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Graphene-Induced Substrate Decoupling and Ideal Doping of a SelfAssembled Iron-phthalocyanine Single Layer Mattia Scardamaglia,*,† Simone Lisi,‡ Silvano Lizzit,§ Alessandro Baraldi,∥,⊥ Rosanna Larciprete,# Carlo Mariani,*,† and Maria Grazia Betti‡ †

Dipartimento di Fisica, CNISM, CNIS, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I - 00185 Roma, Italy Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I - 00185 Roma, Italy § Sincrotrone Trieste, Str. St. 14 km 163.5, I-34149 Trieste, Italy ∥ Physics Department and Center of Excellence for Nanostructured Materials, University of Trieste, Via Valerio 2, I-34127 Trieste, Italy ⊥ IOM-CNR Laboratorio TASC, AREA Science Park, S.S.14 Km 163.5, I-34012 Trieste, Italy # CNR-ISC, Via Fosso del Cavaliere 100, I-00133 Roma, Italy ‡

S Supporting Information *

ABSTRACT: Iron-phthalocyanine molecules self-assemble on the moiré pattern of graphene/Ir(111) as a flat and weakly interacting layer, as determined by core-level photoemission and absorption spectroscopy. The graphene buffer layer decouples the FePc two-dimensional structure from the underlying metal; the electronic structure of the FePc molecular macrocycles is preserved; and the Fe-L2,3 edges present narrower and slightly modified resonances at the FePc single-layer coverage with respect to a thin film. The FePc layer induces a slight electron doping to the Ir-supported graphene resulting in the Dirac cone position expected for an ideal free-standing-like graphene layer with the standard Fermi velocity.

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INTRODUCTION

architectures and tailor the electronic and magnetic properties of organic/inorganic nanostructures on graphene.9−12 A nanoscaled graphene corrugation can be a suitable template for ordered molecular growth.9,10 Such a superstructure can be achieved by epitaxial growth of graphene on the hexagonal-symmetry surfaces of 3d, 4d, and 5d transition metals, exploiting the moiré pattern, with a periodicity of a few nanometers, generated by the lattice mismatch between the two crystalline materials.8,13−17 In particular, the Ir(111) surface is a good template for minimizing the carbon−substrate interaction, keeping a moiré superstructure,8,13−15 as reflected in the lowenergy electron-diffraction (LEED) pattern shown in Figure 1. Metal atoms assemble in clusters on graphene or can form regular superlattices thanks to the corrugated structure,11,12 while an ordered pattern of individual units is more desirable

Graphene is in the spotlight of ongoing research on nanoscale systems, due to its unique electronic, mechanical, and transport properties.1,2 However, the countless strategies of modifying graphene properties by nanopatterning and functionalization are largely unexplored. The assembly of individual organic units3−7 on the graphene sheet represents one of the best possibilities for producing artificial materials with unique properties and functionalities. The preparation of metalsupported graphene with the control of natural corrugation,8 such as to exploit the nanopatterning for molecular selfassembling, is the subject of intense research aiming at tuning the properties of the two-dimensional substrate, minimizing the graphene/surface interaction, and controlling the corrugation, to obtain graphene templates with ideal properties. Modulating the interaction of nanotemplate graphene on suitable substrates and exploiting the corrugation to produce regular patterns is a viable strategy to adsorb regular two-dimensional (2D) © 2012 American Chemical Society

Received: September 6, 2012 Revised: December 13, 2012 Published: December 18, 2012 3019

dx.doi.org/10.1021/jp308861b | J. Phys. Chem. C 2013, 117, 3019−3027

The Journal of Physical Chemistry C

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kagome structure driven by the moiré nanopattern,9,10 and PTCDA forms ordered phases.3 We have recently demonstrated that FePc grown on graphene/Ir(111) lies flat from the single-layer (SL) up to a multilayer coverage, as determined by near-edge X-ray absorption fine-structure spectroscopy (NEXAFS).5 The FePc molecules at the SL arrange in a planar molecular structure, without destroying the moiré periodicity, thus indicating a template-driven supramolecular growth in the first coverage phase.5 Open questions are the degree of interaction of FePc with the underlying graphene layer, the capability of graphene to decouple the MPcs from the underlying Ir metal, and the influence of the molecular adsorption on the graphene electronic states. The present complementary absorption spectroscopy and photoemission experimental approach sheds light on the role of the graphene buffer layer and the influence of the metal-phthalocyanine on the graphene band parameters. Our final goal is to control the interaction process to obtain a molecular layer decoupled from the underlying metal substrates, with unperturbed electronic and magnetic properties and slightly influencing the graphene band parameters.

Figure 1. LEED pattern of the moiré superstructure of graphene/ Ir(111) taken at 142 eV electron energy (left); sketch of the ironphthalocyanine molecule (right).

for obtaining metal/magnetic networks. A viable method for getting regular arrays of individual units is the use of metal atoms embedded into an organic cage, like metal-phthalocyanines (MPcs), adsorbed on single crystalline surfaces.18−23 Supramolecular assembly of metalorganic molecules on graphene is a suitable way to obtain regular nanoarchitectures with the metallic atoms ordered in regular networks. Metalphthalocyanines (M-C32H16N8) are square-shaped planar molecules with a central metal atom, four pyrrole rings, and four benzene rings (see sketch in Figure 1), with π-orbitals ensuring conduction,24 like other aromatic oligomers.25−27 Carbon atoms are present in the pyrrole rings (CP in the sketch image) and in the benzene rings (CB), while the N atoms in the pyrrole rings present different ligands, N1-isoindole linked to the central metal atom and N2-azomethine bridging two carbon atoms. MPcs with a magnetic central atom can be arranged regularly on a naturally patterned surface, like the moiré structure, such as to build up an ordered array of regularly spaced magnetic atoms anchored to the surface through the organic macrocycle,9,10 thus avoiding clustering on graphene. Photoemission experiments of MPcs assembled on metallic substrates suggest a mixing of the 3d open shell MPc central metal atom with the underlying metallic states.19,21−23 Thus, the physical properties of MPcs at the interface with metal substrates appear to be dominated by the interaction process with metallic states, significantly hindering their magnetic response. The presence of a buffer layer to decouple the interaction is a requirement to preserve the magnetic state of the 2D molecular layer. Furthermore, the aromatic oligomers can be used as building blocks for nanometer-sized electronic devices, exploiting the transport properties of π-conjugation of the macrocycle electrons. A variety of organic π-conjugated molecules are being used to functionalize graphene, by tailoring its band structure, with its unprecedented properties like the high mobility,2 by doping and/or charge transfer, for potential nanoelectronic applications.3,4,28 A different aromatic molecule, perylene carboxylic derivative (PTCDA), revealed a weak charge transfer with a slight upward vacuum level shift (0.25 eV) and without any interface state, which has been used for a prototype system facilitating charge transport parallel to the surface plane.3 Only a slight n-type doping has been observed by adsorption of vanadyl-phthalocyanine molecules on graphene,4 and the other phthalocyanine produces a thin planar layer on graphene/ITO.29 π-conjugated oligomers on graphene may assemble on graphene as a supramolecular structure. For example, scanning tunneling microscopy (STM) on iron-phthalocyanine (FePc) deposited on graphene/ Ru(0001) has shown the formation of a supramolecular

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EXPERIMENTAL SECTION High-resolution angular-resolved ultraviolet photoelectron spectroscopy (HR-ARUPS) experiments were carried out at the LOTUS surface physics laboratory at the Physics Department at the University of Roma ″La Sapienza″. HRARUPS spectra have been excited with a He discharge lamp (HeIIα photons, hν 40.814 eV). The photoemitted electrons were analyzed in the plane of incidence, with a high-resolution Scienta SES-200 hemispherical analyzer, by using a twodimensional multichannel-plate detector. The intrinsic energy resolution is 16 meV. Work function measurements have been performed by measuring the low-kinetic energy cutoff, applying a negative bias to the sample. Synchrotron radiation excited XPS spectra have been measured at the SuperESCA beamline of the Elettra synchrotron radiation facility in Trieste, Italy.30 We used 130 eV (valence band energy region, VB), 136 eV (Ir 4f7/2 corelevel), 500 eV (N 1s), and 400 eV (C 1s) photon energies. Photoelectrons were taken at normal emission with a Phoibos electron energy analyzer equipped with a homemade delay line detection system. Data have been normalized to the beam intensity. The binding energy (BE) scale was calibrated using the Ir substrate metal Fermi edge. The NEXAFS data across the C K-edge were also taken at SuperESCA in the Auger yield mode, by tuning the analyzer on the C-KLL transition at 260 eV kinetic energy. The photon energy was spanned in the 280− 320 eV energy range. These data have been taken with linearly polarized radiation (horizontal), with the electric field vector either parallel (incidence angle γ = 0°, transverse electric field) or almost normal (γ = 70°, transverse magnetic field) to the surface plane, by appropriately rotating the sample. The N K-edge and Fe-L2,3-edge NEXAFS data were measured at the ID08 Dragon Beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). The NEXAFS spectra in Grenoble were obtained in the total yield mode. The photon energy was spanned in the 390−440 eV and 690−745 eV energy ranges, for the and N K and Fe-L2,3 edges, respectively. Monochromatized photon energy has been calibrated using a typical gas (N2) adsorption edge. NEXAFS data have been taken with the incident beam at 70° incidence angle and with linearly polarized radiation, by appropriately 3020



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varying the undulator linear polarization from vertical to horizontal, to be in the condition of in-plane or almost out-ofplane excitation, respectively. In all laboratories (NEXAFS, XPS, and HR-ARUPS experiments), we had compatible ultrahigh-vacuum (UHV) chambers equipped with ancillary facilities for sample cleaning and characterization, with base pressures better than 1 × 10−10 mbar. The Ir(111) crystal was cleaned by cycles of sputtering (Ar+ ions at E = 2 keV and T = 300 K), followed by annealing and oxygen/hydrogen-exposures cycles, to get rid of carbon contamination. The graphene single sheet was prepared by repeated cycles of C2H4 dosing followed by annealing up to 1300 K, ensuring a well long-range ordered layer.13−15 Surface quality and cleanness were checked by means of XPS and LEED. The graphene/Ir(111) sample was freshly prepared and measured in situ at the SuperESCA and ID08 beamlines, while it was air-transferred for the HR-ARUPS measurements, and after transfer into UHV, cleaning of the graphene/Ir crystal before FePc deposition was obtained by several subsequent annealing cycles at increasing temperatures up to 900 K: a very good recovering of the clean graphene surface was achieved, as checked by the sharp and low-background LEED pattern presented in Figure 1 (left panel). The FePc molecules were sublimated in UHV in all the laboratories from the same resistively heated quartz crucibles, and the nominal thickness was measured via oscillating quartz microbalances. The FePc deposition rate was about 0.2 Å/min for the first coverage steps till single-layer completion (nominal single-layer thickness ≃3 Å), while the FePc thin film was deposited at a higher rate, with the sample held at room temperature. The overall pressure during deposition was in the low 10−9 mbar range.

Figure 2. X-ray photoelectron spectroscopy (XPS) data of the Ir 4f7/2 (left), N 1s (center), and C 1s (right) core levels for the FePc/ graphene/Ir(111) system, at increasing FePc coverage (from bottom to top). Experimental data (dotted lines) and peaks resulting from a least-squares fitting procedure (continuous lines). Right panel: fitting peaks of C 1s: blue for graphene C, green for benzene-related CB, and purple for pyrrole CP. Spectra vertically stacked for clarity.

discussed in the Supporting Information (SI). The tiny SCLS modification (ΔSCLS/SCLS = 2.6%) upon FePc adsorption is of a similar amount of that found when the graphene sheet is prepared on Ir(111) with respect to the bare Ir surface.13 This evidence suggests a negligible influence of FePc molecules on the buried graphene/Ir(111) interface, with the graphene sheet basically decoupling the Ir surface from the FePc molecular layer. Recent high-resolution electron-energy-loss spectroscopy data on MPcs adsorbed on graphene/Ni(111) observed an energy and line shape variation of the molecular vibrational modes for FePc with respect to CuPc, suggesting an interaction of the Fe-related levels with the Ni states underlying graphene.33 In the present case we do not have evidence of FePc interaction with the underlying Ir surface because of the much lower C−Ir interaction with respect to the C−Ni interaction34 in the graphene layer supporting the FePc molecules. Eventually, when a 5 nm thick film of FePc is grown on graphene/Ir (topmost spectrum), the Ir 4f7/2 double peak is still visible, in agreement with a Stransky Krastanov growth mode.5 The comparison between the N and C 1s core levels for the FePc SL adsorbed on graphene and a FePc thin film (TF) is displayed in Figure 2. The N 1s core-level spectrum for the FePc 5 nm thick TF (top spectrum) is due to the photoexcitation from the two nonequivalent nitrogen atoms (N1 and N2) in the pyrrole rings and in the bridge positions, associated to the N1-isoindole and N2-azomethine atoms (see Figure 1). We fit the N 1s core-level spectrum for the TF with two components at 398.81 and 399.09 eV BE and two shake-up satellites at 1.93 and 3.94 eV higher BE with respect to the first peak, in agreement with the literature on MPcs.22,24,35−37 For the SL FePc on graphene/Ir, the N 1s line shape does not significantly change with respect to the 5 nm thick FePc TF, while a broader structure is observed when FePc is adsorbed directly on the Ir(111) surface, as shown in the SI. There is only a continuous energy shift toward lower BE as a function of coverage, by 0.12 eV (2.5 nm with respect to SL) up to 0.31 eV (5 nm with respect to SL), also for the shake-up satellites. The core-level energy shift can be due to concomitant effects, a band bending in the thin semiconducting film, as also observed in the valence band (see later discussion), and a slight conformational

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RESULTS AND DISCUSSION The adsorption of the FePc molecules on the graphene layer grown on the Ir(111) surface has been investigated by means of core-level photoemission, near-edge absorption spectroscopy, and valence band photoemission. Core-level spectroscopy can unravel the element-specific interaction of the molecule with graphene and the underlying metal. The contribution due to the macrocycle ligands has been deduced from the C and N 1s core-level photoemission signals, while the influence on the metallic substrate is from the Ir 47/2 core level, as shown in Figure 2. The Ir 4f7/2 core level at graphene/Ir(111) presents two main peaks attributed to a bulk and a surface component, at 60.850 and at 60.317 eV binding energy (BE), respectively, as deduced by a least-squares fitting analysis with quasi-Voigt (Gaussian−Lorentzian) line shape. The Ir 4f7/2 surface corelevel shift (SCLS) between the bulk and the surface components for graphene/Ir is of 533 ± 5 meV. The presence of an intense surface-related peak indicates the almost noninteracting nature of the graphene layer, in agreement with previous results.13 In fact, it is well-known that the line shape of the core levels is highly sensitive to impurities on the surface.31,32 The adsorption of the first FePc SL produces a slight decrease of the surface peak intensity with respect to the bulk component by 2.5% and a further reduction of the SCLS by 14 ± 5 meV with respect to the clean graphene/Ir surface, thus resulting in 519 ± 5 meV. On the contrary, adsorption of FePc molecules directly on the Ir(111) surface causes a quenching of the 4f7/2 surface peak already at low molecular density, as 3021



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modification of the FePc molecules in direct contact with the substrate. The C 1s spectrum of graphene/Ir(111) is a narrow peak (0.27 eV full-width at half-maximum, fwhm) centered at 284.12 eV BE, fitted with a Doniach Sunjic line shape with a slight asymmetry (α = 0.11), in agreement with previous determinations.5,13 The C 1s spectrum of the 5 nm thick FePc TF (top spectrum) has been fitted with five pseudo-Voigt-profiled components and with a small residual component due to the underlying graphene signal. The components due to the carbon atoms in the benzene rings (CB) and in the pyrrole rings (CP) are at 284.46 and 285.85 eV BE, respectively, with a relative shift of 1.39 eV. Their relative shake-up satellites (SB and SP), due to the transition from the highest-occupied to the lowestunoccupied molecular orbital (HOMO−LUMO) are at 286.31 and 287.84 eV BE, respectively; the fifth component (Bv) at 284.85 eV is related to the excitation of a vibrational mode for carbon atoms belonging to the benzene rings, in agreement with previous data.18,24 At the FePc SL coverage, the core-level line shape is dominated by the graphene C 1s component, superimposed on the molecular components. In the best-fitted C 1s spectrum, the graphene component is found at 20 meV lower BE value than on the graphene/Ir(111) surface, whereas the CB and CP components appear at 284.32 and 285.58 eV, exhibiting a relative BE shift of 1.26 eV. Within the limit of the experimental uncertainty, the separation between the main peaks and the satellites does not change, upon varying the FePc layer thickness. The CB−CP BE reduction from the TF to the SL (from 1.39 to 1.26 eV) is probably due to a slight molecular deformation. A comparable reduction of the CB−CP energy shift was observed for FePc adsorbed on highly oriented pyrolitic-graphite (HOPG)35 and ascribed to polarization effects. However, the absence of satellite screening in the N and C 1s core levels suggests a lower influence of the polarization effect. A slight modification in the molecular configuration when FePc is adsorbed on graphene can be the reason for the CB−CP energy shift produced by angular deformation of the macrocycles and consequent molecular distortion, as also observed for MPcs adsorbed on semiconductor36 and metal surfaces.22,37 These findings clearly demonstrate the decoupling of the FePc molecules from graphene and from the underlying Ir surface, with the minor effect of the CB−CP BE shift, probably due to a small molecular deformation induced by the moiré superstructure absent in graphite. Further information on the adsorption properties of FePc on graphene was obtained by the absorption spectroscopy at the relevant edges. The carbon K-edge (Figure 3, top left panel) for graphene/Ir(111) presents three main absorption structures: the main peak at 285.5 eV corresponds to C 1s → π* transitions, while the structures at 291.8 and 292.8 eV are due to the threshold of the transitions toward the σ1* and σ2* orbitals, respectively. The pre-edge shoulder at about 284.4 eV is due to the peculiar unoccupied density of states of the single carbon layer.38 The weak shoulder at 289.1 eV can be attributed to an interlayer state of graphite, predicted for single-layer systems,39 experimentally observed in graphene,38,40 and recently theoretically explained as the lowest-energy member of image potential states present even in single-layer graphene.41 The definitely highly dichroic C K-edge signal, due to the intrinsic nature of the electronic levels in graphene, with in-plane σ* states and out-of-plane π* levels, confirms the high-quality well-defined single-layer preparation on Ir(111).

Figure 3. C K-edge (top left panel), N K-edge (top right panel), and Fe-L2,3 edge (lower panel) NEXAFS data for the FePc/graphene/ Ir(111) system, as a function of FePc coverage (spectra from bottom to top). Data taken with linearly polarized radiation, with the electric field vector either parallel (dotted red lines) or almost normal (continuous blue lines) to the crystal surface. Spectra vertically stacked for clarity.

The pre-edge shoulder and the absence of a double main C 1s → π* peak demonstrate that there is a weak interaction of the graphene sheet with the Ir(111) surface. In fact, for graphene grown on other transition metal surfaces, such as Ni,42 there is a splitting of the first C 1s → π* peak due to high pz (graphene)−d (substrate) orbital hybridization which indicates a strong interaction,8,42,43 and the main absorption resonance for graphene prepared on semiconductors presents a much wider peak structure.40,44 Due to the strong C K-edge signal coming from the graphene carbon atoms, the spectra at low FePc coverage are clearly dominated by the graphene contribution. At the completion of the first FePc SL, new features emerge in the near-edge region at 284.5−285 and 287.3 eV due to π* states and at 289.2 eV due to σ1* resonances, respectively,20 associated to the pyrrole and benzene macrocycles, which evolve and eventually become dominant only when the FePc TF is formed. The absorption at the nitrogen K-edge (Figure 3, top right panel) presents a first resonance at about 398.4 eV photon energy, constituted by two structures due to photoexcitation from the N1-isoindole and N2-azomethine nitrogen atoms in the pyrrole ring to the LUMO of π*-symmetry. The higherenergy structures at 400.6, 402.5, and 404.0 eV photon energy are due to transitions to higher-lying unoccupied states mainly involving the pyrrole ring,45 in agreement with the attribution given for FePc planar TF prepared on other substrates.20 As it concerns the N K-edge line shape, it does not present any 3022



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Figure 4. Valence-band photoelectron spectroscopy data for FePc/graphene/Ir(111), as a function of FePc coverage (increasing coverage from top to bottom spectra). Left panel: ±8° angle-integrated data taken at normal emission, with hν = 130 eV photon energy. Central panel: image zoomed on the low binding energy spectral features. Right panel: ±0.9° angle-integrated data taken at the K point of the surface Brillouin zone, with hν = 40.8 eV photon energy (data taken with a slight azimuthal rotation, ≈1.5°, with respect to the exact high-symmetry orientation, to better single out the Dirac cone trace). All the spectra are vertically stacked for clarity.

significant change upon increasing the FePc film thickness. The black ticks in the energy axes represent the BE positions of the N 1s core levels, whose BE is higher than the corresponding absorption peak photon energy, indicating an excitonic effect in the molecular layer,20 which is still present for the FePc-SL on graphene. Since the first unoccupied levels in the FePc molecule are formed by π* states mainly delocalized on the pyrrole macrocycles containing both C and N atoms, they are available for transitions from both the C and the N 1s initial states, justifying the line shape similarity for both C and N Kedges for the FePc thin films, when the graphene contribution to the C K-edge is negligible. The N and C K-edge absorption line shapes do not basically change between the FePc-SL and the FePc-TF, indicating the unperturbed distribution of resonance states in the macrocycles, confirming a negligible interaction with the underlying graphene and iridium surfaces. The adsorption of a SL of FePc on the bare Ir(111) surface influences the absorption line shape, indicating an FePc/Ir interaction (see SI). The Fe-L2,3 edges for the FePc/graphene/Ir system as a function of FePc coverage are shown in the lower panel of Figure 3. The NEXAFS data for the 22 nm thick FePc thin film show a dichroic response associated to the flat-lying configuration and present a structured absorption in the nearedge region due to the ligand field splitting of the Fe-related dlevels, in agreement with previous linearly polarized data on flat molecular FePc multilayers prepared on different substrates.20,46 Despite the controversial theoretical attribution to the actual sequence of resonance states, depending on the use of different exchange-correlation functionals,47,48 the main absorption features derive from the empty b1g state with dx2−y2 symmetry, hybridized with the π orbital localized on the N atoms, the partially empty a1g state with dz2 symmetry, and the doublet eg with dzy,dxz symmetry, mainly localized on the central Fe atom. Taking into account the sequence of spin-split molecular orbitals46 and the expected final low spin state (S = 1), we can assign the different main absorption features considering the symmetry assessed by electrons excited from the 2p 3/2 or 2p 1/2 initial states. In the out-of-plane configuration, the excited electrons accede to states localized

normal to the molecular plane, and therefore the main L3 peak at 707.7 eV photon energy is due to the a1g single empty state; the higher lying structures at 708.8 and 709.3 eV may be attributed to the eg states. These attributions are in agreement with previous studies performed at FePc planar thin films grown on different substrates.20,46 In the in-plane polarized spectrum, the a1g peak is highly reduced, its charge density being localized in the direction perpendicular to the molecular plane; the small pre-edge peak at 707.0 eV can be attributed to the eg state; and the multiplet at higher energy is due to b1g, confirmed by the analogous features present above the L2 edge. The Fe-L2,3 NEXAFS data for the FePc SL on graphene, while maintaining the same dichroic in-plane/out-of-plane response (flat-lying orientation), present a slightly different line shape in the near-edge region, with respect to the FePc TF data. In particular, narrower absorption peaks are present in the FePc SL for both in-plane and out-of-plane spectra in the L3 energy region, with weaker modification across the L2 edge. Moreover, for the in-plane data at the FePc SL, a narrower and more intense pre-edge eg peak at about 707 eV is present, and a quenched structure is observed in correspondence to the a1g structure. Adsorption of FePc and CoPc on metal surfaces gives rise to a stronger interaction specifically involving the central metal ion.19,22,23 In particular, the FePc SL adsorbed on the Au(110) surface shows a strong intensity reduction and a broadening of the resonances, due to hybridization between the dz2-Fe orbitals and the gold metal states.49 Theoretical predictions of Fe adsorption on graphene suggest a strong charge transfer with the electronic structure of the filled and empty states strongly altered and with a reduction of the magnetic moment.50 Our measurements on FePc/graphene/Ir cannot be explained with a relevant orbital hybridization, although the intensity and width variations between the SL and TF suggest orbital localization and probable influence on the spin and orbital configuration of FePc after the adsorption. This suggestion is also supported by the unchanged line shape measured at the N K-edge. Previous results suggesting an interaction of Fe-related orbitals with the underlying metal surface33 may reasonably depend on the substrate, as the Ni 3023



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substrate has a stronger interaction with the graphene sheet,34 influencing both the electronic and vibrational properties. All the experimental results discussed so far suggest an almost unperturbed electronic structure of the layer of FePc molecules assembled on graphene. A further step is the study of the evolution of the graphene band parameters upon FePc adsorption, by ultraviolet and soft X-ray photoelectron spectroscopy. Angle-integrated photoelectron spectroscopy data on the valence band energy region at different photon energies as a function of FePc coverage are reported in Figure 4. We report both angle-integrated (±8°) data around normal emission (left and central panels) and data around the K point (±0.9° integration) of the surface Brillouin zone (SBZ), centered on the Dirac cone vertex at 1.70 Å−1 (right panel). The clean graphene/Ir valence band at normal emission (left panel) at high photon energy (130 eV) is characterized by the main π and σ bands at about 8 and 22 eV, respectively, and by spectral features due to the underlying Ir(111) surface, in particular the peaks at low BE. In the spectrum of clean graphene/Ir taken at low photon energy (40.8 eV) around the K point of the SBZ (Figure 4, right panel), the main peak centered at about 0.4 eV is due to the angle-integrated Dirac cone, while the shoulder at 0.95 eV is due to an underlying Ir(111) surface state.15 Adsorption of the first FePc single layer induces a general intensity reduction of the graphene-associated states; the centroid of the angle-integrated Dirac cone shifts by 80 ± 20 meV to lower BE; and a new electronic state appears above 1 eV BE superimposed to the Ir-related band (Figure 4, right panel). At higher molecular density the weak integrated trace of the Dirac cone (Figure 4, right panel) maintains the −80 meV BE shift. The FePc 5 nm thick film eventually presents the typical molecular orbitals in the whole valence band energy region (Figure 4, left panel), with the HOMO at 1.3 eV BE (a1u state of π-symmetry) and the HOMO-1 at 1.6 eV BE (b2g state of d-character), respectively,19 indicating the formation of a unperturbed molecular thin film. Further details on the graphene band parameters after the FePc single-layer adsorption can be achieved by high-resolution angular-resolved ultraviolet photoelectron spectroscopy (HRARUPS) valence band data around the K point of the surface Brillouin zone. Photoemission spectra scanned along the ΓKM high-symmetry direction are displayed in Figure 5 for clean graphene/Ir (left panels) and for the FePc SL adsorbed on graphene (right panels). The azimuthal angle has been rotated by ≈1.5° with respect to the exact high-symmetry orientation, so that the Dirac cone vertex is slightly lowered in energy and appears just below the Fermi level, allowing us to better follow the FePc influence on the electronic states. A direct comparison between the well-aligned surface and the slightly rotated one is reported in the Supporting Information. The energy distribution curves as a function of the polar angle are shown in the upper part of the figure, while the spectral traces of the Dirac cone (energy vs momentum) are reported in the bottom part of Figure 5. Graphene grown on Ir(111) presents an optimal compromise among weak interaction and quasi-free-standing conformation.8 A tight-binding (TB) calculation fitting the experimental band structure has been performed, considering first-neighbor interaction and a marginal doping as the fitting parameter. We find a hopping parameter of 2.9 ± 0.2 eV, in good agreement with calculation on free-standing graphene,2 and a light p-type doping of 100 ± 20 meV (once considered

Figure 5. Sketch of the graphene Brillouin zone (top image). HRARUPS data for clean graphene/Ir (left panels, (a, c)) and 1 SL FePc/ graphene/Ir (right panels, (b, d)), taken across the K point of the SBZ along the ΓKM direction, at 40.8 eV photon energy. Energy distribution curves (a, b) and corresponding HR-ARUPS Dirac cone intensity plots (c, d). Data taken at the ≈1.5° rotated azimuthal angle with respect to the exact high-symmetry orientation, so that the Dirac cone vertex is slightly lowered in energy and appears just below the Fermi level (see also the Supporting Information), allowing us to better follow the FePc influence on the electronic states.

the well-aligned surface, see SI), in very good agreement with previous observations on graphene/Ir.15,51 We also observe kinks at about 0.7 eV BE, due to expected mini-gap15,52 and crossing of Ir bands in the KM direction,15 and an Ir surface state close to EF.51 By a linear fit of the most intense cone branch (from Γ toward K), we obtain a Fermi velocity of (11.6 ± 0.5) × 105 m/s, as expected,15 while the estimation on the other branch (from K toward M) gives (8.4 ± 0.9) × 105 m/s. This asymmetry on the KM-branch around EF influencing the expected Dirac cone of bare graphene is still controversial, and it may depend on a slight deviation from the purely linear behavior of the π band toward the M point, although it has been recently attributed to a light C-π/Ir-d hybridization.51 Moreover, the slight rounding of the Dirac cone vertex may be also ascribed to a partial hybridization with the underlying Ir states. The adsorption of the FePc SL on graphene causes a slight lowering of the Dirac cone by 80 ± 20 meV toward higher BE. Thus, since graphene/Ir(111) presents the Dirac vertex at about 100 meV above EF, its interaction with a planar array of FePc molecules renders the underlying graphene sheet closer to 3024



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ideal graphene. This shift suggests a very tiny charge transfer from FePc to graphene. By assuming that the doping density is proportional to the amount of dopant molecules,53,54 we estimate about 4 × 10−3 electrons per molecule. Moreover, the Fermi velocity of the graphene π-band after the adhesion of the FePc SL decreases by about 10% (to 10.5 ± 0.5 × 105 m/s), also indicating a weak influence on the graphene band parameters. Finally, the nondispersing molecular band crossing the Dirac cone at about 1 eV BE is associated to the HOMO and HOMO-1 of the molecule. A resuming energy level diagram is reported in Figure 6.

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ASSOCIATED CONTENT

S Supporting Information *

Angular-resolved photoemission data of the Dirac cone at clean graphene/Ir(111) on the well-oriented surface with respect to the slightly rotated one, as well as data showing the much stronger influence exerted by the adsorption of FePc on the bare Ir(111) surface than on the graphene-covered Ir surface. 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]; carlo.mariani@ uniroma1.it. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We kindly thank the Elettra and ESRF synchrotron radiation facilities for the experimental support. In particular, we thank the SuperESCA beamline (P. Lacovig) and the ID08 beamline (V. Sessi) collaborators for experimental assistance. Experimental assistance by Xenia De Lucia, assistance in tightbinding calculations by Giuseppe Forte and useful discussions with Emmanuele Cappelluti are kindly acknowledged. A.B. acknowledges the Università degli Studi di Trieste for the ″Finanziamento per Ricercatori di Ateneo″. Work partially funded by PRIN grants 2008525SC7 and 20105ZZTSE of the Italian Ministery for Research (MIUR) and by Roma ″La Sapienza″ University funds.

Figure 6. Schematic energy level diagram of the FePc/graphene/ Ir(111) interface system, as determined through the photoemission measurements. The reported energy values refer to the well-aligned surface (see the Supporting Information).

The SL FePc on graphene/Ir produces an increase of work function by about 0.25 eV, and such a slightly positive variation has been recently observed for other weakly interacting molecules on graphene.3,29 In the present case, the FePc SL, on one side by inducing a slight n-type doping of the graphene sheet without considerably affecting its mobility, produces a quasi-ideal although supported graphene sheet. On the other side, this SL behaves as an ultrathin slab useful as a planar conduction channel constituting a potential basis for enhancing the adhesion of gate dielectric layers on epitaxial graphene and for further doping by using substituted MPc molecules. In fact, metal-phthalocyanines constitute a very wide class of squareshaped planar aromatic molecules (more than 70 different molecules have been synthesized so far),55 and subsequent doping of graphene may be induced through the ultrathin dielectric MPc layer.

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CONCLUSIONS The FePc adsorption on the graphene sheet prepared on Ir(111) is investigated by absorption spectroscopy and corelevel and valence band photoemission. All spectral evidence brings to light a weak FePc/graphene interaction, with a minor molecular macrocycle deformation on the moiré template. A light electron doping of graphene is induced by FePc adsorption, as revealed by the −80 meV energy shift of the Dirac cone, bringing the graphene sheet to a supported quasiideal condition. The flat monolayer of weakly interacting FePc molecules is an ideal channel for conduction and for doping by using substituted phthalocyanines, rendering it an organic buffer layer decoupled from the underlying metal. It can facilitate the adhesion of gate dielectrics for potential devices, and FePcthanks to the d-state occupation of the Fe atom may be exploited for potential spintronic applications. 3025



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NOTE ADDED AFTER ASAP PUBLICATION This paper published on the Web on February 5, 2013. The first sentence in paragraph two was corrected. The corrected version was reposted with the Issue on February 14, 2013.

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