Direct Evidence of the Temperature-Induced Molecular Reorientation

Article pubs.acs.org/JPCC

Direct Evidence of the Temperature-Induced Molecular Reorientation in Tetracene Thin Films on AlOx/Ni3Al(111)

Michael Naboka,† Serguei Soubatch,‡,§ Alexei Nefedov,† F. Stefan Tautz,‡,§ and Christof Wöll*,† †

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany § Jülich Aachen Research Alliance (JARA)-Fundamentals of Future Information Technology, 52425 Jülich, Germany ‡

S Supporting Information *

ABSTRACT: Control over the optical properties of the fluorescent organic layer plays a key role in the development of organic light-emitting diodes. A combination of near-edge X-ray absorption fine structure spectroscopy and Xray photoelectron spectroscopy was used to study structural changes in thin films of tetracene on AlOx/Ni3Al(111). It is shown that upon deposition onto the cold (100 K) substrate, a monolayer of tetracene molecules adopts a disordered adsorption configuration with the molecular planes orientated almost parallel to the surface. Upon annealing at 280 K, the molecular packing changes and the tetracene units adopt a more upright orientation. The consequences of this orientational change for the luminescent properties of the molecular adlayer are discussed, in particular with regard to a quenching of the optical excitation by an electronic coupling of occupied and unoccupied molecular states to those of the metal substrate.

1. INTRODUCTION In recent years organic semiconductors (OSCs) have attracted considerable attention because of the still increasing interest in organic electronic devices.1 Among the different OSC materials, polyacenes, i.e., polycyclic aromatic hydrocarbons consisting of linearly fused benzene rings, are especially interesting because of high charge carrier mobilities in their crystalline phases. Pentacene, a five-ring polyacene, has been the subject of many works,2−6 but also tetracene (Tc) is of interest with regard to the fabrication of organic field effect transistors (OFETs).7 Because of their unique properties, polyacenes have also found applications in the fabrication of organic light emitting diodes (OLEDs).8 Control over the optical properties of the fluorescent organic layer plays a key role. In this context, the electronic coupling of molecules within thin films to the substrate and in particular the optical quenching of excited states within the molecule by coupling to a supporting metal substrate have to be considered.9,10 To avoid this unwanted quenching, it is common to isolate the organic layer from the metal substrate by a thin insulating oxide barrier.9−12 In the case of tetracene, for instance, a Ni3Al(111) substrate was oxidized prior to tetracene deposition,10−12 leading to the presence of a thin oxide layer with a thickness of about 0.5 nm. Unexpectedly, however, in this work the luminescence of the thin OSC films deposited on AlOx/Ni3Al(111) at low temperature was still found to be quenched. Interestingly, after annealing to 280 K and subsequent recooling to 100 K, quenching was found to be absent.11 To explain this observation it has been suggested that the tetracene layer undergoes a structural transformation upon © 2014 American Chemical Society

postdeposition annealing. Such an explanation is plausible because tetracene molecules oriented parallel to the substrate are expected to show an electronic coupling of occupied and unoccupied states to those of the metal substrate through the thin oxide. For molecules with a significant tilt relative to the substrate, however, one would expect this coupling to be substantially weaker, thus restoring the Tc luminescent properties to that of the bulk. However, solid, definite proof for this reorientation is still lacking. To identify the origin of this unexpected behavior, a number of different experimental methods have been applied, including low-energy electron diffraction with spot-profile-analysis (SPA-LEED), X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy loss spectroscopy (HREELS).12 In the present paper a detailed orientation analysis of the tetracene molecules deposited on the thin oxide insulating film is presented, using a particularly powerful method, near edge X-ray absorption fine structure spectroscopy (NEXAFS), which is very well-suited for studies on the orientation of molecular species deposited on solid substrates and also allows us to derive important information on the position and degree of filling of unoccupied molecular orbitals13 and adsorption-induced rehybridization.14 The potential of the method has been demonstrated before by Reiss et al.15 for the case of antracene and naphthalene deposited on rutile TiO2(110). In addition, NEXAFS can be used to obtain information on molecular conformational changes upon adsorption.16 Received: August 1, 2014 Published: September 10, 2014 22678

dx.doi.org/10.1021/jp507773f | J. Phys. Chem. C 2014, 118, 22678−22682

The Journal of Physical Chemistry C

Article

2. EXPERIMENTAL DETAILS The experiments were carried out using the HE-SGM beamline at synchrotron facility BESSY II of Helmholtz-Zentrum Berlin. The ultrahigh vacuum (UHV) experimental end-station was equipped with a Scienta R3000 electron analyzer.17 The NEXAFS signal was detected using a partial yield detector18 based on Galileo microchannel plates. The base pressure both in analysis and preparation chambers was better than 7 × 10−10 mbar. The Ni3Al(111) crystal was cleaned by repeated Ar+ sputtering and annealing cycles. The AlOx thin film was grown by exposing the crystal surface to 40 L of oxygen at the temperature of 1050 K in the preparation chamber. These preparation conditions are known to guarantee a well-defined two-layer AlOx thin film with a thickness of 0.5 nm.19 After AlOx thin film growth, the substrate was characterized by LEED in the preparation chamber and XPS in the analysis chamber. In Figure S1a of the Supporting Information, the sharp LEED pattern recorded for the clean Ni3Al(111) substrate after the substrate cleaning procedure (several Ar+ sputtering cycles) is shown. The LEED pattern corresponding to the oxidized surface is presented in Figure S1b of the Supporting Information and shows additional diffraction spots which correspond to Al2O3 superstructure with unit cell parameters typical for Al2O3 (a = 18.0 Å, b = 10.6 Å, γ = 91.15°). The corresponding structure of Al2O3 is presented in Figure S2 of the Supporting Information. Moreover, we controlled the AlOx/Ni3Al substrate with XPS. The data recorded at different electron takeoff angles of 0°, 15°, and 45° for Ni 3s and Al 2s lines are presented in Figure S3 of the Supporting Information. The Al 2s peak is split into two components placed at 116.6 and 119.6 eV. The peak at 116.6 eV decreases with increasing electron takeoff angle together with the Ni 3s line and is assigned to Al atoms within the Ni3Al bulk, while the peak at 119.6 eV is assigned to the Al3+ ions in the AlOx oxide layer, in accordance with the data presented in ref 12. Tetracene (Sigma-Aldrich, 99% purity) was sublimated from a resistively heated quartz crucible, and molecular films of different thickness were deposited onto the cold (T = 100 K) AlOx/Ni3Al(111) substrate. It was shown in previous studies that on oxide-free metal substrates like Ag(111), Tc monolayer forms structures with long-range order, referred to as the αphase.20 On AlOx/Ni3Al(111), however, no ordered structures showing long-range order have been observed in previous work.12 For this reason, suitable conditions of Tc monolayer deposition cannot be determined on the basis of LEED. Instead, for the preparation of Tc films we applied a deposition protocol (temperature of Knudsen cell, deposition time) that was developed for ordered monolayers of tetracene on Ag(111), assuming that the sticking coefficient is similar for AlOx/Ni3Al(111) and Ag(111) substrates. This protocol was tested in previous NEXAFS experiments in the same UHV system for Tc deposition on Ag(111),21 where this procedure was found to be highly reproducible. After tetracene was deposited onto the cold AlOx/Ni3Al(111) substrate, the sample was transferred to the analysis chamber. The transfer system of the NEXAFS/XPS setup with precooled receiving station17 allows the transfer to occur in a very short period of time so that the temperature of the sample rises by only about 18−20 K during transfer. In the analysis chamber the sample was characterized with XPS (excitation energy of 400 eV) and NEXAFS (C K-edge) at the substrate

temperature of 100 K. The second set of NEXAFS and XPS data was collected after sample annealing to 280 K and cooling back to 100 K. To determine the molecular orientation, the NEXAFS C K-edge spectra were measured at different incidence angles (θ = 30°, 55°, 75°, 90°) in the partial electron yield mode (PEY) applying a retarding voltage (−150 V) to the detector grid. The NEXAFS spectra were processed according to the following procedure: raw spectra measured for the clean AlOx/Ni3Al substrate were subtracted from raw spectra measured on the organic thin film. The resulting curves were divided by a spectrum measured on a clean gold surface to take into account the energy dependence of photon flux. Finally, the spectra were normalized to the adsorption edge step of 1.

3. RESULTS AND DISCUSSION 3.1. Multilayer of Tetracene. First we evaporated the tetracene at 150 °C for 90 s. These parameters correspond to the growth of a Tc multilayer with a thickness of approximately 3 nm. After the deposition, the sample was characterized by XPS. The presence of the tetracene multilayer on the surface is evidenced by an intense C 1s XPS peak at 285.05 eV (Figure 1a, black curve). The peak shape and position are in good

Figure 1. XPS data for tetracene multilayer measured at 100 K directly after deposition (black) and after annealing to 280 K and subsequent recooling (red): (a) C 1s line and (b) Al 2s and Ni 3s lines.

agreement with previously published results.12 Upon annealing at 280 K, a shift of the C 1s XPS peak from 285.05 to 285.3 eV with a significant decrease of the intensity is observed (Figure 1a, red curve). The decrease of the intensity is explained by the partial desorption of the tetracene upon annealing, while the shift is explained by the higher surface binding energy of the remaining tetracene layer. The XPS Ni 3s and Al 2s lines are completely suppressed for the freshly deposited multilayer (Figure 1b, black curve). From this observation the thickness of the tetracene adlayer can be estimated to be larger than 2.1 nm according to the National Institute of Standards and Technology electron effective-attenuation-length database.22 This is in good agreement with our estimation of the Tc film thickness (see above). Upon annealing of the sample at 280 K, Ni 3s and Al 2s XPS peaks reappear in the spectrum (Figure 1b, red curve) with their intensities being almost the same as those for the clean AlOx/Ni3Al(111) substrate (Figure S3 of the Supporting Information, blue curve). This reappearance, together with the decrease of the C 1s peak intensity, allows us to suppose that only about one Tc monolayer remains on the surface. 22679



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To clarify the question of the molecular orientation for the Tc multilayer we used NEXAFS spectroscopy. The corresponding spectra are presented in Figure 2 (solid lines). The plots

Figure 3. Angle dependence of the first π* resonance intensity recorded for the tetracene multilayer before annealing (blue squares) and the tetracene monolayer before and after annealing (green triangles and red diamonds, respectively) with corresponding fitting curves.

Figure 2. C−K edge NEXAFS spectra for tetracene multilayer measured at 100 K directly after deposition (solid lines) and after annealing to 280 K and subsequent recooling (dashed lines).

130 °C for 60 s in accordance with the developed deposition protocol.21 The XPS analysis was conducted for the samples directly after Tc deposition. It is noticeable that the position, intensity, and full width at half-maximum (fwhm) of the C 1s peak (binding energy (BE), 285.3 eV) of the Tc monolayer (Figure 4a, black curve) are the same as those for the C 1s peak

show the typical NEXAFS absorption resonances of tetracene, the positions of which correlate with results obtained for Tc in previous works.21,23 To provide an assignment of these resonances, we have carried out quantum chemical calculations for the free tetracene molecule using the StoBe program package. The calculated spectra are presented in Figure S4a of the Supporting Information. The NEXAFS spectra are dominated by several pronounced features: two strong π* resonances with an observed fine structure, which are a superposition of at least four C 1s → π* transitions in the photon energy range 283−287 eV. The peaks observed in the photon energy region 287−289 eV are termed here as R/h resonances and are a mixture of resonances coming from Rydberg orbitals and hydrogen-derived σ* orbitals (C−H bonds).21 The broad features observed between 289 and 325 eV refer to the superposition of σ* resonances. The spectra measured after annealing at 280 K and cooling to 100 K have a very similar appearance and are presented in Figure 2 as dotted lines for 90° and 30° only. These spectra show that the orientation of the tetracene molecules remains unchanged after annealing. A quantitative analysis of NEXAFS spectra recorded at different photon incident angles (θ) allows the determination of the average tilt angle (α) of transition dipole moment (TDM) relative to the sample normal. For substrates with 3fold or higher symmetry, the intensity depends only on α and θ and not on the molecular azimuthal angle, as described by the following equation:

Figure 4. XPS data for tetracene monolayer measured at 100 K directly after deposition (black) and after annealing to 280 K and subsequent recooling (red): (a) C 1s line and (b) Al 2s and Ni 3s lines.

for the multilayer upon annealing (see Figure 1a, red curve). In addition, the Ni 3s and Al 2s lines (Figure 4b) are the same as those for the annealed multilayer sample (Figure 1b, red curve) and as for the clean substrate (Figure S3 of the Supporting Information, blue curve). NEXAFS spectra, recorded for the monolayer of Tc directly after deposition, are presented in Figure 5a. These NEXAFS spectra and their angle dependence are the same as for those presented above for multilayer Tc films, and a quantitative analysis yields also the average tilt angle of 43 ± 5° (Figure 3, green triangles). This direct comparison of the XPS and NEXAFS data obtained for the Tc monolayer sample and for annealed multilayer sample allows us to conclude that upon annealing of the multilayer only about one monolayer of the tetracene remains on the surface in accordance with our assumption made above.

⎧1 ⎡ ⎤ 1 I(α , θ ) = A ⎨ P ⎢1 + (3 cos 2θ − 1)(3 cos 2α − 1)⎥ ⎦ ⎩3 ⎣ 2 ⎫ 1 + (1 − P) sin 2α ⎬ ⎭ 2

where A is a constant and P = 0.91 is the polarization factor of the synchrotron light.18 The analysis performed for the spectra taken before as well as after annealing yields an average tilt angle α of approximately 43 ± 5° (Figure 3, blue squares). 3.2. Monolayer Tetracene Films. A more interesting behavior can be observed for the monolayer (ML) coverage of tetracene. To get a ML thickness, tetracene was evaporated at 22680



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Figure 5. C−K edge NEXAFS spectra for tetracene monolayer measured at 100 K directly after deposition (a) and after annealing to 280 K and subsequent recooling (b).

molecular plane relative to the substrate (Figure 3, red diamonds). On the basis of this data, we propose an upright orientation of the tetracene molecules with a tilt angle of α = 63° between the long axis of the Tc molecule and the surface (see Figure 6d). XPS data collected on the annealed monolayer reveal that the BE of the C 1s line changes from 285.3 eV before annealing to 284.1 eV after annealing (Figure 4a, red curve). Such a C 1s binding energy (284.1 eV) is fully consistent with an upright orientation of the planar aromatic compound close to a metal substrate. Similar values have been reported for aromatic SAMs on Au substrates, where the planar aromatic subunits adopt a similar, upright orientation.27 These results correlate also with previous studies,11,12 in which the reorientation of molecules should induce dewetting of the layer and formation of standing molecule islands. Both NEXAFS and XPS results clearly demonstrate the temperature-induced molecular reorientation in the monolayer of tetracene, and this reorientation can explain an absence of luminescence directly after Tc deposition and its appearance after corresponding annealing. A similar behavior was also observed for ethylbenzene and styrene adsorbed on oxygen-terminated FeO(111), where the adsorbate−adsorbate interaction prevailed over adsorbate− substrate interaction. This caused the high tilt angle as well as the growth in the form of two-dimensional islands.28

On the basis of our data, we propose that before annealing the Tc molecules adopt a structure similar to a disordered variant of the β-phase observed for Tc adsorbed on Ag(111).21 In the present case the long axis of the Tc molecule is orientated parallel to the surface with a phenyl ring rotation on 43 ± 5° (Figure 6c). Such a more planar orientation of the

Figure 6. Tetracene adsorption models: a multilayer after deposition at 100 K (a) and after annealing to 280 K and subsequent recooling to 100 K (b); a monolayer after deposition at 100 K (c) and after annealing to 280 K and subsequent recooling to 100 K (d).

molecule relative to the substrate explains the fact that the C 1s binding energy observed for this phase (285.3 eV) is shifted by 0.25 eV relative to the multilayer (285.05 eV). Similar binding energies (∼285 eV) were reported for other planar aromatic compounds adsorbed on 3d metals and strongly interacting with a such substrates, e.g., in the case of graphene on Ni(111), where a hybridization of graphene π-orbitals and 3dz orbitals of nickel takes place.24,25 Although in the present case a thin oxide layer is present, the interaction with the supporting metal is apparently strong enough to cause a ∼1 eV shift in C 1s binding energy. In a next step the deposited monolayer was annealed to 280 K and then cooled back to 100 K. NEXAFS spectra for the annealed Tc monolayer reveal that the dependence of the resonance intensity on the photon incident angle (θ) shows just the opposite trend (Figure 5b) if compared to that of the Tc monolayer before annealing (Figure 5a): The σ*-type resonances are now more intense for grazing incidence, while the π*-type resonances are stronger at the normal incidence. The quantitative analysis of the resonance intensity dependence on angle of incidence yields a tilt angle of 63 ± 5° of the Tc

4. CONCLUSIONS We determined the molecular orientation of tetracene molecules on thin, well-defined aluminum oxide films supported on a Ni3Al(111) substrate using XPS and NEXAFS spectroscopy. A detailed analysis for the corresponding XPS and NEXAFS data of the deposited tetracene layers was carried out. For Tc multilayers, the NEXAFS data reveal an average tilt angle of 43 ± 5° with respect to the surface. This value remains unchanged upon annealing, meanwhile XPS data reveal the partial desorption of the Tc layer (Figure 6a,b). For Tc monolayers, a considerable reorientation of the molecules on annealing is found: After deposition onto a cold (100 K) substrate, a monolayer of tetracene molecules adopts a disordered adsorption configuration similar to one for Tc multilayer (Figure 6b,c) with molecular platelets tilted (43 ± 5°) with respect to the surface. Upon annealing at 280 K and subsequent cooling to 100 K, the film morphology changes and molecular islands with a more up-standing orientation are formed (Figure 6d). These results are a direct experimental confirmation of the hypothesis put forward in previous work,12 22681



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(10) Schneider, M.; Umbach, E.; Langner, A.; Sokolowski, M. Luminescence of Molecules Adsorbed on Surfaces of Wide Gap Materials. J. Lumin. 2004, 110, 275−283. (11) Langner, A.; Su, Y.; Sokolowski, M. Luminescence Quenching of Tetracene Films Adsorbed on an Ultrathin Alumina AlOx Layer on Ni3A(111). Phys. Rev. B 2006, 74, 045428-1−045428-11. (12) Soubatch, S.; Temirov, R.; Weinhold, M.; Tautz, F. S. The Interplay between Molecular Orientation, Film Morphology and Luminescence Properties of Tetracene Thin Films on Epitaxial AlOx/ Ni3Al(111). Surf. Sci. 2006, 600, 4679−4689. (13) Tseng, T-Ch.; Urban, Ch.; Wang, Y.; Otero, R.; Tait, S. L.; Alcamí, M.; Écija, D.; Trelka, M.; Gallego, J. M.; Lin, N.; et al. ChargeTransfer-Induced Structural Rearrangements at Both Sides of Organic/Metal Interfaces. Nat. Chem. 2010, 2, 374−379. (14) Mainka, C.; Bagus, P. S.; Schertel, A.; Strunskus, T.; Grunze, M.; Wöll, Ch. Linear Dichroism in X-ray Absorption Spectroscopy of Strongly Chemisorbed Planar Molecules: Role of Adsorption Induced Rehybridisations. Surf. Sci. 1995, 341, L1055−L1060. (15) Reiß, S.; Krumm, H.; Niklewski, A.; Staemmler, V.; Wöll, Ch. The Adsorption of Acenes on Rutile TiO2(110): A Multi-Technique Investigation. J. Chem. Phys. 2002, 116, 7704−7713. (16) Käfer, D.; Ruppel, L.; Witte, G.; Wöll, Ch. Role of Molecular Conformations in Rubrene Thin Film Growth. Phys. Rev. Lett. 2005, 95, 166602-1−166602-4. (17) Nefedov, A.; Wöll, Ch. Advanced Applications of NEXAFS Spectroscopy for Functionalized Surfaces. In Surface Science Techniques; Bracco, G., Holst, B., Eds.; Springer Series in Surface Science 51; Springer: Heidelberg, Germany, 2013; pp 277−303. (18) Stöhr, J. NEXAFS Spectroscopy; Springer: Berlin, 1992; pp 1− 404. (19) Rosenhahn, A.; Schneider, J.; Becker, C.; Wandelt, K. Oxidation of Ni3Al(111) at 600, 800, and 1050 K Investigated by Scanning Tunneling Microscopy. J. Vac. Sci. Technol., A 2000, A18, 1923−1927. (20) Langner, A.; Hauschild, A.; Fahrenholz, S.; Sokolowski, M. Structural Properties of Tetracene Films on Ag (111) Investigated by SPA-LEED and TPD. Surf. Sci. 2005, 574, 153−165. (21) Sueyoshi, T.; Willenbockel, M.; Naboka, M.; Nefedov, A.; Soubatch, S.; Wöll, Ch.; Tautz, S. F. Spontaneous Change in Molecular Orientation at Order−Disorder Transition of Tetracene on Ag(111). J. Phys. Chem. C 2013, 117, 9212−9222. (22) The National Institute of Standards and Technology. Electron Effective-Attenuation-Length Database; http://www.nist.gov/srd/ nist82.cfm, (accessed September 10, 2014). (23) Jiang, D. T.; Shi, J.; Tersigni, A.; Regier, T.; Qin, X. R. NEXAFS Studies of Pentacene and Tetracene Thin Films. In Canadian Light Source 2007 Activity Report; Dalzell, M., Ed.; pp 32−33. (24) Grüneis, A.; Vyalikh, D. Tunable Hybridization between Electronic States of Graphene and a Metal Surface. Phys. Rev. B 2008, 77, 193401-1−193401-4. (25) Zhang, W.; Nefedov, A.; Naboka, M.; Wöll, Ch. Molecular Orientation of Terephthalic Acid Assembly on Graphene: NEXAFS and XPS Study. Phys. Chem. Chem. Phys. 2012, 14, 10125−10131. (26) Haberer, D.; Giusca, C. E.; Wang, Y.; Sachdev, H.; Fedorov, A. V.; Farjam, M.; Akbar Jafari, S.; Vyalikh, D. V.; Usachov, D.; Liu, X.; et al. Direct Observation of a Dispersionless Impurity Band in Hydrogenated Graphene. Adv. Mater. (Weinheim, Ger.) 2011, 23, 4497−4503. (27) Shaporenko, A.; Terfort, A.; Grunze, M.; Zharnikov, M. A Detailed Analysis of the Photoemission Spectra of Basic Thioaromatic Monolayers on Noble Metal Substrates. J. Electron Spectros. Relat. Phenom. 2006, 151, 45−51. (28) Joseph, Y.; Wühn, M.; Niklewski, A.; Ranke, W.; Weiss, W.; Wöll, Ch.; Schlögl, R. Interaction of Ethylbenzene and Styrene with Iron Oxide Model Catalyst Films at Low Coverages: A NEXAFS Study. Phys. Chem. Chem. Phys. 2000, 2, 5314−5319.

in which it was suggested that the molecules are adsorbed in a more planar fashion which transforms to a more up-standing orientation upon annealing. The C 1s binding energy was found to be almost the same for both Tc monolayers directly after deposition and Tc multilayers after annealing, but as soon as molecules in the thin layer undergo reorientation, the binding energy decreases to values typical for densely packed planar aromatic compounds. To summarize, the obtained results clearly demonstrate that a precise control over the local molecular structure is needed to understand the charge transport in organic semiconductors in close contact with inorganic substrate. The latter is very important for predicting optical properties of the fluorescent organic layer in OLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

LEED images and XPS data for clean and oxidized Ni3Al(111) surfaces, results of StoBe-based calculations of Tc NEXAFS spectra, and the full list of coauthors of refs 13 and 26. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime. This work was partially supported through the “Science and Technology of Nanosystems” Programme (Project 431103-Molecular Building Blocks/Supramolecular Networks). We also thank Dr. Hikmet Sezen for valuable discussion and assistance with the XPS data interpretation.

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REFERENCES

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Direct Evidence of the Temperature-Induced Molecular Reorientation

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