Molecular Spin-crossover Complex Research Articles

Investigation of the molecular switching process between spin crossover states of triazole complexes as basis for optical sensing applications.

With the advent of the first laser sources and suitable detectors, optical sensor applications immediately also came into focus. During the last decades, a huge variety of optical sensor concepts were developed, yet the forecast for the future application potential appears even larger. In this context, the development of new sensor probes at different scales down to the atomic or molecular level open new avenues for research and development. We investigated an iron based triazole molecular spin-crossover complex changing its absorption characteristics significantly by varying environmental parameters such as humidity, temperature, magnetic or electric field, respectively, with respect to its suitability for a new class of versatile molecular sensor probes. Hereby, besides the investigation of synthesized pure bulk material using different analyzing methods, we also studied amorphous micro particles which were applied in or onto optical waveguide structures. We found that significant changes of the reflection spectra can also be obtained after combining the particles with different types of optical waveguides.The obtained results demonstrate the suitability of the material complex for a broad field of future sensor applications.

Variability of the Conductance Changes Associated with the Change in the Spin State in Molecular Spin Crossover Complexes

Here, we examine the conductance changes associated with the change in spin state in a variety of different structures, using the example of the spin crossover complex [Fe(H2B(pz)2)2(bipy)] (pz = (pyrazol-1-yl)-borate and bipy = 2,2′-bipyridine) and [Fe(Htrz)2(trz)](BF4)] (Htrz = 1H-1,2,4-triazole) thin films. This conductance change is highly variable depending on the mechanism driving the change in spin state, the substrate, and the device geometry. Simply stated, the choice of spin crossover complex used to build a device is not the only factor in determining the change in conductance with the change in spin state.

High-Sensitivity Microthermometry Method Based on Vacuum-Deposited Thin Films Exhibiting Gradual Spin Crossover above Room Temperature.

We report on the fabrication, characterization, and microthermometry application of high-quality, nanometric thin films, with thicknesses in the range 20-200 nm, of the molecular spin-crossover complex [Fe(HB(1,2,3-triazol-1-yl)3)2]. The films were obtained by vacuum thermal evaporation and characterized by X-ray diffraction, UV spectrophotometry, and atomic force microscopy. The as-deposited films are dense and crystalline with a preferred [011] orientation of the monoclinic crystal lattice normal to the substrate surface. The films exhibit a gradual spin conversion centered at ca. 374 K spanning the 273-473 K temperature range, irrespective of their thickness. When deposited on a microelectronic device, these films can be used to enhance the UV-light thermoreflectance coefficient of reflective surfaces by more than an order of magnitude, allowing for high-sensitivity thermoreflectance thermal imaging.

Driving Barocaloric Effects in a Molecular Spin-Crossover Complex at Low Pressures.

Barocaloric effects─thermal changes in a material induced by applied hydrostatic pressure─offer promise for creating solid-state refrigerants as alternatives to conventional volatile refrigerants. To enable efficient and scalable barocaloric cooling, materials that undergo high-entropy, reversible phase transitions in the solid state in response to a small change in pressure are needed. Here, we report that pressure-induced spin-crossover (SCO) transitions in the molecular iron(II) complex Fe[HB(tz)3]2 (HB(tz)3- = bis[hydrotris(1,2,4-triazol-1-yl)borate]) drive giant and reversible barocaloric effects at easily accessible pressures. Specifically, high-pressure calorimetry and powder X-ray diffraction studies reveal that pressure shifts as low as 10 bar reversibly induce nonzero isothermal entropy changes, and a pressure shift of 150 bar reversibly induces a large isothermal entropy change (>90 J kg-1 K-1) and adiabatic temperature change (>2 K). Moreover, we demonstrate that the thermodynamics of the SCO transition can be fine-tuned through systematic deuteration of the tris(triazolyl)borate ligand. These results provide new insights into pressure-induced SCO transitions and further establish SCO materials as promising barocaloric materials.

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. The results denote that the expansion of the particles at the spin transition is effectively transferred to the macroscopic composite material, providing ca. 1–3% axial strain for 25% particle load. This strain is in excess of the ‘neat’ particle strain, which we attribute to particle-matrix mechanical coupling. On the other hand, the blocking stress (∼1 MPa) appears reduced by the softening of the composite around the spin transition temperature.

Switching endurance of the molecular spin crossover complex [Fe(HB(tz)3)2]: from single crystals to thin films and electronic devices

The spin-state switching behavior of [Fe(HB(tz)3)2] remains virtually unaltered for more than 104 endurance cycles in ambient air in single crystals, thin films and metal–insulator–metal junctions, highlighting the scope for technological applications.

The emergence of the local moment molecular spin transistor

Local moment molecular systems have now been used as the conduction channel in gated spintronics devices, and some of these three terminal devices might even be considered molecular spin transistors. In these systems, the gate voltage can be used to tune the molecular level alignment, while applied magnetic fields have an influence on the spin state, altering the magnetic properties, and providing insights to the magnetic anisotropy. More recently, the use of molecular spin crossover complexes, as the conduction channel, has led to devices that are both nonvolatile and have functionality at higher temperatures. Indeed, some devices have now been demonstrated to work at room temperature. Here, several molecular transistors, including those claiming to use single molecule magnets (SMM), are reviewed.

Resistance switching in large-area vertical junctions of the molecular spin crossover complex [Fe(HB(tz)3)2]: ON/OFF ratios and device stability

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)3)2]/M (with M = Al or Ca) were fabricated and investigated for their resistance switching properties. Current–voltage–temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

Contactless Spin Switch Sensing by Chemo-Electric Gating of Graphene.

Direct electrical probing of molecular materials is often impaired by their insulating nature. Here, graphene is interfaced with single crystals of a molecular spin crossover complex, [Fe(bapbpy)(NCS)2 ], to electrically detect phase transitions in the molecular crystal through the variation of graphene resistance. Contactless sensing is achieved by separating the crystal from graphene with an insulating polymer spacer. Next to mechanical effects, which influence the conductivity of the graphene sheet but can be minimized by using a thicker spacer, a Dirac point shift in graphene is observed experimentally upon spin crossover. As confirmed by computational modeling, this Dirac point shift is due to the phase-dependent electrostatic potential generated by the crystal inside the graphene sheet. This effect, named as chemo-electric gating, suggests that molecular materials may serve as substrates for designing graphene-based electronic devices. Chemo-electric gating, thus, opens up new possibilities to electrically probe chemical and physical processes in molecular materials in a contactless fashion, from a large distance, which can enhance their use in technological applications, for example, as sensors.

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)3 )2 ] 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 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.

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)3 )2 ] (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.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high-frequency all-optical switches working at room temperature.

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 = 11.5 ± 1.5 GPa), Young's modulus ( Y = 10.9 ± 1.0 GPa), and Poisson's ratio (ν = 0.34 ± 0.04) of the spin crossover complex [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). Crystal structure analysis revealed a pronounced anisotropy of the lattice compressibility, which was correlated with the difference in spacing between the molecules as well as by the distribution of the stiffest C-H···N interactions in different crystallographic directions. Switching the molecules from the low spin to the high spin state leads to a remarkable drop of the Young's modulus to 7.1 ± 0.5 GPa both in bulk and thin film samples. The results highlight the application potential of these films in terms of strain (ε = -0.17 ± 0.05%), recoverable stress (σ = -21 ± 1 MPa), and work density ( W/V = 15 ± 6 mJ/cm3).

A Bistable Microelectromechanical System Actuated by Spin-Crossover Molecules.

We report on a bistable MEMS device actuated by spin-crossover molecules. The device consists of a freestanding silicon microcantilever with an integrated piezoresistive detection system, which was coated with a 140 nm thick film of the [Fe(HB(tz)3 )2 ] (tz=1,2,4-triazol-1-yl) molecular spin-crossover complex. Switching from the low-spin to the high-spin state of the ferrous ions at 338 K led to a reversible upward bending of the cantilever in agreement with the change in the lattice parameters of the complex. The strong mechanical coupling was also evidenced by the decrease of approximately 66 Hz in the resonance frequency in the high-spin state as well as by the drop in the quality factor around the spin transition.

Microelectromechanical systems integrating molecular spin crossover actuators

Silicon MEMS cantilevers coated with a 200 nm thin layer of the molecular spin crossover complex [Fe(H2B(pz)2)2(phen)] (H2B(pz)2 = dihydrobis(pyrazolyl)borate and phen = 1,10-phenantroline) were actuated using an external magnetic field and their resonance frequency was tracked by means of integrated piezoresistive detection. The light-induced spin-state switching of the molecules from the ground low spin to the metastable high spin state at 10 K led to a well-reproducible shift of the cantilever's resonance frequency (Δfr = −0.52 Hz). Control experiments at different temperatures using coated as well as uncoated devices along with simple calculations support the assignment of this effect to the spin transition. This latter translates into changes in mechanical behavior of the cantilever due to the strong spin-state/lattice coupling. A guideline for the optimization of device parameters is proposed so as to efficiently harness molecular scale movements for large-scale mechanical work, thus paving the road for nanoelectromechanical systems (NEMS) actuators based on molecular materials.

Current Switching Coupled to Molecular Spin-States in Large-Area Junctions.

The fabrication of large-area vertical junctions with a molecular spin-crossover complex displaying concerted changes of spin degrees of freedom and charge-transport properties is reported. Fabricated devices allow spin-state switching in the spin-crossover layer to be triggered and probed by optical means, while detecting associated changes in electrical resistance in the junctions.

High-temperature photo-induced switching and pressure-induced transition in a cooperative molecular spin-crossover material

The thermal and photo-induced switching properties of the recently published molecular spin crossover complex [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO () have been investigated in detail through Raman spectroscopy. Magnetometric and single crystal X-ray diffraction kinetic studies within the hysteresis loop have proven its metastable character, which allowed establishing the shape of the true, quasi-static hysteresis loop. This is related to the proximity of the temperatures of the thermal high to low spin SCO (T1/2↓) to those of the relaxation of the thermally or photo-generated metastable states (T(TIESST) and T(LIESST), respectively). Green light irradiation within the hysteresis loop results in a complete low to high spin photo-switch. In addition, the SCO of at room temperature can be induced by applying pressure, as followed by Raman spectroscopy.

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. This observation as well as the rather slow dynamics suggest that the main control parameter is the inhomogeneous accommodation strain, which destabilizes the photoinduced domain.

Soft lithographic patterning of spin crossover complexes. Part 2: stimuli-responsive diffraction grating properties

Surface-relief diffraction gratings of various geometries were fabricated from the molecular spin crossover complex [FeII(hptrz)3](OTs)2 using the soft lithographic method described in Part 1 of this paper [Quintero et al., J. Mater. Chem., DOI: 10.1039/c2jm15662h]. The grating geometry and optical properties were obtained from atomic force microscopy and spectroscopic ellipsometry measurements. The grating diffraction efficiency (η) was determined as a function of the temperature at various wavelengths (400–700 nm) using a conoscopic microscopy setup. A significant effect of the molecular spin state change on η is demonstrated (ca. 3% modulation). Using scalar transmission theory the changes in the diffraction pattern were quantitatively traced back to the decrease of the real part of the refractive index (ΔnHL = –0.01) accompanying the 1A → 5T spin transition. These stimuli-responsive photonic devices exhibit useful properties for various applications. Here a proof of concept gas sensor application is demonstrated for the detection of alcohol vapor.

Pressure-induced two-step spin transition with structural symmetry breaking: X-ray diffraction, magnetic, and Raman studies

We have used single-crystal x-ray diffraction, Raman spectroscopy, and magnetometry to investigate the effect of hydrostatic pressures up to 16 kbar on the molecular spin crossover complex [Fe${}^{\mathrm{II}}$(bapbpy)(NCS)${}_{2}$]. A stepped first-order transition from the high-spin (HS) to low-spin (LS) phase was observed upon compression of single-crystal samples. The intermediate phase (IP) is stable between 4 and 11 kbar at room temperature. This phase is characterized by supercell reflections and tripling of the c-axis of the unit cell (C2/c) due to the formation of a periodic [HS-LS-LS] structural motif, as seen in the thermal stepped transition. The pressure-temperature phase diagram reveals an anomalous increase of the thermal hysteresis widths with increasing pressure and the stabilization of the IP across the investigated P-T space.

Electrical properties and non-volatile memory effect of the [Fe(HB(pz)3)2] spin crossover complex integrated in a microelectrode device

We report on the deposition of thin films of the [Fe(HB(pz)3)2] (pz = pyrazolyl) molecular spin crossover complex by thermal evaporation. By means of impedance measurements and Raman microspectroscopy, we show that the films maintain the structure and properties of the bulk material. The conductivity of the films decreases by ca. 2 orders of magnitude when the freshly deposited compound goes through a first (irreversible) thermal phase change above ca. 380 K. This property can be exploited as a non-volatile (read-only) memory effect.