pubs.acs.org/Langmuir © 2010 American Chemical Society
Pentacene Films on Cu(119) E. Annese,*,† I. Vobornik,† G. Rossi,†,‡ and J. Fujii† †
TASC Laboratory, IOM-CNR, SS 14, km 163.5, I-34149 Trieste, Italy, and ‡Dipartimento di Fisica, Universit a di Modena e Reggio Emilia, via Campi 213/A, I-41100 Modena, Italy Received September 10, 2010. Revised Manuscript Received November 10, 2010
The molecular structure of thin pentacene film grown on a Cu(119) surface has been studied by near-edge X-ray absorption fine structure spectroscopy and scanning tunneling microscopy. The interaction between the π-molecular orbitals delocalized on the aromatic rings and the underlying copper substrate was deduced from XAS spectra. Pentacene molecules arrange with the main axis almost parallel with the Cu terraces according to the measured polarization dependence of the C 1s absorption spectra. For thickness exceeding 4 nm an upright arrangement of the molecules was observed with a dense herringbone-like ordering. The present study thus demonstrates that highly ordered pentacene films can be obtained on a Cu(119) vicinal surface both in a flat orientation for low coverages and in a bulk-like herringbone orientation for higher coverages.
Introduction Substantial research has sought to understand the diverse structural1-4 and electronic properties5 of the molecular film grown on insulator and metallic substrates since molecular films are increasingly implemented as a constituent of electronics device (organic transistors, solar cell, etc.).6,7 Polyacenes, π-conjugated organic molecules, made up of benzene rings, are promising candidates among other molecules since conjugation provides a good conduction channel. Furthermore, the small size of the molecule permits a joint experimental and theoretical research approach. The growth mode of the molecular film has been demonstrated to affect its final electronic structure and transport properties.8 As a consequence, understanding and controlling the molecular orientation and packing are preliminary to the optimization of the properties of the organic film. Pentacene has attracted the interest of the scientific community because of its highly anisotropic charge transport and high carrier mobility. In bulk phase, pentacene has a layered herringbone-like structure,2 whereas at interfaces with single crystalline substrates the first layer molecules lie down when deposited on strongly interacting metallic surfaces4,9,10 and stand up when deposited on weakly interacting substrates.9,11 The molecular arrangement of the first molecular layer is the result of competition between molecule-substrate and lateral molecule-molecule interactions. Several periodic structures can *Corresponding author. E-mail:
[email protected].
(1) Danisman, M. F.; Casalis, L.; Scoles, G. Phys. Rev. B 2005, 72, 85404. (2) S€ohnchen, S.; Lukas, S.; Witte, G. J. Chem. Phys. 2004, 121, 525–534. (3) K€afer, D.; Ruppel, L.; Witte, G. Phys. Rev. B 2007, 75, 085309–13. (4) Satta, M.; Iacobucci, S.; Larciprete, R. Phys. Rev. B 2007, 75, 155401– 155411. (5) Ueno, N.; Kera, S. Prog. Surf. Sci. 2008, 83, 490–557. (6) Sekitani, T.; Takamiya, M.; Noguchi, Y.; Nakano, S.; Kato, Y.; Sakurai, T.; Someya, T. Nature Mater. 2007, 6, 413. (7) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Nature Mater. 2009, 8, 494–499. (8) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. Dev. 2001, 45, 11. (9) Chiodi, M.; Gavioli, L.; Beccari, M.; Di Castro, V.; Cossaro, A.; Floreano, L.; Morgante, A.; Kanjilal, A.; Mariani, C.; Betti, M. G. Phys. Rev. B 2008, 77, 115321–7. (10) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274–1281. (11) Thayer, G. E.; Sadowski, J. T.; Meyer zu Heringdorf, F.; Sakurai, T.; Tromp, R. M. Phys. Rev. Lett. 2005, 95, 256106.
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form at room temperature.10,12 Thermal annealing triggers the transition between different structures, inducing the monolayer reorganization or partial desorption.13,14 The evolution upon further deposition beyond the first layer depends on the competition between the interface bonding and the most stable configuration of pure pentacene crystal. As regards the growth of pentacene films, previous studies about their structure and morphology yielded information on the molecular layer spacing, domain size, and film roughness.3,15,16 It is well-known that first ordered molecular layer favors the formation of stable multilayer structures.2,4 On the other hand, there are also evidence of growth of ordered multilayer in presence of disordered interfaces17 and sensitivity to the kinetic conditions of the molecule deposition (i.e., kinetic energy control of the molecular beam).18 Here, we investigated the growth of pentacene film on Cu(119) by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM), with the aim to determine the molecular orientation at the surface of pentacene at subsequent stages of deposition from the interface layer to a multilayer thickness and to clarify the role of polyacenesubstrate and molecule-molecule interactions in the formation of organic films.
Experimental Section The pentacene film was grown on Cu(119), vicinal surface of Cu(001) with 11.45 A˚ wide terraces separated by monatomic steps. The substrate preparation and the film growth procedure are described in ref 19. During the preparation of the molecular (12) M€uller, K.; Kara, A.; Kim, T. K.; Bertschinger, R.; Scheybal, A.; Osterwalder, J.; Jung, T. A. Phys. Rev. B 2009, 79, 245421. (13) Baldacchini, C.; Allegretti, F.; Gunnella, R.; Betti, M. G. Surf. Sci. 2007, 601, 2603–2606. (14) McDonald, O.; Cafolla, A. A.; Li, Z.; Hughes, G. Surf. Sci. 2006, 600, 1909– 1916. (15) Bouchoms, I. P. M.; Schoonveld, W. A.; Vrijmoeth, J.; Klapwijk, T. M. Synth. Met. 1999, 104, 175. (16) Gundlach, D. J.; Jackson, T. N.; Schlom, D. G.; Nelson, S. F. Appl. Phys. Lett. 1999, 74, 3302. (17) Eremtchenko, M.; Temirov, R.; Bauer, D.; Schaefer, J. A.; Tautz, F. S. Phys. Rev. B 2005, 72, 115430. (18) Casalis, L.; Danisman, M. F.; Nickel, B.; Bracco, G.; Toccoli, T.; Iannotta, S.; Scoles, G. Phys. Rev. Lett. 2003, 90, 206101. (19) Annese, E.; Viol, C. E.; Zhou, B.; Fujii, J.; Vobornik, I.; Baldacchini, C.; Betti, M. G.; Rossi, G. Surf. Sci. 2007, 601, 4242.
Published on Web 11/23/2010
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Figure 1. Scheme of NEXAFS experimental geometry: the orientation of the electric field vector E varies from parallel to upright to surface depending on the incidence angle of the synchrotron light. E can be also switched out of the normal plane of synchrotron radiation source. Insets ii and iii represent the angle between long (short) molecular axis and the substrate. The short and long molecular axes are within the molecular plane. film the substrate was kept at room temperature (RT). The thickness of the sample was monitored by quartz microbalance. The deposition rate was 1 A˚/min. The experiments were performed at the APE beamline of IOMCNR at the ELETTRA Synchrotron Radiation Facility (Trieste), delivering both linearly and circularly polarized light.20 Near-edge X-ray absorption fine structure (NEXAFS) measurements were obtained by measuring the sample drain current with an energy resolution of 150 meV. In this experiment we used linearly polarized radiation oriented in the storage ring plane or perpendicular to it. We will call those cases linearly horizontal (HP) or linearly vertical (VP) polarized light. NEXAFS experiments were performed in the following geometries (see Figure 1): (i) the photon beam is perpendicular to the substrate surface (normal incidence, i.e., θ = 0°) and the electric vector of the light is perpendicular (VP) or parallel (HP) to the step edges; (ii) photon beam impinges on sample surface at number of angles (θ = 0°, 15°, 30°, 45°, 60°) with respect to its normal and with electric vector in the plane containing the light direction and normal surface (HP). In the last geometry the electric vector is oriented from parallel to almost perpendicular with respect to the sample surface depending on θ. X-ray photoemission (XPS) spectra data have been acquired by a seven channel Omicron EA-125 analyzer at normal emission using linearly polarized X-rays with photon energies of 370 eV. The overall energy resolution (photons þ analyzer) was about 200 meV. Scanning tunneling microscopy (STM) images were acquired with a homemade STM, operating in UHV with atomic resolution in the connected suite of UHV chambers at the APE beamline.20 STM measurements were acquired in constant current mode at room temperature (RT) with a tungsten tip, which is capable of atomic resolution. All STM images are acquired at RT. The sample bias voltage is referred with respect to tip.21
Results and Discussion In panel a of Figure 1 selected C 1s core level photoemission spectra versus pentacene thickness are reported. The line shape modification of XPS spectrum from interface to thin film is the fingerprint of site specific change due to different molecular environment of the C atoms within the single molecule and of a molecule-substrate interaction at the interface involving different strength of pentacene C atoms.13,22 Hereafter, we identify the (20) Panaccione, G.; Vobornik, I.; Fujii, J.; Krizmancic, D.; Annese, E.; Giovanelli, L.; Maccherozzi, F.; Salvador, F.; De Luisa, A.; Benedetti, D.; Gruden, A.; Bertoch, P.; Polack, F.; Cocco, D.; Sostero, G.; Diviacco, B.; Hochstrasser, M.; Maier, U.; Pescia, D.; Back, C. H.; Greber, T.; Osterwalder, J.; Galaktionov, M.; Sancrotti, M.; Rossi, G. Rev. Sci. Instrum. 2009, 80, 043105. (21) Horcas, I.; Fernandez, R.; Gomez-Rodrı´ guez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705. (22) Floreano, L.; Cossaro, A.; Cvetko, D.; Bavdek, G.; Morgante, A. J. Phys. Chem. B 2006, 110, 4908–4913.
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sample with a thickness greater than 4 nm as “>4 nm”. This sample shows a XPS line shape similar to one of ∼5 nm of ref 13. STM was used to image the structure of pentacene on Cu(119). Figure 1b-d shows the images measured for 0.3 (1 monolayer, 1 ML), 2, and 4 nm. Because of the in-plane anisotropy of the Cu(119) surface, no rotational domains are present on this substrate in contrast to, for example, Au(111) where three rotational domains have been observed.10,23 The images confirm some previous STM experiments of pentacene film grown on Cu(119) with molecules aligned along the step directions for pentacene coverage lower and/or equal to 1 ML.24 In contrast to pentacene grown on Cu(110), where different structural phases are observed for coverages between 0.2 and 1 ML while using the same growth conditions,12 pentacene 1 ML grown on Cu(119) shows a unique phase. The preferential adsorption direction of the molecules has been already observed upon deposition of slightly more than 1 ML25 and for film thickness up to 2 nm (Figure 1b, ref 19). A two-dimensional unit cell of the observed row structure is drawn in Figure 1b,c. It contains one molecule and has dimensions of a = 7.5 ( 0.8 A˚ and b = 18 ( 0.8 A˚. The same dense packed molecular structure is observed at 4 nm (Figure 1c). Hereafter, we will refer to this growth mode of pentacene on Cu(119) as phase A. This phase can be referred to the “lines” and “wave” phases whose energy is calculated in ref 4. The dense lines phase fills the terraces and the weavy charater in the images may be due to slight disalignment of different terraces. In any case we do not observe the “brick” phase where staggered rows of pentacene are side-by-side halfway displaced along the molecular axis. At coverage >4 nm we observe a high degree of order (panels e and f of Figure 2). The molecules appear as elongated balls with a well-defined periodic arrangement different from the one observed in Figure 2b-d, indicating that a new phase is formed at this stage of film growth. To understand the molecule’s orientation in this film, we consider STM image measured on 2 nm pentacene film grown on Cu(100), reported in panel g of Figure 2. A unit cell with lattice parameters (7.3 5.7 A˚2) describes the periodicity of this molecular film. The small variation in images aspect (Figure 2e-g) is due to tip-induced effect and to the different bias used to measure the images. In the case of pentacene multilayer grown on Cu(100), the strong linear dichroism in the NEXAFS measurements4 is compatible with an angle between the normal to the aromatic ring and the normal to the surface of 65°, giving evidence of a dominant ordering with upright molecules. By comparison with pentacene film grown on Cu(100), the densely packed film on Cu(119) arranges in rectangular adsorption structure with molecules oriented upright with respect to the substrate surface. In order to better characterize this geometry, we define a unit cell between nearest neighboring molecules and measure the lattice parameters of 7.4 6.5 A˚2, as indicated in panel f of Figure 2. Each cell contains two pentacene molecules. The periodicity of upright pentacene molecules on Cu(119) is similar to the one observed2 in pentacene film grown on Cu(110) with thicknesses higher than 2 nm, where the molecules form an angle of 73° with respect to the substrate surface and the molecular side-to-side distances along directions in the surface plane are 7.4 and 5.4 A˚. A triclinic unit cell of a = 7.9 A˚, b = 6.1 A˚, and c = 16.0 A˚ (with angles R = 102°, β = 113°, and γ = 86°) has (23) Gavioli, L.; Fanetti, M.; Sancrotti, M.; Betti, M. G. Phys. Rev. B 2005, 72, 035458. (24) Annese, E.; Fujii, J.; Baldacchini, C.; Zhou, B.; Viol, C. E.; Vobornik, I.; Betti, M. G.; Rossi, G. Phys. Rev. B 2008, 77, 205417. (25) Campbell, R. B.; Robertson, J. M.; Trotter, J. Acta Crystallogr. 1961 14, 705.
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Figure 2. Growth of pentacene film on Cu(119) as a function of film thickness. (a) C 1s core level photoemission spectra recorded with photon energy of 370 eV. The spectra were aligned to a common energy scale by measuring the Fermi edge. The evolution of the C 1s feature vs thickness: 0.3 nm (thin line), 2 nm (dash line), >4 nm (thick line). (b-e) STM images of pentacene at dose: 0.3 nm (a), 2 nm (b), 4 nm (c) and >4 nm (d). Pentacene adopts a periodic, highly ordered arrangement with molecule flat on the substrate (b-d). The pentacene unit cell is drawn in panels c and d. (e) High-resolution STM image shows individual pentacene molecule upright with respect to the substrate surface (>4 nm). (f ) Zoomed imaged of (>4 nm) sample in a different point. A unit cell is traced with two pentacene molecules per cell. (g) STM image of pentacene film (thickness: 2 nm) grown on Cu(100). The step edges of Cu(119) substrate are aligned along the [110] direction and is indicated by the arrow in the images.
Figure 3. (a) C 1s NEXAFS spectra of pentacene in gas phase (blue line, cartoon from ref 28) and pentacene films on Cu(119) of thickness 4 nm (red curve) and >4 nm (black curve). NEXAFS spectra (b-d) were recorded for various orientations of the electric field vector E parallel and upright to surface by using different angles of incidence of the synchrotron light according to the experimental geometry shown in Figure 1i. (b-d) Representative spectra at θ = 0° (thin line), θ = 30° (midspan line), and θ = 60° (thick line) are shown for thicknesses ranging from 0.3 to 4 nm. (e-g) C 1s NEXAFS spectra recorded at θ = 0° with orientation of the electric field vector E perpendicular (blue) and parallel (black) to the step edges. (h) C 1s NEXAFS spectra recorded at θ = 0° with orientation of the electric field vector E perpendicular to the step edges in the π region.
been reported for bulk pentacene (two molecules per unit cell).26,27 A comparison with the density of bulk pentacene suggests that the rectangular unit cell is not primitive yielding a herringbone-type arrangement. Hereafter, we will refer to this stage of growth of pentacene on Cu(119) as phase B, i.e., a dense phase with herringbone packing of pentacene molecules almost perpendicular to the substrate surface and to the underlying parallel-line molecular layers. The B phase appears for thickness >4 nm and changes drastically the lattice parameter perpendicular to the surface, requiring much higher deposit time in order to fill a packed layer, or a collective reorientation of the top underlying layers. As the cohesion energy of the pentacene film depends primarily on density, the competition (26) Tiago, M. L.; Northrup, J. E.; Louie, S. G. Phys. Rev. B 2003, 67, 115212. (27) St€ohr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891–7905.
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between the two dense phases (lines and herringbone) is expected to determine the extension and the location of the interface between phase A and B. NEXAFS measurements were performed on the samples investigated by STM (Figure 2b-d). NEXAFS probes the LUMO orbital of the molecules and reveals the local bonding environment around specific atoms and the chemical state of these atoms (line shape of the spectra). Figure 3 shows representative C 1s NEXAFS spectra for free pentacene in gas phase (blue curve in panel a) and for pentacene dense packed layers as grown on Cu(119) corresponding to different thicknesses ranging from 0.3 (d) to 4 nm (b) (phase A) as measured with electric horizontal polarization (geometry ii). Like other polycyclic hydrocarbons, the NEXAFS spectra of pentacene films are characterized by two different energy regions: the first with well-resolved peaks below 290 eV relative to the 1s-π* transitions Langmuir 2010, 26(24), 19142–19147
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corresponding to LUMO and LUMOþ1 (π* resonance); the second with broad features at higher photon energies are due to the transition 1s-σ* transitions (σ* resonance). In the gas phase C 1s NEXAFS spectrum of pentacene, the different structures below 290 eV reflect π* transition of the six unequivalent C atoms.28 In the adsorbed molecule on a substrate (monolayer), the C K edge presents a strong line shape modification at energies corresponding to the transition to LUMO orbitals, while the LUMOþ1 are less influenced.9,2 The pronounced electronic distortion of the molecular orbitals revealed in NEXAFS measurements upon adsorption is in agreement with the partial filling of the LUMO observed in ARPES spectra.29,30 Unlike pentacene films grown on Cu(110), where a gradual change in NEXAFS line shape occurs, pentacene grown on Cu(119) show similar NEXAFS line shape from the monolayer (0.3 nm) up to 4 nm thick films. The different behavior of pentacene on Cu(119) and on Cu(110) can be attributed to first molecular interface layer. Both experiments and calculations indicate a strong interaction of the pentacene π orbitals with Cu(100) and Cu(119) d bands,30 which results in the hybridization of molecular state with substrate and a partial occupation of LUMO. On the other hand, the strength of molecule-substrate interaction is larger in the case of Cu(119) with respect to Cu(110). Upon adsorption of 1 ML of pentacene, a binding energy shift of Cu(110) surface state occurs toward higher binding energy, which indicates either an electron transfer from molecule to the substrate or a surface state/bulk state mixing.31 The differences in the details of molecule-substrate interaction of the interface layer have consequences on the further growth. We have in fact evidence that the phase A (flat lines) is dense and grows up to 4 nm thickness. Hybridization with copper is certainly limited to the interface layer, but the intralayer bonding of pentacene in the A phase remains strong up to a critical thickness that is much larger than found in Cu(110). In the A phase the upper layer molecules progressively lose the strength of the π-bonding and acquire some angular amplitude both rotating about the long axis and increasing the angle with respect to the surface plane. The C 1s photoemission line shape at 2 nm is broadened because of the coexistence of of chemically shifted C 1s . The overall symmetry of the density of states probed by C K-edge XAS remains nevertheless stable up to 4 nm and characteristic of the phase A. We stress that in cases where the interface interaction is weak, like for pentacene grown on hexagon boron nitride nanomesh and La0.7Sr0.3MnO3 substrates,32,33 the NEXAFS C 1s spectral shape for pentacene is strongly reminiscent of the free pentacene spectrum. In our case only for the film >4 nm the C 1s XAS spectrum displays all the features of the free molecule (Figure 3a). According to the dipole approximation, the absorption intensity is proportional to the square of the scalar product of the electric field of the X-rays and the direction of the final state (28) Alagia, M.; Baldacchini, C.; Betti, M. G.; Bussolotti, F.; Carravetta, V.; Ekstr€om, U.; Mariani, C.; Stranges, S. J. Chem. Phys. 2005, 122, 124305. (29) Ferretti, A.; Baldacchini, C.; Calzolari, A.; Di Felice, R.; Ruini, A.; Molinari, E.; Betti, M. G. Phys. Rev. Lett. 2007, 99, 046802. (30) Baldacchini, C.; Mariani, C.; Betti, M. G.; Vobornik, I.; Fujii, J.; Annese, E.; Rossi, G.; Ferretti, A.; Calzolari, A.; Di Felice, R.; Ruini, A.; Molinari, E. Phys. Rev. B 2007, 76, 245430. (31) Scheybal, A.; M€uller, K.; Bertschinger, R.; Wahl, M.; Bendounan, A.; Aebi, P.; Jung, T. A. Phys. Rev. B 2009, 79, 115406. (32) Ng, M. L.; Preobrajenski, A. B.; Zakharov, A. A.; Vinogradov, A. S.; Krasnikov, S. A.; Cafolla, A. A.; Ma˚rtensson, N. Phys. Rev. B 2010, 81, 115449. (33) Li, F.; Graziosi, P.; Tang, Q.; Zhan, Y.; Liu, X.; Dediu, V.; Fahlman, M. Phys. Rev. B 2010, 81, 205415. (34) Floreano, L.; Cossaro, A.; Gotter, R.; Verdini, A.; Bavdek, G.; Evangelista, F.; Ruocco, A.; Morgante, A.; Cvetko, D. J. Phys. Chem. C 2008, 112, 10794. (35) Yannoulis, P.; Dudde, R.; Frank, K. H.; Koch, E. E. Surf. Sci. 1987, 189-190, 519–528.
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orbitals when exciting from s-like core levels. The spatial distribution of maximum (minimum) unoccupied molecular orbitals (π* or σ*) are detected by varying the relative position of the electric field vector of linearly polarized X-rays and final state orbital (search light effect), hence by rotating the sample surface with respect to X-ray beam (variation of the θ angle in Figure 1) or by varying the polarization of the incoming light. Since π* or σ* orbitals are defined relative to the molecular symmetry axis or plane, the angular dependence of π* and σ* resonance intensities directly reflects the orientation of molecules on the substrate. This search light effect averages to zero when the molecules are oriented randomly. In the case of pentacene adsorbed on Cu(119), the substrate gives a preferential direction for the adsorption of the molecules along the step edges as shown from STM images and previous results.19,24,25 The 1s-π* transition has a maximum when the electric field and final state orbital are aligned (electric field and the benzene ring normal direction are aligned), i.e., at grazing incidence for flat lying molecules, whereas π* resonance has a minimum at normal incidence (θ = 0). The opposite verifies for the 1s-σ* transition. In Figure 3b-d NEXAFS spectra of pentacene film grown on Cu(119) are displayed measured for θ = 0°, 30°, and 60° with the electric field vector E of the synchrotron light in the plane defined by the surface normal and light direction (see Figure 3a). NEXAFS data show a pronounced dichroism of π* region, indicating the high degree of molecular orientation of the pentacene (see Figure 3b-d). The quantitative analysis of the π-resonance angular-dependent intensities gives an average value of the molecule tilting angle R of 5° for 2 and 4 nm sample, whereas 0° for 0.3 nm (R is the angle formed by the normal to the aromatic ring and the normal to the surface). In Figure 3e-g NEXAFS spectra of pentacene film measured at θ = 0° and with polarization vector parallel (HP) and perpendicular (VP) to the step edges. A dichroism is observed by switching the polarization from HP to VP. The latter polarization probes molecular electronic distribution along the other molecular axis (short one). Panel h of Figure 3 shows the zoom of π* resonance for spectra measured with VP linear polarization for three thicknesses and structures at 284.5 and 285 eV. The intensity ratio of these peaks measured with VP and HP, respectively, is converted in j angle ∼10° for 0.3 nm and 15° for 2-4 nm. In the case of unperturbed molecule the dichroism obtained by changing the polarization reveals the angle j between substrate surface and molecular plane. In the present case, this dichroism can be explained satisfactorily as an angular rearrangement j of the molecules as well as the perturbation due to strong interaction with the substrate. However, we propose the first one in the light of previous results of pentacene growth on Cu(110), where the molecules show a tilting of pentacene molecule around its long and short molecular axes in a transition phase before molecules arrange in an upright configuration.2 In panel a is also reported the NEXAFS spectra for phase B (black line) measured at θ = 60°. A change in the spectral line shape occurs, and features resembling the one of gas phase spectra appear in the spectra. To enhance the spectral features related to the orientation of the molecules for growth modes A and B, we report in Figure 3 the difference NEXAFS spectra. Those spectra are obtained by subtracting the absorption intensity measured at polar angle θ = 60° from the one measured at θ = 0° and are displayed in the π* resonance region. The spectra relative to thicknesses ranging from 0.3 to 4 nm show a change in the line shape in low photon energy region (indicated by an arrow in the Figure 4a), a signature of DOI: 10.1021/la1036376
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Figure 4. (a) Difference NEXAFS spectra obtained by subtracting the absorption intensity measured at θ = 60° and at θ = 0° for 0.3 nm (thin line: 1), 2 nm (dash line: 2), 4 nm (thick line: 3), >4 nm (red line: 4). (b) Spectrum (thick line) difference of (3)-(1) spectra of panel (a). (c) Spectrum (red line) difference of (4)-(1) spectra of panel a.
redistribution of the π* state going from monolayer to thicker film. Thanks to NEXAFS probing depth (several nanometers at C absorption edge) it is possible to observe in the difference spectra contributions relative to (i) molecules in the outmost molecules layer (upright) and (ii) those of layers underneath. A positive sign is expected in correspondence of flat lying molecules, whereas a negative sign in the case of upright molecules. In the case molecules preserve the same arrangement (flat lying or upright) throughout all the layers, the sign and shape of the difference spectra are unaffected. In the presence of change of molecules arrangement between different layers, the resulting spectra sign and shape depend on the extent of the contribution relative to the layer with standing up or upright molecules. In the investigated samples the line shape of the difference spectra is similar up to 4 nm; the spectral features preserve energy position and sign. Having subtracted the interface layer, these line shapes reflect the electron states of the additional pentacene layers, i.e., layers in the dense phase lying horizontal (line phase) or dense layers of the up-standing herringbone molecules. A reversal in the NEXAFS linear dichroism occurs beyond the 4 nm thickness. For thickness >4 nm minima in the dichroism signal are observed for energies 284.5 and 285 eV. This is consistent with the almost vertical orientation of the molecules and identifies the transition from phase A to phase B as well as the critical thickness of stability of phase A that, within our error bar on thicknesses, is about 4 nm. To highlight the significant change in NEXAFS spectra between phase A and B, we made the difference of spectra (3) and (1), (4) and (1) of Figure 3a. The features of these difference spectra directly reflect the orientation of the molecules in the multilayer. The contribution of disordered part of >4 nm pentacene film are ruled out since they are canceled out in the difference spectra. The reversal of dichroism for phase B indicates the change in the orientation of the molecules in the outermost layer. The sign change of dichroism was also observed in pentacene films grown on Cu(110), where a gradual modification of NEXAFS aspect and dichroism corresponds to a progressive modification of molecular orientation from flat to upright (tilting angle changing from 27° to 73°).2 We note that the red curve in Figure 4a indicates that the magnitude of dichroism corresponding to the herringbone B phase for the >4 nm sample equals, and cancels, the dichoism due to the 19146 DOI: 10.1021/la1036376
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underlying A phase. This indicates that for that sample roughly the same number of molecules is in the A as in the B phase. The STM results only show phase A for thicknesses up to 4 nm (with some increasing angular disorder) and only dense phase B for >4 nm. We argue that the transition from phase A to phase B is actually a phase transition involving both the newly deposited molecules and the upper part of the A-type film. The energetic of a 5-6 layer thick phase A and the energetics of a thin bulk-like B phase are similar from cohesion energy calculations.4 The experimental evidence of formation of a dense up-standing phase B for thicknesses exceeding the 4 nm phase A points to the reorientation of the upper part of the phase A film when the critical thickness of 4 nm is overcome. We know from the combined result of NEXAFS dichroism and STM that the long axis of the pentacene in the upper layers of the A phase has an average angle of 5° with respect to the surface normal, indicating that a vertical tilt builds up as the molecules are farther away from the surface. Nevertheless, when the bulk-like B phase becomes energetically favored, it can grow both in the direction of newly supplied molecules (deposit) and in the inner direction by converting to B phase the less stable top A phase layers. Although the evidence of this A to B phase transition of some layers is indirect, it is a consistent phenomenology with the measured relative magnitude of dichroism for incremental coverages and for the STM evidence of a dense up-standing B phase that requires conversion of previously A phase molecules. According to this interpretation, the actual interface between A and B phase is likely to be established at 1-2 nm distance from the copper substrate (3-5 molecular layers). In Figure 5, we briefly review the recently published results on the growth of pentacene films on different Cu surfaces in the multilayer regime. In the central panel it is reported the phase A of pentacene film on Cu(110), where pentacene molecules long molecular axis is parallel to the substrate and form an angle of 21° with respect to [110] the substrate direction. Our results for growth mode A differ from the one of ref 2 by the tilting angle and by the much larger critical thickness. In the right panel it is displayed the theoretical growth model on Cu(100),4 where an upright arrangement (side view) occurs starting at the second layer. The pentacene upright geometry (reported only as a top view for the growth mode B of Figure 5) is similar for all the surfaces, since it mimics the pentacene bulk-like arrangement. In the present case of growth on Cu(119) a stronger interface interaction stabilizes the first pentacene layer in a dense line-wave geometry that is followed by a few layers of dense flat lying molecules, progressively acquiring some tilt with rotations both about the long axis and about the short axis. Beyond 4 nm a substantial part of the film converts to the B phase, including both the newly deposited pentacene molecules and partially from the topmost layers of the A phase. The molecular orientation of acenes yielding an upright standing phase for thicker layers has been observed before also for tetracene, pentacene films deposited on Cu(100),4,32 and SiO2,13 independently from the substrate symmetry. On the basis of NEXAFS data of phase B, it is difficult to state the tilting angle of the molecules within reasonable error bar. However, the combined results of STM image and NEXAFS on the same sample confirm the switch in the molecular configuration from flat lying to upright molecules in the bulk-like herringbone structure at a critical thickness and the partial conversion of the previously A type film into B type. The herringbone type of packing appears in many molecular crystals of polycyclic aromatic hydrocarbons13,32 when the intermolecular potential of molecules predominates over the π-bonding to the substrate. Therefore, the observed molecular reorientation Langmuir 2010, 26(24), 19142–19147
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Article
Figure 5. Pentacene growth mode scheme for different substrates: Cu(119), left side panel; Cu(110), central panel; Cu(100), right panel.
from a templated interface structure toward a bulk-like structure is a common growth mode of such molecules on metal surfaces. What appears peculiar of Cu(119) is the rather large thickness reached by the A phase (4 nm) and its partial reorientation transition when the critical thickness is exceeded by adding further pentacene.
Conclusions By combining the results obtained from STM and NEXAFS measurements, we are able to propose the local structure details and molecules orientations for the different growth phases in the pentacene film on Cu(119). Up to 4 nm, the molecules are aligned along the preferential direction driven by the in-plane anisotropy of the substrate. A different phase is observed with molecules upright, with a herringbone-like structure, at thickness >4 nm,
Langmuir 2010, 26(24), 19142–19147
which can be understood as due to the conversion of the upper part of the flat lying A phase film. This establishes an internal interface between A and B phases that is at a few molecular layers from the substrate Cu(119) surface. For device applications it is important that also on metal substrates the growth of ordered pentacene film with their molecular axes perpendicular to the substrate surface is possible and that by choosing the orientation of the metal substrate the thickness of the flat laying layer can be modified. Acknowledgment. One of the authors acknowledges G. Di Santo for his help in the preparation of growth model representation. This work was carried out in the framework of the PRIN 2008 525SC7 contract coordinated by M. G. Betti.
DOI: 10.1021/la1036376
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