# Enhanced magnetic properties in a copper *β*-diketonato complex film stabilized by I××× I interactions on a graphite surface at 2 K for molecular spintronics

Fabien Silly, Emilio Vélez-Fort, Mathieu Silly, Philippe Ohresser, Jacques

Jean Bonvoisin

► To cite this version:

Fabien Silly, Emilio Vélez-Fort, Mathieu Silly, Philippe Ohresser, Jacques Jean Bonvoisin. Enhanced magnetic properties in a copper *β*-diketonato complex film stabilized by I××× I interactions on a graphite surface at 2 K for molecular spintronics. ACS Applied Nano Materials, 2024, 7 (23), pp.26737-26742. 10.1021/acsanm.4c04426. cea-04861317

HAL Id: cea-04861317

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# Magnetic Properties of a Copper β-Diketonato Complex Film stabilized by I···I interactions on a Graphite Surface at 2 K

Fabien Silly,*,† Emilio Vélez-Fort,‡ Mathieu G. Silly,¶ Philippe Ohresser,*,¶ and Jacques Bonvoisin*,§

†Université Paris-Saclay, CEA, CNRS, SPEC, TITANS F-91191 Gif sur Yvette, France
‡European Synchrotron Radiation Facility (ESRF), Avenue des Martyrs 71, 38043 Grenoble, France

¶Synchrotron SOLEIL, L'Orme des Merisiers, F-91190 Saint-Aubin, France
§CEMES, CNRS UPR 8011, Université de Toulouse, 29 rue Jeanne Marvig, B.P. 94347, 31055 Toulouse Cedex 4, France

E-mail: fabien.silly@cea.fr, Tel:+33(0)169088019, Fax:+33(0)169088446;
philippe.ohresser@synchrotron-soleil.fr; jacques.bonvoisin@cemes.fr

## Abstract

The structural and magnetic properties of a drop-cast film of flat a β-diketonato complex functionalized with iodine atoms (C₃₀H₁₈CuI₄O₄) on a graphite surface are investigated using scanning tunneling microscopy, synchrotron X-ray absorption spectroscopy and X-ray magnetic circular dichroism. Experimental measurements reveal that the Cu-complexes self-assembled into a halogen-bonded arrangement on the graphite surface and that the molecules preferentially remain flat in the organic film. The magnetic hysteresis loops show that the organic film is paramagnetic at 2 K with an easy axis of magnetization perpendicular to the graphite surface, which is also perpendicular to the plane of the Cu-complex skeleton.

## Introduction

Engineering novel nanomaterials with improved physical and chemical properties is a challenge in nanosciences.¹⁻⁶ The fabrication of ordered arrays of magnetic building blocks has attracted scientific interest to create new devices for information processing, high density information storage and permanent magnets.⁷⁻¹²

Molecular self-assembly is an appealing technique to spontaneously engineer well-ordered organic nanoarchitectures.¹³⁻¹⁹ Magnetic nano-materials can thus be achieved through the self-assembly of single molecular magnets (SMMs).¹⁹⁻²²

Selective and directional intermolecular interactions as halogen bonds,²³⁻³¹ metal-organic interactions,³²⁻³⁷ organic-ionic interactions³⁸ and hydrogen bonds¹³,³⁹,⁴⁰,⁴⁰⁻⁴⁷ have been successfully used to trigger the formation of non-covalent porous self-assembled organic 2D nanoarchitectures.¹⁶,⁴⁸⁻⁵⁰

Molecular magnets consisting of metal ions bonded together by various organic ligands, which ensure their cohesion, with various designs have now been successfully synthesized.⁵¹⁻⁵⁶
netic complexes based on porphyrin and phthalocyanine skeleton have been extensively studied in the past, but new alternative molecular designs are now emerging. We recently synthesized β-diketonato copper complex with bromine substituents ($C_{54}H_{34}Br_4CuO_4$).²⁰ We observed that this molecule self-assemble into a Br···Br halogen-bond arrangement on a graphite surface and the resulting organic film has a small out-of-plane magnetic anisotropy. The versatility of the β-diketonato copper skeleton allows to add or change molecular substituents to trigger the formation of different molecular arrangements, which is expected to influence the magnetic properties of the resulting organic film.

In this paper, we investigate the structural and magnetic properties of an organic film based on flat β-diketonato copper complex with iodine substituents ($C_{30}H_{18}CuI_4O_4$) deposited on a graphite surface. Molecular self-assembly is investigated using scanning tunneling microscopy (STM) and X-ray Natural Linear Dichroism (XNLD). The magnetic properties of the organic layer is assessed using the high sensitivity of X-ray Magnetic Circular Dichroism (XMCD).

## Experimental

The synthesis of complex has been described in Ref.[⁵⁷]. Solutions of the complexes in 1-phenyloctane (Aldrich) were prepared. A droplet of the solutions was then deposited on a graphite substrate. STM imaging of the samples was performed at the liquid-solid interface using a PicOSPM (Molecular Imaging, Agilent Technology) scanning tunneling microscope. Cut Pt/Ir tips were used to obtain constant current images at room temperature with a bias voltage applied to the sample. STM images were processed and analyzed using the application FabViewer.⁵⁸

The chemical selected magnetization loops, the XMCD and the XNLD have been measured on the DEIMOS beamline at SOLEIL Synchrotron.⁵⁹,⁶⁰ All the X-ray Absorption Spectroscopy (XAS) measurement presented in this paper have been performed at a low temperature of 2 K in a cryomagnet with a base pressure of $1.10^{-10}$ mbar.

The photon flux has been reduced at the lowest (typically $6.10^9$ ph/s) to avoid any possible beam damage. The typical edge jumps are around few percent of the background and to obtain a good statistic a typical dichroism measurement (XMCD or XNLD) is obtained by taking thirty two XAS spectra recorded with opposite orientation of the magnetic field (for the case of XMCD) and flipping the helicity of the light. XAS has been measured for horizontal and vertical linear polarizations at grazing incidence to determine the adsorption geometry of the molecules on the surface. The XMCD spectrum corresponds to the difference of the XAS spectra measured with left and right helicity of the light while XNLD is obtained using vertical and horizontal linear polarized light. For the sake of simplicity, we will call $\mu^+$, $\mu^-$, $\mu^H$ and $\mu^V$ the absorption coefficient of respectively left-, right-circular, horizontal and vertical linear polarized light.

## Results

### Cu-complex design

<figure><img src="image_2.png" /><figcaption>Figure 1: Scheme of the β-diketonato copper complex, $C_{30}H_{18}CuI_4O_4$ in square-planar complex $[Cu(dipd)_2]⁰$ conformation in front and side views (a,b). Carbon atoms are gray, iodine atoms are purple, hydrogen atoms are white, oxygen atoms are red and the copper atom is green, respectively.</figcaption></figure>

The chemical structure of the β-diketonato copper complex ($C_{30}H_{18}CuI_4O_4$) is presented in Figure 1. This 2-fold symmetry complex is a flat X-shaped complex. Its skeleton consists of a central copper atom connected to two acetylacetonate type ligands. Iodine atoms are located at the apex
of the four complex arms. The iodine atom separations are 13.8 Å and 11.4 Å.

The synthesized molecule adopts a flat $[\text{Cu(dipd)}_2]^0$ conformation. However a non-planar $[\text{Cu(dipd)}_2]^{-1}$ is also possible.⁶¹

## Scanning Tunneling Microscopy (STM)

<figure><img src="image_3.png" /></figure>

Figure 2: STM image of the complex-1 self-assembled porous network at the 1-phenyloctane/graphite interface, (a) 35×25 nm², $V_s = 0.55$ V, $I_t = 9$ pA, (b) 5×6 nm², $V_s = 0.75$ V, $I_t = 9$ pA. Model of the molecular assembly is superimposed to the STM image in (b). The square unit cell is represented by dashed-purple lines.

The STM image in Figure 2(a) shows the graphite surface after deposition of a droplet of the Cu-complex in 1-phenyloctane. Molecules self-assemble into a large-scale 2D organic nanoarchitecture. Bright spots and dark areas are visible in the STM image.

A high resolution STM image of the molecular self-assembly is presented in the Figure 2(b). The STM image shows that the organic structure is porous. The dark areas in the STM image correspond to empty cavities in the organic arrangement. STM also reveals that each molecule has four side-by-side neighbors. The bright features observed in the STM images correspond to squares composed of four bright spots (Figure 2a,b). These bright spots are observed at the apex of molecular arms. These bright spots therefore correspond to the molecular iodine atoms. It has been previously experimentally observed that halogen atoms, such as bromine and iodine, appear brighter than molecular carbon atoms in the STM images.³¹,⁶²,⁶³ The molecular arrangement in Figure 2(a,b) is stabilized by $X_4$ synths. Two molecular orientations are observed in the organic nanoarchitecture, as it can be observed in the high resolution STM image presented in Figure 2(b). Molecular closest neighbors are rotated by 90°. The model of the Cu-complex self-assembly is superimposed to the STM image in Figure 2(b). The network unit cell, containing two molecules is represented by dashed purple lines in Figure 2(b). The unit cell of this porous structure is a square with 2.2±0.1 nm unit cell constants. The molecular architecture is stabilized by I···I halogen interactions between neighboring molecules, forming $X_4$ synths. The angle between I-C groups of neighboring molecules is ~95°.
## X-ray Natural Linear Dichroism

After STM measurements, the sample is introduced into the UHV chamber, achieving solvent evaporation. The orientation of the molecules adsorbed on the surface is revealed using XAS spectroscopy.<sup>64,65</sup> The electronic anisotropy of the organic film structure is thus assessed by linear polarization at a grazing incidence of $\theta = 45^\circ$, at the Cu-$L_{2,3}$ edge at a temperature of 2 K (Figure 3). The $\mu^V$ and $\mu^H$ polarizations are probing the in-plane and out-of plane electronic configuration, respectively. The XNLD green curve in Figure 3 corresponds to the difference between the two XAS spectra ($\mu^H$ - $\mu^V$).

<figure><img src="image_4.png" /><figcaption>Figure 3: Experimental XAS (top) and XNLD (bottom) spectra of the copper $\beta$-Diketonato complex film on HOPG, recorded at a temperature of 2 K, an applied external magnetic field of 1 T and an incidence angle of 45°.</figcaption></figure>

A noticeable linear dichroism is observed in the XAS spectra recorded at Cu-$L_{2,3}$ edges, between the $\mu^H$ horizontal and $\mu^V$ vertical polarization (Figure 3). This is highlighting a strong molecular electronic anisotropy. The Cu-$L_3$ peak mean feature is maximum with the $\mu^V$ polarization (Figure 3,top). As the main Cu-$L_3$ multiplet results from the complex $d_{x^2-y^2}$ orbitals, this XAS dichroism indicates that the film molecules are preferentially lying flat on the surface.

## X-ray Magnetic Circular Dichroism

<figure><img src="image_5.png" /><figcaption>Figure 4: (top) Polarization dependent XAS of the Cu-complex film on HOPG recorded at the Cu $L_{2,3}$ edges at normal incidence (0°) as a function of circular polarization ($\mu^+$, red spectrum, and $\mu^-$, black spectrum). (bottom) XMCD spectra recorded at normal incidence (0°, green spectrum) and grazing incidence (45°, purple spectrum). XAS and XMCD spectra have been recorded at a temperature of 2 K and an applied external magnetic field of 3 T.</figcaption></figure>

The magnetic properties of the Cu-complex thin film on HOPG are now investigated using XAS with circularly polarized light to determine the XMCD signal, Figure 4top. The XAS spectra at the Cu $L_{2,3}$ edges are recorded at the normal ($\theta = 0^\circ$) and a temperature of 2 K and under a magnetic field $B = 3$ T.

Two peaks are observed in the spectra, one at 935.2 eV and the other one at 955.2 eV. Additional features with lower amplitude soon appear at 937.5 eV and 957.8 eV, Figure 4top. XAS is a powerful technique to reveal the existence of different copper oxidation states (Cu$^+$ and Cu$^{2+}$).<sup>66</sup> Comparison with the literature highlights that peaks at ~935 eV and ~955 eV correspond to the signature of the Cu$^{2+}$ species, whereas the small features at ~937.5 eV and 957.8 eV correspond to the contribution of the lower oxidation state of Cu$^+$.<sup>66-68</sup>

Significant circular dichroism is visible in the XAS spectra for the Cu$^{2+}$ state of the Cu-complex at normal incidence ($\theta = 0^\circ$), Figure 4top. This in-
dicates that the organic film has a non-zero magnetization under an applied field of 3 T. This dichroism is especially visible in the resulting XMCD spectra (difference between the absorption of $\mu^-$ and $\mu^+$ circular polarized light) recorded at normal incidence ($\theta = 0^\circ$) and grazing incidence ($\theta = 0^\circ$), Figure 4bottom. The XMCD spectra also highlight a larger contribution for the normal geometry, with a value of ~41% at the Cu-L$_3$ edge at normal incidence and 32% at the $\theta = 45^\circ$ grazing incidence.

## Hysteresis

<figure><img src="image_6.png" /></figure>

Figure 5: Magnetic hysteresis loops measured at 2 K on the copper β-Diketonato complex film on HOPG at the normal incidence ($\theta = 0^\circ$, red curve) and the grazing incidence ($\theta = 45^\circ$, blue curve). The measurements have been normalized in intensity to the corresponding XMCD signal (see Figure 4).

Figure 5 shows the magnetization curves of the Copper β-Diketonato complex film on HOPG at a normal incidence ($\theta = 0^\circ$) and at a grazing incidence ($\theta = 45^\circ$) at 2 K. These curves are obtained by recording the variation of the maximum Cu-L$_3$ XMCD intensity with the applied magnetic field.

Figure 5 reveals that the magnetization curves are not saturated at 2 K for 5 T. The curves adopt a paramagnetic shape for both incidence angles. This shows that the molecular film has a paramagnetic behavior. Figure 5 also highlights that the magnetic anisotropy favors the out-of-plane geometry.
Discussion

A solution of β-Diketonato complexes with iodine substituents (Figure 1) has been deposited onto a HOPG surface. STM at the solid-liquid interface reveals that the molecules adsorb flat on the HOPG surface and self-assembled into a porous halogen-bonded nanoarchitecture stabilized by I···IX₄ synths, Figure 2. The angle between the complex halogen-carbon covalent bond (X-C) of neighboring complexes is 95° in the X₄ synthon. The strength of the halogen···halogen interaction depends of the C-X···X-C binding angle.⁶⁹ When the C-X···X-C angle is ~90±30°, the halogen···halogen interaction is the strongest and is similar to the one of a hydrogen-bond. This bond is called a type-II halogen interaction, and this is the only halogen interaction experimentally observed in the β-Diketonato complex assembly (Figure 2).

A signal of ~40% is observed in the XNLD spectrum at the Cu-L₃ edge at grazing incidence, Figure 3. The central Cu²⁺ cation surrounded by for oxygen atom in the flat β-Diketonato skeleton has an in-plane $d_{x^2-y^2}$ orbital geometry. The XNLD spectrum thus confirms that the molecule preferentially remain flat on the surface once the sample is then transferred to the UHV chamber.

The Figure 4 reveals that the XMCD signal is maximum at normal incidence (41%). This shows that the magnetic anisotropy of the Cu²⁺-complex thin film has an easy axis of magnetization preferentially perpendicular to the HOPG surface. The magnetic anisotropy is therefore also perpendicular to the plane of the Cu²⁺-complex skeleton. The magnetisation curves of the Cu-complex thin film also show that the organic film on HOPG remains paramagnetic at 2 K.

We previously investigated the magnetic properties of a drop-casted film of another β-Diketonato complexes with bromine substituents (C₅₄H₃₄Br₄CuO₄) on HOPG.²⁰ This molecule shares the same central structure as the present molecule (C₃₀H₁₈CuI₄O₄, Figure 1). We observed that the a maximum of 32% in the XMCD signal at normal incidence at 2 K and 3 T was measured for the C₅₄H₃₄Br₄CuO₄ complex film, to be compared with the 41% measured for the C₃₀H₁₈CuI₄O₄ complex film (Figure 4) measured

using the same conditions. The two β-Diketonato complexes are forming different self-assemblies on the HOPG surface. One is also stabilised by X₂ bromine synths, whereas the other one is stabilized by X₄ iodine synths (Figure 2). The resulting XNLD signal is also stronger for the I-β-Diketonato complex film. These observations thus highlight that the molecular iodine substituents modify the molecular arrangement of the β-Diketonato complexes on HOPG and thus improve the horizontal orientation of the molecules in the organic film. This leads to an increase of the out-of-plan magnetic signal of the organic film.

A small amount of Cu⁺-complexes is detected by the X-ray spectroscopy. The XMCD spectrum reveals that this species is non-magnetic, in contrast with the Cu²⁺ species (Figure 4).

Conclusion

To summarize, we have engineered a drop-cast film of flat β-diketonato complexes with iodine substituents on a non-magnetic graphite surface. Scanning tunneling microscopy and synchrotron radiation spectroscopies show that the molecules are forming halogen-bonds and are preferentially laying flat on the graphite surface. The organic Cu²⁺-complex film is paramagnetic at 2 K and its magnetic anisotropy has an easy axis of magnetization perpendicular to the molecular plane. These observations open up new opportunities for engineering magnetic halogen-bonded nanoarchitectures through halogen-bonding.

**Acknowledgement** The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n° 259297.

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