Ligand Influence on Local Magnetic Moments in Fe-Based Metal

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Ligand Influence on Local Magnetic Moments in Fe-Based Metal-Organic Networks Manel Mabrouk, Adrien Savoyant, Luca Giovanelli, Sylvain Clair, Roland Hayn, and Rafik Ben Chaabane J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10709 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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The Journal of Physical Chemistry

Ligand Influence on Local Magnetic Moments in Fe-based Metal-Organic Networks Manel Mabrouk,†,‡ Adrien Savoyant,† Luca Giovanelli,† Sylvain Clair,† Roland Hayn,∗,† and Rafik Ben Chaabane‡ Aix-Marseille Univ., CNRS, IM2NP-UMR 7334, 13397 Marseille Cedex 20, France, and Université de Monastir, Faculté des Sciences, Laboratoire des Interfaces et Matériaux Avancés, Avenue de l’environnement, 5019 Monastir, Tunisie E-mail: [email protected]

Abstract Planar metal-organic networks are highly promising materials due to their modular nature and wide-ranging possible applications from spintronics up to bio sensing. Spin state transitions connect local magnetic properties with structural modifications. In this paper, we report on ab-initio calculations for two metal-organic planar networks, the Fe-phthalocyanine (Pc) polymer and its precursor material Fe-tetracyanobenzene (TCNB). The spin-polarized generalized gradient approximation to density functional theory with an explicit treatment of the Hubbard-U correction (SGGA+U) indicates a spin state transition between the well confirmed S = 1 state for Fe-Pc and a local, high spin S = 2 state at the Fe site for Fe-TCNB. The high spin state at the Fe site is confirmed by X-ray absorption spectroscopy (XAS) measurements of the Fe-TCNB network on the Au(111) substrate in connection with a multiplet analysis. We † Aix-Marseille

Univ., CNRS, IM2NP-UMR 7334, 13397 Marseille Cedex 20, France de Monastir, Faculté des Sciences, Laboratoire des Interfaces et Matériaux Avancés, Avenue de l’environnement, 5019 Monastir, Tunisie ‡ Université

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Manel Mabrouk et al.

Ligand Influence on Local Magnetic . . .

propose a possible spin state transition between Fe-TCNB and Fe-Pc by the on-surface synthesis of the latter compound. The ab-initio results prove also a high chemical stability of the Fe-TCNB network, metallic and ferromagnetic behavior, as well as a partial screening of the Fe spin S = 2 by two antiparallel electrons on the ligand sites to a state with total spin of S = 1. All this makes the Fe-TCNB network an interesting material for spintronics applications.

Introduction The magnetic state of Fe-ions surrounded by organic ligands is of high interest in a very broad range of scientific domains ranging from molecular spintronics, 1 over catalysis 2–4 up to biochemistry. 5 To reach the final aim of artificially nanostructured magnetic memory devices one may use organic molecules like tetracyanobenzene (TCNB), tetracyanoquinodimethane (TCNQ), phthalocyanine (Pc), or many other molecules in combination with transition metal ions. On the other hand, metal porphyrins and especially iron porphyrins are of highest importance in numerous biological processes. 6 The local environment of the iron ion is very similar in both cases. Depositing these iron organic networks on metallic or insulating substrates opens the way to numerous applications ranging from molecular memory devices to bio sensing. The crucial point to understand and to control these iron organic networks is the spin state of the iron ion. It depends on the local environment and can be easily tuned. That will be shown in the present article by comparing two metal-organic networks, the Fe-Pc polymer and Fe-TCNB (see Fig. 1). The second one is the precursor material to reach the on-surface synthesis of the first, they are in fact two modifications of the same system. This synthesis was first proposed in Ref. 7 but the realized structure remained Fe-TCNB. A modified approach led finally to the thermally activated on-surface synthesis of the Fe-Pc molecule 8,9 and to the polymerized network Mn-Pc. 10,11 The synthesis of the Fe-Pc polymer network is still missing but we see no fundamental obstacle to reach it in the next future. The starting point to understand these Fe organic networks is the Fe-Pc molecule. Its magnetic structure is rather well known. It has the electron configuration d 6 but a large ligand field such that 2 ACS Paragon Plus Environment

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the spin state S = 1 is realized. It also has a large orbital moment close to L = 1 due to a twofold orbital degeneracy of the states dxz and dyz at the Fermi level in fourfold symmetry but filled with only one electron. 12 That is confirmed by XAS and XMCD measurements even if the orbital order of the Fe-Pc molecule remains disputed. 13–15,17,18 A further complication is that the local magnetic moment and the orbital order of the Fe-Pc molecule may be influenced by the metallic substrate. That is especially visible for the Ag(001) substrate leading to an important reduction of the spin moment. 19 The influence of the Au(111) substrate on the Fe-Pc molecule is much less remarkable: its spin moment and the orbital character are not significantly affected by the adsorption process at least above the Kondo temperature to be discussed below. That experimental result 13 is confirmed by ab-initio calculations 20 and supported by symmetry arguments, since Au(111) has a C3 axis in difference to the C4 axis of the molecule and the Ag(100) surface. Therefore, we feel justified to neglect the substrate in the following theoretical ab-initio simulations completely. Last, but not least, the observed moment of Fe-Pc on metallic substrates may even depend on the temperature. STM measurements indicate a Kondo screening of the local moment at low temperatures with a remarkable high Kondo temperature of about 200 K. 9,21,22

Figure 1: The global framework of 2D Fe-Pc polymer (left) and of the Fe-TCNB network (right). The Fe, N, C, and H atoms are highlighted in black, cyan, green, and blue, respectively. The black line represents the unit cell and the red bars indicate bonds that are broken in the Fe-TCNB network. The large orbital moment of the Fe-Pc molecule makes it an interesting candidate for nanospintronics since one might expect a large single-ion anisotropy which is necessary to stabilize the direction of the magnetic moment. Building a stable, nano-ordered monolayer i.e. the Fe-Pc



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polymer, out of these molecules would open interesting applications. However, as we will show in the theoretical part of the present study, the orbital order changes in going from the Fe-Pc molecule to the Fe-Pc polymer. That change of orbital order explains the reduced orbital moment of the FePc polymer. 23 On the other hand, we confirm the large chemical stability and the local spin moment of S = 1 for the Fe-Pc polymer. Very interesting for future applications is also our finding that the Fe-Pc polymer is close to a metallic state. The precursor material Fe-TCNB is chemically less stable but has the advantage that it can be easily produced on metallic substrates. In the following we will report on ab-initio calculations for Fe-TCNB showing that it is a ferromagnetic and metallic monolayer with a quite large, spin polarized, density of states at the Fermi level. Furthermore, our calculations indicate a spin tuning by switching between Fe-TCNB and the Fe-Pc polymer. The local spin states at the Fe site change between the high spin value S = 2 for Fe-TCNB to S = 1 for the Fe-Pc polymer. On the other hand, the total magnetic moment per unit cell is nearly the same for both compounds close to 2 µB (or S = 1). That is explained by a partial screening of the Fe moment by unpaired electrons in the ligand bands of Fe-TCNB, in a similar manner as proposed for Mn-TCNB. 24 Here, we concentrate on the free-standing metal-organic networks Fe-TCNB and the Fe-Pc polymer. A metal substrate like Au(111) would provide minimal interaction with the organic layer. Insulating substrates would have the advantage that the monolayer could be used as a twodimensional conductor. Aligning its magnetic moments could then give rise to two dimensional spin polarized transport. We present now at first the theoretical results of the DFT calculations and check then the calculated spin switching of the local Fe spin between S = 1 and S = 2 by X-ray absorption spectroscopy (XAS) measurements. Since the value of S = 1 for Fe-Pc was confirmed numerous times, we concentrate on the Fe-TCNB network. It has been deposited on the Au(111) surface to minimize the network-substrate interactions. The XAS measurements are simulated by a multiplet model and confirm the theoretical prediction of a high spin value S = 2 for the Fe spin.


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DFT Calculations Method The calculations have been performed with the Vienna ab initio simulation package (VASP) 26 being based on density functional theory (DFT). The VASP code (version 5.2.11) uses a plane-wave basis set, more specifically the projected augmented plane wave (PAW) method. 27,28 The spinpolarized generalized gradient approximation (SGGA) of Perdew and Wang (PW91) 29,30 was used with the interpolation formula of Vosko, Wilk, and Nusair (VWN). 31 We used the PAW pseudo potentials in the GGA class and we have chosen the following valence electron configurations: Fe - d7s1, N - s2p3, C - s2p2, and H - 1s1 from the VASP data set. The Coulomb correlation in the d shell was taken into account by a Hubbard term in the form proposed by Liechtenstein et al., 32 i.e. we used the SGGA+U functional (and the LSDA+U one for comparison). The correlation and exchange energies for Fe d orbitals are taken as U = 5 eV and J = 0.90 eV, respectively. The value of J is fixed to a value close to that one of a free Fe ion. The U parameter is necessary to split occupied and unoccupied Fe 3d states. We checked that a variation of U in the region of ±2 eV does not change the conclusions of our paper outlined below. The kinetic energy cutoff was set to 400 eV and the convergence criterium for the energy was set to 10−7 eV. The Brillouin zone utilized for the unit cell optimization was represented by the set of 6×6×1 k-points using the MonkhorstPack grid. After preparing the two structures shown in Fig. 1 all positions had been relaxed until equilibrium. All relaxations had been performed with the SGGA+U (or the LSDA+U) functional.

Magnetic moments for Fe-Pc polymer and Fe-TCNB The chemical stability of the Fe-TCNB network can be explained by a charge transfer from Fe to the TCNB molecules. The Fe atom becomes Fe2+ and gives two electrons, i.e. one electron to each of the two TCNB molecules in the Fe(TCNB)2 formula unit (or one 1/2 electron to each of the four surrounding C=N ligands). The cohesive energy of the Fe-TCNB network or the Fe-Pc



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polymer is defined by: Ecoh = EFe−TCNB/Pc − EFe − 2ETCNB and its value for Fe-TCNB was found to be -5.252 eV. That clearly demonstrates the chemical stability of the Fe-TCNB network which may eventually be synthesized also on other than metallic substrates and be even stable without any substrate at all. 33 By turning the benzene hexagons by 30 degrees and creating new N-C bonds one arrives at the Fe-Pc polymer (Fig. 1). Correspondingly, the binding energy per formula unit increases and Ecoh goes down by -7.88 eV, in connection with a higher chemical stability. Both structures are magnetic. Figs. 2 and 3 demonstrate the variation of the cohesive energy as a function of lattice parameter (three parabola in each case) for the Fe-Pc and Fe-TCNB systems, respectively, via the SGGA+U method. The Fe-Pc polymer has a magnetic ground state solution with a magnetic moment of 2 µB per unit cell corresponding to S = 1. That magnetic moment is nearly exclusively concentrated at the Fe site (see Table 1) where the local magnetic moment is 1.9 µB . The same is true for the LSDA+U solution where Md = 1.86 µB . Those values of the local magnetic moments are based on a Bader charge analysis separately for the 3d spin up and spin down electrons. The lattice constant in LSDA+U is much smaller, only 10.56 Å in contrast to 10.68 Å for SGGA+U, and also the Fe-N distance reduces from 1.96 Å to 1.93 Å. The S = 1 solution does not correspond to the maximal possible spin value for a d 6 configuration which is S = 2. That solution can be stabilized for larger lattice parameters (Fig. 2) as well as a S = 0 solution for smaller lattice parameters. All solutions are locally stable, i.e. they can be found by a proper choice of initial conditions. The spin value S = 1 of the ground state of the Fe-Pc polymer coincides with that one for the Fe-Pc molecule 12 but there are changes in the orbital order to be discussed below. At first sight, the magnetic state of Fe-TCNB (see Fig. 3) is very similar to that one of the Fe-Pc polymer with a S = 1 ground state. But a closer inspection of the local moments reveals a magnetic moment of 3.52 µB at the Fe site (Table 1), close to a high spin S = 2 state. That high spin state is explained by a much weaker ligand field in Fe-TCNB than in Fe-Pc. An indication of 6 ACS Paragon Plus Environment

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Table 1: Lattice constants a, magnetic moments per unit cell M and local magnetic moments Md at the Fe site, as well as the Fe-N distances and the total energies for the two systems (Fe-Pc and Fe-TCNB) as calculated with the SGGA+U functional (GS: ground state, ES: excited state). system

state a (Å) Fe-Pc GS 10.68 Fe-Pc ES 10.75 Fe-Pc ES 10.64 Fe-TCNB GS 12.08 Fe-TCNB ES 11.82 Fe-TCNB ES 11.82

M (µB ) 2 3.95 0 2.21 0.20 1.00

Md Fe-N (µB ) (Å) 1.90 1.96 3.74 2.00 0 1.92 3.52 2.05 1.79 1.91 1.77 1.91

energy (eV) 0 0.95 2.37 7.88 7.91 7.96

Figure 2: Cohesive energy Ecoh of the Fe-Pc polymer as a function of the lattice parameter. The spin values correspond to the rounded magnetization M = 2SµB per unit cell (see Table 1), S = 1 is the ground state. 7 ACS Paragon Plus Environment




F e -T C N B /P A W -S G G A + U

-5 .1 2 5 -5 .1 5 0

C o h e s iv e e n e r g y ( e V )

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-5 .1 7 5

S = 1 /2

-5 .2 0 0 -5 .2 2 5

S = 0

-5 .2 5 0

S = 1 -5 .2 7 5 1 1 .7

1 1 .8

1 1 .9

1 2 .0

1 2 .1

1 2 .2

1 2 .3

L a ttic e p a r a m e te r ( Å ) Figure 3: Cohesive energy Ecoh as a function of lattice parameter of the Fe-TCNB network. The stable state correspond to S = 1. the weaker ligand field is the increase of the Fe-N distance from 1.96 Å (ground state of Fe-Pc) to 2.05 Å (ground state of Fe-TCNB). For the ground state of Fe-TCNB, the local high-spin state S = 2 at Fe is partially screened by two antiparallel electrons with S = 1/2 at the ligand sites. The local S = 1 state can also be observed for Fe-TCNB at a smaller lattice constant of 11.82 Å but at higher energy with a screening by one or two antiparallel electrons at the ligand sites leading to a total moment of 1 µB (S = 1/2) or 0.2 µB (S = 0), respectively (see Fig. 3 and Table 1). So, our SGGA+U results for the Fe-TCNB network predict a high spin S = 2 state at Fe and a partial screening by ligand electrons to a total moment corresponding to S = 1 per formula unit. The screening is absent for the Fe-Pc polymer. For a better illustration we present the spin density distribution of the two-ground states for Fe-Pc polymer and Fe-TCNB as Supplementary Material. A similar behavior was recently calculated for Mn-TCNB and the Mn-Pc polymer 24 with the Fe2+ d 6 configuration to be replaced by the Mn2+ d 5 configuration having a S = 5/2 high spin state. The screening is static in the SGGA+U calculations but is expected to be dynamic in reality to account for the reduced magnetic moment that was observed in XMCD data for Mn-TCNB. 25 The


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details of the screening and its absence for the Pc polymer had been explained in detail in Ref. 24 and will not be repeated here.

Densities of states We now analyze the electronic structure, i.e. the total and partial densities of states (DOS), of the two ground-state solutions for the Fe-Pc polymer and the Fe-TCNB network more in detail. The corresponding atomic positions of the two ground-state solutions are presented in the supplementary material. It is remarkable that both structures are metallic or close to it. The DOS at Fermi level EF of the Fe-Pc polymer is zero and there is a small gap of 98 meV for spin down. The corresponding gap for spin up is 357 meV (Fig. 4). On the contrary, the spin-polarized DOS at EF is non-zero for Fe-TCNB (Fig. 6), so its metallicity might be interesting for future applications. 25 20

Density of States (states/eV)

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spin up

Total DOS PAW-SGGA+U

15 10 5 0 -5 -10 -15 -20 -25 -30 -10

spin down -9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (eV)

Figure 4: Total DOS of Fe-Pc monolayer (S = 1). Here, and in the following DOS figures, the Fermi level is at zero energy. Very instructive is the orbital analysis. For the Fe-Pc polymer, the dx2 −y2 orbital is completely empty due to the large ligand-field splitting in that case (Fig. 5). Furthermore, the partial dxz/yz DOS is very broadened and away from the Fermi level for both spin directions. The majority9 ACS Paragon Plus Environment




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dxy_u dxy_d dyz/dxz_u dyz/dxz_d dz2_u dz2_d dx2-y2_u dx2-y2_d

Partial DOS PAW-SGGA+U, Fe-d

spin up

5 4 3 2 1 0 -1 -2 -3

spin down

-4 -5 -6

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (eV)

Figure 5: PDOS of the d orbitals on the Fe atom in the 2D Fe-Pc polymer.

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Total DOS PAW-SGGA+U spin up

15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -10

spin down -9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

Energy (eV)

Figure 6: Total DOS of Fe-TCNB network in the stable state.


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6

dxy_u dxy_d dyz/dxz_u dyz/dxz_d dz2_u dz2_d dx2-y2_u dx2-y2_d

Partial DOS PAW-SGGA+U, Fe-d

spin up

5


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4 3 2 1 0 -1 -2 -3

spin down

-4 -5 -10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

Energy (eV)

Figure 7: PDOS of the d orbitals on the Fe atom in the 2D Fe-TCNB sheet. spin component of dxz/yz is completely filled and the minority one completely empty. That is fundamentally different from the results of the Fe-Pc molecule 12 where the minority-spin dxz/yz peak appears at EF and is partially filled with one electron leading to orbital degeneracy, orbital moment and large magnetic anisotropy. 14,17,18 The difference between Fe-Pc polymer and Fe-Pc molecule can be explained by hybridization with ligand p orbitals which build a broad band in the former case and push the dxz/yz orbitals away from EF . For minority spin, the dxy and the dz2 2 2 d 2 (or orbitals are completely filled (Fig. 5) leading to an electronic configuration close to dxz/yz dxy z2

e2g b22g a21g ) having no orbital degeneracy. For Fe-TCNB (Fig. 7) the ligand field splitting is much smaller and there are now all Fe 3d majority-spin levels filled, including the 3dx2 −y2 one. For minority spin, mainly the dxy level is occupied. As for the Fe-Pc polymer, the 3dxz/yz levels are broadened due to hybridization with ligand orbitals. Those bands with dxz/yz character lie at the Fermi level for minority spin and take part in the screening of the central Fe moment as explained in Ref. 24.



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Nearest neighbor magnetic coupling It is highly instructive to study the nearest neighbor magnetic exchange coupling between two Fe spins in both networks. For that purpose we compared the total energies of a 2×1 supercell with antiferromagnetic and ferromagnetic arrangements. The energy was well converged with a k-point mesh of 5×10×1. The difference to a k-point mesh of 4×8×1 is less than 0.1 meV per Fe ion and the results were confirmed for a 2×2 supercell. It turns out that the Fe-Pc polymer shows an antiferromagnetic coupling with an energy gain of 0.8 meV per Fe ion with respect to the ferromagnetic one similar to previous results. 34 The Fe-TCNB, however, prefers the ferromagnetic arrangement by a surprisingly large value of 79 meV per Fe ion in comparison to the antiferromagnetic arrangement. That difference in the magnetic couplings is connected with different ground states: the metallic state of the Fe-TCNB network leads to a ferromagnetic arrangement of the spins, whereas the insulating Fe-Pc polymer shows a small antiferromagnetic exchange.

Summary of magnetic and electronic structure The magnetic moments of ground- and excited-states of Fe-Pc polymer and Fe-TCNB are presented in Table 1. It contains the total moments as well as the local ones at the Fe site and allows to distinguish between the partial screening for Fe-TCNB and its absence for the Fe-Pc polymer. Most interestingly, we propose that the local spin state can be tuned by switching between FeTCNB and the Fe-Pc polymer. The total spin of the Fe-Pc polymer per formula unit cell is very close to its local value S = 1. The ligand influence on the Fe 3d levels (the ligand field) is very much reduced in Fe-TCNB in comparison to the Fe-Pc polymer. As a consequence, the local spin adopts for Fe-TCNB its maximal value for a d 6 configuration S = 2. However, the total magnetic moment per unit cell remains close to 2 µB (or S = 1) due to the partial screening effect. For a complete understanding, we summarize in Fig. 8 the different level orders and include also the Fe-Pc molecule which was already intensively treated in the scientific literature. We underline that Fig. 8 is a schematic representation of the level order (in the one-electron mean-field picture) which can be concluded from the partial DOS of several DFT calculations and which is 12 ACS Paragon Plus Environment

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in agreement with the available experimental data. The level scheme of the Fe-Pc molecule in Fig. 8 is based on the DFT results 12 showing orbital degeneracy of the dxz/yz orbital at EF . The S = 1 and the orbital degeneracy leading to a large orbital moment close to L = 1 are confirmed by several XMCD measurement. 13–15 It should be noted, however, that the level ordering of the Fe-Pc molecule is still debated 16,17 and the level ordering which results of the multiplet analysis 18 of the XMCD measurements of a thin film of α-FePc 14 is slightly different from the DFT results of an isolated molecule, 12 confirming, however, the 3 Eg ground state with orbital degeneracy. The discrepancy might be due to the difference between multiplet analysis and DFT or between α-FePc and an isolated molecule and should be clarified by further research work but it is not essential for the present analysis. A survey of the available theoretical and experimental data of FePc is presented in Ref. 17. The orbital degeneracy of the Fe-Pc molecule is expected to lead to a considerable magnetic anisotropy, making that molecule an interesting candidate for molecular magnetic memory devices. However, our SGGA+U results for the Fe-Pc polymer indicate a level change between the dxz/yz and the dz2 orbital and a broadening of the former levels such that the orbital degeneracy disappears. As a consequence, we expect a reduced orbital moment in the Fe-Pc polymer in comparison to the isolated Fe-Pc molecule which is indeed confirmed by DFT-calculations including the spin-orbit coupling. 23 For Fe-TCNB, the ligand field splitting reduces such that the high-spin S = 2 solution is realized at the Fe site (Fig. 8). As it is seen in Table 1, the total moment is however only 2.21 µB , i.e. more close to a S = 1 solution due to the partial screening effect by ligand electrons as explained above.

XAS measurements and multiplet analysis To check the calculated spin state transition between S = 1 and S = 2 in going from Fe-Pc (molecule or polymer) to Fe-TCNB we performed X-ray absorption measurements (XAS). The local spin S = 1 for the Fe-Pc molecule was already confirmed by X-ray measurements several times 13–15 in



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Figure 8: Schematic summary of level orders in Fe-Pc molecule and polymer and in Fe-TCNB. The main difference between Fe-Pc and Fe-TCNB is the position of the dx2 −y2 orbital which is too high to be occupied in Fe-Pc. very different environments. Furthermore, our calculations for the Fe-Pc polymer and its comparison with those for the Fe-Pc-molecule, show no essential difference concerning the local magnetic moment of the Fe-ion. Therefore, we concentrate the XAS measurements on the Fe-TCNB network. The measurements were performed at APE beamline at the Elettra synchrotron (Trieste) on samples prepared in situ and characterized by STM. The Au(111) substrate was prepared by standard cycles of sputtering and annealing. TCNB was sublimated from a Mo crucible and the evaporation rate was calibrated with a quartz microbalance. Each step of sample preparation was monitored by STM. First a single layer of TCNB was deposited on clean Au(111) to form an ordered self-assembled layer. Fe was then incrementally deposited. Patches of Fe(TCNB)4 compounds were observed among original supramolecular domains at low Fe dose. The Fe dose was then increased to the optimum 1:2 stoichiometry, i.e. the formation of large islands of Fe(TCNB)2 2D metal-organic network similar to what found previously 7 and schematized in Fig. 1. No trace of Fe clustering was found by STM. The as grown sample was transferred in the synchrotron end-station without breaking the UHV conditions. XAS spectra were measured by collecting the sample drain current while varying continuously the incident photon energy. Fig. 9 displays the Fe-L2,3 edge absorption spectrum (full black line) taken in normal incidence


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with horizontally polarized light at room temperature. The two peaks white lines correspond to local transitions between the core 2p electrons and the 3d shell which are split by the spin-orbit coupling of the 2p states.

To extract information about the spin state, we simulated the XAS absorption spectra for S = 1 and S = 2 cases, in the framework of the multiplet model. Ultimately, the ground-state spin of a localized d n configuration (n = 6 for iron) depends on the interplay between the d-d Coulomb interaction (Slater-Condon or Racah parameters), the spin-orbit coupling within the valence d-shell, and the ligand-field (LF) interaction. For D4h symmetry this latter interaction is described by three parameters, Dq , Dt and Ds . The Coulomb and spin-orbit terms are intra-atomic interactions, and are relatively well-known from free ion optical spectra: their parameters are only slightly reduced by the environment’s effect. On the contrary, the LF describes the influence of the environment on the localized d-electrons: its parameters can take many different values, depending on the particular environment, and finally dictate the ground-state spin value.

To distinguish between the S = 1 and S = 2 spin values, we started from two sets of LF parameters previously used to reproduce XAS and XMCD spectra of the Fe ion in similar environments. The first set describes the high spin state of Fe in the supramolecular network Fe-TPA with the terephthalate acid (TPA) molecule 1 and the second set was reported for the Fe-Pc molecule 15 (see Table 2). Each of these sets gives an XAS spectra, for which the line widths of the L2 and L3 lines have been adjusted to best fit the experimental ones (1.3 and 0.5 eV, respectively).

We then searched for criteria allowing to determine the ground-state spin of the Fe-TCNB molecule on the basis of experimental data. A common way to determine the spin state from the XAS spectra is the analysis of the branching ratio 35 (BR), I(L3 )/[I(L3 ) + I(L2 )]. However, in the present case both the S = 1 and S = 2 XAS simulations calculated by the CTM4XAS code 36,37 give a value of the BR (0.78 and 0.79, respectively) very close to the experimental one (0.79 ±



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Table 2: The two sets of ligand field parameters leading to S = 1 (high ligand field) and S = 2 (low ligand field), for Fe2+ ion in D4h symmetry. Dq Dt Ds S

Fe-Pc 15 0.270 0.247 0.860 1

Fe-TPA 1 0.058 0.030 0.290 2

0.02). Therefore, if on one side the closeness of the calculated and experimental BRs found here validates the multiplet model approach, on the other it appears impossible to conclude about the spin state on the basis of the BR analysis only. In the present case a better way to assess the spin state is to appreciate the whole effect of the spin state on the line shape. When looking at Fig. 9, one can see that the line shape of the L3 edge changes appreciably between intermediate and low spin. Clearly, the S = 2 simulation is closer to the experimental XAS spectra: the main shape of the spectrum as well as the shoulder at about 706 eV are well reproduced whereas the S = 1 simulation has a pre-peak at 702 eV at the L3 edge that is absent in the experimental spectra. So, we conclude that the S = 2 fit gives the best description of the experimental spectra. Although it is difficult to span the whole LF parameter space, we consider the above fit, together with the ab-initio study, as a strong indication that the ground state spin of Fe-TCNB is indeed S = 2.

Discussion and Conclusions Summarizing we propose a possible tuning of the local Fe spin by going from the Fe-TCNB network to the more closed packed Fe-Pc polymer. It changes from a local high-spin value S = 2 to S = 1. The high-spin value S = 2 is confirmed by XAS measurements in connection with a multiplet analysis. In Fe-TCNB, the Fe spin is partially screened by two antiparallel electrons on the ligands leading to a total magnetic moment of 2.21 µB , i.e. close to S = 1. The spin state transition


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Figure 9: XAS intensity as a function of photon energy for a monolayer of Fe-TCNB on Au(111) in comparison to a multiplet analysis for a S = 2 or S = 1 Fe state. between S = 2 and S = 1 can be used to detect the polymerization reaction leading to the Fe-Pc polymer. A detailed ab-initio investigation of the substrate influence on Fe-TCNB or the Fe-Pc polymer is beyond the scope of the present article. However, such an influence certainly exists and can be discussed by analogy to similar metal-organic networks. For instance, there exist small differences in the XAS-spectra of Mn-TCNB on Au(111) or on Ag(111) concerning the nearest neighbor exchange coupling but not the value of the local Mn magnetic moment. 25 In the present case of FeTCNB the neglect of the substrate in the theoretical part is justified a-posteriori by the experimental confirmation of the local high spin value S = 2. On the other hand, whether the substrate can be neglected also for the magnetic nearest neighbor coupling constants is much less sure. A proper treatment of the substrate influence should be reserved for further studies. The same is true for



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a DFT calculation of the crystal field parameter, or the magnetic anisotropies, as well as for an experimental verification of the ferromagnetic nearest neighbor exchange in Fe-TCNB. Keeping in mind that the formation of the organic monolayers is more straightforward for the Fe-TCNB network than for the Fe-Pc polymer, we would like to point out some additional advantages of Fe-TCNB which can be concluded from our study. Of course its cohesive energy of about -5.2 eV could be improved by -7.9 eV by the polymerization process. However, Fe-TCNB is predicted to be metallic whereas the Fe-Pc polymer has a gap of 98 meV. Also, our ab-initio study points to a ferromagnetic nearest neighbor exchange coupling. So we expect the planar FeTCNB network to be a ferromagnetic metal at low temperatures. It should allow for spin-polarized transport which is interesting for spintronics applications.

Supporting Information Atomic positions of the relaxed two-dimensional structures of the Fe-Pc polymer and the Fe-TCNB network and its spin density contributions.

Acknowledgements We thank M.D. Kuzmin, A. Arnau and M. Abel for useful discussions and R.H. thanks the Donostia Physics Center (Spain) for hospitality. This work was granted access to the High Performance Computing (HPC) resources of Aix-Marseille University financed by the project Equip@Meso (ANR-10-EQPX-29-01) and to the computer resources of the Centre Informatique National de l’Enseignement Superieur (CINES), Project No. c2015096873.

References (1) Gambardella, P.; Stepanow, S.; Dmitriev, A.; Honolka, J.; de Groot, F.M.F.; Lingenfelder, M.; Gupta, S.S.; Sarma, D.D.; Bencok, P.; Stanescu, S.; et al. Supramolecular Control of the 18 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




Magnetic Anisotropy in Two-dimensional High-Spin Fe Arrays at a Metal Interface. Nat. Mater. 2009, 8, 189-193. (2) Wang, Y.; Yuan, H.; Li, Y.; Chen, Z. Two-Dimensional Iron-Phthalocyanine (Fe-Pc) Monolayer as a Promising Single-Atom-Catalyst for Oxygen Reduction Reaction: a Computational Study. Nanoscale 2015, 7, 11633-11641. (3) Grumelli, D.; Wurster, B.; Stepanow, K. Bio-inspired Nanocatalysts for the Oxygen Reduction Reaction. Nature Communications 2013, 4, 2904. (4) Deng, Q.; Pan, J.; Yin, X.; Wang, X.; Zhao, L.; Kang, S.; Jimenez-Cruz, C.A.; Zhou, R.; Li, J. Toward High Permeability, Selectivity and Controllability of Water Desalination with FePc Nanopores. J. Phys. Chem. Phys. 2016, 18, 8140-8147. (5) Consani, C.; Auböck, G.; Bräm, O.; van Mourik, F., Chergui, M. A Cascade Through Spin States in the Ultrafast Haem Relaxation of Met-Myoglobin. J. Chem. Phys. 2014, 140, 025103025108. (6) Gottfried, J. M. Surface Chemistry of Porphyrins and Phthalocyanines. Surface Science Reports 2015, 70, 259-379. (7) Abel, M.; Clair, S.; Ourdjini, O.; Mossoyan, M.; Porte, L. Single Layer of Polymeric FePhthalocyanine: an Organometallic Sheet on Metal and Thin Insulating Film. J. Am. Chem. Soc. 2011, 133, 1203-1205. (8) Kezilebieke, S.; Amokrane, A.; Boero, M.; Clair, S.; Abel, M.; Bucher, J.-P. Steric and Electronic Selectivity in the Synthesis of Fe-1,2,4,5-Tetracyanobenzene (TCNB) Complexes on Au(111): from Topological Confinement to Bond Formation. Nano Res. 2014, 7, 888-897. (9) Kezilebieke, S.; Amokrane, A.; Abel, M.; Bucher, J.-P. Hierarchy of Chemical Bonding in the Synthesis of Fe-Phthalocyanine on Metal Surfaces: a Local Spectroscopy Approach. J. Phys. Chem. Lett. 2014, 5, 3175-3182. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



(10) Koudia, M.; Abel, M. Step-by-Step on-Surface Synthesis: from Manganese Phthalocyanines to their Polymeric Form. Chem. Commun. 2014, 50, 8565-8567. (11) Piantek, M.; Serrate, D.; Moro-Lagarest, M.; Algarabel, P.; Pascual, J.I.; Ibarro, R. Manganese Phthalocyanine Derivatives Synthesized by on-Surface Cyclotetramerization. J. Phys. Chem. C 2014, 118, 17895-17899. (12) Kuzmin, M.D.; Hayn, R.; Oison, V. Ab Initio Calculated XANES and XMCD Spectra of Fe (II) Phthalocyanine. Phys. Rev. B 2009, 79, 024413(1)-024413(5). (13) Miedema, P.S.; Stepanow, S.; Gambardella, P.; de Groot, F.M.F. 2p X-Ray Absorption of Iron-Phthalocyanine. J. Phys. Conf. Ser. 2009, 190, 012143(1)-012143(6). (14) Bartolomé, J.; Bartolomé, F.; García, L.M.; Filoti, G.; Gredig, T.; Colesniuc, C.N.; Schuller, I.K.; Cezar, J.C. Highly Unquenched Orbital Moment in Textured Fe-Phthalocyanine Thin Films. Phys. Rev. B 2010, 81, 195405(1)-195405(8). (15) Stepanow, S.; Miedema, P.S.; Mugarza, A.; Ceballos, G.; Moras, P.; Cezar, J.C.; Carbone, C.; de Groot, F.M.F.; Gambardella, P. Mixed-Valence Behavior and Strong Correlation Effects of Metal Phthalocyanines Adsorbed on Metals. Phys. Rev. B 2011, 83, R220401(1)-R220401(4). (16) Mugarza, A.; Robles, R.; Krull, C.; Korytar, R.; Lorente, N.; Gambardella, P. Electronic and Magnetic Properties of Molecule-Metal Interfaces: Transition-Metal Phthalocyanines Adsorbed on Ag(111). Phys. Rev. B 2012, 85, 155437(1)-155437(13). (17) Kuzmin, M.D.; Savoyant, A.; Hayn, R. Ligand Field Parameters and the Ground State of Fe(II) Phthalocyanine. J. Chem. Phys. 2013, 138, 244308(1)-244308(9). (18) Fernandéz-Rodríguez, J.; Toby, B.; van Veenendaal, M. Mixed Configuration Ground State in Iron(II) Phthalocyanine. Phys. Rev. B 2015, 91, 214427(1)-214427(7).


Page 20 of 23

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




(19) Stepanow, S.; Rizzini, A.L.; Krull, C.; Kavich, J.; Cezar, J.C.; Yakhou-Harris, F.; Sheveryaeva, P.M.; Moras, P.; Carbone, C.; Ceballos, G.; et al. Spin Tuning of Electron-Doped Metal-Phthalocyanine Layers. J. Am. Chem. Soc. 2014, 136, 5451-5459. (20) Zhang, Y.Y.; Du, S.X.; Gao, H.-J. Binding Configuration, Electronic Structure, and Magnetic Properties of Metal-Phthalocyanines on a Au(111) Surface Studied with Ab Initio Calculations. Phys. Rev. B 2011, 84, 125446(1)-125446(8). (21) Gao, L.; Ji, W.; Hu, Y.B.; Cheng, Z.H.; Deng, Z.T.; Liu, Q.; Jiang, N.; Lin, X.; Guo, W.; Du, S.; et al. Site-Specific Kondo Effect at Ambient Temperatures in Iron-Based Molecules. Phys. Rev. Lett. 2007, 99, 106402(1)-106402(4). (22) Minamitani, E.; Tsukahara, N.; Matsunaka, D.; Kim, Y.; Takagi, N.; Kawai, M. SymmetryDriven Novel Kondo Effect in a Molecule. Phys. Rev. Lett. 2012, 109, 086602(1)-086602(5). (23) Li, L.-H.; Li, J.-Q.; Wu, L.-M. Polymeric Fused-Ring Type Iron Phthalocyanine Nanosheet and its Derivative Ribbons and Tubes. J. Phys. Chem. C 2012, 116, 9235-9242. (24) Mabrouk, M.; Hayn, R. Magnetic Moment Formation in Metal-Organic Monolayers. Phys. Rev. B 2015, 92, 184424(1)-184424(8). (25) Giovanelli, L.; Savoyant, A.; Abel, M.; Maccherozzi, F.; Ksari, Y.; Koudia, M.; Hayn, R.; Choueikani, F.; Otero, E.; Ohresser, P.; et al. Magnetic Coupling and Single-Ion Anisotropy in Surface-Supported Mn-Based Metal-Organic Networks. J. Phys. Chem. C 2014, 118, 1173811744. (26) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (27) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (28) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



(29) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671-6687. (30) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244-13249. (31) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: a Critical Analysis. Can. J. Phys. 1980, 58, 1200-1211. (32) Liechtenstein, A. I.; Anisimov, V. I.; Zaanen, J. Density-Functional Theory and Strong Interactions: Orbital Ordering in Mott-Hubbard Insulators. Phys. Rev. B 1995, 52, R5467-R5470. (33) Bauer, T.; Zheng, Z.; Renn,A.; Enning, R.; Stemmer, A.; Sakamoto, J.; Schlüter, D. Synthesis of Free-Standing, Monolayered Organometallic Sheets at the Air/Water Interface. Angewandte Chemie 2011, 50, 7879-7884. (34) Zhou, J.; Sun, Q. Magnetism of Phthalocyanine-Based Organometallic Single Porous Sheet. J. Am. Chem. Soc. 2011, 133, 15113-15119. (35) Thole, B. T.; van der Laan, G. Branching Ratio in X-Ray Absorption Spectroscopy. Phys. Rev. B 1988, 38, 3158-3171. (36) de Groot F.M. Multiplet Effects in X-Ray Spectroscopy. Coord. Chem. Rev. 2005, 249, 31-63. (37) Stavitski E.; de Groot F.M. The CTM4XAS Program for EELS and XAS Spectral Shape Analysis of Transition Metal L Edges. Micron 2010, 41, 687-694.


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Ligand Influence on Local Magnetic Moments in Fe-Based Metal

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