Temperature-, Light-, and Soft X-ray-Induced Spin Crossover in a

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Temperature, Light and Soft X-ray Induced Spin Crossover in a Single Layer of Fe-Pyrazolylborate Molecules in Direct Contact with Gold II

Kaushik Bairagi, Amandine Bellec, Cynthia Fourmental, Olga Iasco, Jérôme Lagoute, Cyril Chacon, Yann Girard, Sylvie Rousset, Fadi Choueikani, Edwige Otero, Philippe Ohresser, Philippe Sainctavit, Marie-Laure Boillot, Talal Mallah, and Vincent Repain J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11874 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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The Journal of Physical Chemistry

Temperature, Light and Soft X-ray Induced Spin Crossover in a Single Layer of FeII-pyrazolylborate Molecules in Direct Contact with Gold Kaushik Bairagi,†,⊥ Amandine Bellec,∗,† Cynthia Fourmental,†,‡ Olga Iasco,¶ Jérôme Lagoute,† Cyril Chacon,† Yann Girard,† Sylvie Rousset,† Fadi Choueikani,§ Edwige Otero,§ Philippe Ohresser,§ Philippe Sainctavit,k Marie-Laure Boillot,¶ Talal Mallah,¶ and Vincent Repain† †Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot, Sorbonne Paris Cité, CNRS, UMR 7162, 75013 Paris, France. ‡Synchrotron SOLEIL,L’orme des Merisiers,Saint-Aubin-BP48,91192 Gif-sur-Yvette Cedex, France ¶Institut de Chimie Moléculaire et des Matériaux d’Orsay, Université Paris Sud, Université Paris Saclay, CNRS, UMR 8182, 91405 Orsay cedex, France. §Synchrotron SOLEIL,L’orme des Merisiers,Saint-Aubin-BP48,91192 GIF-sur-YVETTE CEDEX, France kInstitut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS, Université Pierre et Marie Curie, UMR 7590, Paris, France ⊥Current address: CIC nanoGUNE Consolider Tolosa Hiribidea 76 , 20018 Donostia-San Sebastian, Spain E-mail: [email protected]

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Abstract Understanding the properties of spin-crossover molecules in direct contact with metals is crucial for their future integration in electronic and spintronic devices. Here, by xray absorption spectroscopy, we investigate the properties of FeII ((3, 5-(CH3 )2 Pz)3 BH)2 molecules in the form of monolayer islands on a metallic substrate, namely Au(111). We demonstrate that the spin crossover transition can be thermally induced from the high spin state to a mixed spin state phase containing one third of high spin state and two third of low spin state molecules in agreement with previous work by scanning tunneling microscopy. In addition, at 4.6 K, the spin crossover from the low spin state to the high spin state can also be induced by x-ray and by light excitations.

Introduction Spin crossover (SCO) molecules present the remarkable property to have two spin states that can be manipulated by external stimuli such as temperature, light or pressure. 1–3 They thus are promising building blocks for molecular electronics and spintronics. 4–6 Even if the behavior of SCO molecules is well documented as bulk material, their incorporation in devices implies to study their properties in the form of nanoparticules 7–9 or thin films 10–12 and down to single layers. The size reduction together with the effect of their environment may drastically impact their transition temperature and the width of their thermal hysteresis. 13,14 As demonstrated recently at such scale, devices including a thin layer of SCO (thicknesses from 10 to 100 nm) between two metallic electrodes show a change in electrical conduction depending on the molecule spin state. 15 Ultimately, the SCO properties have to be understood down to the interface with the electrodes, i.e. in the form of monolayer or single molecule in direct contact with a substrate. Recent studies by scanning tunneling microscopy (STM) investigated the properties of SCO molecules on various substrates in the form of mono- and bilayers 16–18 or single molecules. 19–22 The possibility to control the molecular spin state in a bilayer 18 or at the 2


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level of single molecules 17,19 has been demonstrated once the molecules are decoupled from the metallic substrate. Complementary, x-ray absorption spectroscopy (XAS) is a unique tool to investigate the magnetic properties of spin crossover molecules. It is thus possible to follow the thermal transition or the light induced excited spin state trapping (LIESST) of molecules from powder state 23 down to a few monolayers and sub-monolayer coverages, 24–30 which provide valuable information concerning the size and environment effects on their physical behavior. In our present study we focus on the FeII ((3, 5-(CH3 )2 Pz)3 BH)2 molecule (inset of Figure 1a) which is suitable for sublimation under ultra high vacuum (UHV) conditions. In the form of a powder, the molecules present a complete thermal SCO with an asymmetric hysteresis, transition temperatures Tdown and Tup of c.a. 174 K and c.a. 205 K respectively and a width of c.a. 31 K. 31,32 In the form of thin films on SiO2 /Si 31 or on quartz 32 the thermal hysteresis is preserved even for film thicknesses of a hundred nm. 32 XAS experiments on such thin films demonstrate that part of the molecules are pinned in HS state at low temperature which was attributed to the interaction of the molecules with the substrate. 31 However, recent studies using complementary optical and magnetic measurements on films with thicknesses ranging from a hundred to thousand nm demonstrate that the residual HS proportion at low temperature is due to a metastable phase which can be transformed into the stable one by annealing. 32 Focusing on monolayer samples, some of us showed by STM that for sub-monolayer deposition, the molecules self-assembled on Au(111) in a stable long-range-ordered mixed spin-state phase at 4.6 K. 33 Here we report a XAS study on a sub-monolayer of FeII ((3, 5-(CH3 )2 Pz)3 BH)2 molecules in direct contact with a metallic Au(111) substrate. The results confirm the presence of the mixed phase of HS and LS below 100 K with one third of the molecules in HS state as evidence by the STM. Moreover, the thermal spin crossover is evidenced and at 4.4 K, the SCO process can also be induced by x-ray and light.

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Methods The Au(111) single crystal was first cleaned by sputtering (Ar+ , 1.2 keV) and annealing (400 ◦ C) cycles (base pressure: 10−10 mbar). The molecules were sublimated on Au(111) at a temperature of 90 ◦ C from a homemade evaporator (effusion cell type). To obtain a submonolayer deposition, we reproduced the experimental conditions (sample-crucible distance, sublimation temperature and deposition time) used in our previous work by STM. 33 In addition, the molecular deposition on the Au(111) was monitored by a quartz balance to have a constant flux from which we can estimate a coverage around 0.2 ML. Finally, the submonolayer coverage is confirmed by comparing the L3 edge jump on our sample to the one on a molecular thin layer (130 nm) grown on a Si wafer. From the ratio of the L3 edge jumps in the thin film sample and sub-monolayer sample and the distance between molecular layers in the molecular film, we can estimate an approximate coverage between 0.1 and 0.3 ML of the FeII ((3, 5-(CH3 )2 Pz)3 BH)2 compound on Au(111). These estimations, detailed in the SI, give strong evidence that the molecular coverage is less than one monolayer. All the experiments have been realized by XAS and x-ray magnetic circular dichroism (XMCD) on the DEIMOS beamline at SOLEIL Synchrotron. 34 The technique is elementspecific and enables to precisely distinguish at the Fe L2,3 edges the HS and the LS state of the molecules even for a sub-monolayer coverage. 25,27 The x-ray beam (800 × 800 µm2 ) enables us to have an average measurement over several molecular islands on the contrary of the STM measurements realized inside a given island. The XAS spectra were acquired in total electron yield mode. The typical photon flux density during our experiments was 108 photons.sec−1 .mm−2 . The acquisition time for a whole L2,3 edge XAS spectrum has been reduced to the minimum that can be afforded on DEIMOS, namely three minutes in order to reduce the influence of the x-ray beam. 35 Thus above 70 K, no SOXIESST effect was observed.

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Results and discussion XAS spectra are recorded at the Fe L2,3 edges for different temperatures after thermalisation of the sample while cooling down (for each temperature the thermalisation is reached in about 30 minutes). Figure 1.a presents typical spectra obtained at 300 K (black line) and 80 K (red line). The spectrum at 300 K presents one peak at 709.9 eV (L3 edge) and one at 722.5 eV (L2 edge) that are characteristic of the HS state of the compound. Decreasing the temperature to 80 K leads to the intensity decrease of the peaks at 709.9 eV and at 722.5 eV to the profit of higher energy peaks (711.1 eV and 723 eV for the L3 and L2 edges respectively). This is the signature of the molecular SCO from the HS to the LS state. Nonetheless, the spectrum at 80 K still presents a residual peak at 709.9 eV which implies that the transition is not complete and that a mixture of both spin states is present. To access the proportions of molecules in HS and LS we simulate the LS spectrum by the linear combination of the spectra at 300 K and 80 K (see SI for details). To avoid any possible soft x-ray induced excited spin state trapping (SOXIESST) effect, 36 the spectrum at 80 K was taken at a virgin spot. The simulated LS spectrum is presented in Figure 1.a in gray line. Determining the HS proportion from this simulated spectrum introduced a systematic error of 6% (see SI). We thus determine a HS proportion of 32% at 80 K. This value is in agreement with the STM experiments that we performed at 4.6 K. 33 Moreover, new STM experiments realized at 78 K (see inset of Figure 1.b) show that the ordered lattice of one molecule in HS state and two in LS state is identical to the one at 4.6 K. The HS proportions for intermediate temperatures (see SI for the spectra) are thus extracted from the linear combination of the HS and the simulated LS spectra. The error bars on xHS account for the HS fluctuations between the first and the last measured spectra at a given temperature. As visible in Figure 1.b (red down triangles), a Tdown of 150 ± 1 K is determined for the cooling branch (average speed around 0.2 K min−1 ). Here the transition temperature is defined when 2/3 of the molecules are in HS state what corresponds to half conversion 5



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Figure 1: Temperature induced spin crossover (a) Normalized XAS spectra of submonolayer of FeII ((3, 5-(CH3 )2 Pz)3 BH)2 on Au(111) at 300 K (HS), 80 K and the simulated LS spectrum obtained from a linear combination of the spectra at 300 K and 80 K (see SI for details). Inset Molecular structure. The C atoms are in gray, the B in pink, the Fe in red and the H in white. (b) HS proportion as a function of temperature for the sub-monolayer of FeII ((3, 5-(CH3 )2 Pz)3 BH)2 (for both cooling and warming branches) obtained from the linear combination of the HS and simulated LS spectra. The dashed line marks the 33 % value of HS proportion. Inset presents a 5×5 nm2 topographic STM image of sub-monolayer of FeII ((3, 5-(CH3 )2 Pz)3 BH)2 on Au(111) acquired at V=0.3 V (I = 50 pA) at 78 K. Only one molecule over three is appearing as a bright spot (molecule in HS state) at this bias voltage.

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between the fully HS spin phase at room temperature and the mixed spin state phase at 80 K (the temperature error is the maximal standard deviation measured on the thermocouple). XAS spectra were also recorded while increasing the temperature after the experiments described below (black up triangles in Figure 1.b). Going back to room temperature, the system recovers its full HS state which demonstrates that the SCO process is reversible. A Tup value of 157 ± 1 K is determined for the warming branch (average speed around 1 K min−1 ). The transition takes place in the same temperature range compared to thin films - 130 nm 32 - but is smoother. Our results evidence the presence of a hysteresis. Nonetheless, complementary measurements are needed to confirm and understand the origin of this hysteresis. Especially to know if it is related to a structural transition or if it is the signature of cooperativity in the molecular monolayer. At low temperature, soft x-ray and visible light can induce the molecular SCO from LS to HS state known as the SOXIESST 36 and the LIESST effects, respectively. We thus cooled down the sample to 4.4 K to study the effect of x-ray and light. Figure 2.a reports the evolution of the XAS spectrum at the L3 edge due to exposure to soft x-ray. The black curve has been recorded on a pristine spot before any exposure to X-ray and the red curve were acquired 96 min later with the sample under constant exposure. One can clearly see that the peak at 711.1 eV decreases to the profit of the peak at 709.9 eV. This indicates that the LS to HS conversion can be induced on gold surface by a SOXIESST effect. The spectra was saturated after 72 minutes and no evolution was observed during the next 24 minutes meaning a saturated SOXIESST state was reached with a proportion of molecules in the HS spin state of 63 % (see SI for details on the dynamics). We have taken XMCD measurements under high magnetic field (± 6.5 T) after the saturated SOXIESST state is reached. We observed a XMCD signal which confirms that part of the molecules are in HS state (see SI). The LIESST effect while illuminating with a blue laser (405 nm, 20 mW) has been investigated at 4.4 K. On a second spot the spectra were only recorded from time to time during the laser irradiation thus illuminating the spot only by the blue laser and not by the

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an

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Figure 2: Soft x-ray and light induced spin crossover. Normalized XAS spectra at 4.4K (a) before (black curve) and after (red curve) a continuous exposure to X-ray during 96 minutes and (b) before (black curve) and after (red curve) laser irradiation during 216 minutes (405 nm).

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x-ray beam to avoid any further SOXIESST effect. As reported in Figure 2.b the initial state (black curve) was partially affected by x-ray (42% HS). The spectrum acquired after 216 minutes of laser irradiation shows the conversion from LS to HS under light up to a value of 69 % of molecules in HS state (the stationary state isn’t reached here, see SI). This confirms that the SCO can be induced by light for molecules in direct contact with a metallic substrate. 25,33 100

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Figure 3: Soft x-ray and light excitation and thermal relaxation. HS proportion as a function of the temperature while cooling down the sample from 300 K to 4.4 K (red down triangles), exposing it to soft x-ray (eight green points) and light (three blue squares) at 4.4 K and warming it up to 250 K (black up triangles).

Figure 3 summarizes the evolution of the HS proportion while cooling the sample from 300 K to 4.4 K, then under x-ray and light exposure and finally while warming it up from 4.4 K to 250 K. As one can see, below 80 K the HS proportion is already affected by the SOXIESST effect. Indeed, if the HS proportion is 32 % at 80 K it increases up to 40 % at 40 K. As detailed previously, at 4.4 K the LS to HS conversion can be induced by SOXIESST (green points) or LIESST (blue squares) effects. Exposing a given spot first to x-ray then to light enables us to obtain up to 80 % of the molecules in HS state. But even though, it was not possible to obtain a full conversion to the HS state. In our previous study by STM, we evidenced that the photoexcited state is also a mixed spin state phase with only one half of 9



the molecules in HS. 33 In both cases (STM and XAS experiments), the laser intensity was of the order of a few mW.cm−2 . But, the main difference between both setups is that for the STM experiments the illumination is realized in the near field regime while for the XAS experiments it is in the far field regime. This might explain why the LS to HS conversion is more efficient in the reported case than under the STM tip. Increasing the temperature from the x-ray and light excited phase leads to the relaxation of the system evidenced by the decrease of the HS proportion. Indeed, we observe that after thermal relaxation the HS proportion becomes 29% at 80 K. This prove that the x-ray as well as the blue light excitation is reversible and that the molecules are very little affected. Further increase of temperature induces the SCO from LS to HS as presented above in Figure 1.

Conclusions In conclusion, we investigated the magnetic properties of sub-monolayers of FeII ((3, 5(CH3 )2 Pz)3 BH)2 molecules adsorbed on a metallic substrate. With the XAS and XMCD measurements, we confirmed that at low temperature the molecules arranged in a mixed spin-state phase with one third of the molecules in HS state. 33 On the contrary of thin films with thicknesses of hundred to few thousand nm that are presenting a complete SCO once annealed, 32 the sub-monolayer deposition on Au(111) presents a unique long-range ordered mixed spin-state phase. We have also demonstrated the thermal, soft x-ray and light induced SCO of the molecules. The thermal relaxation to the initial state proves that the molecular properties are kept on the metallic substrate and that mixed spin-state phase is robust. Manipulating molecules in direct contact with a metal is one of the key point for the development of devices.

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Supporting Information Available Estimation of the molecular coverage, Determination of the low spin spectrum, XAS spectra during the thermal transition, Dynamics of the SOXIESST and LIESST effects, X-ray magnetic circular dichroism spectrum of the excited phase.

Acknowledgement The authors thank the french national research agency ANR (ANR-BLANC-12 BS10006, SPIROU project) and the labex SEAM (ANR-11-LABX-086, HEFOR project) for support, DEIMOS staff for setting the beamline and helping during the beamtime and Eric Rivière for setting the blue laser.

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(29) Kipgen, L.; Bernien, M.; Nickel, F.; Naggert, H.; Britton, A. J.; Arruda, L. M.; Schierle, E.; Weschke, E.; Tuczek, F.; Kuch, W. Soft-x-ray-induced spin-state switching of an adsorbed Fe(II) spin-crossover complex. Journal of Physics: Condensed Matter 2017, 29, 394003. (30) Ossinger, S.; Naggert, H.; Kipgen, L.; Jasper-Toennies, T.; Rai, A.; Rudnik, J.; Nickel, F.; Arruda, L. M.; Bernien, M.; Kuch, W. et al. Vacuum-Evaporable SpinCrossover Complexes in Direct Contact with a Solid Surface: Bismuth versus Gold. The Journal of Physical Chemistry C 2017, 121, 1210–1219. (31) Davesne, V.; Gruber, M.; Studniarek, M.; Doh, W. H.; Zafeiratos, S.; Joly, L.; Sirotti, F.; Silly, M. G.; Gaspar, A. B.; Real, J. A. et al. Hysteresis and change of transition temperature in thin films of Fe[Me2 Pyrz]3 BH2 , a new sublimable spin-crossover molecule. The Journal of Chemical Physics 2015, 142, 194702. (32) Iasco, O.; Boillot, M.-L.; Bellec, A.; Guillot, R.; Riviere, E.; Mazerat, S.; Nowak, S.; Morineau, D.; Brosseau, A.; Miserque, F. et al. The disentangling of hysteretic spin transition, polymorphism and metastability in bistable thin films formed by sublimation of bis(scorpionate) Fe(II) molecules. Journal of Materials Chemistry C 2017, 5, 11067– 11075. (33) Bairagi, K.; Iasco, O.; Bellec, A.; Kartsev, A.; Li, D.; Lagoute, J.; Chacon, C.; Girard, Y.; Rousset, S.; Miserque, F. et al. Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer. Nat Commun 2016, 7, 12212. (34) Ohresser, P.; Otero, E.; Choueikani, F.; Chen, K.; Stanescu, S.; Deschamps, F.; Moreno, T.; Polack, F.; Lagarde, B.; Daguerre, J.-P. et al. DEIMOS: A beamline dedicated to dichroism measurements in the 350-2500 eV energy range. Review of Scientific Instruments 2014, 85, 013106. (35) Joly, L.; Otero, E.; Choueikani, F.; Marteau, F.; Chapuis, L.; Ohresser, P. Fast contin15



uous energy scan with dynamic coupling of the monochromator and undulator at the DEIMOS beamline. Journal of Synchrotron Radiation 2014, 21, 502–506. (36) Collison, D.; David Garner, C.; M. McGrath, C.; Frederick W. Mosselmans, J.; D. Roper, M.; M. W. Seddon, J.; Sinn, E.; A. Young, N. Soft X-ray induced excited spin state trapping and soft X-ray photochemistry at the iron L2,3 edge in [Fe(phen)2 (NCS)2 ] and [Fe(phen)2 (NCSe)2 ] (phen = 1,10-phenanthroline). J. Chem. Soc., Dalton Trans. 1997, 4371–4376.

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Graphical TOC Entry Temperature

LS

HS HS HS HS HS HS

XAS

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


HS LS

HS LS LS LS

HS HS HS 710

720

Energy (eV)

17

LS HS

730


Temperature-, Light-, and Soft X-ray-Induced Spin Crossover in a

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