12742
Langmuir 2008, 24, 12742-12744
Three-Dimensional Molecular Packing of Thin Organic Films of PTCDI-C8 Determined by Surface X-ray Diffraction Tobias N. Krauss,† Esther Barrena,*,†,‡ Xue N. Zhang,† Dimas G. de Oteyza,† Ja´nos Major,† Volker Dehm,§ Frank Wu¨rthner,§ Leide P. Cavalcanti,| and Helmut Dosch†,‡ Max-Planck-Institut fu¨r Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart, Germany, Institut fu¨r Theoretische and Angewandte Physik, Pfaffenwaldring 57, UniVersita¨t Stuttgart, 70550 Stuttgart, Germany, Institut fu¨r Organische Chemie, Am Hubland, UniVersita¨t Wu¨rzburg, 97074 Wu¨rzburg, Germany, and European Synchrotron Radiation Facility (ESRF), 6 Rue Jules Horowitz, 38043 Grenoble, France ReceiVed September 15, 2008. ReVised Manuscript ReceiVed October 5, 2008 We have determined the full molecular 3D packing of thin organic films of the archetypical organic n-type semiconductor N,N′-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (PTCDI-C8) by surface X-ray crystallography. We show that PTCDI-C8 forms smooth layered films on Al2O3 (11-20) with an outstanding degree of molecular order. The thin-film structure is found to consist of a triclinic unit cell with the plane of the aromatic core tilted by 67 ( 2° with respect to the surface plane, which differs significantly from the bulk structure. The 3D crystallites extend with vertical coherent order across the entire film thickness.
Research on organic semiconductors has undergone rapid progress driven by the demand for lightweight, inexpensive electronic components. The current research focus is on highperformance organic field-effect transistors (OFETs) built from small conjugated organic molecules because they offer potential low-cost building blocks for the fabrication of flexible displays or radio frequency identification (RFID) tags.1-8 Progress in the attaining OFETs with high charge carrier mobility hinges on the ability to grow organic films with the highest crystallinity and lowest defect concentration.9-13 The particular challenge in the design of functional organic thin films is related to the highly anisotropic properties of the organic molecules that, in turn, define the anisotropic properties of organic films on the * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +49 (0)711 689 1846. Fax: +49 (0)711 689 1902. † Max-Planck-Institut. ‡ Universita¨t Stuttgart. § Universita¨t Wu¨rzburg. | European Synchrotron Radiation Facility.
(1) Forrest, S. R. Nature 2004, 428, 911. (2) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. (3) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bre´das, J. L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (4) Wang, Y.; Chen, Y.; Li, R.; Wang, S.; Su, W.; Ma, P.; Wasielewski, M. R.; Li, X.; Jiang, J. Langmuir 2007, 23, 5836. (5) Horowitz, G. AdV. Mater. 1998, 10, 365. (6) Dodabalapur, A. Mater. Today 2007, 10, 24. (7) Ling, M. M.; Bao, Z. Chem. Mater. 2004, 16, 4824. (8) Facchetti, A. Mater. Today 2007, 10, 28. (9) Meyer zu Heringdorf, F.-J.; Reuter, M. C.; Tromp, R. M. Nature 2001, 412, 517. (10) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K.-C.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R. L.; Iannotta, S.; Malliaras, G. G. Chem. Mater. 2004, 16, 4497. (11) de Oteyza, D. G.; Barrena, E.; Osso´, J. O.; Sellner, S.; Dosch, H. J. Am. Chem. Soc. 2006, 128, 15052. (12) Wu, Y.; Toccoli, T.; Koch, N.; Iacob, E.; Pallaoro, A.; Rudolf, P.; Iannotta, S. Phys. ReV. Lett. 2007, 98, 076601. (13) Hlawacek, G.; Puschnig, P.; Frank, P.; Winkler, A.; Ambrosch-Draxl, C.; Teichert, C. Science 2008, 321, 108. (14) Nabok, D.; Puschnig, P.; Ambrosch-Draxl, C.; Werzer, O.; Resel, R.; Smilgies, D.-M. Phys. ReV. B 2007, 76, 235322. (15) Doi, K.; Yoshida, K.; Nakano, H.; Tachibana, A.; Tanabe, T.; Kojima, Y.; Ozaki, K. J. Appl. Phys. 2005, 98, 113709. (16) Parisse, P.; Ottaviano, B.; Delley, B.; Picozzi, S. J. Phys.: Condens. Matter 2007, 19, 106209. (17) Wu¨rthner, F.; Schmidt, R. ChemPhysChem 2006, 7, 793.
macroscopic scale. Hence, the determination of the molecular orientation and packing of organic semiconductors in the thinfilm geometry (which may be significantly different from its bulk structure) is a crucial input for modeling the electronic band structure and the associated intrinsic charge-transport properties.14-17 Only recently, the 3D structure determination of organic thin films has been made possible by virtue of controlled thin film growth and powerful analysis by surface-sensitive X-ray diffraction methods. Until now, reports have existed for the most important organic p-type semiconductor, pentacene on silicon dioxide,14,18,19 and for two other p-type organic materials.20,21 This has enabled for the first time the computation of the electronic band structure of the pentacene thin-film phase disclosing larger intermolecular bandwidths than for the bulk phase.14 In this letter, we report for the first time the 3D thin-film structure of the archetype organic n-type semiconductor N,N′dioctyl-3,4:9,10-perylene tetracarboxylic diimide (PTCDI-C8) depicted in Figure 1a. The strongly increasing interest in this organic system has been sparked by the most compelling results obtained for its OFET performance with electron field mobility values of 0.6-1.7 cm2/Vs.22,23 Nowadays, perylene diimide (PTCDI) derivatives are considered to belong to the most promising and versatile materials to fabricate n-channel OFETs.24-27 The molecules were evaporated by organic molecular beam deposition (OMBD) in ultrahigh vacuum (UHV) with a growth (18) Schiefer, S.; Huth, M.; Dobrinevski, A.; Nickel, B. J. Am. Chem. Soc. 2007, 129, 10316. (19) Yoshida, H.; Inaba, K.; Sato, N. Appl. Phys. Lett. 2007, 90, 181930. (20) Yuan, Q.; Mannsfeld, S. C. B.; Tang, M. L.; Tooney, M. F.; Lu¨ning, J.; Bao, Z. J. Am. Chem. Soc. 2008, 130, 3502. (21) Yuan, Q.; Mannsfeld, S. C. B.; Tang, M. L.; Roberts, M.; Tooney, M. F.; DeLongchamp, D. M.; Bao, Z. Chem. Mater. 2008, 20, 2763. (22) Malenfant, P. R. L.; Dimitrakopoulos, C. D.; Gelorme, J. D.; Kosbar, L. L.; Graham, T. O.; Curioni, A.; Andreoni, W. Appl. Phys. Lett. 2002, 80, 2763. (23) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Ewbank, P. C.; da Silva Filho, D. A.; Bre´das, J.-L.; Miller, L. L.; Mann, K. R.; Frisbie, C. D. J. Phys. Chem. B 2004, 108, 19281. (24) Weitz, R. T.; Amsharov, K.; Zschieschang, U.; Villas, E. B.; Goswami, D. K.; Burghard, M.; Dosch, H.; Jansen, M.; Kern, K.; Klauk, H. J. Am. Chem. Soc. 2008, 130, 4637. (25) Schmidt, R.; Ling, M. M.; Oh, J. H.; Winkler, M.; Ko¨nemann, M.; Bao, Z.; Wu¨rthner, F. AdV. Mater. 2007, 19, 3692.
10.1021/la8030182 CCC: $40.75 2008 American Chemical Society Published on Web 10/17/2008
Letters
Figure 1. (a) Scheme of the PTCDI-C8 molecule. (b) Specular X-ray intensity of PTCDI-C8 grown on Al2O3 (11-20) at 150 °C. Bragg reflections are seen with almost undamped Laue oscillations around the first, second, and third Bragg peaks corresponding to a distance between molecular layers of ∼ 20.8 Å.
rate of 3-6 Å/min on an Al2O3 (11-20) substrate kept at 150 °C. PTCDI-C8 shows a weak van der Waals interaction with the substrate, giving rise to films with an outstanding degree of crystalline order. The crystalline domains in the organic film exhibit a random azimuthal orientation on the surface. In what follows, we present 3D crystallography of the structure based on the data of PTCDI-C8 grown on Al2O3 (11-20). As a direct monitor of the film quality, we recorded the specular X-ray intensity, shown in Figure 1b, with the corresponding scattering geometry illustrated in the inset of Figure 1b. The scattered intensity exhibits Bragg reflections with well-defined Laue oscillations that are characteristic of a highly ordered lamella structure. From the data, we extract a distance between adjacent layers of d ≈ 20.8 Å (as measured at 150 °C) and a total film thickness of D ≈ 182 Å (about 8.7 layers). The width of the Bragg rocking scans reveal excellent alignment (better than 0.005°) of the (001) crystalline planes with respect to the growth direction. Films grown at room temperature exhibit similar layered texture and in-plane unit cell (as revealed by grazing incidence X-ray diffraction, Figure S1, Supporting Information). Because better structural order can be observed at 150 °C, as disclosed by more pronounced Laue oscillations, we have performed a detailed analysis of the 3D molecular packing at this temperature.
Langmuir, Vol. 24, No. 22, 2008 12743
Figure 2. (a) Reciprocal map of the PTCDI-C8 film on Al2O3 (11-20) measured by grazing incidence using a 1D detector (geometry is shown schematically in the inset). The Bragg reflections are assigned to a triclinic lattice with one molecule per unit cell (values given in the text). (b) Best agreement between the integrated intensity and the calculated intensity from the structural model.
Three-dimensional X-ray crystallography was carried out at beamline ID10B of the European Synchrotron Radiation Facility (ESRF) with a wavelength of λ ) 1.5461 Å and at an incident angle close to the critical angle of the substrate (0.20°) as schematically shown in the inset of Figure 2a.28 The scattering intensity recorded by a 1D detector is plotted as a function of the momentum transfer q|, qz parallel and perpendicular to the surface, respectively (Figure 2a). The pronounced intensity rods with Bragg reflections up to fourth order, which can be observed at different q| positions, disclose the extraordinary crystalline order of the organic film. In the first step of data analysis, the unit cell parameters have been deduced from the positions of the Bragg reflections, resulting (26) Tatemichi, S.; Ichikawa, M.; Koyama, T.; Taniguchi, Y. Appl. Phys. Lett. 2006, 89, 112108. (27) Rolin, C.; Vasseur, K.; Schols, S.; Jouk, M.; Duhoux, G.; Mu¨ller, R.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2008, 93, 033305. (28) The value of the momentum transfer parallel to the surface, q|, is selected by rotating the 1D detector around the normal to the surface. The off-specular scattering intensity collected by the 1D detector exhibits a small component in q| that has to be taken into account.
12744 Langmuir, Vol. 24, No. 22, 2008
Figure 3. (a) Structure of PTCDI-C8 thin films obtained from the analysis of the intensity of the X-ray scattering pattern in Figure 2. The aromatic PTCDI-C8 core is tilted with respect to the surface by an angle of 67°. (b) Projections of the a-c and b-c planes.
in a primitive triclinic unit cell with a ) 9.00 Å, b ) 4.89 Å, c ) 21.65 Å, R ) 95.0°, β ) 100.7°, and γ ) 112.8° where the a-b plane is parallel to the surface. The crystal structure for the PTCDI-C8 thin film differs considerably from the bulk structure recently obtained from millimeter-long needles of PTCDI-C8 (a ) 8.50 Å, b ) 4.68 Å, c ) 19.72 Å, R ) 88.43°, β ) 94.01°, and γ ) 97.21°).29 The major differences are found in the larger values of the c lattice parameter and the γ angle for the thin-film structure. The resulting Miller indices of the Bragg reflections are given in Figure 2a. In the second step, the orientation of the molecule within the unit cell was determined by a model fit to the Bragg intensities. Because the PTCDI-C8 molecule is large and strongly anisotropic, the intensity pattern is strongly modulated by the molecular structure factor. We have integrated the intensity of each Bragg reflection and then, after standard corrections,30,31 compared each to the calculated square modulus of the molecular structure factor (29) Briseno, A. L.; Mannsfeld, S. C. B.; Reese, C.; Hancock, J. M.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Nano Lett. 2007, 7, 2847. (30) Vlieg, E. J. Appl. Crystallogr. 1997, 30, 532. (31) Smilgies, D.-M. ReV. Sci. Instrum. 2002, 73, 1706.
Letters
associated with all possible rotations of PTCDI-C8 within the unit cell (Figure 2b). Six angles have been used to define the orientation of the PTCDI-C8 molecule: three rotational angles for the orientation of the aromatic core within the unit cell and three for the alkyl chains considered to be rigid (all-trans configuration).32 The molecular arrangement that gives the best agreement with the experimental intensities is shown in Figure 3. The PTCDI-C8 molecules stack in cofacial packing along the b axis with a distance between adjacent aromatic planes of 3.58 Å, which is a typical intermolecular distance of perylene molecules along the π-π stacking direction.33,34 Consequently, the overlap of the molecular π orbitals, which is crucial for charge transport, is expected to be larger along the [010] crystal direction. Thus, the anisotropic growth of PTCDI-C8 along this direction (assisted by external methods such as the use of templates or anisotropic substrates) could be exploited to induce 1D structures with enhanced effective charge carrier mobility.35-37 The molecular plane of the aromatic core is tilted by 67 ( 2° with respect to the surface plane. Note that this structure implies a tilt of each alkyl chain of 50 ( 3° with respect to the molecular plane. A similar value is found for the bulk phase. However, the tilt angle of the aromatic core relative to the a-b plane, which is more relevant for intermolecular π-π overlap, differs slightly from that of the bulk phase (69.3° between the aromatic core and surface plane).29 In addition, the average size L of the crystallites perpendicular to the surface can be assessed from the full width at half-maximum (∆q) of the Bragg reflections along qz according to L ≈ 2π/∆q. We obtain a vertical coherence length L of ∼180 Å that coincides with the film thickness. This implies that 3D crystallites extend across the entire thickness of the layered film. The determination of the molecular packing and microstructure of this organic thin film is a key input in calculating the associated electronic band structure and in modeling the charge-transport properties in organic thin film transistor devices. This is one of the essential key capabilities for a comprehensive understanding of the structure-property relationships and, ultimately, for control of the macroscopic electrical properties of polycrystalline organic thin films. Acknowledgment. We acknowledge the ESRF for the provision of the synchrotron radiation facility. We are very thankful to Stephan Hirschmann (Universita¨t Stuttgart, Germany) for purifying the molecules by vacuum gradient sublimation and to Dr. Me´lissa Delheusy for profound discussions. Supporting Information Available: In-plane scans measured by grazing incidence X-ray diffraction for PTCDI-C8 films grown on Al2O3 (11-20) at 150 °C and room temperature. This material is available free of charge via the Internet at http://pubs.acs.org. LA8030182 (32) Because this is the close-packed configuration of alkyl chains, the assumption of rigid alkyl chains can be justified, thereby keeping the number of fit parameters low. However, a certain bending (as found in the bulk structure of PTCDI-C8) or gauche defects could exist that may become detectable by extending the crystallographic study to higher Fourier components. It turns out that only two rotational angles of the alkyl chains have to be considered because of steric reasons between single atoms of adjacent molecules. (33) Ha¨dicke, E.; Graser, F. Acta Crystallogr., Sect. C 1986, 42, 195. (34) Klebe, G.; Graser, F.; Ha¨dicke, F.; Berndt, J. Acta Crystallogr., Sect. B 1989, 45, 69. (35) Briseno, A. J.; Mannsfeld, S. C. B.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Mater. Today 2008, 11, 38. (36) Tang, Q.; Li, H.; Liu, Y.; Hu, W. J. Am. Chem. Soc. 2006, 128, 14634. (37) Xiao, K.; Tao, J.; Pan, Z.; Puretzky, A. A.; Ivanov, I. N.; Pennycook, S. J.; Geohegan, D. B. Angew. Chem., Int. Ed. 2007, 46, 2650.
