Origin of giant negative piezoelectricity in a layered van der Waals ferroelectric

Abstract

Recent research on piezoelectric materials is predominantly devoted to enhancing the piezoelectric coefficient, but overlooks its sign, largely because almost all of them exhibit positive longitudinal piezoelectricity. The only experimentally known exception is ferroelectric polymer poly(vinylidene fluoride) and its copolymers, which condense via weak van der Waals (vdW) interaction and show negative piezoelectricity. Here we report quantitative determination of giant intrinsic negative longitudinal piezoelectricity and electrostriction in another class of vdW solids—two-dimensional (2D) layered ferroelectric CuInP2S6. With the help of single crystal x-ray crystallography and density-functional theory calculations, we unravel the atomistic origin of negative piezoelectricity in this system, which arises from the large displacive instability of Cu ions coupled with its reduced lattice dimensionality. Furthermore, the sizable piezoelectric response and negligible substrate clamping effect of the 2D vdW piezoelectric materials warrant their great potential in nanoscale, flexible electromechanical devices.

Purely in-plane ferroelectricity in monolayer SnS at room temperature

“…In contrast to the 3D materials, 2D layered materials have a dangling-bond-free surface with van der Waals (vdW) gap, and hence, they maintain the intrinsic properties even at an ultrathin thickness. Recent intensive works on 2D ferroelectrics [7][8][9][10][11][12][13][14][15][16][17][18][19][20] have experimentally demonstrated stable ferroelectricity down to the ultimate monolayer thickness for out-of-plane ferroelectrics (MoTe 2 7 and WTe 2 8 ) and in-plane/out-of-plane intercorrelated ferroelectrics (α-In 2 Se 3 13,14 and SnTe 15 ). Even in 2D ferroelectrics, however, the spontaneous polarization is also degraded with decreasing thickness when the 2D ferroelectric layer is vertically sandwiched with metals, owing to the depolarization field at metal/ferroelectric interfaces 21 , as in 3D ferroelectrics.…”

Ferroelectric “gourd” goes into vdW atomic cage

“…Moreover, the combination between "hard" phosphorus-sulfur framework and "soft" vdW *Received September 8, 2020. This article also can be found at http://journal.hep.com.cn/fop/EN/10.1007/s11467-020-0998-9 layer lead to exotic negative piezoelectricity, which is the second experimentally confirmed one (and the first in inorganic materials) [5]. Also, thanks to the high mobility of small copper ions and loose framework, the ionic conduction is prominent in this system [7], an uncommon property for ferroelectric materials and important for ionic battery applications.…”

“…The charge asymmetry of copper-indium pairs is switchable within the cage. Surprisingly, the copper can even move outside the cage a little, leading to rare quadruple polarization states [5,6]. Moreover, the combination between "hard" phosphorus-sulfur framework and "soft" vdW *Received September 8, 2020.…”

Ion adsorption-induced reversible polarization switching of a van der Waals layered ferroelectric

“…Recently CuInP 2 S 6 (CIPS), a typical room-temperature van der Waals (vdW) layered ferroelectric crystal 19,20,21 , has attracted intensive attention due to its unexpected features including giant negative piezoelectricity 22 , tunable quadruple-well ferroelectric nature 23 , negative capacitance characteristic 24 and the interplay between ferroelectricity and ionic conductivity 25,26,27 . These unique characteristics not only bring new insights into the fundamental research 28,29 , but also provide new opportunities for diverse applications of layered ferroelectrics 30,31 .…”