RESUMEN
Inherent symmetry breaking at the interface has been fundamental to a myriad of physical effects and functionalities, such as efficient spin-charge interconversion, exotic magnetic structures and an emergent bulk photovoltaic effect. It has recently been demonstrated that interface asymmetry can induce sizable piezoelectric effects in heterostructures, even those consisting of centrosymmetric semiconductors, which provides flexibility to develop and optimize electromechanical coupling phenomena. Here, by targeted engineering of the interface symmetry, we achieve piezoelectric phenomena behaving as the electrical analogue of the negative Poisson's ratio. This effect, termed the auxetic piezoelectric effect, exhibits the same sign for the longitudinal (d33) and transverse (d31, d32) piezoelectric coefficients, enabling a simultaneous contraction or expansion in all directions under an external electrical stimulus. The signs of the transverse coefficients can be further tuned via in-plane symmetry anisotropy. The effects exist in a wide range of material systems and exhibit substantial coefficients, indicating potential implications for all-semiconductor actuator, sensor and filter applications.
RESUMEN
Hybrid perovskites have two properties that are absent in traditional inorganic photovoltaic materials, namely, polarization and mobile ionic defects, the interaction between which may introduce new features into the materials. By using the first-principles calculations, we find that the formation energies of the vacancy defects at a tetragonal MAPbI3(110) surface are highly related to the surface polarization. The positive total polarization and local polarization of MA facilitate the formation of surface MA vacancies, whereas the negative total polarization and local polarization of MAI are favorable for the formation of surface iodine vacancies. The phenomena can be explained quantitatively on the basis of the two kinds of Coulomb interactions between the charged defect and the polarization-induced electrostatic field. The comprehensive insights into the interaction between the polarization and the ionic defects in hybrid perovskites can provide a new avenue for defect control for high-performance perovskite solar cells via surface polarization.
RESUMEN
Cr2O3 is generally considered as an impurity phase with negative effects on the electrochemical performance of NaCrO2 because it may cause a certain degree of capacity loss. In this study, however, we have found the bright side of Cr2O3 as a protective coating material, which greatly improves the Na+ storage capability, especially the cycling stability, of NaCrO2. After 1000 cycles at 10C, a capacity of 100.4 mAh g-1 with a high capacity retention of 84.8% can be achieved for a Cr2O3-coated NaCrO2 sample. The optimal sample exhibits a rate performance with 108.0 mAh g-1 at a high rate of 60C. Cyclic voltammetry analysis indicates that such an in situ-formed inactive Cr2O3 layer has little influence on Na+ diffusion in NaCrO2 electrodes, but it prevents the direct contact between the active material and the electrolyte, suppressing the side reactions effectively.