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Self-Organized Organic Microdots of Fluorescent Diaminodistyrylbenzene Molecules Hisao Yanagi,*,†,‡ Naoki Matsuoka,† Masatoshi Kondo,† Michifumi Nagawa,§ and Yoshio Taniguchi§ Faculty of Engineering, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan, PRESTO, Japan Science and Technology Corporation (JST), Rokkodai, Nada-ku, Kobe 657-8501, Japan, and Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan Received March 28, 2001. In Final Form: June 20, 2001 Organic microdots of fluorescent bis(N,N-di-p-tolylamino-p-styryl)benzene molecules were self-organized by vapor deposition onto the KCl (001) surface kept at 180 °C. In situ optical microscopy revealed that the microdots preferably nucleated themselves along the KCl [100] step so that they were aligned partly in a linear manner. As the deposition was continued, the number of nuclei reached the saturation point and their diameters increased from a few submicrometers to several micrometers. Finally, the adjacent microdots coalesced resulting in a diameter larger than 10 µm. Such microdot growth was attributed to molecular migration on the substrate surface with a long diffusion length based on the molecular bearing effect of the peripheral bulky group. The as-deposited microdots exhibited a bluish-green fluorescence, which was considerably quenched under UV excitation in air due to photooxidation of the π-conjugating backbone. When the microdots were covered with a MgF2 layer, this fluorescence quenching was thoroughly prevented.
Introduction Recently, organic light-emitting materials have attracted practical interest as electroluminescence devices1,2 as well as organic semiconductor lasers.3 To overcome the instability of organic materials under excitation with a high current injection, further considerations have to be given to device engineering in the area of improving light amplification. In view of wellorganized quantum structures and crystal engineering of semiconductor devices, it is important to introduce a mesoscopically ordered structure in organic materials for the efficient confinement of excitons and light. One can attempt to amplify their light emission by introduction of external resonators such as the distributed Bragg reflector (DBR),4 distributed feedback (DFB),5,6 microdisks,7 etc. Moreover, mesoscopic structures with a dielectric periodicity in a dimension comparable to the wavelength of light have attracted interest as photonic crystals.8-10 Radiative properties of photonic-crystal materials can be controlled by the concept of a photonic band gap. For * To whom correspondence should be addressed. E-mail:
[email protected]. † Kobe University. ‡ Japan Science and Technology Corporation. § Shinshu University. (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884. (3) (a) Scho¨n, J. H.; Kloc, Ch.; Dodabalapur, A.; Batlogg, B. Science 2000, 289, 599. (b) Scho¨n, J. H.; Dodabalapur, A.; Kloc, Ch.; Batlogg, B. Science 2000, 290, 963. (4) Tessler, N.; Denton, G. J.; Friend, R. H. Nature 1996, 382, 695. (5) McGehee, M. D.; Dı´az-Garcı´a, M. A.; Hide, F.; Gupta, R.; Miller, E. K.; Moses, D.; Heeger, A. J. Appl. Phys. Lett. 1998, 72, 1536. (6) Nagawa, N.; Ichikawa, M.; Koyama, T.; Shirai, H.; Taniguchi, Y.; Hongo, A.; Tsuji, S.; Nakano, Y. Appl. Phys. Lett. 2000, 77, 2641. (7) Frolov, S. F.; Fujii, A.; Chinn, D.; Vardeny, Z. V.; Yoshino, K.; Gregory, R. V. Appl. Phys. Lett. 1998, 72, 2811. (8) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (9) John, S. Phys. Rev. Lett. 1987, 58, 2486. (10) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143.
example, the introduction of a microcavity by the localized defect mode11,12 and the anomaly of the group velocity13,14 in photonic crystals have enabled the realization of optical amplification15 and lasing.16-18 Most of such mesostructures of submicrometer scale have been prepared by chemical etching, patterned growth, and lithography of inorganic semiconductors.19-21 Microlithographic patterning has also been successfully performed in emissive conjugated polymers.22 The encapsulation of dyes and polymers in inorganic mesoscopic media is an alternative method to fabricate hybrid materials. Another possibility is the fabrication of self-assembled mesostructures of surfactant moleculessfor example, organosilicons on a hydroxylated surface and alkanethiols on gold.23-25 The latter method is limited to molecules that have chemically active headgroups and alkyl chains. Fabrication of selfassembled mesostructures is also performed by the patterning of substrate surfaces or the segregation of different molecular components. In this paper, to meet (11) McCall, S. L.; Platzman, P. M.; Dalichaouch, R.; Smith, D.; Schultz, S. Phys. Rev. Lett. 1991, 67, 2017. (12) Yablonovitch, E.; Gmitter, T. J.; Meade, R. D.; Rappe, A. M.; Brommer, K. D.; Joannopoulos, J. D. Phys. Rev. Lett. 1991, 67, 3380. (13) Yeh, P. J. Opt. Soc. Am. 1979, 69, 742. (14) Dowling, J. P.; Scalora, M.; Bloemer, M. J.; Bowden, C. M. J. Appl. Phys. 1994, 75, 1896. (15) Vlasov, Yu. A.; Luterova, K.; Pelant, I.; Hoenerlage, B.; Astratov, N. Appl. Phys. Lett. 1997, 71, 1616. (16) Inoue, K.; Sasada, M.; Kawamata, J.; Sakoda, K.; Haus, J. W. Jpn. J. Appl. Phys. 1999, 38, L157. (17) Imada, M.; Noda, S.; Chutinan, A.; Tokuda, T.; Murata, M.; Sasaki, G. Appl. Phys. Lett. 1997, 75, 316. (18) Paintner, O.; Lee, R. K.; Scherer, A.; Yariv, A.; O’Brien, J. D.; Dapkus, P. D.; Kim, I. Science 1999, 284, 1819. (19) Gru¨ning, U.; Lehmann, V.; Ottow, S.; Busch, K. Appl. Phys. Lett. 1996, 68, 747. (20) Kawakami, S. Electron. Lett. 1997, 33, 1260. (21) Baba, T.; Matsuzaki, T. Jpn. J. Appl. Phys. 1996, 35, 1348. (22) Renak, M. L.; Bazan, G. C.; Roitman, D. Adv. Mater. (Weinheim, Ger.) 1997, 9, 392. (23) Zisman, W. A. Adv. Chem. Ser. 1964, No. 43, 1. (24) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (25) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuaao, R. G. J. Am. Chem. Soc. 1989, 111, 321.
10.1021/la010474r CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
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Figure 1. (a) Chemical structure and (b) optimized geometry of DADSB.
the requirements of simpler alternative methods for mesostructure preparation, we report the spontaneous growth of organic microdots in fluorescent molecules by conventional vapor deposition. Experimental Section The fluorescent molecule used here is a styrylbenzene derivative,26 bis(N,N-di-p-tolylamino-p-styryl)benzene (DADSB, Figure 1). An optimized structure of the DADSB molecule was estimated by molecular mechanics (MM3) and molecular orbital (PM3) calculations using a CAChe program (Oxford Molecular Group, Inc.). The fabrication of the microdots was carried out by vapor deposition onto a freshly cleaved (001) surface of a KCl single crystal. Under a vacuum of 5 × 10-4 Pa, the DADSB molecules that were loaded on a resistively heated quartz crucible were deposited onto the KCl substrate kept at 180 °C. In situ observation of the microdot growth was performed by setting a stereomicroscope with an 1× objective lens (W.D. ) 74 mm) at a viewing port outside the vacuum chamber. Under the illumination of a halogen lamp, transmission optical images were recorded on video with a CCD camera from the backside of the substrate through a hole made on a substrate heating holder. The deposition rate was monitored with a quartz-crystal microbalance (QCM). The morphology of the as-deposited microdots was observed using an atomic force microscope (AFM) (JEOL JSPM-4200) in the ac mode with a Si cantilever. Transmission electron microscopy (TEM) and electron diffraction were performed at an acceleration voltage of 100 kV using a transmission electron microscope (Hitachi H-7100). For TEM observation, the as-deposited DADSB microdots, reinforced with an amorphous carbon film, were transferred onto a copper mesh after dissolving the KCl substrate on a water surface. Some of the microdot specimens on the KCl substrate were encapsulated in a MgF2 layer. After the DADSB microdots were formed on the KCl surface at 180 °C, the substrate was cooled to room temperature, and then MgF2 powder that was loaded on a coiled tungsten basket was vapor deposited onto the microdots on KCl. The thickness of the MgF2 layer was controlled to be approximately 1 µm by QCM. Fluorescence properties of the as-deposited and MgF2-coated microdots were investigated using an inversion fluorescence microscope (Olympus IX-70) equipped with a 250 W high-pressure Hg lamp and a CCD multichannel spectrometer (Hamamatsu Photonics PMA-11). The stability of their fluorescence spectra was investigated in air and in a nitrogen atmosphere using a sealed glass cell. The (26) Okumura, Y.; Nagawa, M.; Adachi, C.; Satsuki, M.; Suga, S.; Koyama, T.; Taniguchi, Y. Chem. Lett. 2000, 754.
Figure 2. In situ optical micrographs of microdot growth of DADSB vapor deposited on the KCl (001) surface kept at 180 °C. Images were taken at (a) 0, (b) 5, (c) 10, (d) 15, (e) 23, and (f) 30 min after starting the deposition. microdot specimens on KCl substrate were provided for UV/ visible (UV-vis) and Infrared (IR) spectroscopy.
Results and Discussion In situ optical microscopy shown in Figure 2 reveals sequential growth processes of DADSB microdots that were vapor deposited on the KCl (001) surface kept at 180 °C. The deposition was controlled to give a thickness rate of approximately 10 nm/min on the QCM monitor. On the fleshly cleaved KCl surface, many steps running almost parallel are visible (Figure 2a). Small nuclei begin to appear a few minutes after deposition (Figure 2b). Most of them seem to nucleate along the KCl steps. As the deposition continues, the nuclei become clear and grow to
Microdots of Diaminodistyrylbenzene Molecules
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Figure 4. Schematic representation of microdot growth of DADSB vapor deposited on the KCl (001) surface.
Figure 3. AFM images of DADSB microdots as-deposited on the KCl (001) surface. Images a and b were obtained from microdots grown for 5 and 15 min after starting deposition, respectively.
microdots (Figure 2c). In 15 min after deposition the number of the microdots is already saturated but their diameters grow larger (Figure 2d). Further deposition causes coalescence of the adjacent microdots (as marked with arrows), resulting in an increase in the diameter and a decrease in the number of microdots (Parts e and f of Figure 2). The morphology of the deposited microdots was examined by AFM. Figure 3a shows a representative AFM image of the microdots grown at the initial stage (corresponding to those in Figure 2b). At the KCl steps, with a height comparable to that of the microdots (∼100 nm), some nuclei grow sticking to the step wall. Although the other microdots seem to grow on the terrace region of KCl, most of them probably nucleate over the steps of an atomic level height or other kink sites. AFM profiles of the microdots at this stage indicated that a typical diameter and height were 0.5 µm and 25 nm, respectively. The AFM image of further grown microdots, as shown in Figure 3b, represents their diameter and height larger than 5 and 0.4 µm, respectively. From this topography, it is observed that the microdots have a rather disklike morphology. Further AFM estimations of microdots at different growth stages demonstrated that the ratio of the height to the diameter being about 1/20 at the initial stage reached a value of 1/8 in 30 min after deposition. The above findings suggest the growth process of the DADSB microdots to be as shown schematically in Figure 4. The molecules adsorbed on the KCl substrate easily migrate on the surface and nucleate at the 〈100〉 steps
Figure 5. (a) Absorption and fluorescence spectra and (b) fluorescence micrograph of the as-deposited DADSB microdots on the KCl surface.
and kink sites existing on the (001) terrace. After all the nucleation sites are saturated, the microdots continue to grow larger and thicker by the inclusion of the diffused molecules. The diffusion length at the substrate temperature of 180 °C is at least several micrometers, as estimated from micrographs and AFM images (Figures 2 and 3). Such a long diffusion length can be attributed to the molecular structure of DADSB, as is shown in Figure 1b. The bulky, peripheral groups of N,N-di-p-tolylamine act as “molecular bearings”, which prevent the π-conjugating distrylbenzene chain from coming in direct contact with the KCl ionic surface. This steric effect of decrease in π-electron interaction with the substrate surface enables the molecules to migrate with long distance. Thus, as-deposited DADSB microdots exhibit bluishgreen fluorescence under excitation at wavelengths below 450 nm, as shown in absorption/fluorescence spectra in Figure 5a and a fluorescence micrograph underUV excitation (λex ) 365 nm) in Figure 5b. This fluorescence spectrum of the microdots is essentially same as that in a dilute solution of DADSB. It suggests that neither significant spectral shift nor band splitting is caused by the aggregation or crystallization of the molecules in the microdots. As shown in Figure 6, an electron diffraction pattern obtained for the as-deposited microdots gives a
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Figure 8. IR absorption spectra of (a) the as-deposited DADSB microdots and (b) those after UV irradiation for 1 h.
Figure 6. TEM image and electron diffraction pattern of the as-deposited DADSB microdots on the KCl (001) surface.
Figure 7. Changes in fluorescence intensity with time period of UV excitation of the DADSB microdots as-deposited on the KCl surface measured in (a) air and (b) nitrogen atmosphere and on (c) MgF2-covered microdots measured in air.
hollow reflection indicating that they are amorphous. This amorphous structure is also attributed to the lesser intermolecular-packing nature of the DADSB molecules, as a result of their bulky peripheral groups. The stability of fluorescence of the microdots under UV excitation is strongly affected by atmospheric conditions. Figure 7 depicts changes in the fluorescence intensity of the DADSB microdots deposited on the KCl surface at the time of UV excitation (λex ) 365 nm). In air, the fluorescence intensity at the maximum wavelength (λ ) 500 nm) of the as-deposited microdots (curve a) quickly decreased upon excitation. TEM imaging and electron diffraction of the microdots, subjected to this fluorescence quenching, indicated that the amorphous structure of the microdots was not changed upon UV irradiation. Therefore, this fluorescence quenching is not due to crystallization in the microdots. However, UV-vis and IR spectra of the quenched microdots changed when compared to those before UV irradiation. The electronic spectral band of the as-deposited microdots that appeared around 400 nm (Figure 5a), which is assigned to the π-π* transition of the π-conjugating molecular backbone, shifted to 280-
Figure 9. Fluorescence micrographs of (a) photopatterned DADSB microdots as-deposited on the KCl surface and (b) those covered with a MgF2 layer.
330 nm. As shown in Figure 8, the IR spectra taken from the as-deposited and quenched microdots after UV irradiation indicate that the stretching band of the πconjugation backbone of DADSB at 1510 cm-1 decreased while a broad band assigned to carbonyl groups appeared around 1700 cm-1 upon UV irradiation. These results suggest that fluorescence quenching is ascribed to photo-
Microdots of Diaminodistyrylbenzene Molecules
oxidation at the backbone of the DADSB molecule resulting in a breakup of the π-conjugation. This photooxidative reaction is also confirmed by the fact that fluorescence quenching is thoroughly prevented when UV excitation is performed in nitrogen (Figure 7b). The fluorescence quenching was also suppressed even in air when the microdots were covered with a MgF2 layer, as shown in Figure 7c. In contrast to the as-deposited microdots in nitrogen (curve b), the fluorescence intensity of the MgF2-covered microdots gradually increases with excitation time. Electron diffraction obtained from this specimen after UV irradiation indicated that the amorphous structure of the embedded microdots was not significantly changed. Although the reason for this increase of fluorescence intensity is still under investigation, we assume that the MgF2 layer not only prevents the microdots from being exposed to atmospheric oxygen but also excludes residual oxygen or moisture from the microdots upon UV irradiation. This stabilized fluorescence of the MgF2-covered microdots is preserved for a long time at ambient conditions. On the basis of this phenomenon, Figure 9 demonstrates a fluorescence patterning where the as-deposited DADSB microdots and those covered with the MgF2 layer are UV irradiated
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through a 400-mesh photomask for 30 min. Their fluorescence micrographs show that the fluorescence intensity at the UV-irradiated areas of the former is quenched while that of the latter is patterned in a bright manner. By means of such stabilized fluorescence of the DADSB microdots encapsulated in the low-refractive dielectric layer, we have observed amplified emission based on light confinement inside the microdot cavity, which will be reported elsewhere. Furthermore, the above-mentioned growth mechanism of the DADSB microdots suggests a possibility of arranging them in a periodic array, for example, by forming regular kink sites on the substrate surface using an AFM-indentation technique. Such a twodimensional fluorescent microdot array is interesting in the formation of emissive photonic crystals. Acknowledgment. This work was partly supported by the Toray Science and Technology Grant. This work was also part of a project performed under the Photonics Materials Program by the Venture Business Laboratory of Kobe University. LA010474R
