NEXAFS investigations of NTCDA monolayers on different metal substrates

Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 523–528

NEXAFS investigations of NTCDA monolayers on different metal substrates D. Gador, Y. Zou, C. Buchberger, M. Bertram, R. Fink*, E. Umbach ¨ Wurzburg ¨ ¨ , Am Hubland, D-97074 Wurzburg , Germany Experimentelle Physik II, Universitat

Abstract The interaction of naphthalene-tetracarboxylic acid dianhydride (NTCDA) monolayers with Ag(111), Cu(100) and Ni(111) substrate surfaces has been investigated by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The C and O K-edge spectra drastically change for monolayers compared to multilayers thus reflecting a strong covalent bonding to the metal substrate. Whereas for Ag substrates the p*-resonances attributed to the naphthalene core are mainly affected by the substrate interaction, the anhydride groups are more involved in the molecular bonding to Cu substrates and even more for Ni substrates. In the latter case one may even speculate about molecular dissociation, in particular upon thermal treatment of the films.  1999 Elsevier Science B.V. All rights reserved. Keywords: Near-edge X-ray absorption fine structure spectroscopy; Metal substrates; Ag(111); Cu(100); Ni(111); Naphthalene-tetracarboxylic acid anhydride

1. Introduction Well-ordered, highly oriented, in some cases even epitaxial thin films of large organic molecules have recently attracted considerable attention, in particular since they exhibit interesting physical properties, e.g. extremely sharp photoluminescence peaks comparable to single molecules or perfect organic single crystals. This has, for instance, been demonstrated for ultrathin films of quaterthiophene, which can be grown epitaxially on a Ag(111) substrate by vacuum sublimation under ultrahigh vacuum conditions [1]. Organic molecular beam deposition (OMBD) allows, in contrast to the frequently employed Langmuir– Blodgett (LB) or self-assembled monolayer (SAM) *Corresponding author. Tel.: 149-931-888-5163; fax: 149-931888-5158. E-mail address: [email protected] (R. Fink)

techniques, a larger flexibility with respect to the growth conditions. In particular, structurally welldefined and clean substrates like single crystal surfaces prepared in-situ by sputtering and annealing can be utilized. In addition, the sublimation process itself allows the deposition of very pure organic substances with almost any deposition rate. This flexibility even allows the structural manipulation of films, as has, e.g., been demonstrated for two planar molecules, namely naphthalene-dicarboxylic-anhydride (NDCA) and the related molecule naphthalene-tetracarboxylic acid-dianhydride (NTCDA). In the first case, monolayer deposition on a Ni(111) substrate yielded molecules with their molecular planes parallel to the substrate whereas for monolayer adsorption on an oxygen-precovered Ni(111)-surface (with a p(232)-oxygen superstructure) NEXAFS revealed upright standing molecules, bonding to the substrate via the anhydride groups

0368-2048 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00486-1

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[2]. In the second case, different adsorption kinetics occurring during the preparation of NTCDA multilayers yielded molecules with their molecular plane parallel and perpendicular to the substrate for low substrate temperatures (T ,200 K) and room temperature deposition, respectively. In the second case, the monolayer is always oriented parallel to the substrate, irrespective of the preparation conditions (e.g. deposition rate or substrate temperature). This latter finding was explained by the p-interaction with the Ag substrate mainly via the naphthalene core [3]. Since the strength of the substrate interaction not only influences the molecular orientation but also the mobility of the molecules upon adsorption and thus the domain size of the adsorbed monolayer, knowledge about the bonding to the substrate is important for the epitaxy of organic films, i.e. the formation of large, well-ordered domains in the first and subsequent layers. We therefore started a comprehensive study on this topic by varying the substrate materials and in some cases the substrate orientation. The molecule–substrate interaction is usually investigated by photoelectron spectroscopy (XPS / UPS) in combination with thermal desorption spectroscopy (TDS). A very useful technique for this purpose also is near-edge-X-ray absorption spectroscopy (NEXAFS) which not only allows the straightforward determination of the molecular orientation, but also the observation of modifications of the molecular orbitals upon covalent interaction or reaction and thus the identification of the molecular subunits mostly involved in the interaction. In this paper we present a comparative NEXAFS investigation of NTCDA monolayers adsorbed on Ag(111), Cu(100), and Ni(111). We will focus on the more reactive substrates Cu(100) and Ni(111) since in these two cases the monolayers are more strongly bound as concluded from thermal desorption. Whereas NTCDA monolayers can be completely removed from Ag substrates by heating above 390 K [4], no monolayer desorption is observed from Cu and Ni substrates. An interesting aspect concerns the modification of the NEXAFS spectra upon thermal treatment of the Ni(111) substrate, which may be interpreted as molecular dissociation. Structural aspects derived from low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and photoemission data obtained for Ag and Cu

substrates have been published recently [3–5] and shall not be discussed here.

2. Experimental All preparations and measurements were conducted in ultrahigh vacuum at a base pressure of 2310 210 mbar. The X-ray absorption (NEXAFS) measurements were performed at the PM-1 (SX 700-I) beamline at the BESSY storage ring in Berlin (Germany). The Auger yield mode (FWHM of the energy window: 12 eV) was used as secondary detection signal of the X-ray absorption. The photocurrent signal of a gold grid in front of the sample was used for normalization of the synchrotron radiation intensity. The minima of this signal at 284.7 and 291.0 eV (due to the carbon contaminants on the optical elements) was used for energy calibration. The films were produced by organic molecular deposition (OMBD) on clean metal single crystals which were prepared by sputtering and annealing cycles. X-ray photoelectron spectroscopy (XPS) was used to control all preparation steps and for film thickness calibration in combination with thermal desorption spectroscopy. Deposition rate, total amount and purity of the organic substance were monitored by a quadrupole mass spectrometer. The NTCDA monolayers were produced in two different ways: firstly, the correct amount of NTCDA was dosed onto the the single crystal at a substrate temperature of 150 K, and secondly several NTCDA layers were deposited at 150 K on the substrate followed by subsequent desorption of the multilayers upon annealing at 310 K [3]. The temperature during the NEXAFS measurements was kept below 200 K.

3. Results and discussion Fig. 1 shows the C K- and O K-edge spectra of NTCDA monolayers adsorbed on Ag(111), Cu(100), and Ni(111) at 150 K, together with a monolayer on Ni(111), which was prepared by thermal desorption (t ML), and a spectrum of a 50 ML thick film for comparison. The C K-edge spectrum of condensed NTCDA multilayers shows four distinct p* resonances. As for the related molecule NDCA, consist-

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Fig. 1. Comparison of C K- (left) and O K-edge (right) NEXAFS spectra for 50 monolayers (ML) NTCDA adsorbed on Ag(111) (uppermost spectra), 1 ML NTCDA on Ag(111), Cu(100), Ni(111), and an NTCDA monolayer on Ni(111) prepared by thermal desorption of multilayers (t-ML). All spectra were obtained for grazing incidence of the synchrotron radiation. The inset shows the NTCDA molecule; different atoms are labeled.

ing of a naphthalene core and only one dianhydride group, the resonances (1), (2) and (4) are assigned to the excitations of C 1s electrons of the naphthalene core to different unoccupied molecular orbitals. According to Xa-SW calculations of NDCA [6], resonance (3) stems from the excitation of the anhydride carbon [3]. The corresponding multilayer O K-edge spectrum consists of two p* resonances (a) and (b) at 531 eV and 534 eV, respectively, and a broader s* resonance at higher photon energy. Resonance (b) is assigned to the excitation of the terminal oxygen atoms O b (see inset of Fig.1) into the lowest unoccupied molecular orbital (LUMO), whereas resonance (a) is assigned to the excitation of the central oxygen atom O a into the LUMO plus the excitation of O b into the LUMO11 [2,6]. The NEXAFS spectra of NTCDA monolayers are strongly modified compared to multilayer films. They also differ for the various substrates reflecting a different chemical interaction. On Ag(111) and

Cu(100) the spectra do not depend on the preparation method, i.e. monolayer preparation by exact dosing or thermal desorption of multilayers (t-ML) [5]. In contrast, the preparation method for the monolayer on Ni(111) has a strong influence on the NEXAFS spectra. The C K-edge spectra show a reduction of the intensity of resonance (1) for all substrates, whereas resonance (3) (anhydride carbon) is not reduced accordingly. Only for the monolayer on Cu(100) and for the ‘thermal monolayer’ on Ni(111) (t-ML) this resonance is strongly affected. Besides these changes in the relative intensities, the resonances are broadened and shifted in their energy positions. The oxygen K-edge spectra show even more pronounced modifications. Firstly, the difference in photon energies of resonances (a) and (b) is smaller for the monolayers than for condensed NTCDA. The relative intensities of the resonances are drastically changed compared to the multilayer. As for the C

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K-edge spectra, there are also pronounced differences for the different substrates. For the monolayer on Ag(111) the intensity of resonance (b) is strongly reduced compared to that of resonance (a). In contrast, the intensity of resonance (a) is also significantly decreased for the monolayers on Cu(100) and Ni(111). These changes in the monolayer spectra compared to condensed NTCDA multilayer films indicate a strong covalent bonding of the NTCDA molecules to the metal substrates. The results of the NEXAFS spectra lead to the following conclusions for the bonding to the substrate. The reduction of the intensity of resonance (1) indicates that the LUMO localised at the naphthalene core is involved in the substrate bonding. The fact that the intensity of resonance (3), which is also assigned to an excitation into the LUMO does not decrease for Ag(111) and Ni(111) for monolayer dosage indicates that the localization of this molecular orbital must be modified due to bond formation to the substrate. The broadening of the resonances may be explained by the formation of (unoccupied) metal–adsorbates states or bonds and / or by a reduced life time of the core-excited states, because the excitation can also relax into the substrate, which is not possible for the multilayers. The shift of the resonances is a direct consequence of the modifications and interactions of the molecular orbitals. Core level shifts alone, due to the substrate bonding, cannot explain the changes in the spectra because such shifts should affect the resonances (1), (2) and (4) simultaneously, and this is obviously not the case. The differences in the O K-edge monolayer spectra indicate that for Ag(111) only the terminal oxygen O b is affected by the bonding to the substrate, whereas both oxygen atoms are involved in the covalent bonding to the Cu(100) and Ni(111) substrates. The differences in the spectra may lead to the conclusion that the naphthalene core is similarly involved in the bonding (pure p-interaction), whereas the contribution of the anhydride groups is different for the different substrates. From the comparison with the multilayer spectra we conclude that on Ag(111) the oxygen atoms of the anhydride group have the weakest interaction with the substrate, whereas the whole anhydride group is mostly involved in the covalent bonding to the Ni(111)

substrate. In the case of Ag(111) the d-bands are (relatively) far away from the Fermi edge (binding energy EB of the Ag d-electrons 4–8 eV), whereas for Ni(111), which represents the most reactive amongst the three investigated substrates, the d-band has significant DOS at the Fermi edge. In the Cu case, the binding energy of d electrons is between those of Ag and Ni (EB 2–6 eV). Obviously, the energetic position of the metal d-band is an important parameter for the degree of covalent bonding of the NTCDA monolayer. A possible dissociation of the NTCDA molecules can be excluded in the Ag(111) case from thermal desorption spectroscopy [4]. From the present data sets NTCDA dissociation cannot be excluded for the other substrates. For the Cu substrate, however, dissociation is unlikely as concluded from the following comparison: NEXAFS, thermal desorption, and XPS reveal intact molecules of the second and subsequent layers [5]. The LEED patterns obtained for the monolayer and all film thicknesses up to 5 ML, however, are identical. In case of dissociation of the monolayer, one would expect different superstructures and probably a disordered layer growth, which is apparently not the case. For the Ni(111) substrate and especially for the thermally prepared monolayer (t-ML) a dissociation process is more plausible, which would also be compatible with the different spectra obtained for the two preparation methods on the Ni substrate. A dissociation of the anhydride species was already suggested from XPS and infrared spectroscopy (FTIR) for the adsorption of pyromellitic dianhydride (PMDA) on Cu and Pt in Refs. [7–9]. Similar results were also obtained for the adsorption of ultrathin NDCA films (,1 ML) on Ni(111) [2]. The covalent bonding of the NTCDA monolayer to the substrate was also observed in XPS and UPS spectra [3,5]. However, the XPS (in comparison to the NEXAFS) measurements do not give such a detailed insight into the molecular subunits and hence into the bonding mechanism to the metallic substrate. Using the NEXAFS linear dichroism it is also possible to deduce the molecular orientation of NTCDA on Ni(111) from the dependence of p* intensities as a function of incident angle Q of the synchrotron radiation [10]. For the preparation at 150

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Fig. 2. Angular dependent C and O K-edge NEXAFS spectra of NTCDA monolayers on Ni(111) prepared by top: exact dosage of 1 ML at 150 K and bottom: thermal desorption of multilayers by heating to 320 K. Data are shown for s-polarization (Q508) and p-polarization (Q5708).

K (upper spectra), the maximum p* intensity is obtained for grazing angles (Q5708, mostly p-polarization) (see Fig. 2 and Refs. [3,5]) which is only compatible with molecules that are preferentially oriented parallel to the substrate. For the C K-edge spectra the p* intensities vanish almost completely for normal incidence (Q508, s-polarization), which indicates an almost perfect parallel orientation. This is also the case for the O K-spectra for Ag(111) and Cu(100) [3,5]. For Ni(111) some O-K intensity remains for normal incidence (Fig. 2, right side). This can be interpreted as due to a mixing of metal states to those molecular orbitals which are mainly located at the oxygen atoms of the anhydride group. In other words, due to the strong covalent bonding the planar symmetry of the involved molecular orbitals is lost. For monolayers prepared by thermal desorption of multilayers at 320 K, the molecular orientation depends on the substrate. NTCDA monolayers on Ag(111) and Cu(100) are oriented with their molec-

ular plane parallel to the substrate irrespective of the preparation method or substrate temperature. In contrast, almost no angular dependence is detected for the thermally prepared monolayer on Ni(111) (see lower spectra of Fig. 2). Thus no preferential molecular orientation exists (or, less likely, the molecular plane is oriented close to the magic angle of 528). The fact that no LEED patterns are obtained for the thermally prepared monolayer on Ni(111) indicates that no order and hence no preferential orientation of the molecules exists. The loss of molecular orientation is most likely due to dissociation or fragmentation similar to PMDA, which leads to a benzoate-like species with a more upright orientation [8]. Note, that the drastic change of polarization dependence (Fig. 2) occurs only after thermal treatment of the films. Thus we conclude that dissociation is not immediately caused by the bonding of the anhydride oxygen atoms to the substrate which apparently weakens the intramolecular bonds, but by a thermally activated process.

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4. Conclusions From the direct comparison of the C and O Kedge NEXAFS spectra of differently prepared NTCDA monolayers on different metal substrates, the molecular subunits involved in the covalent substrate interaction could be identified. The lower the d-band binding energy of the substrate, the more reactive is the substrate and the more involved in the bonding is the dianhydride group. Thermal activation may even lead to molecular dissociation as demonstrated for NTCDA on Ni(111). Compared to photoemission the investigation of the molecule–substrate interaction by NEXAFS is more detailed and yields additional information.

Acknowledgements This work was financially supported by the Bun¨ Bildung, Erziehung, Forschung desministerium fur und Technologie (contracts 05 625 WWA 9, 05 SF8

WWA 7). One of us (E.U.) acknowledges support by the Fond der Chemischen Industrie.

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