state, in the vicinity of the spincrossovertransition, 13 is possible and might be associated with a
change in the dielectric constant. Steric hindrance, including crystal packing, sample
conductance and flux density would all then play a role in determining at what temperatures, this
transient, not necessarily even metastable, electronic transition would occur. Yet the very
2 was then immersed in isopropyl alcohol and placed in an ultrasonic bath for 20 min. and
then the solvent was evaporated. This procedure resulted in a mixture of 1 and 2 with the majority of SCO molecules locked in the low spin state (as is evident in magnetometry and as discussed below). The resulting material also exhibits a spincrossovertransition, leading to an increase in the HS state population by 5 to 10%, but remaining far from the nearly 100% occupancy in the HS state, as is observed in 1 alone. The molar ratio 1/2 of 1:2 was chosen for the studies here because, over many combinations, it resulted in the largest fraction of molecules in the LS state, ca. 60% from X-ray absorption measurements, at temperatures above 160 K.
room temperature. Does this mean coexistence of locked high-spin and low-spin states during annealing through the SCO transition temperature? As noted above, it is already known [16, 18] that a conducting substrate, like Au(111), tends to pin more than 50% of several SCO complexes in the high spin state even well below the SCO transition temperature. To investigate this point further, we have utilized X-ray absorption spectroscopy. The Fe L-edge X-ray absorption (XAS) spectra are representative of resonant state-to-state transitions of electrons from the occupied Fe 2p orbital to unoccupied 3d orbitals. Other intra-atomic Fe transitions from 2p to 4s are of low probability, while excitations to 4p are dipole forbidden. Figure 4(a) illustrates temperature-induced changes in the XAS features across the spincrossovertransition temperature of 80 K to 340 K for [Fe(H 2 B(pz) 2 ) 2 (bipy)]. Like in prior studies ,
hysteresis width and the abruptness of the transition decrease. These phenomena become more pronounced in sample 4. Figure S3 shows representative Mössbauer spectra of sample 4 at 340 and 80 K, while the corresponding hyperfine Mössbauer parameters at various temperatures are reported in table S2 for samples 1-4. For each sample, the residual HS fraction measured at 80 K is about 5-10 % and thus does not seem to be influenced by the substitution of the triazole ligand (up to 10 % alloying). For all samples, a constant relative proportion of the two spin states was evidenced in the investigated temperature
Overall, these results provide clear proof for the possibility of combining the pyroelectric response of the polymer with the piezoelectric eﬀect arising due to the SCO. Nevertheless, to exploit this combined eﬀect for thermal energy harvesting the following issues will have to be considered. First, a real gain with respect to state-of-the-art pyroelectric harvesters can be expected only for small and slow temperature excursions around the spintransition temperature. Second, the ideal SCO com- pound should display a relatively abrupt spintransition without hysteresis in the temperature range between ca. 30 and 50 1C and an associated large volume change (ca. 10%). Third, it is obviously important to find an arrangement where the sign of the two eﬀects (pyro and piezo) are such that they sum up and not cancel each other. Indeed, in our diﬀerent samples we observed both situations (see for example Fig. 7b and d). This means that diﬀerent eﬀects are in competition within the material. Notably, since the pyroelectric coeﬃcient is negative, and upon heating the SCO material exhibits tensile strain (expansion) the two eﬀects will sum up only if the piezoelectric coeﬃcient has a positive sign. Since in P(VDF–TrFE) d33 o 0 whereas d31 , d32 4 0, various situations may arise depending on the effective anisotropy of the SCO strain. Besides, other phenom- ena may also contribute to the piezoelectric response, such as the variation of the dielectric permittivity upon the SCO (Fig. S23, ESI†). Material and mechanical engineering must consider these effects in order to optimize the harvesting output.
spin states of adsorbed spin-crossover complexes, we re- frain from doing so in the present case for two reasons. First, the flat adsorption of the tpy ligands significantly changes the symmetry and presumably the ligand field such that different spin states relative to the bulk com- pound may be expected. Second, the coordination of the Fe to the tpy ligand appears to be weak and unsta- ble. Indeed, the position of the central Fe relative to the molecule was observed to easily change at moderate tunneling conditions (V = 0.5 V and I = 100 pA) as shown in Figures 6b and d.
[Fe II (Htrz)2(trz)](BF4) spincrossover particles of 85 nm mean size are dispersed in an SU-8 polymer matrix and spray-coated onto silicon microcantilevers. The subsequent photothermal treatment of the polymer resist leads to micrometer thick, smooth, and homogeneous coatings, which exhibit well-reproducible actuation upon the thermally induced spintransition. The actuation amplitude as a function of temperature is accurately determined by combining integrated piezoresistive detection with external optical interferometry, which allows for the assessment of the associated actuation force (9.4 mN), stress (28 MPa), strain (1.0%), and work density (140 mJ cm −3 ) through a stratified beam model. The dynamical mechanical characterization of the films evidences an increase of the resonance frequency and a concomitant decrease of the damping in the high-temperature phase, which arises due to a combined effect of the thickness and mechanical property changes. The spray-coating approach is also successfully extended to scale up the actuators for the centimeter range on a polymer substrate providing perspectives for biomimetic soft actuators.
arising from the strain ε SCO is described by the well-known
Timoshenko formula,  from which the Young’s modulus E of
the nanocomposite film was extracted by taking into account the cantilever geometry. The strain is different in the HS → LS and LS → HS directions, corresponding to different Young’s moduli in the two spin states. For the other actuation param- eters, such as the volumetric and gravimetric work densities (W/V and W/m), actuating stress (σ), and blocking force (F), this difference can be neglected within the experimental uncer- tainty. It is important to note that the bending of the cantilever between the LS and HS states refers to the same temperature inside the hysteresis loop (353 K), i.e., no contribution arises from ordinary thermal expansion.
configuration can exhibit a reversible switching between the molecular low-spin (LS) and high-spin (HS) states upon the application of an external stimulus such as temperature, pressure, intense magnetic field, or light irradiation. This phenomenon is accompanied by a spectacular modification of the magnetic, optical, electrical, and mechanical properties of the material. Due to the strong electron-lattice coupling, the molecule occupies a smaller volume in the LS state, which implies a higher density and stiffness when compared to the HS form. In bulk SCO solids, this misfit between the HS and LS molecular volumes leads to strong elastic interactions, which play a major role in the cooperativity of spintransition [ 2 ] and its spatiotemporal dynamics [ 3 , 4 ]. In addition, new attractive applications of SCO materials such as microactuators [ 5 , 6 ], magnetostrictive heterostructures [ 7 ], or bistable composites [ 8 ] are based on the important spontaneous strain accompanying the SCO. Hence the knowledge of the elastic constants of these materials and their spin-state dependence is of significant importance. Unfortunately, the lattice dynamics and, in particular, the acoustic phonon modes of SCO materials remain largely unknown and their elastic constants have been determined only in a few occasions (and usually in only one spin state), using AFM [ 9 ], x-ray diffraction [ 10 , 11 ], and Brillouin spectroscopy [ 12 ]. In this context, nuclear inelastic scattering (NIS) is a very suitable technique [ 13 ], which allows to extract many lattice dynamical parameters and this even in nano-objects, which are of current interest in the SCO field.
Using the field-effect transistor structure, we now focus on the application of a gate voltage to the Ge channel to modulate the spin signal [ 34 ]. At negative gate voltage to a maximum of V G ¼ 50 V, the carrier density is lowered in the n-Ge channel and its resistivity is enhanced [ 35 ]. At 10 K, we find R=R ¼ ½Rð50 VÞ Rð0 VÞ=Rð0 VÞ ¼ þ68:2% whereas R=R ¼ þ21:9% at 300 K. The result- ing spin signal variation at 300 K is reported in Fig. 4(a) . We can clearly see the effect of the gate voltage with a significant spin signal increase whereas almost no variation is observed at 10 K (not shown). All the measurements are summarized in Fig. 4(b) as a function of temperature: a clear transition occurs again between 150 and 200 K (171 K from a linear fit to the finite values of V=V above 200 K). Again these findings are in good agreement with a transition from spin injection into interface states to the Ge conduction band. To be more quantitative, in the case of spin injection in the Ge conduction band and in the frame of the diffusive regime model [ 12 ], the spin resistance-area product is given by R S A ¼ V S =IA ¼ ðTSP l cb sf Þ 2 ð=t Ge Þ. Hence, if we assume that TSP and l cb sf remain constant under the application of an electric field, V S scales as ( =t Ge ) which is proportional to the channel resistance R. We thus expect V=V to scale with R=R in the event that spin polarized carriers are injected in the Ge conduction band. Below 150 K, we obtain negligible values of V=V for large values of R=R which is compatible with spin accumulation into interface states. Above 200 K, V=V starts to increase as spins start to accumulate in the Ge conduction band and at room temperature we find V=V R=R which means that we have fully achieved spin injection in the Ge conduction band.
we succeeded in elaborating smooth, homogeneous films of nanoparticles of the molecule-based spincrossover complex [Fe(Htrz) 2 (trz)](BF 4 ) in an SU-8 polymer matrix with thick-
nesses in the micrometer range. Interestingly, the composite films exhibited SCO with thermal hysteresis loops twice as large as the initial nanoparticles. This effect possibly stems from the mechanical interaction with the crosslinked polymer matrix and provides scope to use these actuators in a “catch-state” (i.e., without consuming energy). Actuation of MEMS devices with the SCO/SU-8 nanocomposite films led to well-reproducible (both device-to-device and cycle-to-cycle), large actuation ampli- tude and stress upon the spintransition. The associated high work density (140 mJ cm −3 ) provides the real scope for applica- tions. Besides microsystems, we have also constructed macro- scopic (centimeter scale) actuator devices based on a bilayer polymer architecture. These soft actuators displayed large deflections and perceptible color changes upon the SCO, which might be exploited in biomimetic artificial muscles.
states (< 10 –6 S cm –1 ), which is a useful property as it allows
to keep small the leakage currents in the composite material. In fact, the observation of such important discharge peaks would be impossible with a conducting filler. Hence, the poor conductivity, considered in general as a drawback for SCO com- pounds, becomes here a key advantage. Overall this DMA and BDS analysis of the composites reveals that the excellent elec- tromechanical properties of the P(VDF-TrFE) and PVDF (see Figures S24 and S25 in the Supporting Information) matrices have been preserved to a large extent in the composite material. The DMA results confirm the effective strain coupling between the SCO particles and the polymer matrix providing support for our initial hypothesis on the mechanical origin of the “anom- alous” discharge current peaks at the spintransition. On the other hand, the BDS data reveal also a strong modulation of the permittivity of the composite between the two spin states, which contributes obviously also to the “pseudo-pyroelectric” currents. Theoretical modeling will be necessary to disentangle these contributions.
Nano-electromanipulation of SpinCrossover Nanorods: Towards Switchable Nanoelectronic Devices
Switching the electronic or magnetic states of molecules or assemblies of molecules is one of the foremost paradigms in molecular electronics. Up to now numerous switchable mole- cular compounds have been synthesized, involving different physical phenomena, but the common challenge remains the construction of active devices. In particular, devices based on charge transport properties attract a great deal of attention, due to the substantial technological developments accomplished to probe charge transport down to the single molecule level. Here we focus on the charge transport properties of nano-objects dis- playing molecular spin-state switching. We used electric-fi eld- assisted directed assembly to organize high aspect-ratio spincrossover nanorods between interdigitated electrodes with a very high degree of alignment. The temperature-dependent cur- rent-voltage characteristics of each device revealed a bistability of the current intensity associated with the spin-state switching, providing appealing perspectives for nano-scale switching and memory devices.
B istable molecular complexes that can exist in two interchangeable states can act as switches under external stimuli. In this context, molecular spincrossover (SCO) compounds present a special interest due to their response to various external stimuli that might lead to a wide range of potential applications. 1 , 2 In these systems, the electronic conﬁguration of the metal can be conveniently switched from the so-called low-spin (LS) to a high-spin (HS) electronic conﬁguration in response to an external stimulus such as temperature, pressure, light irradiation, and so forth. 3 This spin state switching leads to a pronounced change of various material properties, including optical, magnetic, mechanical, and electrical characteristics. Because bulk SCO materials are in general highly insulating, 4 their electrical properties have been largely ignored and received growing interest only in the past few years, in parallel with the emergence of nanoscale SCO materials. 5 To date, most of the studies have been focused on the thermal bistability of the electrical properties. 4 − 12 A few
E 2000 cm -1 et l’état BS est maintenant l’état fondamental du point de vue quantique et reste thermodynamiquement stable à haute température. Enfin dans le cas où 10Dq HS ~ 11000- 12500 cm -1 et 10Dq BS ~ 19000-22000 cm -1 , alors E 0 ~ 0-2000 cm -1 donc l’état fondamental du point de vue quantique est l’état BS et la transition de spin thermique y est attendue. Ces plages énergétiques sont relativement faibles, c’est pourquoi la force du champ cristallin est comparable à l’énergie d’appariement des électrons et les configurations HS ou BS sont énergétiquement proches. Cependant cette énergie n’est pas optiquement accessible car l’excitation directe de l’état BS vers l’état HS (et réciproquement) est interdite. On ne peut donc pas mesurer la longueur d’onde/l’énergie correspondant à une excitation directement d’un état à l’autre par de simples méthodes spectroscopiques. Il existe toutefois une méthode de photo-excitation induisant une transition de spin indirecte, par exemple l’effet LIESST. Cet effet sera décrit plus précisément dans le paragraphe I.6.c. Par contre si les énergies des états HS et BS sont proches, le système pourra basculer d’un état de spin à un autre sous l’effet d’une perturbation extérieure, soit une conversion entre les deux états de spin sous l’effet de cette perturbation. L’effet d’une perturbation extérieure la plus commune pour un changement d’état de spin d’un complexe donné, est une variation de température. Mais une transition de spin peut être également provoquée par une variation de pression ou d’un champ magnétique extérieur, ou par irradiation.
Conventional experimental approaches used to characterize bulk SCO materials (magnetometry, X-ray diffraction, calorimetry, Mössbauer, electronic and vibrational spectroscopies), are often not well adapted to investigate nanoscale SCO objects, gen- erally due to the low amount of matter, and new technics are needed to characterize them. In particular, there is a need for high spatial resolution microscopy tools as well as for high sensitivity methods able to detect molecular spin-state changes in very small amounts of matter, ideally in a single, isolated nano-object. Beyond their high resolution and/or high sensitivity, these new experimental approaches can provide also information on material properties, which are either difficult to access by conventional methods or not so relevant at other size ranges. Far-field optical microscopy techniques have already been employed with success to monitor the spin state changes in a single nanometric ob- ject. For example, single SCO nanoparticles were studied using fluorescence, Raman and differential interference contrast microscopy. On the other hand, nanometric thin films of SCO complexes were analyzed by different photonic methods (ellipsometry, surface plasmon resonance, etc.). In order to surmount the rather limited spatial resolution of far-field optical methods one may use electron or X-ray beams, which can provide struc- tural and spectroscopic information with high spatial resolution. In the case of relatively brittle molecular SCO materials care must be taken, however, due to the invasive nature of these techniques: sample heating and radiation damage are in fact frequently encoun- tered. These problems have been largely decreased in a very recent work, which used aperture-based time-resolved electron microscopy to follow the spintransition in individ- ual nanoparticles . Another possible approach, which we explored in this thesis work, is based on scanning probe microscopies (SPM). Although SPM has been already used to study phase transition in different materials, SPM studies on spincrossover materials are very scarce. Actually only scanning tunneling microscopy (STM) has been used in this field, but STM is more relevant in the context of single molecule studies than nanomate- rials which are the scope of our work. Indeed, the principal objective of this work is to explore the possible use of SPM techniques to characterize and manipulate the spin state of SCO complexes at the nano- and micrometric scales. We focused on two SPM methods: near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM). This latter was used either for imaging sample topography or mechanical properties.
isostructural and that no lattice solvent is involved makes 1 a benchmark compound for diﬀerent investigations.
Magnetic measurements were used to conﬁrm the thermal SCO in a polycrystalline sample at around 365 K as well as the well-known thermal hysteresis of about 40 K width 27 (see Figures S2 and S3 in the Supporting Information (SI) for the magnetic data and other sample characterization details). The conductivity, the electric modulus, and the dielectric permittivity of the sample recorded at atmospheric pressure at diﬀerent temperatures are also reported in the SI (Figures S4−S6) and are comparable to the data recorded on the same complex previously. 28 It should also be noted that, prior to the measurements under pressure, the good reproducibility of the spintransition was controlled over eight consecutive thermal cycles ( Figure S7 ). For high-pressure measurements, the powder was slightly pressed to a thickness of 1.96 mm between two parallel electrodes of 15 mm diameter within a Teﬂon ring and sealed by epoxy glue so that no losses or changes in the thickness of the sample may occur. The electrodes were connected by ﬂexible leads to high-pressure feedthrough connectors in a commercial high-pressure cell (Novocontrol Technologies). Silicone oil was used as a pressure-transmitting medium, and the electrode assembly, including the inner powder layer, was fully immersed in oil to provide hydrostatic conditions. It was conﬁrmed that the silicone oil has negligible eﬀect on the measured electrical signal. Pressure can be changed in small steps of ∼10 bar up to 3 kbar upon both compression and decompression. A schematic of the high- pressure setup is presented in the SI (Figure S1).
∗ Corresponding author.
E-mail address: email@example.com (E. Freysz).
both a change in the absorption coefﬁcient and the index of refrac- tion. While the change in the absorption of SCO NPs during a spin state transition can be easily monitored, very little is known about the evolution of their index of refraction  . Different experiments have been dedicated to the investigation of the evolution of the dielectric properties of SCO bulk materials [14–19] . However up-to now and to the best of our knowledge, the evolution of the dielec- tric constant associated with spintransition of SCO NPs in the UV, visible and near IR spectral range has been barely investigated  . Hereafter, we propose a simple technique which makes it possible to determine the optical dielectric constant of a solution of SCO NPs. A simple model is used to compute the dielectric constant of the SCO NPs and their evolution versus temperature and high spin fraction. While over the studied temperature we demonstrate that spin state transition can result in variation of the relative change of the index of refraction ( n/n) that can be as high as 7%. Such SCO NPs can be used to adjust on demand (or ‘tune’) the optical dielectric constant of a given dielectric material.
Spincrossover particles dispersed in a piezo/ferroelectric poly(vinylidene fluoride-co-trifluoro-ethylene), P(VDF TrFE), rnatrix give rise to inspiring rnechano-electric phenomena, with possible applications for energy harvesting and sensing. Particles of different chemical compositions. morphologies and concentrations were loaded into copolyrners with different TrFE molar contents to investigate the effect of these pararneters on the mechano-electric coupling between the polymer and the spincrossover rnaterial The samples were characterized by elernental analysis, powder X-ray diffraction, Raman spectroscopy, optical reflectivity, differential scanning calorimetry, and electron microscopy and were studied also for their piezo/pyroelectric properties. We show that it is possible to tune simultaneously the spintransition temperature of the particles and the Curie temperature of the rnatrix in such a way that the piezoelectric effect from the spintransition and the pyroelectric response of the polyrner can be concomitantly observed. This result provides prospects for the development of smart, multifunctional electroactive polymer composites and for increasing their thermal electrical harvester output.
of the ith site. This local quantity allows to access to the spatial distribution of bond vibrations and its knowledge can also probe the impact of surface vibrations on the crystal stiffness. The coordination defects at the surface (missing bonds due to the creation of surface) can be simply taken into account by applying free boundary conditions. Other boundary conditions can also be used to simulate the difference between the bulk and the surface in terms of chemical properties, either with a local modification of the ligand field at the surface [ 32 ] or by fixing molecules at the surface in the HS state (specific boundary conditions) [ 24 , 28 ], which forms a core-shell system. These latter conditions explain in a simple manner the down-shift of the transition temperature and the existence of a residual HS fraction at low temperatures and are interpreted as a “negative pressure,” which acts on the nanoparticle core. A physical origin of the size evolution of the transition temperature has been proposed through a core-shell thermodynamic model, which introduces a spin state dependent surface energy [ 31 ]. This interface energy can be developed in chemical and mechanical (matrix and local surface stresses...) terms, even if all these contributions are correlated. The resulting difference between the HS and the LS surface energies constitutes the driving force of the spintransition at the nanoscale [ 31 ]. In this present work, we are interested in the influence of a free interface on the lattice dynamics of nanoparticles, i.e., without additional constraints. The calculation of thermal quantities n HS and u i by Monte