Maria D. Manrique-Juarez

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.

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 140u2005nm 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 338u2005K 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 66u2005Hz 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 200u2009nm thin layer of the molecular spin crossover complex [Fe(H2B(pz)2)2(phen)] (H2B(pz)2u2009=u2009dihydrobis(pyrazolyl)borate and phenu2009=u20091,10-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 10u2009K led to a well-reproducible shift of the cantilevers resonance frequency (Δfru2009=u2009−0.52u2009Hz). 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...

In Situ AFM Imaging of Microstructural Changes Associated with The Spin Transition in [Fe(Htrz)2(Trz)](Bf4) Nanoparticles

Topographic images of [Fe(Htrz)2(trz)](BF4) nanoparticles were acquired across the first-order spin transition using variable-temperature atomic force microscopy (AFM) in amplitude modulation mode. These studies revealed a complex morphology of the particles consisting of aggregates of small nanocrystals, which expand, separate and re-aggregate due to the mechanical stress during the spin-state switching events. Both reversible (prompt or slow recovery) and irreversible effects (fatigue) on the particle morphology were evidenced and correlated with the spin crossover properties.

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

We report large-area (∼3u2009mm2), pinhole free crossbar junctions of thin films of the molecular complex [Fe(HB(tz)3)2] displaying spin transition around 336u2009K. The charge transport in the thinner junctions (10 and 30u2009nm) occurs by a tunneling mechanism, which is not affected substantially by the spin transition. The thicker junctions (100 and 200u2009nm) 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.

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.