Spin-crossover Nanoparticles Research Articles

Enhancing Magnetic Cooperativity in Fe(II) Triazole-based Spin-crossover Nanoparticles by Pluronic Matrix Confinement.

Polymeric one-dimensional (1D) triazole-based FeII spin crossover nanoparticles have been entrapped in pluronic P123 matrix, forming nanorods in which the interaction between host (P123) and guest (FeII complex) promoted high reproducibility of the spin crossover process, significant shifts of the transition temperatures (T↑=370 K, T↓=338 K for the P123 entrapped material vs the literature values of T↑=358 K, T↓=341 K for the neat/polymer free system) and larger magnetic hysteresis width.

Elastic-frustration-driven unusual magnetoelastic properties in a switchable core-shell spin-crossover nanostructure

Spin-crossover (SCO) solids have been studied for their fascinating properties, exhibiting first-order phase transitions and macroscopic bistabilities, accompanied by significant magnetic, structural, and optical changes. These exceptional properties make these materials promising for applications as high-density information storage and optical switches. Recently, the critical progress made in chemistry allowed the design of spin-crossover nanocomposites, combining the properties of two types of spin-crossover solids having different properties, like different lattice parameters, bulk moduli, transition temperatures, ligand fields, etc. In this paper, we consider a microscopic electroelastic description of a SCO nanostructure made of a SCO active core surrounded by a SCO active shell, for which we impose an unconventional elastic frustration at the core-shell interface. The detailed examination of the thermodynamic properties of such a nanocomposite, as a function of the lattice parameter misfit between the two constituents, revealed that the frustration causes unexpected behaviors on the thermal dependence of the average bond lengths, such as the emergence of two- or three-step spin transitions, with self-organization of the spin states in the plateau regions. These results highlight the nontrivial character of the magnetoelastic properties in switchable SCO nanoparticles.

Confined Crystallization of Spin-Crossover Nanoparticles in Block-Copolymer Micelles.

Nanoparticles of the spin‐crossover coordination polymer [FeL(bipy)]n were synthesized by confined crystallization within the core of polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin‐crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as‐synthesized product (T 1/2↓=163 K and T 1/2↑=170 K) to the annealed product (T 1/2↓=203 K and T 1/2↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin‐crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.

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)4]. Magnetic measurements reveal both high-spin and low-spin residual fractions at atmospheric pressure. A pressure-induced transition of the high-spin residue is observed at around 1.5 GPa by synchrotron Mössbauer spectroscopy. We show that it is equivalent to a downshift of the transition temperature by ca. 400 K due to the size reduction. Unexpectedly, small-angle neutron scattering experiments demonstrate that these high-spin residual centers are not confined to the surface, which contradicts general theoretical considerations.

Sensing of the Molecular Spin in Spin-Crossover Nanoparticles with Micromechanical Resonators

In the past years, the use of highly sensitive silicon microelectromechanical cantilevers has been proposed as a tool to characterize the spin-crossover phenomenon by employing fast optical readout of the motion. In this work, FeII-based spin-crossover nanoparticles of the well-known [Fe(Htrz)2(trz)](BF4) complex wrapped with thin silica shells of different sizes will be studied by means of silicon microresonators. The silica shell will enhance its chemical stability, whereas the low thickness will allow a proper mechanical coupling between the cantilever and the spin-crossover core. To maximize the sensing of the spin-crossover phenomena, different cantilever geometries and flexural modes were employed. In addition, the experimental observations were also compared with COMSOL numerical simulations, which are in close agreement with them. The probe of spin-crossover phenomena with micro- and nanoelectromechanical actuators offers the possibility of preparing smart sensing memory devices near/above room temperature.

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, 0.2 and 0.3) were synthesized in concentrated solutions without using any surfactant or polymer. The nanoparticle powders were investigated by transmission electron microscopy, powder X-ray diffraction, magnetometry, calorimetry, Raman/IR spectroscopies, elemental analysis and 57Fe Mössbauer spectrometry. Remarkably, the latter revealed a large decrease of the lattice stiffness when incorporating a small amount of amino-triazole ligand, reflected by the drop of the Debye temperature from 285 K (x = 0) to 205 K (x = 0.3). This collapse of the lattice cohesion was attributed to a reorganization of the supramolecular interactions between the Fe-triazole chains. This effect on the SCO properties is also discussed.

Downsizing of robust Fe-triazole@SiO2 spin-crossover nanoparticles with ultrathin shells.

A chemical protocol to design robust hybrid [Fe(Htrz)2(trz)](BF4)@SiO2 nanoparticles (NPs) with sizes as small as 28 nm and ultrathin silica shells below 3 nm has been developed. These NPs present a characteristic abrupt spin transition with a subsequent decrease in the width of the thermal hysteresis upon reducing the NP size.

Facile preparation of hybrid thin films composed of spin-crossover nanoparticles and carbon nanotubes for electrical memory devices.

In this study, organic solvent-dispersible nanoparticles of an FeII-1,2,4-triazole spin-crossover complex were synthesized. On mixing the suspension of the spin-crossover nanoparticles with a solution of single-walled carbon nanotubes (SWCNTs), the nanoparticles were strongly adsorbed on the hydrophobic SWCNT bundles, resulting in hybrid network structures. Variable temperature DC electrical conductivity measurements of the hybrid network thin films demonstrated that the conductivities of the composite films were switched by the spin transition of the nanoparticles.

Ising-like model applied to a triangular lattice of 2D spin-crossover nanoparticles: evidence of a re-entrant phase transition

We consider a 2D spin crossover (SCO) nanoparticles configuration under the influence of an external environment. Each nanoparticle is at the vertex of a triangular lattice leading to hexagonal-shaped systems. Each molecule located in the bulk is then connected to six nearest-neighbours (nn) while those situated at the surface have three or four nn.Adapting the entropic sampling Monte Carlo method, using three parameters: nHS (the total high spin fraction), s (the correlation: the sum of nn interactions the molecules) and nsHS (the high-spin fraction at the surface), we determined the density of macrostates D(nHS, nsHS,s) by scanning the full spin configurations. This information is then used to analyse the thermal behaviour of the system taking into account for the short- (J) and long-range (G) interactions as well for the interactions (L) between the molecules at the surface and the matrix (environment). The present contribution is devoted to the investigations of the required physical conditions which allow this triangular 2D lattice to exhibit a re-entrant phase transition, in particular when the size of the nanoparticles is reduced.

Size effect on the three state thermal hysteresis of a 2D spin crossover nanoparticles

In this report first we show, in a framework of the Ising-like model, a numerical simulation of a typical two step thermal transition obtained for a square lattice 12x12: a “first-step hysteresis” for a high spin fraction Nhs between 0 - 0,5 and at a higher temperature a “second-step hysteresis” with Nhs between 0.5 and 1. As long as we decrease the number of molecules the temperature range of the “second-step hysteresis” moves to a lower temperature, until is obtained, for a square of 4x4, a clear overlapped case with a three state behaviour. A detailed analysis on the role of the size system (4x4, 5x5, 6x6, 8x8 and 12x12) on the stability of this “Three state behaviour” is presented in this contribution. We study the influence of the surrounding environment for this specific thermal hysteresis. To solve the self-consistent equation related to the average value of the spin-operator <σ>, we use the density of the states calculated using a dynamic programming algorithm that will be presented in this paper.

Spin-crossover nanoparticles and nanocomposite materials

This contribution reports on the state of the art of the elaboration and the application of nanoparticles (NPs) and nanohybrid/nanocomposite materials based on spin-crossover (SCO) complexes. The first part of this review concerns the syntheses and the characterizations of the physical properties of SCO NPs. All of the methods including homogeneous and heterogeneous media syntheses developed for the elaboration of such NPs and the associated methods used for their morphological characterization are presented. A particular attention is paid on the effects of the size reduction and the influence of the environment on the SCO properties and to specific and recent remarkable advanced physical measurements realized on a batch of NPs or on an isolated object. The second part presents the elaboration of various nanocomposite or nanohybrid materials for which SCO NPs have been associated with magnetic entities, noble metals, different fluorescent dyes, and different active polymers with the objectives to go toward specific applications based on synergistic effects between the two components. Cette contribution rend compte de l'état de l'art sur l'élaboration et la mise en application de nanoparticules et de matériaux nanohybrides/nanocomposites à base de complexes à transition de spin. La première partie de cette revue concerne la synthèse et la caractérisation des propriétés physiques de nanoparticules à transition de spin. Toutes les méthodes, y compris les synthèses en milieux homogène et hétérogène développées pour l'élaboration de telles nanoparticules et les méthodes utilisées pour leurs caractérisations morphologiques, sont présentées. Une attention particulière est portée aux effets de la réduction de taille et de l'influence de l'environnement sur les propriétés de transition de spin et sur des mesures physiques récentes remarquables réalisées sur un lot de nanoparticules ou sur un objet unique. La deuxième partie présente l'élaboration de divers matériaux nanocomposites ou nanohybrides au sein desquels des nanoparticules à transition de spin ont été associées à des entités magnétiques, des métaux nobles, divers fluorophores et différents polymères actifs dans le but d'aller vers des applications spécifiques basées sur des effets synergétiques entre les deux composants.

Application of spin-crossover water soluble nanoparticles for use as MRI contrast agents

Water soluble spin-crossover (SCO) iron(II) nanoparticles (NPs) were synthesized by the polyethylene glycol (PEG) coating of [Fe(Htrz)3-3×(NH2trz)3×](BF4)2 (x = 0, 0.1, 0.5 and 1). The NPs with x = 0.1 show gradual SCO behavior over 280–330 K in water. The relaxation times, T1 and T2, were determined and the thermally-responsive T2 values making these NPs a candidate for use as a MRI contrast agent.

Charge Mobility and Dynamics in Spin-Crossover Nanoparticles Studied by Time-Resolved Microwave Conductivity.

We use the electrodeless time-resolved microwave conductivity (TRMC) technique to characterize spin-crossover (SCO) nanoparticles. We show that TRMC is a simple and accurate means for simultaneously assessing the magnetic state of SCO compounds and charge transport information on the nanometer length scale. In the low-spin state from liquid nitrogen temperature up to 360 K the TRMC measurements present two well-defined regimes in the mobility and in the half-life times, in which the former transition temperature TR occurs near 225 K. Below TR, we propose that an activationless regime taking place associated with short lifetimes of the charge carriers points at the presence of shallow-trap states. Above TR, these states are thermally released, yielding a thermally activated hopping regime where longer hops increase the mobility and, concomitantly, the barrier energy. The activation energy could originate not only from intricate contributions such as polaronic self-localizations but also from dynamic disorder due to phonons and/or thermal fluctuations of SCO moieties.

Microscopic models of spin crossover

The basic model for thermal spin crossover (SCO) is discussed in its microscopic and thermodynamic formulation. Compared to the basic model, its more elaborated forms formulated in course of almost 50 years are briefly reviewed with emphasis on their additional features. A separate section is devoted to the newer developments in the field of modelling of the SCO nanoparticles. The presentation of models is led in a comparative way to provide an accessible outline of the foundations of modern theoretical research on SCO and a simple applicability in quantitative interpretation of experiments.

Elastic models, lattice dynamics and finite size effects in molecular spin crossover systems

The experimental studies of spin-crossover compounds switched in the last decade from bulk measurements and macroscopic observations to the nanoscale and microscopic approaches. In this context, new and sometimes unexpected behaviours have been documented, which could be partially described only by the classical phenomenological models developed in the last period of the last century. In this context, the development of more complex models, able to reproduce the nucleation and domain propagation within the material, has proved to be not a whim of some theoreticians but a necessity, which facilitated the full understanding of observed phenomena and even made premises for further experiments. Here, we present and analyse various elastic models identifying their common points and differences and discuss how they can be used for the study of microscopic phenomena as the cluster formation, stability and propagation or for the study of finite size effects in spin-crossover nanoparticles, with open boundary conditions or embedded in various matrices.

Functionalization of wood/plant-based natural cellulose fibers with nanomaterials: a review

Being the most abundant natural biopolymer on earth, cellulose has been vastly exploited in a range of applications, from writing paper to high-end biosensors. Natural cellulose fibers can be isolated from wood or non-woody plants such as hemp, jute, flax, and bamboo by chemical or mechanical treatments. To make it suitable for targeted applications, cellulose fibers are modified with functional moieties in the nanometer scale. Cellulose has been functionalized with noble metals such as silver and gold nanoparticles for catalysis and antimicrobial applications. A number of metal oxides, such as zinc oxide, titanium dioxide, and tin dioxide have been incorporated into cellulose. The porosity, hydrophilicity, and roughness of cellulose surface makes it an ideal substrate for a plethora of sensing applications. Further, it can be made into a lightweight, portable, foldable, and disposable device, which provides an excellent platform for various point-of-care purposes. Cellulose fibers have also been immobilized with carbon nanomaterials, including carbon nanotubes and graphene oxide. For optical applications, [Fe(hptrz)3](OTs)2 spin-crossover nanoparticles have also been immobilized on cellulose fibers. Likewise, many enzymes, macromolecules, and some polymers have been used to modify natural cellulose for specific end uses. This review focuses on recent developments in the modification or immobilization of functional materials on cellulose fibers, in macro-scale only, obtained from wood or plant sources.

Evidencing size-dependent cooperative effects on spin crossover nanoparticles following their HS→LS relaxation

The size dependence of the relaxation mechanism after photo-excitation through the Light-Induced Excited Spin-State Trapping (LIESST) effect of [Fe(pz)Pt(CN)4] micro- and nanoparticles is evidenced experimentally and simulated in the framework of the mechanoelastic model.

Extreme downsizing in the surfactant-free synthesis of spin-crossover nanoparticles in a microfluidic flow-focusing junction.

A new surfactant-free, flow-focusing droplet microfluidic approach was developed as an important alternative to existing synthesis techniques for the preparation of spin crossover nanoparticles. It enables great control of the mixing of the reactants, and produces crystals of [Fe(pyrazine)(Pt(CN)4)] with an unexpected 20-fold downsizing compared to classical bulk synthesis.

An electro-elastic theory for the mechanically-assisted photo-induced spin transition in core-shell spin-crossover nanoparticles.

The development of heterostructure materials may lead to new features that cannot be obtained with natural materials. Here we simulate a model structurally hybrid core-shell nanoparticle with different lattice parameters between an electronically inert shell and an active spin crossover core. The nanoparticle consists of a 2D core with 20 × 20 size with square symmetry, surrounded by a shell made of 10 atomic layers. The low temperature photoexcitation of the core shows a significant environment-dependent behavior. In particular, we demonstrate that a shell with a large lattice parameter accelerates the low-spin to high-spin photoexcitation process of the core through the single domain nucleation mechanism while a moderate shell lattice parameter leads to spatially-homogeneous growth of the high-spin fraction. We found that the mechanical retro-action of the shell may cause elastic instability of the core leading to efficient control and manipulation of its photo-conversion.

Numerical Investigations of the Thermal, Pressure and Size Effects on 2D Spin Crossover Nanoparticles

In the framework of the Ising-like model, the thermal and pressure effects on the spin crossover systems are evaluated through two-states fictitious spin operators σ with eigenvalues \U0001d70e = −1 and \U0001d70e = +1 respectively associated with the low-spin (LS) and highspin (HS) states of each spin-crossover (SCO) molecule. Based on each configurational state, the macroscopic SCO system, is described by the following variables: m=Σ σi, s=Σ σi σj and c=Σ σk standing respectively for the total magnetization, the short-range correlations and surface magnetization. To solve this problem, we first determine the density of macrostates d[m][s][c], giving the number of microscopic configurations with the same m, s and c values. In this contribution, two different ways have been performed to calculate this important quantity: (i) the entropic sampling method, based on Monte Carlo simulations and (ii) a new algorithm based on specific dynamic programming. These two methods were tested on the 2D SCO nanoparticles for which, we calculated the average magnetization < σ> taking into account for short-, long-range interactions as well as for the interaction between surface molecules with their surrounding matrix. We monitored the effect of the pressure, temperature and size on the properties of the SCO nanoparticles.