dramatically on dimerization,42 so that the dimerization of Rh B molecules should be avoided in practical applications (e.g., as a dye laser). The photostability and thermal stability of dye molecules are dramatically enhanced when they are adsorbed onto silica.29 Yamashita et al. have shown that mesoporous molecular sieves with large cavities (FSM-16 and MCM-41) are better than microporous materials (ZSM-5 zeolite) in realizing a high dispersion of dye molecules.30 On the basis of these findings, MMSS with ordered mesopores and a high specific surface area is anticipated to be good candidate as a support for Rh B for use as a laser medium. The pore size of MMSS (2.1 nm) is slightly smaller than those of FSM-16 or MCM-41 (2.7 nm), so that the dye concentration at which the fluorescence intensity drops due to dimerization is lower than that for the Rh B-MMSS conjugates. In this study, the amount of Rh B was adjusted to 0.2 mg per 100 mg of MMSS because Rh B molecules are considered to adsorb on silica walls as monomers with high dispersion at this concentration. Stability of the Adsorbed Rh B. To investigate the stability of the adsorbed Rh B in the pores of MMSS, the time-resolved absorbance of the Rh B-MMSS conjugates (sample a; the amount of Rh B is 0.2 mg/100 mg of MMSS) dispersed in chloroform was measured. The result is shown in Figure 6 together with that for free Rh B in chloroform. We prepared a dilute Rh B solution, the initial absorbance of which was nearly the same as that of sample A. The absorbance of free Rh B in chloroform decreased drastically, almost becoming zero after about 100 min; Rh B has three distinct molecular forms,43 and it changes its form according to its microenvironment. Since the lactone form has only very weak adsorption and fluorescence and is colorless, it is considered to become the dominate form in the

dilute chloroform solution of Rh B over time. By contrast, the absorbance of the Rh B-MMSS conjugates in chloroform did not vary with time. The Rh B in the MMSS is prevented from changing into the lactone form, implying that it is more stable than free Rh B. The appearance of the Rh B-MMSS conjugates and that of nonporous SiO2 which was introduced Rh B by simple infiltration is compared (Supporting Information, Figure S4). It is clearly seen that the former is very homogeneous whereas the uniformity of the latter is low. These results together with previous findings strongly suggest that Rh B is homogeneously distributed inside the MMSS, contributing the increase in stability. Reflection and Photoluminescence Spectra. The effect of the stop band on the spontaneous emission from the embedded fluorophores can be clarified by preparing two samples: one in which the fluorescence spectrum overlaps with the Bragg reflection wavelength and the other in which it does not. By comparing the results from these two samples, we can discuss the effect of the stop band on the emission from embedded fluorophores. We used two Rh B-MMSS conjugates with different diameters (sample 2: 320 nm; sample 3: 564 nm). On the basis of the particle diameters of the prepared Rh B-MMSS conjugates, the stop band position of sample 2 is thought to overlap the fluorescence profile of Rh B at a certain angle. This is verified in Figure 7. In Figure 7a, the reflection spectra of sample 2 at six different angles (θ = 16°, 20°, 24°, 28°, and 32°) are indicated. As the angle increases, the stop band shifts to shorter wavelengths and gets across the fluorescence spectrum in the wavelength range 550650 nm. The fluorescence spectrum completely overlaps the stop band at an angle of 24°. The fluorescence spectrum of Rh B is not as broad as that of Alq3; hence, the reflection spectra of sample 2 are wider than its fluorescence profile. By contrast, the reflection spectra of sample 3 lie in the wavelength range 900-1100 nm, which is distant from the fluorescence band at 550-650 nm. Hence, sample 3 can be used as a reference, for which no stop band effect is expected. Angle-Resolved Emission Intensity. We conducted angleresolved emission measurements of sample 2 using the experimental setup depicted in Figure 1 with an excitation energy of 0.04 mJ/pulse. On the basis of the results obtained for the Alq3MMSS conjugate, a definite change in the emission profile was anticipated. This expectation was not fulfilled, but a change in the emission peak intensity (defined as emission intensity) is clearly observed. The dependence of the emission intensity on the detection angles for sample 2 is shown in Figure 8. Several raw emission data on typical detection angles are also shown in the Supporting Information (Figure S5). The emission intensity gradually decreases as the detection angle is increased to an angle of 29°. The intensity is a minimum at an angle of 29°, and at angles larger than 29°, the intensity recovers along with the detection angle. The emission intensities are strongly affected by the overlapping area

(41) Minakata, S.; Tsuruoka, R.; Komatsu, M. J. Am. Chem. Soc. 2008, 130, 1536. (42) Wakai, F.; Kodama, Y.; Sakaguchi, S. J. Am. Ceram. Soc. 1990, 73, 457. (43) Negishi, N.; Fujino, M.; Yamashita, H.; Fox, M. A.; Anpo, M. Langmuir 1994, 10, 1772.

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Figure 7. (a) Fluorescence spectrum and angle-resolved reflection spectra for sample 2. Reflection spectra at angles of incidence of (from right to left) 12°, 16°, 20°, 24°, 28°, and 32°. (b) Reflection spectra at angles of incidence of (from right to left) 15°, 20°, 25°, and 35° for sample 3. The inset of (b) represents fluorescence spectrum for sample 3. The peak in the reflection spectrum shifts steadily to shorter wavelengths with an increase in the angle of incidence, as indicated by the arrow.

Figure 8. Dependence of emission peak intensity on the detection angle of sample 2.

Figure 9. Changes in the emission peak intensity of sample 2 (filled circles) and sample 3 (open circles) as a function of excitation pulse energy.

between the reflection spectrum and the fluorescence spectrum. The overlapping area between these two spectra increases with increasing detection angle and becomes a maximum at an angle of 24° (Figure 7). This angle differs slightly from that at which the emission intensity is a minimum (i.e., 29°). Two reasons are proposed for this discrepancy. One reason is a difference between the measurement devices used, while the other reason relates to the tendency of MMSS to absorb moisture. In the case of the colloidal crystals fabricated from MMSS, the reflection spectrum was found to change when water vapor was introduced into the mesopores of MMSS.37 The samples were dried in the same manner prior to each measurement in this present study; however, environmental conditions, especially humidity, may cause a shift of several nanometers in the reflection spectra. Irrespective of its cause, a misalignment of a mere 5° does not mask the fact that the emission intensity decreases as the overlap between the stop band and the fluorescence spectrum increases. In the case of the Rh B-MMSS conjugate, its emission profile did not change but its emission intensity changed as a result of the stop band effect. This is because the reflection spectra of the Rh B-MMSS conjugate has a wider band than its fluorescence profile. The synthetic opals fabricated from the Alq3-MMSS conjugate and from the Rh B-MMSS conjugate do not have a complete PBG, but instead they have a photonic stop band; therefore, the influence of changes in the DOS is assumed to be small. However, partial suppression of the spontaneous emission caused by a reduction in the DOS within the stop band has been clearly observed in both conjugates. Amplified Spontaneous Emission. Figure 9 shows the dependence of the emission intensity of samples 2 and 3 on the excitation intensity. It clearly shows that the emission intensity of sample 2 increases as the excitation energy is increased, whereas no amplification is observed for sample 3. A full width at half-maximum

(fwhm) of 5 nm is obtained for sample 2 when the excitation intensity is 1.7 mJ/pulse, which is about 15 nm narrower than that at a excitation intensity of 0.04 mJ/pulse. This obvious narrowing and enhancement of the emission occur only for sample 2, in which the stop band effect is expected to occur. However, the enhancement of the emission from sample 2 is independent of the detection angle. This is thought to stem largely from multiple scattering, which is caused when the wavelength of the exciting light is close to the periodicity of the photonic bandgap material. On the basis of particle size, the effect of the multiple scattering is expected to be significant for sample 2 but not for sample 3. Multiple scattering increases the optical path length not only for the emission profile but also for the excitation light,44,45 resulting in a large enhancement in the emission intensity. The combination of the stop band effect and multiple scattering are conjectured to cause the huge enhancement in the emission intensity of sample 2. In the case of sample 3, the emission intensity increases at the low excitation energy and seems to become saturated at pump intensities greater than 0.5 mJ/pulse. We suspect that this is caused by gain saturation, which has been observed in laser excitation at a high pumping energy.46,47 In order to demonstrate the reproducibility of the stop band effect on this study, we have investigated the effect of pump energy on the emission intensity for another synthetic opal (MMSS with diameter of 370 nm) in which the fluorescence spectrum overlaps with the stop band also (Figure S6). The result strongly supports our notion that the manipulation of the spontaneous emission is caused by the stop band effect of MMSS synthetic opals impregnated with various fluorescent guests.

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(44) Vlasov, Yu. A.; Luterova, K.; Pelant, I.; Honerlage, B. Appl. Phys. Lett. 1997, 71, 1616. (45) Usami, A.; Ozaki, H. J. Phys. Chem. B 2005, 109, 2591. (46) Dorrington, A. A.; Jones, T. W.; Danehy, P. M. Appl. Opt. 2004, 43, 6629. (47) Hunga, J.; Castilloa, J.; Olaizola, A. M. J. Lumin. 2003, 101, 263.

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The fwhm of sample 2 decreases to 5 nm, which is narrower than that reported previously in the combination of Rh B and a colloidal crystal with stop band.48 Despite this narrowing and the enhancement in the emission intensity, lasing was not observed in sample 2 in this present study. Two reasons are offered to explain this phenomenon. One reason concerns the quality of the reflection spectrum. A high-quality reflection spectrum is crucial for achieving efficient lasing from a photonic structure containing fluorescent dyes. The greater the reflectance of the photonic structure is, the more efficient is luminescent energy feedback, thereby promoting stimulated emission. It is also important that the reflection band completely overlaps the fluorescence spectrum. The reflectance of our photonic system (sample 2) is significantly less than 100%, and the band in its reflection spectra is wider than its fluorescence profile. Thus, the conditions for enhanced stimulated emission are not satisfied in sample 2. Another factor is the spot size of the excitation beam. We used a circular spot with a diameter of 1 mm, which is larger than that used when lasing was observed.27,28 This relatively wide spot size might have been detrimental to efficient feedback of the emission energy. Finally, the emission spectrum with an excitation energy of 0.029 mJ/pulse and a laser spot size of 100 μm was collected from sample 2 (Figure S7). The narrowing of the fwhm proceeded only to 5 nm; however, the top of the emission peak is detected in isolation. It appears that several narrowed emissions, like laser emission, are simultaneously detected. Although definite laser emission could not be obtained from our photonic system constructed from a Rh B-MMSS conjugate, coherent control of spontaneous emission by the stop band effect was successfully demonstrated. Impregnation of the pores of MMSS is not restricted to Rh B. Any kind of light-emitting materials (e.g., other organic dyes, rare-earth atoms, and quantum dots) is a potential candidate for an active media in a new photonic device in which the spontaneous emission can be controlled. This study is (48) Li, M. Z.; Xia, A. D.; Wang, J. X.; Song, Y. L.; Jiang, L. Chem. Phys. Lett. 2007, 444.

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believed to make an important step toward realizing new photonic devices, such as miniature lasers, and work is currently underway to achieve this goal.

4. Conclusions In this study, we fabricated synthetic opals made up of homogeneous single building blocks of MMSS. The fluorescent guests, Alq3 and Rh B, were uniformly adsorbed in the mesopores of MMSS in a stable form and acted as efficient light-emitting materials. The spontaneous emission can be easily controlled in MMSS photonic systems. In the Alq3-MMSS conjugate, a substantial dip in the broad Alq3 emission band is observed and changed in accordance with a shift in the stop band. In the Rh BMMSS conjugate, the suppression of the emission intensity was observed. Moreover, obvious narrowing and enhancement of the emission were observed as the excitation intensity was increased in the Rh B-MMSS system. These results are explained by the inhibition of the spontaneous emission caused by a reduction in the DOS within the stop band. This study demonstrates that MMSS can provide a promising strategy for designing innovative optical applications, including low-threshold miniature lasers and light-emitting diodes. Acknowledgment. The authors are grateful to Dr. Hirozumi Azuma for assistance with the angle-resolved emission study. The authors also thank for Dr. Masahiko Ishii for thoughtful and insightful discussions. Supporting Information Available: SEM images of MMSS, nitrogen adsorption properties, fluorescence spectra of Rh B-MMSS conjugates, photo images of Rh B-MMSS synthetic opal under natural light and UV light, angle-resolved emission spectra of Rh B-MMSS synthetic opal on typical detection angles, results regarding emission study for MMSS with diameter of 370 nm, and an emission spectrum with a laser spot size of 100 μm. This material is available free of charge via the Internet at http://pubs.acs.org.

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