Locking and Unlocking the Molecular Spin Crossover Transition

The Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)] (pz = pyrazol‐1‐yl, bipy = 2,2′‐bipyridine) can be locked in a largely low‐spin‐state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO₂ and Al₂O₃, or in powder samples by mixing with the strongly dipolar zwitterionic p ‐benzoquinonemonoimine C₆H₂(—? NH₂)₂(—? O)₂. Remarkably, it is found in both cases that incident X‐ray fluences then restore the [Fe{H₂B(pz)₂}₂(bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light‐ or X‐ray‐induced excited‐spin‐state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.     

Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure

2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in‐plane polarization. Remarkably, they exhibit transition temperatures (T_c ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the T_c enhancement is unveiled, which relies on the formation of γ‐SnTe, a van der Waals orthorhombic phase with antipolar inter‐layer coupling in few‐AL thick SnTe films. In this phase, 4n ? 2 AL (n = 1, 2, 3…) thick films are found to possess finite in‐plane polarization (space group Pmn2₁), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ‐SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS‐type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin‐orbit coupling and van der Waals structure, underlines the potential of atomically thin γ‐SnTe films for the development of novel spontaneous polarization‐based devices.     

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.     

High Field Magnetism of Sr0.78Y0.22CoO3−δ Under High Pressure

We performed high field magnetization measurements up to 50 T on polycrystalline samples of Sr_0.78Y_0.22CoO_3?δ at 4.2 K under high pressures up to 0.48 GPa. The magnetization at 0.21 GPa or 0.48 GPa increases with field and shows an anomaly around 30 T which is ascribed to be a field-induced spin-state transition, while the magnetization at ambient pressure shows no such anomaly and increases gradually at high fields. This peculiar magnetic behavior of this compound under pressure is attributed to small bulk modulus and susceptible spin state of Co³⁺ ions.     

Topological Quantum Phase Transition and Superconductivity Induced by Pressure in the Bismuth Tellurohalide BiTeI

A pressure‐induced topological quantum phase transition has been theoretically predicted for the semiconductor bismuth tellurohalide BiTeI with giant Rashba spin splitting. In this work, evolution of the electrical transport properties in BiTeI and BiTeBr is investigated under high pressure. The pressure‐dependent resistivity in a wide temperature range passes through a minimum at around 3 GPa, indicating the predicted topological quantum phase transition in BiTeI. Superconductivity is observed in both BiTeI and BiTeBr, while resistivity at higher temperatures still exhibits semiconducting behavior. Theoretical calculations suggest that superconductivity may develop from the multivalley semiconductor phase. The superconducting transition temperature, T_c, increases with applied pressure and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa for BiTeBr), followed by a slow decrease. The results demonstrate that BiTeX (X = I, Br) compounds with nontrivial topology of electronic states display new ground states upon compression.     

Coupling Mechanical and Electrical Properties in Spin Crossover Polymer Composites

Spin crossover particles of formula [Fe{(Htrz)₂(trz)}_0.9(NH₂‐trz)_0.3](BF₄)_1.1 and average size of 20 nm ± 8 nm are homogeneously dispersed in poly(vinylidene fluoride‐co‐trifluoro‐ethylene), P(VDF‐TrFE), and poly(vinylidene fluoride) (PVDF) matrices to form macroscopic (cm‐scale), freestanding, and flexible nanocomposite materials. The composites exhibit concomitant thermal expansion and discharge current peaks on cycling around the spin transition temperatures, i.e., new “product properties” resulting from the synergy between the particles and the matrix. Poling the P(VDF‐TrFE) (70–30 mol%) samples loaded with 25 wt% of particles in 18 MV m^?1 electric field results in a piezoelectric coefficient d₃₃ = ?3.3 pC N^?1. The poled samples display substantially amplified discharges and altered spin transition properties. Analysis of mechanical and dielectric properties reveals that both strain (1%) and permittivity (40%) changes in the composite accompany the spin transition in the particles, giving direct evidence for strong electromechanical couplings between the components. These results provide a novel route for the deployment of molecular spin crossover materials as actuators in artificial muscles and generators in thermal energy harvesting devices.     

Mechanical properties of materials based on MAX phases of the Ti-Al-C system

By studies of materials based on the Ti₃AlC₂ MAX phase containing inclusions of titanium carbide it has been shown that as a titanium carbide content increases from 2 to 99%, the nanohardness and Young modulus of the material increase from 2.0 ± 0.4 to 23.6 ± 1.2 GPa and from 137 ± 21 to 447 ± 11 GPa, respectively. The exponent in the equation of creep for these samples has been found to vary from 104 to 140, which indicates that mechanical properties of the material and, hence, of the Ti₃AlC₂ MAX phase depend on the strain rate only slightly. The formation of broad hysteresis loops has been observed in the cyclic loading/unloading of the indenter for samples consisting mainly of the Ti₃AlC₂ MAX phase. This points to serious losses in elastic energy of the MAX phase in strain cycling and, hence, the prospects of the MAX phase application as a damping material. It has been found that the microhardness of a sample consisting of 98% Ti₃AlC₂ produced by sintering under a load of 4.9 N was 2.1 GPa and its fracture toughness was high (no cracks from the indent corners were observed even under a load of 149 N). Microhardness and fracture toughness of the material consisting of 71% Ti₃AlC, 6% Ti₂AlC, and 23% TiC were 3.0 ± 0.6 GPa and 4.3 ± 1.4 MPa·m^1/2, respectively.     

Development of Multi-extreme ESR Measurement System in Kobe

Development of multi-extreme electron spin resonance (ESR) measurement system is in progress in Kobe. We have installed the pulsed magnetic field up to 55 T and our high field ESR system can cover the frequency region from 30 GHz to 7 THz. Our aim is to extend our high field ESR system to the multi-extreme ESR measurement system by combining low temperature, high pressure and nano techniques, and the combination with the high pressure is focused in the paper. High pressure can be achieved up to 1.4 GPa by using the transmission type piston cylinder pressure cell. We have succeeded in combining the high field and the high pressure. As an example, the pressure dependence of the spin gap in SrCu₂(BO₃)₂, S=1/2 Shustally-Sutherland dimer system, observed by our multi-extreme ESR measurement is presented. This measurement is the direct and precise measurement of the spin gap, and the spin gap decreased from 723 GHz (34.7 K) to 581 GHz (27.9 K) at 1.2 GPa.     

Ultrastiff,Strong, and Highly Thermally Conductive Crystalline Graphitic Films with Mixed Stacking Order

A macroscopic film (2.5 cm × 2.5 cm) made by layer‐by‐layer assembly of 100 single‐layer polycrystalline graphene films is reported. The graphene layers are transferred and stacked one by one using a wet process that leads to layer defects and interstitial contamination. Heat‐treatment of the sample up to 2800 °C results in the removal of interstitial contaminants and the healing of graphene layer defects. The resulting stacked graphene sample is a freestanding film with near‐perfect in‐plane crystallinity but a mixed stacking order through the thickness, which separates it from all existing carbon materials. Macroscale tensile tests yields maximum values of 62 GPa for the Young's modulus and 0.70 GPa for the fracture strength, significantly higher than has been reported for any other macroscale carbon films; microscale tensile tests yield maximum values of 290 GPa for the Young's modulus and 5.8 GPa for the fracture strength. The measured in‐plane thermal conductivity is exceptionally high, 2292 ± 159 W m^?1 K^?1 while in‐plane electrical conductivity is 2.2 × 10⁵ S m^?1. The high performance of these films is attributed to the combination of the high in‐plane crystalline order and unique stacking configuration through the thickness.     

Hot-pressed ceramic SiC–YAG materials

Dense SiC-based ceramic materials containing yttrium aluminum garnet (YAG) as an oxide sintering aid have been prepared by hot pressing in the temperature range 1750–1850°C. As a result of melting, the oxides fill spaces between the SiC particles, contributing to the densification of the material and mass transport during the hot pressing process. The present results demonstrate that relatively small amounts of the oxides (≤5 wt %) are needed to ensure a high degree of densification of the SiC–YAG materials. The best physicomechanical properties are offered by the SiC + 3 wt % YAG material sintered at a temperature of 1850°C: ρ = 3.24 ± 0.01 g/cm³, П = 1.1 ± 0.1%, σ_b = 640 ± 10 MPa; K_Ic = 6.4 ± 0.2 MPa m^1/2, Еel = 410 ± 20 GPa, and HV = 26.0 ± 0.2 GPa. This material experiences predominantly intercrystalline fracture.     

Evidence of low-symmetry phases in pressure-dependent Raman spectroscopic study of BaTiO<Subscript>3</Subscript>

In the earlier pressure-dependent Raman spectroscopic studies, it has been reported that BaTiO₃ undergoes a tetragonal to cubic phase transition above ~ 2 GPa, whereas pressure-dependent X-ray absorption, X-ray diffuse scattering studies and pair distribution function studies have reported the presence of a low-symmetry rhombohedral phase above ~ 2.3 GPa. In this report, we present our pressure-dependent Raman spectroscopic studies on polycrystalline BaTiO₃ which shows that it first undergoes a transition from tetragonal to orthorhombic/rhombohedral phase above ~ 2.6 GPa and then finally goes to the cubic phase above 8.4 GPa. Pressure-dependent synchrotron X-ray diffraction (SXRD) studies have also been carried out that provided rate of change of volume as a function of pressure resulting in bulk modulus of 215 ± 9 GPa.     

Abnormal Pressure‐Induced Photoluminescence Enhancement and Phase Decomposition in Pyrochlore La2Sn2O7

La₂Sn₂O₇ is a transparent conducting oxide (TCO) material and shows a strong near‐infrared fluorescent at ambient pressure and room temperature. By in situ high‐pressure research, pressure‐induced visible photoluminescence (PL) above 2 GPa near 2 eV is observed. The emergence of unusual visible PL behavior is associated with the seriously trigonal lattice distortion of the SnO₆ octehedra, under which the Sn–O1–Sn exchange angle θ is decreased below 22.1 GPa, thus enhancing the PL quantum yield leading to Sn ³P₁ → ¹S₀ photons transition. Besides, bandgap closing followed by bandgap opening and the visible PL appearing at the point of the gap reversal, which is consistent with high‐pressure phase decomposition, are discovered. The high‐pressure PL results demonstrate a well‐defined pressure window (7–17 GPa) with flat maximum PL yielding and sharp edges at both ends, which may provide a great calibration tool for pressure sensors for operation in the deep sea or at extreme conditions.     

Growth and Properties of Fully Strained SrCoOx (x ≈ 2.8) Thin Films on DyScO3

High quality epitaxial SrCoO_x (oxygen deficient SrCoO₃) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X‐ray diffraction as well as the XAS at the O K‐edge indicates an oxygen content of x ≈ 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic‐type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in‐plane strain in SrCoO_x thin films.     

Rutile TiO<Subscript>2</Subscript> films as electron transport layer in inverted organic solar cell

Titanium dioxide (TiO₂) thin films were prepared by sol–gel spin coating method and deposited on ITO-coated glass substrates. The effects of different heat treatment annealing temperatures on the phase composition of TiO₂ films and its effect on the optical band gap, morphological, structural as well as using these layers in P3HT:PCBM-based organic solar cell were examined. The results show the presence of rutile phases in the TiO₂ films which were heat-treated for 2 h at different temperatures (200, 300, 400, 500 and 600 °C). The optical properties of the TiO₂ films have altered by temperature with a slight decrease in the transmittance intensity in the visible region with increasing the temperature. The optical band gap values were found to be in the range of 3.28–3.59 eV for the forbidden direct electronic transition and 3.40–3.79 eV for the allowed direct transition. TiO₂ layers were used as electron transport layer in inverted organic solar cells and resulted in a power conversion efficiency of 1.59% with short circuit current density of 6.64 mA cm^?2 for TiO₂ layer heat-treated at 600 °C.     

Large‐Area Atomic Layers of the Charge‐Density‐Wave Conductor TiSe2

Layered transition metal (Ti, Ta, Nb, etc.) dichalcogenides are important prototypes for the study of the collective charge density wave (CDW). Reducing the system dimensionality is expected to lead to novel properties, as exemplified by the discovery of enhanced CDW order in ultrathin TiSe₂. However, the syntheses of monolayer and large‐area 2D CDW conductors can currently only be achieved by molecular beam epitaxy under ultrahigh vacuum. This study reports the growth of monolayer crystals and up to 5 × 10⁵ µm² large films of the typical 2D CDW conductor—TiSe₂—by ambient‐pressure chemical vapor deposition. Atomic resolution scanning transmission electron microscopy indicates the as‐grown samples are highly crystalline 1T‐phase TiSe₂. Variable‐temperature Raman spectroscopy shows a CDW phase transition temperature of 212.5 K in few layer TiSe₂, indicative of high crystal quality. This work not only allows the exploration of many‐body state of TiSe₂ in 2D limit but also offers the possibility of utilizing large‐area TiSe₂ in ultrathin electronic devices.     

Effect of annealing temperature on structural,optical and humidity sensing properties of indium tin oxide (ITO) thin films

Tin doped indium oxide (ITO) thin films were prepared by sol–gel spin coating method with In (NO₃)·3H₂O and SnCl₄·5H₂O as indium and tin sources, respectively. The as deposited samples were annealed at various temperature such as, 300, 400, 500 and 600?°C for 2 h in ambient atmosphere. The grown ITO thin films are polycrystalline in nature with cubic structure of In₂O₃ with the space group La3 and the results are in good agreement with the standard JCPDS data (card no#06-0416). In addition crystalline size increases with increasing annealing temperature from 25 to 55 nm. Polycrystalline with uniform smooth surface was observed by SEM micrographs. The optical band gap energy was found to be decreased from 3.85 to 3.23 eV as the annealing temperature is increased from 300 to 600?°C. The humidity sensing performance (high sensitivity and fast response time) was significantly improved for 600?°C thin films samples, which is probably due to smaller energy band gap and physisorption between the water molecules and the surface of the thin films. The films were further characterized by PL and EDS analysis. The effect of temperature on humidity sensing mechanism of ITO thin films is also discussed.     

Optical properties of nanostructured ruthenium dioxide thin films via sol–gel approach

High purity ruthenium dioxide (RuO₂) nanoparticles with the average size is about 9 nm in diameter are readily synthesized through a low cost sol–gel method. RuO₂ thin films have been deposited on SiO₂ substrates by sol–gel spin coating techniques at room temperature, followed by annealing at 500 °C for 2 h. The result of X-ray diffraction indicates that the RuO₂ nanoparticles are well crystallized with a rutile tetragonal structure. Morphological of RuO₂ films were characterized using atomic force microscopy (AFM), transmission electron microscopy and high resolution transmission electron microscopy. The AFM images confirmed a spherical-shape nanoparticles with diameter of 9 nm and surface roughness of 12 nm of the films. The optical absorption studies showed the presence of direct band transition with band gap equal to 1.87 eV. Refractive index and dielectric properties of the films were estimated from optical measurements. Room temperature photoluminescence of RuO₂ film showed an emission band at 432 nm.     

High‐Temperature Shape Memory Effect in Ti‐Ta Thin Films Sputter Deposited at Room Temperature

Ti‐Ta based alloys are potential high‐temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti‐Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as‐deposited co‐sputtered Ti₇₀Ta₃₀, Ti₆₈Ta₃₂ and Ti₆₇Ta₃₃ films, independent of the substrate. A Ti₇₀Ta₃₀ free‐standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one‐way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti‐Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.     

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 °C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni_24.7Ti_50.3Pd_25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 °C and ∼8.5 °C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 °C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82 GPa and hardness of ∼4.7 GPa in martensite phase.     

Effect of high pressure and torsional strain on the structure,microstresses, 55Mn NMR,and magnetoresistance of La0.6Sr0.3Mn1.1O3 ± δ nanopowders

The effects of high pressure (3 GPa) and torsional strain on the structure and properties of compacts prepared from nonstoichometric lanthanum-strontium manganite La_0.6Sr_0.2Mn_1.2O_{3 ± δ} (LSMO) nanopowder with a perovskite structure have been studied for the first time using X-ray diffraction, low-temperature Ar adsorption, electron microscopy, and magnetic (χ_ac, ⁵⁵Mn NMR, and magnetoresistance) measurements. The laws governing the influence of this treatment on the structure, local valence and magnetic states of manganese ions and their inhomogeneous environment (involving other ions and defects), and on the resistivity, microstresses, coercivity, Curie temperature, and magnetoresistance of LSMO nanopowder compacts have been determined. A positive effect of several cycles of torsion under pressure on the magnetoresistance of this material has been observed for the first time.