On-Surface Synthesis of Spin Crossover Polymeric Chains - American

Subscriber access provided by TUFTS UNIV

C: Physical Processes in Nanomaterials and Nanostructures

On-Surface Synthesis of Spin Crossover Polymeric Chains Hassan Denawi, Mathieu Koudia, Roland Hayn, Olivier Siri, and Mathieu Abel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04171 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 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

The Journal of Physical Chemistry

On-Surface Synthesis of Spin Crossover Polymeric Chains Hassan Denawi1, *, Mathieu Koudia1, Roland Hayn1, *, Olivier Siri2, Mathieu Abel1, * 1

Aix Marseille Université, CNRS, IM2NP, UMR 7334, Campus de St Jérôme, 13397, Marseille, France

2

Aix Marseille Université, CNRS, CINAM, UMR 7325, Campus de Luminy, 13288, Marseille, France

ABSTRACT

We report on the successful on-surface polymerization reaction of co-deposited zwitterionic quinones with Fe atoms on Au(110) at an appropriate temperature. The resulting covalent onedimensional (1D) polymer chains arrange in a well ordered two-dimensional (2D) structure as proven by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The ordered regions can reach the micrometer size. Furthermore, the electronic and magnetic properties of the zwitterionic quinoidal polymers on Au(110) were investigated using density functional theory with an explicit inclusion of the Hubbard U term. The spin polarized generalized gradient approximation plus U method (SGGA+U) has been used and the free standing isolated polymer chain, the 2D arrangement and the adsorbed polymers have been calculated. From ab-initio calculations, we predict the zwitterionic quinoidal polymer chains to be one-dimensional spin cross-over compound. For the free-standing chains, we find two local minima with comparable energies but different spin states: a high spin state (S=2 per Fe) and a Fe-Fe distance of 7.9 Å and an intermediate spin state (S=1) with a Fe-Fe distance of 7.72 Å. The experimental and theoretical results show that the substrate dictates the lattice constant and

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 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

Page 2 of 26

the adsorbed polymer on Au(110) has a Fe-Fe distance of 8.16 Å and is in the high-spin state. The exchange coupling in the polymeric chain with the Au(110) lattice constants was found to be antiferromagnetic. The adsorption on Au(110) removes the surface reconstruction of a free surface and the ab-initio simulation gives the short-bridge position for Fe as the most stable one.

Introduction Spin chains with antiferromagnetic nearest neighbor Heisenberg exchange are famous model systems for one-dimensional quantum behavior. Their theoretical exact solution goes back as far as 19311 and they represent probably the best investigated interacting quantum system today. They show an unconventional spinon continuum of excitations2,3 in contrast to the magnon spectrum of higher dimensional magnetic systems. Experimentally, they can be realized in very different ways. They are found in inorganic, anisotropic crystals (like e.g. cuprates or vanadates) or in organic molecular crystals (as for instance the famous Bechtgaard salts). Finite spin chains can also be realized with the help of ultra-cold trapped ions. Spin chains represent a very convenient way to realize spin qubits with low damping factor being of importance for quantum computing. One possibility is the use of an impurity in a spin chain which behaves as an effective spin ½ system.4,5 Alternatively, it was recently proposed that a finite Heisenberg spin chain realized by cold trapped ions can act as a quantum spin transistor.6 And a very promising route is to construct bound state Majorana fermions by a spin chain attached to a metallic substrate in its superconducting state.7,8 The realization of a spin chain by metal-organic polymers on a metallic surface could have many advantages, for instance it would be able to address the individual spins by a STM tip. Supramolecular (i.e. reversible) metal organic coordination networks have been extensively studied in the past for the realization of 1D or 2D arrays.9,10


2

Page 3 of 26 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


On-surface synthesis has recently emerged11,12 as a new technic to develop 1D or 2D covalent polymers incorporating metallic atoms in the pi-electron system of the organic molecules.13,14 So, it was shown that adsorption on the Ag(111) surface provokes a co-polymerization reaction between zwitterionic quinoidal molecules and metallic Fe which is not possible in the gas phase or in wet conditions.15 Furthermore, the resulting polymers build a well ordered arrangement of spin chains on the Ag(111) surface up to micrometer size. We report now on the synthesis of the same polymer on the Au(110) surface with a much larger lattice constant and show that the adaption to different substrates may be explained by a near degeneracy of two different spin states with different lattice constants. That is similar to the microscopic explanation of the invar effect in FeNi or FeCo alloys for which the thermal lattice dilatation disappears.16 The Ag(111) surface allows for 6 different directions of the growth. In contrast, we demonstrate here an analogous polymerization reaction on the Au(110) surface where only one direction for the alignment of the polymer chains is possible. Furthermore, we show that the Fequinoidal chains adapt their lattice constant to the substrate which is very different for the two substrates. To clarify the surprising adaption to very different lattice constants, we performed abinitio calculations, for the free standing polymer, its two-dimensional arrangement, and for the adsorbed chains on the reconstructed and non-reconstructed Au(110) surfaces. We discover two local minima being close in energy with S=2 (Fe-Fe distance = 7.95 Å) and S=1 ( = 7.72 Å) per Fe for the free standing chain. These two spin states correspond to the high spin (HS) S=2 and intermediate spin (IS) S=1 state of the ion with its configuration. So, it is clear that the synthesized spin chains are new spin cross over materials.17,18,19 These materials show transitions between different spin states by change of some external parameters. In our case, it would be tensile or compressive stress of a free-


3


Page 4 of 26

standing polymer or some surface modification of the adsorbed phase. The bi-stability between two different spin states is promising for different applications like data storage, sensors or nonlinear optics. The plan of the paper is as follow. After presenting the computational methods, we show the details of the polymer synthesis and the experimental characterization by scanning tunneling spectroscopy (STM), low energy electron diffraction (LEED) and X-ray photoemission spectroscopy. Then, we give the results of numerical simulations of a free-standing polymer and its adsorbed state on the Au(110) surface. We report on the calculation of the (small) nearest neighbor exchange couplings, within the chains and between neighboring chains in the 2D arrangement on Au(110). In the last chapter we compare the HS and the IS state by a detailed analysis of the density of states (DOS), showing its semiconducting behavior, the gap values and indicating a rather broad valence band of 3d character. So, the proposed polymer has also the possibility to combine conducting behavior with magnetic moments. These, and other perspectives are shown together with our conclusions. Computational Methods The first-principle calculations are performed using the Vienna ab-initio simulation package (VASP)20 being based on density functional theory (DFT). We use pseudopotentials of the form of projector augmented waves (PAW)21 and the generalized gradient exchange-correlation potential proposed by Perdew and Wang in its spin-polarized version (SGGA). The Fe ion has an incompletely filled d-shell leading to a local magnetic moment. It is well known that such a situation is not correctly captured by the SGGA method which leads for instance to the wrong metallic ground state of transition metal oxides in contrast to the correct insulating behavior. Therefore, we treat the electron correlation in the incompletely filled d-shell of the transition


4

Page 5 of 26 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


metal ion by an explicit inclusion of the Hubbard U term in the rotational invariant version of Liechtenstein et al, i.e. by the SGGA+U method.22,23,24 A common recipe for the k-point grid was developed by Monkhorst and Pack,25 and we use here a grid of (8×1×1) for the one-dimensional (1D) free standing polymer and of (8×5×1) for the 2D arrangement without substrate. The smearing was set to 0.01 eV and the energy cutoff to 480 eV. The convergence criteria for ionic steps was set to 10 eV/Å and that one for electronic steps to 10 eV which is sufficient for the polymers studied here. We have modelled the Au(110) substrate with the experimental lattice constant by a slab consisting of 5 layers of Au and with an additional vacuum layer of 10 Å between the slabs to avoid interaction between the surfaces along the z-axis. The investigated supercell corresponds to a (4×4) superstructure of Au(110). The same cut-off energy of 480 eV was chosen as for the free-standing polymer and the structures were relaxed until the residual forces were smaller than 0.02 eV/Å. Only a single k-point, namely the Г-point, was used for the calculations of the polymer with substrate. All other parameters were the same without and with substrate. Results and Discussion On-Surface Polymerization, STM, LEED, AND XPS Studies In a vacuum chamber, the precursor molecules of zwitterionic quinone 1 and Fe are evaporated on the Au(110) surface with the co-deposition process which was previously used to produce well-ordered polymers by on-surface synthesis on Ag(111).15 The chosen temperature is short above the desorption temperature of the quinone molecules to provide adsorption-controlled growth of the polymer by reactive epitaxy. The zwitterionic quinone molecules have a large electric dipole moment. To minimize the energy, these dipole moments are arranged in an alternating manner for one polymer chain (see Figure 1).


5


Page 6 of 26

Figure 1: Schematic representation of the polymerization reaction from quinoid zwitterion (1) with M=Fe forming dimers (2) and 1D covalent chains (3).

The assembly of iron atoms and zwitterionic quinone to a polymer deposited on the Au(110) surface was studied by scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED). The zwitterionic quinone molecules were synthesized as described.26,27,28 Co-deposition of the quinone molecule (1) with Fe atoms on a substrate maintained at 250 °C leads to well-crystalized domains between 40 nm to 400 nm in length (Figure 2a).


6

Page 7 of 26 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


Figure 2: (a, b) STM topography of co-vaporized Fe atoms and zwitterionic quinone on the Au(110) surface held at 250°C. The direction of the Au(110) substrate is determined by surface reconstruction underlined by dashed lines. The surface reconstruction is visible in the regions which are not covered by the polymer. (c) The N 1s and Fe 2p3/2 XPS spectra of the studied polymer film on Au(110): from bottom to top: Molecule (orange line) is obtained from one monolayer deposition of molecules (1) on Au(110) at room temperature whereas Fe is obtained from Fe deposition on Au(110); then the two upper spectra correspond to codeposition of molecule (1) and Fe atoms at 100 °C (blue line) and 250 °C (green line). The dashed lines correspond to 399.4 eV, 397.8 eV for N1s and 710 eV, 707.3 eV for Fe 2p3/2. The XPS spectra of Figure 2c track the progress of the reaction upon temperature. The N1s core level is shifted from 399.4 eV to 397.8 eV while increasing the substrate temperature from room temperature to 250 °C. This is in good agreement with amine deprotonating reaction occurring above 100 °C.29,30 In the same time the overall Fe 2⁄ peak is shifted from 707.3 eV to 710 eV confirming the incorporation of Fe atoms in the polymer, since the shift is compatible with an evolution of the oxidation state from Fe(0) to Fe(II) at 250 °C.31,32 Sub-monolayer


7


Page 8 of 26

deposition at 250 °C allows an accurate description of the polymer with respect to the underlying reconstructed substrate. The regions in Figure 2b which are not covered by the polymer show a (1×2) reconstruction visible as atomic rows in the [1-10] direction which is indicated by an arrow in Figure 2b. A remarkable feature of this system is the presence of polymer chains all aligned in the same direction oriented perpendicularly to the [1-10] direction. The STM image of the polymer chain (Figure 2b) distinguishes two species as an alternation of bright and dark protrusions with the same length as the interline distance of the substrate (dashed line in Figure 2b) but shifted by a quarter of this periodicity in the [001] direction. The inter-chain distance is 5.8 Å corresponding to two interatomic distances in the [1-10] direction. This description is confirmed by diffraction experiments (LEED) where the elementary mesh correspond to an epitaxial c(4 × 2) reconstruction (Figure 3a). (A;B) are the basis vectors of the superstructure A = 7.06 Å et B= 8.16 Å represented schematically in the figure 3b. It is important to note here that the polymer adopts an epitaxial configuration with the selection of a unique adsorption site. As detailed below, the bridge site occupied by Fe atoms is found to be the most stable. This implies a large adaptation of the polymer to the substrate lattice constant. In the case of the polymer deposition on Au(110) the Fe-Fe distance is found to 8.16 Å whereas it was obtained to 7.65 Å when deposited on Ag(111). This very large difference of 6% cannot be interpreted in the framework of linear elasticity but should imply some other process such as magneto-elastic coupling involved in spin crossover compounds.33,19


8

Page 9 of 26 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


Figure 3: (a) LEED pattern obtained after co-deposition of molecule (1) with Fe atoms on (1x2) Au(110) at 250°C, (b) Schematic representation of the c(4x2) superstructure of the polymer with respect to the (1 x 2) reconstruction (top view) dark grey spheres representing the outermost atoms of the Au-surface. (c) Side view of the schematic representation.

Ab-Initio Calculation of The Free-Standing Polymer In order to get an idea about the possible origin of the surprising adaptability of Fe-quinoidal polymer chains to different substrates we calculate the electronic and magnetic properties of free-standing polymer chains. The necessity of the SGGA+U method for metal-organic compounds with transition metal ions is proven by many examples as X-TCNB10,34 or X-TCNQ35 where X is a transition metal. We choose in the following different values of the correlation energy U between 3 and 5 eV and an exchange energy of J = 0.9 eV for the Fe d-orbitals. The geometry optimization was performed with the variable lattice parameter and the full relaxation of the coordinates (Figure 4).


9


Page 10 of 26

Figure 4: Ab-initio structure of a free-standing 1D polymer chain of Fe atoms and zwitterionic quinone after relaxation. The Fe, N, C, O and H atoms are highlighted in orange, blue, gray, red and white, respectively.

In all calculations, Fe has the valence state of +2. We find three possible spin states, a low spin state S=0, an intermediate spin (IS) state with S=1 per Fe and a high-spin (HS) state S=2. The IS and HS states are shown in Figure 5 by variation of the lattice constant, but the LS state is much higher in energy (by 0.7 eV) and not considered further on. Without U, in the SGGA method, the IS state at an equilibrium lattice constant of 15.3 Å (two times the Fe-Fe distance) is the most stable one, but both magnetic states are very close in energy for U between 3 and 5 eV (Table 1). For U = 5 eV the HS state is lowest in energy at a lattice constant of 15.90 Å. The equilibrium lattice constants for HS and IS states depend very weakly on U. Table 1. Energy difference ∆E=E(HS)-E(IS) between HS and IS states, as well as the energy gaps for spin up ( ) and spin down ( ) as a function of U (in eV) in the SGGA+U method.

SGGA (U=0)

U=3

U=4

U=5

∆E (meV)

1387.54

595.60

108.35

-145.45

(IS: S = 1) Ea (eV) Eb (eV)

0.56 0.10

0.68 0.07

0.71 0.32

0.85 0.65

(HS: S = 2) Ea (eV) Eb (eV)

no gap no gap

0.98 1.00

1.20 1.33

1.20 1.30

Both magnetic solutions show different distances between Fe and its neighboring ligands. The Fe-N (Fe-O) distances are 1.93 Å (1.93 Å) and 2.02 Å (2.08 Å) for the IS and HS states of Fe,


10

Page 11 of 26 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


respectively. The Hubbard U parameter increases the gap at Fermi level in a considerable way. As visible in Table 1, the gaps are very small or even absent without the Hubbard-U correction. That seems to be unrealistic, and they increase with U to values of the order of 0.65 eV (1.2 eV) for the IS (HS) state and U = 5 eV.

Figure 5: Total energy vs lattice constant for the free-standing Fe-quinone polymer calculated with the SGGA+U method for U = 5 eV.

It is interesting to note that the equilibrium lattice constant for the HS solution of the freestanding polymer 15.90 Å is very close to the lattice constant imposed by the Au(110) substrate 16.32 Å. And we find indeed the HS state for the adsorbed polymer (see next Chapter). On the contrary, the lattice constant 15.30 Å which is imposed by the Ag(111) substrate15 is close to the minimum for the IS state. Polymer Adsorption The polymer adsorption was simulated with the same SGGA+U method taking U = 5 eV in all of the calculations. Additional tests were performed to check that small changes of U in the region of ± 1 … 2 eV do not change our conclusions.


11


Page 12 of 26

Table 2. Adsorption energies in eV for different sites of the 2D Fe-zwitterionic quinone on the Au(110) surface without and with (1×2) reconstruction. The magnetic state is ferromagnetic (FM).

T

SB

H

LB

Au(110)-( 1×1)

-2.450

-3.175

-1.535

-1.465

Au(110)-( 1×2)

-1.809

-1.889

-0.739

-1.386

The adsorption energy for Fe-zwitterionic quinone on Au(110) is defined as = !"#$%&/()(++,) − ()(++,) − !"#$%& , where ()(++,) and !"#$%& are the total energies of the Au(110) surface and the polymer, respectively, and !"#$%&/()(++,) is the energy of the adsorbed polymer on the surface. These energies for the above mentioned (4×4) superstructure containing 4 Fe atoms are summarized in Table 2. We simulated both, the adsorption on the (1×2) reconstructed surface of Au(110), as well as on the non-reconstructed one for the following adsorption sites of the Fe atom: on top (T) of the Au atom, at the short bridge (SB) of the rectangular unit cell of the Au(110) surface, the long bridge (LB) and the hollow (H) site. One can remark that the adsorption on the non-reconstructed surface is always preferred by a substantial energy gain with respect to the reconstructed one. That is true for all adsorption sites and can be explained by the fact that each second Fe atom is far above the Au surface in the case of a reconstructed surface. The most preferred adsorption site is the short-bridge position on the non-reconstructed surface with each Fe atom being close to two Au atoms with a Fe-Au distance of 2.95-3.01 Å. The corresponding relaxed configuration is shown in Figure 6. We conclude from Table 2 that the surface reconstruction disappears when the polymer is deposited. Or, in other words, the energy gain due to surface reconstruction36 is less than the energy gain due to adsorption. We should remark, however, that we have no direct experimental proof that the reconstruction disappears since the STM picture can only show the uppermost atoms. But we can


12

Page 13 of 26 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


notice that the electron density is identically observed on all Fe atoms Figure 2b. Also, the SB position of the Fe atom leads to the schematic figure of the adsorbed polymer with respect to the surface reconstruction presented in Figures. 3b and 3c. It explains the shift of the Fe lines with respect to the Au lines of the reconstructed surface by ¼ of the interline distance which is well visible in the STM topography (Figure 2b).

Figure 6: Calculated adsorption configuration of Fe-zwitterionic quinone on the unreconstructed Au(110) surface. The Fe atom is in the energetically preferred short-bridge (SB) position.

For the adsorbed polymer on the Au(110) surface without surface reconstruction we also compare the anti-ferromagnetic (AFM) with the ferromagnetic (FM) order along one chain and find that AFM is preferred by 15 meV for the (4×4) supercell containing 4 Fe ions. The small AFM nearest neighbor exchange integrals are also confirmed by calculations without substrate but higher computational precision presented below. The vertical distance between the Fe atoms and the Au(110) substrate is 2.59 Å independent of the magnetic structure. There are 4 Fe atoms in the (4×4) superstructure (Figure 6) which are all crystallographically different and denoted by + and in one chain and and / in the neighboring one. The distance along the polymer chains ( + − ) is dictated by the substrate and coincides with


13


Page 14 of 26

the B vector measured by LEED (Figure 3). But instead of one inter-chain distance A=7.06 Å of the c(4×2) superstructure we find two distances of 7.12 Å and 6.87 Å after the ab-initio relaxation. We explain that difference between theory and experiment by temperature effects since the LEED data were taken at room temperature but the ab-initio calculations gives the hypothetical zero temperature structure. The crystallographic difference between the 4 Fe positions in one (4×4) superstructure is also confirmed by slightly different local magnetization values varying between 3.672 µB and 3.676 µB as calculated in the VASP code by projection on the local 3d orbitals. Figure 6 shows also a considerable buckling of the 2D arrangement of adsorbed polymer chains. The Fe atoms are attracted by the substrate providing strong bonds between polymer and Au. But some H atoms have not enough space and are pushed out of the polymer plane. That is connected with the small inter-chain distance of 5.77 Å of the polymer chains on the Au(110) substrate in comparison to the much larger inter-chain distance of 6.55 Å for a free-standing 2D arrangement. Finally, in Figure 7, the projected density of states (PDOS) for Fe d electrons is shown. The spin up and spin down electrons are well distinguished in the ferromagnetic state. One clearly observes the HS state with S=2 since the 3d spin up electron states of Fe are all occupied like also the spin down 31 orbital. The remaining 3d spin down orbitals are empty. Therefore, the PDOS in Figure 7 proves very nicely the S=2 state, even better than the local magnetization values of the VASP code which deviate by 0.32 µB from 4 µB. The Fermi level is in a region of vanishing Fe 3d DOS. By comparing with the DOS of a free-standing polymer, we can conclude that there is no remarkable charge transfer between the Au substrate and the adsorbed polymer.


14

Page 15 of 26 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


Figure 7: Projected DOS of the d orbitals on the Fe atom in the Fe-zwitterionic quinoidal polymer (2D) deposited on the Au(110) surface in the FM spin configuration.

Exchange Couplings To estimate the magnetic exchange couplings we calculated the free-standing polymer and its 2D arrangements with the same lattice parameters as dictated by the Au(110) substrate. The neglect of the substrate allows to increase the number of k-points to (8×5×1) for the 2D arrangement. The total energy calculations of the AFM and FM spin structures for the 1D polymer are performed with (8×1×1) k-points. It was tested that the total energy is well converged for the chosen k-point mesh. Without substrate, the relaxed structure shows no buckling as in Figure 6 and is perfectly plan. We have checked that the buckling due to the substrate influences the exchange couplings by about 40% but does not change the qualitative conclusion below. The AFM structure of the isolated 1D polymer chain is preferred by 8.83 meV per Fe ion with respect to the FM one. From that energy difference one concludes the nearest neighbor exchange couplings in the Hamiltonian


15


3 = ∑:67; 567 896 897 2

Page 16 of 26

(1)

where the sum over < => > counts each bond only once and where 896 are the spin operators for S = 2. Neglecting quantum corrections, the energy difference between FM and AFM states per Fe ion Δ = A − (A = 8.83 CD = 4 5+ 8 can be used to determine the nearest neighbor exchange coupling within chain direction 5+ to be 0.55 meV. That AFM exchange is confirmed by a calculation of the 2D arrangement. In that case, we have to distinguish between the in-chain exchange 5+ (between + and ) and the inter-chain exchange 5 (the coupling of + to and / being approximately equal). The energy difference between the AFM and FM spin arrangements is Δ = A − (A = 10.46 CD per Fe ion leading to 5+ = 0.65 CD if we neglect 5 completely. But 5 is not small. That is visible if we calculate a third magnetic structure with parallel spins in one chain but antiparallel ones between neighboring chains (alternating chains). Its energy is denoted (G in difference to A and (A with the following energy values per Fe spin S = 2 for the spin Hamiltonian (1): A = 2 5+ 8 + 4 5 8 (A = − 2 5+ 8

(2)

(G = 2 5+ 8 − 4 5 8 The corresponding energy gain Δ′ = A − (G = 15.50 CD exceeds even Δ = 10.46 CD for the 2D arrangement and leads to 5+ = 0.17 CD and 5 = 0.48 CD from Eqns. (2). So, we can conclude that all nearest neighbor exchange couplings for the 2D arrangement of polymer chains on the Au(110) surface are antiferromagnetic but quite small, below 1 meV. However, the estimation presented here is very rough, neglects the substrate, might depend on the parameter U, and is at the border of the numerical precision of the VASP code. Nevertheless


16

Page 17 of 26 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


it becomes clear that the spin chains are magnetically not well separated from each other on the Au(110) surface. The distance between neighboring spins is too small. There is the perspective that the spin chains can be better separated since the chemical bonding is exclusively directed within chain direction. That is visible in the polymerization energy per Fe atom which is defined as +

J"#$%&61K6"L = GM6L + NO − − A"#(GP QORO NP )

(3)

corresponding to the reaction shown in Figure 1 and calculated to be J"#$%&61K6"L = −3.34 D. On the contrary, the arrangement of 2D chains (without substrate) gives a smaller energy gain per Fe atom defined as +

+

T T &&LS = / J"#$%& − GM6L

(4)

T and found to be &&LS = −0.105 D due to the hydrogen bonds for a distance of 6.55 Å

between the chains. For an inter-chain distance of 5.77 Å on the Au(110) substrate that energy T T gain &&LS is nearly zero (&&LS = −7.33 CD for the AFM spin structure) due to an

accidental cancelation between the binding energy of the hydrogen bonds and the electrostatic repulsion due to the small inter-chain distance dictated by the substrate. Comparison of High Spin and Intermediate Spin States To understand the electronic and magnetic structures in more detail, we present the spinpolarized total density of states (DOS), the projected densities of states (PDOS) onto the different atoms Fe, C, O, N or H, and the orbital resolved PDOS onto Fe d electrons in Figure 8 for the IS (S=1) and in Figure 9 for the HS state (S=2). The DOS at the Fermi level of the polymer zwitterionic quinoidal is zero. The DOS are shown for the FM spin arrangement for the equilibrium lattice constants of the HS and IS states of a free-standing polymer calculated at U =


17


Page 18 of 26

5 eV. The gap values which are visible in Figures 8 and 9 are also given in Table 1 and vary between 0.65 eV and 1.30 eV.

a

b


18

Page 19 of 26 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


c

Figure 8: (a) Total DOS, (b, c) Projected DOS of the atoms (Fe, C, O, N, H) and the d orbitals at the Fe atoms in the 1D Fe zwitterionic quinoidal polymer with intermediate-spin (S = 1).

One remarks valence and conduction bands of about 1 eV width in Figures 8 and 9. Their 1D character manifests itself by high Van-Hove singularities at the band edges and opens the perspective of directed 1D electron transport. But the band character is different for the different spin up or down valence and conduction bands, it is of Fe d character for the spin down valence band but of dominantly N p character for both conduction bands and the spin up valence band. Also, the difference between S=1 and S=2 solutions is clearly visible by an unoccupied U$ spin up band for S=1 in Figure 8 whereas there is no unoccupied 3d spin up band for S=2 (Figure 9) at all. There is a large similarity between the partial d-DOS of a free-standing polymer (Figure 9) and the adsorbed polymer (Figure 7) proving the absence of charge transfer between substrate and ad-layer.


19


Page 20 of 26

a

b


20

Page 21 of 26 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


c

Figure 9: (a) Total DOS, (b, c) Projected DOS of the atoms (Fe, C, O, N, H) and the d orbitals at the Fe atoms in the 1D Fe zwitterionic quinoidal polymer with high-spin (S = 2).

Conclusions and Perspectives The Au(110) surface allows the synthesis of well-ordered covalent one-dimensional (1D) polymer chains with equally spaced magnetic ions. The surface reconstruction disappears during the adsorption process and the polymeric chains build a well ordered supramolecular twodimensional (2D) arrangement. The preferred adsorption site for the Fe-ion is the short-bridge site. We performed an accurate calculation of the electronic and magnetic properties of these 1D and 2D polymers. Without substrate, the electronic structure corresponds to an antiferromagnetic semiconductor with a gap of 1.2 eV. The antiferromagnetic state is preferred by a very small energy difference of only 9…10 meV per Fe ion with respect to the ferromagnetic one. The surprising adaptability of the synthesized polymer to substrates with very different (by 6%) lattice constants may be explained by the presence of two local minima being close in energy but having different spin states (HS and IS) and different lattice constants. We discover by our ab-initio calculations two states with S=1 ( = 7.72 Å) and S=2 ( = 7.95 Å)


21


Page 22 of 26

for the free-standing chain. The lattice constant which is imposed by the Au(110) substrate ( = 8.16 Å) leads to the HS state which is confirmed by the ab-initio calculations of the adsorbed polymer. On the other hand, the Ag(111) substrat is more close to the IS state ( = 7.65 Å) which should play an essential role in the adsorption process on Ag(111). The presented synthesis of Fe zwitterion quinoidal polymers opens many perspectives. At first, one can substitute Fe by other transition metals and synthesize a large diversity of spin chains with different values of the local spin. Also, a synthesis on insulating substrates would allow to use the 1D electronic conductance properties, eventually spin resolved. A better separation between the spin chains on other substrates or in a free-standing polymer spin chain would allow to test and to exploit the special properties of 1D spin-chain compounds. But the first interesting experimental verification would be to synthesize the Fe zwitterion quinoidal polymer chain on different substrates and to measure the spin state transition between S=1 and S=2 by X-ray magnetic circular dichroism (XMCD).

AUTHOR INFORMATION Corresponding Authors *(H.D.) E-mail: [email protected] *(R.H.) E-mail: [email protected] *(M.A.) E-mail: [email protected]

ACKNOWLEDGMENTS This work was supported by the Computer resources of the Centre Informatique National de l’Enseignement Supérieur (CINES), Project No. A0020906873 and the High Performance Computing (HPC) resources of Aix-Marseille University financed by the project Equip@Meso (ANR-10-EQPX-29-01).


22

Page 23 of 26 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


REFERENCES (1) Bethe, H. Zur Theorie der Metalle. Z. Phys. 1931, 71, 205-226. (2) Solyom, J. The Fermi gas model of one-dimensional conductors. Adv. Phys. 1979, 28, 201303. (3) Lorenzana, J.; Eder, R. Dynamics of one-dimensional Heisenberg model and optical absorption of spinons in cuprate antiferromagnetic chains. Phys. Rev. B 1997, 55, R3358R3361. (4) Lüscher, A.; Läuchli, A. M. Exact diagonalization study of the antiferromagnetique spin1/2 Heisenberg model on the square lattice in a magnetic field. Phys. Rev. B 2009, 79, 195102. (5) Levy, J. Universal quantum computation with spin-1/2 pairs and Heisenberg exchange. Phys. Rev. Lett. 2002, 89, 147902. (6) Marchukov, O. V.; Volosniev, A. G.; Valiente, M.; Petrosyan, D.; Zinner, N. T. Quantum spin transistor with a Heisenberg spin chain. Nat. commun. 2016, 7, 13070. (7) Nadj-Perge, S.; Drozdov, I.; Li, J.; Chen, H.; Jeon, S.; Seo, J.; MacDonald A.; Bernevig B.; Yazdani, A. Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor. Science 2014, 346, 602-607. (8) Andolina, G.M.; Simon, P. Topological properties of chains of magnetic impurities on a superconducting substrate: Interplay between the Shiba band and ferromagnetic wire limits. Phys. Rev. B 2017, 96, 235411. (9) Lin, N.; Stepanow, S.; Vidal, F.; Kern, K., Alam, M. S.; Stromsdorfer, S.; Dremov, V.; Muller, P.; Landa, A.; Ruben, M. Surface-assisted coordination chemistry and selfassembly. Dalton Trans. 2006, 0, 2794-2800. (10) Mabrouk, M.; Savoyant, A.; Giovanelli, L.; Clair, S.; Hayn, R.; Ben Chaabane, R. Ligand influence on local magnetic moments in fe-based metal-organic networks. J. Phys. Chem. C 2017, 121, 4253-4260. (11) Lindner, R.; Kühnle, A. On-surface reactions. ChemPhysChem 2015, 16, 1582-1592. (12) Goiri, E.; Borghetti, P.; El-Sayed, A.; Ortega, J. E.; G. de Oteyza, D. Multi-component organic layers on metal substrates. Adv. Mater. 2016, 28, 1340-1368. (13) Nardi, E.; Chen, L.; Clair, S.; Koudia, M.; Giovanelli, L.; Feng, X., Müllen, K.; Abel, M. On-surface reaction between tetracarbonitrile-functionalized molecules and copper atoms. J. Phys. Chem. C 2014, 118, 27549-27553. (14) Koudia, M.; Abel, M. Step-by-step on-surface synthesis: From manganese phthalocyanines to their polymeric form. Chem. Commun. 2014, 50, 8565-8567. (15) Koudia, M.; Nardi, E.; Siri, O.; Abel; M. On-surface synthesis of covalent coordination polymers on micrometer scale. Nano Res. 2017, 10, 933-940.


23


Page 24 of 26

(16) Ruban, A.V.; Khmelevskii, S.; Mohn, P.; Johansson, B. Magnetic state, magnetovolume effects, and atomic order in Fe65Ni35 Invar alloy: A first principles study, Phys. Rev. B 2007, 76, 014420. (17) Harding, D. J.; Harding, P.; Phonsri, W. Spin crossover in iron(III) complexes. Coord. Chem. Rev. 2016, 313, 38-61. (18) Kumar, K. S.; Ruben, M. Emerging trends in spin crossover (SCO) based functional materials and devices. Coord. Chem. Rev. 2017, 346, 176-205. (19) Lebègue, S.; Pillet, S.; Ángyán, J. G. Modeling spin-crossover compounds by periodic DFT+U approach. Phys. Rev. B 2008, 78, 024433. (20) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758-1775. (21) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979. (22) 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, R5467R5470. (23) 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. (24) Perdew, J. P.; Wang, Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. B 1992, 45, 13244-13249. (25) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. (26) Braunstein, P.; Siri, O.; Taquet, J.-P.; Rohmer, M.-M.; Benard, M.; Welter, R. A 6π+6π potentially antiaromatic zwitterion preferred to a quinoidal structure: Its reactivity toward organic and inorganic reagents. J. Am. Chem. Soc. 2003, 125, 12246–12256. (27) Yang, Q.-Z.; Siri, O.; Braunstein, P. Tunable n-substitution in zwitterionic benzoquinonemonoimine derivatives: Metal coordination, tandemlike synthesis of zwitterionic metal complexes, and supramolecular structures. Chem. -Eur. J. 2005, 11, 7237-7246. (28) Siri, O.; Braunstein, P. Unprecedented zwitterion in quinonoid chemistry. Chem. Commun. 2002, 0, 208-209. (29) Lin, Y. P.; Ourdjini, O.; Giovanelli, L.; Clair, S.; Faury, T.; Ksari, Y.; Themlin, J.; Porte, L.; Abel, M. Self-assembled melamine monolayer on Cu (111). J. Phys. Chem. C 2013, 117 , 9895–9902.


24

Page 25 of 26 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


(30) Bai, Y.; Buchner, F.; Wendahl, M. T.; Kellner, I.; Bayer, A.; Steinrück, H. P.; Marbach, H.; Gottfried, J. M. Direct metalation of a phthalocyanine monolayer on Ag(111) with coadsorbed iron atoms. J. Phys. Chem. C 2008, 112 (15), 6087–6092. (31) Descostes, M.; Mercier, F.; Thromat, N.; Beaucaire, C.; Gautier-Soyer, M. Use of XPS in the determination of chemical environment and oxidation state of iron and sulfur samples: constitution of a data basis in binding energies for Fe and S reference compounds and applications to the evidence of surface species of an oxidized pyrite in a carbonate medium. Appl. Surf. Sci. 2000, 165, 288-302. (32) Conradie, J.; Erasmus, E.; XPS Fe 2p peaks from iron tris (β-diketonates): Electronic effect of the β-diketonato ligand. Polyhedron 2016, 119, 142–150. (33) Ksenofontov, V.; Gaspar, A. B.; Gütlich P., Pressure Effect Studies on spin crossover and valence tautomeric systems. Top. Curr. Chem. 2004, 235, 23-64. (34) Mabrouk, M.; Hayn, R.; Denawi, H.; Ben Chaabane, R. Possibility of a ferromagnetic and conducting metal-organic network. J. Magn. Magn. Mater. 2018, 453, 48-52. (35) Ma, Y.; Dai, Y.; Wei, W.; Yu, L.; Huang, B. Novel two-dimensional tetragonal monolayer: Metal-TCNQ networks. J. Phys. Chem. A 2013, 117, 5171-5177. (36) Reiff, S.; Block, J. H. Oxygen chemisorption and surface reconstruction on Ag(110) invertigated with second-harmonic generation. Surf. Sci. 1996, 345, 281-289.


25


Page 26 of 26

TOC Graphic


26

On-Surface Synthesis of Spin Crossover Polymeric Chains - American

Recommend Documents