William Nicolazzi

Emerging properties and applications of spin crossover nanomaterials

In the past few years we have witnessed the spectacular development of nano-materials of spin crossover complexes of 3d4–3d7 transition metal ions, including thin films, nanoparticles, nanopatterns and nanoscale assemblies. These developments were primarily motivated by fundamental questions concerning size reduction effects on the bistability properties. On the other hand, spin crossover nano-objects require new characterization techniques (scanning probe microscopy, micro-magnetometry, etc.), revealing unknown details of the SCO mechanism at the nanoscale and giving access also to properties, which have been largely unexplored until now. In this Highlight, we review these recent experimental developments and discuss perspectives for societal applications as well.

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.

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.

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. On the other hand, a re-opening of the hysteresis was observed for smaller (2 nm) particles. A detailed (57)Fe Mössbauer spectroscopy analysis was used to correlate this unusual phenomenon to the modification of the stiffness of the nanoparticles thanks to the determination of their Debye temperature.

Finite size effects in molecular spin crossover materials

In recent years, study of nano-objects with first-order phase transitions has raised new interesting questions. Indeed, the phase stability and the transformation kinetics are highly dependent on the object size. In the case of the spin crossover phenomenon size effects may lead to a loss of the hysteresis (bistability) properties, a shift of the transition temperature or the occurrence of an incomplete transition and also reappearance of the hysteresis upon extreme size reduction (∼1–4 nm). Research in this area is growing, but the mechanisms of the spin transition at the nanoscale and in particular the size dependence of the collective behaviour are still not completely understood. The aim of this Focus paper is to point out new issues brought about by the size-reduction effects. We also review size-induced effects for a few selected phase transition phenomena, such as structural or electronic transitions, with the aim of highlighting the possible analogies with the spin crossover phenomenon.

Hybrid spin-crossover nanostructures.

Summary 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.

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.

Critical temperature and correlation length of an elastic interaction model for spin-crossover materials

It has previously been pointed out that the coexistence of infinite-range and short-range interactions causes a system to have a phase transition of the mean-field universality class, in which the cluster size is finite even at the critical point. In the present paper, we study this property in a model of bistable molecules, whose size changes depending on the bistable states. The molecules can move in space, interacting via an elastic interaction. It is known that due to the different sizes, an effective long-range interaction between the spins appears, and thus this model has a mean-field type of phase transition. It is found that the scaling properties of the shift of the critical temperature from the pure short-range limit in the model with infinite-range and short-range interactions hold also in the present model, regarding the ratio of the size of the two states as a control parameter for the strength of the long-range interaction. By studying the structure factor, it is shown that the dependence of the cluster size at the critical temperature also shows the same scaling properties as a previously studied model with both infinite-range and short-range interactions. We therefore conclude that these scaling relations hold universally in hybrid models with both short-range and weak long-range interactions.

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.

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.