  Van der Waals (vdW) polytypes of broken inversion and mirror symmetries were
recently shown to exhibit switchable electric polarization even at the ultimate
two-layer thin limit. Their out-of-plane polarization was found to accumulate
in a ladder-like fashion with each successive layer, offering 2D building
blocks for the bottom-up construction of 3D ferroelectrics. Here, we
demonstrate experimentally that beyond a critical stack thickness, the
accumulated polarization in rhombohedral polytypes of molybdenum disulfide
(r-MoS2) saturates. The underlying saturation mechanism, deciphered via DFT and
self-consistent Poisson-Schr\"odinger calculations, point to a purely
electronic redistribution involving: (1) polarization-induced bandgap closure
that allows for cross-stack charge transfer and the emergence of free surface
charge; (2) reduction of the polarization saturation value, as well as the
critical thickness at which it is obtained, by the presence of free carriers.
The resilience of polar layered structures to atomic surface reconstruction,
which is essentially unavoidable in polar 3D crystals, potentially allows for
the design of new devices with mobile surface charges. Our findings, which are
of general nature, should be accounted for when designing switching and/or
conductive devices based on ferroelectric layered materials.
