Andrea Luigi
Sorrentino
ab,
Lorenzo
Poggini
*bc,
Giulia
Serrano
ab,
Giuseppe
Cucinotta
b,
Brunetto
Cortigiani
b,
Luigi
Malavolti
d,
Francesca
Parenti
e,
Edwige
Otero
f,
Marie-Anne
Arrio
g,
Philippe
Sainctavit
fg,
Andrea
Caneschi
a,
Andrea
Cornia
e,
Roberta
Sessoli
bc and
Matteo
Mannini
*b
aDepartment of Industrial Engineering - DIEF - and INSTM Research Unit, University of Florence, Via Santa Marta 3, 50139 Florence, Italy
bDepartment of Chemistry “U. Schiff” - DICUS - and INSTM Research Unit, University of Florence, Via della Lastruccia 3-13, 50019 Sesto Fiorentino, FI, Italy. E-mail: lpoggini@iccom.cnr.it; matteo.mannini@unifi.it
cInstitute for Chemistry of Organo-Metallic Compounds (ICCOM-CNR), Via Madonna del Piano, 50019 Sesto Fiorentino, FI, Italy
dMax Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany
eDepartment of Chemical and Geological Sciences and INSTM Research Unit, University of Modena and Reggio Emilia, Via G. Campi 103, 41125 Modena, Italy
fSynchrotron-SOLEIL, L'Orme des Merisiers, 91192 Saint-Aubin, France
gCNRS UMR7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université/MNHN, 4 place Jussieu, 75252 Paris Cedex 5, France
First published on 1st July 2024
The decoration of technologically relevant surfaces, such as metal oxides, with Single-Molecule Magnets (SMMs) constitutes a persistent challenge for the integration of these molecular systems into novel technologies and, in particular, for the development of spintronic and quantum devices. We used UHV thermal sublimation to deposit tetrairon(III) propeller-shaped SMMs (Fe4) as a single layer on a TiO2 ultrathin film grown on Cu(001). The properties of the molecular deposit were studied using a multi-technique approach based on standard topographic and spectroscopic measurements, which demonstrated that molecules remain largely intact upon deposition. Ultralow temperature X-ray Absorption Spectroscopy (XAS) with linearly and circularly polarized light was further employed to evaluate both the molecular organization and the magnetic properties of the Fe4 monolayer. X-ray Natural Linear Dichroism (XNLD) and X-ray Magnetic Circular Dichroism (XMCD) showed that molecules in a monolayer display a preferential orientation and an open magnetic hysteresis with pronounced quantum tunnelling steps up to 900 mK. However, unexpected extra features in the XAS and XMCD spectra disclosed a minority fraction of altered molecules, suggesting that the TiO2 film may be chemically non-innocent. The observed persistence of SMM behaviour on a metal oxide thin film opens new possibilities for the development of SMM-based hybrid systems.
The vastness of available oxide materials and their widespread use for technological applications leaves additional space for the exploration of alternative materials which can support molecular spin functionalities. In this context, TiO2 has a high technological significance thanks to its photocatalytic31,32 and electron transport properties,33,34 which are of relevance for sensing,35 catalysis, and photovoltaics.36–38 The nature of the surface plays a decisive role in promoting these functionalities, and the richness of TiO2 structural phases that are accessible by finely tuning the preparation technique makes this material particularly versatile for multifunctional electronic devices.39 In fact, TiO2 can be nanostructured as a thin film grown on metals to control its electronic properties,40–42 which depend on the structural phase, the surface stoichiometry, and the presence of defects.43–45 For instance, some of us demonstrated that Cu(001) surface is a good playground for the growth of continuous TiO2 films having different structural phases and electronic properties depending on growth condition parameters.46 In parallel, we showed that SMM behaviour of TbPc2 molecules might persist on sub-monolayer TiO2 islands with a lepidocrocite-like structure grown on Ag(100).47
Here, we investigated the chemical, structural, and magnetic properties of tetrairon(III) propeller-like SMMs (Fe4) deposited on a single layer of TiO2 grown on Cu(001) (hereafter called TiO2/Cu). The Fe4 complexes are archetypal, low-temperature SMMs with general formula [Fe4(LR)2(dpm)6], where a central FeIII ion is surrounded by three peripheral FeIII ions arranged at the vertices of a triangle. The dipivaloylmethanido ligands (dpm−) bind exclusively to peripheral ions, while the magnetic core is held together by two tripodal ligands (LR)3− = [RC(CH2O)3]3−. The bridging oxygen atoms of the tripods promote antiferromagnetic interactions between the s = 5/2 spins of the central and peripheral FeIII ions, yielding a molecular spin S = 5 ground state.48 Complexes of this family exhibit slow magnetic relaxation only below 1 K, but have good chemical stability and can be easily functionalized by proper choice of the R substituent. In this way, derivatives suitable for deposition on surfaces by either a wet-chemistry approach, electrospray, or thermal sublimation in UHV have been designed.13,14,16,49–54 Recently, a Fe4 derivative with R = CH2SMe (hereafter called Fe4SMe) was sublimated on Pb(111), showing great chemical stability and enhanced organization on the surface promoted by the short S-functionalized tethering group.16 This derivative was indeed originally designed for deposition on metal surfaces (e.g., Au, Pb) to form a well-ordered and assembled monolayer. Although it is not specifically tailored for TiO2 substrates, it exhibits superior stability during thermal sublimation and improved magnetic behavior compared to all other Fe4 compounds.51,55
In this paper, we deposited Fe4SMe molecules on the TiO2/Cu surface and used both STM and X-ray Photoelectron Spectroscopy (XPS) to check their chemical and structural integrity after deposition. Furthermore, we used synchrotron-based X-ray Absorption Spectroscopy (XAS) methods, namely X-ray Natural Linear Dichroism (XNLD) and X-ray Magnetic Circular Dichroism (XMCD), to probe their electronic structure and magnetic properties. This spectroscopic study showed that the main fraction of Fe4SMe complexes are intact and feature magnetic hysteresis and QTM up to 900 mK, while a minority fraction of molecules contain reduced FeII metal centers and become paramagnetic.
![]() | ||
Fig. 1 (a) Ti 2p XPS spectrum of the TiO2 single layer grown on Cu(001) single crystal. (b) STM image of the TiO2/Cu surface at RT (V = 1.5 V, It = 150 pA). |
STM images at Room Temperature (RT) were acquired to evaluate the film morphology and to confirm the TiO2 coverage estimated by XPS. The wide-area image in Fig. 1b shows a complete layer of TiO2, characterized by domain boundaries that suggest the presence of the quasi-hexagonal (QH) structure with squared bright spot ascribable to the growth of a second TiO2 layer.46 The formation of the single layer of TiO2-QH grown on Cu(001) structure is confirmed by Low Energy Electron Diffraction (LEED) showing two p(2 × 7) domains rotated by 30 degrees (Fig. S2†).46,65,66
A monolayer of Fe4SMe was deposited on TiO2/Cu following the protocol described in Methods. After molecular deposition, the XPS spectrum in the Ti 2p region (Fig. S3†) shows no significant variation as compared with that of the pristine substrate, thus confirming the stability of the TiO2 surface upon further processing.47,64,67 The small decrease (0.3%) of the TiIII fraction lies well within the limits of semiquantitative analysis by XPS (Table S1†). The C 1s spectrum reported in Fig. 2a reveals two main contributions attributed to the aliphatic and oxygen-bound carbon atoms of the molecular layer at 284.9 and 286.5 eV (Fig. 2a, in yellow and dark blue, respectively). An additional shake-up component is present at 289.6 eV (Fig. 2a, in cyan).16,68 The S 2p signal consists in two contributions located at 163.7 eV and at 164.9 eV (spin–orbit coupled component) (Fig. 2b), indicating the presence of intact CH2SMe functional groups.69–71 The C/S atomic ratio (46.0 ± 2.3) is in gross agreement with the expected value of 39 based on the molecular formula (C78H136Fe4O18S2), and suggests the overall integrity of Fe4SMe molecules on the TiO2/Cu surface. Unfortunately, a more comprehensive semiquantitative elemental analysis of the molecular species cannot be carried out due to the overlap of the molecular Fe 2p and O 1s signals with the Cu LMM Auger peaks and the oxygen signals, respectively, of the TiO2/Cu substrate.46,72 However, successful sublimation of Fe4SMe molecules has been demonstrated in a previous study.16
STM images of the TiO2/Cu surface decorated with Fe4SMe molecules acquired at 30 K and different magnifications are displayed in Fig. 2c and d. An almost complete coverage of the surface by Fe4SMe molecules is clearly visible. Molecules have a quasi-spherical shape but do not form densely packed and ordered islands, as observed on metals.16 Uncovered parts of the TiO2 surface and a few additional molecules sitting on top of the molecular film are visible, confirming that the desired monolayer coverage was reached. The statistical height distribution in the Fe4SMe monolayer is shown in Fig. S4† and provides a medium height of 0.65 ± 0.06 nm. This value is slightly lower than that observed for the same Fe4 derivative on Pb(111)16 and for the related complex with R = Ph (Fe4Ph) on Au(111)51 and Cu2N22 (∼0.8 nm). Such a slightly reduced molecular height could be attributed either to a different molecular orientation or to a stronger molecule/surface interaction, which would be consistent with the absence of dense and ordered molecular packing. Furthermore, within the limits of our STM investigation, the presence of molecular fragments can be excluded, confirming the enhanced stability of Fe4SMe16vs. Fe4Ph51,55 upon sublimation.
From the STM image at higher magnification (Fig. 2d) we estimated a lateral dimension of 1.80 ± 0.15 nm for individual molecules (Fig. S5†), a value in close agreement with the X-ray structure (1.7 nm) and consistent with literature data for Fe4 complexes on other surfaces.22,48,51 Additionally, the STM image in Fig. 2d evidences the internal resolution of single Fe4 units, with a triangle of bright features separated by 0.58 ± 0.06 nm (see line profile in Fig. S5a and S5b†). These features are similar to those observed on Pb(111)16 and can be attributed to the –CH2SMe group pointing out of the plane, a tert-butyl group of the topmost dpm− ligand, and the surrounding envelope of dpm− ligands.
The electronic and magnetic properties of the Fe4SMe deposit were investigated by synchrotron radiation at the DEIMOS beamline (SOLEIL, France) using a dilution cryostat to reach sub-kelvin temperatures.73 These experiments were carried out with linearly and circularly polarized light (Fig. 3) by monitoring the absorption at the Ti L2,3 and Fe L2,3 edges in the Total Electron Yield detection mode (TEY). Experiments were performed between 220 and 900 mK and in magnetic fields up to 30 kOe (see Methods). The XAS profile at the Ti L2,3 edges confirms the QH structure of the TiO2 monolayer deposited on Cu (see ESI and Fig. S6† for additional details). The XAS spectrum at the Fe L3 edge exhibits a fine structure with two main signals at 707.6 and 709.2 eV (Fig. 3a). We notice that the first peak has an additional shoulder approximately centered at 707.1 eV and marked with a green arrow in Fig. 3a. This extra XAS component was not detected in our previous investigations of Fe4 arrays at surfaces.16,51,74,75 Significantly, it was absent in monolayers of the same Fe4SMe complex prepared using identical thermal sublimation conditions but a different substrate.16 Therefore, the thermal sublimation process is unlikely to be responsible for this feature.55 Radiation damage is ruled out as a possible cause of the XAS profile alteration, since there is no evidence of time evolution of this feature under X-ray irradiation.71,76,77 A plausible explanation is the partial reduction of molecules interacting with TiIII sites on the TiO2 surface to give a minority fraction of FeII containing species55,64,78,79 and this aspect will be the subject of further discussion later.
The XNLD contribution (Fig. 3a) was extracted from the XAS signals recorded using linearly polarized X-rays with propagation vector directed at θ = 45° from the surface normal (see Methods and inset in Fig. 3a). The overall XNLD profile is essentially in line with that reported earlier for other Fe4 derivatives with short tethering groups.14,16,75 From the maximum XNLD amplitude, which reaches 6% of the average isotropic XAS signal, we conclude that the extent of preferential orientation on the surface is similar to that found for the same complex on Pb(111).16
The XMCD profile (Fig. 3b), obtained from the XAS signals recorded using circularly polarized X-rays at 220 mK and θ = 0, is dominated by two negative dichroic signals at the L3 edge. Although the position of the two XMCD minima (707.6 and 709.2 eV, β and α, respectively) agrees with that typically observed in Fe4 complexes, the exact shape and amplitude of the XMCD profile are slightly different. In the ground S = 5 state of Fe4 complexes, the opposing contribution of the central FeIII spin results in three important spectral fingerprints:16,48,51,74,75,80,81 (i) an intensity ratio of ca. 0.30 between the two XMCD minima at the L3 edge, (ii) saddle point between the two intense negative signals (708.3 eV, γ), and (iii) a maximum XMCD amplitude amounting to ca. 40% of the average isotropic XAS signal. In Fig. 3b, the intensity ratio between the two XMCD minima at the L3 edge is 0.55, hence higher than typically found in Fe4 complexes. In addition, the XMCD signal at 708.3 eV (−8%) remains significantly different from zero, and the normalized XMCD intensity at 709.2 eV is as large as 47%. It is worth stressing that heteronuclear CrIII-centered Fe3Cr complexes, in which the opposing spin contribution of the central FeIII ion is absent, also exhibit a non-zero XMCD signal at 708.3 eV and a much increased XMCD amplitude at 709.2 eV.74,80 Here, the Fe4 spectral fingerprints are presumably partly obscured by the surface-induced reduction of some FeIII centers to FeII.
To support this interpretation of the XMCD fine structure, we performed Ligand Field Multiplet (LFM) calculations. Starting from our previous knowledge of the XAS and XMCD signals for pristine Fe4 molecules, we replaced FeIII with variable amounts of FeII in the calculations. We reached a good agreement with the measured XAS spectra assuming that 30% of the Fe centers are reduced to FeII. With this percentage, the ratio between the first peak at 707.6 eV and the main peak at 709.2 eV in the XAS plots is nicely reproduced (Fig. S7†). In order to determine the speciation of FeII, we examined various situations where the amount of FeII was fixed to 30% of the total Fe ion content. It should be underlined that if one supposes that all the FeII ions are present as Fe oxyhydroxides at the TiO2 surface, the XMCD signal at 220 mK and 30 kOe would be much larger than measured. Thus, one can exclude this situation as the main location for FeII ions and suppose that FeII ions primarily occur in the Fe4 molecular structure. There are only five distinct distributions of oxidation states which are compatible with a 30% concentration of FeII ions, as fully detailed in the ESI (see Fig. S7†). Among these five different distributions, the one with 70% of pristine Fe4 molecules and 30% of FeII4 molecules yields by far the best agreement with the measured XAS and XMCD signals. Note that in the present fit we always considered that the central ion, whether it is an FeII or an FeIII ion, is coupled antiferromagnetically with the three peripheral Fe ions (see ESI† for the precise values of the FeII LFM parameters). The resulting simulation features an intensity ratio of ≈0.51 between the two XMCD minima at the L3 edge, a normalized XMCD intensity of ≈50% at 709.2 eV, and a negative XMCD signal of −4% at 708.3 eV. The three above features rather closely match the experimental spectra, strongly supporting the presence of a fraction of FeII ions in the molecular deposit, which however primarily comprises intact Fe4SMe molecules. An additional and strong indication that the monolayer contains a fraction of altered molecules was extracted by monitoring the magnetic field dependence of the XMCD signal at different energies to selectively address the magnetic behaviour of the different species on the surface. We expect that intact Fe4SMe molecules mainly contribute to the XMCD signal at 709.2 eV, and not at 708.3 eV. The temperature dependence of XMCD at 709.2 eV (α, Fig. 4a) mirrors the typical magnetic behaviour of Fe4 SMMs, whose hysteresis loop is open below 1 K and becomes wider with decreasing temperature.48 In particular, the sharp magnetization steps at 0 and 5 kOe show that Fe4SMe undergoes resonant QTM on TiO2/Cu, similarly to what has been observed on Pb(111).16 Additionally, in agreement with XNLD evidence, the angular-dependent experiment at 220 mK (Fig. 4b) confirms that the complex is preferentially oriented with the easy axis close to the surface normal. As the incidence angle θ increases, saturation is reached more slowly and the resonant condition for QTM at a nonzero field broadens and shifts to higher fields.1451
When the photon energy is decreased to 708.3 eV (γ), the intensity of the XMCD signal at 220 mK decreases considerably, primarily due to the contribution of the reduced species only (Fig. 4c). Crucially, its field dependence is markedly different from that of Fig. 4b, evidencing a closed hysteresis loop. We attribute this paramagnetic response at 708.3 eV to magnetic species containing FeII centres resulting from surface-induced reduction but still embedded in a Fe4-like structure, in accordance with STM measurements and LFM calculations. For the sake of completeness, setting the photon energy at 707.6 eV (β) yields a field-dependent XMCD signal with intermediate characteristics as compared with those recorded at 709.2 (α) and 708.3 (γ) eV. In this case, the hysteresis loop is still detectable but smeared out due to the additional paramagnetic contribution.
To summarize, our combined spectroscopic studies by XAS, XNLD, and XMCD converge in indicating that Fe4SMe monolayers on TiO2/Cu contain a dominant fraction of intact molecules and a minor fraction of molecules undergoing significant modification. XAS spectra suggest that such a modification consists of a reduction of FeIII centers to FeII. Theoretical calculations already predicted a similar effect for atoms on TiO2.82 A strong molecule–surface interaction was also observed when mono and bis-phthalocyaninato complexes are deposited on TiO2,64,79,83–86 leading in extreme circumstances to surface–molecule charge transfer processes.64,86–88 The latter could be favoured by TiO2 reactive sites, such as oxygen vacancies.89 We can tentatively associate the partial modification of Fe4SMe complexes on TiO2/Cu with the presence of TiIII active sites on the surface. Considering the overwhelming number of TiIII atoms compared to Fe4SMe molecules, such a hypothesis does not contradict the observation that the amount of TiIII remains substantially unaltered after Fe4SMe deposition (Fig. S3 and Table S1†).
The electronic and magnetic characterization of the Fe4SMe complex on TiO2/Cu was carried out at the DEIMOS beamline (SOLEIL synchrotron).73,90 The XNLD was extracted as the difference between the cross sections recorded using vertically (σV) and horizontally (σH) polarized light (σV − σH) at θ = 45°, H = 30 kOe, and T = 220 mK. The XNLD contribution was normalized with respect to the L3 edge jump of the isotropic spectrum (1/3σV + 2/3σH) and expressed as percentage (XNLD%). Analogously, XMCD was obtained as the difference between the XAS spectra measured using negative (σ−) and positive (σ+) circular light polarizations (σ− − σ+) at θ = 0, H = 30 kOe, and T = 220 mK. The dichroic signal was normalized with respect to the L3 edge jump of (σ+ + σ−)/2 and expressed as percentage (XMCD%). The magnetic hysteresis measurements at specific photon energies were made at θ = 0 or 45° by cycling the magnetic field between −15 kOe and 15 kOe with a scan rate of 0.2 kOe s−1 and working at temperatures from 220 to 900 mK. All the samples were prepared in Florence and transferred to the beamline employing a home-made suitcase equipped with a D100 SAES Nextorr Neg-Ion Combination Pump that guarantees a pressure P < 10−10 mbar during the transport. All the steps from sample preparation to synchrotron measurements were accomplished without breaking the vacuum connection (P < 10−9 mbar).16 Data analysis was performed using pyDichroX software.91
The XAS and XMCD spectra were calculated within the Ligand Field Multiplet (LFM) theory using Quanty92 (see ESI† for details).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr02234c |
This journal is © The Royal Society of Chemistry 2024 |