Langmuir 2006, 22, 4853-4855
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Simultaneous Alignment and Micropatterning of Organic Crystallites under a Modulated Magnetic Field Guangzhe Piao,† Fumiko Kimura,† and Tsunehisa Kimura*,†,‡ Tsukuba Magnet Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan, and Department of Applied Chemistry, Tokyo Metropolitan UniVersity, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan ReceiVed December 28, 2005. In Final Form: March 10, 2006 In a previous paper, we reported the micropatterning of magnetically isotropic particles using a microscopically modulated magnetic field. In this paper, we report that the alignment occurs simultaneously if the particles have magnetic anisotropy. An oil-in-water emulsion of p-terphenyl or anthracene was subjected to the modulated magnetic field and allowed to evaporate the solvent to obtain a line pattern consisting of the crystallites with alignment. The patterned samples exhibited an emission strongly polarized in the direction of the applied magnetic field that is perpendicular to the patterning lines.
Introduction In recent years, micropatterning has become a focus of interest in various areas of fabrication, including display units,1-3 optical information storage, optical switching, diffractive optical elements,4 miniaturized sensors,5 biosensors,6-8 integrated circuits,9 and photonic crystals.10 A number of techniques, including photo or electron-beam/ion-beam lithography and microcontact printing techniques,11-15 have been used to provide two-dimensional patterning. We have recently developed a novel method of micropatterning feeble magnetic particles.16 In this method, chemical, physical, or biological modifications of the substrate surface are not required. The substrate surface is exposed to a microscopically modulated magnetic field that traps diamagnetic particles in locations where the field strength is weak. An advantage of this method is that the particle alignment could also be achieved concomitantly if the particles have magnetic anisotropy. By combining these two magnetic effects, it is possible to obtain patterning of particles with alignment. In many aspects, control of alignment is of great importance. For example, one-dimensional aromatic π-conjugated materials such as p-terphenyl (p-TP) and anthracene (AT) used in the
present study have potential applications in optoelectronic devices, including thin-film transistors and light-emitting diodes.17-19 The alignment of these molecules is an important factor, since their optical and electrical properties are strongly affected by their alignment. For example, if the long axes of π-conjugated molecules are uniaxially aligned in a film, the resultant emission and absorption will be highly anisotropic because the transition dipole moments lie parallel to the long axis.20-24 In this study we demonstrate the simultaneous alignment and line patterning of p-TP and AT crystallites on a glass substrate and the resultant patterned and polarized fluorescent emission. Experimental Section
* To whom correspondence should be addressed. Tel: +81-426-77-2845. Fax: +81-426-77-2821. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Tokyo Metropolitan University.
An anion-type surfactant solution was prepared by mixing sodium dodecylbenzene sulfonate (SDBS, Wako, 2.03 g, 5.9 mmol), sodium dodecyl sulfate (SDS, Wako, 0.20 g, 0.70 mmol), and poly(ethylene oxide) 6000 (PEO, Wako, Mw ) 7,500, 0.2 g) with 5 mL of water under strong agitation. A cation-type surfactant solution was prepared by dissolving hexadecyltrimethylammonium bromide (HDTMAB, Wako, 0.97 g, 2.6 mmol) in 5 mL of water. p-TP (Aldrich, 23.5 mg, 0.10 mmol) or AT (Aldrich, 18.0 mg, 0.10 mmol) was dissolved in 50 mL of benzene or toluene solvent to obtain the solutions. A total of 2 mL of the solution of p-TP was added to a mixture of 2 mL of water and 0.2-0.4 mL of the anion-type surfactant solution and agitated strongly to prepare the emulsion. A total of 2 mL of the solution of AT was added to a mixture of 2 mL of water and 0.2-0.4 mL of the cation-type surfactant solution and agitated strongly to prepare the emulsion. A total of 50-100 µL of aqueous manganese
(1) Kim, J. M.; Yoneya, M.; Yokohama, H. Nature 2002, 420, 159. (2) Sinha, G. P.; Rosenblatt, C.; Mirantsev, L. V. Phys. ReV. E 2002, 65, 041718. (3) Wen, B.; Mahajan, M. P.; Rosenblatt, C. Appl. Phys. Lett. 2000, 76, 1240. (4) Yang, S. Z.; Yang, K.; Niu, L. G.; Nagarajan, R.; Bian, S. P.; Jain, A. K.; Kumar, J. AdV. Mater. 2004, 16, 693. (5) Semancik, S.; Cavicchi, R. Acc. Chem. Res. 1998, 31, 279. (6) Pandey, A.; Mann, M. Nature 2000, 405, 837. (7) Bayley, H.; Cremer, P. S. Nature 2001, 413, 226. (8) Lauks, I. R. Acc. Chem. Res. 1998, 31, 317. (9) Ogino, T.; Hibino, H.; Homma, Y.; Kobayashi, Y.; Prabhakaran, K.; Sumimoto, K.; Omi, H. Acc. Chem. Res. 1999, 32, 447. (10) Masuda, Y.; Itoh, T.; Itoh, M.; Koumoto, K. Langmuir 2004, 20, 5588. (11) Geissler, M.; Xia, Y. AdV. Mater. 2004, 16, 1249. (12) Yee, C. K.; Amweg, M. L.; Parikh, A. N. AdV. Mater. 2004, 16, 1184. (13) Werts, M. H. V.; Lambert, M.; Bourgoin, J.-P.; Brust, M. Nano Lett. 2002, 2, 43. (14) Ancona, M. G.: Kooi, S. E.; Kruppa, W.; Snow, A. W.; Foos, E. E.; Whitman, L. J.; Park, D.; Shirey, L. Nano Lett. 2003, 3, 135. (15) Jacobs, H. O.; Whitesides, G. M. Science 2001, 291, 1763. (16) Kimura, T.; Yamato, M.; Nara, A. Langmuir 2004, 20, 572.
(17) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friends, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (b) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (18) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, 30, L1938. (19) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, 30, L1938. (20) (a) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.-G.; Scherf, U.; Yasuda, A. AdV. Mater. 1999, 11, 671. (b) Grell, M.; Bradley, D. D. C. AdV. Mater. 1999, 11, 895. (21) Jandke, M.; Strohriegl, P.; Gmeiner, J.; Bruˆtting, W.; Schwoerer, M. AdV. Mater. 1999, 11, 1518. (22) Era, M.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1995, 67, 2436. (23) Hagler, T. W.; Pakbaz, K.; Voss, K. F.; Heeger, A. J. Phys. ReV. B 1991, 44, 8652. (24) (a) Yoshida, Y.; Tanigaki, N.; Yase, K.; Hotta, S. AdV. Mater. 2000, 12, 1587. (b) Hotta, S. “The Thiophene/Phenylene Co-Oligomers: Integrated Functionalities of Electronics and Photonics”, presented at Int. Conf. on Science and Technology of Synthetic Metals 2004 (ICSM2004), Wollongong, New South Wales, Australia, June 28 to July 2, 2004.
10.1021/la053505h CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006
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Figure 1. Experimental setup. A plastic vessel whose bottom was replaced with a round cover glass (167 µm) was placed on a field modulator, consisting of alternating aluminum and iron sheets (300µm thickness), and then fixed in the center of a superconducting magnet generating a horizontal field of 10 T so that the direction normal to the layers coincided with the field direction. (II) chloride solution (2.9 mmol/L) was added to 50-100 µL of the prepared emulsion, just before the emulsion was poured into a plastic vessel whose bottom was covered with a cover glass (Ø22 mm, 0.167 mm thickness). The vessel containing the emulsion was then placed on the top surface of the field modulator installed in the center of a JASTEC cryogen-free superconducting magnet generating a horizontal field of 10 T, and the solvent was allowed to evaporate overnight at room temperature (see Figure 1). The direction normal to the layers of the modulator coincided with the field direction. Optical observation was carried out using a polarized optical microscope (Olympus) connected to a digital camera. The sample was irradiated with a UV lamp with a maximum wavelength of 254 nm. The photographs were taken through a polarizing filter with a transmittance of 42%. The fluorescence emission spectra were measured using a Hitachi F-2500 fluorescence spectrophotometer with 354 nm excitation wavelength. The emitted light from the patterned p-TP was detected along the direction parallel (|) and perpendicular (⊥) to the magnetic field, respectively. The polarization ratio of the emissions was defined as I|/I⊥.
Results and Discussion A field modulator composed of alternating 300 µm-thick aluminum and iron layers was placed in a 10-T homogeneous horizontal magnetic field to generate a field gradient over a glass substrate that was in contact with the modulator (Figure 1). In this experimental setup, the magnetic flux density is lower over the iron layers than the aluminum layers because the flux is selectively absorbed by the ferromagnetic iron layers. If the applied external field is higher than is necessary to saturate the magnetic moment induced in the iron layers, the horizontal component of the magnetic flux remains over the modulator surface, enabling the magnetic alignment of the particles. The strength of 10 T used in the present study is high enough for this purpose. The field modulation over the modulator surface persists approximately only over the distance of the thickness of the alternating layers (300 µm). The 167-µm glass substrate used in the present study is thin enough to meet this requirement. The modulated field produces two effects.16 One is the microMoses effect. A thin liquid layer on a substrate exposed to a modulated field exhibits undulation of its surface. If the liquid is diamagnetic, the tops of the undulations develop over the iron layers where the field strength is weak, whereas if the liquid is paramagnetic, they are formed over the aluminum layers. The other effect is the trapping of particles. If particles (with magnetic susceptibility χp) suspended in a liquid medium (with magnetic susceptibility χm) are exposed to a modulated field B, they experience a force ∆χV∇B2, where ∆χ ) χp - χm and V is the volume of the particle. Depending on the sign of ∆χ, the particles are trapped in locations with a high field strength (∆χ > 0) or a low field strength (∆χ < 0). A typical case of the latter is a suspension where diamagnetic particles (χp < 0) are suspended
Figure 2. Polarizing optical image (with a color plate) of patterning lines obtained after drying the emulsion of p-TP (with SDBS, SDS, and PEO) in the presence of the magnetic field. Different colors indicate the alignment. Scale bar is 600 µm.
Figure 3. Schematic figure showing the alignment of the dipole moments of the molecules (parallel to the molecular long axes of p-TP and AT) in relation to the crystal alignment in a patterning line. Arrows indicate the directions of the dipole moment.
in a paramagnetic medium (χm > 0). In this case, the particles are trapped at locations where the field strength is low. Because the trapping force is proportional to ∆χV, the trapping efficiency is low for smaller particles and/or for particles whose magnetic susceptibility is close to that of the surrounding medium. In such cases, the micro-Moses effect could become dominant over the trapping. It was found that precipitation of the crystallites took place centered on the aluminum layers. Because the magnetic susceptibility (χp < 0) of the benzene droplets containing p-TP or AT is smaller than that of the aqueous medium (χm > 0) containing manganese chloride, i.e., ∆χ ) χp - χm < 0, the droplets would be trapped over the iron layers if the trapping mechanism were dominant. However, the obtained result is opposite to this, indicating that the micro-Moses effect is dominant. Investigations are under way regarding at what stage of solidification the patterning occurs. Figure 2 shows a polarizing optical microscope observation of the obtained pattern. The change in colors under different rotation angles (45 and 135°) clearly demonstrates the presence of optical anisotropy. Rietveld et al. and Cruickshank25,26 reported on the crystalline structure of p-TP and AT using X-ray and neutron analyses. The space group for p-TP and AT is P21/a, with two molecules per monoclinic unit cell. It is reported that the long axes of p-TP and AT molecules in the crystal are inclined by 9° and 3°, respectively, with respect to the c axis. Due to the large magnetic anisotropy of the aromatic rings in these molecules,27,28 the magnetic energy is significantly reduced when the long axes are aligned parallel (25) Rietveld, H. M.; Maslen, E. N.; Clews, C. J. B. Acta Cryst. 1970, B26, 693. (26) Cruickshank, D. W. J. Acta Cryst. 1956, 9, 915. (27) Mori, T.; Mori, K.; Mizutani, T. Thin Solid Films 2001, 393, 143. (28) Pauling, L. J. Chem. Phys. 1936, 4, 673.
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Figure 5. Polarized photoluminescence from the patterned AT (with HDTMAB) film excited by depolarized 354 nm light.
Figure 4. Polarized emission from the patterned and aligned p-TP (a) and AT (b) upon irradiation with a UV lamp. The direction of the analyzer (shown with a white arrow) is parallel (upper) or perpendicular (bottom) to the direction of the applied magnetic field when preparation (shown with a black arrow), demonstrating that the emission is anisotropic. Light blue and pink colors indicate that the emission is strong and weak, respectively.
to the applied magnetic field. In this alignment, the planes of the aromatic ring in the crystal are all directed parallel to the field, which is magnetoenergetically stable. Figure 3 schematically shows the alignment of the dipole moments of the molecules in the patterned lines. The molecular long axes, parallel to the dipole moment, are arranged in the crystal such that they lie approximately parallel to the crystal c axis. Under the applied magnetic field, the dipole moments align approximately parallel to the field. The magnetic field does not affect the orientation of the a and b axes. Upon irradiation of the ultraviolet light, the patterned p-TP and AT exhibit polarizing emission as shown in Figure 4, parts a and b. The sample was irradiated from the side with unpolarized light from a UV lamp with a maximum wavelength at 254 nm, after which the emitted light was observed via a polarizing filter with a transmittance of 42%. The transmitting light intensity is stronger (light blue) and the lines are better resolved when the analyzer is set parallel to the magnetic field, indicating the polarizing emission. Figure 5 shows the corresponding polarized
emission spectra of the patterned AT. The polarization ratio I|/I⊥ was 1.3. Here, the effect of instrumental polarization is excluded due to the observation that photoluminescence from the AT suspension, assumed to be randomly oriented, showed no polarization dependence. The patterned films exhibited an emission strongly polarized in the field direction perpendicular to the patterning lines. This anisotropic emission is attributed to the alignment of the dipole moment in the direction of the applied field.
Conclusion We have demonstrated a novel, readily applied method for simultaneous alignment and line patterning of fluorescent crystallites on a glass substrate using a microscopically modulated magnetic field. This method is superior to other methods in that it does not require pretreatment of the substrate to ensure patterning and alignment. This process has potential for wide use in the patterning of polarizing semiconductor materials with one or higher-dimensional periodicity, which will help improve the quality of optoelectronic devices, including thin-film transistors and light-emitting diodes. Acknowledgment. This work was partially supported by Grant-in-Aid for Scientific Research on Priority Area “Innovative utilization of strong magnetic fields” (Area 767, No. 15085207) from MEXT of Japan. We are grateful to Dr. M. Yamato for instrument usage and assistance. LA053505H
