Intrinsic flexoelectricity of van der Waals epitaxial thin films

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO3 and SrTiO3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films.This work was supported by the National Natural Science Foundation of China (Grants No. 12174174, No. 51962020, No. 51972157, No. 51702149, No. 11604135, and No. 11964017) and the Natural Science Foundation of Jiangxi Province (Grants No. 20212ACB214011, No. 20202ZDB01006, and No. 20192ACB21017) were also acknowledged. G.C. also acknowledges financial support from Project No. PID2019-108573GB-C21 funded by MCIN/AEI/10.13039/501100011033 and Severo Ochoa Grant No. SEV-2017-0706. L.S. thanks for support from Nanchang University.Peer reviewe

Inhibiting oxygen vacancies and twisting NbO6 octahedron in erbium modified KNN-based multifunctional ceramics

It is a challenge to obtain highly tunable multifunctional performances in one ferroelectric system by a simple approach to meet the miniaturization, integration, and functionalization requirements of advanced electronic components. Herein, rare earth erbium (Er) modulated 0.9K0.5Na0.5NbO3-0.1Sr(1-x)ErxTi(1-x/4)O3, (0.9KNN-0.1ST: xEr) transparent-photoluminescent-ferroelectric energy storage multifunctional ceramics are prepared to solve this problem. The effect of lattice distortion and oxygen vacancies by Er doping on the optical and electrical properties is systematically investigated. The Er3+ ions can introduce a large distortion of the NbO6 octahedron by replacing the A-site in KNN-based ceramics. Thanks to the higher c/a ratio and lower oxygen vacancy content are simultaneously obtained in 0.9KNN-0.1ST: 0.1Er ceramics. The effective energy storage density (Wrec) of 0.86 J/cm3, excellent near-infrared transmittance of 51.7% (1 100 nm) and strong green upconversion photoluminescence are achieved in this multifunctional ceramic. This study provides a solid basis for rare earth ions doped ferroelectric ceramics with tunable multifunctional properties and has significant potential for applications in optoelectronic devices

Strong metal-support interaction for 2D materials: application in noble metal/TiB₂ heterointerfaces and their enhanced catalytic performance for formic acid dehydrogenation

Strong metal-support interaction (SMSI) is a phenomenon commonly observed on heterogeneous catalysts. Here, direct evidence of SMSI between noble metal and 2D TiB2 supports is reported. The temperature-induced TiB2 overlayers encapsulate the metal nanoparticles, resulting in core-shell nanostructures that are sintering-resistant with metal loadings as high as 12.0 wt%. The TiOx -terminated TiB2 surfaces are the active sites catalyzing the dehydrogenation of formic acid at room temperature. In contrast to the trade-off between stability and activity in conventional SMSI, TiB2 -based SMSI promotes catalytic activity and stability simultaneously. By optimizing the thickness and coverage of the overlayer, the Pt/TiB2 catalyst displays an outstanding hydrogen productivity of 13.8 mmol g-1 cat h-1 in 10.0 m aqueous solution without any additive or pH adjustment, with >99.9% selectivity toward CO2 and H2 . Theoretical studies suggest that the TiB2 overlayers are stabilized on different transition metals through an interplay between covalent and electrostatic interactions. Furthermore, the computationally determined trends in metal-TiB2 interactions are fully consistent with the experimental observations regarding the extent of SMSI on different transition metals. The present research introduces a new means to create thermally stable and catalytically active metal/support interfaces for scalable chemical and energy applications.National Research Foundation (NRF)The authors are grateful for financial supports from the National Natural Science Foundation of China (Nos. 21872123, 21902027, and U19B2003) and Zhejiang Provincial Natural Science Foundation of China (No. LY18B030007). T.S.C. and W.L. acknowledge funding from National Research Foundation of Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme