Negative piezoelectricity in quasi-two/one-dimensional ferroelectrics

Abstract In recent years, the investigation of low-dimensional ferroelectrics has attracted great attention for their promising applications in nano devices. Piezoelectricity is one of the most core properties of ferroelectric materials, which plays an essential role in micro-electromechanical systems. Very recently, anomalous negative piezoelectricity has been predicted/discovered in many quasi-two-dimensional layered ferroelectric materials. In this Topical Review, we will briefly introduce the negative piezoelectricity in quasi-two/one-dimensional ferroelectrics, including its fundamental concepts, typical materials, theoretical predictions, as well as experimental phenomena. The underlying physical mechanisms for negative piezoelectricity are divergent and vary case by case, which can be categorized into four types: first, the soft van der Waals (vdW) layer is responsible for the volume shrinking upon pressure while the electric dipoles are from the non vdW layer; second, the noncollinearity of local dipoles creates a ferrielectricity, which leads to orthogonal ferroelectric and antiferroelectric axes; third, the electric dipoles come from interlayer/interchain couplings, which can be enhanced during the volume shrinking; fourth, the special buckling structure contributes to local dipoles, which can be enhanced upon pressure. In real materials, more than one mechanism may work together. Finally, future directions of negative piezoelectricity and their potential applications are outlined.