Institut de Chimie Moléculaire et des Matériaux d'Orsay

Spin transition nanomaterials and molecular switching on surfaces

Marie-Laure Boillot (Research Dir.), Eric Rivière (Research Engineer, 10%)

PhD student : Arthur Tauzin

The research program targets spin-transition molecular magnetic materials whose properties can be manipulated by an external stimulus (for instance light, temperature and electric field). We design, synthesize and study these important materials for applications (sensors, information storage, electronics and molecular spintronics) with the objective of controlling the processes (switching, bistability of electronic states) at reduced scales of length and involving interfaces with the environment. This work is done with physicists and theoreticians to achieve a better understanding of mechanisms and to develop innovative systems.

• Axis 1 - Spin crossover nanomaterials
• Axis 2 - Mechnism and dynamic of the spin transition
• Axis 3 - Molecular switching on surfaces

Bistable spin-transition microparticles : Influence of the surrounding medium

The study of the relationship between the cooperativity of the spin transition and the particle size shows general trends but a few puzzling observations remain unclear. The difficulty arises from the combination of size effects with other factors including the particle surface and its interaction with the surrounding environment. First, we observed that the embedding in polymers, molecular liquids or glasses of microcrystals characterized by an abrupt transition was non-innocent. The transition curves are less cooperative and shift towards lower temperature, or give rise to thermal hysteresis. The magnetic study of these cycles (coll. E. Enachescu, Univ. Iasi, Romania) demonstrated that the hysteresis loop did not result from cooperativity but from (i) particle-matrix elastic interactions (b) that were modeled by negative variable pressure effects, (ii) their cutting off (c) – restoration (a) according to the change of volume driven by the spin transition and the glass-liquid transition. At low temperature, the particle environment also influences the spin switching induced by light and the kinetics of the LIESST relaxation, observation also modeled with a variable pressure effect. The matrix being active, the question to be studied now concerns the combined effects of interface, matrix and confinement taking place in the case of spin-transition nano-objects.

First-Order Reversal Curves (FORC) analysis of the hysteresis loop revealing a matrix effect rationalized with a combination of a variable negative pressure effect and the switching on/cutting off of elastic interaction

Spin transition thin films formed by sublimation : Size reduction effect on the bistability

The effect of dimension on the physical properties of spin transition materials have been studied with thin films formed by molecular sublimation. The films, with a thickness of 130 nm and more, exhibit spin transitions with thermal hysteresis of 17 to 43 K-width depending on the increasing size of the crystallites constituting the films, transitions which are incomplete. A combination of structural, magnetic and optical measurements shows the formation of a metastable high-spin polymorph which co-crystallizes with the previously known spin-transition polymorph. By a thermal annealing of the film, this first polymorph was completely converted into the thermodynamically stable form, characterized by the spin transition with hysteresis. The phase transition process which here improves the material crystallinity makes it possible to control the opening of a remarkable hysteresis of 19 K-width in the case of the 130 nm-thick film.

AFM image of the Fe(HB(Me₂-pz)₃)₂ 130 nm-thick film and spin-crossover curves of the 130 nm- and the 8020 nm-thick films compared to the bulk powder

Spin transition nanoparticles : elastic cooperativity in the out-of-equilibrium switching induced by an ultra-short laser pulse

One challenge in materials science is to control the cooperative phase transformation occurring in solids on ultra-short time scales. Bistable spin-transition materials that may be switched with light according to the LIESST effect are good candidates for this mechanistic and dynamical study done in collaboration with physicists (E. Collet, IPR, Univ. Rennes) and theoreticians (C. Enachescu, Iasi Univ., Romania). This phenomenon takes place via the coupling between the electronic (spin multiplicity) and structural (bonds, molecular volume) change at the origin of cooperativity. The simultaneous excitation of a large number of molecules within a spin-transition material requires a femtosecond optical technique and nanocrystals. It thus generates an elastic wave capable of self-amplifying the ultra-fast photo-switching of the material. This elastically-driven cooperativity triggered by a light pulse leads to an efficient route towards the generation and stabilization of photoinduced phases in volume-changing materials.

Dynamic characterizing the low-spin to high-spin transition triggered by a femtosecond laser pulse in [Fe^III(3-MeOSalEen)₂]PF₆ in the form of single-crystal, micro- and nanocrystals

Structural dynamics and coherence upon d-d vs MLCT light-induced spin state trapping

The photo-induced spin transition (LIESST effect), like the low-temperature photomagnetism, involves a ΔS = 2 spin change which due to the selection rules takes place by intersystem crossing involving intermediate excited states and the structural relaxation of metal-ligand bonds. Mechanism and ultrafast dynamics have been studied in Fe^IIN₆ complexes under ¹MLCT excitation. To compare several excitation pathways, we have designed a Fe^IIN₄O₂ spin-crossover material of lower symmetry whose d-d transitions may also be probed. By combining femtosecond optical absorption measurements, ab-initio DFT and TD-DFT calculations, we demonstrated that shorter intermediate states enhanced coherent structural dynamics and therefore ¹d-d excitation induced faster low spin to high spin switching compared to MLCT.

Self-assembled monolayer of molecules on gold surface : spin state switching detected by Scanning Tunneling Microscopy and studied by X ray absorption Spectroscopy

Spin-crossover molecules, whose spin states switch in response to an electric field and a light irradiation, are promising for spin electronics (or integration into electronic devices). However, it is mandatory to preserve the switchability on metallic surfaces at the scale of a few molecules (or single molecule), that is scarcely observed. This question was studied with T. Mallah, Scanning Tunneling Microscopy (STM) specialists (V. Repain, MPQ, Paris-Diderot Univ.) and theoreticians (C. Barreteau, CEA Paris-Saclay), the aim being to modulate the spin-polarized current by switching the magnetic moment of a spin-crossover molecule or vice versa. The formation of self-assembled monolayer of molecules on a gold surface was exploited to analyze by STM, a local technique, and theoretical calculations, the electronic structure of molecules, its change upon light irradiation and its dynamics. Moreover X-ray Magnetic Circular Dichroism and X-ray absorption spectroscopy measurements have provided evidences for the thermal spin-crossover and the presence of S = 2 iron(II) residue at low temperature. These observations rise the question of the importance of surface-molecules interactions with respect to the molecule-molecule interactions.

Last update on Feb. 28^th 2022