Spin Control Induced by Molecular Charging in a Transport Junction

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Spin control induced by molecular charging in a transport junction Sujoy Karan, Carlos garcia, Michael Karolak, David Jacob, Nicolas Lorente, and Richard Berndt Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03411 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Spin control induced by molecular charging in a transport junction Sujoy Karan,⇤,†,‡ Carlos Garc´ıa,¶ Michael Karolak,§ David Jacob,k,? Nicolás Lorente,¶,# and Richard Berndt† †Institut f¨ ur Experimentelle und Angewandte Physik, Christian-Albrechts-Universität, 24098 Kiel, Germany ‡Institute of Experimental and Applied Physics, University of Regensburg, 93053 Regensburg, Germany ¶Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain §Institut f¨ ur Theoretische Physik und Astrophysik, Universität W¨ urzburg, Am Hubland, 97074 W¨ urzburg, Germany kDepartamento de F´ısica de Materiales, Universidad del Pa´ıs Vasco, UPV/EHU, Av. Tolosa 72, 20018 San Sebastián, Spain ?IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013 Bilbao, Spain #Centro de F´ısica de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 Donostia-San Sebastián, Spain E-mail: [email protected]

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Abstract The ability of molecules to maintain magnetic multistability in nanoscale-junctions will determine their role in downsizing spintronic devices. While spin-injection from ferromagnetic leads gives rise to magnetoresistance in metallic nano-contacts, nonmagnetic leads probing the magnetic states of the junction itself have been considered as an alternative. Extending this experimental approach to molecular junctions, which are sensitive to chemical parameters, we demonstrate that the electron affinity of a molecule decisively influences its spin transport. We use a scanning tunneling microscope to trap a meso-substituted iron porphyrin, putting the iron center in an environment that provides control of its charge and spin states. A large electron affinity of peripheral ligands is shown to enable switching of the molecular S = 1 ground state found at low electron density to S = 1/2 at high density, while lower affinity keeps the molecule inactive to spin-state transition. These results pave the way for spin control using chemical design and electrical means.

Keywords: molecular spintronics, electronic transport, spin excitations, electron affinity, single molecule, STM

The electron transport through molecular junctions is usually determined by the moleculeelectrode contact geometry and the corresponding hybridization of molecular orbitals with the electrodes. 1–5 Ferromagnetic electrodes, that provide spin-polarized current, are conducive to a stronger hybridization, 6–8 which can even lead to the quenching of molecular spin in the junction. 8 If the spin is not quenched, the coupling of the molecular spin to the electrode magnetization reduces its relaxation time. This limitation does not appear to hold when nonmagnetic leads are used. There have been examples demonstrating the detection and control of magnetic anisotropy and spin state in two-terminal device geometry 9–15 as well as in the junction of a three-terminal settings where electrical gating by a third terminal was used for stabilizing a particular state. 16–19 Structural changes may also a↵ect molecular spin states or their conductance fingerprints. 20–22 Here we show that the polarizability of 2


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molecular ligands can provide control of the magnetic state in di↵erent transport regimes with no electrical gating required. We built junctions with R4-type iron porphyrins (FeTPP, Fig. 1), involving di↵erent meso sub-substituents, namely, sulfonyl hydroxide (–SO3 H) and methoxy (–OCH3 ), in a scanning tunneling microscope (STM). Figure 2 summarizes the results of tetraphenylporphine sulfonate (denoted FeTPP-S below). When deposited (see Methods) on Au(111), the molecules appear to lie flat featuring a quatrefoil shape in STM topographs (Fig. 2a) with one of their c2 axes preferentially oriented along h1¯10i direction of the substrate. However, deviations up to ⇡ ±15 from this orientation are observed. R

R

R = ── SO3H

N N

Fe

N

── OCH3

N

R

R

Figure 1: Schematic of meso-substituted porphyrins. Molecules host an Fe ion at the center of tetraphenylporphyrin with four meso-subgroups occupying para positions of the phenyl rings. H atoms are not shown. Phenyl rings are substituted with sulfonyl hydroxide (–SO3 H) or methoxy (–OCH3 ) groups. Pink and grey colors are used in STM images below for these molecules. Directions connecting opposite pyrroles correspond to c2 axes. Figure 2b shows a conductance-displacement curve acquired while bringing the STM tip closer to the center of a molecule until contact was reached. The decreasing width of the tunneling barrier is reflected by an exponential increase of the conductance followed by an abrupt rise indicating the transition from tunneling to contact. Next, di↵erential conductance (dI/dV ) spectra were recorded while holding the tip at predetermined heights. Spectra acquired in the tunneling regime (Fig. 2c, black curves) exhibit steps at ⇠ ±7 meV. We attribute these features to inelastic spin-flip excitations, similar to those of iron porphyrin and octaethylporphyrin, 23,24 which are consistent with our calculations revealing a S = 1 3


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state of the molecule exhibiting magnetic anisotropy (details below). The possibility that these features are due to vibrational excitation may be excluded because the changes of conductance exceed several tens of percent that are typical of magnetic excitations as shown for similar molecular systems. 23,25–28 In contrast, the largest conductance changes caused by vibration excitation rarely reach 20% (in the cases of Ce-H 29,30 and CO-molecular cascades 31 ) because of partial cancellation of the changes of the elastic and inelastic conductances. 32 Once the tip is brought into contact with the iron center, the spectrum (light blue) develops a dip as typical of the Kondo e↵ect. 33,34 Further approach of the tip does not change the shape of the spectrum until destruction of the junction occurs. The spectrum may be fit by a Frota line 35,36 (red curve) as expected for the spin-1/2 Kondo e↵ect. 37–40 This requires a change of either the magnetic anisotropy or the spin state of the molecule in order to have a degenerate ground state. The data tentatively rule out the former scenario, since the transition from two excitation steps to a Kondo-dip is abrupt, and no intermediate regime with a splitting of the Kondo feature was observed, in contrast to earlier results from a Co-terpyridine derivative. 41 Our calculations suggest that it is indeed a S = 1/2 ground state (details below), ruling out inelastic spin flips by virtue of Kramers theorem that forces a degenerate S = 1/2 ground state. When lobes are contacted, the spectrum remains virtually unchanged (Fig. 2d). On the lobes, where the tip mainly couples to the molecular ⇡ system, the inelastic steps appear more symmetrical around zero-bias than at the center of the molecule, where the d orbitals of Fe, particularly dz2 , prevail. We therefore moved to less polarizable methoxy (–OCH3 ) groups as meso-substituents to investigate the influence of the peripheral ligands on the Kondo physics, while keeping the environment of the Fe atom unchanged. The molecules (denoted FeTPP-Me below) agglomerate into small domains (Fig. 3a) along h11¯2i and h1¯10i directions starting from fcc regions of the herringbone reconstruction. Individual molecules are imaged as ‘crabs’ with ‘claws’ representing the methoxy groups (Fig. 3b). Both tunneling and contact regimes

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Figure 2: (a) Constant-current image (I = 100 pA, sample voltage V = 2.56 V, 3.4⇥4.6 nm2 ) of FeTPP-S on Au(111). (b) Conductance measured as while the STM tip is brought closer to the center of a FeTPP-S molecule. Positive displacements indicate a reduced tip-molecule distance. (c) Spectra of the di↵erential conductance dI/dV recorded at displacements indicated with stars in (b). The inset schematically shows the experimental setting. Zero displacement ( z = 0) corresponds to I = 100 pA and V = 0.1 V. The lower spectra (black curves) have been multiplied by 2000, 170, and 17, respectively. (d) dI/dV data recorded at a molecular lobe. The lower three spectra have been scaled by factors of 5000, 300, and 20, respectively. were explored under experimental conditions similar to those used before. Data from a molecule with protruding opposite pyrrole units in the h1¯10i direction are shown in Fig. 3d. The tunneling characteristics of the molecule are virtually identical to those of FeTPP-S. However, in the contact regime, the spectrum retains the inelastic steps. These results are remarkable raising the question how the proximity of metallic leads influences the magnetic state of the molecule and which chemical parameter becomes predominant at contact.

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Figure 3: (a) STM image (11.5 ⇥ 11.4 nm2 , I = 100 pA, V = 0.05 V) of a square-shaped island of FeTPP-Me molecules on Au(111). (b) Image of a FeTPP-Me chain. (c) Conductance measured during tip approach to the molecular center. (d) dI/dV spectra recorded at displacements indicated with colored stars in (c). The upper spectra (black) have been multiplied by 4500, 275, and 65, respectively. Density functional theory (DFT) calculations give a clear picture of the di↵erent conductance regimes. In the tunneling regime, FeTPP-Me and FeTPP-S have similar electronic structures and bind in the same way to Au(111). The interaction is basically driven by dispersion forces and negligible charge transfer takes place (see

Q in Tab. 1). 42 As a conse-

quence, the spin distribution over the molecule is hardly perturbed by the substrate. Most of the spin is localized at the Fe atom, Table 1 and red isocontour in Figs. 4a–c, with some small redistribution of spin in the adjacent C and N atoms. The molecule is in a S = 1 ground state, in good agreement with a Fe(II) configuration of the molecule. 42,43 The experimental conductance shows the signature of an excitation from the ground state spin lying in the molecular plane to an out-of-plane excited state. This interpretation coincides with our magnetic-anisotropy energy (MAE) calculations (Supporting Information, section 2), that show the easy-axis character, i. e. Sz = ±1 ground state, of the S = 1 system. The contact regime is modeled using the above system plus an FCC pyramid of gold atoms 6


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Figure 4: (a–c) Calculated spin-density isosurfaces (0.01 e/˚ A3 ) for FeTPP-Me (a), FeTPP-S (b) on Au(111), and gas-phase FeTPP-S (c). Red and yellow indicate spin directions. These calculations represent the system for a tunneling junction where the STM tip does not perturb the surface. The three systems show qualitative resemblance. (d,e) Corresponding results for the “contact” situation, calculated with a [111] pyramid of 10 Au atoms placed above the Fe atom. The pyramid is depicted with smaller grey balls in order not to obscure the molecule. (f) shows a negatively charged gas-phase FeTPP-S molecule, which resembles (e) indicating a larger charge transfer for FeTPP-S than for FeTPP-Me at contact. grown along the crystallographic [111] direction (see Methods). The apex atom is placed at 2.35 ˚ A from the Fe atom for both molecules. During the relaxation of forces on the tip apex and the molecule, the Fe atom is displaced towards the surface, until the vertical distance with the surface plane is 2.40 ˚ A. These values correspond to typical Fe-Au covalent-bond distances, and represent a strong, non-destructive interaction with the metallic tip. This simulation setup corresponds to the large measured contact conductance. Despite the similarity in molecular structure, particularly in the frontier orbitals and spin distribution calculated for the tunneling range, the contact regime shows qualitative di↵erences. The presence of the tip brings in enough electrons and a change in the electrostatic balance for the FeTPP-S molecule, while it basically leaves FeTPP-Me unaltered. Indeed, FeTPP-S takes a full electron while FeTPP-Me does not. The electron is largely placed in the C and N atoms next to Fe. In a first approximation, FeTPP-S looks much like its anion, Figs. 4e and f, where the Fe core holds a magnetic moment of 2 Bohr magnetons (S = 1)

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while the C and N rings polarize their extra electron to create an antiferromagnetic coupling. Consequently, the anion is a doublet (S = 1/2). For the FeTPP-S trapped between tip and substrate, the Fe atom captures charge (⇡ 0.2e, Table 1) reducing its magnetic moment. The charge on C and N carries significant magnetic moment, oriented opposite to the total moment, such that the molecule also becomes a doublet in contact. However, FeTPP-Me captures less charge with a consequently smaller reduction of its magnetic moment, Table 1. These results show that the valence state of the Fe ion remains unchanged regardless of the electrode configuration, despite the di↵erent charge transfer to the ligands. The ligands behave as ‘innocent’ ligands. 44 Calculations for a tip-apex centered 2.45 ˚ A above a benzene ring of FeTPP-S do not show significant changes of the molecular states except for those caused by the new constraint to the geometry of the molecule (Supporting Information, section 4). The magnetic moment (Tab. 1, last row) is hardly changed from the tunneling case, although there is a small charge transfer comparable to the one of the FeTPP-Me molecule in the contact range. These results fully agree with unaltered excitation steps observed experimentally. The main e↵ect of the tip contacting the Fe atom is to force an extra electron into the FeTPP-S molecule, reducing its spin to S = 1/2, while FeTPP-Me maintains its S = 1 configuration. This is clearly seen in the experiment, where the di↵erential conductance displays a zero-bias anomaly for FeTPP-S characteristic of a S = 1/2 Kondo e↵ect. However, FeTPP-Me maintains unaltered spin transitions at the same energy, reflecting an unchanged S = 1 configuration. The qualitative di↵erences between the molecules despite their electronic similarity can be traced back to their di↵erent electron affinities, which result from the di↵erent polarizabilities of the –OCH3 and –SO3 H substituents. We evaluate the affinity as the energy di↵erence between the ground states of the anion and the neutral molecule. As expected, the –SO3 H group is highly polarizable, more than the –OCH3 group, and the affinity of FeTPP-S is 0.87 eV larger. Consequently FeTPP-S easily captures and stabilizes one extra electron in

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Table 1: Magnetic moment (in Bohr magnetons, µB ) for the full system (µT otal ) and the Fe atom using Bader charges (µF e ), Fe charge di↵erence with respect to the gas-phase molecule ( Q, in electrons, positive values mean the adsorbed molecule is negatively charged with respect to the gas-phase one), and affinity energy as the di↵erence of total energy between the anion and the neutral gas-phase molecule. For the two first cases the contact regime is simulated by locating the pyramidal tip apex at 2.35 ˚ A above the Fe atom. On the lobes ˚ of the FeTPP-S molecule, the tip is centered 2.45 A above a benzene ring. The tunneling regime is not considered since the calculation does not include any tip in the tunneling configuration. µT otal (µB ) –OCH3 2.05 –SO3 H 1.93 –SO3 H Lobe -

Tunneling µF e (µB ) 2.28 2.15 -

Q (e) -0.013 0.012 -

µT otal (µB ) 1.67 0.99 1.82

Contact µF e (µB ) 2.00 1.54 2.08

Q (e) 0.11 0.19 0.12

Affinity (eV) -2.13 -3.00 -3.00

an environment of increased electron density. In conclusion, the tunneling conductance spectra of FeTPP-Me and FeTPP-S show magnetic excitations corresponding to a S = 1 system with easy-plane anisotropy. When the tip contacts the Fe core of the molecules, FeTPP-Me shows unchanged magnetic structure (S = 1) but FeTPP-S displays a zero-bias anomaly typical of a S = 1/2 system. The change from S = 1 to

1 2

in the case of FeTPP-S is due to more polarizable end groups resulting in

larger electron affinity, which stabilizes an electron from the electron-rich metallic environment at contact. The lower electron affinity of FeTPP-Me however preserves its spin even at contact. The main observation is that the observed Kondo features of FeTPP-S are only compatible with a S < 1 state of the molecule in good agreement with the computed value of its total spin. This work shows that a degree of control on the molecular magnetism can be achieved by judiciously choosing their ligands and their electronic environment.

Methods

Sample Preparation. Purified iron meso-tetra(4-sulfonatophenyl)porphine chloride and meso-tetra(4-methoxyphenyl)porphine chloride were dissolved separately in aqueous methanol 9


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in the presence of 1 vol% acetic acid. Singly charged molecular cations originated from the dissolution of Cl ions were electrosprayed onto clean Au(111) surfaces in ultra-high vacuum. 45 STM. Imaging and contact experiments were performed using a scanning tunneling microscope operated at ⇠ 5 K. An electrochemically etched tungsten wire —indented in situ into the substrate to get it coated with gold— was used as the tip. The di↵erential conductance spectra were recorded adding a sinusoidal modulation (2 mVrms , 1.2 kHz) to the bias. DFT. Ab-initio calculations have been performed in the framework of density functional theory (DFT) as implemented in the VASP code (details in the Supporting Information, section 1). The molecules were relaxed on a 3-layer slab with a 10 ⇥ 10 Au (111) unit cell. To model the tip, a [111] pyramid of 10 atoms was chosen. Due to the lack of coordination, the DFT electronic structure predicts magnetic moments on the edge atoms. However, when the tip contacts the molecule, the magnetism appears only in the apex atom on top of the Fe atom. This shows that, in our calculations, the pyramid is in interaction with the substrate and does not develop magnetism contrary to the free-standing pyramid. The calculation of molecular spins within DFT can be problematic. Typically spin contamination is a minor problem, 46 but the total spin is not a good quantum number in the monoconfigurational implementations of DFT. However, Sz is a good quantum number. Fixing di↵erent Sz solutions in the absence of spin-orbit coupling (hence keeping spin degeneracies), leads to di↵erent total energies, showing that the only possible ground state compatible with Sz = 1/2 corresponds to the antiferromagnetic coupling of the ligand and Fe spins.

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Supporting Information Available Density functional theory calculations; magnetic anisotropy; total spin-dependent charge on the adsorbed molecule; tip contacting FeTPP-S lobe; spatial evolution of dI/dV spectra; dI/dV spectra of molecules over a wider range of sample voltages. Acknowledgements Financial support of the Deutsche Forschungsgemeinschaft (DFG) via Sonderforschungsbereich 677 and Forschergruppe 1162, MINECO (Grant No. MAT2015-66888-C3-2-R) and FEDER funds is acknowledged.

Notes The authors declare no competing financial interests.

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Spin Control Induced by Molecular Charging in a Transport Junction

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