Measurement of the Flexoelectric Coefficients in van der Waals Materials with Separation of Piezoelectricity

The flexoelectric coefficient is a key material parameter describing the interaction between theelectric polarization and strain gradient, which is of significance to design high-performance flexoelectricdevices. The macroscopic cantilever bending and truncated pyramid compression are common approaches tomeasure the flexoelectric coefficients of bulk materials. However, these conventional methods are challengingfor the small-sized van der Waals (vdW) materials that have recently shined in the field of flexoelectricity,especially for piezoelectric ones to separate flexoelectric and piezoelectric contribution. In this work, we designvdW materials-embedded multilayer structures for accurately measuring its flexoelectricity. An oscillatory four-point bending deformation is applied onto the multilayer structures and induces stable flexoelectric current.Combined with a theoretical model, the contribution of the piezoelectricity is separated through measuring thecurrent variation among the multilayer structures in which the vdW material is embedded in different planeposition. The flexoelectric coefficients of two typical vdW materials, piezoelectric CuInP₂S₆ (CIPS) and non-piezoelectric 2H-MoS₂, are measured as -25.6 nC/m and 174.1 nC/m, respectively. And large flexocouplingcoefficients are found in the both vdW materials. This work provides a new method for the intrinsic flexoelectricmeasurements of small-sized vdW materials with separating piezoelectric contribution and brings new insightsinto the exploration of high-performance flexoelectric materials.