J. Phys. Chem. C 2008, 112, 4569-4572
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X-ray Diffraction Study of Directionally Grown Perylene Crystallites Dag W. Breiby,*,† Henrik T. Lemke,‡ Peter Hammershøj,‡ Jens W. Andreasen,§ and Martin M. Nielsen‡,§ Department of Physics, Norwegian UniVersity of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway, Centre for Molecular MoVies, UniVersity of Copenhagen, UniVersitetsparken 5, 2100 Copenhagen Ø, Denmark, and Polymer Department, Risø National Laboratory, P.O. Box 49, FrederiksborgVej 399, 4000 Roskilde, Denmark ReceiVed: August 6, 2007; In Final Form: December 21, 2007
Using grazing incidence X-ray diffraction, perylene crystallites grown on thin highly oriented poly(tetrafluoroethylene) (PTFE) films on silicon substrates have been investigated. All the perylene crystallites are found to orient with the ab plane of the monoclinic unit cell parallel to the substrate. The scattering data is interpreted as a trimodal texture of oriented perylene crystallites, induced by interactions between the perylene molecules and the oriented PTFE substrate. Three families of biaxial orientations are seen, with the 〈h10〉 axes (h ) 1, 2, or 3) parallel to the PTFE alignment, all having the ab-plane parallel to the substrate. About 92% of the scattered intensity corresponds to a population with 〈110〉 highly parallel to 〈001〉PTFE.
Introduction A prerequisite for exploitation of intrinsic anisotropic molecular properties for organic electronics and nanotechnology is that paths are found to induce and characterize macromolecular order.1-12 The remarkable ability of friction-deposited poly(tetrafluoroethylene) (PTFE) films in inducing uniaxial alignment order in a wide range of materials is increasingly exploited.4-10 Several mechanisms have been suggested to explain the aligning properties of PTFE, ranging from chemical interactions with the hydrophobic PTFE surface to epitaxy7,8 and mechanical confinement effects by the highly corrugated PTFE surface, which consists of mesoscopic “bundles”/fibers of many helical PTFE molecular strands (see Scheme 1).1,7-10 The planar conjugated molecule perylene (C20H12) is much studied for nanoapplications because of its chemiophysical properties, notably photoinduced excimer formation. Perylene is reported to crystallize in two different monoclinic crystal forms. The R-variant, which exhibits excimer formation, has the molecules arranged in dimers, with two dimers per monoclinic unit cell, a ) 11.277 Å, b ) 10.826 Å, c ) 10.263 Å, and β ) 100.55° (see Scheme 1 for an illustration).13 The β-variant is a herringbone structure with unpaired molecules and a ) 9.78 Å, b ) 5.9 Å, c ) 10.59 Å, and β ) 96.75°.14 In this paper, we report on perylene grown by chemical vapor deposition (CVD) onto friction-deposited PTFE. The crystallites forming on the substrate are found to exhibit a fascinating interplay with the PTFE substrate. A detailed, yet conceptually simple, structural model is presented, which gives excellent quantitative agreement with the experimental data. Experimental Section PTFE alignment layers were obtained as described in the literature,1,7-10 by sliding a PTFE rod at a constant pressure (∼1 MPa) along a microscopy slide at 280 °C with a speed of †
Norwegian University of Science and Technology. University of Copenhagen. § Risø National Laboratory. ‡
SCHEME 1: (a) Perylene Chemical Structure and Unit Cell and (b) Chemical Structure of PTFE and an Illustration of Its Helical Chain, Which in the FrictionDeposited Films Is Hexagonally Packed and Uniaxially Oriented
about 0.7 mm/s. Perylene crystallites (“films”) were deposited using CVD (base pressure 10-6 mbar, room temperature, perpendicular deposition). Atomic force microscopy (AFM) in tapping mode was performed using a Digital Instruments Dimension 3100. Grazing incidence X-ray diffraction (GIXD)15 was performed using the diffractometer at the BW2 wiggler beamline at HASYLAB, with wavelength λ ) 1.2398 Å and an incidence angle of 0.158°, which is just below the critical angle for total reflection from the Si substrate. The diffractometer has a rotation φ about the (vertically oriented) surface normal.16 The sample cell was made of kapton and flushed with helium to reduce background and beam damage. The incoming beam had dimensions of 1 mm (horizontal, H) × 0.1 mm (vertical, V). A pinhole of ∼1 mm diameter was used at the entrance of the detector flight tube, ∼20 cm from the sample. Just in front of the Cyberstar point detector, 1.2 m from the sample, slits (H ×
10.1021/jp076290o CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008
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Figure 1. (a) GIXD exposure with the Risø instrument of a perylene sample with no in-plane orientation, being equivalent to a rotation image of a single crystal. The image has been converted to reciprocal space coordinates. (1) and (2) denote positions (Qxy, Qz) used for the rocking scans in Figure 2. (b) Synchrotron radial GIXD scan of an oriented sample, performed with Q parallel to the PTFE alignment (Qz ) 0.03 Å-1, Qx ) 0). The inset shows the lowest Q peak on a rescaled axis, emphasizing the rectangular shape of the peaks.
Simulations Simulations of the GIXD data were done using in-house software (Breiby et al., J. Appl. Crystallogr. 2008, in press) that first calculates a superposition of reciprocal space lattices (containing nodes with structure factors |Fhkl|2) from the real space crystal lattices (one for each main crystallite orientation). Throughout, the unit cell with dimer structure, advanced by Camerman et al.13 was employed, as this unit cell accounts well for the scattering data. The simulated diffraction scans are then obtained by calculating a trace through reciprocal space, where for every discrete point j of the trace, the intensity was obtained by the approximation
Ij )
∑n (LPV)|Fhkl|2 P1(WQ, Qj - Qn) × P2(Wχ, χj - χn) P3(Wφ, φj - φn) (1)
Figure 2. (a) Rocking scan about the sample normal (φ-rotation) for reciprocal coordinates (Qxy, Qz) ) (0.803, 0.106) Å-1, corresponding to {110} reflections. The inset is a magnification with a logarithmic intensity scale. (b) Rocking scan at (Qxy, Qz) ) (0.579, 1.246) Å-1, corresponding to {012} (logarithmic intensity scale). See the main text for further details.
V) of 0.5 × 4 mm2 were used. The scattering vector is defined by Q ≡ kout - kin, where kin and kout are the in- and out-going wave vectors, respectively, and k ) |kout| ) |kin| ) 2π/λ. Q can be decomposed into a Cartesian reciprocal coordinate system (Qx, Qy, Qz), with Q2 ) Qx2 + Qy2 + Qz.2 We take Qz along the film normal and Qx perpendicular and Qy parallel to the PTFE alignment. Q may be associated with a real space distance d ) 2π/Q. Additional grazing incidence X-ray scattering images with lower resolution were recorded with Fuji image plates at the rotating anode based (Cu KR, λ ) 1.5418 Å) diffractometer at Risø. These measurements were done in vacuum for perylene cast directly onto Si, i.e., without the PTFE alignment layer.
where the sum is over all reciprocal lattice points n. LPV denotes the Lorentz, polarization, and volume corrections, all depending on the detector angles.15 Because the detector has a fixed position for in-plane rocking scans (constant Q), the LPV corrections contribute only a multiplicative factor in this case. The weighting factors Pi in (1) are peak functions of width Wi, with the second argument being the offset from the peak maximum. P1 (discussed below) measures the radial distance (Q mismatch) from the current point j to lattice point n, and the width WQ arises from mosaic spread and instrumental resolution. Similarly, out-of-plane and in-plane preferred orientations are accounted for by Lorentzians P2 and P3, respectively. Attempts of using pseudo-Voigt peak functions did not further improve the fits. The (arbitrary) zero of the azimuth angle φ is chosen to correspond to the direction perpendicular to the PTFE alignment. Results and Discussion An in-house GIXD exposure recorded for a sample without a PTFE alignment layer, thus having no in-plane orientation, is shown in Figure 1a. As the diffraction peaks are points rather than arcs, the sample has a high degree of out-of-plane orientation. By simulations it is found that this diffraction pattern corresponds to a fiber symmetry unit cell orientation with the ab plane of the unit cell in ref 13 parallel to the substrate. This geometry is sometimes referred to as a “2D powder”; the crystallites are highly oriented along one axis, but have random
Directionally Grown Perylene Crystallites
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Figure 3. (a) Sketch (top view) of the four {110} A orientations of the perylene crystallites, illustrated with one unit cell per orientation. The underlying stripe patterns indicate the 5.66 Å intrachain and 1.3 Å interchain repetition distances and orientation of the highly oriented PTFE alignment layer. Note that the size of the perylene unit cell is incommensurate with the PTFE repetitions. (b) 5 × 5 µm2 AFM amplitude image of perylene crystallites on bundles of numerous PTFE chains, the latter seen as stripes in the background. Most crystallites orient with their longest side parallel to the PTFE bundles. Examples of crystallites with facet directions consistent with orientations B and C deduced from the diffraction measurements are indicated accordingly.
orientations with respect to the substrate. In Figure 1b, a scan at the synchrotron of an aligned sample, with Q being parallel to the PTFE alignment, is shown (direction Qy). Because this is an in-plane scan (Qz ≈ 0.03 Å-1), which according to the pattern in Figure 1a should not yield any Bragg peaks, the peaks seen in Figure 1b are mosaic tails of peaks with nonzero Qz (technically, P2 in (1) contributes significantly). The detailed structural model behind the simulation is discussed below. The rather square-shaped peaks (see Figure 1b) indicate that the diffraction signal is dominated in Q by the instrumental resolution. The fit was obtained by convoluting the simulated diffraction signal, using a Gaussian of width ∼0.008 Å-1 for P1 in (1), by a square function of width 0.02 Å-1. By the Scherrer formula,17 this suggests a correlation length ξ ) 0.9 (2π/0.008) Å ≈ 700 Å. Representative in-plane rocking scans (rotation about the sample normal, Q constant) performed with synchrotron radiation are shown in Figure 2, with intriguing intensity patterns arising from the coexistence of several well-defined crystallite orientations. The main peaks “A” in Figure 2a can be accounted for by the model shown in Figure 3, comprising a family of four equally probable (energetically equivalent) crystallite orientations, which are related by two symmetry operations, a rotation of the crystallite by 180° and a mirroring about the in-plane symmetry axis provided by the aligned PTFE. All the crystallites are apparently in the R-phase and with the ab plane parallel to the substrate. The “setting angle” η of the a axis was fitted to yield η ) 46.1 ( 0.2°. This angle corresponds exactly to arctan(a/b) ) 46.17° of the unit cell of ref 13, being strong evidence that the 〈11〉ab axis of the ab plane is aligned along the PTFE, as visualized in Figure 3. This axis is almost parallel to the 〈110〉 axis of the monoclinic unit cell. Upon closer inspection of Figure 2, much weaker peaks marked “B” and “C” are seen. We find it unlikely that these minor peaks are mosaic tails of higher order peaks, remembering the narrow radial peak width of ∼0.02 Å-1 (see Figure 1b). It thus appears that the model in Figure 3 is not sufficient to account for the much weaker peaks B and C, suggesting that there are more crystallite orientations present. The fits shown in Figures 1 and 2 were obtained assuming small amounts of η
) 27.51° (B, 6%) and η ) 19.15° (C, 2%), implying that ∼92% of the intensity originates from orientations A as in Figure 3a). These less abundant crystallite orientations can be understood as arctan((a/2)/b) and arctan((a/3)/b), respectively, suggesting families with 〈21〉ab and 〈31〉ab oriented parallel to the PTFE. Again, these orientations are close to 〈210〉 and 〈310〉. As it was found that the unit cells with dimers (ref 13) describe the observed scattering well, the only fitting parameters were the orientation distribution widths, the setting angles η, and a relative weighting between the different families of crystallite orientations. The distribution widths obtained are as narrow as Wφ ) 3.29 ( 0.2° for {110}, and Wφ ) 9.9 ( 2° for the other domains. The simulations are less sensitive to Wχ; 2.5° was used. AFM shows that the crystallites have sharp facets (see Figure 3), indicating that they were formed by direct solidification from the gas phase, as an intermediary liquid phase would be expected to yield crystals with rounder shapes. The height of the perylene crystals was estimated to be 100 nm by AFM, corresponding to ∼100 unit cell repetitions. Judging from the figure, the crystallites have a lateral characteristic size of roughly 300 nm. The AFM images further show that the crystallites are elongated along the PTFE alignment, suggesting that this is the easy growth direction. In other words, it is the easiest for the crystallites to grow parallel to the bundled PTFE molecules, and the crystallites tend to be oriented to give this growth in 110 directions. This is substantiated by the fact that the aVerage size of crystallites assuming alternative orientations appears to be smaller (see Figure 3b). Thus, whereas about 92% of the scattered intensity is in accordance with an orientation having 〈110〉 parallel to the PTFE chains (PTFE c axis), the number ratio of crystallites having this orientation is smaller. A few crystallites having facet orientations differing from the majority and with angles (43.83° - 27.51° ≈ 16° for B and 43.83° 19.15° ≈ 25° for C) consistent with the orientation families suggested by diffraction are indicated in Figure 3b. Orientation patterns resembling those reported here have been reported for, e.g., sexithiophene on PTFE, using transmission electron microscopy (TEM).4 However, these observations revealed only one single orientation family, and to the best of our knowledge, patterns similar to those observed here with
4572 J. Phys. Chem. C, Vol. 112, No. 12, 2008 sharply defined, but less populated, orientations also along higher order lattice planes have not previously been reported. It is worth mentioning that because the molecules are approximately at 90° angles to each other within the unit cell, the perylene crystallites do not exhibit optical dichroism. With the presented fits and the results of Wittmann et al.4-7 in mind, it seems natural to coin the alignment mechanism as “ledge-directed nucleation”, with the ledges of the corrugated PTFE film serving as nucleation centers. This growth mode is related to graphoepitaxy.18 For those crystallite seeds having a favorable orientation (particularly 〈110〉 || 〈001〉PTFE), facilitated growth will make these crystallites grow at the expense of others. A further note on the growth mechanism is the possible interpretation that whereas the primary nucleation is with the 〈11〉ab || 〈001〉PTFE orientation, i.e., by interaction with the PTFE, the other orientations observed are formed later by epitaxial relations to the primary crystallites. Support for this conjecture is given by the AFM images, where the disoriented crystallites are always in close contact with crystallites having the dominant orientation. Conclusions In summary, perylene crystallites grown by vacuum deposition onto thin PTFE alignment substrates were found to exhibit well-defined crystal orientations, all having the ab plane parallel to the substrate. A large majority of the crystallites were oriented with a 〈110〉 axis highly parallel to the PTFE alignment. Much weaker diffraction signals were interpreted as smaller populations aligned with 〈210〉 or 〈310〉 || 〈001〉PTFE, supported by AFM. These extraordinarily detailed orientation patterns of perylene crystallites may contribute to an increased understanding of the mechanisms behind substrate-induced ordering, which is crucial information for the further development of structured functional surfaces.
Breiby et al. Acknowledgment. The Danish Research Foundation and DanSync are acknowledged for financial support, and the HASYLAB staff is acknowledged for technical assistance. References and Notes (1) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414-417. (2) Farchioni, R., Grosso G., Eds. Organic Electronic Materials; Springer-Verlag: Berlin, Heidelberg, 2001. (3) Breiby, D. W.; Bunk, O.; Pisula, W.; Sølling, T. I.; Tracz, A.; Pakula, T.; Mu¨llen, K.; Nielsen, M. M. J. Amer. Chem. Soc. 2005, 127, 11288-11293. (4) Wittmann, J. C.; Straupe, C.; Meyer, S.; Lotz, B.; Lang, P.; Horowitz, G.; Garnier, F. Thin Solid Films 1998, 333, 272-277. (5) Moulin, J-F.; Brinkmann, M.; Thierry, A.; Wittmann, J. C. AdV. Mater. 2002, 14, 436-439. (6) Brinkmann, M.; Graff, S.; Straupe´, C.; Wittmann, J. C.; Chaumont, C.; Nuesch, F.; Aziz, A.; Schaer, M.; Zuppiroli, L. J. Phys. Chem. B 2003, 107, 10531-10539. (7) Brinkmann, M.; Wittmann, J. C.; Barthel, M.; Hanack, M.; Chaumont, C. Chem. Mater. 2002, 14, 904-914. (8) Bunk, O.; Nielsen, M. M.; Solling, T. I.; van de Craats, A. M.; Stutzmann, N. J. Am. Chem. Soc. 2003, 125, 2252-2258. (9) Hansma, H.; Motamedi, F.; Smith, P.; Hansma, P.; Wittmann, J. C. Polymer 1992, 33, 647-649. (10) Dietz, P.; Hansma, P. K.; Ihn, K. J.; Motamedi, F.; Smith, P. J. Mater. Sci. 1993, 28, 1372-1376. (11) Breiby, D. W.; Hansteen, F.; Pisula, W.; Bunk, O.; Kolb, U.; Andreasen, J. W.; Mu¨llen, K.; Nielsen, M. M. J. Phys. Chem. B 2005, 109, 22319-22325. (12) Breiby, D. W.; Samuelsen, E. J. J. Polym. Sci., Part B: Polym. Phys. 2003, 43, 2375-2393. (13) Camerman, A.; Trotter, J. Proc. R. Soc. London 1964, 279, 129. (14) Kerr, A. Acta Crystallogr. 1966, 21, A119-. (15) Als-Nielsen, J.; McMorrow, D. Modern X-ray Physics; John Wiley & Sons, Ltd.: West Sussex, U.K., 2004. (16) Bunk, O.; Nielsen, M. M. J. Appl. Crystallogr. 2004, 37, 216222. (17) Warren, B. E. X-ray Diffraction; Addison-Wesley Publishing Co., Inc.: Reading, MA, 1969. (18) Givargizov, E. I. In Handbook of Crystal Growth; Hurle, D. T. J., Ed.; Elsevier Science B.V.: New York, Vol. 3, pp 943-992.
