Interface between FePc and Ni(111): Influence of Graphene Buffer

Article pubs.acs.org/JPCC

Interface between FePc and Ni(111): Influence of Graphene Buffer Layers Johannes Uihlein,† Heiko Peisert,*,† Hilmar Adler,† Mathias Glaser,† Małgorzata Polek,† Ruslan Ovsyannikov,‡ and Thomas Chassé† †

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-Str. 15, 12489 Berlin, Germany

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ABSTRACT: The alteration of the electronic interface properties of iron phthalocyanine (FePc) on Ni(111) by graphene interlayers is studied using both photoemission techniques (XPS and UPS) and X-ray absorption spectroscopy (XAS). Both XPS and XAS clearly indicate a charge transfer from Ni(111) to the Fe ion of FePc. In contrast to CoPc, this charge transfer can be completely suppressed by the introduction of a graphene buffer layer between FePc and Ni(111). Further UPS measurements indicate that the interfacial charge transfer include also the FePc macrocycle. Surprisingly, for FePc and CoPc the energy level alignment after the formation of the interface dipoles is rather unaffected by both the introduction of a graphene buffer layer and charge transfer processes between the central metal atom of the Pc and the substrates.

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INTRODUCTION In recent years, extensive research efforts have been devoted to the investigation of organic semiconducting molecules for application in optoelectronic devices such as organic lightemitting devices (OLEDs), 1−3 organic photovoltaics (OPVs),1,4 organic field effect transistors (OFETs),5−7 and for application in spintronic devices like molecular based processing8−10 and information storage devices.11,12 Transition metal phthalocyanines (TMPcs) represent a highly promising class of molecules in these fields. They outperform others because of their high stability against humidity, light, heat, and oxygen13 and offer a broad possibility to tune their optical and electronic properties.14 It is known that the behavior of organic semiconducting molecules at the interface with substrate materials can highly influence the performance of devices based on these molecules. Interactions appearing at these interfaces are playing an important role in electron transport processes. It has been recently shown that the magnetic properties of TMPcs can drastically be influenced by interactions with metallic substrates; e.g., the molecular spin can be strongly influenced15,16 or even be quenched17,18 on metallic substrate surfaces. The strength of this interaction depends on the nature of the TMPc central metal atom.19−22 The aim of the present work is the tuning of such interactions at interfaces by the introduction of a buffer layer in between the metal substrate and the adsorbed molecules. A promising candidate is graphene with its outstanding electronic properties, its high flexibility, stability, and easy experimental accessibility.23 Recently, for CoPc it has been shown that a charge transfer associated with a redistribution of the d© 2014 American Chemical Society

electrons at the Co central atom of the phthalocyanine occurs at the interface to Ni(111) and that graphene buffer layers do not prevent the charge transfer at the interface to Ni(111); however, the detailed electronic situation is different.24 Similar to the interfaces between TMPcs and metal substrates, it can be expected that the influence of graphene on these interactions depends on the filling nature of the d-band in the central metal atom of the TMPcs, as recently suggested from electron energy loss spectroscopic investigations.25,26 In the present work we focus on the electronic structure of FePc on Ni(111) and graphene/Ni(111) and compare the results to CoPc. In order to study the occupied and unoccupied electronic structure, we apply X-ray absorption spectroscopy (XAS) and photoemission spectroscopies (X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS)).

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EXPERIMENTAL SECTION The XAS measurements have been executed at the third generation synchrotron radiation source BESSY II (Berlin) using the endstation SurICat at the Optics-beamline. For calibrating the photon energies the binding energy (BE) of Au 4f7/2 and Ag 3d5/2 peaks excited by first- and second-order light have been compared. The energy resolution for photoemission (PES) and XAS was manually set to ≈100 meV at a photon energy of 400 eV. The absorption was monitored indirectly by measuring the total electron yield (sample current). The XAS Received: February 5, 2014 Revised: April 17, 2014 Published: April 23, 2014 10106

dx.doi.org/10.1021/jp501269q | J. Phys. Chem. C 2014, 118, 10106−10112

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spectra have been normalized to the same absorption step height. 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 have been taken using Mg Kα radiation (1253.6 eV) and X-ray power of 300 W; the UPS have been measured using He I radiation (21.22 eV). The Ni(111) single crystal has been cleaned by argon ion sputtering and annealing cycles. XPS and UPS measurements have been performed to check the cleanliness. For graphene preparation the Ni(111) single crystal has been heated up to 520 °C in a separate preparation chamber and exposed to propene (99.5%, Westfalen AG) at a pressure of 9 × 10−6 mbar for 5 min (∼2000 L).27 The quality of the resulting graphene layers has been controlled by XPS. Angle-resolved UPS measurements showed the typical angular dependence for graphene.28 FePc has been evaporated at evaporation rates between 0.1 and 0.4 nm/min, estimated from a quartz microbalance prior to the deposition. The nominal layer thicknesses were estimated from XPS intensity ratios using photoemission cross sections from Yeh and Lindau29 assuming layer-by-layer growth. To prove the integrity of the evaporated FePc molecules, the C 1s photoemission spectra of the FePc layers have been examined regularly and found to correspond very well with the literature.30,31

Figure 1. Angular dependent N K edge XAS spectra of a 3.1 nm thick FePc film on Ni(111) (a) and a 1.5 nm thick FePc film on graphene/ Ni(111) (b). Spectra were taken at different incidence angles of the incoming p-polarized synchrotron light (θ = 90° corresponds to normal and θ = 10° to grazing incidence of the beam).

to normal and θ = 10° to grazing incidence of the beam). In the spectra it can be distinguished between two groups of N 1s excitations showing different polarization dependence: in the region between 398 and 405 eV photon energy a set of signals occurs that decreases with increasing angle θ whereas the features in the region between 405 and 420 eV photon energy increase with increasing angle θ. The signals at lower photon energies can be essentially assigned to transitions into π* states and the latter into σ* states (e.g., ref 33). The angular dependence of the N K absorption spectra indicates an orientation of the FePc molecules with their molecular plane flat on both substrates (lying orientation), in good agreement with related systems, including FePc on graphene/Ir(111).38 Reasons for weak remaining intensity in the region of the π* transitions at normal incidence might be associated with a slight tilt of the molecule with respect to the substrate surface. However, in particular for organometallic phthalocyanines, small in-plane resonances superimposed on the π* region of nitrogen K-edges due to hybridization with states related to central metal atom of the phthalocyanine were recently discussed.33,39,40 Comparing however FePc on Ni(111) with FePc on graphene/Ni(111), the remaining intensity in the energy range of π* transitions is clearly higher in the normal incidence spectrum of Figure 1a, which hints to a slightly tilted adsorption geometry of FePc on Ni(111). A detailed analysis of intensity ratios is however hindered by interfering second order Ni L3 edge signals at photon energies higher 425 eV. The preferred lying adsorption geometry of FePc on both substrates results also in a strong angular dependence of polarization dependent Fe L-edge absorption spectra. In Figure 2a, the absorption spectra of FePc on Ni(111) with a layer thickness of 3.1 nm are depicted for different angles with respect to the incident light. In these spectra two sets of signals are clearly visible: between 705 and 715 eV transitions from Fe 2p3/2 levels into unoccupied d orbitals occur (Fe L3 transitions), whereas the signals in the region of 718 eV up to 726 eV can be assigned to transitions from Fe 2p1/2 levels (Fe L2 transitions). Considering transitions from Fe 2p3/2 more in detail, we can distinguish features with different polarization dependence: the features labeled A1 (705.9 eV) and A2 (707.6 eV) show maximum intensity at grazing incidence (10°) and decrease in

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RESULTS AND DISCUSSION Orientation and Electronic Structure of FePc in Thin Films. On single crystalline substrates, a highly ordered growth of phthalocyanines is often observed.32−36 A high degree of a known orientation supports the assignment of the different polarization dependent XAS transitions from core level states into lower unoccupied states as a function of energy. Thus, polarization-dependent XAS provides information about the electronic situation of the investigated sample on the one hand and information about the orientation of the molecules on the other hand. The molecular orientation of planar molecules like transition metal phthalocyanines can be probed looking at XAS transitions at the macrocycle of the Pc; both C K edge and the N K edge absorption spectra are commonly used. In our case, common carbon contaminations of beamline components hinder the determination of the orientation from C K edge spectra; hence the N K edge spectra are analyzed. If the electric field vector E is perpendicular to the molecular plane of the flat, π-conjugated system of the phthalocyanine (i.e., parallel to the 2pz orbitals), the intensity of the excitation from N 1s into a π* orbital is maximal, whereas transitions from N 1s to σ* are strongest if E is parallel to the molecular plane and to the chemical bonds. By monitoring the relative intensities of excitations from occupied molecular levels into unoccupied molecular levels as a function of the direction of the electric field vector of the synchrotron, radiation relative to the sample surface the molecular orientation can be determined.33,37 In Figure 1, N K spectra for FePc on Ni(111) and graphene/ Ni(111) for different angles of incidence of the p-polarized synchrotron light are shown, where θ describes the angle between the electric field vector of the incident p-polarized synchrotron light and the surface normal (θ = 90° corresponds 10107



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Figure 2. Angular dependent Fe L2,3 edge XAS spectra of a 3.1 nm thick FePc film on Ni(111) (a) and a 1.5 nm thick FePc film on graphene/Ni(111) (b). Spectra were taken at different incidence angles of the incoming p-polarized synchrotron light (θ = 90° corresponds to normal and θ = 10° to grazing incidence).

Figure 3. Fe 2p3/2 photoemission spectra of FePc on (a) Ni(111) and (b) graphene/Ni(111) for layers of different film thickness. A distinct interface component occurs at lower binding energies for FePc on Ni(111). However, on graphene/Ni(111) the interface component does not occur.

intensity with increasing angle of incidence. Simultaneously, feature B1 (705.1 eV), B2 (705.9 eV), and a broad feature with a maximum denoted B3 (708.4 eV) arise with increasing angle. All features are in good agreement with FePc films of similar thicknesses of a few nanometers on Ag(111) and on Au(100).41 Feature A consists of multiplet features involving transitions out of the molecular plane (e.g., transitions into the dz2 orbital), and feature B can be assigned to in-plane transitions (e.g., into dxy), due to their angular dependence. The multiplet shape of these features is mainly affected by the overlap of the wave function of the ionized core orbital and the wave functions of partly filled valence orbitals.42 It appears that the angular dependence of B1 and B2 behaves different. This might be understood by the proposed 3E1g symmetry configuration for FePc, where transitions at the lowest photon energy (B1) are attributed to e1g with a dxz, dyz symmetry while for transitions at the energy of B2 an a1g (dz2) symmetry is expected.43 Comparing now the angular dependence of A features for FePc on Ni(111) and on graphene/Ni(111) in Figure 2, the intensity of B2 seems to be slightly increased at θ = 90° in the first case. This can be understood, taking into account the possible small tilt angle of the molecules discussed above and a resulting overlapping of B2 with A1 for not perfectly ordered molecules. Interaction at Interfaces. Generally, interactions between TMPc and substrates including charge transfer can be concerned to the macrocycle or the central metal atom of the TMPc as shown for related systems.44 We will focus first on interactions between the central metal atom of FePc and Ni(111) and graphene/Ni(111) looking at the corresponding core level photoemission and absorption spectra. Fe 2p core level spectra for FePc layers of different thickness on both substrates, Ni(111) and graphene/Ni(111), are displayed in Figure 3. For both substrates, the spectra of the thickest films are dominated by bulk related features and show a broad signal with an intensity maximum at 708.5 eV. The shape of this signal can be described by multiplet effects as explained above and is in good agreement with FePc bulk spectra reported in the literature.41 For FePc on Ni(111) a change of the spectra is clearly visible: Going to lower film

thicknesses the intensity of the bulk signal is decreasing and a new peak at lower binding energy (707.2 eV) appears and dominates the spectrum of the FePc film with 0.6 nm. This behavior reminds to FePc on Ag(111),41 and the interface species point analogously to a charge transfer from the substrate to the metal central atom resulting in a chemical reduction associated with a change of the electronic configuration.44,45 In contrast to the recently reported results for CoPc on graphene/Ni(111), where a clear change of Co 2p core level spectra was observed as a function of the layer thickness,24 the Fe 2p photoemission spectra show no distinct interface species for FePc on graphene/Ni(111) (Figure 3b). With decreasing layer thickness no significant change in the peak shape and position is observable, leading to the assumption that the graphene buffer layer hinders the resulting charge transfer from the Ni(111) substrate. In other words, we observe a decoupling of FePc due to the presence of the graphene buffer layer, which hinders the interaction of the d-molecular orbitals with the Ni metallic states. The presence of a slight tail at 0.3−0.4 FePc coverage in the energy region of the interface peak at 707.2 eV might be justified by little portions of Ni surface where the graphene layer is not well formed and the molecules are in direct contact with the metallic substrate. We note, however, that the graphene layer for both systems, CoPc and FePc on graphene/Ni(111), has been prepared applying the same procedure and additionally XPS and UPS measurements were reproduced in our home lab. The thickness of the graphene layer determined from XPS intensities even exceeds the expected value for a single graphene layer slightly, which is might be related to the high carbon solubility of Ni.46 Therefore, we rule out that the observed different interaction of CoPc and FePc on graphene/Ni(111) is related to a different quality of the graphene layer, in particular due to the presence of large domains of uncovered Ni(111). Since both changes of the peak shape and the energetic position in photoemission might be also affected by processes such as final state charge transfer and polarization,47 we confirm the results by complementary polarization-dependent XAS studies, probing the unoccupied electronic structure as a function of the layer thickness. To compare the electronic 10108



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depends on both the TMPc and the substrate under consideration. Also, the in-plane transitions B1 and B2 in Figure 4b probed at normal incidence (θ = 90°) show significant changes as a function of the film thickness: Most obviously, B1 vanishes at the interface and the shape of the spectrum changes. The remaining feature between 704.5 and 706.5 eV for the lowest film thickness is found at 705.7 eV and thus not at the same position as B2; we denote this feature therefore B0. Simultaneously, the feature with the maximum B3 slightly changes its shape, as the shoulder on the higher photon energy side decreases in intensity until it is completely disappeared in the spectrum of the 0.6 nm layer. The change of both in-plane (A) and out-of-plane (B) features demonstrates that not only a charge transfer across the interface involving the Fe ion occursrather the overall electronic configuration of Fe is changed at the interface to Ni(111). The Fe L3 edge absorption spectrum of thickest FePc film (1.5 nm) on graphene/Ni(111) recorded at grazing incidence in Figure 5a shows the same shape as the spectrum of 3.1 nm FePc film on Ni(111) and can therefore be analogously related to the bulk. In contrast to FePc on Ni(111) (Figure 4), however, changes of the peak shape as a function of the layer thickness are hardly visible: at grazing incidence (Figure 5a) the A1 feature does not decrease with decreasing film thickness and still dominates the spectrum for the monolayer film (0.3 nm). Also, the absorption spectra at normal incidence excitation do not change, and even the shoulder on the high energy side of feature B3 persists. Comparing the bulk related spectra at normal incidence for FePc on Ni(111) (Figure 4a) and FePc on graphene/Ni(111) (Figure 5a), a slight change in the peak shape can be recognized: the intensity ratio of the two peaks assigned as feature B1 and B2 is inverted. We ascribe this difference to the slightly tilted adsorption geometry of FePc on Ni(111) gained from the N K edge spectra. As a result, the Fe L3 edge spectra at normal incidence for FePc on Ni(111) contain remaining intensity of A1 (observed at grazing incidence) in superposition with B2, resulting in an apparent higher intensity of this feature. In summary, we observe that the charge transfer between Fe of FePc and Ni(111) can be essentially suppressed, in contrast to CoPc on graphene/Ni(111). On the other hand, similarly for FePc on graphene/Ir(111),38 a decoupling of FePc from the underlying metal system was observed. Ir(111), however, is known as a substrate where the graphene−substrate interaction is comparably weak.48 Interface Energetics. The question now arises if the suppression of the charge transfer due to the graphene buffer layer has any consequences on the electronic interface parameters such as the interface dipole. The formation of interface dipoles can be monitored by the measurement of the work function (and thus the position of the vacuum level respective the Fermi level) as a function of the film thickness. The work function of a sample can be obtained from the position of the secondary electron cutoff in UPS spectra. In Figure 6a, the development of the work function with increasing film thickness of FePc on Ni(111) and graphene/ Ni(111) is compared. For the clean Ni(111) substrate a work function of 5.3 eV is obtained, which is in very good agreement with the literature.49 With increasing FePc film thickness the work function of the sample decreases and approaches a value of 4.3 eV corresponding to a large interface dipole of about Δ = −1.0 eV. The dipole formation is completed at a film thickness

situation of Fe in FePc at the interface to Ni(111) and graphene/Ni(111), we compare in Figures 4 and 5 the Fe L3

Figure 4. Absorption spectra of FePc on Ni(111) and on graphene/ Ni(111) for the prominent angles of (a) θ = 10° (grazing incidence) and (b) θ = 90° (normal incidence) for different layer thicknesses.

Figure 5. Absorption spectra of FePc on Ni(111) and on graphene/ Ni(111) for the prominent angles of (a) θ = 10° (grazing incidence) and (b) θ = 90° (normal incidence) for different layer thicknesses.

XAS spectra for both systems at the prominent angles θ = 10° (grazing incidence) and θ = 90° (normal incidence) as a function of the layer thickness. The spectrum of the 3.1 nm thick FePc film on Ni(111) recorded at grazing incidence (Figure 4a, topmost spectrum) has already been discussed above, showing the features A1 (705.9 eV) and A2 (707.6 eV). Going to lower film thicknesses the relative intensity of A1 and A2 decreases, whereas A1 seems to decrease faster than A2 and a new feature A0 (706.3 eV) arises and is dominating the spectrum of the 0.6 nm film. The presence of such an interface feature A0 was already observed for related systems like FePc on Ag(111)41 and Au(110)15,16 and CoPc on Ni(111).24 It was proposed that A0 originates from transitions into new electronic levels formed by hybridization of the Fe dz2 orbitals with substrate states at the interface. A0 shows the same angular dependence as A1 and A2, indicating that the hybrid state is formed by orbitals with out-of-plane character. The smaller distance A0−A1 compared to the related systems (see above) may imply that the energetic position of the hybrid state 10109



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however a charge transfer in the opposite direction, so we are left with a scenario where complicated charge-transfer processes have to be considered including a back-transfer from the phthalocyanine macrocycle to the substrate as suggested for related systems.42,58 Introducing now graphene as an intermediate layer, during the first steps of deposition of TMPcs an increase of the work function is observed, which might be partially related to the charge transfer to the central metal atom of the Pcthe increase of 0.7 eV for CoPc is somewhat larger than for FePc (0.4 eV), in agreement with the final interaction strength at these interfaces. However, for both FePc and CoPc the energy level alignment after the formation of the interface dipoles is rather unaffected by both the introduction of a graphene buffer layer and charge-transfer processes between the central metal atom of the Pc and the substrate. It seems that such a charge transfer is compensated by back-transfer processes between the macrocycle of the Pc and the substrate in order to reach the equilibrium conditions after dipole formation.

Figure 6. Development of work function of FePc (a) and CoPc (b) on Ni(111) and graphene/Ni(111) with increasing film thickness.

of about 1 nm; in other words, after the adsorption of about 3 molecular layers. We note that changes of the work function are not accompanied by rigid shifts of Fe 2p core level spectra (Figure 3) pointing to a more complex nature of the interface formation, as discussed below. The energy level alignment agrees very well to reported values for TMPcs on Au(001) where the position of the Fermi level of the metal substrate is similarly found close to the midgap position of the organic semiconductor.50,51 Since the work functions of both substrates are similar, the measured interface dipoles are comparable. In view of the fact that no charge transfer involving the central metal atom of FePc was observed for FePc on Au(100), this result appears surprising. The development of the work function of FePc on graphene/ Ni(111) in Figure 6a is quite different. First, graphene reduces the Ni(111) work function to 4.2 eV, which could be explained by the so-called push-back effect (see below), but also a charge transfer resulting in a doping of the graphene layer is known for Ni(111).52 Subsequently with increasing FePc film thickness the work function increases up to a value of 4.6 eV (coverage of about 1 monolayer), and finally a slight decrease is observed, resulting in a value of 4.4 eV at 1.1 nmcomparable to Ni(111). Thus, the energy level alignment is almost not affected by the graphene intermediate layer; the Fermi level of the substrate is found almost at the same position within the gap of the organic semiconductor FePc. The modification of the substrate work function is compensated by the interface dipole of Δ = +0.2 eV (on graphene/Ni(111)) instead of Δ = −1.0 (on Ni(111)). In order to verify whether this behavior is more general, we will discuss also the development of the work function for a related system: CoPc on Ni(111) and graphene/Ni(111). The corresponding data are displayed in Figure 6b. Similar to FePc the work function on Ni(111) decreases from 5.3 eV to about 4.4 eV with increasing CoPc film thickness, resulting in an interface dipole of Δ = −0.9 eV. Also, the development of the work function of CoPc on graphene/Ni(111) is similar to FePc. Several effects contribute to the formation of dipoles at interfaces, such as the change of the metal work function due to adsorption of organic molecules (also known a push-back effect),44,51 polarization effects, or an adsorption-induced geometric distortion of the molecules.53−55 In many cases, however, complicated charge-transfer and back-transfer mechanism are essential to understand the dipole formation at interfaces.56,57 In order to explain a large negative interface dipole in this manner, as observed for FePc and CoPc on Ni(111), a charge transfer from the molecule to the substrate is expected. At the respective central metal atom, we observe

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SUMMARY Interactions at interfaces between FePc and Ni(111) graphene/ Ni(111) were investigated. On Ni(111) a slightly tilted adsorption geometry of FePc is found accompanied by a charge transfer and a redistribution of the central metal delectrons. The graphene buffer layer prevents the charge transfer; the electronic situation of the Fe ion remains unaltered while the molecules are now flat lying. However, the development of the work function measured by UPS shows also that not only the central metal atom of the Pc is involved in charge transfer processes. For FePc as well as for CoPc, the energy level alignment after the formation of the interface dipoles is rather unaffected for both by the introduction of a graphene buffer layer. We propose that back transfer processes between the macrocycle of the Pc take place resulting in comparable equilibrium conditions after the dipole formation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (+49) 07071/2976931; Fax (+49) 07071/29-5490 (H.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the German Research Council Ch 132/20-2 and the doctoral training network “Kohlenstoff auf Substraten: vom Molekül zur Schicht”. We acknowledge the Helmholtz Zentrum Berlin GmbH, Elektronenspeicherring BESSY II, for providing synchrotron radiation at the Optics beamline. We thank the group of Prof M. Hanack (Tübingen) for providing the FePc and W. Neu (Tübingen) as well as M. Bauer (BESSY II) for technical support. Financial travel support by Helmholtz Zentrum Berlin GmbH is gratefully acknowledged.

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REFERENCES

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Interface between FePc and Ni(111): Influence of Graphene Buffer

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