  Twisted heterostructures of two-dimensional crystals offer almost unlimited
scope for the design of novel metamaterials. Here we demonstrate a
room-temperature ferroelectric semiconductor that is assembled using mono- or
few- layer MoS2. These van der Waals heterostructures feature broken inversion
symmetry, which, together with the asymmetry of atomic arrangement at the
interface of two 2D crystals, enables ferroelectric domains with alternating
out-of-plane polarisation arranged into a twist-controlled network. The latter
can be moved by applying out-of-plane electrical fields, as visualized in situ
using channelling contrast electron microscopy. The interfacial charge transfer
for the observed ferroelectric domains is quantified using Kelvin probe force
microscopy and agrees well with theoretical calculations. The movement of
domain walls and their bending rigidity also agrees well with our modelling
results. Furthermore, we demonstrate proof-of-principle field-effect
transistors, where the channel resistance exhibits a pronounced hysteresis
governed by pinning of ferroelectric domain walls. Our results show a potential
venue towards room temperature electronic and optoelectronic semiconductor
devices with built-in ferroelectric memory functions.
