Ni(111) - American Chemical Society

Article pubs.acs.org/JPCC

Interface Properties of VOPc on Ni(111) and Graphene/Ni(111): Orientation-Dependent Charge Transfer Hilmar Adler,†,‡ Mateusz Paszkiewicz,† Johannes Uihlein,† Małgorzata Polek,† Ruslan Ovsyannikov,§ Tamara V. Basova,||,⊥ Thomas Chassé,†,‡ and Heiko Peisert*,† †

Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Center for Light−Matter Interaction, Sensors and Analytics (LISA+), University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany § Institute for Methods and Instrumentation in Synchrotron Radiation Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany || Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Lavrentiev Avenue, 630090 Novosibirsk, Russia ⊥ Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: Interface properties of VOPc films on Ni(111) and graphene/Ni(111) were investigated by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and X-ray absorption spectroscopy (XAS). The XAS spectra reveal that the molecules grow highly ordered on both substrates, flat lying on the substrate surface. On Ni(111), strong interactions between vanadium ions of the VOPc molecules and the substrate were observed. It seems, however, that not all molecules of the first monolayer interact with the substrate. Scanning tunneling microscopy (STM) suggests that the strength of the interaction depends on the molecular orientation (oxygen-up or oxygen-down). The interaction can be completely prevented by graphene as an intermediate layer. Graphene affects not only the strength of the interaction at the interface, but also the energy level alignment.

1. INTRODUCTION In previous years, transition metal phthalocyanines (TMPcs) gained increasing attention, resulting in various applications in the field of optoelectronic devices, such as light-emitting diodes, field-effect transistors, and solar cells and, most recently, possible future spintronic nanodevices are discussed increasingly (see, e.g., refs 1−4). Besides planar molecules with D4h symmetry, nonplanar complexes with a perpendicular component of the permanent electric dipole moment are known. A representative of these polar phthalocyanines is vanadyl phthalocyanine (VOPc) where the vanadyl group is located perpendicular to the macrocycle and a distinct angle between outer (phenyl) and inner (pyrrole) rings is observed (Figure 1). Both the nonplanarity and the dipolar character of VOPc and the perfluorinated VOPcF16 render a specific, temperaturedependent polymorphism that differs significantly from those of planar nonpolar phthalocyanines.5−7 Moreover, the molecular alignment of further oxo-metal phthalocyanines might be tuned by external factors, such as temperature or even magnetic and electric fields.6,8,9 The nonplanarity of the molecules results not necessarily in a low charge transport mobility, just the example of VOPc shows that high performance thin-film transistors can be based on these materials.10 © 2015 American Chemical Society

Figure 1. Chemical structure (left) of VOPc and three-dimensional visualization of the geometry (right). Color code: red, oxygen; blue, vanadium; pink, nitrogen; gray, carbon; green, hydrogen.

At interfaces, possible dipole-up and dipole-down adsorption configurations of polar phthalocyanines allow tuning of the energy level alignment for the respective interface, as recently also demonstrated for VOPc.11 Moreover, it was suggested that due to the different oxygen-up and oxygen-down orientation VOPc may be acting as a molecular switch, where changing the Received: February 12, 2015 Revised: March 24, 2015 Published: March 26, 2015 8755

DOI: 10.1021/acs.jpcc.5b01485 J. Phys. Chem. C 2015, 119, 8755−8762

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The Journal of Physical Chemistry C

system with a base pressure of 3 × 10−10 mbar. Electrochemically etched tungsten tips were used for STM analysis. Tungsten tips were treated with hydrofluoric acid before transferring in to the vacuum system to reduce the oxide layer.

surface orientation blocks (or turns on) the local electrical conductivity.12 In addition, the transition metal may interact with substrates; for VOPc an effective antiferromagnetic coupling of the central V ions with the ferromagnetic Fe and Co (but not Ni) surfaces was recently reported.13 In the case of TMPcs, molecule−substrate interactions can drastically influence both electronic and magnetic properties of the first layer of the organic semiconductor. For related, planar TMPcs, charge transfer processes were observed depending on both the central metal atom of the TMPc and the substrate.14−20 The aim of the present work is the detailed study of interface properties of VOPc on Ni(111) and graphene/Ni(111) using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and scanning tunneling microscopy (STM). It was shown recently for TMPc/metal interfaces that interface properties such as charge transfer or chemical reactions can be tuned by the insertion of graphene as a buffer layer.21,22

3. RESULTS AND DISCUSSION 3.1. Molecular Orientation in Thin Films and Electronic Structure. In particular for anisotropic molecules, the orientation can significantly affect interface properties due to a different strength of the interaction. For planar TMPcs on single crystalline metal substrates, the molecules grow often in a flat lying geometry with respect to the substrate surface (see, e.g., ref 27). For nonplanar Pcs such as VOPc an additional degree of freedom exists, the orientation of the molecular dipole moment. It was reported that VOPc adsorbs on Ag(111) in a parallel orientation to the surface with an oxygen-up configuration whereas an oxygen-down configuration was observed on Si(111).28 Also, for the relatively inert graphite surface the formation of well-ordered, unidirectionally aligned VOPc layers with oxygen-up configuration was reported.29 Different results were obtained for Au(111): Whereas scannedenergy mode photoelectron diffraction studies suggest that VOPc is oriented flat lying with oxygen-up configuration,30 a mixed geometry with (less stable or site-dependent) oxygendown and oxygen-up orientation was obtained from STM.12,31 In contrast to the other single crystalline surfaces, on Pt(111) VOPc might even grow perpendicular to the substrate surface.32 It seems therefore that both the kind of substrate and the preparation conditions significantly affect the growth of VOPc. In order to study the molecular orientation in thin films we performed polarization-dependent XAS measurements. For planar π-conjugated carbon systems, the molecular orientation can be probed by monitoring the relative intensities of excitations from occupied C 1s core levels into unoccupied molecular levels (π* or σ*). For phthalocyanines, in addition, N K edge absorption spectra can be analyzed, avoiding contributions of common carbon contaminations of beamline components. If the electric field vector E is perpendicular to the molecular plane (i.e., parallel to the 2pz orbitals), the intensity of the excitation from C 1s or N 1s core levels into a π* orbital is maximal, whereas transitions from C 1s or N 1s to σ* are strongest if E is parallel to the molecular plane and to the chemical bonds.27,33 Although for polar phthalocyanines such as VOPc or TiOPc (titanyl phthalocyanine) the angle between outer (phenyl) and inner (pyrrole) rings can be about 7°,34,35 the preferred molecular orientation can be determined from N K edge absorption spectra. In Figure 2 we show N K XAS spectra for VOPc thin films on Ni(111) and graphene/Ni(111). Analogously to other phthalocyanines, features below photon energies of 401 eV (A and B) arise from transitions with mainly π* character.27 For related TMPcs, the most intense π* resonances at the lowest photon energy are assigned to transitions from N 1s to LUMO eg orbitals.36,37 The more complex structure of this feature indicates therefore an involvement of the ligand LUMO in the hybridization with metal d-states, which is most evident for transition metals with partly occupied eg d-levels. Accordingly, for VOPc, these features A and B in Figure 2 were recently assigned to transitions from the N 1s state to the LUMO and LUMO+1 states of N character.38 Generally, the spectral shape of the lowest lying feature is very similar to copper

2. EXPERIMENTAL SECTION The Ni(111) single crystal has been cleaned by argon ion sputtering and annealing. The cleanliness was checked by XPS. For graphene preparation, the Ni(111) single crystal has been heated up to 450 °C in a adjacent preparation chamber and treated with propene (99.5%, Westfalen AG) at a pressure of 9 × 10−6 mbar for 5 min. The quality of the formed graphene layers has been controlled by XPS and ARUPS measurements. VOPc was synthesized by heating a 4:1 mixture of 1,2dicyanobenzene and V2O5 at 220 °C.23 VOPc has been deposited at evaporation rates between 1 and 4 Å/min, estimated from a quartz microbalance and XPS intensity ratios using sensitivity factors from Yeh and Lindau24 assuming layerby-layer growth. The pressure during deposition of the VOPc was typically 4 × 10−9 mbar. The XAS measurements have been performed at the third generation synchrotron radiation source BESSY II (Berlin) using the endstation SurICat at the Optics-beamline. The energy resolution for photoemission (PES) and XAS was set to about 100 meV at a photon energy of 400 eV. The absorption was monitored indirectly by measuring the total electron yield (sample current). For photon energy calibration the binding energies (BE) of the Au 4f7/2 peak excited by first and second order light have been compared and/or the energy of the Ni L3 edge (852.7 eV) was taken as a reference.25 The XAS spectra have been normalized to the same absorption step height. Due to second order excitation of the Ni L edge in the N K edge region (about 427 eV) the normalization of the spectra was carried out at 426 eV resulting in some imprecisions due to residual intensity of σ* transitions at this energy. Therefore, the main attention was set on the qualitative evaluation of the spectral features. Peak fitting of XPS spectra was performed using the program Unifit 2008.26 XPS and UPS measurements performed at SurICat were reproduced in the home lab using a multichamber UHV system with a base pressure of 2 × 10−10 mbar. The spectrometer is equipped with an Omicron hemispherical analyzer (EA 125), a conventional X-ray tube (Omicron DAR 400) and a helium discharge lamp (Leybold-Hereaus UVS10/35). The XPS spectra have been taken using Mg Kα radiation (1253.6 eV), the UPS spectra have been measured using He I (21.22 eV) or He II (40.8 eV) radiation. STM measurements were performed at an Omicron VTSTM at room temperature in a separate multichamber UHV 8756


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(see Supporting Information, Figure S1). Generally, the spectra are composed of the L3 (513−520 eV) and L2 (522−527 eV) absorption features attributed to 2p3/2 → 3d and 2p1/2 → 3d electron transitions, respectively. In addition, the O K edge is visible around 530 eV. We will discuss more in detail the wellresolved L3 absorption edge. Resonances excited in the molecular plane, probed at normal incidence of the incoming synchrotron light are denoted B1−B3, out-of-plane transitions are A1 and A2. It was predicted that the dxy (b2g) orbital of V (formally a d1 configuration) may contribute significantly to the HOMO of VOPc, whereas the LUMO contains only a small contribution from the V and O atoms.38 Other calculations suggest that both the HOMO and the LUMO of the molecule are of pure ligand π and π* character, respectively.39 The lowest lying absorption features in Figure 3 are visible at normal incidence (90°), that is, the transitions in the molecular plane. Although core level excitation spectra of the central metal atom of the TMPcs are determined to a large extent by multiplet effects due to the strong overlap of the core wave function with the valence wave functions,40 the observed angular dependence of B1 and B2 indicates that the lowest orbital of the vanadium central ion is dxy (b2g) and not an orbital with distinct out of plane character (dyz, dzx). 3.2. Interface Interactions. As mentioned above, it has been shown that the central metal atom of TMPcs can be involved in interfacial charge transfer processes that can be tuned by graphene intermediate layers, depending on the system under consideration. In order to study a possible interfacial charge transfer or chemical interactions for VOPc we performed core level photoemission measurements. V 2p XPS spectra for VOPc on Ni(111) and graphene/Ni(111) are compared in Figure 4 as a function of the film thickness. The

Figure 2. Polarization-dependent N K edge XAS spectra for VOPc on Ni(111) and graphene/Ni(111). The strong angular dependence indicates a highly ordered growth with flat lying adsorption geometry in both cases.

phthalocyanine (CuPc), indicating that the interaction between the cation and the phthalocyanine ligand is relatively weak.14,38 It is clearly visible that the relative intensities of the spectral features A and B vary strongly with the angle θ of the incident radiation; the maximum intensity of the π* transitions is found at grazing incidence (θ = 10°) of the p-polarized light (see inset for geometry). The almost vanishing remaining intensity at normal incidence indicates that the VOPc molecule planes are well oriented on both substrates, oriented flat lying on the substrate surface. The remaining intensity is mainly found in the energy range of feature B. This may indicate that, in particular, this feature contains transitions, which are not of pure out-of-plane character due to the nonplanar geometry of the molecule. Due to the high degree of orientation, also the related vanadium L2,3 absorption spectra show a distinct angular dependence. As an example, we discuss in Figure 3 VOPc on graphene/Ni(111); spectra for VOPc/Ni(111) are very similar

Figure 4. Comparison of V 2p core level spectra for VOPc on Ni(111) and graphene/Ni(111). In the case of Ni(111), an interface component is clearly visible, indicating a chemical interaction at the interface for a part of the first monolayer.

spectra were fitted assuming a single doublet for the 2p3/2 and 2p1/2 components (red curve), resulting in a spin−orbit splitting of 7.3 eV. Satellite lines due to the multiplet structure can be described by a single broad peak at about 520 eV (black line). For the thickest films we found a V 2p3/2 binding energy (EB) of 516.4 eV on Ni(111), whereas EB(V 2p3/2) on graphene/Ni(111) was slightly higher (516.7 eV). A possible reason for this difference might be a different energy level

Figure 3. Polarization-dependent V L edge XAS spectra for VOPc on graphene/Ni(111). The strong angular dependence of the flat lying molecules is caused by excitations in orbitals with in-plane (90°) and out-of-plane (10°) character. 8757


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Figure 5. VOPc (∼ monolayer) on Ni(111): (a) STM image (50 × 50 nm) showing the ordered growth of the molecules, bright and dark molecules are related to a different orientation (oxygen-up or oxygen-down) adsorbed VOPc. Vtip = −200 mV, I = 50 pA; (b) apparent height for the average line scan indicated in (a); (c) Fourier analysis showing the hexagonal arrangement; (d) PSD analysis to reveal the dominant frequency.

indicates therefore that the graphene buffer layer can prevent the strong interaction between VOPc and Ni(111). The question now arises, whether or not the molecular orientation of the first VOPc layer on Ni(111) may explain that only a part of the molecules undergo strong interactions at the interface to Ni(111). For this reason, we performed STM measurements for a monolayer coverage of VOPc on Ni(111). The STM image in Figure 5 shows that the Ni(111) surface is uniformly covered by VOPc; due to the submolecular resolution the 4-fold D4h symmetry of the molecule is visible. The molecules adopt a hexagonal packaging within this first monolayer in contrast to the known well-ordered rectangular unit cell on less reactive surfaces.12,31 The Fourier analysis shown in Figure 5c reveals more clearly the ordered arrangement with 6-fold symmetry. The average distance between the molecules was determined by the dominant frequency of the power-spectral-density (PSD) analysis to be about 1.79 nm, a value that is somewhat higher compared to VOPc on other less-reactive surfaces.29 It is also visible from Figure 5 that the molecules appear with a different brightness which is almost randomly distributed among them. The apparent height difference between the bright and dark molecules is about 0.05 nm (see line scan in Figure 5a) and the analysis in Figure 5b) and, thus, significantly lower than the expected value for a molecule adsorbed in a second layer on the surface. Moreover, about one-third of the molecules show a protrusion, whereas the other two-thirds show an apparent dip in the center of the molecule. The different brightness is therefore ascribed, in agreement with the literature,12,29 to a different oxygen-up and oxygen-down adsorption geometry of VOPc. The assignment of the oxygen-up and oxygen down configuration to the bright and dark molecules is discussed differently: The dark central region can be attributed to the oxygen atom reducing the number of available states near the Fermi energy;12 on the other hand, the central bright dot was ascribed to oxygen-up VOPc molecules with the O atom pointing toward the vacuum.29 However, in any case, our STM data reveal that both VOPc configurations, oxygen-up and oxygen-down, are present on the Ni(111) surface, and thus, the different reactivities of molecules within the first monolayer might be understood. In addition, the data show that even if XAS data reveal that the molecular planes are

alignment at the interface, as discussed below. Only at the interface to Ni(111), we find a lowering of EB of the main component by 0.5 eV, which can be understood analogously to other TMPcs by polarization screening at the interface.41 However, most visibly, an additional V 2p component (blue curve) appears for the 0.4 nm coverage on Ni(111), which can be described as an additional doublet with EB (V 2p3/2) = 513.3 eV. This film thickness corresponds to about one monolayer of flat lying molecules considering the intermolecular distance of about 0.34 nm in the triclinic crystal structure.35 In addition, at the monolayer coverage the relative intensity of the oxygen signal is increased and cannot be described by a single component, pointing to the presence of additional oxygen on the comparable reactive Ni(111) surface. The oxygen might come from a contamination during the VOPc evaporation or alternatively due to a reaction of VOPc with the surface and a subsequent desorption of parts of the molecule. We note however that we estimate from the additional O 1s intensity one oxygen atom per 50 nickel surface atoms (2% coverage), thus, this influence on the interaction of other VOPc molecules at the interface might be neglected. The V 2p interface peak for VOPc on Ni(111) reminds to other TMPc/Ni(111) interfaces where additional peaks, shifted up to 2 eV lower EB were observed. The origin of these was discussed in terms of a charge transfer from the substrate to the central metal atom of the TMPc.14 However, in contrast to other TMPcs, only a part of the VOPc molecules seem to undergo a possible charge transfer; the main V 2p peak for the monolayer coverage (0.4 nm) of VOPc on Ni(111) in Figure 4 (red curve) can still be assigned to unaffected molecules. We note that from photoemission we have no clear hints for the involvement of the macrocycle in a strong interaction at the interface such as a chemical reaction or local charge transfer to N or C atoms (see Supporting Information, Figure S2). This is in contrast to VOPc on Ag(111), where a strong interaction between the N and C atoms of VOPc and surface Ag atoms was observed.28 The introduction of graphene as an intermediate layer changes the situation drastically: The V 2p and O 1s spectra in Figure 4 are almost independent of the film thickness. We note that there is also no indication from C 1s and N 1s core level spectra for a strong interaction at the interface (see Supporting Information, Figure S3). The absence of an interface peak 8758


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Figure 6. Comparison of UPS valence band spectra (hν = 40.8 eV) for VOPc on Ni(111) and graphene/Ni(111). Right: Development of the work function with increasing layer thickness, determined from the high binding energy cutoff of valence band spectra (hν = 21.22 eV). Although the work function is very similar for a coverage of about 2 nm, the position of the HOMO is different.

compared to He I excitation. The respective bottom spectra are representative of the clean substrate, with the Ni 3d bands being clearly visible. The superposition of substrate related features with VOPc related features complicates in particular the analysis of possible interface states close to the Fermi level. A reliable evaluation of the energetic position of the HOMO for coverages in the nm range is however ensured, an example is shown as Supporting Information (Figure S4). We note that, according to ref 11, a slightly lower binding energy of the HOMO might be expected for molecules with oxygen-up configuration which cannot be resolved in our measurements. The evaluation of the onset of the HOMO implies however that contributions of these molecules determine discussed energetic values. At the highest film thickness (3.7 nm) VOPc related features have been developed, the residual intensity in the region of Ni 3d bands suggests island growth mode. Most notably, the position of the HOMO relative to the Fermi level depends on the substrate: In the case of VOPc on Ni(111) EHOMO is 0.9 eV for coverages between 1 and 3.7 nm and, thus, 0.4 eV lower compared to VOPc on graphene/Ni(111) (EHOMO = 1.3 eV). The development of the work function Φ with the film thickness, determined from the high binding energy cutoff of UPS spectra (He I excitation), is displayed in Figure 6 (right). First, the formation of graphene on the Ni(111) substrate results in a distinct lowering of Φ from 5.3 to 4.1 eV. The adsorption of VOPc induces clear changes of Φ on both substrates: On Ni(111), Φ is lowered by 0.9 eV, whereas on graphene/Ni(111) an increase of 0.3 eV is observed. Such a change of Φ at the interface indicates the formation of an interface dipole, pointing in different directions for both substrates. Generally, potential drops at interfaces to metals can be understood by the push back of the electron cloud of the metallic substrate (often also called Pauli repulsion or pillow effect). The size of this effect is discussed controversially, but for several systems, often values in the order of ∼0.3 eV are found.42−45 For the formation of the large interface dipole in the case of VOPc on Ni(111) (−0.9 eV), additional mechanisms have to be considered, most likely a charge (electron) transfer from VOPc to the metal takes place. Since from V 2p spectra an opposite charge transfer was concluded,

highly oriented, a kind of disorder might be induced by the arbitrary oxygen-up or oxygen-down orientation. The presence of an oxygen-up and oxygen-down might be understood by the strength of the interaction of the molecule on the substrate surface. As an example, in ref 13 it was concluded that VOPc shows an oxygen-up configuration on a Ni(001) film grown epitaxially on Cu(001) due to strong interaction between Pc π orbitals and Ni d levels. On the other hand, as discussed above, different orientations were observed, even if the substrates are similar. It seems therefore that both the adsorption energy for each configuration and the energy of the molecule approaching the substrate surface can distinctly affect the final adsorption geometry. Initially, upon arrival of the molecules on the substrate surface the probabilities for oxygenup and oxygen-down orientations should be the same. If the energy of the initially adsorbed molecule is not high enough to overcome the activation energy for the desorption, it may remain in the less favored energetic adsorption geometry and a mixture of oxygen-up and oxygen-down configurations on Ni(111) is observed. Since the different initial orientations can affect significantly the energy level alignment (ELA) at the respective interface,11 the influence of graphene on ELA becomes important. We carried out UPS measurements in order to study the energy level alignment for VOPc on Ni(111) and graphene/Ni(111). In UPS, the work function of a sample Φ can be determined by the difference between the photon energy and the width of the UPS spectrum, the latter being given by the energetic separation of the high binding energy (secondary electron) cutoff and the Fermi energy. The HOMO position EHOMO relative to Fermi energy is directly extracted from the spectra and the ionization potential IP corresponds to the sum of EHOMO and Φ. The energetic position of the cutoff and of the HOMO was determined from the respective onset in the UPS spectra. In Figure 6 we compare selected UPS spectra using He II excitation (hν = 40.8 eV) for VOPc on Ni(111) and graphene/ Ni(111) as a function of the film thickness. At this excitation energy graphene related features at about 10 eV are clearly visible and the secondary electron background is lower 8759


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Figure 7. Energy level diagrams for 2.6 nm VOPc on Ni(111) and graphene/Ni(111). Although the work function Φ is about the same for VOPc on both substrates, the energetic position of the HOMO and the ionization potential (IP) is clearly different. All values are given in eV.

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the situation seems to be more complex and a bidirectional interfacial charge transfer mechanism involving both occupied and unoccupied molecular orbitals might be considered, analogously to other TMPc interfaces.19,46−48 Interestingly, values for Φ reach about the same value of 4.4 eV at coverages of 1−2 nm and they stay almost constant at higher film thickness, indicating that the dipole formation is completed at a thickness of 1−2 nm. The energy level alignment is summarized in Figure 7 for a VOPc film thickness of 2.6 nm. From Figure 7 it is clearly visible that the different positions of the HOMO on both substrates results not in a change of Φ, but in a change of the ionization potential (IP). The behavior reminds to other organic films, where IP depends strongly on the preparation condition. As example, it was shown that the IP of sexiphenyl is 0.7 eV lower if grown on disordered instead of ordered TiO2. Also, the HOMO position is 0.6 eV closer to the Fermi level for disordered films. The effect can be understood by a different extramolecular screening of the photohole created in photoemission49 or by intrinsic surface dipoles due to the πelectron clouds over the molecular planes.50 In our case, in addition the permanent dipole of the molecule has to be considered. The higher IP (Figure 7) for VOPc on graphene/ Ni(111) points therefore to a more ordered growth compared to VOPc on Ni(111), possibly due to a uniform oxygen-up orientation of the first monolayer, as reported for VOPc on graphite.11,29

ASSOCIATED CONTENT

S Supporting Information *

Polarization-dependent V L edge X-ray absorption spectra for VOPc on Ni(111), C 1s and N 1s core level spectra as well as UPS valence band spectra are shown. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 7071 2976931. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the German Research Council (PE 546/5-1 and CH 132/23-1). We acknowledge the Helmholtz Zentrum Berlin GmbH, Elektronenspeicherring BESSY II for providing synchrotron radiation at the Optics beamline. Financial travel support by Helmholtz Zentrum Berlin GmbH is gratefully acknowledged. We thank W. Neu for technical support.

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REFERENCES

(1) Martinez-Diaz, M. V.; de la Torrea, G.; Torres, T. Lighting Porphyrins and Phthalocyanines for Molecular Photovoltaics. Chem. Commun. 2010, 46, 7090−7108. (2) Reineke, S.; Thomschke, M.; Lussem, B.; Leo, K. White Organic Light-Emitting Diodes: Status and Perspective. Rev. Mod. Phys. 2013, 85, 1245−1293. (3) Mei, J. G.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. N. Integrated Materials Design of Organic Semiconductors for FieldEffect Transistors. J. Am. Chem. Soc. 2013, 135, 6724−6746. (4) Sanvito, S. Molecular Spintronics. Chem. Soc. Rev. 2011, 40, 3336−3355. (5) Yanagi, H.; Mikami, T.; Tada, H.; Terui, T.; Mashiko, S. Molecular Stacking in Epitaxial Crystals of Oxometal Phthalocyanines. J. Appl. Phys. 1997, 81, 7306−7312. (6) Basova, T. V.; Kiselev, V. G.; Dubkov, I. S.; Latteyer, F.; Gromilov, S. A.; Peisert, H.; Chasse, T. Optical Spectroscopy and Xrd Study of Molecular Orientation, Polymorphism, and Phase Transitions in Fluorinated Vanadyl Phthalocyanine Thin Films. J. Phys. Chem. C 2013, 117, 7097−7106. (7) Fronk, M.; Bräuer, B.; Zahn, D. R. T.; Salvan, G. Temperature Dependence of the Optical Anisotropy of Vanadyl Phthalocyanine Films. Thin Solid Films 2008, 516, 7916−7920.

4. CONCLUSION We studied the electronic structure of VOPc and interface properties to Ni(111) and graphene/Ni(111). Polarizationdependent XAS measurements reveal that VOPc planes are highly oriented, flat lying on both substrate surfaces. Further from XAS we confirmed experimentally that the lowest transition has in plane character indicating that the lowest orbital of the vanadium central ion is dxy (b2g) and not an orbital with distinct out of plane character (dyz, dzx). The central metal atom of VOPc undergoes charge transfer at the interface to Ni(111) most likely depending on the molecular orientation at the interface (oxygen-up or oxygen-down). The charge transfer can be completely prevented by the introduction of a graphene intermediate layer. Graphene affects also the energy level alignment at the interface−the relative position of the HOMO to the Fermi level was distinctly lowered due to the buffer layer between Ni(111) and VOPc. 8760


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Ni(111) - American Chemical Society

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