Influence of Graphene on Charge Transfer between CoPc and Metals

Article pubs.acs.org/JPCC

Influence of Graphene on Charge Transfer between CoPc and Metals: The Role of Graphene−Substrate Coupling Johannes Uihlein,†,∥ Małgorzata Polek,†,∥ Mathias Glaser,† Hilmar Adler,† Ruslan Ovsyannikov,‡ Maximilian Bauer,‡ Milutin Ivanovic,† Alexei B. Preobrajenski,§ Alexander V. Generalov,§ Thomas Chassé,† and Heiko Peisert*,† †

Institute of Physical and Theoretical Chemistry, 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-Strasse 15, 12489 Berlin, Germany § MAX-Laboratory, Lund University, Box 118, 22100 Lund, Sweden ‡

S Supporting Information *

ABSTRACT: The electronic structure of cobalt phthalocyanine (CoPc) on Pt(111), graphene/Pt(111), and Au-intercalated graphene/Ni(111) is investigated by photoexcited electron spectroscopies: photoemission (XPS and UPS) and X-ray absorption spectroscopy (XAS or NEXAFS). For CoPc on Pt(111), significant changes of the shape of XPS and XAS spectra indicate a charge transfer from the metal substrate to the Co ion of CoPc. The strong interaction between CoPc and Pt(111) can be completely prevented by the insertion of a graphene buffer layer. For CoPc on graphene/ Ni(111), the charge transfer is only prevented if the graphene on Ni(111) is intercalated by gold. Therefore, the disturbance of the graphene electronic structure by the interaction with underlying substrate and the corresponding charge doping of graphene has been found to affect the electronic properties of adsorbed CoPc considerably.

1. INTRODUCTION In recent years, huge efforts have been put into the investigation of organic semiconducting materials for application in electronics and optoelectronics as well as, more recently, spintronic processing and data storage devices.1−9 The class of transition metal phthalocyanines (TMPcs) provides molecules highly suitable for these requirements, which is in large parts due to the tuning possibility of their electronic and optical properties as well as their stability against environmental conditions (e.g., humidity, oxygen, light, and heat).10,11 The behavior and performance of the TMPc molecules in devices can be strongly influenced by interactions at interfaces, as observed for many systems.12−15 Furthermore, the contact to specific substrate materials can alter or even quench the magnetic properties of the TMPcs.16,17 Such interactions can be influenced in strength and nature by systematic variation of both the substrate material and the central metal atom of the TMPc.17,18 For first row transition metal phthalocyanines, such interactions at interfaces can be systematically tuned. Depending on the transition metal, the d-levels can considerably contribute to the highest occupied molecular orbital (HOMO) and/or lowest unoccupied molecular orbital (LUMO) of the TMPc. We note however that the detailed electronic configuration is still controversially discussed in the literature, in particular at interfaces.19−26 © 2015 American Chemical Society

Instead of changing the substrate or varying the adsorbed molecule desired, a further possible way of tuning the interactions at the interface is provided by the insertion of a buffering layer between. Graphene has turned out to be a useful material for such an application and pleases with its easy accessibility by simple catalytic decomposition of propene gas on hot metal surfaces as well as its stability, flexibility, and remarkable electronic properties. This has been reported for the interactions of several TMPcs on Ni(111); graphene interlayer may transmit even a magnetic coupling between iron phthalocyanine and nickel.27 However, the influence of the graphene buffer layer likewise depends on the central metal atom used. CoPc for its part experiences a distinct charge transfer from the Ni(111) substrate to its central metal atom, which is not prevented by the insertion of the graphene layer resulting in a comprehensive variation of the interfacial electronic structure.28 At this point, it has to be mentioned that graphene prepared on a Ni(111) surface strongly differs from freestanding graphene due to a modification of the electronic structure and a doping by the substrate leading to a shift of the Received: March 26, 2015 Revised: June 5, 2015 Published: June 5, 2015 15240

DOI: 10.1021/acs.jpcc.5b02912 J. Phys. Chem. C 2015, 119, 15240−15247

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The Journal of Physical Chemistry C graphene’s band structure to higher energies.29 In particular, the close lattice matching between graphene and Ni(111) yields to a strong orbital hybridization at the interface and therefore to a substantial charge transfer from Ni to graphene.30,31 By intercalating the graphene with noble metals (e.g., gold, silver, or copper) or even other elements, this interaction can be modified affecting the degree of doping. In this manner, the coupling of the evaporated TMPc molecules at the interface might be tuned systematically. Moreover, the kind of substrate affects significantly graphene chemical activity and the degree of doping, offering an additional route for a systematic variation. The focus of the present work is set on the influence of the degree of doping of graphene on interactions between CoPc and metals. The graphene was modified by intercalation with Au and by the variation of the substrate (Ni(111) and Pt(111)). For investigating the occupied and unoccupied electronic structure at interfaces, X-ray photoemission and absorption spectroscopies (XPS and XAS) have been used.

3. RESULTS AND DISCUSSION 3.1. Molecular Orientation. Because of the anisotropic nature of XAS, a detailed knowledge of the molecular orientation is beneficial for the understanding of the electronic structure. The orientation can be extracted from polarizationdependent carbon or nitrogen K edge absorption spectra. Regarding the planar, discoid shape of the phthalocyanine molecules, the transitions from the 1s orbitals into the unoccupied π* and σ* orbitals are out of the molecular plane or in-plane, respectively. Therefore, the absorption intensity of these transitions changes in the case of oriented molecules as a function of the angle θ between the sample normal and the electric field vector of the exciting linearly p-polarized light. In Figure 1 we show N K edge spectra of a 2 nm thick CoPc film

2. EXPERIMENTAL SECTION XAS and XPS measurements have been performed at the third generation synchrotron radiation sources BESSY II in Berlin using the SurICat endstation at the Optics-beamline (CoPc on intercalated graphene/Ni(111)) and at MAX IV Laboratory (Lund) at the beamline D1011 (CoPc on graphene/Pt(111)). For photon energy calibrations, the binding energies (BE) of Au 4f7/2 or Pt 4f7/2 excited by first- and second-order light have been compared. Further, the Ni L3 of the Ni(111) substrate edge at 852.7 eV32 has been used for referencing the absorption spectra. The energy resolution of beamline and analyzer for XPS and XAS measurements was manually set to ∼100 meV at a photon energy of 400 eV. The X-ray absorption was monitored indirectly in total electron yield (TEY) mode by measuring the sample current or in partial electron yield (PEY) mode using a microchannel plate detector. The N K edge spectra were normalized to the same step height. For N K edge absorption on Ni substrates, the second-order excitation of the Ni L edge at about 426.4 eV is in superposition with (weaker) σ* transitions resulting in some imprecisions. The main attention was therefore set on the qualitative evaluation of the spectral features. To discuss changes of the spectral shape, also Co L3 edge spectra were normalized to the most intense feature. The single crystal substrates have been cleaned by multiple cycles of argon ion sputtering and annealing; the cleanliness has been checked by photoemission. Graphene has been prepared by annealing the single crystals up to 460 °C and exposing to propene (99.5%, Westfalen AG) for 5 min at a pressure of 9 × 10−6 mbar (approximately 2000 L).33 The quality of the resulting graphene layers has been controlled by XPS and angle resolved UV photoemission spectroscopy (ARUPS) measurements. The intercalation of the graphene on Ni(111) was carried out by the evaporation of 1−2 monolayers of gold (approximately 0.5 nm, estimated by a quartz microbalance) and a subsequent annealing step for 60 min at 380 °C. CoPc (Sigma-Aldrich) has been evaporated at a rate of approximately 1 Å/min, estimated by a quartz microbalance prior the deposition. The final layer thickness has been calculated from XPS intensity ratios assuming layer-by-layer growth and using the sensitivity factors from Yeh and Lindau.34 The peak fit analysis was performed using UNIFIT.35

Figure 1. N K edge absorption spectra of CoPc on gold intercalated graphene on Ni(111) for the two prominent angles of incidence of the p-polarized synchrotron light. The measurement geometry is schematically displayed as an inset. Blue curve, normal incidence (90°); black curve, grazing incidence (10°).

on Au intercalated graphene grown on Ni(111) for the two prominent angles grazing (θ = 10°) and normal incidence (θ = 90°). The measurement geometry is schematically displayed as an inset. The spectra can be divided into two regions reaching from 395 to 404 eV and from 404 to 424 eV photon energy. The lower energy region can be assigned to transitions into π* orbitals, which show their maximum intensity at grazing incidence, decreasing with increasing θ. The most distinct features are labeled with A−D in close agreement with the literature, although not all studies resolve the shoulder on the high energy side of feature A, here labeled with B.36−38 The signals in the region of higher photon energy labeled with E and F can be attributed to transitions into the in-plane σ* orbitals and show highest intensity at normal incidence, decreasing in intensity with decreasing θ. The angular behavior indicates a preferred flat lying orientation of TMPcs, similar to TMPc films on single crystalline metal substrates or graphene/ Ni(111).39−42 The residual intensity in the region of out-ofplane transitions is mainly due to an overlap with the Co L2 edge, excited by second-order light (broader feature at 397 eV, discussed more in detail in Supporting Information Figure S1 and ref 43); also, weak in-plane transitions with excitation energies in the range π* can be expected.37,44−46 The low intensity in this range points to only a small tilt angle of the molecules with respect to the substrate surface for CoPc on intercalated graphene. In other words, the preferred flat molecular orientation observed for CoPc on graphene/ Ni(111)28 is not affected distinctly by the intercalation process. 15241


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The Journal of Physical Chemistry C For thin films of CoPc on Pt(111) and on graphene/ Pt(111), similarly the maximum intensity in the region of π* transitions is found at grazing incidence, indicating a preferred flat lying molecular orientation (Figure 2, see also Supporting

Figure 3. Photoemission spectra of the valence band (a) and C 1s region (b) as a function of ongoing substrate preparation (1, clean Ni(111) surface; 2, graphene on Ni(111); 3, Au evaporated on graphene; 4, Au intercalated underneath graphene after annealing).

compare the data to results obtained in the home-lab, we chose the excitation energy of 40.8 eV according to the He II emission line. The spectrum for the clean Ni(111) surface labeled with 1 in Figure 3 is dominated by the intense Ni 3d states close to the Fermi energy EF. After graphene preparation by catalytic reaction of propene on the annealed Ni(111) surface, a single broad signal of the graphene π-band occurs at a binding energy of 9.8 eV, shown in spectrum 2 of Figure 3. Both substrate and graphene spectra are in good agreement with the literature.51−53 The evaporation of a thin layer of gold (about 0.5 nm) attenuates the signal of the graphene π-band and gives rise to distinct features in the region from 2 to 7 eV, assigned to Au 5d states.53 The intercalation of this gold layer underneath the graphene after annealing up to 640 K leads to a decrease of the Au signals. The graphene π-band, now dominating spectrum 4 shown in Figure 3, is shifted by about 2.0 eV to lower binding energies due to decoupling from the Ni(111) substrate.53 Similar but smaller energetic shifts were also observed in the according C 1s core level spectra displayed in Figure 3 (right). Such shifts are generally expected due to the shift of the Fermi level (i.e., the reference level in photoemission) upon (de-)doping of graphene. The smaller energetic shift of the C 1s core level as compared to carbon derived valence band features points however to a significant influence of the varied charge density at the corresponding atoms (chemical shift). The C 1s signal is shifted about 0.6 eV from 284.7 eV for graphene on Ni(111) to 284.1 eV after the intercalation with gold, a value that can be expected according to the literature.54−56 For the small shoulder at lower binding energy (283.5 eV) in spectrum 2 of graphene on Ni(111), several possible origins are discussed, like unreacted propene or propene fragments adsorbed to the Ni(111) surface or the formation of nickel carbide. Among these, the formation of propene fragments is assumed to be most likely.55 3.2.2. Interface Properties of CoPc on Intercalated Graphene. We will discuss now whether or not the decoupling of graphene by Au intercalation affects the interface properties of the subsequently evaporated CoPc. We note that there is no hint for interactions of the CoPc macrocycle from both N 1s and C 1s spectra. The C 1s spectra in Figure 4a can be well described by a superposition of graphene (red curves) and phthalocyanine related features (blue curves). The bottom spectrum of Figure 4a shows the spectrum of the C 1s region of

Figure 2. N K edge absorption spectra of CoPc on Pt(111) (a) and graphene/Pt(111) (b) for the two prominent angles of incidence of the p-polarized synchrotron light. The measurement geometry is schematically displayed as an inset. Black curve, grazing incidence (30°); blue curve, normal incidence (90°). The remaining intensity in the energy range of π* transitions in the normal incidence spectra indicates the presence of tilt angles in the case of thin films in the nanometer range.

Information Figure S1). However, the remaining π* intensity at normal incidence (90°) is pointing to an increased average tilt angle or a higher degree of disorder. For related systems, such as FePc on Ni(111)47 or zinc phthalocyanine (ZnPc) on Pt(111)48 analogously for molecules in films with a film thickness of few nanometers, XAS measurements indicate the presence of an increasingly tilted adsorption geometry. It seems that the introduction of graphene between FePc and Ni(111) results in a weaker physisorption associated with an ordered growth of the molecules oriented parallel to the substrate surface.47 Possibly as a consequence of the increased corrugation of graphene on Pt(111),49 the initial orientation may change more distinctly as a function of the layer thickness, because substrate roughness has a substantial impact on the molecular orientation in phthalocyanine thin films.50 For interactions at the interfaces, such as charge transfer, in particular the orientation of the first molecular layer has to be considered. We compare therefore in Figure 2 also N K XAS spectra for CoPc coverages in the monolayer range on Pt(111) and graphene/Pt(111). It is clearly visible that the remaining intensity in the energy range of π* transitions is clearly lower in the normal incidence spectra as compared to the thin film. This hints to an almost perfect flat lying orientation of the first molecular layer and the formation of tilt angles during the growth of the thin film. However, in all cases, in view of the adsorption geometry CoPc may interact similarly with the substrates. 3.2. CoPc on Au-Intercalated Graphene. 3.2.1. The Influence of Intercalation on the Doping of Graphene. To validate the substrate preparation and the influence of the intercalation of the graphene layers on the energetic position of valence band features, UPS spectra have been recorded at BESSY II after each preparation step displayed in Figure 3. To 15242


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Figure 4. C 1s (a) and Co 2p (b) core level spectra of CoPc on gold intercalated graphene on Ni(111) as a function of the CoPc film thickness. No interface peak is visible pointing to the absence of charge transfer involving the Co ion. The C 1s spectra can be well described by a superposition of graphene (red curves) and phthalocyanine related features (blue curves).

Figure 5. Co L3 edge XAS spectra of CoPc on gold intercalated graphene on Ni(111) for the two prominent incidence angles of ppolarized synchrotron light (a, grazing 10°; b, normal 90°) as a function of the film thickness. As a reference, spectra for CoPc on graphene/Ni(111) are included (bottom spectra),28 showing clear hints for charge transfer.

intercalated graphene on Ni(111) as discussed in Figure 3b; the peak shape was kept constant for all spectra. With increasing thickness of the adsorbed CoPc layer, a second peak arises on the high binding energy side; at a thickness of 2.0 nm, the shape shows clearly the characteristic features known for phthalocyanine C 1s spectra.57,58 The phthalocyanine related features were described by components for the aromatic carbon of the benzene rings, pyrrole carbon linked to nitrogen and corresponding satellites as described in ref 58. To analyze possible interactions at the interface involving the central metal atom of CoPc, we will discuss now Co 2p core level photoemission spectra on intercalated graphene as a function of the film thickness shown in Figure 4b. The spectra of films in the nm range show the typical multiplet structure as known for CoPc.14,17 Charge transfer processes between CoPc and several metals including silver, gold, and even graphene/Ni(111) were recently observed;14,17,28,59−64 such interactions result in an additional interface component at about 2 eV lower binding energy visible for coverages in the monolayer range.14,28 However, in contrast to the other substrates in Figure 4, no distinct change in both the shape and the binding energy is recognizable with decreasing film thickness, pointing to the absence of charge transfer between the Co ion of CoPc and Au intercalated graphene. Hardly visible is a small shoulder on the low binding energy side of the Co 2p spectrum for 0.5 nm CoPc on intercalated graphene, which might be due to adsorption on defect sites or traces of Au on top of the graphene layer. Complementary results can be drawn from the corresponding Co L edge absorption spectra. In Figure 5 we focus on the Co L3 edge and two prominent angles, grazing (left) and normal incidence (right), of the p-polarized synchrotron light. For comparison, spectra for a coverage of about a monolayer of CoPc on (nonintercalated) graphene/Ni(111) are included (bottom spectra, blue).28 The clearly different shape of these reference spectra on graphene/Ni(111) as compared to the data for CoPc on intercalated graphene/Ni(111) hints clearly to a charge transfer only at the interface between CoPc and graphene/Ni(111).28 According to the molecular orientation, feature A at 778.2 eV in Figure 5a arises from transitions into an unoccupied orbital with a component out of the molecular

plane (e.g., dz2 (a1g)), whereas features B2 (780.2 eV) and B3 (781.6 eV) result from transitions into orbitals with in-plane symmetry as their intensities are weakest at grazing incidence but dominate the spectrum at normal incidence (Figure 5b). We note however that multiplet effects determine the peak shape, and thus the features cannot easily be assigned to distinct single transitions. For a more detailed and partial controversial discussion of the assignment of spectral XAS features, we refer to the literature (e.g., refs 21,65−71). Most visibly from Figure 5 are only minor changes of the spectral shape for both measurement geometries, grazing and normal incidence. The relative intensity of the B2 and B3 features at grazing incidence seems to be somewhat weaker for the lowest film thickness of 0.5 nm (1−2 monolayer), but such changes may result from uncertainties in the background correction procedure and/or from the slightly different molecular orientation discussed above. In contrast to CoPc on reactive Ag(111) or Ni(111) substrates,14 no interface feature A0, most likely due to hybridization of Co d-orbitals with substrate states, is visible. Also, the relative intensity of feature B3 is not decreased at low coverages as observed for many substrates including Au(100) and graphene/Ni(111), cf., bottom spectrum of Figure 5b. The appearance of such clear changes of the peak shape can be understood by charge transfer processes involving the Co ion of CoPc and/or a more complex mixed-valence behavior at the interface.14,66 Vice versa, the absence of distinct changes of the peak shape in XAS and Co 2p XPS spectra as a function of the film thickness confirms the absence of such a charge transfer process across the interface. Thus, in contrast to the doped graphene on Ni(111), the intercalated graphene can prevent a charge transfer from the substrate to the central metal atom of CoPc. 3.3. CoPc on Pt(111) and on Graphene/Pt(111). The electronic properties of the graphene can be varied by the substrate used for the growth. As an example, the degree of interfacial orbital hybridization between graphene and metal states is rising in the series Pt(111)−Ir(111)−Rh(111)− Ru(0001) accompanied by a gradual change in graphene morphology from strongly corrugated to nearly flat.49 As a 15243


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Figure 6. (a,b) Co L3 edge XAS spectra for CoPc on graphene/Pt(111) and on Pt(111) (c) taken at different angles θ of the incoming synchrotron light. In (a), a film thickness of about 2.2 nm was chosen to rule out contributions of the interface.

consequence, for graphene on Ir(111) the Dirac point is localized just below the Fermi level.72 We prepared therefore graphene on the weakly interacting Pt(111) surface and study interactions between CoPc and the metal substrate with focus on the Co ion of CoPc as well as the influence of a graphene buffer layer. In good agreement with the literature,49 the C 1s signal for graphene on Pt(111) is found at 284.0 eV and thus close to the value found for Auintercalated graphene (284.1 eV, see section 3.2.1), indicating a very weak doping of graphene. Polarization-dependent XAS spectra at the Co L3 edge for CoPc layers with different thickness on Pt(111) and graphene/ Pt(111) are shown in Figure 6. The spectra in Figure 6a correspond to a 2.2 nm thick CoPc layer on Graphene/ Pt(111); they are very similar to 1.5 nm CoPc on Pt(111) (Supporting Information Figure S2). The typical angular dependence of the multiplet features is clearly visible in Figure 6a, typically for thin films, where mainly bulk properties are probed (see, e.g., ref 14). At grazing incidence the intensity of feature A is maximal, whereas at normal incidence the group of excitations denoted B2 and B3 dominates the spectrum. Going to a monolayer coverage CoPc on graphene/Pt(111), almost no changes were observed, similar to CoPc on intercalated graphene/Ni(111). On the other hand, for a monolayer CoPc on Pt(111), clear changes of the peak shape are visible, indicating a change of the electronic configuration of the Co ion of CoPc at the interface to Pt(111). Most visibly, at normal incidence the relative intensity of B3 is significantly lower at the interface, and at grazing incidence also the relative intensity, and shape of A, is decreased as compared to the thicker film. This effect is more clearly visible in the corresponding spectra normalized to the same step height shown as Supporting Information (Figure S3). Such changes may indicate a partial filling of the orbitals related to A and B3 transitions, due to a charge transfer from the Pt(111) substrate toward the metal atom of the organic molecule. In addition, slight changes of the energetic position of spectral features can be observed as a function of the film thickness: feature A is shifted from 778.2 eV (1.5 nm) to 778.6 eV (0.4 nm) and B2 from 780.2 to 780.4 eV, respectively. Such energetic shifts may be understood by hybridization at the interface; the effect is however much less as

compared to CoPc on Ni(111) or Ag(111) where A was shifted by approximately 1 eV.14 The fact that the spectral changes do not only affect xy- or zpolarized transitions and therefore the entire electronic distribution seems to change at the interface resulting in a different multiplet structure. We note however that for CoPc on Au(111) a similar peak shape in the XAS spectra at the interface66 could neither be described by a pure d7 nor by a pure d8 configuration but by a more complex mixed-valence behavior based on a Co d7 configuration with an additional electron in a separate orbital that can hop to the Co ion, thereby creating a d8 configuration at a higher energy.66 The comparison of Figure 6a and b illustrates the clearly different behavior of CoPc on Pt(111) and graphene/Pt(111). Strong interactions between CoPc and Pt(111) can be prevented by the insertion of a graphene buffer layer.

4. SUMMARY We have shown that the electronic configuration of the central metal atom of CoPc is significantly altered at the interface to Pt(111). However, such a strong interaction between CoPc and Pt(111) can be prevented by the insertion of a graphene buffer layer. This is in contrast to CoPc on graphene/Ni(111), where a charge transfer occurs from Ni(111) through the graphene intermediate layer to the Co ion of CoPc. However, the situation is very similar to CoPc on graphene/Pt(111) if the graphene on Ni(111) is intercalated by gold. Because both graphene on Pt(111) and intercalated graphene on Ni(111) are weakly bonded and weakly doped, the degree of chemical interaction with the substrate and the level of doping of the graphene intermediate layer are considered crucial for the interaction between CoPc and the substrate.

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ASSOCIATED CONTENT

S Supporting Information *

Additional polarization-dependent N K and Co L edge X-ray absorption spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b02912. 15244


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(13) Ahmadi, S.; Shariati, M. N.; Yu, S.; Göthelid, M. Molecular Layers of ZnPc and FePc on Au(111) Surface: Charge Transfer and Chemical Interaction. J. Chem. Phys. 2012, 137, 084705. (14) Peisert, H.; Uihlein, J.; Petraki, F.; Chasse, T. Charge Transfer between Transition Metal Phthalocyanines and Metal Substrates: The Role of the Transition Metal. J. Electron Spectrosc. 2015, DOI: 10.1016/j.elspec.2015.1001.1005. (15) Cai, Y. L.; Song, J. J.; Bao, S. N.; He, P. M.; Hu, F.; Zhang, H. J. The Electronic and Transport Property of the CoPc on Au(111) Surface. Chem. Phys. Lett. 2014, 609, 142−146. (16) Zhao, A.; Li, Q.; Chen, L.; Xiang, H.; Wang, W.; Pan, S.; Wang, B.; Xiao, X.; Yang, J.; Hou, J. G.; et al. Controlling the Kondo Effect of an Adsorbed Magnetic Ion through Its Chemical Bonding. Science 2005, 309, 1542−1544. (17) Schmid, M.; Kaftan, A.; Steinruck, H. P.; Gottfried, J. M. The Electronic Structure of Cobalt(II) Phthalocyanine Adsorbed on Ag(111). Surf. Sci. 2012, 606, 945−949. (18) Gargiani, P.; Angelucci, M.; Mariani, C.; Betti, M. G. MetalPhthalocyanine Chains on the Au(110) Surface: Interaction States Versus d-Metal States Occupancy. Phys. Rev. B 2010, 81, 085412. (19) Brumboiu, I. E.; Totani, R.; de Simone, M.; Coreno, M.; Grazioli, C.; Lozzi, L.; Herper, H. C.; Sanyal, B.; Eriksson, O.; Puglia, C.; et al. Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine. J. Phys. Chem. A 2014, 118, 927−932. (20) Kroll, T.; Aristov, V. Y.; Molodtsova, O. V.; Ossipyan, Y. A.; Vyalikh, D. V.; Büchner, B.; Knupfer, M. Spin and Orbital Ground State of Co in Cobalt Phthalocyanine. J. Phys. Chem. A 2009, 113, 8917−8922. (21) Kroll, T.; Kraus, R.; Schonfelder, R.; Aristov, V. Y.; Molodtsova, O. V.; Hoffmann, P.; Knupfer, M. Transition Metal Phthalocyanines: Insight into the Electronic Structure from Soft X-Ray Spectroscopy. J. Chem. Phys. 2012, 137, 054306. (22) Liao, M.-S.; Scheiner, S. Electronic Structure and Bonding in Metal Phthalocyanines, Metal=Fe, Co, Ni, Cu, Zn, Mg. J. Chem. Phys. 2001, 114, 9780−9791. (23) Marom, N.; Kronik, L. Density Functional Theory of Transition Metal Phthalocyanines, I: Electronic Structure of NiPc and CoPc-SelfInteraction Effects. Appl. Phys. A: Mater. Sci. Process. 2009, 95, 159− 163. (24) Marom, N.; Kronik, L. Density Functional Theory of Transition Metal Phthalocyanines, Ii: Electronic Structure of MnPc and FePcSymmetry and Symmetry Breaking. Appl. Phys. A: Mater. Sci. Process. 2009, 95, 165−172. (25) Maslyuk, V. V.; Aristov, V. Y.; Molodtsova, O. V.; Vyalikh, D. V.; Zhilin, V. M.; Ossipyan, Y. A.; Bredow, T.; Mertig, I.; Knupfer, M. The Electronic Structure of Cobalt Phthalocyanine. Appl. Phys. A: Mater. Sci. Process. 2009, 94, 485−489. (26) Kuz’min, M. D.; Hayn, R.; Oison, V. Ab Initio Calculated XANES and XMCD Spectra of Fe(II) Phthalocyanine. Phys. Rev. B 2009, 79, 024413. (27) Candini, A.; Bellini, V.; Klar, D.; Corradini, V.; Biagi, R.; De Renzi, V.; Kummer, K.; Brookes, N. B.; del Pennino, U.; Wende, H.; et al. Ferromagnetic Exchange Coupling between Fe Phthalocyanine and Ni(111) Surface Mediated by the Extended States of Graphene. J. Phys. Chem. C 2014, 118, 17670−17676. (28) Uihlein, J.; Peisert, H.; Glaser, M.; Polek, M.; Adler, H.; Petraki, F.; Ovsyannikov, R.; Bauer, M.; Chasse, T. Communication: Influence of Graphene Interlayers on the Interaction between Cobalt Phthalocyanine and Ni(111). J. Chem. Phys. 2013, 138, 081101. (29) Gruneis, A.; Vyalikh, D. V. Tunable Hybridization between Electronic States of Graphene and a Metal Surface. Phys. Rev. B 2008, 77, 193401. (30) Weser, M.; Rehder, Y.; Horn, K.; Sicot, M.; Fonin, M.; Preobrajenski, A. B.; Voloshina, E. N.; Goering, E.; Dedkov, Y. S. Induced Magnetism of Carbon Atoms at the Graphene/Ni(111) Interface. Appl. Phys. Lett. 2010, 96, 012504. (31) Abtew, T.; Shih, B.-C.; Banerjee, S.; Zhang, P. GrapheneFerromagnet Interfaces: Hybridization, Magnetization and Charge Transfer. Nanoscale 2013, 5, 1902−1909.

AUTHOR INFORMATION

Corresponding Author

*Phone: +49 7071 2976931. Fax: +49 7071 295490. E-mail: [email protected]. Author Contributions ∥

J.U. and M.P. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support by the German Research Council (PE 546/5-1, CH 132/23-1) and the doctoral training network “Kohlenstoff auf Substraten: vom Molekül zur Schicht”. W. Neu contributed significantly by technical support. We acknowledge the Helmholtz Zentrum Berlin GmbH, Elektronenspeicherring BESSY II, and MAX IV Laboratory for providing synchrotron radiation and user support. Financial travel support by Helmholtz Zentrum Berlin GmbH is gratefully acknowledged. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) CALIPSO under grant agreement no. 312284.

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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) Domingo, N.; Bellido, E.; Ruiz-Molina, D. Advances on Structuring, Integration and Magnetic Characterization of Molecular Nanomagnets on Surfaces and Devices. Chem. Soc. Rev. 2012, 41, 258−302. (5) Cinchetti, M.; Heimer, K.; Wustenberg, J. P.; Andreyev, O.; Bauer, M.; Lach, S.; Ziegler, C.; Gao, Y. L.; Aeschlimann, M. Determination of Spin Injection and Transport in a Ferromagnet/ Organic Semiconductor Heterojunction by Two-Photon Photoemission. Nat. Mater. 2009, 8, 115−119. (6) Schmitt, F.; Sauther, J.; Lach, S.; Ziegler, C. Characterization of the Interface Interaction of Cobalt on Top of Copper- and IronPhthalocyanine. Anal. Bioanal. Chem. 2011, 400, 665−671. (7) Liu, Y.; Lee, T.; Katz, H. E.; Reich, D. H. Effects of Carrier Mobility and Morphology in Organic Semiconductor Spin Valves. J. Appl. Phys. 2009, 105, 07C708. (8) Serri, M.; Wu, W.; Fleet, L. R.; Harrison, N. M.; Hirjibehedin, C. F.; Kay, C. W. M.; Fisher, A. J.; Aeppli, G.; Heutz, S. HighTemperature Antiferromagnetism in Molecular Semiconductor Thin Films and Nanostructures. Nat. Commun. 2014, 5, 9. (9) Javaid, S.; Bowen, M.; Boukari, S.; Joly, L.; Beaufrand, J. B.; Chen, X.; Dappe, Y. J.; Scheurer, F.; Kappler, J. P.; Arabski, J.; et al. Impact on Interface Spin Polarization of Molecular Bonding to Metallic Surfaces. Phys. Rev. Lett. 2010, 105, 077201. (10) Hanack, M.; Lang, M. Conducting Stacked Metallophthalocyanines and Related Compounds. Adv. Mater. 1994, 6, 819−833. (11) Arillo-Flores, O. I.; Fadlallah, M. M.; Schuster, C.; Eckern, U.; Romero, A. H. Magnetic, Electronic, and Vibrational Properties of Metal and Fluorinated Metal Phthalocyanines. Phys. Rev. B 2013, 87, 165115. (12) Caplins, B. W.; Suich, D. E.; Shearer, A. J.; Harris, C. B. Metal/ Phthalocyanine Hybrid Interface States on Ag(111). J. Phys. Chem. Lett. 2014, 5, 1679−1684. 15245


Article

The Journal of Physical Chemistry C (32) Hufner, S.; Yang, S. H.; Mun, B. S.; Fadley, C. S.; Schafer, J.; Rotenberg, E.; Kevan, S. D. Observation of the Two-Hole Satellite in Cr and Fe Metal by Resonant Photoemission at the 2p Absorption Energy. Phys. Rev. B 2000, 61, 12582−12585. (33) Addou, R.; Dahal, A.; Sutter, P.; Batzill, M. Monolayer Graphene Growth on Ni(111) by Low Temperature Chemical Vapor Deposition. Appl. Phys. Lett. 2012, 100, 021601. (34) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization CrossSections and Asymmetry Parameters - 1 Less-Than-or-Equal-to Z Less-Than-or-Equal-to 103. At. Data Nucl. Data Tables 1985, 32, 1− 155. (35) Hesse, R.; Chasse, T.; Streubel, P.; Szargan, R. Error Estimation in Peak-Shape Analysis of XPS Core-Level Spectra Using Unifit 2003: How Significant Are the Results of Peak Fits? Surf. Interface Anal. 2004, 36, 1373−1383. (36) Koch, E. E.; Jugnet, Y.; Himpsel, F. J. High-Resolution Soft XRay Excitation Spectra of 3d-Metal Phthalocyanines. Chem. Phys. Lett. 1985, 116, 7−11. (37) Rocco, M. L. M.; Frank, K. H.; Yannoulis, P.; Koch, E. E. Unoccupied Electronic Structure of Phthalocyanine Films. J. Chem. Phys. 1990, 93, 6859−6864. (38) Petraki, F.; Peisert, H.; Latteyer, F.; Aygul, U.; Vollmer, A.; Chasse, T. Impact of the 3d Electronic States of Cobalt and Manganese Phthalocyanines on the Electronic Structure at the Interface to Ag(111). J. Phys. Chem. C 2011, 115, 21334−21340. (39) Wang, Y. F.; Wu, K.; Kroger, J.; Berndt, R. Structures of Phthalocyanine Molecules on Surfaces Studied by Stm. AIP Adv. 2012, 2, 041402. (40) Peisert, H.; Schwieger, T.; Auerhammer, J. M.; Knupfer, M.; Golden, M. S.; Fink, J.; Bressler, P. R.; Mast, M. Order on Disorder: Copper Phthalocyanine Thin Films on Technical Substrates. J. Appl. Phys. 2001, 90, 466−469. (41) Petraki, F.; Peisert, H.; Biswas, I.; Aygul, U.; Latteyer, F.; Vollmer, A.; Chasse, T. Interaction between Cobalt Phthalocyanine and Gold Studied by X-Ray Absorption and Resonant Photoemission Spectroscopy. J. Phys. Chem. Lett. 2010, 1, 3380−3384. (42) Hamalainen, S. K.; Stepanova, M.; Drost, R.; Liljeroth, P.; Lahtinen, J.; Sainio, J. Self-Assembly of Cobalt-Phthalocyanine Molecules on Epitaxial Graphene on Ir(111). J. Phys. Chem. C 2012, 116, 20433−20437. (43) Glaser, M.; Peisert, H.; Adler, H.; Aygül, U.; Ivanovic, M.; Nagel, P.; Merz, M.; Schuppler, S.; Chassé, T. Electronic Structure at Transition Metal Phthalocyanine-Transition Metal Oxide Interfaces: Cobalt Phthalocyanine on Epitaxial MnO Films. J. Chem. Phys. 2015, 142, 101918. (44) Floreano, L.; Cossaro, A.; Gotter, R.; Verdini, A.; Bavdek, G.; Evangelista, F.; Ruocco, A.; Morgante, A.; Cvetko, D. Periodic Arrays of Cu-Phthalocyanine Chains on Au(110). J. Phys. Chem. C 2008, 112, 10794−10802. (45) Holland, B. N.; Peltekis, N.; Farrelly, T.; Wilks, R. G.; Gavrila, G.; Zahn, D. R. T.; McGuinness, C.; McGovern, I. T. NEXAFS Studies of Copper Phthaloyanine on Ge(001)-2 × 1 and Ge(111)-C(2 × 8) Surfaces. Phys. Status Solidi B 2009, 246, 1546−1551. (46) Willey, T. M.; Bagge-Hansen, M.; Lee, J. R. I.; Call, R.; Landt, L.; van Buuren, T.; Colesniuc, C.; Monton, C.; Valmianski, I.; Schuller, I. K. Electronic Structure Differences between H2-, Fe-, Co-, and CuPhthalocyanine Highly Oriented Thin Films Observed Using Nexafs Spectroscopy. J. Chem. Phys. 2013, 139, 034701. (47) Uihlein, J.; Peisert, H.; Adler, H.; Glaser, M.; Polek, M.; Ovsyannikov, R.; Chassé, T. Interface between FePc and Ni(111): Influence of Graphene Buffer Layers. J. Phys. Chem. C 2014, 118, 10106−10112. (48) Ahmadi, S.; Agnarsson, B.; Bidermane, I.; Wojek, B. M.; Noel, Q.; Sun, C.; Gothelid, M. Site-Dependent Charge Transfer at the Pt(111)-ZnPc Interface and the Effect of Iodine. J. Chem. Phys. 2014, 140, 174702. (49) Preobrajenski, A. B.; Ng, M. L.; Vinogradov, A. S.; Martensson, N. Controlling Graphene Corrugation on Lattice-Mismatched Substrates. Phys. Rev. B 2008, 78, 073401.

(50) Peisert, H.; Biswas, I.; Knupfer, M.; Chasse, T. Orientation and Electronic Properties of Phthalocyanines on Polycrystalline Substrates. Phys. Status Solidi B 2009, 246, 1529−1545. (51) Eastman, D. E.; Himpsel, F. J.; Knapp, J. A. Experimental Band Structure and Temperature-Dependent Magnetic Exchange Splitting of Nickel Using Angle-Resolved Photoemission. Phys. Rev. Lett. 1978, 40, 1514−1517. (52) Nagashima, A.; Tejima, N.; Oshima, C. Electronic States of the Pristine and Alkali-Metal-Intercalated Monolayer Graphite/Ni(111) Systems. Phys. Rev. B 1994, 50, 17487−17495. (53) Shikin, A. M.; Adamchuk, V. K.; Rieder, K. H. Formation of Quasi-Free Graphene on the Ni(111) Surface with Intercalated Cu, Ag, and Au Layers. Phys. Solid State 2009, 51, 2390−2400. (54) Susi, T.; Pichler, T.; Ayala, P. X-Ray Photoelectron Spectroscopy of Graphitic Carbon Nanomaterials Doped with Heteroatoms. Beilstein J. Nanotechnol. 2015, 6, 177−192. (55) Grüneis, A.; Kummer, K.; Vyalikh, D. V. Dynamics of Graphene Growth on a Metal Surface: A Time-Dependent Photoemission Study. New J. Phys. 2009, 11, 073050. (56) Haberer, D.; Giusca, C. E.; Wang, Y.; Sachdev, H.; Fedorov, A. V.; Farjam, M.; Jafari, S. A.; Vyalikh, D. V.; Usachov, D.; Liu, X.; et al. Evidence for a New Two-Dimensional C4h-Type Polymer Based on Hydrogenated Graphene. Adv. Mater. 2011, 23, 4497−4503. (57) Ruocco, A.; Evangelista, F.; Gotter, R.; Attili, A.; Stefani, G. Evidence of Charge Transfer at the Cu-Phthalocyanine/Au(100) Interface (Reprinted from J. Phys. Chem a, Vol 111, 2007). J. Phys. Chem. C 2008, 112, 2016−2025. (58) Peisert, H.; Knupfer, M.; Fink, J. Electronic Structure of Partially Fluorinated Copper Phthalocyanine (CuPcF4) and Its Interface to Au(100). Surf. Sci. 2002, 515, 491−498. (59) Baran, J. D.; Larsson, J. A.; Woolley, R. A. J.; Cong, Y.; Moriarty, P. J.; Cafolla, A. A.; Schulte, K.; Dhanak, V. R. Theoretical and Experimental Comparison of SnPc, PbPc, and CoPc Adsorption on Ag(111). Phys. Rev. B 2010, 81, 075413. (60) Baran, J. D.; Larsson, J. A. Theoretical Insights into Adsorption of Cobalt Phthalocyanine on Ag(111): A Combination of Chemical and Van Der Waals Bonding. J. Phys. Chem. C 2013, 117, 23887− 23898. (61) Betti, M. G.; Gargiani, P.; Frisenda, R.; Biagi, R.; Cossaro, A.; Verdini, A.; Floreano, L.; Mariani, C. Localized and Dispersive Electronic States at Ordered FePc and CoPc Chains on Au(110). J. Phys. Chem. C 2010, 114, 21638−21644. (62) Toader, M.; Shukrynau, P.; Knupfer, M.; Zahn, D. R. T.; Hietschold, M. Site-Dependent Donation/Backdonation Charge Transfer at the CoPc/Ag(111) Interface. Langmuir 2012, 28, 13325−13330. (63) Mugarza, A.; Robles, R.; Krull, C.; Korytar, R.; Lorente, N.; Gambardella, P. Electronic and Magnetic Properties of Molecule-Metal Interfaces: Transition-Metal Phthalocyanines Adsorbed on Ag(100). Phys. Rev. B 2012, 85, 155437. (64) Salomon, E.; Amsalem, P.; Marom, N.; Vondracek, M.; Kronik, L.; Koch, N.; Angot, T. Electronic Structure of Copc Adsorbed on Ag(100): Evidence for Molecule-Substrate Interaction Mediated by Co 3d Orbitals. Phys. Rev. B 2013, 87, 075407. (65) Kroll, T.; Aristov, V. Y.; Molodtsova, O. V.; Ossipyan, Y. A.; Vyalikh, D. V.; Buchner, B.; Knupfer, M. Spin and Orbital Ground State of Co in Cobalt Phthalocyanine. J. Phys. Chem. A 2009, 113, 8917−8922. (66) Stepanow, S.; Miedema, P. S.; Mugarza, A.; Ceballos, G.; Moras, P.; Cezar, J. C.; Carbone, C.; de Groot, F. M. F.; Gambardella, P. Mixed-Valence Behavior and Strong Correlation Effects of Metal Phthalocyanines Adsorbed on Metals. Phys. Rev. B 2011, 83, 220401. (67) Stepanow, S.; Lodi Rizzini, A.; Krull, C.; Kavich, J.; Cezar, J. C.; Yakhou-Harris, F.; Sheverdyaeva, P. M.; Moras, P.; Carbone, C.; Ceballos, G.; et al. Spin Tuning of Electron-Doped Metal− Phthalocyanine Layers. J. Am. Chem. Soc. 2014, 136, 5451−5459. (68) Gargiani, P.; Rossi, G.; Biagi, R.; Corradini, V.; Pedio, M.; Fortuna, S.; Calzolari, A.; Fabris, S.; Cezar, J. C.; Brookes, N. B.; et al. 15246


Article

The Journal of Physical Chemistry C Spin and Orbital Configuration of Metal Phthalocyanine Chains Assembled on the Au(110) Surface. Phys. Rev. B 2013, 87, 165407. (69) Kuz’min, M. D.; Hayn, R.; Oison, V. Ab Initio Calculated XANES and XMCD Spectra of Fe(II) Phthalocyanine. Phys. Rev. B 2009, 79, 024413. (70) Johnson, P. S.; Garcia-Lastra, J. M.; Kennedy, C. K.; Jersett, N. J.; Boukahil, I.; Himpsel, F. J.; Cook, P. L. Crystal Fields of Porphyrins and Phthalocyanines from Polarization-Dependent 2p-to-3d Multiplets. J. Chem. Phys. 2014, 140, 114706. (71) Willey, T. M.; Bagge-Hansen, M.; Lee, J. R. I.; Call, R.; Landt, L.; van Buuren, T.; Colesniuc, C.; Monton, C.; Valmianski, I.; Schuller, I. K. Electronic Structure Differences between H-2-, Fe-, Co-, and CuPhthalocyanine Highly Oriented Thin Films Observed Using Nexafs Spectroscopy. J. Chem. Phys. 2013, 139, 034701. (72) Scardamaglia, M.; Lisi, S.; Lizzit, S.; Baraldi, A.; Larciprete, R.; Mariani, C.; Betti, M. G. Graphene-Induced Substrate Decoupling and Ideal Doping of a Self-Assembled Iron-Phthalocyanine Single Layer. J. Phys. Chem. C 2013, 117, 3019−3027.

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