  Two-dimensional materials with ferroelectric properties break the size effect
of conventional ferroelectric materials and unlock unprecedented potentials of
ferroelectric-related application at small length scales. In this work, using
density functional theory (DFT) calculations, we discover a
tribo-ferroelectricity behavior in a group of bilayer group-IV
monochalcogenides (MX, with M = Ge, Sn and X = S, Se). Upon interlayer sliding
over an in-plane unit cell length, the top layer exhibits a reversible
intralayer ferroelectric switching, leading to a reversible transition between
the ferroelectric (electric polarization of 40$\mu$C/cm$^2$) and
antiferroelectric states in the bilayer MXs. Our results show that the
interlayer van der Waals interaction, which is usually considered to be weak,
can actually generate an in-plane lattice distortion and thus cause the
breaking/forming of intralayer covalent bonds in the top layer, leading to the
observed tribo-ferroelectricity phenomenon. This unique property has several
advantages for energy harvesting over existing piezoelectric and triboelectric
nanogenerators. The interlayer sliding-induced polarization change is as high
as 40$\mu$C/cm$^2$, which can generate an open-circuit voltage two orders of
magnitude higher than that of MoS$_2$-based nanogenerators. The polarization
change occurs over a time period for interlayer sliding over a unit-cell
length, leading to an ultrahigh polarization changing rate and thus an
ultrahigh short-circuit current. The theoretical prediction of power output for
the tribo-ferroelectric bilayer MXs at a moderate sliding speed 1 m/s is four
orders of magnitude higher than the MoS$_2$ nanogenerator, indicating great
potentials in energy harvesting applications.
