  Nanoscale functional materials with ferroelectricity based on two-dimensional
(2D) van der Walls (vdW) materials have been a booming field due to their
underlying new physics and potential applications. Here, using DFT-based
first-principles calculations, we discovered an interlayer sliding induced
reversible phase transition between intralayer ferroelectric (FE) and
antiferroelectric (AFE) in bilayer group-IV monochalcogenides MX (M=Ge, Sn,
X=S, Se). This unusual behavior originates from the interlayer vdW interaction
leading to the in-plane lattice distortion and thus promoting intralayer
covalent bond breaking/formation in the top layer. The consequent
ultrahigh-frequent (MHz) and high electric polarization switching
(~40$\mu$C/cm$^2$) and the resultant electric output can be, in principle,
induced by interlayer sliding. It provides a promising way to design the
multifunctional 2D ferroelectric materials using vdW forces and broaden their
applications in nanogenerator or energy harvesting nanodevices via mechanical
sliding.
