Anisotropic Polymerization of a Long-Chain Diacetylene Derivative

May 27, 2006 - Ryo Onoki,Keiji Ueno,*Hiroo Nakahara,Genki Yoshikawa,Susumu Ikeda,Shiro Entani,Tetsuhiko Miyadera,Ikuyo Nakai,Hiroshi Kondoh,Toshiaki ...
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Langmuir 2006, 22, 5742-5747

Anisotropic Polymerization of a Long-Chain Diacetylene Derivative Langmuir-Blodgett Film on a Step-Bunched SiO2/Si Surface Ryo Onoki,† Keiji Ueno,*,† Hiroo Nakahara,† Genki Yoshikawa,‡ Susumu Ikeda,§ Shiro Entani,§ Tetsuhiko Miyadera,‡ Ikuyo Nakai,‡ Hiroshi Kondoh,‡ Toshiaki Ohta,‡ Manabu Kiguchi,| and Koichiro Saiki‡,§ Department of Chemistry, Saitama UniVersity, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan, Department of Chemistry, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, Department of Complexity Science and Engineering, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan, and Department of Chemistry, Hokkaido UniVersity, N10W8 Kita-ku, Sapporo, Hokkaido 060-0812, Japan ReceiVed February 20, 2006. In Final Form: April 21, 2006 Alternating facet/terrace nanostructures were fabricated on a SiO2 surface by step-bunching and thermal oxidation of a vicinal Si(111) substrate, and their influence upon the polymerization direction of a long-chain diacetylene derivative monolayer film was investigated by angle-dependent polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that the peak intensity of the C 1s-π* transition was stronger when the electric vector plane of the incident X-ray was parallel to the direction of the periodic facet/terrace structures rather than perpendicular to them. On the contrary, a polymer film fabricated on a flat SiO2 surface showed no in-plane anisotropy of the peak intensity. These results indicate that the diacetylene groups in the diacetylene derivative monolayer are preferentially photopolymerized in the direction not across but along the periodic one-dimensional structures on the step-bunched and thermally oxidized SiO2/Si(111) surface.

Introduction It is well-known that diacetylene derivatives (R-CtC-Ct C-R′) photopolymerize under ultraviolet (UV) irradiation and form polymers with conjugated π-electron systems. Many types of diacetylene polymers have drawn considerable attention to their physical properties, and future applications such as nonlinear optics and electronic devices have been widely studied.1-4 To develop these potential capabilities of the polymerized film, control of the ordering of the molecular structure is the fundamental requirement. Therefore, a wide variety of diacetylene derivative polymers has been studied for about thirty years5,6 to clarify the mechanism of the polymerization reaction, which has been found to depend on the orientation and packing of diacetylene derivative molecules.7-12 * To whom correspondence should be addressed. Telephone: +81-48858-3388. Fax: +81-48-858-3388. E-mail: [email protected]. † Saitama University. ‡ Department of Chemistry, The University of Tokyo. § Department of Complexity Science and Engineering, The University of Tokyo. | Hokkaido University. (1) Sauert, C.; Herman, J. P.; Fer, R.; Predieere, F.; Ducing, J.; Baughman, R. H.; Chance, R. R. Phys. ReV. Lett. 1976, 36, 956. (2) Bloor, D., Chance, R. R., Eds.; Polydiacetylenes; Martinus Nijhoff: Dordrecht, The Netherlands, 1985. (3) Chemla, D. S., Zyss, J., Eds.; Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: Orlando, FL, 1987. (4) Kasai, H.; Tanaka, H.; Okada, S.; Oikawa, H.; Kawai, T.; Nakanishi, H. Chem. Lett. 2002, 31, 696. (5) Wegner, G. Z. Naturforsch., B: Chem. Sci. 1969, 24, 824. (6) Matsumoto, A.; Matsumoto, A.; Kunisue, T.; Tanaka, A.; Tohnai, N.; Sada, K.; Miyata, M. Chem. Lett. 2004, 33, 96, and references therein. (7) Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. Angew. Chem. Int. Ed. 1976, 15, 764. (8) Batchelder, D. N.; Evans, S. D.; Freemen, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Worf, H. J. Am. Chem. Soc. 1994, 116, 1050. (9) Kanetake, T.; Tokura, Y.; Koda, T. Solid State Commun. 1985, 56, 803. (10) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117, 3963. (11) Mowery, M. D.; Kopta, S.; Ogletree, D. F.; Salmeron, M.; Evans, C. E. Langmuir 1999, 15, 5118.

Highly ordered thin molecular films containing diacetylene groups in the principal chain have been well prepared by the Langmuir-Blodgett (LB) method at air/water interfaces.13,14 In previous studies, it has been found that the polymerization reaction of amphiphilic monomers such as long-chain esters and longchain dienoic acids markedly proceeds in their LB films, and the structure of the resulting polymers can be controlled.15,16 In addition, the structure of a polymer film made from a LB film of a long-chain diynoic acid cadmium salt was extensively studied and compared to that of the metal-free film by UV photoelectron spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.17-21 Recently, the efficiency of the monomer-to-polymer conversion in the diacetylene derivative thin film was measured by fluorescence yield near-edge spectroscopy (FYNES),22 and it was found that rearrangement of the conjugated backbone in the film during the photopolymerization induced the color phase transition.23 In these studies, many kinds of multilayer films highly ordered along the surface normal (c axis) direction were used for the characterization, (12) Britt, D. W.; Hofmann, U. G.; Mo¨bius, D.; Hell, S. W. Langmuir 2001, 17, 3757. (13) Gains, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (14) Petty, M. C. Langmuir-Blodgett Films; Cambridge University Press: Cambridge, 1996. (15) Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid Films 1983, 99, 87. (16) Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid Films 1985, 133, 39. (17) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. J. Chem. Phys. 1988, 88, 4076. (18) Nakahara, H.; Fukuda, K.; Seki, K.; Asada, S.; Inokuchi, H. Chem. Phys. 1987, 118, 123. (19) Seki, K.; Morisada, I.; Tanaka, H.; Edamatsu, K.; Yoshiki, M.; Takata, Y.; Yokoyama, T.; Ohta, T.; Asada, S.; Inokuchi, H.; Nakahara, H.; Fukuda, K. Thin Solid Films 1989, 179, 15. (20) Seki, K.; Yokoyama, T.; Ohta, T.; Nakahara, H.; Fukuda, K. Mol. Cryst. Liq. Cryst. 1992, 218, 85. (21) Sto¨hr, J. NEXAFS spectroscopy; Springer: Berlin, 1992. (22) Evans, C. E.; Smith, A. C.; Burnett, D. J.; Marsh, A. L.; Fischer, D. A.; Gland, J. L. J. Phys. Chem. B 2002, 106, 9036. (23) Fujimori, A.; Ishitsuka, M.; Nakahara, H.; Ito, E.; Hara, M.; Kanai, K.; Ouchi, Y.; Seki, K. J. Phys. Chem. B 2004, 108, 13153.

10.1021/la060482d CCC: $33.50 © 2006 American Chemical Society Published on Web 05/27/2006

Anisotropic Polymerization of a Diacetylene DeriVatiVe

because the network of the conjugated π-electron systems is built within each two-dimensional unit plane in the c-axis ordered film. In really, the multilayer film highly ordered along its c axis consists of many domains. In each domain, the in-plane ordering of the diacetylene derivative molecules is highly established, and most of the conjugated π-electron backbones induced by the UV irradiation aligns parallel to each other within a single domain.24-27 Recently, one-dimensional chain polymerization with a nanometer scale was realized using a scanning tunneling microscope (STM) in a diacetylene derivative monolayer film on graphite.28,29 The polymerization direction aligned each other in one domain, but it was randomly distributed in the entire film, because there were a lot of domains in the film. Thereby no in-plane anisotropic feature exists in a wide range on the polymerized film. Thus, it has been desired to establish a method which controls the in-plane polymerization direction in the entire region of the film. If the polymerization direction can be freely controlled, the diacetylene derivative polymer can be utilized as a real model of the one-dimensional “molecular wire”, and a novel functionality will be found in the one-dimensionally polymerized film. To control the in-plane polymerization direction, two promising methods have been proposed: irradiation of polarized UV light and utilization of an anisotropic template. In the former case, it was reported that the direction of the π-conjugation in the photopolymerized diacetylene derivative LB film could be arranged in the electric vector plane of the polarized UV light. In the latter case, it is known that the molecular orientation and packing in a LB film are often affected by the surface structure of the substrate, which results in anisotropic features of the polymerized film. Today, nanoscale fabrication of an anisotropically patterned surface can be achieved by the photolithography or electron-beam lithography method. However, these top-down lithographic methods are expensive and require complicated processes. As a simple and low-cost method, growth of a polyimide film followed by surface rubbing is practically used in order to control the alignment of the liquid crystal molecules on a glass substrate. The anisotropic surface structures made by the rubbing process, however, lack accuracy and reproducibility. Instead of these methods, we tried to use a self-organized periodic nanostructure fabricated on a thermally oxidized amorphous SiO2 layer on a vicinal Si(111) substrate. First, we cleaned a vicinal Si(111) surface by direct current (DC) heating in an ultrahigh vacuum (UHV), and successively oxidized it in a dry O2 furnace. It has been reported that the vicinal Si(111) surface tilted toward the [112h] direction has a self-organized alternating structure of flat terraces and facets of bunched steps after the DC-heating with the current flowing along the [1h10] direction.30-34 Even after the thick oxidization, these one-dimensional periodic structures are almost preserved, and the surface has alternating (24) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1478. (25) Tieke, B.; Weiss, K. J. Colloid Interface Sci. 1984, 101, 129. (26) Carpick, R. W.; Sasaki, D. Y.; Burns, A. R. Langmuir 2000, 16, 1270. (27) Takami, K.; Mizuno, J.; Akai-kasaya, M.; Saito, A.; Aono, M.; Kuwahara, Y. J. Phys. Chem. B 2004, 108, 16353. (28) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (29) Okawa, Y.; Aono, M. Surf. Sci. 2002, 514, 41. (30) Williams, E. D.; Bartelt, N. C. Science 1991, 251, 393. (31) Viernow, J.; Lin, J.-L.; Petrovykh, D. Y.; Leibsle, F. M.; Men, F. K.; Himpsel, F. J. Appl. Phys. Lett. 1998, 72, 948. (32) Lin, J.-L.; Petrovykh, D. Y.; Viernow, J.; Men, F. K.; Seo, D. J.; Himpsel, F. J. J. Appl. Phys. 1998, 84, 255. (33) Men, F. K.; Liu, F.; Wang, P. J.; Chen, C. H.; Cheng, D. L.; Lin, J.-L.; Himpsel, F. J. Phys. ReV. Lett. 2002, 88, 096105. (34) Himpsel, F. J.; McChesney, J. L.; Chain, J. N.; Kirakosian, A.; Pe´rezDieste, V.; Abbott, N. L.; Luk, Y.-Y.; Nealey, P. F.; Petrovykh, D. Y. J. Phys. Chem. B 2004, 108, 14484 and reference therein.

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Figure 1. Schematic illustrations of substrate surfaces; (a) after the cleaning by DC heating in UHV and (b) after the thermal oxidation of (a).

facet and terrace structures elongated along the [1h10] direction as shown in Figure 1. We now report the influence of the underlying surface nanostructure on the topochemical photopolymerization of diacetylene groups in the LB film deposited on the abovementioned anisotropic SiO2/Si(111) surface. The molecular orientation in the polymer film was characterized by polarized NEXAFS spectroscopy. The NEXAFS spectroscopy is sensitive to a particular element in the molecule, and it is possible to explore the spatial arrangement of functional groups in the polymerized film.21,23 In the NEXAFS spectra, in-plane anisotropy was found in the polymerized diacetylene derivative film on the specially processed periodic surface but not on a nominally flat SiO2 surface. It is suggested that the one-dimensional periodic structures on the substrate surface can control the in-plane photopolymerization direction of the diacetylene derivative LB film on a macroscopic scale. In addition, we can directly use the template SiO2 layer as the gate capacitor of a field effect transistor (FET). If the anisotropic photopolymerization is achieved over a wide range on the surface, for example between a source electrode and a drain electrode of FET, it will be possible to fabricate a defect-free π-conjugated active channel. Such a defectfree organic channel in the FET may realize a much higher carrier mobility than ever reported, because the carrier transport in the organic FET is mainly disturbed by the poor crystallinity and the enormous domain boundaries in the grown organic film. We think the highly oriented π-conjugated backbone will be a better active channel of organic FET than such small molecules as pentacene, C60, metal-phthalocyanine, etc. Experimental Section Template Substrate Fabrication. The periodic facet/terrace surface structure was fabricated on a Si substrate as follows; first, a Si substrate was cut from a P-doped n-type (1∼10 Ω cm resistivity) Si(111) wafer tilted toward the [112h] direction by 4°. It was first degreased by dipping in an ultrasonic bath of acetone and methanol, and rinsed in ultrapure water. It was then introduced into an UHV chamber with the base pressure of 2 × 10-8 Pa, and cleaned by DC heating with the current flowing only along the [1h10] direction.

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Figure 2. AFM images and section profiles of 4°-off Si(111) surfaces before and after the thermal oxidation. Samples were cleaned and step-bunched by the DC-heating in UHV before the oxidation. Panels a and b show surfaces before and after the oxidation, respectively. Panel c shows the surface of a commercially available flat SiO2/Si substrate. The scanned area of images is 1 µm × 1 µm. Panels d-f are section profiles at lines in panels a-c, respectively. After 12 h of heating at 600 °C, we repeated flash heating from 930 to 1250 °C of 3 ∼ 4 times, then annealed the sample at 930 °C for 30 min to produce the step bunching. The substrate temperature was then slowly cooled by 3 °C min-1, which leads to the formation of a uniform 7 × 7 reconstruction on the (111) terrace. Next, the sample was taken out of the UHV chamber and oxidized in a dry O2 furnace at 1100 °C for 6 h to form an amorphous SiO2 layer as thick as 300 nm. In addition, a commercially obtained Si(111) wafer covered with a thermally grown SiO2 layer (700 nm) was used as a flat surface for comparison. The surface morphology of the samples was observed by atomic force microscopy (AFM, SII SPA300 with SPI3800 probe station). LB Film Deposition. The diacetylene derivative compound, 10,12pentacosadiynoic acid (abbreviated as DA, CH3(CH2)11-CtCCtC-(CH2)8COOH, purchased from Dojin Chemical Laboratories), was first purified by recrystallization from a chloroform:ethanol ) 1:9 (v/v) mixed solvent and repeated several times. The amphiphile was dissolved in chloroform with the concentration of 1 × 10-4 mol L-1 to make a spreading solution. A monolayer was spread onto an aqueous subphase (pH ) 6.8) containing CdCl2 (3 × 10-4 mol L-1) and KHCO3 (5 × 10-5 mol L-1) in Milli-Q water (R ) 18 MΩ). Z-type monolayer films were deposited by the LB method using a computer-controlled KSV Mini trough (KSV Instruments Ltd.) in the form of a cadmium salt (cadmium 10,12-pentacosadiynoate, abbreviated as CdDA) on SiO2/Si(111) substrates at a surface pressure of 25 mN m-1 and a temperature of 15 °C. During the LB film deposition the substrate was perpendicular to the water surface, and the dipping direction was parallel to the elongated facets/terraces on the surface. Photopolymerization of the diacetylene derivative monolayer was carried out by exposing the LB films to a 500 W Xe lamp at a distance of 40 cm from the surface for 1 h. NEXAFS Spectroscopy. The NEXAFS spectra of polymerized films were measured on the BL-7A soft X-ray beamline at the Photon Factory in the National Laboratory for High-energy Accelerator Research Organization (KEK-PF).35 The carbon K-edge NEXAFS spectra were measured in the photon energy region of 270∼340 eV under the partial electron yield (PEY) mode36 with the retarding voltage of -180 V by varying the incident beam angle. The relative intensity of the NEXAFS spectra was normalized using the intensity (35) Amemiya, K.; Kondoh, H.; Yokoyama, T.; Ohta, T. J. Elec. Spectrosc. Relat. Phenom. 2002, 124, 151. (36) Seki, K.; Matsumoto, R.; Ito, E.; Araki, T.; Sakurai, Y.; Yoshimura, D.; Ishii, H.; Ouchi, Y.; Miyamae, T.; Narita, T.; Nishimura, S.; Takata, Y.; Yokoyama, T.; Ohta, T.; Suganuma, S.; Okino, F.; Touhara, H. Mol. Cryst. Liq. Cryst. 2001, 355, 247.

of the edge-jump transition from the C 1s core to the continuous state above the vacuum level. The polarization factor of the incident X-ray was estimated as 92%.

Results and Discussion AFM Observation. Topographic AFM images of the DCcleaned Si(111) surfaces tilted in the [112h] direction by 4° observed just after being taken out of the UHV chamber were similar to those of previous reports,32,33 as shown in Figure 2a. Very flat terraces and facets of bunched steps alternately appear on the cleaned vicinal Si(111) surface, and they run along the [1h10] direction longer than several tens of micrometers. The clean Si(111) surface, however, is highly reactive so that a native oxide layer had already grown on the surface by exposure to air. We then rapidly oxidized the step-bunched substrate in the dry O2 furnace to grow an artificial SiO2 layer, which may be used as the gate capacitor of the organic FET. Figure 2b shows an AFM image of the oxidized surface of the step-bunched vicinal Si(111) surface. For comparison, Figure 2c shows an AFM image of a thermally grown SiO2 surface on a flat Si(111) wafer. Although each bunched step in the facet could be no longer separately recognized, the alternating structure of the wide terraces and facets, which looks like a “washboard”, still existed even after the 300 nm oxidation. The height of the oxidized facets was about 7 nm, and the widths of the oxidized terraces were around 60∼80 nm, which extended more than several tens of micrometers along the [1h10] direction as before the oxidation. Panels d and e in Figure 2, which are section profiles of panels a and b in Figure 2, reveal that the edges of the “washboard” structure were rounded off after the oxidation. The surface of the wide terraces on the oxidized surface was very smooth, and the root-meansquare (rms) value of the surface roughness was about 0.2 nm. Meanwhile, a thermally oxidized SiO2 surface of a nominal Si(111) substrate has a similar roughness as shown in Figure 2f. This result shows that the terrace of the thermally grown SiO2 on the step-bunched Si(111) surface is as smooth as the commercially available flat SiO2/Si(111) surface. Through the presented processes, we have succeeded in fabricating alternating facet/terrace structures on the amorphous SiO2 surface. As the next step, we expected in-plane ordering of the photopolymerization direction along these one-dimension-

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Figure 3. AFM images of CdDA films deposited on a step-bunched SiO2/Si surface; (a) before the photopolymerization and (b) after the photopolymerization. The area of the images is 2.5 µm × 2.5 µm.

ally elongated template structures. To confirm the influence of the periodically elongated surface structures, a CdDA LB film was deposited and photopolymerized by the UV light irradiation and characterized by the AFM and NEXAFS spectroscopy. First, we observed the surface morphology of a polymerized LB film on the step-bunched substrate by AFM. Figure 3 shows AFM images of a CdDA film before and after the photopolymerization. Before the UV irradiation, many small domains of CdDA were found on the template surface as shown in Figure 3a. These domains had various shapes and sizes, and no tendency of elongating along the direction parallel to the one-dimensional template structure was observed. This image indicates that there was no anisotropy during the film deposition. Additionally, most domains continuously extended over the step-bunched facets, which suggests that the growth of the CdDA LB film was not affected by the periodic surface structure on the template substrate. From the isotropy of the domain structure, we consider that the CdDA molecules in the LB film absorbed almost vertically on the substrate, and the in-plane anisotropy due to the reorientation of the molecules during the vertical deposition process did not occur in the present films. After the UV light irradiation, slight changes of the domain structure were found as shown in Figure 3b. Some polymerized domains seemed to be restricted in their regions within the flat terrace bounded by two parallel facets. Such a domain vertically expanded just on one flat terrace, and its length along the onedimensional template structure was longer than the width. Since the CdDA monomers in the LB film topochemically react with neighboring molecules, it is suggested that the polymerization reaction proceeded just along the direction of the domain elongation. In the case of a multilayered film or an evaporated film on a flat surface, elongating large domains with high crystallinity can be grown. When polymer backbones developed by the irradiation of UV light on the highly crystalline domains, very long bright lines and cracks can be seen on the domain surface by AFM.27 They are straight and parallel to each other and indicate the direction of the photopolymerization. However, these significant changes of the morphology could not be observed in the present AFM images, because the domain size was too small. It is expected that the π-conjugated systems in the polymerized film is extended along the [1h10] direction, but the present AFM observation could not offer physical evidence of the polymerization direction nor confirm the molecular orientation in the film because of its restricted resolution. NEXAFS Spectroscopy. To investigate the effect of the onedimensional template structure on the photopolymerization direction from the viewpoint of the direction of the electron orbitals, the NEXAFS spectra of the photopolymerized CdDA LB films were measured. Figure 4 shows the dependence of the C K-edge NEXAFS spectra of the polyCdDA films on the incident

Figure 4. p-polarized C K-edge NEXAFS spectra of polyCdDA films deposited on (a) a flat SiO2/Si(111) surface and (b) a stepbunched SiO2/Si(111) surface. The irradiated X-ray was parallel to the direction of the step/terrace structures.

angle θ of the p-polarized X-ray. For comparison, the NEXAFS spectra of a polyCdDA film on a flat SiO2/Si(111) substrate are indicated in Figure 4a. According to previous reports,22,37-39 peaks in the spectra were assigned to the transitions from the C 1s core level to π*(285.5 eV) and to σ*(C-C, 292.6 eV). The transition in the polymerized diacetylene backbone contains multiple components in the π* region (below ∼286 eV); conjugated p orbitals perpendicular to the molecular plane and included both in the CdC and CtC bonds, and the unconjugated p orbitals within the molecular plane, included only in CtC bonds. It is known that the peak intensity of the conjugated π component at lower energy is much weaker than that of the unconjugated π component at higher energy in the π* region. Hence, the peak at 285.5 eV can be mainly attributed to the π*(CtC, nonconjugated) transition. We eliminated the effect of the charging with electricity, because the peak shift was not observed throughout the NEXAFS measurement. In Figure 4a, the spectra show no dependence on θ. This means there is a random conformation of molecules in the polymerized film. It is now known that the molecular orientation in the diacetylene derivative thin film rearranges due to the UV light irradiation. In the present measurement, we used a “red” polymer film, which is the most stable among the various photopolymerized states. The structure of the “red” film is rather more disordered than those of the other colors.23 Therefore, it is concluded that the molecular orientation of the polymerized polyCdDA film on the usual flat SiO2 surface is almost random. In addition, when we rotated the substrate by 90° around the surface normal, no change was observed in the NEXAFS spectra at any incident X-ray angle θ. This result also suggests that the polyCdDA film on the flat SiO2 surface has no in-plane anisotropy of the π-electron conjugation on the macroscopic scale. Figure 4b shows the NEXAFS spectra of the polyCdDA film on the step-bunched SiO2/Si(111) surface. In these spectra, the plane of the electric vector E of the incident X-ray was set parallel to the [1h10] direction, that is, parallel to the one-dimensional facet/terrace periodic structures. In the polarized NEXAFS (37) Eckhardt, D. S.; Boudreaux, D. S.; Chance, R. R. J. Phys. Chem. 1986, 85, 4116. (38) Outka, D. A.; Sto¨hr, J.; Madix, R. J.; Rotermund, H. H.; Hermsmeier, B.; Solomon, J. Surf. Sci. 1987, 53, 185. (39) Seki, K.; Morisada, I.; Edamatsu, K.; Tanaka, H.; Yokoyama, T.; Ohta, T.; Nakahara, H.; Fukuda, K. Phys. Scr. 1990, 41, 173.

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Figure 6. Orientation distribution function of polyCdDA molecules in the thin films on the step-bunched SiO2/Si(111) surfaces.

Figure 5. Azimuthal incident angle dependence of polarized C K-edge NEXAFS spectra of a polyCdDA film on the step-bunched SiO2/Si(111) surface at the magic angle incidence (θ ) 35°). In the left figure, the inset shows the relationship between the direction of the elongated facet/terrace structures on the surface (indicated by arrows) and the E vector direction of the incident X-ray. The right figure enlarges the rectangular region indicated by broken lines in the left figure.

spectra, the peak intensity of the σ*(C-C) transition should be stronger when the C-C bonds in the hydrocarbon side chains are aligned within the E vector plane of the incident X-ray. Similarly, the peak intensity of the π*(CtC, nonconjugated) transition becomes stronger when the E vector of the incident X-rays is parallel to the molecular plane of the conjugated polymer, because the nonconjugated π* orbitals of CtC bonds exist within the molecular plane. If the C-C or CtC bonds are located out of the polarized E vector plane, the transition peaks related to them in the NEXAFS spectra show only a slight dependence on the incident angle θ of the X-rays, and the peak intensity should be small. As shown in Figure 4b, the σ* transition peak is the largest at the normal incidence (NI, θ ) 0°) and smallest at the grazing incidence (GI, θ ) 75°). These changes in the peak intensity suggest that the hydrocarbon side chains in the polyCdDA film are mainly elongated within the E vector plane of the incident X-ray. Accordingly, the molecular orientation in the polyCdDA film on the step-bunched SiO2/Si(111) surface is fairly ordered in the direction along the facets. Furthermore, the intensity of the C 1s-π*(CtC, nonconjugated) transition peaks at NI and the magic angle incidence (MI, θ ) 35°) of the X-rays was stronger than at the GI, and it was also stronger compared to the spectra of the polyCdDA LB film on the flat SiO2/Si substrate. These results also reveal that regular π-conjugated systems were formed by the photopolymerization reaction on the “washboard” substrate, and they were elongated parallel to the one-dimensional facet/terrace structures. Next we rotated the substrate by 90° around the surface normal so that the E vector plane of the incident X-ray was parallel to the [112h] direction, namely, perpendicular to the periodic facet/ terrace structures. In this case, only a slight dependence of the peak intensity on the incident X-ray angle θ was observed, suggesting that the molecular orientation and π-conjugation in the polyCdDA film are less ordered in the direction across the facets than along them. Figure 5 shows the NEXAFS spectra before and after the azimuth angle rotation by 90° at the magic angle incidence (θ ) 35°) on the step-bunched SiO2/Si surface, and enlarged the ones around the C 1s-π* transition region. They show significant differences in the peak intensities, and

especially the C 1s-π* peak intensity was about 4 times stronger when the E vector plane was parallel to the one-dimensional structures than perpendicular to them. Thus, we can directly confirm the in-plane anisotropy in the polyCdDA film on the macroscopic scale. Quantitative Analysis. We tried the quantitative evaluation of the π-conjugation anisotropy as follows; we applied the method described in a previous literature40 to obtain the orientation distribution function of molecules with 2-fold symmetry. We assume that the orientation distribution function can be approximated by the following Gauss function:

f(φ) )

1

2

e(φ-π/2) /2σ

2

x2πσ

where φ is the azimuthal angle between the polymer main chain axis and the [112h] direction of the substrate, and σ2 is its dispersion. Here, the intensity of the NEXAFS signal is proportional to cos2 R, where R is the angle between the E vector of the X-ray and the transition moment of the peak. Accordingly, the intensity of a transition peak observed by NEXAFS is represented as

I(β) )

∫02πf(φ) cos2(β - φ) dφ

where β is the azimuthal angle between the E vector of the X-ray and the [112h] direction of the substrate. When we compare the intensities of the C 1s-π*(CtC, nonconjugated) transition peaks in Figure 5, the ratio of absorption intensity (I(0°)/I(90°)) is about 0.24. By fitting the experimental results with the fitting function, the dispersion angle σ was determined to be 28° for the present polyCdDA film. Figure 6 shows the calculated orientation distribution function f(φ) of the polymer film with 2-fold symmetry on the step-bunched substrate. This result suggests that the π-conjugated chains in the polyCdDA film mainly align parallel to the [1h10] direction within the angular dispersion of 28°. Additionally, the intensity of the C 1s-σ*(CC) transition peaks was stronger when the E vector plane was parallel to the [1h10] direction than perpendicular to them, as shown in Figure 5. This difference also supports the onedimensionality of the polymer chain direction indicated in the calculated distribution above. Here, we calculated the directional dispersion of the backbones in the polymer film only from the dichroic ratio of I(0°) and I(90°). Further NEXAFS spectra at different in-plane orientations are required in order to obtain an accurate estimate of the angular dispersion. (40) Kiguchi, M.; Entani, S.; Saiki, K.; Yoshikawa, G. Appl. Phys. Lett. 2004, 84, 3444.

Anisotropic Polymerization of a Diacetylene DeriVatiVe

Langmuir, Vol. 22, No. 13, 2006 5747

although there was the in-plane anisotropy of the polymerization direction. The larger miscut angle of the vicinal Si(111) substrate is able to produce narrower terraces and a higher density of facets on the DC-cleaned step-bunched surface, so that a higher anisotropic photopolymerization of the diacetylene derivative film will occur on its oxidized surface. To completely determine the effect of the periodic structures on the photopolymerization reaction, the NEXAFS spectra of the LB film before the polymerization should be measured. Namely, it is necessary to investigate whether the molecular orientation in the monomer film has the in-plane anisotropy or not and compare the molecular orientation of the LB films before and after the polymerization. Additionally, we have to examine the LB films deposited by dipping the substrate in the direction perpendicular to the elongated facets/terraces structures. We are now pushing forward the above-mentioned subjects. It is obvious, however, that the in-plane anisotropy of the LB film due to the surface structure of the substrate or the dipping direction was not observed in the AFM observation, and the π-conjugated systems in the polymerized film had the in-plane anisotropy.

Conclusions

Figure 7. Schematic illustrations of the anisotropic polymerization of diacetylene derivatives on the periodic step/terrace structures.

These results indicate that the arrangement of the π-conjugated systems in the polyCdDA film was more highly ordered along the alternating facet/terrace structures on the step-bunched SiO2/ Si surface than those on the flat SiO2 surface. It is considered that the diacetylene groups in the LB film were preferentially photopolymerized along the periodic facet/terrace structures, as schematically shown in Figure 7. The diacetylene groups in the LB films easily polymerize on the same terrace, since each CtC bond in the neighboring DA molecules are located at the same height. At the initial stage of the chain reaction induced by the UV irradiation, the polymerization of CdDA molecules propagated to random directions on the terrace surface. The polymerization chain reaction, however, could not propagate beyond the edge of a facet because of the misalignment of the CtC bond positions on different terraces. Therefore, a longer chain reaction of the CdDA molecules occurred only along the elongated template structures, and parallel conjugated backbones were fabricated on the step-bunched SiO2/Si(111) substrate as revealed by the NEXAFS spectroscopy. In the present case, we fabricated LB films on vicinal Si(111) substrates with the miscut angle of 4°. The widths of the oxidized terraces on this surface were far wider in comparison with the size of the DA monomers. Accordingly, some parts of the CdDA film seemed to polymerize in the direction across the facets,

Nanometer-scale self-organized periodic structures were fabricated by step bunching by the DC-cleaning of a vicinal Si(111) substrate. These periodic structures containing flat terraces and bunched steps still existed after the thermal oxidization, and the surface of the oxidized terraces was as smooth as the generally available SiO2/Si surface. The conformation of a photopolymerized diacetylene derivative LB film deposited on the alternating facet/terrace structures was determined by angledependent polarized NEXAFS spectroscopy. The NEXAFS spectra clearly revealed the in-plane anisotropy in the polymerized film fabricated on the periodic washboard-like surface but not on the flat SiO2 surface. The π-electron conjugation in the polymerized LB film was more regular in the direction along the periodic structures than across them. These results indicate that diacetylene groups in the LB films were preferentially photopolymerized in the direction not across but along the step structures. Thus, we have confirmed that the control of the polymerization direction can be accomplished by the formation of one-dimensional periodic nanostructures on the amorphous SiO2/Si substrate, although isotropic random polymerization occurs on the usual flat SiO2/Si surface. Acknowledgment. The authors greatly appreciate Dr. Kenta Amemiya (The University of Tokyo) for his help with the NEXAFS measurements and discussions. This research was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2003G259). This work was supported by a Grant-in-Aid for Creative Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 14GS0207). LA060482D