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Thin films with thicknesses in the range between ca. 10–50 nm of the spin crossover (SCO) compound {Fe(pyrazine)[μ4-M(CN)4]} (M = Ni, Pt) have been deposited on fused silica substrates using a sequential assembly method and 4-pyridinecarboxylic acid as anchoring layer. Film morphology and crystallinity were assessed by means of atomic force microscopy and grazing incidence X-ray diffraction, respectively. The intensity of the π–π* transition of the pyrazine ligand at 270 nm, being rather insensitive to the spin state of the complex, was used to follow the film growth as a function of different deposition parameters. On the other hand, the spin state changes were inferred from the temperature dependence of absorption bands appearing at 540, 490 and 310 nm in the low spin state. In line with their amorphous nature, each film displays a very gradual thermal spin crossover between ca. 100–300 K, independently of its thickness and deposition conditions. These results are not only interesting to better understand the effects of size reduction and organization on the SCO phenomenon, but the deposition of these SCO compounds on electrically insulating and/or optically transparent oxide surfaces opens also the door for various photonic or electronic applications. [ABSTRACT FROM AUTHOR]

We used a spray-coating process to cover silicon microcantilevers with ca. 33 wt% [Fe(Htrz)2(trz)](BF4)@P(VDF70-TrFE30) nanocomposite thin films of 1500 nm thickness. The bilayer cantilevers were then used to investigate the thermomechanical properties of the composites through a combined static and dynamic flexural analysis. The out-of-plane flexural resonance frequencies were used to assess the Young's modulus of the spray-coated films (3.2 GPa). Then, the quasi-static flexural bending data allowed us to extract the actuation strain (1.3%) and an actuation stress (7.7 MPa) associated with the spin transition in the composite. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on the vacuum thermal deposition of bilayer thin films of the luminescent complex Ir(ppy)3, tris[2-phenylpyridinato-C2,N]iridium(III), and the spin crossover complex [Fe(HB(tz)3)2], bis[hydrotris(1,2,4-triazol-1-yl)borate]iron(II). Switching the spin state of iron ions from the low spin to the high spin state around 337 K leads to a reversible jump of the luminescence intensity, while the spectrum shape and the luminescence lifetime remain unchanged. The luminescence modulation occurs due to the different UV light absorption properties of the iron complex in the two spin states and its magnitude can therefore be precisely adjusted by varying the film thickness. These multilayer luminescence switches hold potential for micro- and nanoscale thermal sensing and imaging applications. [ABSTRACT FROM AUTHOR]

We report large-area (~3mm2), pinhole free crossbar junctions of thin films of the molecular complex [Fe(HB(tz)3)2] displaying spin transition around 336 K. The charge transport in the thinner junctions (10 and 30 nm) occurs by a tunneling mechanism, which is not affected substantially by the spin transition. The thicker junctions (100 and 200 nm) exhibit rectifying behavior and a reproducible drop of their electrical resistance by ca. 65-80% when switching the molecules from the high-spin to the low-spin state. This current modulation is ascribed to a bulk-limited charge transport mechanism via a thermally activated hopping process. The demonstrated possibility of resistance switching in ambient conditions provides appealing prospects for the implementation of molecular spin crossover materials in electronic and spintronic devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

From the development of new methods for the synthesis and patterning of nanometre-scale thin films and particles, to the first investigations of charge transport and photonic properties, there has been a proliferation of research concerning spin crossover nanomaterials in recent years. Studies have aimed at addressing fundamental questions concerning size-reduction effects, as well as striving towards practical applications in this important class of bistable molecular materials. This microreview describes the most recent achievements and highlights possible future directions in this field. [ABSTRACT FROM AUTHOR]

An optimised procedure was developed for the layer-by-layer deposition of the Hofmann clathrate-like coordination compound {Fe(pyrazine)[Pt(CN)4]} either as continuous or as nano-patterned thin films. Characterization of the thickness and topography of the thin films by atomic force microscopy (AFM) and by surface plasmon resonance (SPR) spectroscopy, which also yields the layer's refractive index and losses, are reported. We found that the films are of good optical quality and the results of both AFM and SPR experiments are in good agreement with the theoretical predictions of the films thicknesses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigated the solvent vapor annealing process of thermally evaporated thin films of the [FeII(HB(tz) 3) 2 ] (tz = 1,2,4-triazol-1-yl) spin crossover complex. Using grazing incidence X-ray diffraction, atomic force microscopy, and variable-temperature UV spectrophotometry, we show how the crystallinity, morphology, and spin crossover properties of the resulting films are altered by exposure to vapors of five different solvents: diethyl ether, acetone, dichloromethane, ethanol, and water. Remarkably, most solvents yielded highly oriented crystalline films displaying an abrupt and complete spin transition around 336 K. A notable exception is the case of dichloromethane—suggesting that the solvent ability to accept hydrogen bonding might be a key factor in controlling the film crystallinity. On the other hand, only water vapor treatment affords for smooth and continuous film morphology. The recrystallization process in controlled humidity was thus followed in situ by monitoring the UV absorbance of the films. This study revealed the critical importance of the relative humidity (RH): for values above a threshold of ca. 72% RH (at 298 K) a very fast (seconds) crystallization occurs leading to robust, crystalline films, whereas crystallization at lower RH values is slower (hours) and leads to less stable, semicrystalline films displaying incomplete spin transitions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Two different nanofabrication processes of SERS substrates are proposed. [•] Both fabrication pathways permit to enhance Raman scattering of spin-crossover ultrathin films. [•] The two approaches probe the molecules at the close vicinity of the SERS substrate. [•] Thermal study of SERS spectrum indicates that the thin films remain in the high spin state regardless of the temperature. [Copyright &y& Elsevier]

Nano-objects and thin films displaying molecular spin-crossover phenomena have recently attracted much attention. However, the investigation of spin crossover at reduced sizes is still a big challenge. Here we demonstrate that surface plasmon polariton waves propagating along the interface between a metal and a dielectric layer can be used to detect the spin-state changes in the latter with high sensitivity, even at the nanometer scale. [ABSTRACT FROM AUTHOR]