  Van der Waals (vdW) materials have greatly expanded our design space of
heterostructures by allowing individual layers to be stacked at non-equilibrium
configurations, for example via control of the twist angle. Such
heterostructures not only combine characteristics of the individual building
blocks, but can also exhibit emergent physical properties absent in the parent
compounds through interlayer interactions. Here we report on a new family of
emergent, nanometer-thick, semiconductor 2D ferroelectrics, where the
individual constituents are well-studied non-ferroelectric monolayer transition
metal dichalcogenides (TMDs), namely WSe2, MoSe2, WS2, and MoS2. By stacking
two identical monolayer TMDs in parallel, we obtain electrically switchable
rhombohedral-stacking configurations, with out-of-plane polarization that is
flipped by in-plane sliding motion. Fabricating nearly-parallel stacked
bilayers enables the visualization of moir\'e ferroelectric domains as well as
electric-field-induced domain wall motion with piezoelectric force microscopy
(PFM). Furthermore, by using a nearby graphene electronic sensor in a
ferroelectric field transistor geometry, we quantify the ferroelectric built-in
interlayer potential, in good agreement with first-principles calculations. The
novel semiconducting ferroelectric properties of these four new TMDs opens up
the possibility of studying the interplay between ferroelectricity and their
rich electric and optical properties.
