Spin Switching in Electronic Devices Based on 2D Assemblies of Spin‐Crossover Nanoparticles

Abstract

Spin-crossover (SCO) materials form an intriguing class of compounds in which various external physical or chemical stimuli can change their ground spin state between a low-spin (LS) and a high-spin (HS) near room-temperature. [1][2][3][4] It has been a long standing dream to use this phenomenon in solid-state electronic devices. [5] Only recently several research groups succeeded in electrically addressing SCO materials based on Fe (II) coordination compounds at the nanoscopic [6][7][8] and microscopic [8][9][10] levels. This electric control of the spin has even been achieved in individual SCO molecules. [11,12] 2 The most investigated SCO systems are based on six-coordinated octahedral iron (II) complexes. In such systems, the spin state changes from the LS configuration, S = 0, to the HS one, S = 2; this SCO is accompanied by a change in the magnetic moment, the structure and color of the compound. Although the origin of this HS-LS transition is purely molecular, the SCO manifests cooperatively at the solid state and is strongly influenced by short/long range elastic interactions between neighboring SCO molecules. Such a cooperativity can result in a thermal hysteresis cycle, a property that can be used for memory applications in electronic devices. [5,13] In recent years the size of the SCO solid has been reduced to the nanoscale, while preserving the memory effect near room temperature. In particular, nanoparticles (NPs) of ~10 nm [14,15] have been obtained, based on the polymeric one-dimensional (1D) compound ([Fe(Htrz) 2 (trz)](BF 4 )), (Htrz = 1,2,4-triazole), [16,17] which is one of the most promising SCO systems for designing electronic devices working at room temperature. [14] SCO NPs of other materials forming 3D networks have also been studied in this context, but in most cases they undergo a fast decrease of the hysteresis width upon size reduction, which has been accompanied by a higher residual high spin fraction and a downshift of the transition temperatures. [19] Recently, the memory effect of the [Fe(Htrz) 2 (trz)](BF 4 ) NP(s) has electrically been demonstrated, [6,8,9] showing their potential as switching elements of nanoscale devices.Nevertheless, opposite behaviour in the conductivity change during the SCO transition has been reported, which remains unclear mainly due the scare experimental studies of the effects of an electric field on the charge transport properties. [20,21] In particular, measurements on single NP devices (NP mean size of 10 nm) show an increase in the electric conductivity when the SCO occurs, [6] whereas the reverse situation is observed for an assembly of NPs (mean size 15 nm) [8] and for larger particles [8][9][10] (from 100 to 300 nm in diameter and from 250 to a couple of microns in length). Although it is tempting to attribute this difference to 3 separate transport mechanisms operating in these devices, i.e, single-step tunnelling vs. multistep hopping, differences in the chemical and structural NP features as well as in the experimental conditions may not...

Charge Transport and Electrical Properties of Spin Crossover Materials: Towards Nanoelectronic and Spintronic Devices

“…The I-V characteristics recorded at 373 K in both spin states showed a non-linear behavior, which probably originates in a voltage activation of the charge transport. Overall, fairly similar results were reported later by Dugay et al [61] for [Fe(Htrz) 2 (trz)](BF 4 ) nanoparticles integrated in a similar device. Lefter et al [62] completed the study of [Fe(Htrz) 2 (trz)](BF 4 ) based devices by the analysis of the robustness of the thermal spin transition and that of the electronic device on successive thermal cycling.…”

Activating the molecular spinterface

“…Exciton population 100 might constitute a somewhat universal strategy for the optical modulation of the second molecular layer. A more specific response could be achieved by using photoswitchable 101-103 spin crossover 104,105 or molecules with an intrinsic magnetic moment 57,58,106 . Intermolecular coupling, on the other hand, can be achieved, for example, by using bi-planar π-conjugated molecules as proposed in a theoretical study 69 .…”

Preserving the spin transition properties of iron-triazole coordination polymers within silica-based nanocomposites

“…10,13 These works have evidenced the importance of the dimensional structure (two-or three-) in the cooperativity effect associated with the spin transition, where the temperature dependence of the high spin molar fraction strongly depends on intermolecular interactions. 36 Alternatively, coordination clusters can be incorporated within matrices to obtain nanocomposites. Such approaches allow tuning the cooperativity between spin-active sites that should be strongly connected to ensure an abrupt spin transition.…”