Finite Size Effects on the Switching Dynamics of Spin-Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge-Transfer Phase Transition

We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge-transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+-N-C-Fe2+ tetragonal phase to the high temperature Mn2+-N-C-Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out-of-equilibrium dynamics leading to this phase is multi-scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N-C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+-N-C-Fe2+ phase creates small charge-transfer polarons [Mn2+-N-C-Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge-transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+-N-C-Fe2+ species. Above a threshold fluence, when a critical fraction of small CT-polarons is reached, the macroscopic phase transition to the persistent Mn2+-N-C-Fe3+ cubic phase occurs within ≃ 100 ps. This non-linear photo-response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.

Spin-State Switching of Spin-Crossover Complexes on Cu(111) Evidenced by Spin-Flip Spectroscopy

Spin-crossover compounds can be switched between two stable states with different magnetic moments, conformations, electronic, and optical properties, which opens appealing perspectives for technological applications including miniaturization down to the scale of single molecules. Although control of the spin states is crucial their direct identification is challenging in single-molecule experiments. Here we investigate the spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl)3)2] on a Cu(111) surface with scanning tunneling microscopy and density functional theory calculations. Spin crossover of single molecules in dense islands is achieved via electron injection. Spin-flip excitations are resolved in scanning tunneling spectra in a magnetic field enabling the direct identification of the molecular spin state, and revealing the existence of magnetic anisotropy in the HS molecules.

Observation of a Picosecond Light-Induced Spin Transition in Polymeric Nanorods

Spin transition (ST) materials are attractive for developing photoswitchable devices, but their slow material transformations limit device applications. Size reduction could enable faster switching, but the photoinduced dynamics at the nanoscale remains poorly understood. Here, we report a femtosecond optical pump multimodal X-ray probe study of polymeric nanorods. Simultaneously tracking the ST order parameter with X-ray emission spectroscopy and structure with X-ray diffraction, we observe photodoping of the low-spin-lattice within ∼150 fs. Above a ∼16% photodoping threshold, the transition to the high-spin phase occurs following an incubation period assigned to vibrational energy redistribution within the nanorods activating the molecular spin switching. Above ∼60% photodoping, the incubation period disappears, and the transition completes within ∼50 ps, preceded by the elastic nanorod expansion in response to the photodoping. These results support the feasibility of ST material-based GHz optical switching applications.

Magnetoelectric and MIESST effects in spin crossover materials exhibiting symmetry-breaking

Giant magnetoelectric coupling and magnetic-field-induced spin state trapping (MIESST) were recently reported in spin crossover materials with polar phases. We discuss these phenomena considering the distinct contributions of the change of the molecular spin state, driven by the magnetic field, and the coupled structural symmetry-breaking during the stepwise change of electric polarisation or MIESST.

Ultrafast and persistent photoinduced phase transition at room temperature monitored by streaming powder diffraction

Ultrafast photoinduced phase transitions at room temperature, driven by a single laser shot and persisting long after stimuli, represent emerging routes for ultrafast control over materials' properties. Time-resolved studies provide fundamental mechanistic insight into far-from-equilibrium electronic and structural dynamics. Here we study the photoinduced phase transformation of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material, designed to exhibit a 75 K wide thermal hysteresis around room temperature between MnIIIFeII tetragonal and MnIIFeIII cubic phases. We developed a specific powder sample streaming technique to monitor by ultrafast X-ray diffraction the structural and symmetry changes. We show that the photoinduced polarons expand the lattice, while the tetragonal-to-cubic photoinduced phase transition occurs within 100 ps above threshold fluence. These results are rationalized within the framework of the Landau theory of phase transition as an elastically-driven and cooperative process. We foresee broad applications of the streaming powder technique to study non-reversible and ultrafast dynamics.

Surface-Induced Electronic and Vibrational Level Shifting of [Fe(py)2bpym(NCS)2] on Al(100)

It is essential that one understands how the surface degrees of freedom influence molecular spin switching to successfully integrate spin crossover (SCO) molecules into devices. This study uses density functional theory calculations to investigate how spin state energetics and molecular vibrations change in a Fe(II) SCO compound named [Fe(py)2bpym(NCS)2] when deposited on an Al(100) surface. The calculations consider an environment-dependent U to assess the local Coulomb correlation of 3d electrons. The results show that the adsorption configurations heavily affect the spin state splitting, which increases by 10-40 kJmol-1 on the surface, and this is detrimental to spin conversion. This effect is due to the surface binding energy variation across the spin transition. The preference for the low-spin state originates partly from the strong correlation effect. Furthermore, the surface environment constrains the vibrational entropy difference, which decreases by 8-17 Jmol-1K-1 (at 300 K) and leads to higher critical temperatures. These results suggest that the electronic energy splitting and vibrational level shifting are suitable features for characterizing the spin transition process on surfaces, and they can provide access to high-throughput screening of spin crossover devices.

Reversible Switching of Strong Light-Matter Coupling Using Spin-Crossover Molecular Materials

The formation of hybrid light-matter states through the resonant interaction of confined electromagnetic fields with matter excitations has emerged as a fascinating tool for controlling quantum-mechanical states and then manipulating the functionalities and chemical reactivity landscape of molecular materials. Here we report the first observation of switchable strong light-matter coupling involving bistable spin-crossover molecules. Spectroscopic measurements, supported by transfer-matrix and coupled-oscillator simulations, reveal Rabi splitting values of up to 550 meV, which at 15% of the molecular excitation energy enter the regime of ultrastrong coupling. We find that the thermally induced switching of molecules between their low-spin and high-spin states allows fine control of the light-matter hybridization strength, offering the appealing possibility of reversible switching between the ultrastrong- and weak-coupling regimes within a single photonic structure. Through this work, we show that spin-crossover molecular compounds constitute a promising class of active nanomaterials in the burgeoning context of tunable polaritonic devices and polaritonic chemistry.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule-substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

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.

Broadband high-contrast visible optical switches based on a spin-crossover material

Visible optical switches embedding a spin-crossover thin film and exploiting the frustrated total internal reflection operation principle are studied and optimized numerically with a view to obtain broadband high-contrast devices. A practical implementation using uncoated SF11 prisms embedding a 1-µm-thick layer of iron-triazolyl-borate complex as the thermo-active phase-change material is shown to support p-polarized modulation with contrast in excess of 90% over a spectral bandwidth greater than 270 nm and over an angular acceptance bandwidth of 0.45°, surpassing the performance achievable with optically resonant devices.

Dynamics of Spin Crossover Molecular Complexes

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion.

Thermal Stability and Kinetics of Single I2@ZIF-8 Particles

Thermogravimetric analysis (TGA) is a key material characterization method for studying the thermal stability and thermochemical process. However, the common TGA for bulk samples lacks sufficient spatial information, which blurs the intrinsic thermal decomposition characteristic and limits the understanding of the structure-performance relationship. Here, we report a dark-field microscope (DFM) method for studying thermal desorption process of I₂ from I₂-loaded zeolitic imidazolate framework-8 (I₂@ZIF-8). Because of the high spatial resolution, DFM enables the imaging and tracking of the local mass loss of I₂ in single I₂@ZIF-8 particles at different reaction temperatures. We obtain from the DFM images the single-particle thermogravimetric and differential thermogravimetric curves to evaluate the inherent thermal stability of single I₂@ZIF-8 particles. We also find the heterogeneous thermal decomposition property among different I₂@ZIF-8 particles. Furthermore, we demonstrate the capacity of DFM to quantitatively determine thermal kinetics parameters such as the diffusion coefficient and activation energy of I₂ in individual and multiple ZIF-8 particles. These useful results are essential for developing high-efficient porous adsorbents for the capture of I₂.

Sequential Activation of Molecular and Macroscopic Spin-State Switching within the Hysteretic Region Following Pulsed Light Excitation

Molecular spin-crossover (SCO) compounds constitute a promising class of photoactive materials exhibiting efficient photoinduced phase transitions (PIPTs). Taking advantage of the unique, picture-perfect reproducibility of the spin-transition properties in the compound [Fe(HB(1,2,4-triazol-1-yl)₃ )₂ ], the spatiotemporal dynamics of the PIPT within the thermodynamic metastability (hysteretic) region of a single crystal is dissected, using pump-probe optical microscopy. Beyond a threshold laser-excitation density, complete PIPTs are evidenced, with conversion rates up to 200 switched molecules per absorbed photon. It is shown that the PIPT takes place through the sequential activation of two (molecular and macroscopic) switching mechanisms, occurring on sub-microsecond and millisecond timescales, governed by the intramolecular and free energy barriers of the system, respectively. The main finding here is that the thermodynamic metastability has strictly no influence on the sub-millisecond switching dynamics. Indeed, before this millisecond timescale, the response of the crystal to the laser excitation involves a gradual, molecular conversion process, as if there were no hysteresis loop. Consequently, in this regime, even a 100% photoinduced conversion may not give rise to a PIPT. These results provide new insight on the intrinsic dynamical limits of the PIPT, which is an important issue from a technological perspective.

Room temperature optoelectronic devices operating with spin crossover nanoparticles

Molecular systems can exhibit multi-stimuli switching of their properties, with spin crossover materials having unique magnetic transition triggered by temperature and light, among others. Light-induced room temperature operation is however elusive, as optical changes between metastable spin states require cryogenic temperatures. Furthermore, electrical detection is hampered by the intrinsic low conductivity properties of these materials. We show here how a graphene underlayer reveals the light-induced heating that triggers a spin transition, paving the way for using these molecules for room temperature optoelectronic applications.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex

We investigate giant magnetoelectric coupling at a Mn³⁺ spin crossover in [Mn^IIIL]BPh₄ (L = (3,5-diBr-sal)₂323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη² coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

High-Repetition Rate Optical Pump-Nuclear Resonance Probe Experiments Identify Transient Molecular Vibrations after Photoexcitation of a Spin Crossover Material

Phonon modes play a vital role in the cooperative phenomenon of light-induced spin transitions in spin crossover (SCO) molecular complexes. Although the cooperative vibrations, which occur over several hundreds of picoseconds to nanoseconds after photoexcitation, are understood to play a crucial role in this phase transition, they have not been precisely identified. Therefore, we have performed a novel optical laser pump-nuclear resonance probe experiment to identify the Fe-projected vibrational density of states (pDOS) during the first few nanoseconds after laser excitation of the mononuclear Fe(II) SCO complex [Fe(PM-BiA)₂(NCS)₂]. Evaluation of the so obtained nanosecond-resolved pDOS yields an excitation of ∼8% of the total volume of the complex from the low-spin to high-spin state. Density functional theory calculations allow simulation of the observed changes in the pDOS and thus identification of the transient inter- and intramolecular vibrational modes at nanosecond time scales.

Sublimable Spin-Crossover Complexes: From Spin-State Switching to Molecular Devices

Spin-crossover (SCO) active transition metal complexes are an important class of switchable molecular materials due to their bistable spin-state switching characteristics at or around room temperature. Vacuum-sublimable SCO complexes are a subclass of SCO complexes suitable for fabricating ultraclean spin-switchable films desirable for applications, especially in molecular electronics/spintronics. Consequently, on-surface SCO of thin-films of sublimable SCO complexes have been studied employing spectroscopy and microscopy techniques, and results of fundamental and technological importance have been obtained. This Review provides complete coverage of advances made in the field of vacuum-sublimable SCO complexes: progress made in the design and synthesis of sublimable functional SCO complexes, on-surface SCO of molecular and multilayer thick films, and various molecular and thin-film device architectures based on the sublimable SCO complexes.

Nonvolatile Voltage Controlled Molecular Spin-State Switching for Memory Applications

Nonvolatile, molecular multiferroic devices have now been demonstrated, but it is worth giving some consideration to the issue of whether such devices could be a competitive alternative for solid-state nonvolatile memory. For the Fe (II) spin crossover complex [Fe{H2B(pz)2}2(bipy)], where pz = tris(pyrazol-1-yl)-borohydride and bipy = 2,2′-bipyridine, voltage-controlled isothermal changes in the electronic structure and spin state have been demonstrated and are accompanied by changes in conductance. Higher conductance is seen with [Fe{H2B(pz)2}2(bipy)] in the high spin state, while lower conductance occurs for the low spin state. Plausibly, there is the potential here for low-cost molecular solid-state memory because the essential molecular thin films are easily fabricated. However, successful device fabrication does not mean a device that has a practical value. Here, we discuss the progress and challenges yet facing the fabrication of molecular multiferroic devices, which could be considered competitive to silicon.

Low-spin and spin-crossover iron(II) complexes with pyridyl-benzimidazole ligands: synthesis, and structural, magnetic and solution study

Two tridentate ligands (L1 = 2,6-bis(1-(3,5-di-tert-butylbenzyl)-1H-benzimidazol-2-yl)pyridine and L2 = 2,6-bis(1-(4-tert-butylbenzyl)-1H-benzimidazol-2-yl)pyridine) and one didentate ligand (L3 = 1-(4-tert-butylbenzyl)-2-pyridine-2-yl-1H-benzimidazol) were used for the synthesis of eight mononuclear Fe(ii) compounds 1-8 containing miscellaneous counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported coordination compounds and revealed the octahedral geometry of metal centres in the complex dications of 1-8. Compounds 1-6 prepared from tridentate ligands were low-spin and, therefore, diamagnetic up to 400 K. On the other hand, compounds 7 and 8, in which the Fe(ii) centre was coordinated with didentate ligand L3, exhibited temperature and light triggered spin-crossover behaviour. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal and light induced spin state switching. In addition, the solution redox properties of compounds 1-8 were investigated by cyclic voltammetry.

Magnetic-Pole Flip by Millimeter Wave

In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, "focused-millimeter-wave-assisted magnetic recording (F-MIMR)," is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau-Lifshitz-Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.

Spin-Crossover Complexes in Direct Contact with Surfaces

The transfer of the inherent bistability of spin crossover compounds to surfaces has attracted considerable interest in recent years. The deposition of the complexes on surfaces allows investigating them individually and to further understand the microscopic mechanisms at play. Moreover, it offers the prospect of engineering switchable functional surfaces. We review recent progress in the field with a particular focus on the challenges and limits associated with the dominant experimental techniques used, namely near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). One of the main difficulties in NEXAFS-based experiments is to ascertain that the complexes are in direct contact with the surfaces. We show that molecular coverage determination based on the amplitude of the edge-jump of interest is challenging because the latter quantity depends on the substrate. Furthermore, NEXAFS averages the signals of a large number of molecules, which may be in different states. In particular, we highlight that the signal of fragmented molecules is difficult to distinguish from that of intact and functional ones. In contrast, STM allows investigating individual complexes, but the identification of the spin states is at best done indirectly. As quite some of the limits of the techniques are becoming apparent as the field is gaining maturity, their detailed descriptions will be useful for future investigations and for taking a fresh look at earlier reports.

Unprecedented switching endurance affords for high-resolution surface temperature mapping using a spin-crossover film

Temperature measurement at the nanoscale is of paramount importance in the fields of nanoscience and nanotechnology, and calls for the development of versatile, high-resolution thermometry techniques. Here, the working principle and quantitative performance of a cost-effective nanothermometer are experimentally demonstrated, using a molecular spin-crossover thin film as a surface temperature sensor, probed optically. We evidence highly reliable thermometric performance (diffraction-limited sub-µm spatial, µs temporal and 1 °C thermal resolution), which stems to a large extent from the unprecedented quality of the vacuum-deposited thin films of the molecular complex [Fe(HB(1,2,4-triazol-1-yl)₃)₂] used in this work, in terms of fabrication and switching endurance (>10⁷ thermal cycles in ambient air). As such, our results not only afford for a fully-fledged nanothermometry method, but set also a forthcoming stage in spin-crossover research, which has awaited, since the visionary ideas of Olivier Kahn in the 90’s, a real-world, technological application.

Developing novel thermometry techniques for nanoscale temperature measurements are vital for realizing efficient thermal management of nanoscale devices. Here, the authors report thermally stable spin-crossover material-based nanothermometers for high-resolution surface temperature mapping.

Heat Capacity and Thermal Damping Properties of Spin-Crossover Molecules: A New Look at an Old Topic

The thermally induced spin-crossover (SCO) phenomenon in transition metal complexes is an entropy-driven process, which has been extensively studied through calorimetric methods. Yet, the excess heat capacity associated with the molecular spin-state switching has never been explored for practical applications. Herein, the thermal damping effect of an SCO film is experimentally assessed by monitoring the transient heating response of SCO-coated metallic microwires, Joule-heated by current pulses. A damping of the wire temperature, up to 10%, is evidenced on a time scale of tens of microseconds due to the spin-state switching of the molecular film. Fast heat-charging dynamics and negligible fatigability are demonstrated, which, together with the solid-solid nature of the spin transition, appear as promising features for achieving thermal energy management applications in functional devices.

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.