Langmuir 2008, 24, 11605-11610
11605
Nanotransfer of the Polythiophene Molecular Alignment onto the Step-Bunched Vicinal Si(111) Substrate Ryo Onoki,† Genki Yoshikawa,‡ Yuki Tsuruma,§ Susumu Ikeda,§ Koichiro Saiki,§ and Keiji Ueno*,† Department of Chemistry, Saitama UniVersity, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan, and Department of Complexity Science and Engineering, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ReceiVed May 30, 2008. ReVised Manuscript ReceiVed July 27, 2008 A poly(3-dodecylthiophene-2,5-diyl) film having in-plane anisotropic molecular arrangement was successfully fabricated by transferring its Langmuir-Blodgett film onto a step-bunched Si(111) substrate. Polarized near-edge X-ray absorption fine structure measurements revealed that the polythiophene main chains are preferentially orientated along periodic facet/terrace nanostructures on the step-bunched substrate, whereas less anisotropy was found on a flat substrate. The step-bunched Si substrate has been proved to be effective for controlling the in-plane molecular arrangement in the polymer thin film.
Introduction Conjugated organic polymers are the subject of a great deal of attention for their electrical and optical properties, reasonable chemical stability, and the easy processability of thin films. Solution-processable conjugated polymers have been widely investigated because of their potential advantage in developing large-area and low-cost electronic devices such as light-emitting diodes,1,2 photovoltaic cells,3-5 and field effect transistors (FETs).6-8 Among the conjugated organic polymers, poly(3alkylthiophene)s (PATs) are one of the most promising solutionprocessable organic semiconductors. The film structure and the molecular packing in the PAT thin film have been widely studied to elucidate their fundamental properties and their relationship with the performance of PAT-based devices. It is well-known that the carrier transport in PAT thin films is dramatically affected by regioregularity and the length of alkyl side chains at the “3”-position, both of which cause improvement in the carrier mobility in their FETs by several orders of magnitude.9 In addition, film crystallinity and the FET mobility largely depend on the average molecular weight (MW) of PATs.10,11 A low-MW polymer film contains highly ordered nanorods with a self-organized lamellar structure, whereas a highMW film shows irregular topographies with a random, network* To whom correspondence should be addressed. E-mail: kei@ chem.saitama-u.ac.jp. † Saitama University. ‡ National Institute for Materials Science. § The University of Tokyo.
(1) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (3) Brabec, C. J.; Serdar Sariciftci, N.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (4) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (5) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (6) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (7) Katz, H. E.; Bao, Z. J. Phys. Chem. B 2000, 104, 671. (8) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108. (9) Kaneto, K.; Lim, W. Y.; Takashima, W.; Endo, T.; Rikukawa, M. Jpn. J. Appl. Phys. 2000, 39, L872. (10) Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawapirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. AdV. Funct. Mater. 2004, 14, 757. (11) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fre´chet, J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312.
like structure. FETs based on the low-MW PAT polymer, however, exhibit lower mobility than those of the high-MW polymer FETs, although the low-MW PAT films have better crystallinity. This result has been attributed to the fact that a large number of domain boundaries exist among the nanorods in the low-MW PAT film, whereas long polymer chains are well connected to each other in the high-MW PAT film. Therefore, if the well-ordered molecular alignment in the high-MW PAT film is achieved, FET devices with much higher carrier mobility are expected to be realized. It is, however, difficult to control the molecular alignment in the high-MW polymer thin film because the polymer main chain is subject to increasing entanglement and twisting with increase in the number of MWs. In the present study, we have utilized the Langmuir technique as the method to prepare a highly ordered PAT film. Recently, PAT thin films have been fabricated using solution-processing techniques such as dip coating, drop casting, or spin coating. Although these processes have considerable advantages in the simple deposition process, they are not adequate for the fine control of film thickness and molecular orientation. The regioregular PAT molecule has a hairy-rod-like polymer structure and has ability to form stable and transferable rigid and stiff Langmuir monolayers despite the lack of the polar hydrophilic group. Actually, the preparation of ordered PAT ultrathin films has been demonstrated by the Langmuir-Blodgett (LB) technique.12-14 The ordering of the PAT film deposited on a flat substrate by the LB method, however, was still incomplete. The best dichroic ratio of the PAT film measured by the polarized UV-vis absorption was 1.60.13 We have succeeded in regularly controlling the in-plane alignment of high-MW PAT polymer using the substrates with one-dimensional nanostructures, that is, step-bunched Si substrates. When a vicinal Si(111) surface tilted toward the [112j] direction is thermally cleaned in an ultrahigh vacuum (UHV) by a direct current (dc) flowing in the [1j10] direction and successively annealed at an appropriate temperature, the vicinal Si(111) surface has a self-organized alternating structure of flat terraces and facets (12) Watanabe, I.; Hong, K.; Rubner, M. F. Langmuir 1990, 6, 1165. (13) Xu, G.; Bao, Z.; Groves, J. T. Langmuir 2000, 16, 1834. (14) Matsui, J.; Yoshida, S.; Mikayama, T.; Aoki, A.; Miyashita, T. Langmuir 2005, 21, 5343.
10.1021/la8016722 CCC: $40.75 2008 American Chemical Society Published on Web 09/09/2008
11606 Langmuir, Vol. 24, No. 20, 2008
of bunched steps.15-19 The alternating facet and terrace structures elongate straight along the [1j10] direction over tens of micrometers. The molecular orientation of transferred PAT thin films onto step-bunched Si(111) substrates using LB technique was characterized by polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectroscopy is element-sensitive, and it is possible to explore the spatial arrangement of functional groups in the polymer film.20 In previous studies, molecular orientation and electronic structure in thin films of polythiophene derivative materials were successfully analyzed by NEXAFS spectroscopy.21-23 In the present study, we measured sulfur (S) K edge NEXAFS spectra. In the case of target PAT molecules, azimuthal-angle-dependent NEXAFS measurement of the S 1s-σ* transition moment of S-C bonds, which is parallel to the polymer backbone, gives the orientation of polymer main chains. Our S K edge NEXAFS observation revealed that the PAT thin film on the step-bunched Si(111) substrates with periodic straight nanostructures has better in-plane anisotropy than that on a flat Si substrate. We have succeeded in demonstrating that the one-dimensional structures on the substrate surface can induce straight extension of the conjugated backbones in the PAT thin film, resulting in the inplane anisotropic alignment in the high-MW polymer film.
Experimental Section Substrate Fabrication. Periodic facet/terrace surface structures were fabricated on a vicinal Si(111) substrate as follows:17-19 a Si substrate was cut from a P-doped n-type Si(111) wafer tilted toward the [112j] direction by 4° or 7°. It was introduced into an UHV chamber with a base pressure of 2 × 10-8 Pa and cleaned by dc heating with the current flowing along the [1j10] direction. After 12 h of heating at 600 °C, we repeated flash heating to 1250 °C 3-4 times and annealed the sample at 930 °C for 30 min. Then, the temperature of the substrate was slowly cooled by the rate of 3 °C min-1. The surface morphology of the step-bunched substrate was observed by atomic force microscopy (AFM; SEIKO Instruments SPI-3800/SPA-300 system, contact mode using a Si3N4 cantilever) just after the sample was taken out of the UHV chamber. As shown in Figure 1a, very flat terraces and facets of bunched steps alternately appear on the cleaned vicinal Si(111) surface, and these structures run along the [1j10] direction for a longer distance than several tens of micrometers. The height of surface roughness on these flat terraces is determined to be less than 0.3 nm. Self-Assembled Monolayer Fabrication. Before the deposition of the polythiophene film, the step-bunched Si surface was made hydrophobic with a self-assembled monolayer (SAM) of octadecyltriethoxysilane (OTES, Tokyo Chemical Industry Co., Ltd.) according to the previously reported method.24,25 Briefly, the stepbunched surface was first exposed to UV ozone cleaning. Then, the substrate was soaked in the cyclohexane containing hydrolyzed OTES (15) Williams, E. D.; Bartelt, N. C. Science 1991, 251, 393. (16) Viernow, J.; Lin, J.-L.; Petrovykh, D. Y.; Leibsle, F. M.; Men, F. K.; Himpsel, F. J. Appl. Phys. Lett. 1998, 72, 948. (17) Lin, J.-L.; Petrovykh, D. Y.; Viernow, J.; Men, F. K.; Seo, D. J.; Himpsel, F. J. J. Appl. Phys. 1998, 84, 255. (18) 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. (19) Onoki, R.; Ueno, K.; Nakahara, H.; Yoshikawa, G.; Ikeda, S.; Entani, S.; Miyadera, T.; Nakai, I.; Kondoh, H.; Ohta, T.; Kiguchi, M.; Saiki, K. Langmuir 2006, 22, 5742. (20) Sto¨hr, J. NEXAFS Spectroscopy; Springer: Berlin, 1992. (21) Yoshikawa, G.; Kiguchi, M.; Ikeda, S.; Saiki, K. Surf. Sci. 2004, 559, 77. (22) Kiguchi, M.; Entani, S.; Saiki, K.; Yoshikawa, G. Appl. Phys. Lett. 2004, 84, 3444. (23) Kim, D. H.; Jang, Y.; Park, Y. D.; Cho, K. Langmuir 2005, 21, 3203. (24) Peanasky, J.; Schneider, H. M.; Granick, S.; Kessel, C. R. Langmuir 1995, 11, 953. (25) Ulman,A.AnIntroductiontoUltrathinOrganicFilmsfromLangmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991.
Onoki et al.
Figure 1. (a) AFM image of the step-bunched 4°-off Si(111) surface. The scanned area of the image is 1 µm × 1 µm. (b) Schematic illustration of deposition by the horizontal lifting technique.
for 30 min at room temperature (RT). The treated Si substrate was taken out of the solution and rinsed in cyclohexane. Poly(3-dodecylthiophene-2,5-diyl) Film Deposition. The polythiophene compound, regioregular poly(3-dodecylthiophene-2,5diyl) (PDT, Sigma-Aldrich Corporation), was purified several times by precipitation. The weight-average MW of PDT was approximately 162 000. Purified PDT was dissolved in toluene with a concentration of 5 × 10-5 mol L-1 to prepare a spreading solution. The PDT monolayer was spread onto Milli-Q water (F ) 18 MΩ cm) and compressed by the trough barrier. The compressed film was deposited by the Langmuir-Schaefer (LS) horizontal lifting technique on a Si substrate at 20 °C with a surface pressure of 18 mN m-1. The LS deposition was repeated by several times to fabricate a multilayer film. Surface structures of PDT films were observed by AFM in the same manner as for the Si substrate. During the LS deposition, the elongated facet/terrace structures of the step-bunched substrate were set precisely parallel or perpendicular to the barrier. In the case shown in Figure 1b, the elongated periodic structure is set parallel to the barrier, namely, perpendicular to direction of the compression of the spread PDT film. Additionally, multilayer PDT films were deposited in the same manner on nominally flat (0°-off) Si surfaces covered with the native oxide layer and OTES-SAM in order to examine the effect of the periodic surface nanostructures on the ordering of the deposited PDT molecules. Hereafter, these deposition conditions will be abbreviated as, for example, “5-LS/4°/parallel”, which means “5times deposited PDT film by the LS technique on a 4°-off Si(111) substrate with the elongated periodic structures parallel to the compression barrier”. The sample on a flat Si substrate will be abbreviated as “5-LS/0°”. Since the deposited film was not completely uniform due to some defects induced during transfer of Langmuir film onto the substrate, the thickness of the 5-LS film is estimated to be 3-5 monolayers. NEXAFS Spectroscopy. The NEXAFS spectra of polymer films were measured at the BL-11B soft X-ray beam line at the Photon Factory in the National Laboratory of the High-Energy Accelerator Research Organization (KEK-PF).26,27 This beam line is equipped with a Ge(111) crystal monochromator (X-ray energy range: 2020-3911 eV), which enables the S K edge excitation measurement. The degree of polarization of the X-ray beam is more than 95%. The S K edge NEXAFS spectra were measured under the fluorescence yield mode by varying the incident angle of the p-polarized X-ray beam. In-plane orientation of PDT films was also measured with rotating the specimen around the normal incident X-ray. (26) Amemiya, K.; Kitajima, Y.; Ohta, T.; Ito, K. J. Synchrotron Radiat. 1996, 3, 282. (27) Kitajima, Y.; Yonamoto, Y.; Amemiya, K.; Tsukabayashi, H.; Ohta, T.; Ito, K. J. Electron Spectrosc. Relat. Phenom. 1999, 101, 927.
Polythiophene Alignment on Step-Bunched Si(111)
Langmuir, Vol. 24, No. 20, 2008 11607
Figure 2. AFM images of PDT films deposited on (a) a flat Si substrate and (b) a step-bunched 4°-off Si(111) substrate.
Results and Discussion AFM Observation. As shown in Figure 1a, flat terraces and facets of bunched steps alternately appear on the cleaned 4°-off Si(111) surface, and these structures run along the [1j10] direction for a longer distance than several tens of micrometers. The height of the facets is ∼7.5 nm, whereas the width of the terraces is ∼100 nm. In the case of the 7°-off Si(111) substrate, the stepbunched surface has slightly higher facets with ∼9 nm height, and the width of terraces is narrower than the 4°-off Si(111) substrate, ∼70 nm. Figure 2 shows AFM images of (a) a 5-LS/0° and (b) a 5-LS/ 4°/parallel samples. On the flat substrate, each PDT domain has a random structure and no anisotropy can be recognized. On the step-bunched surface, on the other hand, each domain has an elongated shape along the periodic step/terrace structures, and it extends just within the width of a single terrace. Thus, PDT molecules in the film are expected to be arranged more orderly on the step-bunched periodic surface than on the flat substrate. Since each molecule in the polymer films, such as PDT, is difficult to be resolved with AFM, orientation of PDT molecules in the deposited films has been determined by means of NEXAFS, which can provide the information on the spatial orientation of the orbitals related to the specific element. Incident Angle Dependence of NEXAFS. Polarized S K edge NEXAFS spectra of the 5-LS/4°/parallel PDT film are shown in Figure 3a for two different incident angles of X-rays. The plane of the incident p-polarized X-rays was set parallel to the [1j10] direction of the substrate, that is, parallel to the onedimensional facet/terrace periodic structures. The intensity of the NEXAFS spectrum was normalized with the height of the S 1s absorption edge-jump. In every NEXAFS spectrum, an intense peak appears around 2473 eV. According to previous studies, this peak consists of two components which can be assigned to the transitions from the S 1s core level to π*(SdC bond, 2472.4 eV) and σ*(S-C bond, 2473.3 eV), respectively.21,22 Therefore, the main peak around 2473 eV was curve-fitted using two Gaussian functions which represent the S 1s-π*(SdC) and S 1s-σ*(S-C) transitions and an edge-jump function. For the edge-jump function, the integral of a normal (Gaussian) distribution was selected. The energy and variance of S 1s-π*(SdC) and S 1s-σ*(S-C) peaks as well as the step edge were fixed for all fitted spectra. In Figure 3a, fitted curves are superposed on raw spectra. As schematically shown in Figure 3b, the S 1s-σ*(S-C) transition moment is parallel to the polymer main chain axis within the molecular plane (thiophene ring faces), and the S 1s-π*(SdC) transition moment is perpendicular to the molecular plane. As shown in Figure 3a, peak intensity of the S 1s-σ*(S-C) transition shows noticeable incident angle (θ) dependence, the highest at the normal incidence (NI, θ ) 0°) and the lowest at the grazing incidence (GI, θ ) 75°). This result indicates that the deposited PDT film includes the component of S 1s-σ*(S-C) transition moment which run parallel to the [1j10] direction, that
Figure 3. (a) Polarized S K edge NEXAFS spectra of a PDT film deposited on a step-bunched 4°-off Si(111) substrate. The E vector plane of incident X-rays was parallel to the direction of the periodic step/terrace structures. The incident X-ray angle from the surface normal (θ) was set to 0° (normal incidence; NI) and 75° (grazing incidence, GI). (b) Schematic view of S 1s-σ*(S-C) and S 1s-π*(SdC) transition moments in a PDT molecule.
is, parallel to the elongated periodic step-bunched structures. The angular distribution of PDT main chains, however, cannot be obtained only by the incident-angle-dependent NEXAFS measurements. In Figure 3a, peak intensity of the S 1s-π*(SdC) transition also shows the incident angle (θ) dependence; it was the highest at NI and the lowest at GI. If all main chains of the PDT molecules are parallel to the [1j10] direction and molecules stand vertically on the substrate, the 1s-π*(SdC) transition peak should not be observed in the NEXAFS spectrum because the E vector of the incident p-polarized X-rays is perpendicular to the 1s-π*(SdC) transition moment. The appearance of the 1s-π*(SdC) transition peak and its incident angle dependence suggests the existence of disordered PDT main chains. Details about the 1s-π*(SdC) transition will be discussed later. Azimuthal Angle Dependence of NEXAFS. In order to determine the distribution of the in-plane orientation of PDT molecules, we performed azimuthal-angle-dependent NEXAFS measurements by rotating the samples around the NI X-ray beam (θ ) 0°). Figure 4a shows the result for a 5-LS/4°/parallel PDT film. Here, φ denotes the azimuthal angle between the E vector of the X-rays and [112j] direction of the step-bunched Si(111) substrate. The main peak around 2473 eV was curve-fitted as mentioned above. In Figure 4b, fitted curves are superposed on raw spectra measured at φ ) 0°, 45°, and 90°. The intensity of the S 1s-σ*(S-C) transition peak exhibits a noticeable azimuthal angle dependence, and the fitted peak intensity is the highest at φ ) 90° and the lowest at 0°. As previously shown in Figure 3b, the S 1s-σ*(S-C) transition moment exists in the molecular plane of PDT. Therefore, it is concluded that most of the molecular planes of thiophene rings in the polymer backbones are orientated parallel to the [1j10] direction of the step-bunched Si(111) substrate. S 1s-π*(SdC) transition peak intensity also shows azimuthal angle dependence as shown in Figure 4b although the quantitative
11608 Langmuir, Vol. 24, No. 20, 2008
Onoki et al.
Figure 5. Schematic view of LS-deposited PDT molecules on a stepbunched Si(111) substrate.
Figure 4. (a) Rotational angle (φ) dependence of polarized S K edge NEXAFS spectra of PDT film with normal X-ray incidence. Here, the [112j] and [1j10] directions of the Si substrate are defined as φ ) 0° and 90°, respectively. (b) Peak-fitting for the main absorption peaks (2473 eV) shown in (a). Each NEXAFS spectrum is fitted using two Gaussian functions, which represent the S 1s-π*(SdC) and S 1s-σ*(S-C) transitions, and an edge-jump function. In the figure, fitted curves are superposed on raw spectra measured at φ ) 0°, 45°, and 90°.
analysis is difficult due to its smaller signal-to-noise ratio. The fitted S 1s-π*(SdC) transition peak intensity is the highest at φ ) 0° and the lowest at 90°. This result is in agreement with the model reported by Xu et al.13 for the horizontally deposited LB film of poly(3-hexylthiophene) on a hydrophobic substrate, in which the PAT film has the edge-on structure and most of the molecular planes are orientated parallel to each other. In our case, PDT molecules with the edge-on structure are supposed to be orientated along the [1j10] direction, along the step-bunched structures as shown in Figure 5. NEXAFS Spectra on Different Substrates. The φ-dependent NEXAFS spectra were also measured for a 5-LS/0°, 5-LS/7°/ parallel, and 5-LS/4°/perpendicular PDT films; results are shown in Figure 6a-c, respectively. On the flat Si substrate (Figure 6a), φ denotes the azimuthal angle between the E vector of the X-rays and the compressing direction which is perpendicular to the trough barrier. On the step-bunched substrate (Figure 6, parts b and c), φ denotes the azimuthal angle between the E vector of the X-ray and [112j] direction, which is perpendicular to the elongated facet/ terrace structures. On the flat substrate, the main peak shows azimuthal angle (φ) dependence, where the peak intensity is slightly higher at φ ) 90°, but the azimuthal angle dependence is much smaller than that of the 5-LS/4°/parallel PDT film. On the 7°-off Si(111) substrate, on the other hand, NEXAFS spectra shows significant φ dependence, and the azimuthal angle dependence is larger than that on the 4°-off substrate. In addition, the azimuthal angle dependence almost diminishes on the 5-LS/4°/perpendicular film, which was deposited with elongated periodic structures being
set perpendicular to the trough barrier. Changes in the intensity of curve-fitted S 1s-σ*(S-C) transition peaks against the azimuthal angle φ are summarized in Figure 7a for all types of samples. Quantitative Analysis. We tried the quantitative evaluation of the in-plane anisotropy of PDT main chains using the data shown in Figure 7a as follows: we applied the method described in a previous literature22 to obtain the orientation distribution function of molecules with twofold symmetry. We assume that the orientation distribution function can be approximated by the following Gauss function:
f(R) )
( ) 1
√2πσ
e-
(R-π/2)2 2σ2
(1)
where R is the azimuthal angle between the molecular main chain of PDT and the direction of φ ) 0° defined above and σ2 is its variance. This assumption is not adequate for sample iv in Figure 7a, because it has almost no anisotropy of the peak intensity. Therefore, this sample is excluded from the following analysis. The intensity of the NEXAFS peak is proportional to cos2 γ, where γ is the angle between the electric vector of the X-ray and the transition moment of the peak. The intensity of the S 1s-σ*(S-C) transition peak observed by NEXAFS is thus represented as eq 2:
I(φ) )
∫02π f(R) cos2(φ - R) dR
(2)
where φ is the azimuthal angle of the electric vector E of the incident X-rays defined above. With the use of these equations, σ can be calculated from the peak intensity ratio R ) I(90°)/I(0°) as eq 3:
σ)
ln
R+1 R-1 2
(3)
Calculated σ values for three samples shown in Figure 7a are 54° (i, flat), 39° (ii, 4° off |), and 31° (iii, 7° off |). Figure 7b shows the obtained orientation distribution functions of these samples. It is clearly revealed that the PDT film on a 7°-off step-bunched substrate has the best in-plane ordered main chains
Polythiophene Alignment on Step-Bunched Si(111)
Langmuir, Vol. 24, No. 20, 2008 11609
Figure 7. (a) Changes in the intensity of curve-fitted S 1s-σ*(S-C) transition peaks against the azimuthal angle φ: (i) on a flat Si substrate, (ii) on a 4°-off step-bunched substrate, (iii) on a 7°-off step-bunched substrate, and (iv) on a 4°-off step-bunched substrate, deposited with setting the elongated periodic structures perpendicular to the barrier. Ratios between peak intensities at φ ) 90° and φ ) 0° are shown in the figure. (b) Orientation distribution functions of PDT molecules in samples i-iii.
Figure 6. Rotational angle (φ) dependence of polarized S K edge NEXAFS spectra of PDT films with normal X-ray incidence. (a) PDT film deposited on a flat Si substrate. The direction parallel to the trough barrier is defined as φ ) 90°. (b) PDT film on a 7°-off step-bunched substrate deposited with setting the elongated periodic structures on the substrate parallel to the trough barrier. (c) PDT film on a 4°-off stepbunched substrate deposited with setting the elongated periodic structures on the substrate perpendicular to the trough barrier. In (b) and (c), [112j] and [1j10] directions of the Si substrate are defined as φ ) 0° and 90°, respectively.
along the [1j10] direction, although some PDT molecules still have random directions. As shown in Figure 4b, the peak of the S 1s-π*(SdC) transition, which is orthogonal to the S 1s-σ*(S-C) one, is observed even at φ ) 90°. In addition, this peak is higher at NI than GI as shown in Figure 3a, which indicates that observed π* orbitals are nearly parallel to the substrate surface. The partial in-plane random distribution of PDT molecules results in the observation of this S 1s-π*(SdC) transition at φ ) 90°. Here it must be noted that the average intensity of the S 1s-σ*(S-C) transition peak over the azimuthal angle φ is the highest on sample i and the lowest on sample iii, as shown in Figure 7a. If all main chains of PDT molecules are parallel to the substrate surface, the average intensity of the 1s-σ*(S-C) transition peak over φ should be equal, because all samples are assumed to have the same film thickness with 5-times LS deposition and their spectra are normalized by the height of the edge-jump. However, if some parts of the main chains have vertical corrugation or defects, the intensity of the 1s-σ*(S-C)
transition peak will become lower for the entire azimuthal angles with the normal X-ray incidence (θ ) 0°). Thus, we suppose that some PDT main chains have corrugation or defects on the stepbunched Si(111) surfaces probably due to the existence of bunched steep steps, whereas most of the PDT main chains are parallel to the surface and orientated along the [1j10] direction on the step-bunched Si surfaces as can be seen in Figures 3a and 7b. Origin of the In-Plane Anisotropy. In the present study, we have directly measured the in-plane anisotropy of polymer molecules in the PDT film. These results indicate that most of the PDT main chains align parallel to the elongated periodic structures on the step-bunched vicinal Si(111) surfaces only when these elongated structures are set parallel to the trough barrier during the LS deposition. It is also revealed that only small parts of the PDT main chains align parallel to the trough barrier on the flat Si surface, whereas the rest have random in-plane directions. It is reported that the polythiophene backbones in the PAT film prepared by the Langmuir technique tend to align parallel to the trough barrier, and this preferential alignment is induced by the pressure generating from the barrier compression.13 Therefore, it is suggested that the aligned polythiophene backbones in the PDT Langmuir film can be transferred by the LS deposition method onto the step-bunched Si(111) substrate retaining the in-plane anisotropy, and the polymer chains in the LB film are mainly orientated along the facet/terrace periodic structures on the step-bunched substrate. This preservation of the in-plane ordering in the PDT Langmuir film is possible only when the elongated step-bunched structures are set parallel to the trough barrier. Otherwise, the anisotropic structure almost collapses during the LS deposition. On the flat Si substrate, a large part of the in-plane ordering collapses during the LS deposition processes, and the LB film exhibits only small anisotropy, as revealed by the small dichroic ratios: 1.60 in ref 13 and 1.41 in our case. The in-plane arrangement in the LS-deposited PDT film can be attributed to two types of steric effects. One is the property of such long linear molecules as PATs, which tend to fit snugly in the straight grooves, such as those caused by the step-bunching, and extend along them. The other is the narrow width of the flat terrace area separated by facets on the step-bunched surface. It is supposed that the PDT molecules cannot entangle or aggregate beyond the edge of the facets because the height of the facets is about 8 nm, higher than that of the PDT monolayer, ∼3 nm.
11610 Langmuir, Vol. 24, No. 20, 2008
Hence, the polymer main chains of PDT arrange themselves along the one-dimensional periodic structures, and the large anisotropy appears in the film deposited on the step-bunched surface as revealed by the NEXAFS spectroscopy. Since the step-bunched 7°-off substrate has narrower width of terraces, the PDT main chains can arrange more orderly along the step-bunched structures than on the 4°-off substrate. On the flat substrate, however, the compressed Langmuir film can easily collapse and lose the anisotropy due to the absence of the force for the arrangement during the deposition. It is also revealed that the alignment of the direction of the elongated groove on the stepbunched substrate with that of the orientated backbone in the compressed PDT Langmuir film during the LS deposition is critical for the transfer of the arrangement onto the substrate. In the case where these directions are misaligned with each other, the arrangement in the PDT film is completely collapsed by the “bumpy” substrate. As previously reported by us, the step-bunched anisotropic substrate surface is also useful for controlling the photopolymerization direction in the LB film of a long-chain diacetylene derivative compound.19 In this case, the photoassisted chain reaction of diacetylene derivative monomers progresses mainly along the facet/terrace periodic structures, and the anisotropic polymerization was revealed by polarized carbon K edge NEXAFS spectroscopy. Thus, the step-bunched Si(111) substrate is useful for straightly arranging the main chains of polymer molecules fabricated on its surface. Straight alignment of the π-conjugated polymer molecules enables the ordered arrangement of π-orbitals and reduces the hopping barrier against the carrier transport. Then it will be possible to increase the carrier mobility in the organic FET or improve the efficiency of an organic polymer
Onoki et al.
photovoltaic cell. Furthermore, the periodic structures on the step-bunched Si(111) substrate can be transferred onto the surface of such spin-coated polymer films as PDT, poly(methyl methacrylate), and poly(vinyl alcohol) by the imprinting method. Investigation of the arrangement of polymer backbones deposited on these imprinted, “low-cost” substrates is under the experiment.
Conclusions We succeeded in demonstrating the in-plane anisotropic molecular arrangement in the PDT film using the step-bunched vicinal Si(111) substrate and the LS deposition technique. NEXAFS measurements have revealed that the polythiophene main chains are preferentially orientated along the self-assembled periodic facet/terrace structures when the elongated step-bunched structures are set parallel to the trough barrier, whereas they take almost random in-plane orientations on the flat Si substrate or on the step-bunched substrate set perpendicular to the barrier. This nanotransfer technique utilizing the step-bunched substrate will open the door to the further applications of LB films. Acknowledgment. The authors greatly appreciate Mr. Y. Kitajima (KEK-PF) for his help with the NEXAFS measurements and discussions. We also thank Professor H. Nakahara (Saitama University) for helpful discussions about the fabrication of LB films. This research was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2006G103). It was supported by a Grand-in-Aid for Creative Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 14GS0207). LA8016722
