Article pubs.acs.org/JPCC
Transition-Metal Phthalocyanines on Transition-Metal Oxides: Iron and Cobalt Phthalocyanine on Epitaxial MnO and TiOx Films
Mathias Glaser,† Heiko Peisert,*,† Hilmar Adler,† Małgorzata Polek,† Johannes Uihlein,† Peter Nagel,‡ Michael Merz,‡ Stefan Schuppler,‡ and Thomas Chassé† †
Eberhard Karls Universität Tübingen, Institut für Physikalische und Theoretische Chemie, Auf der Morgenstelle 18, 72076 Tübingen, Germany ‡ Karlsruher Institut für Technologie, Institut für Festkörperphysik, 76021 Karlsruhe, Germany S Supporting Information *
ABSTRACT: The interaction at interfaces between transition-metal phthalocyanines and ultrathin transition-metal oxide films is studied by means of photoemission (PES) and X-ray absorption spectroscopy (XAS). Our results are compared to the recently investigated system CoPc on MnO. A flat-lying adsorption geometry of iron and cobalt phthalocyanines (FePc and CoPc) on the different oxide substrates was observed: ultrathin epitaxially grown MnO films and ultrathin TiOx films. A charge transfer from the MnO, in particular, to the Fe atom of the FePc molecule is observed by both PES and XAS. X-ray absorption spectra of the N K-edge of FePc do not hint at a nitrogen involvement in the interaction process. As a consequence of the charge transfer, a shift of the Fermi level of the semiconducting MnO films is observed, which is visible as a shift of MnO related core levels to lower binding energy. In contrast, CoPc and FePc deposited on TiOx show no hints for a charge transfer, although the flat-lying adsorption geometry allows in principle a maximum interaction between the π-system and the substrate. Increased surface roughness (compared to MnO) and an oxygen termination of the surface of the TiOx films is considered to suppress a possible strong interaction between the organic molecules and the substrate. the first layer of the organic material.20 In view of applications such as spintronic devices or solar cells, transition-metal oxides (TMO) might be promising substrates. For instance, on epitaxially grown ultrathin MnO films, CoPc recently showed charge-transfer (CT) processes across the interface, which were also reflected in an energy shift of the oxide related core levels.21 In special photocatalytic and Grätzel cells, TMPc are prone to transfer electrons to the TiO2 substrate under irradiation.22,23 Another study revealed a strong coupling of the first-layer FePc to rutile TiO2(110) resulting as well in a CT from the molecules to the TiO2 substrate.24 Also for FePc on defect-rich MoOx, a CT was observed recently, which might be understood as an electron transfer from FePc molecules to the substrate.25 A possible route for the preparation of TMO substrates is the growth of ultrathin oxide films. One advantage of using thinfilm TMO substrates is avoiding charging effects during photoemission and X-ray absorption measurements. Ultrathin films of MnO grow epitaxially on Ag(001), initially forming strained pseudomorphic layers with tetragonal distorted unit cells to compensate the lattice mismatch (ca. 9%). At a thickness of several nanometers, the structure of the MnO films relaxes to the bulk lattice structure. The electronic as well as the geometric structure of these films was well-characterized in the
1. INTRODUCTION Transition-metal phthalocyanines (TMPcs), a fundamental class of organic semiconductors, are promising candidates for optoelectronic devices, field-effect transistors, and organic photovoltaics as well as for spintronic devices. The performance of devices based on these organic materials highly depends on the interaction processes at the interface between the organic material and the substrate. Depending on the nature of the substrate and the metal center of the phthalocyanine, in particular electronic properties and thus transport processes as well as magnetic properties involving molecular spins can be affected drastically. Interfacial local charge transfer between the transition metal of the TMPc and metal substrates was reported. For instance, for CoPc at the interface to metals such as gold,1−4 silver,3,5−9 or nickel,3 a local charge transfer into empty Co states was observed, which can be accompanied by a back-donation via the ligands resulting in a positive charge accumulation at the ligands.2,10,11 Comparable interactions occur at the interface between FePc and metals such as silver,12−14 gold,15 or nickel.16 Such interactions can be tuned by the introduction of graphene as an intermediate layer, e.g., between the TMPc and a nickel or iridium substrate3,17,18 For gold and silver substrates, a change of the molecular spin of TMPc after adsorption on the substrate was reported.4,6,9 Furthermore, recent studies investigating magnetic couplings revealed a strong intermolecular antiferromagnetic coupling of CoPc in the form of powder and thin films19 and, in the case of MnPc, a ferromagnetic coupling between cobalt substrate and © 2015 American Chemical Society
Received: October 1, 2015 Revised: November 13, 2015 Published: November 13, 2015 27569
DOI: 10.1021/acs.jpcc.5b09612 J. Phys. Chem. C 2015, 119, 27569−27579
Article
The Journal of Physical Chemistry C literature.26−29 Depending on the preparation parameters, e.g., evaporation time, oxygen pressure, post-growth annealing temperature, the TiOx thin films are accessible in different modifications (from x = 2 to x = 1.1), and offer therefore a good alternative for single-crystalline TiO2 substrates.30,31 The aim of the present work is comparison of the nature of the interaction for two TMPcs on MnO and TiOx. FePc and CoPc were chosen as TMPcs because of their interesting interface properties observed at related systems.
considering the molecule−molecule distance from powder diffraction data of TMPc α-polymorph.33−35 The preparation of the oxide films and the organic films was carried out in different UHV chambers to avoid possible cross contaminations during preparation. All preparation steps were performed in situ in the described UHV system of the WERA beamline; all preparation steps for the additional measurements were performed in the described UHV system of our group. All measurements were performed at room temperature.
2. EXPERIMENTAL SECTION X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed at the soft X-ray beamline WERA at the synchrotron radiation source ANKA (Karlsruhe, Germany). Polarization-dependent X-ray absorption spectra at the N K-, Fe L-, Mn L-, Co L- and Ti Ledges were acquired at different angles of the incident ppolarized light with respect to the surface plane (with θ = 90°, normal incidence; 20°, grazing incidence) in both total-electron yield (TEY, drain current) and partial electron yield (PEY, retarding field, channeltron detector) mode. For background correction, the photon flux, I0, of the incident synchrotron light was monitored using a gold mesh. The photon energies were calibrated comparing the binding energies (BEs) of core-level photoemission peaks excited by first- and second-order light. Moreover, the photon-energy calibration was ensured by adjusting the Ni L3 peak position measured on a NiO single crystal to the established peak position.32 The energy resolution was set to ∼100 meV at the photon energy of 400 eV. Additional X-ray photoemission spectroscopy (XPS) measurements using a standard source with twin anodes (Al Kα and Mg Kα excitation 1486.6 and 1253.6 eV) were performed in a multichamber ultrahigh vacuum (UHV) system (base pressure, ∼1 × 10−10 mbar) equipped with a Phoibos 100 hemispherical analyzer (SPECS). The binding energy (BE) scale was calibrated to the BEs of the signals of Cu 2p3/2 (932.6 eV), Ag 3d5/2 (BE 368.2 eV), and Au 4f7/2 (BE 84.0 eV). Epitaxial MnO thin films were prepared in situ by thermal evaporation of Mn on a single-crystalline Ag(001) surface in a reactive oxygen atmosphere (2.0 × 10−7mbar) as described in ref 28. The Ag(001) single crystal (Matek GmbH) was kept at 475 K during growth of the oxide films and during a consecutive annealing in UHV for at least 2 h. Well-ordered oxide films were obtained, showing sharp low-energy electron diffraction (LEED) spots, in good agreement with literature data.27 The TiOx films were prepared by evaporation of titanium atoms from a titanium filament on a single-crystalline Pt(111) surface in a reactive oxygen atmosphere (2.0 × 10−7 to 2.0 × 10−6 mbar). Prior to preparation of either MnO or TiOx film, the silver and respectively platinum single crystal was cleaned by repeated sputtering−annealing treatment. The cleanliness of the single crystals was verified by XPS and the quality of the surface by LEED. Both FePc and CoPc films were deposited on the oxide substrate in UHV from a temperature-controlled evaporation cell at evaporations rates in the range of 0.1−0.4 nm per minute. The rates were estimated by a quartz microbalance. Determination of the film thickness was done by comparing photoemission intensities of C 1s, Mn 2p, and Ag 3d or C 1s, Ti 2p, and Pt 4f core-level signals, in consideration of the according sensitivity factor, assuming a layer-by-layer growth mode. The nominal thickness of the FePc or CoPc films was varied between about 0.3 and 3 nm. The monolayer (ML) thickness is assumed to be 0.34 nm for flat-lying molecules
3. RESULTS AND DISCUSSION 3.1. Molecular Orientation. The planar shape (cf. Figure 1) of the conjugated π-system of TMPcs enables the
Figure 1. Chemical structure of iron and cobalt phthalocyanine.
determination of the molecular orientation of the organic molecules using XAS. The preferred molecular orientation results in a strong angular dependence in polarizationdependent XAS C K- and N K-edge absorption spectra. If the electric field vector E is parallel to the molecular plane (and so to the chemical bonds), the intensity of transitions into σ* orbitals are maximal, whereas transitions into π* orbitals are observed if E is perpendicular to the molecular plane and thus parallel to the 2pz orbitals. Because the analysis of C K-edges may considerably be complicated because of common carbon contaminations of beamline components, the molecular orientation of the organic films was determined from N K absorption spectra. In the N K-edge spectra, the signals of the π* transitions are between 398 and 405 eV, whereas the σ*transitions show signals above 405 eV. FePc on MnO (Figure 2), as well as CoPc on TiOx and on Pt(111) (Figure 3) show a strong angular dependence in the spectra as expected for a molecular orientation, where the molecular plane is almost parallel with respect to the surface of the substrate (face-on or flat-lying molecular orientation). At grazing incidence (θ = 20°, where θ is the angle between incident light and sample plane) the intensity of π* transitions is maximal, whereas at normal incidence (θ = 90°) σ* transitions are most intense. At normal incidence (θ = 90°), signals with low intensity are still visible in the region of the π* resonances, in particular for multilayer films of CoPc (nom. thickness > 1.0 nm) on TiOx (i.e., Figure 3a top). This points to a tilt of the molecules with respect to the sample surface or lower degree of ordering. We note that additional second-order excitations of the Co L3-edge are also visible between 388 and 391 in the N K-edge spectra with a strong angular dependence (cf. Figure 3) as discussed for related systems in more detail in ref 21. For monolayer coverages, the remaining weak signals in the π* region do not appear at exactly the same energetic positions compared to the thin film, pointing to a different origin such as a hybridization of unoccupied nitrogen-related states with d orbitals of the central TM or a distortion of some N bonds.36−39 Thus, the appearance of additional features at normal incidence does not contradict our conclusion regarding an essentially flat-lying 27570
DOI: 10.1021/acs.jpcc.5b09612 J. Phys. Chem. C 2015, 119, 27569−27579
Article
The Journal of Physical Chemistry C
Figure 2. Linear polarized N K-edge spectra for a series of FePc films with different nominal thicknesses on a MnO thin-film substrate with a nominal thickness of 4.2 nm. (a) Normal incidence (θ = 90°): σ* transitions (at photon energies >404 eV) are maximized. (b) Grazing incidence (θ = 20°): π* transitions (
