Ordered Structures of Tetracene and Pentacene on Cu(110) Surfaces

Oct 29, 2003 - At low coverage, 0.7 ML, LEED shows an oval pattern, which is related to a slightly disordered surface, while STM indicates molecules i...
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Ordered Structures of Tetracene and Pentacene on Cu(110) Surfaces Q. Chen,* A. J. McDowall, and N. V. Richardson School of Chemistry and Ultrafast Photonics Collaboration, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K. Received June 15, 2003. In Final Form: September 1, 2003 The adsorption of tetracene on a Cu(110) surface has been studied with high-resolution energy loss spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Vibrational spectra confirm that at monolayer (ML) coverage the adsorbed molecules are in a flat-lying geometry with their molecular plane parallel to the substrate. At low coverage, 0.7 ML, LEED shows an oval pattern, which is related to a slightly disordered surface, while STM indicates molecules in different azimuthal orientations. At saturation coverage, a c(10 × 2) structure is formed, which gradually transfers into a p(5 × 2) structure on annealing at 340 K. STM reveals the molecular arrangement within both these unit cells and clearly indicates that the long molecular axis is aligned along 〈110〉. Molecular rows show a long-range wavelike behavior related to the interdigitation of C-H bonds of adjacent molecules. Similar structures, c(12 × 2) and p(6 × 2), are formed for pentacene on Cu(110).

1. Introduction The limitation in the use of organic materials as semiconductors in light-emitting devices is their low conductivity or the low mobility of their charge carriers. The development and control of the ordering of organic semiconductor systems is currently of considerable interest as a means of optimizing the carrier mobilities.1-4 Improving charge transport and optical confinement of these crystalline organic films depends crucially on the molecular orientation and the long-range ordering,4-9 which, in turn, determine their suitability for molecular electronic devices such as FETs, LEDs,1,10 etc. It has been suggested11,12 that improved internal ordering of the organic thin film could enhance field-effect carrier mobilities, together with increased electrical conductivity and reduced activation energy for electrical conduction. The orientation and structure of the first monolayer are particularly important in determining the structural characteristics of a thin organic film. Planar aromatic hydrocarbons typically have a shallow and broad intermolecular interaction potential energy surface dominated by van der Waals interactions. Thus, the molecule-substrate interaction can play a significant role in determining the commensurability and subsequent 3D crystalline structure of thin films.13-16 The molecular * To whom correspondence should be addressed. Fax: +44 1334-467285. E-mail: [email protected]. (1) Ozaki, H. J. Chem. Phys. 2000, 113, 6361. (2) Morozov, A. O.; Kampen, T. U.; Zahn, D. R. T. Surf. Sci. 2000, 446, 193. (3) Mu¨ller, E.; Ziegler, C. J. Mater. Chem. 2000, 10, 47. (4) Bo¨hler, A.; Urbach, P.; Scho¨bel, J.; Dirr, S.; Johannes, H. H.; Wiese, S.; Ammermann, D.; Kowalsky, W. Physica E 1998, 2, 562. (5) Toda, Y.; Yanagi, H. Appl. Phys. Lett. 1996, 69, 2315. (6) Cho, K. J.; Shim, H. K.; Kim, Y. I. Synth. Met. 2001, 117, 153. (7) Colle, M.; Tsutsui, T. Synth. Met. 2000, 111, 95. (8) Feng, W.; Fujii, A.; Lee, S.; Wu, H.; Yoshino, K. J. Appl. Phys. 2000, 88, 7120. (9) Gerasimova, N. B.; Komolov, A. S.; Aliaev, Y. G.; Sidorenko, A. G. Phys. Low-Dimens. Struct. 2001, 1-2, 119. (10) Yamaguchi, T. J. Phys. Soc. Jpn. 1999, 68, 1321. (11) Salih, A. J.; Lau, S. P.; Marshall, J. M.; Maud, J. M.; Bowen, W. R.; Hilal, N.; Lovitt, R. W.; Williams, P. M. Appl. Phys. Lett. 1996, 69, 2231. (12) Karl, N.; Marktanner, J. Mol. Cryst. Liq. Cryst. 2001, 355, 149.

arrangement of the monolayer structure reflects the balance between molecule-substrate interactions and lateral molecule-molecule interactions. The size of aromatic molecules is usually much larger than the underlying lattice of the metal substrate. Normally, the diffusion barrier for the aromatic molecules on a metal substrate is lower than or comparable to the thermal energy at room temperature. Therefore, it is likely to form ordered structure at room temperature. Previous studies have shown that, on Cu(111) surfaces, both naphthalene and anthracene molecules form ordered structures with their longer molecular axis aligned along the close-packed [110] azimuth of the substrate,17 which is also found for anthracene on Ag(111) and Ag(001) surfaces in perchloric acid solution.18 However, on the Ag(110) surface, this molecular axis is found to be perpendicular to the [110] azimuth.18 This difference might be related to the difference in lattice parameters between Cu (2.56 Å) and Ag (2.89 Å). The preference for the molecular longer axis aligned along the close-packed azimuth of the Cu single crystal indicates that the interaction between the aromatic system and the substrate atoms can be maximized by matching the lattice spacing and the distance between adjacent phenyl rings. The structure of a low coverage of pentacene on a Cu(110) surface has shown19 that the adsorbed, flat-lying pentacene molecules form straight molecular stripes separated by 28 Å, running along the 〈001〉 azimuth. Within the rows, the molecules are oriented with their longer axis parallel to the 〈110〉 azimuth. To our knowledge, only early synchrotron-based, angleresolved photoemission and X-ray absorption studies have been carried out on the tetracene/Cu(110) and indeed (13) England, C. D.; Collins, G. E.; Schuerlein, T. J.; Armstrong, N. R. Langmuir 1994, 10, 2748. (14) Lomas, J. R.; Baddeley, C. J.; Tikhov, M. S.; Lambert, R. M. Langmuir 1995, 11, 3048. (15) Chen, Q.; Rada, T.; McDowall, A.; Richardson, N. V. Chem. Mater. 2002, 14, 743. (16) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274. (17) Wan, L.; Itaya, K. Langmuir 1997, 13, 7173. (18) Shimooka, T.; Yoshimoto, S.; Wakisaka, M.; Inukai, J.; Itaya, K. Langmuir 2001, 17, 6380. (19) Lukas, S.; Witte, G.; Wo¨ll, C. Phys. Rev. Lett. 2002, 88, 028301.

10.1021/la035052r CCC: $25.00 © 2003 American Chemical Society Published on Web 10/29/2003

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tetracene/Cu(100) systems.20-22 These investigations concluded that the long molecular axis was perpendicular to the copper substrate and found evidence of a c(2 × 2) structure by low-energy electron diffraction (LEED). These results differ markedly from the results presented here, and we return to this controversy in the Results and Discussion. In this paper, we present the results of high-resolution energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and LEED studies of ordered monolayer structures of tetracene (I) and pentacene (II) on Cu(110). We demonstrate that the steric interactions play a significant role in driving the interdigitation of adjacent molecules.

2. Experimental Section Experiments were carried out in two different UHV instruments, base pressure ∼1 × 10-10 mbar, equipped with LEED and either variable-temperature (vt)-STM (Omicron) or HREELS (VSW HIB 1000 double pass spectrometer) instruments. Both systems have a Hiden quadrupole mass spectrometer to monitor the molecular deposition. The Cu(110) crystals were cut and polished mechanically, and for STM experiments electrochemically, to a mirror finish before insertion into UHV, where they were cleaned by standard Ar+ bombardment (typically, 500 eV and 30 µA cm-2 ) and annealing (773 K) procedures until a clean surface was obtained, characterized by sharp (1 × 1) LEED patterns, the absence of loss features in HREELS, and large flat terraces in STM. Tetracene (Sigma, 99%) was degassed for 3 h at 333 K before dosing. The doser consists of a glass tube heated with Ta wire (0.25 mm diameter), and a K-type thermocouple sensor is inserted into the middle of the sample, so the dosing temperature is well controlled and the reproducibility is ensured. Tetracene was dosed at 400 K, corresponding to a dosing pressure of 1 × 10-9 mbar, with the substrate at room temperature. A similar doser was used for pentacene (Sigma, 99%). The chemical was dosed at 410 K, slightly higher than that for tetracene. LEED was used to monitor the surface coverage. The IR spectrum of crystalline tetracene on a KBr disk was collected using a Nicolet 860 spectrometer.

3. Results and Discussion Focusing first on the orientation of the tetracene molecule with respect to the substrate, by vibrational spectroscopy, we present a comparison among HREELS of a monolayer of tetracene on Cu(110), the IR spectrum of the crystalline solid, previous work on matrix-isolated tetracene molecules,23and the results of ab initio calculations on an isolated molecule using GAUSSIAN 98W.24 Next, the LEED patterns of a Cu(110) surface with a monolayer tetracene are presented, which demonstrate a gradual phase transition during subsequent annealing of the surface. The STM images of both room temperature and annealed surfaces are also presented. Finally, models for these ordered structures are proposed. Previous infrared studies of tetracene have compared the measured frequencies and intensities of matrix(20) Yannoulis, P.; Frank, K. H.; Koch, E. E. Surf. Sci. 1991, 243, 58. (21) Yannoulis, P.; Dudde, R.; Frank, K. H.; Koch, E. E. Surf. Sci. 1987, 189/190, 519. (22) Yannoulis, P.; Koch, E. E.; La¨hdeniemi, M. Surf. Sci. 1987, 192, 299. (23) Hudgins, D. M.; Sandford, S. A. J. Phys. Chem. A 1998, 102, 329.

Figure 1. Comparison of the HREELS vibrational spectrum from the tetracene on a Cu(110) surface with the IR and ab initio spectra of free tetracene molecules. The EELS spectrum has been magnified against the elastic peak intensity. The CH stretch region has been further magnified.

isolated tetracene23 with those from DFT calculations, at a level of approximation and basis set similar to those we have used.25 The conclusion from those studies was that the DFT results are in excellent agreement with the measured frequencies and allow a confident assignment of the IR bands. The band intensities are also well reproduced although the relative intensity of the C-H stretching modes is significantly overestimated by the calculation. A complete tabulation of experimental and corresponding calculated frequencies and intensities is provided by Hudgins and Sandford.23 After dosing for 15 min, an ordered monolayer of tetracene is formed on Cu(110). Figure 1 shows the HREELS spectrum (the full width at half-maximum is 47 cm-1) of this surface together with the results of our ab initio calculations on the free molecule and the IR spectrum of polycrystalline tetracene. The ab initio calculations were carried out on a fully optimized free molecule with the B3LYP density functional theory26 and 6-31g basis set.24 The on-specular HREELS spectrum is completely dominated by three loss peaks at 469, 743, and 893 cm-1, with very much weaker features between 1000 and 1600 cm-1 and a low-intensity peak at 3050 cm-1. The vibrational frequencies from the EELS spectrum of the adsorbate are in excellent agreement with the IR spectrum of the solid (470, 740, and 902 cm-1), those of the matrix-isolated species (-, 743 and 895 cm-1) 23 and ab initio calculated frequencies of the free molecule (479, 763, and 911 cm-1, uncorrected), which is clear evidence that the molecules (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98W, Revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (25) Langhoff, S. R. J. Phys. Chem. 1996, 100, 2819. (26) Becke, A. D. Phys. Rev. A 1988, 38, 3098.

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Figure 2. LEED pattern of low-coverage tetracene on Cu(110) recorded at (A) 51 eV and (B) 40 eV. The sample was tilted downward to show the (0, 0) spot. (C) Idealized diffraction intensity around each integer order spot with measured dimensions in reciprocal space and (D) corresponding real space dimension.

Figure 4. (A) LEED pattern of a monolayer of tetracene on Cu(110) with a c(10 × 2) periodicity. (B) Reconstructed LEED pattern with substrate spots.

Figure 3. STM image (130 Å × 130 Å, bias voltage -0.41 V, tunneling current 0.99 nA) of the lower coverage tetracene on a Cu(110) surface. The inset shows the FFT of the image.

remain chemically intact and indeed rather little perturbed upon adsorption. An isolated, planar tetracene molecule, C18H12, belongs to the D2h point group and has 84 vibrational modes. Of these 35 are infrared active and can be classified into three groups, each characterized by a different orientation of the dynamic dipole relative to the molecular axes. b1u (b2u) modes are polarized parallel to the short (long), inplane axis of the molecule, while b3u modes are polarized perpendicular to the molecular plane. Assignments based on the ab initio calculations show that the three strong modes observed in EELS are exclusively associated with out-of-plane bending modes of b3u symmetry. The first of these at 469 cm-1 is an out-of-plane deformation of the carbon skeleton, while the two higher ones are out-ofplane C-H wagging modes. For tetracene, the IR spectrum of the solid and that of the matrix-isolated species indicate that the in-plane modes are inherently much weaker than the strong outof-plane modes around 740 and 900 cm-1. For example, the strongest in-plane modes at 1120 and 1290 (two bands combined), 1389, and 1465 cm-1 have intensities of 11%,

19%, 27%, and 15%, respectively, compared with the 740 cm-1 mode. However, in EELS the relative intensity of these modes is much lower still, reaching at most 1-2%. Also, focusing on the C-H stretching region around 3050 cm-1, shown magnified in Figure 1, the IR data suggest an intensity around 12% that of the 740 cm-1 mode, whereas the EELS has an intensity less than 0.2%. Even allowing for the bias in EELS toward low-frequency modes, such a high intensity ratio in EELS can be attributed only to the fact that most of the molecules are in flat-lying geometry relative to the substrate with excitation of inplane modes forbidden by the surface selection rule. The weak intensity of the in-plane modes could even be contributed from other electron scattering mechanisms, such as the impact scattering27 and the negative resonance,28 or contributed from the molecules adsorbed on the surface defects with different orientations. Our conclusion from the EELS that, up to monolayer coverage, the molecules take up an orientation in which the molecular plane is parallel to the substrate, maximizing van der Waals interactions with the substrate, agrees with the widely accepted view for adsorption of aromatics on many metal surfaces, including that proposed for pentacene on Cu(110).19 However, it is in clear contradiction to earlier studies of tetracene adsorption on Cu(110) and Cu(100) surfaces using X-ray absorption and angleresolved photoemission.20-22 A clue to understanding this discrepancy may be that the coverage was only poorly controlled in the earlier work, where it was suggested that 2-3 monolayers were present, on the basis of quartz (27) Frederick, B. G.; Jones, T. S.; Pudney, P. D. A.; Richardson, N. V. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 115. (28) Chen, Q.; Frederick, B. G.; Richardson, N. V. J. Chem. Phys. 1998, 108, 5942.

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Figure 5. (A-E) A series of LEED patterns recorded at different annealing temperatures. (F) Diffraction intensity measured along the half-order spots.

crystal microbalance measurements. Recent and preliminary evidence from IR measurements of tetracene adsorption on Cu(110), which we have undertaken, suggests that deposition beyond one monolayer, which is stable at room temperature in UHV, triggers a structural phase transition involving reorientation of the molecule in the first and subsequent layers such that b2u modes are observed.29 This would suggest that the long molecular axis is perpendicular to the surface as proposed in the earlier study. Parts A and B of Figure 2 show the LEED patterns of the room-temperature low-coverage surface recorded at 51 and 40 eV. An oval shape is found surrounding the integer order spots of the substrate. The lack of distinctive diffraction spots suggests poor long-range ordering. The oval pattern has dimensions along the principal axes of 0.15a and 0.35b in reciprocal space (Figure 2C), where a and b are the reciprocal axes of the substrate unit cell (29) McDowall, A. J.; Francis, S. M.; Richardson, N. V. Unpublished results.

along the 〈110〉 and 〈001〉 azimuths, respectively. These two dimensions of the oval correspond to lengths of 17.0 and 10.3 Å in real space, shown in Figure 2D. The free tetracene molecular plane has a rectangular shape with van der Waals dimensions of 13.7 Å × 7.0 Å. This suggests that the mean spacing between neighboring molecules is larger than the van der Waals dimension but there is preferential alignment of the long molecular axis along 〈110〉. This scenario is confirmed with STM observation of the corresponding surface (Figure 3). The inset shows the FFT of the image, which closely resembles the corresponding LEED pattern. Most of the molecules (90%) are aligned with their longer axis aligned along the 〈110〉 azimuth, while occasionally molecules (10%) can be found being aligned along the 〈001〉 azimuth. The space between molecules suggests that the lateral intermolecular interactions are rather weak or even repulsive; otherwise, closely packed islands should be formed. On the other hand, the azimuthal alignment of the molecular axes is relatively well defined, which suggests a preferential

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Figure 6. (A) STM image (530 Å × 290 Å, bias voltage 0.06V, tunneling current 1.68 nA) of the c(10 × 2) periodicity of the tetracene/Cu(110) surface. (B) STM image of a magnified small area (37 Å × 37 Å). (C) FFT image of the STM image. (D) 1D autocorrelation function profile measured along the 〈001〉 azimuth.

molecule-substrate interaction. The high probability for the longer molecular axis parallel to the 〈110〉 azimuth suggests that this interaction is more favorable than that for the molecular longer axis parallel to the 〈001〉 azimuth. The image also shows some streaks along the scanning direction (45° against the 〈110〉 azimuth), which might be due to tip-induced diffusion of surface species. At saturation coverage, a closely packed ordered structure is formed. Figure 4 shows the LEED pattern of this surface recorded at 19 eV at room temperature. The sample was tilted slightly downward to show the (0, 0) spot. Figure 4B shows the idealized LEED pattern together with substrate spots not visible in Figure 4A. The ordered structure has a c(10 × 2) periodicity with elongated spot profiles along the 〈001〉 azimuth, which indicates that the ordering along the 〈001〉 azimuth is poorer than that along the 〈110〉 azimuth. The c(10 × 2) periodicity corresponds to a rectangular unit cell with dimensions of 25.6 Å × 7.2 Å, containing 20 Cu atoms. Because the flat-lying molecule has a projected area of 95.9 Å2 () 13.7 Å × 7.0 Å), it is likely that the centered unit cell contains two equivalent molecules. Thus, the surface coverage at this stage is 0.1 molecule per Cu atom. There is some slight diffraction intensity between the first half-order spots, which is possibly contributed by some p(5 × 2) structure. Nevertheless, the overall surface is dominated by the c(10 × 2) periodicity. The ordering and spacing at saturation coverage are expected to be driven by a combination of commensurate molecule-substrate interactions compatible with the van der Waals size of the molecule. Annealing the surface to 340 K, a gradual phase transition from the c(10 × 2) to p(5 × 2) periodicity is observed. Figure 5 shows a sequence of LEED patterns as a function of surface temperature. The intensity profiles measured along the 〈110〉 azimuth along the half-order spots are shown in Figure 5F. As the surface temperature increases, the (1/10, 1/2) spots’ intensities decrease, while the intensity of the (0, (1/2) spots increases. Clearly, the nature of this phase transition arises from molecular rearrangement along the 〈001〉 azimuth, while the peri-

odicity along the 〈110〉 azimuth is maintained during the phase transition. Finally, a primitive (5 × 2) periodicity is formed at 340 K, which is thermally stable up to 600 K. Above this temperature, molecules decompose and/or desorb from the surface and no LEED pattern can be recovered after cooling to room temperature. The p(5 × 2) periodicity has dimensions of 12.8 Å × 7.2 Å, which is comparable to that of a single molecule. Therefore, the orientation of the tetracene molecule is also strictly defined. Although the periodicity at this stage is different from the room-temperature structure, the surface coverage is unchanged. The STM image of a large area (660 Å × 340 Å) of the c(10 × 2) structure is shown in Figure 6A, while the enlarged small-area image (60 Å × 60 Å) is shown in Figure 6B. A very low bias voltage (