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J. Phys. Chem. B 2009, 113, 4578–4581
Three-Dimensional Chirality Transfer in Rubrene Multilayer Islands on Au(111) Marina Pivetta,* Marie-Christine Blu¨m,‡ Franc¸ois Patthey, and Wolf-Dieter Schneider Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institut de Physique de la Matie`re Condense´e CH-1015 Lausanne, Switzerland ReceiVed: NoVember 26, 2008; ReVised Manuscript ReceiVed: January 19, 2009
The growth of rubrene (C42H28, 5,6,11,12-tetraphenylnaphthacene) multilayer islands up to a thickness of six layers on a Au(111) surface has been investigated by scanning tunneling microscopy. The molecules selforganize in parallel twin rows, forming mirror domains of defined local structural chirality. Each layer is composed of twin-row domains of the same structural handedness rotated by 120° with respect to each other. Moreover, this structural chirality is transferred to all successive layers in the island, resulting in the formation of three-dimensional objects having a defined structural chirality. The centered rectangular surface unit cell differs from the one characteristic for the single-crystal orthorhombic phase. Organic materials have been identified as potential substitutes for inorganic semiconductors in electronic devices.1-4 Rubrene (C42H28, 5,6,11,12-tetraphenylnaphthacene) is a polycyclic aromatic hydrocarbon, formed by a tetracene backbone and four out-of-plane rotated phenyl groups. Owing to its specific electronic structure, characterized by an extended system of delocalized π electrons, rubrene constitutes one of the most promising candidates for the realization of organic field-effect transistors (OFET). Indeed, OFETs based on rubrene single crystals grown by physical vapor deposition reveal very high carrier mobility.5-7 Thus, many investigations have been dedicated to the growth of rubrene crystals and to their characterization with different techniques.8-12 However, for the implementation of rubrene in large-scale device production, processing of crystalline thin films is essential. It has been reported that the choice of the substrate strongly influences the growth of rubrene thin films.13-18 Moreover, the deposition technique can also lead to different results. In particular, the conformation of the rubrene molecules may play an important role in the thin film growth. In fact, as shown in Figure 1a,b, the rubrene tetracene backbone is planar in single crystals,19,20 while it is twisted in the gas phase, conferring chirality to the molecule.13,21 Here we present the results of a scanning tunneling microscopy (STM) investigation of the growth of rubrene on a Au(111) surface. An accurate analysis of STM images displaying submolecular resolution permits identifying the surface unit cell and determining the molecular arrangement within the cell. The rubrene islands are formed by mirror-imaged rotational domains of a twin-row structure characterized by a local structural handedness. Each island is composed exclusively by either lefthanded or right-handed domains, resulting in the formation of three-dimensional objects with defined structural chirality. Owing to the twisted conformation and to the adsorption geometry of the molecules, the structure of these thin films is different from the one typical for single crystals. These results demonstrate that ordered rubrene multilayer islands grow on gold using standard evaporation conditions. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Present address: Center for NanoScience (CeNS), Ludwig-MaximiliansUniversita¨t Mu¨nchen, 80539 Munich, Germany.
Figure 1. (a) Ball-and-stick model of rubrene, formed by a tetracene backbone and four out-of-plane rotated phenyl groups. (b) In the gas phase, the tetracene backbone is twisted. (c) STM image (360 nm × 180 nm) of rubrene multilayer islands grown on Au(111). Ordered regions are surrounded by amorphous ones. (d) Detail (90 nm × 45 nm) of c. (e) STM image (90 nm × 45 nm) of a sample with lower rubrene coverage. (Inset) Fourier transform of the image in e.
Rubrene was deposited in ultrahigh vacuum from a crucible heated to ∼200 °C on a Au(111) surface held at room temperature. The measurements were carried out with mechanically cut PtIr tips and etched W tips in a home-built STM
10.1021/jp8104024 CCC: $40.75 2009 American Chemical Society Published on Web 03/06/2009
3D Chirality Transfer in Rubrene Multilayer Islands
J. Phys. Chem. B, Vol. 113, No. 14, 2009 4579
Figure 2. (a) STM image (22 nm × 11 nm) showing the detail of the twin-row structure and a layer edge. Different layers and domain orientations are visible. (b) Model reproducing the region of the uppermost layer of the island delimited by a white rectangle in a, sketched using the unit cell described in Figure 3b. A model of a molecular pair is superimposed on the STM data shown in a.
operated at a temperature of 50 K. Typical bias voltages were - 3 V g V g - 4 V at tunneling currents of I ) 20 pA. Figure 1c shows a survey STM image of rubrene grown on Au(111), where well-ordered multilayer islands are surrounded by amorphous regions. The general shape of the islands is determined by the substrate steps and terraces. The magnified view in Figure 1d shows in detail several layers of the same island, each with an apparent height of ∼0.5 nm. Different orientational domains of a stripe structure rotated by 120° with respect to each other coexist in the top layer, as well as in the successive ones. The investigation of samples with lower rubrene coverage for which both the first and second layer are visible demonstrates that the orientation of the stripe domains is related to the symmetry of the first rubrene monolayer, which has a hexagonal close-packed (hcp) structure with an intermolecular distance of ∼1.3 nm and apparent height of ∼0.25 nm.22 In Figure 1e, the second rubrene monolayer characterized by the stripe domains as well as the first one are shown. The bright spots visible on the first layer correspond to a 2 × 2 superstructure, emerging at the used tunneling conditions. The three orientations of the stripe structure coincide with the highsymmetry directions of the underlying hcp arrangement of the rubrene monolayer. The Fourier transform of the image, shown in the inset to Figure 1e, confirms that the stripe arrangement of the second layer and the hexagonal superstructure of the first one are aligned. (It has been verified that the Fourier pattern originates from both structures.) For the first hcp layer, two oblique alignments rotated by ca. (9° with respect to the Au(111) atomic lattice directions are observed on distinct terraces. This type of close-packed arrangement, reported for different classes of adsorbed molecules,23-26 induces the formation of two mirror orientations with respect to the surface atomic lattice, i.e., the first molecular layer possesses a substrate-induced orientational chirality.27,28 In order to determine the molecular arrangement in the rubrene islands, an accurate analysis of the STM data has been performed. In particular, the study of the layer edges provides useful insight. The image shown in Figure 2a reveals that the stripes are formed by paired circular protrusions, where each bright spot is identified as a single rubrene molecule. This fact is deduced from the presence of unpaired protrusions, for instance, at layer edges or at domain boundaries. The apparent intermolecular distance in the twin rows is slightly different
Figure 3. (a) Results of the statistical analysis for the unit cell: lattice parameters a and b and angle R. (b) Corresponding average centered rectangular unit cell.
for opposite directions along the stripes, leading to two possible ways of forming pairs of molecules. We define the building block of the twin rows as the two molecules which seem to be further apart in the row but which are bridged by a bright region appearing to link them. A statistical analysis performed on many distinct islands, displayed in Figure 3a, allows us to identify an average unit cell for the observed organization. A distribution of distances and angles is found for the twin-row arrangement, indicating that this structure is characterized by a certain degree of accommodation. The average unit cell shown in Figure 3b is a centered rectangular cell with a ) 2.15 ( 0.1 nm and b ) 1.25 ( 0.1 nm, containing two molecules. This rectangular unit cell is closely related to a hexagonal periodicity: indeed, it can also be viewed as a slightly distorted hexagonal structure, as demonstrated by the Fourier transform pattern shown in Figure 1e, with paired molecules forming parallel rows. Its lattice parameters almost match the ones of the hcp first monolayer. The nearest-neighbor distance between two molecules is ∼1.25 nm, very close to the typical values found for rubrene directly adsorbed on metal surfaces.22,29-32 Hence, we infer that the adsorption geometry is similar to the one observed on bare gold, i.e., with the tetracene backbone facing the surface, but tilted with respect to the surface plane.21,29 Moreover, the molecules retain the twisted conformation typical of the gas phase, as deduced from X-ray adsorption fine structure spectroscopy measurements for rubrene films of thickness smaller than ∼10 monolayers.21,33 The two nonequivalent molecules in each cell are oriented differently, as depicted in Figure 3b. This arrangement, where the two tetracene backbones form an angle of ∼30° with respect to each other, is deduced from images with submolecular resolution as the one shown in Figure 2a. Owing to the inclination of the tetracene backbone with respect to the surface, the upper part of the molecule appears as a brighter protrusion, as schematized in Figure 3b. Furthermore, the twisted conformation with a phenyl ring pointing upward can accentuate this appearance. This arrangement allows the formation of a CH · · · π bond between the edge of the backbone of the corner molecule
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Figure 4. STM images (15 nm × 23 nm) acquired on two distinct rubrene islands demonstrating the island chirality: (a) left-handed domains; (b) right-handed domains. Each island is formed by domains of only one chirality. (c) Mirror unit cells for left-handed (λ) and righthanded (F) domains.
and the twisted phenyl ring of the center one, giving rise to the bright region linking the two molecules. On the basis of this model the observed twin-row structure is reproduced satisfactorily. For example, the edge of the uppermost layer in Figure 2a, delimited by the white rectangle, is well represented in Figure 2b. The most common crystallization polymorph of rubrene is an orthorhombic phase in a herringbone arrangement of the molecules with lattice constants: a ) 2.7 nm, b ) 0.72 nm, c ) 1.44 nm.19,20 In the ultrathin islands, both the unit cell and the molecular arrangement are different from the ones expected for this single-crystal phase. This result is attributed to the conformational difference of rubrene molecules in the gas phase and in crystals and to the fact that in the present deposition conditions the molecules retain their chiral geometry.21,33 Another indication of the persistence of the molecular twisted conformation is the chiral nature of the rubrene islands. A closer look into the different molecular layers of Figure 2a reveals that, taking the row direction as reference, the paired molecules define an angle of about 45° clockwise. In both images of Figure 4a,b, acquired on two distinct islands, two rotational domains are visible. The local structural chirality is defined by the angle formed by the row direction and the paired molecules: 45° clockwise for right-handed domains, 45° counterclockwise for left-handed ones. The existence of domains of opposite structural handedness is likely related to the presence of different molecular enantiomers. A possible scenario is the following: each molecular pair in the twin rows is formed by one left- and one right-handed molecule but exchanged: the right enantiomer in the corner of the unit cell and the left one in the center for right-handed domains and conversely for left-handed domains, as indicated in Figure 4c. This kind of organization has already been reported for a racemic mixture of molecules which do not resolve spontaneously but form chiral mirror domains.26 An analysis of several distinct islands demonstrates that each of them is formed exclusively by rotational domains of the same structural chirality. Interestingly, the underlying layers present the same structural chirality as the top one, as deduced from the study of islands edges, for example, of those shown in
Pivetta et al. Figures 1d and 2a. Moreover, depending on the orientational chirality ((9°) of the first hcp layer, second layer islands with either left-handed or right-handed domain structure are formed. This behavior is deduced from the analysis of samples with low coverage, as the one shown in Figure 1e. The same structural chirality is then maintained in the successive molecular layers. This observation demonstrates that the structural handedness is not limited to monomolecular layers but that it is transferred to the third dimension, i.e., each three-dimensional rubrene island possesses a defined structural chirality. Since islands composed of both mirror-imaged structural domains are formed with equal probability, the rubrene thin film is overall nonchiral. To summarize, in this work three types of chirality have been described: (i) the chirality of single molecules, left and right enantiomers exist; (ii) the substrate-induced orientational chirality of the first hcp molecular layer; (iii) the structural chirality of the stripe domains, determined by orientation of the paired molecules with respect to the row direction. Specifically, the formation of ordered multilayer islands of rubrene on Au(111) has been observed. Owing to the twisted geometry of the rubrene molecules in the gas phase, the islands grown in the present conditions possess a structure different from the one expected for orthorhombic crystals. Moreover, the rubrene conformational adaptability permits the formation of slightly distorted lattices, as demonstrated by the statistical analysis of the unit cell. Each multilayer island is composed of rotational domains of a twinrow structure and has a defined structural chirality. Acknowledgment. This work was supported by the Swiss National Science Foundation. References and Notes (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (2) Horowitz, G. AdV. Mater. 1998, 10, 365. (3) de Boer, R. W. I.; Gershenson, M. E.; Morpurgo, A. F.; Podzorov, V. Phys. Status Solidi A 2004, 201, 1302. (4) Reese, C.; Bao, Z. N. Mater. Today 2007, 10, 20. (5) Podzorov, V.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 82, 1739. (6) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644. (7) Briseno, A. L.; Tseng, R. J.; Ling, M.-M.; Falcao, E. H. L.; Yang, Y.; Wudl, F.; Bao, Z. N. AdV. Mater. 2006, 18, 2320. (8) da Silva, D. A.; Kim, E. G.; Bredas, J. L. AdV. Mater. 2005, 17, 1072. (9) Menard, E.; Marchenko, A.; Podzorov, V.; Gershenson, M. E.; Fichou, D.; Rogers, J. A. AdV. Mater. 2006, 18, 1552. (10) Chapman, B. D.; Checco, A.; Pindak, R.; Siegrist, T.; Kloc, C. J. Cryst. Growth 2006, 290, 479. (11) Kim, K.; Kim, M. K.; Kang, H. S.; Cho, M. Y.; Joo, J.; Kim, J. H.; Kim, K. H.; Hong, C. S.; Choi, D. H. Synth. Met. 2007, 157, 481. (12) Zeng, X. H.; Zhang, D. Q.; Duan, L. A.; Wang, L. D.; Dong, G. F.; Qiu, Y. Appl. Surf. Sci. 2007, 253, 6047. (13) Ka¨fer, D.; Witte, G. Phys. Chem. Chem. Phys. 2005, 7, 2850. (14) Haemori, M.; Yamaguchi, J.; Yaginuma, S.; Itaka, K.; Koinuma, H. Jpn. J. Appl. Phys. 2005, 44, 3740. (15) Itaka, K.; Yamashiro, M.; Yamaguchi, J.; Yaginuma, S.; Haemori, M.; Koinuma, H. Appl. Surf. Sci. 2006, 252, 2562. (16) Ribicˇ, P. R.; Bratina, G. J. Vac. Sci. Technol. B 2007, 25, 1152. (17) Zeng, X. H.; Wang, L. D.; Duan, L.; Qiu, Y. Cryst. Growth Des. 2008, 8, 1617. (18) Campione, M. J. Phys. Chem. C 2008, 112, 16178. (19) Henn, D. E.; Williams, W. G.; Gibbons, D. J. J. Appl. Crystallogr. 1971, 4, 256. (20) Jurchescu, O. D.; Meetsma, A.; Palstra, T. T. M. Acta Crystallogr., Sect. B 2006, 62, 330. (21) Ka¨fer, D.; Ruppel, L.; Witte, G.; Wo¨ll, Ch. Phys. ReV. Lett. 2005, 95, 166602. (22) Blu¨m, M.-C.; Pivetta, M.; Patthey, F.; Schneider, W.-D. Phys. ReV. B 2006, 73, 195409. (23) Glo¨ckler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bo¨hringer, M.; Berndt, R.; Schneider, W.-D. Surf. Sci. 1998, 405, 1.
3D Chirality Transfer in Rubrene Multilayer Islands (24) Berner, S.; de Wild, M.; Ramoino, L.; Ivan, S.; Baratoff, A.; Gu¨ntherodt, H.-J.; Suzuki, H.; Schlettwein, D.; Jung, T. A. Phys. ReV. B 2003, 68, 115410. (25) Humblot, V.; Raval, R. Appl. Surf. Sci. 2005, 241, 150. (26) Fasel, R.; Parschau, M.; Ernst, K.-H. Nature 2006, 439, 449. (27) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (28) Ernst, K.-H. Top. Curr. Chem. 2006, 265, 209. (29) Blu¨m, M.-C.; C´avar, E.; Pivetta, M.; Patthey, F.; Schneider, W.D. Angew. Chem., Int. Ed. 2005, 44, 5334.
J. Phys. Chem. B, Vol. 113, No. 14, 2009 4581 (30) Pivetta, M.; Blu¨m, M.-C.; Patthey, F.; Schneider, W.-D. Angew. Chem., Int. Ed. 2008, 47, 1076. (31) Miwa, J. A.; Cicoria, F.; Bedwani, S.; Lipton-Duffin, J.; Perepichka, D. F.; Rochefort, A.; Rosei, F. J. Phys. Chem. C 2008, 112, 10214. (32) Miwa, J. A.; Cicoria, F.; Lipton-Duffin, J.; Perepichka, D. F.; Santato, C.; Rosei, F. Nanotechnology 2008, 19, 424021. (33) Wang, L.; Chen, S.; Liu, L.; Qi, D. C.; Gao, X. Y.; Subbiah, J.; Swaminathan, S.; Wee, A. T. S. J. Appl. Phys. 2007, 102, 063504.
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