Effects of the surface energy and surface stress on the phase stability of spin crossover nano-objects: a thermodynamic approach.

Size-induced phase transformation at the nanoscale is a common phenomenon whose understanding is essential for potential applications. Here we investigate phase equilibria in thin films and nanoparticles of molecular spin crossover (SCO) materials. To calculate the size-temperature phase diagrams we have developed a new nano-thermodynamic core-shell model in which intermolecular interactions are described through the volume misfit between molecules of different spin states, while the contributions of surface energy and surface stress are explicitly included.

Pressure Tuning of Coupled Structural and Spin State Transitions in the Molecular Complex [Fe(HB(pz))(phen)].

The large volume change, which accompanies the molecular spin crossover (SCO) phenomenon in some transition metal complexes, prompts frequently the coupling of the SCO with other instabilities. Understanding the driving mechanism(s) of such coupled phase transitions is not only important for fundamental reasons but also provides scope for the development of multifunctional materials. The general theoretical expectation is that the coupling has elastic origin, and the sequence of transitions can be tuned by an externally applied pressure, but dedicated experiments remain scarce.

Thermal hysteresis of stress and strain in spin-crossover@polymer composites: towards a rational design of actuator devices.

Polymer composites of molecular spin crossover complexes have emerged as promising mechanical actuator materials, but their effective thermomechanical properties remain elusive. In this work, we investigated a series of iron(ii)-triazole@P(VDF-TrFE) particulate composites using a tensile testing stage with temperature control. From these measurements, we assessed the temperature dependence of the Young's modulus as well as the free deformation and blocking stress, associated with the thermally-induced spin transition.

Sequential Activation of Molecular and Macroscopic Spin-State Switching within the Hysteretic Region Following Pulsed Light Excitation.

Molecular spin-crossover (SCO) compounds constitute a promising class of photoactive materials exhibiting efficient photoinduced phase transitions (PIPTs). Taking advantage of the unique, picture-perfect reproducibility of the spin-transition properties in the compound [Fe(HB(1,2,4-triazol-1-yl) ) ], the spatiotemporal dynamics of the PIPT within the thermodynamic metastability (hysteretic) region of a single crystal is dissected, using pump-probe optical microscopy. Beyond a threshold laser-excitation density, complete PIPTs are evidenced, with conversion rates up to 200 switched molecules per absorbed photon.

Direct Visualization of Local Spin Transition Behaviors in Thin Molecular Films by Bimodal AFM.

Thin films of the molecular spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl) ) ] undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.

Finite Size Effects on the Switching Dynamics of Spin-Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse.

Using ultrafast optical absorption spectroscopy, the room-temperature spin-state switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover (SCO) complex [Fe(HB(tz) ) ] (tz = 1,2,4-triazol-1-yl) are studied. These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out-of-equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.

Drastic lattice softening in mixed triazole ligand iron(ii) spin crossover nanoparticles.

A series of spin-crossover (SCO) coordination nanoparticles (ca. 60 nm) with the general formulae [Fe(Htrz)1+y-x(trz)2-y(NH2trz)x](BF4)y·nH2O (x = 0, 0.1, 0.

Phase Stability of Spin-Crossover Nanoparticles Investigated by Synchrotron Mössbauer Spectroscopy and Small-Angle Neutron Scattering.

Spin-crossover nanomaterials have been actively studied in the past decade for their potential technological applications in sensing, actuating, and information processing devices. Unfortunately, an increasing number of the metallic centers become inactive at reduced sizes, presumably due to surface effects, limiting their switching ability and thus the scope of applications. Here we report on the investigation of "frozen" metallic centers in nanoparticles (2-80 nm size) of the spin-crossover compound Fe(pyrazine)[Ni(CN)].

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.

Scan-rate and vacuum pressure dependence of the nucleation and growth dynamics in a spin-crossover single crystal: the role of latent heat.

Using optical microscopy we studied the vacuum pressure dependence (0.1-1000 mbar) of the nucleation and growth dynamics of the thermally induced first-order spin transition in a single crystal of the spin-crossover compound [Fe(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). A crossover between a quasi-static hysteresis regime and a temperature-scan-rate-dependent kinetic regime is evidenced around 5 mbar due to the change of the heat exchange coupling between the crystal and its external environment.

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.

Investigation of surface energies in spin crossover nanomaterials: the role of surface relaxations.

We analyse in detail the role of surface relaxations on the spin transition phenomenon through an Ising-like model solved in the inhomogeneous mean field approach. We show the surface relaxation tends to decrease the energy cost of missing bonds. Cooperative phenomena are also affected, leading to an asymmetric hysteresis loop.

High Spatial Resolution Imaging of Transient Thermal Events Using Materials with Thermal Memory.

The working principle of a new kind of nanothermometer is experimentally demonstrated using bistable materials with thermal memory. This thermometry approach allows for acquiring sub-wavelength resolution images of fast, transient heating events.

Hybrid spin-crossover nanostructures.

This review reports on the recent progress in the synthesis, modelling and application of hybrid spin-crossover materials, including core-shell nanoparticles and multilayer thin films or nanopatterns. These systems combine, often in synergy, different physical properties (optical, magnetic, mechanical and electrical) of their constituents with the switching properties of spin-crossover complexes, providing access to materials with unprecedented capabilities.

Re-appearance of cooperativity in ultra-small spin-crossover [Fe(pz){Ni(CN)₄}] nanoparticles.

A reverse nanoemulsion technique was used for the elaboration of [Fe(pz){Ni(CN)4}] nanoparticles. Low-temperature micellar exchange made it possible to elaborate ultra-small nanoparticles with sizes down to 2 nm. When decreasing the size of the particles from 110 to 12 nm the spin transition shifts to lower temperatures, becomes gradual, and the hysteresis shrinks.

Non-extensivity of thermodynamics at the nanoscale in molecular spin crossover materials: a balance between surface and volume.

The spin transition behavior in nanoparticles of molecular spin crossover (SCO) materials is investigated theoretically using a two-variable microscopic Ising-like model solved by Monte Carlo simulations. The extensive nature of the energy, and therefore the whole thermodynamics is affected by the increasing role of surface energetic parameters. As a consequence the pressure inside the nanoparticle is different from the external pressure of the bath.

Atomic force microscopy and near-field optical imaging of a spin transition.

We report on atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) investigations of single crystals of the spin crossover complex {Fe(pyrazine)[Pt(CN)4]} across the first-order thermal spin transition. We demonstrate for the first time that the change in spin state can be probed with sub-micrometer spatial resolution through various topographic features extracted from AFM data. This original approach based on surface topography analysis should be easy to implement to any phase change material exhibiting sizeable electron-lattice coupling.

Enhanced cooperative interactions at the nanoscale in spin-crossover materials with a first-order phase transition.

We analyzed the size effect on a first-order spin transition governed by elastic interactions. This study was performed in the framework of a nonextensive thermodynamic core-shell model. When decreasing the particle size, differences in surface energies between the two phases lead to the shrinking of the thermal hysteresis width, the lowering of the transition temperature, and the increase of residual fractions at low temperature, in good agreement with recent experimental observations on spin transition nanomaterials.

Synergistic switching of plasmonic resonances and molecular spin states.

Plasmonic resonance properties of a series of lithographically patterned gold nanorod arrays, spin coated by thin films of an iron(II)-triazole type spin crossover complex, were investigated upon heating/cooling and also under 633 nm laser irradiation. In both cases a reversible shift of the localised surface plasmon resonance wavelength was observed and quantitatively linked to the refractive index change accompanying the spin transition. These results show that molecular spin state switching can be very efficiently triggered by the photo-thermal effect, which - in turn - allows for an active tuning of the plasmon resonance.

Triggering a phase transition by a spatially localized laser pulse: role of strain.

We report here the optical microscopic imaging of a first-order phase transition induced by a nanosecond laser pulse (532 nm) in a single crystal of the molecular spin-crossover complex [Fe(bapbpy)(NCS)(2)]. The transition starts with the formation of a high spin domain in the region irradiated by the focused laser beam, followed by the subsequent growth or contraction of the initial domain. Remarkably, in otherwise identical experimental conditions one can observe either the irreversible transition of the whole crystal or merely the formation of a transient domain-depending on which region of the crystal is excited.

Surface plasmons reveal spin crossover in nanometric layers.

Nano-objects and thin films displaying molecular spin-crossover phenomena have recently attracted much attention. However, the investigation of spin crossover at reduced sizes is still a big challenge. Here we demonstrate that surface plasmon polariton waves propagating along the interface between a metal and a dielectric layer can be used to detect the spin-state changes in the latter with high sensitivity, even at the nanometer scale.

Molecular spin crossover phenomenon: recent achievements and prospects.

Recently we assisted a strong renewed interest in the fascinating field of molecular spin crossover complexes by (1) the emergence of nanosized spin crossover materials through direct synthesis of coordination nanoparticles and nanopatterned thin films as well as by (2) the use of novel sophisticated high spatial and temporal resolution experimental techniques and theoretical approaches for the study of spatiotemporal phenomena in cooperative spin crossover systems. Besides generating new fundamental knowledge on size-reduction effects and the dynamics of the spin crossover phenomenon, this research aims also at the development of practical applications such as sensor, display, information storage and nanophotonic devices. In this critical review, we discuss recent work in the field of molecule-based spin crossover materials with a special focus on these emerging issues, including chemical synthesis, physical properties and theoretical aspects as well (223 references).

Cooperative spin crossover phenomena in [Fe(NH2trz)3](tosylate)2 nanoparticles.

Nanoparticles of the [Fe(NH(2)trz)(3)](tosylate)(2) spin crossover complex of 3-4 nm size were synthesized and dispersed in a nonionic surfactant. This colloidal system exhibits thermal spin crossover phenomena associated with a virtually first order transition. By thermal treatment the colloidal suspension can be transformed into a fibrous structure forming a gel.