Sylvain Rat

Current Switching Coupled to Molecular Spin-States in Large-Area Junctions

The fabrication of large-area vertical junctions with a molecular spin-crossover complex displaying concerted changes of spin degrees of freedom and charge-transport properties is reported. Fabricated devices allow spin-state switching in the spin-crossover layer to be triggered and probed by optical means, while detecting associated changes in electrical resistance in the junctions.

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

Nanoscale spin crossover materials capable of undergoing reversible switching between two electronic configurations with markedly different physical properties are excellent candidates for various technological applications. In particular, they can serve as active materials for storing and processing information in photonic, mechanical, electronic, and spintronic devices as well as for transducing different forms of energy in sensors and actuators. In this progress report, a brief overview on the current state-of-the-art of experimental and theoretical studies of nanomaterials displaying spin transition is presented. Based on these results, a detailed analysis and discussions in terms of finite size effects and other phenomena inherent to the reduced size scale are provided. Finally, recent research devices using spin crossover complexes are highlighted, emphasizing both challenges and prospects.

A Bistable Microelectromechanical System Actuated by Spin-Crossover Molecules

We report on a bistable MEMS device actuated by spin-crossover molecules. The device consists of a freestanding silicon microcantilever with an integrated piezoresistive detection system, which was coated with a 140 nm thick film of the [Fe(HB(tz)3 )2 ] (tz=1,2,4-triazol-1-yl) molecular spin-crossover complex. Switching from the low-spin to the high-spin state of the ferrous ions at 338 K led to a reversible upward bending of the cantilever in agreement with the change in the lattice parameters of the complex. The strong mechanical coupling was also evidenced by the decrease of approximately 66 Hz in the resonance frequency in the high-spin state as well as by the drop in the quality factor around the spin transition.

Vacuum deposition of high-quality thin films displaying spin transition near room temperature

We report on [Fe(HB(tz)3)2] (tz = triazolyl) (1) thin films with thicknesses in the range of 20–200 nm, which were thermally evaporated on fused silica substrates. Using X-ray diffraction, Raman spectroscopy, UV spectrophotometry, magnetometry and atomic force microscopy, we show that the as-deposited amorphous films can be recrystallized by means of solvent–vapour annealing. The resulting crystalline films are dense, homogenous, highly oriented (with the orthorhombic c-axis normal to the substrate) and exhibit an abrupt and fully complete spin transition around 338 K for each film thickness. The films show stable morphology and spin crossover properties upon thermal cycling and also upon long-term storage in ambient air providing appealing prospects for possible applications in a range of nanoscale devices.

Microelectromechanical systems integrating molecular spin crossover actuators

Silicon MEMS cantilevers coated with a 200 nm thin layer of the molecular spin crossover complex [Fe(H2B(pz)2)2(phen)] (H2B(pz)2 = dihydrobis(pyrazolyl)borate and phen = 1,10-phenantroline) were actuated using an external magnetic field and their resonance frequency was tracked by means of integrated piezoresistive detection. The light-induced spin-state switching of the molecules from the ground low spin to the metastable high spin state at 10 K led to a well-reproducible shift of the cantilevers resonance frequency (Δfr = −0.52 Hz). Control experiments at different temperatures using coated as well as uncoated devices along with simple calculations support the assignment of this effect to the spin transition. This latter translates into changes in mechanical behavior of the cantilever due to the strong spin-state/lattice coupling. A guideline for the optimization of device parameters is proposed so as to efficiently harness molecular scale movements for large-scale mechanical work, thus paving the road...

Solvatomorphism and structural-spin crossover property relationship in bis[hydrotris(1,2,4-triazol-1-yl)borate]iron(II)

Two solvatomorphs of the mononuclear bis[hydrotris(1,2,4-triazol-1-yl)borate]iron(II) complex, [Fe(HB(tz)3)2] (1) and [Fe(HB(tz)3)2]·6H2O (1·6H2O), were obtained by modifying the nature of the crystallization solvent. The crystal structure, thermal stability and spin crossover properties of the crystals and associated bulk powder samples were analysed using variable-temperature single-crystal and powder X-ray diffraction, thermogravimetry, calorimetry, and Raman and 57Fe Mossbauer spectroscopy as well as by means of magnetic susceptibility and optical microscopy measurements. The orthorhombic (Cmca) solvatomorph 1·6H2O loses water between ca. 323–353 K, leading to the disintegration of the crystals into the polycrystalline sample 1. The solvent-free crystals of 1 crystallize in the orthorhombic space group Pbca with half a complex molecule in the asymmetric unit. They exhibit a remarkably abrupt phase transition around 334 K between the high spin and low spin states. This (isostructural) spin transition is accompanied by a nearly isotropic change of the FeII–N bond lengths (8.3 ± 0.5%) and a highly anisotropic unit cell volume change (4.6 ± 0.1%). The very high cooperativity of the spin transition in 1 (Γ = 5700 ± 50 J mol−1) is rather unusual in mononuclear SCO compounds. This property can be related to the relatively high stiffness of the lattice (Debye temperature θD = 198 ± 2 K), which involves numerous C–H⋯N hydrogen contacts between each molecule with fourteen neighboring molecules.

Room temperature current modulation in large area electronic junctions of spin crossover thin films

We report large-area (∼3 mm2), pinhole free crossbar junctions of thin films of the molecular complex [Fe(HB(tz)3)2] displaying spin transition around 336 K. The charge transport in the thinner junctions (10 and 30 nm) occurs by a tunneling mechanism, which is not affected substantially by the spin transition. The thicker junctions (100 and 200 nm) exhibit rectifying behavior and a reproducible drop of their electrical resistance by ca. 65–80% when switching the molecules from the high-spin to the low-spin state. This current modulation is ascribed to a bulk-limited charge transport mechanism via a thermally activated hopping process. The demonstrated possibility of resistance switching in ambient conditions provides appealing prospects for the implementation of molecular spin crossover materials in electronic and spintronic devices.

Investigation of nucleation and growth phenomena during the thermal and light induced spin transition in the [Fe(1-bpp)2][BF4]2 complex

Abstract Optical microscopy measurements have been realized on single crystals of the [Fe(1-bpp)2][BF4]2 complex (1-bpp=2,6-di(pyrazol-1-yl)pyridine). The thermal spin transition around 253 K occurs by a heterogeneous nucleation and growth mechanism and involves a clear phase separation and a small hysteresis. This very abrupt and complete thermal transition is preceded by a premonitory spin conversion, which implies only a small fraction (ca. 2–3 %) of the molecules. This peculiar behavior may be the sign of heterophase fluctuations. The light-induced spin transition from the stable low spin (LS) to the metastable high spin (HS) phase was achieved at 80 K by focusing laser light into a small volume fraction of the crystal. Under continuous irradiation this photo-converted HS “nucleus” then grows and the whole crystal converts to the HS phase providing evidence for a light-induced instability in the system due to long-range elastic interactions. The relaxation of the light-induced metastable HS phase at 83 K follows a sigmoidal decay – typical of cooperative spin crossover systems. Nevertheless the spatial development of this relaxation process appears very homogeneous and no phase separation could be detected within the resolution of the optical microscope.

Complete Set of Elastic Moduli of a Spin-Crossover Solid: Spin-State Dependence and Mechanical Actuation

Molecular spin crossover complexes are promising candidates for mechanical actuation purposes. The relationships between their crystal structure and mechanical properties remain, however, not well understood. In this study, combining high pressure synchrotron X-ray diffraction, nuclear inelastic scattering, and micromechanical measurements, we assessed the effective macroscopic bulk modulus ( B = 11.5 ± 1.5 GPa), Youngs modulus ( Y = 10.9 ± 1.0 GPa), and Poissons ratio (ν = 0.34 ± 0.04) of the spin crossover complex [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). Crystal structure analysis revealed a pronounced anisotropy of the lattice compressibility, which was correlated with the difference in spacing between the molecules as well as by the distribution of the stiffest C-H···N interactions in different crystallographic directions. Switching the molecules from the low spin to the high spin state leads to a remarkable drop of the Youngs modulus to 7.1 ± 0.5 GPa both in bulk and thin film samples. The results highlight the application potential of these films in terms of strain (ε = -0.17 ± 0.05%), recoverable stress (σ = -21 ± 1 MPa), and work density ( W/V = 15 ± 6 mJ/cm3).

Spin crossover materials for MEMS actuation: Film integration and characterization

This work describes the integration of molecular spin crossover (SCO) compound [Fe(H<inf>2</inf>B(pz)<inf>2</inf>)<inf>2</inf>(phen)] 1 (H2B(pz)<inf>2</inf> = dihydrobis(pyrazolyl)borate, phen = 1,10-phenantroline) into silicon MEMS with the aim to determine the mechanical properties of the SCO thin film. Analytical methods using the experimental resonance frequency before and after deposition of 1 are used to extract the elastic modulus (E) and residual stress (σ) induced by the film deposition, leading to values of E = 6.9 ± 0.1 GPa and σ = 74.8 MPa respectively. Additional mechanical parameters as a consequence of the expected spin transition were also predicted. These results provide a step towards the integration of SCO materials for future applications as actuators in MEMS/NEMS devices.