Energetics and Hierarchical Interactions of MetalâPhthalocyanines

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Energetics and Hierarchical Interactions of MetalPhthalocyanine Adsorbed on Graphene/Ir(111) Mattia Scardamaglia, Claudia Struzzi, Silvano Lizzit, Matteo Dalmiglio, Paolo Lacovig, Alessandro Baraldi, Carlo Mariani, and Maria Grazia Betti Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401850v • Publication Date (Web): 23 Jul 2013 Downloaded from http://pubs.acs.org on July 29, 2013

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Energetics and Hierarchical Interactions of Metal-Phthalocyanines Adsorbed on Graphene/Ir(111) Mattia Scardamaglia,† Claudia Struzzi,‡ Silvano Lizzit,‡ Matteo Dalmiglio,‡ Paolo Lacovig,‡ Alessandro Baraldi,¶,§ Carlo Mariani,∗,† and Maria Grazia Betti∥ Dipartimento di Fisica, CNISM, CNIS, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I - 00185 Roma, Italy, Elettra - 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, and Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I - 00185 Roma, Italy E-mail: [email protected]

Abstract The adsorption of Metal-Phthalocyanine (MPc) layers (M = Fe, Co, Cu) assembled on graphene/Ir(111) is studied by means of temperature programmed X-ray Photoemission Spec∗ To

whom correspondence should be addressed di Roma La Sapienza, CNISM, CNIS ‡ Sincrotrone Trieste ¶ Università di Trieste § IOM-CNR, Trieste ∥ Università di Roma La Sapienza † Università

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troscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS). The balance between interaction forces among the organometallic molecules and the underlying graphene gives rise to flat-lying molecular layers, weakly interacting with the underlying graphene. Further MPc layers pile up face-on onto the first layer, up to a few nm thickness, as deduced by NEXAFS. The FePc, CoPc and CuPc multilayers present comparable desorption temperatures, compatible with molecule-molecule interactions dominated by van der Waals forces between the π -conjugated macrocycles. The MPc single layers desorb from graphene/Ir at higher temperatures. The CuPc single-layer desorbs at lower temperature than the FePc and CoPc single-layers, suggesting a higher adsorption energy of the FePc and CoPc single-layers on graphene/Ir with respect to CuPc, with increasing molecule-substrate interaction in the order ECuPc < EFePc ∼ ECoPc .

Introduction Aromatic oligomers can be used as building blocks for nanometer-sized devices on graphene, exploiting the transport properties of π -conjugation of the macrocycle electrons, by tailoring the band structure, by doping and/or charge transfer, for potential nanoelectronic applications. 1–4 Organic molecule deposition on patterned surfaces is a viable method for inducing self-assembling and for tuning the charge transport, depending on the specific molecule-substrate interaction. MetalPhthalocyanines (M-C32 H16 N8 , MPcs) are square-shaped planar molecules with a central metal atom, four pyrrole rings and four benzene rings, 5 with the π -orbitals extended on the macrocycles ensuring conduction on substrates. 6 MPcs, embedding a metal atom in the organic cage, also constitute an useful shuttle for organizing individual metal/magnetic units when they are adsorbed on crystalline surfaces. 7–12 Planar arrays of MPcs with different motifs can actually form on the graphene moiré pattern, 13–18 as it has been recently shown by scanning tunneling microscopy measurements. 19–22 In fact, graphene acts as a buffer layer decoupling the metal substrate from the molecular layer, 23 inducing different configurations depending on the molecule-graphene interaction. 2 ACS Paragon Plus Environment

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An important parameter determining the specific molecular adsorption is the interaction strength between the molecule and graphene. A high graphene moiré corrugation can drive long-range ordered superstructures of MPcs, like the kagome super-lattice observed for FePc assembled on graphene prepared on Ru. 19,21 On the other hand, when the moiré corrugation plays a minor role, the molecule-molecule interaction dominates over the molecule-graphene interaction, like in the case of MPcs grown on graphene prepared on Ir, where square closely-packed long-range ordered molecular structures are formed. 21,22 We have recently demonstrated that FePc grown on graphene/Ir(111) lies flat from the single-layer (SL) 24 up to a multilayer coverage, 23 as determined by near-edge X-ray absorption fine-structure spectroscopy (NEXAFS). The FePc molecules of the SL arrange in a planar molecular structure, without modifying the substrate periodicity, and induce a slight electron doping to the Ir-supported graphene, while at the same time graphene acts as a buffer layer efficiently decoupling the molecules from the metallic substrate. 23 A strong mixing of the d molecular orbitals with metallic substrate have been recently reported for the adsorption of FePc and CoPc on metallic substrates, due to a rehybridization of the electronic states localized on the central atoms, breaking the 4-fold symmetry of the molecular orbitals of the MPc molecules. The molecular adsorption is controlled by a symmetry-determined mixing between the electronic states of the MPc metal center and of the metal substrate, as recently observed for MPcs adsorbed on Au(110) 8,25 Despite the weak influence of these π -conjugated molecules on the unperturbed graphene electronic properties, different central metal atoms in the MPc organic cage result in different adsorption energies on the same substrate. The adsorption energy of a single molecular layer on graphene is an important parameter for determining the actual molecule-graphene interaction, and temperature-programmed desorption experiments represent an useful way to investigate the interaction strength between a molecule and a substrate. It is possible to follow in-situ and in real-time the evolution of the molecular layer adsorbed on the substrate, by performing Temperature-Programmed XPS 26 measurements on the relevant core-levels. In this work, we present a systematic fast-XPS study of three MPcs adsorbed on graphene/Ir(111): FePc, CoPc and CuPc. These three molecules differ for the occupation of the


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molecular orbitals involving the central metal states, thus for the symmetry of the partially filled orbitals involved in the interaction with the substrate. Graphene grown on the Ir surface presents a lower interaction than on other surfaces, 27 thus the graphene/Ir substrate can act as an efficient buffer layer for molecular adsorption. Nevertheless, we bring to light a higher interaction of the first FePc and CoPc layers in contact with graphene/Ir than the first CuPc SL, thus suggesting a role of the metal-derived states reinforcing the π -π van der Waals interaction, in the binding to graphene.

Experimental Synchrotron-radiation XPS data have been measured at the SuperESCA beamline of the Elettra synchrotron radiation facility in Trieste, Italy. 28 Photon energy of 130 eV, 500 eV and 400 eV were used for the valence band (VB), the N 1s and the C 1s core-levels, respectively. Photoelectrons were collected at normal emission, with a Phoibos electron energy analyzer equipped with a home-made delay line detection system. Photoemission data have been normalized to the beam intensity and the BE scale was calibrated using the Fermi edge of the Ir substrate. NEXAFS measurements across the C and N K-edges were taken at SuperESCA in Auger yield mode, 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 base pressure in the ultra-high-vacuum (UHV) chamber of the experimental station was better than 1×10−10 mbar. The UHV chamber is equipped with all necessary ancillary facilities for sample cleaning and characterization. The Ir(111) surface was cleaned by cycles of sputtering (Ar+ at E = 2 keV and T = 300 K), followed by annealing and oxygen/hydrogen-exposures cycles, to quench any carbon contamination. The graphene single-layer was prepared by repeated cycles of C2 H4 dosing followed by annealing up to 1300 K. With this procedure, we achieved a long-range ordered layer, 13–15 whose quality and cleanness were checked by means of XPS and


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low-energy electron diffraction (LEED). The MPc molecules were sublimated in UHV by means of home-made resistively heated quartz crucibles. The nominal thickness of the molecular layer was measured by using an oscillating quartz microbalance, and a single-layer is defined as a flat and compact molecular layer, 29 corresponding to about 3.4 Å nominal coverage. During MPc evaporation the sample was kept at room temperature (RT) and the overall pressure during deposition was in the low 10−9 mbar range. Fast-XPS measurements were taken after deposition of a few nm-thick MPc molecular layer. We linearly increased the sample temperature with a rate of 0.3 K/s, following in-situ and in realtime the desorption process. With this procedure, we followed the temperature evolution of the molecular layers adsorbed on the substrate, by analyzing the N 1s core-level intensity, binding energy and lineshape. The N 1s core-level represents a clear signature of the MPc molecule, because the molecular C 1s component is difficult to deconvolve from the graphene signal. 24 Finally, we estimate the desorption energy of the molecular film deposited on the graphene/Ir surface, by appropriately treating the data within the Redhead equation. 30

Results and Discussion The Metal-Phthalocyanine layers have been grown on freshly prepared graphene on Ir(111), by subsequent deposition steps at room-temperature. The adsorption geometry of CoPc and CuPc layers adsorbed on graphene can be investigated by the dichroic response of the C and N absorption edges, as a function of the incident linearly polarized radiation. The N K-edge near-edge absorption spectra for the CoPc and CuPc SL and thin-films prepared on graphene/Ir(111) are shown in Fig. Figure 1. Data show a definite dichroic response for both systems, due to the welldefined molecular orientation of both MPcs. In particular, the N K-edge spectra for both systems presents a first resonance at about 398.4 eV photon energy, constituted by two close structures due to photoexcitation from the 1s level of the two nitrogen atoms in the pyrrole ring to the lowestunoccupied molecular-orbital (LUMO) of π ∗ -symmetry. The higher-energy structures at 400.6,


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402.5, and 404.0 eV photon energies, respectively, are due to transitions to higher-lying unoccupied states mainly involving the pyrrole ring, 31 while the resonances above 406 eV photon energy are associated to transition to unoccupied orbitals with σ ∗ symmetry, in agreement with the attribution given for FePc and CoPc planar thin-film (TF) grown on other crystalline substrates. 9 The clear dichroic response of the signal is fingerprint of flat-lying molecules, from the SL up to the formation of a thin-film on the graphene-Ir substrate. Furthermore, the absorption resonances maintain the same energy position and lineshape in the different coverage phases, indicating a negligible interaction of the molecules with graphene and confirming the decoupling role played by graphene with respect to the bare Ir surface. 23

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N-K edge

N-K edge

TF CoPc

TF CuPc

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2SL CuPc

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Photon Energy (eV) Figure 1: NEXAFS across the N K-edge for CoPc SL and TF (left) and CuPc (≃ 2 SL and TF (right) adsorbed on graphene/Ir(111). Red curves correspond to γ =0◦ and blue curves correspond to γ =70◦ , where γ is the incidence angle between the incident radiation and the surface normal, as reported in the sketch in the inset. The NEXAFS C K-edge data at the SL coverage for CoPc and CuPc on graphene/Ir are shown 6 ACS Paragon Plus Environment

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C-K edge

SL CoPc

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Photon Energy (eV) Figure 2: NEXAFS across the C K-edge for a CoPc SL (left) and 2 SL of CuPc (right) adsorbed on graphene/Ir(111), and for graphene/Ir(111) for comparison (bottom curves). Red curves correspond to γ =0◦ and blue curves correspond to γ =70◦ , where γ is the incidence angle between the incident radiation and the surface normal, as reported in the sketch in the inset.


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in Fig. Figure 2, compared with the C K-edge from clean graphene/Ir. The absorption structures in clean graphene are fingerprint of formation of a good single graphene sheet. 23,32,33 The adsorption of the first CoPc and CuPc layer determines the emergence of new C-related features characteristic of metal-phthalocyanines, 23 superimposed onto the intense graphene signal, maintaining the dichroic signal among the two polarizations, characteristic of flat layers. These NEXAFS results on CoPc and CuPc grown on graphene/Ir, are analogous to the data recently taken and discussed for FePc grown on graphene/Ir, building-up planar layers from the SL to a multi-layer coverage, while maintaining unperturbed resonance lineshape. 23 Thus, MPc growth on graphene/Ir is characterized by a flat molecular orientation and weak interaction of the organic macrocycles with the underlying graphene. The N 1s core-levels for CoPc and CuPc thin films grown on graphene/Ir, taken at room temperature, are reported in the middle panels of Fig. Figure 3. Their lineshape is due to the presence of two non-equivalent nitrogen atoms in the phthalocyanine molecule, the isoindole nitrogen linked to the central atom, and the azomethine nitrogen bridging two carbon atoms in the pyrrole rings, respectively, as shown in the fit superimposed to the experimental data, in agreement with previous determinations in MPc thin-films. 6,34,35 The slightly different N 1s binding energy measured for CoPc (5 nm-thick film) and CuPc (2.4 nm-thick film) is due to a small band bending variation in the semiconducting molecular films, depending on the TF thickness. In order to follow the N 1s core-level intensity evolution as a function of temperature, we fit the data in the following as a single peak. 36 The intensity evolution of the N 1s core levels as a function of temperature is shown through two-dimensional (2D) projected plots of the fast-XPS spectra, as shown in the upper panels of Fig. Figure 3. The single spectra of the time-lapsed sequence have been fitted with single Gaussian curves and the results of the fitting procedure, full-width at half-maximum (FWHM) and binding energy (BE) for both molecular films, are reported in the bottom panels of Fig. Figure 3. The N 1s peak intensity stays constant up to about 590 K for CoPc and 570 K for CuPc on graphene/Ir. Above these temperatures, we observe a drop of the intensity, a shift to lower binding energy of the N 1s peak position, and no effects on the peak width. We attribute this intensity drop


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Figure 3: N 1s core-level of CoPc (left) and CuPc (right) thin-films adsorbed on graphene/Ir(111). Top panels: fast-XPS data as 2D trace of the spectral intensity as a function of BE and temperature (higher to lower intensity signal represented in false colors from red to blue). Middle panels: N 1s core-level data (dotted lines) taken at the initial temperature, RT; experimental data fitted with two 9 N atoms. Bottom panels: BE (blue squares) and components associated to the twoACS non-equivalent Paragon Plus Environment FWHM (green squares) evolution of the N 1s core level as a function of temperature.

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to the desorption of the flat molecular thin-film adsorbed onto the graphene/Ir(111) surface. The desorption temperature for these thin-films is comparable among different MPcs and basically due to the weak intermolecular forces. An analogous temperature was found for NiPc TF desorption from highly-oriented pyrolitic graphite (HOPG), 37 and for FePc TF desorption from from a variety of substrates, from Au to HOPG. 38 It is thus compatible with the multilayer desorption of physisorbed molecular MPc multi-layers dominated by van der Waals interactions. Increasing the temperature above 620 K, the N 1s core-level intensity for the MPcs reaches a plateau, while the BE reaches the same value obtained for a single-layer as-deposited on graphene/Ir at RT. The desorption of the MPc thin films leaves a stable SL adsorbed on the graphene/Ir surface. 23 We report in Fig. Figure 4 the fast-XPS data for FePc (left), CoPc (center) and CuPc (right) on graphene/Ir, as a function of temperature, in order to determine the desorption temperature of the first molecular layer in direct contact with the graphene/Ir substrate. The CuPc SL (right panel) presents a fast and total desorption at about 670 K, corresponding to the disappearance of the N 1s peak. The N 1s core-level shape evolution for the CuPc SL is consistent with the full desorption of the single-layer in contact with graphene. It has be fitted with a single component 36 which starts to decrease above 670 K temperature, that we consider as the desorption temperature. The N 1s peak lineshape evolution for FePc and CoPc as a function of temperature shows an intensity decrease above 820 K, taken as desorption temperature for the SLs, while maintaining a lower decreasing rate with respect to the case of CuPc. We notice that the molecular layer resulting from annealing a multi-layer at 775 K, just below the desorption temperature of the SL, presents the same spectroscopic signatures (binding energy, absorption resonances and spectral density of states in the valence band) of the SL as deposited at RT (data presented in the Supplementary Information section). After the first desorption stage, the N 1s cannot be fitted with a single component above 900 K and it has been fitted with two components, as reported in Figure 5 and Figure 6 for FePc and CoPc, respectively. In fact, there is an apparent shift of the N 1s core-level to lower binding energies actually due to the growth of a second N 1s component at lower BE for both FePc and


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Figure 4: Top panels: 2D plot of the fast-XPS data of the N 1s core-level of FePc (left), CoPc (center) and CuPc (right) single-layers adsorbed on graphene/Ir(111). Higher to lower intensity signal represented in false colors from red to blue. Bottom panels: BE (blue squares) and FWHM (green squares) evolution of the N 1s core level as a function of temperature.


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CoPc systems, which becomes dominant at very high temperature (above 1100 K). This lower BE component appears above the SL desorption temperature, it grows in intensity till the highest temperature reached in the experiment, and is present only at high temperatures and only under high-brilliance and focussed x-ray beam. Above 900 K, also the spectroscopic signature of the C 1s core-level presents a modified shape with respect to the typical graphene layer (see Supplementary Information). The lineshape modification of C 1s and the emerging of the new N 1s component can be attributed to doping of the graphene layer with pyridinic nitrogen 39,40 deriving from the FePc ad CoPc molecular fragmentation. The determination of the desoption temperature is independent from the presence of the doping N defects, as explained in the Supplementary information section, and the majority of molecules of the SLs desorb in the temperature range where the radiationinduced molecular fragmentation is not yet efficient, while at higher temperature the cooperation of temperature and radiation damage induces the uptake of doping N. The undesired effect of beam damage, introducing N defects in graphene, may be exploited as a possible method for doping and patterning, once performed in a controlled way. The desorption temperatures in each phase, shown in Table Table 1, can be correlated to the activation energies for desorption, thus to the adsorption energy on the surface. We can estimate the activation energy EA for the desorption of the MPc TF and of the MPc SL on graphene/Ir(111), by using the Redhead equation: 30 ] [ ν Tmax EA EA = KB Tmax ln − ln ; β KB Tmax

(1)

where KB is the Boltzmann constant, ν a frequency factor, Tmax the desorption temperature,

β the heating rate. In our experiment, the heating rate β is 0.3 K/s, the quantity ln(EA /KB Tmax ) is a small constant value compared to the first term in brackets. These approximations are valid for first order kinetic processes that are non dissociative desorptions without multiple process of re-adsorption. In these processes, the desorption temperature does not change with the initial layer thickness, as in the present case, discussed in the Supplementary Information section. The frequency factor for large molecules with many internal degrees of freedom can be much higher 12 ACS Paragon Plus Environment

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Figure 5: Fast-XPS N 1s core-level spectra of FePc on graphene/Ir(111), as a function of temperature. Left panel: single energy distribution curve data, vertically stacked for clarity. Central panel: fit of significant spectra at different temperatures. Right panel: intensity evolution of the two N 1s components as a function of temperature (molecular N, red lines, doping N, blue lines); N 1s peak intensity normalized to the SL intensity.


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Figure 6: Fast-XPS N 1s core-level spectra of CoPc on graphene/Ir(111), as a function of temperature. Left panel: single energy distribution curve data, vertically stacked for clarity. Central panel: fit of significant spectra at different temperatures. Right panel: intensity evolution of the two N 1s components as a function of temperature (molecular N, red lines, intercalated N, blue lines); N 1s peak intensity normalized to the SL intensity.


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than in small molecules, 41 and we can assume ν to be 1018 s−1 . The choice of this frequency factor has been taken in order to obtain an activation energy for the CuPc thin-film grown on graphene-Ir in agreement with the measured sublimation enthalpy of CuPc thin-films. 42 With this assumption on the frequency factor, we estimated activation energies for the desorption of the thin-films and of the SL of three MPcs adsorbed on graphene/Ir, resumed in Table Table 1. The thin-films of all MPc molecular species present very close activation energies of about 2.2-2.4 eV, compatible with the van der Waals interaction between the π -organic macrocycles of the metalphthalocyanines 42,43 producing the molecular stacking into molecular planes, and common to all three molecules, basically independent on the specific central metal atom. Table 1: Activation energies for desorption, calculated with the Redhead equation (1) for the adsorption of FePc, CoPc and CuPc on graphene prepared on Ir(111): thin-film and single-layer activation energies. Temperature error ±15 K, energy error ±0.1 eV. (a) from Ref. [ 38 ]. TF SL

FePc 2.4 eV (600 K(a) ) 3.2 eV (820 K)

CoPc CuPc 2.3 eV (590 K) 2.2 eV (570 K) 3.2 eV (820 K) 2.6 eV (670 K)

On the other hand, the activation energy of the SL on graphene/Ir is different among the three molecules, i.e. 2.6 eV for CuPc, and 3.2 eV for FePc and CoPc. The activation energy estimated for of the CuPc SL (2.6 eV) is comparable to that of similar organic oligomer adsorbed on metal surfaces 44,45 or on organic buffer layer onto a metal, 46 and it is consistent with an estimation of activation energy basically exerted between aromatic species, driven by π − π interaction. 47 The activation energy of the FePc and CoPc SLs (3.2 eV), instead, presents a stronger interaction than the CuPc SL, and is dominated by stronger molecule-substrate forces. Comparably adsorption energies are found in molecular systems on surfaces with stronger interaction due to a particular surface morphology like vicinal surfaces, 44,48 where the surface step morphology creates preferentially and energetically favorable adsorption site, or for different large organic molecules adsorbed on graphene/Ru(0001), 49 where the lateral variations in the molecule-substrate interactions play an important role. The frequency factor ν for large molecules can also change due to different growth morphology (planar, up-right geometry). 49 Even considering a modification by two orders 15 ACS Paragon Plus Environment

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of magnitude of the pre-factor, associated to a freezing of the modes, in contact with the substrate, the absolute energy value would change by less than 10% However, it does not affect the relative energy difference between the various MPcs studied here in each coverage phase, because the three MPc molecular layers on graphene/Ir maintain the planar geometry from sub-SL to the multi-layer coverage, and they present the same organic cage embedding the central metal atom. The changes in the absolute energy estimation due to the specific choice of a frequency factor, do not modify the definite energy difference observed among FePc and CuPc SL on graphene/Ir. We have assumed the same frequency pre-factor in the Redhead equation to evaluate the adsorption energy for the multilayer and the single layers. The MPc layers, in contact with the graphene, can have degree of freedom frozen by the adsorption geometry and this can induce increasing values for the frequency factor. Even assuming a frequency factor by orders of magnitude higher than the average value used for this estimation, which affects the absolute energy, the relative increase in activation energy between FePc and CoPc, and CuPc SLs, is maintained. In general, for aromatic molecules adsorbed on graphene, both graphene-like (CC) and benzene-like (CH) carbon atoms contribute to the bonding, which also depends on the competition between dipolar, nonlocal correlation and electrostatic energy contributions to the total energy, varying as a function of the molecule-to-graphene distance. 47 Recently, the lowest-energy linear assembly of coplanar unsupported FePc chains has been predicted by means of density functional theory (DFT) calculations by computing the interaction energy as a function of the intermolecular distance, and stable long-range ordered chain configurations have been obtained, without incurring any energy barrier. 50 The intermolecular attractive electrostatic potential is the major driving force to stabilize the molecular assembly, and the molecule-molecule distance is dictated by the repulsion barrier. Long-range ordered configurations, mainly driven by the MPc-MPc mutual electrostatic interactions, have been observed for CoPc assembled on graphene/Ir(111) 22 and graphite. 51 State-of-the-art first principle DFT calculations evaluate a negligible interaction of MPc with graphene if van der Waals forces are not included. 51,52 The inclusion of van der Waals functional in the DFT calculations of CuPc on


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graphene induces an energy minimum for flat-lying molecules with a binding energy of 3.37 eV. 53 Very recent preliminary calculations within the DFT approach, including different van der Waals functionals, obtain basically undistorted flat-lying MPc molecules on graphene, whose minimum energy configuration is a hollow site for the central metal atom on the carbon network, with a weak dependence on a specific adsorption site, 52 and where the main contribution to the adsorption energy is due to the organic macrocycles without the metallic center (2 eV). 52 These considerations can rationalize the formation of stable ordered flat MPc layers on graphene. As far as it concerns the role of the central metal atom in the adsorption on graphene, there is not yet any theoretical analysis enlightening a clear hybridization and/or interaction process explaining the observed adsorption energies. On the other hand, theoretical and experimental data have rationalized the interaction of MPc on metallic substrates, where the energy landscape is strongly influenced from the MPc central metal atoms orbitals mixed and hybridized with the underlying metallic states, as for example on Au and Ag surfaces. 50,54,55 Previous experimental analysis of FePc adsorption on graphene-Ir enlightened the absence of molecular orbital occupation, hybridization or significant charge transfer between graphene and the molecules. 23 Thus, we suggest that very slight deformation of the molecules with subsequent broken symmetry of the crystal field might play a role in the case of FePc and CoPc on graphene-Ir, claiming for refined theoretical analysis.

Conclusions Temperature-Programmed XPS was used to follow the evolution of MPc (M = Fe, Co, Cu) molecular layers adsorbed onto the graphene/Ir(111) surface to determine the desorption temperatures. Different phases have been singled out, by following the N 1s core-level intensity, binding-energy and lineshape evolution, upon linearly increasing the temperature, and estimating the adsorption energy. Very close values of activation energy (≃ 2.2-2.4 eV) is determined for all MPcs for the desorption of the flat multi-layer film, while a higher activation energy is obtained for the SL of FePc and CoPc with graphene/Ir (3.2 eV), with respect to SL of CuPc (2.6 eV). The flat-lying MPc


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single-layers are stabilized by cooperative molecule-molecule interactions, van der Waals forces and dipolar interaction with the underlying graphene. The higher interaction strength experienced by FePc and CoPc SLs with graphene/Ir can be related to tiny molecular deformation and crystal field effects, also depending on the metal-associated d-orbital occupancy. The energetics and the role of competing interaction in graphene functionalized with organic molecules is a key challenge in graphene research, to move from model structures to more complex systems. A comprehensive theoretical contribution in the determination of adsorption energy in van der Waals systems with competing and hierarchical interactions is demanding, but highly wished.

Acknowledgements We kindly thank the Elettra synchrotron radiation facility for the experimental assistance. Marco Buongiorno Nardelli, Arrigo Calzolari and Andrea Pelissetto are gratefully acknowledged for useful scientific discussions. Work funded by PRIN grant 20105ZZTSE "GRAF" of the Italian Ministery for Research (MIUR), by Roma "La Sapienza" University funds, and by the Eletra user support.

Supporting Information Available Valence band, core-level photoemission and NEXAFS data for the FePc SL as-deposited on graphene/Ir(111) at RT, and as obtained after a thermal treatment of a FePc multi-layer, together with core-level data taken at very high temperatures, and temperature programmed desorption mass spectra, are available in the SI. This information is available free of charge via the Internet at http://pubs.acs.org.

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