Manipulating electron redistribution to achieve electronic pyroelectricity in molecular [FeCo] crystals

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

Polarity-dependent nonlinear optics of nanowires under electric field

Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. Among these properties are piezoelectricity, ferroelectricity, pyroelectricity, and the bulk photovoltaic effect. Here we report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively. This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. EFM-SHG may be observed in any type of polar nanostructures and used to map the absolute polarity of materials at the nanoscale.

Stacking-engineered ferroelectricity in bilayer boron nitride

2D ferroelectrics with robust polarization down to atomic thicknesses provide building blocks for functional heterostructures. Experimental realization remains challenging because of the requirement of a layered polar crystal. Here, we demonstrate a rational design approach to engineering 2D ferroelectrics from a non-ferroelectric parent compound via employing van der Waals assembly. Parallel-stacked bilayer boron nitride exhibits out-of-plane electric polarization that reverses depending on the stacking order. The polarization switching is probed via the resistance of an adjacently stacked graphene sheet. Twisting the boron nitride sheets by a small angle changes the dynamics of switching thanks to the formation of moiré ferroelectricity with staggered polarization. The ferroelectricity persists to room temperature while keeping the high mobility of graphene, paving the way for potential ultrathin nonvolatile memory applications.

Manipulating electron redistribution to achieve exotic electronic pyroelectricity in dynamic [FeCo] crystals

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for the practical uses remains a formidable challenge, because in conventional ferroelectric materials the pyroelectric effect promptly declines upon cooling from the transition temperature. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice resulting in the electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin transition and redistribution of electron density between redox isomers of high-spin Fe through a charge transfer between the Fe atom and redox active ligand. As a result, temperature dependence of the pyroelectric behavior reported here is opposite and originates from the interconversion between three distinct electronic states. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

K3V2O3F4(IO3)3: A High-Performance SHG Crystal Containing Both Five and Six-coordinated V5+ Cations

The combination of d0 transition metal oxofluoride group with iodate anions assist to synthesize the polar crystals. Herein, a novel polar crystal, K3V2O3F4(IO3)3, which is the first metal vanadium iodate...