Polarization Saturation in Multi-layered Interfacial Ferroelectrics

TL;DR

This paper investigates the saturation of out-of-plane polarization in multilayered ferroelectric materials, revealing a critical thickness beyond which polarization saturates due to electronic redistribution, impacting device design.

Contribution

It experimentally demonstrates polarization saturation in rhombohedral MoS2 polytypes and elucidates the electronic mechanisms behind this phenomenon using DFT and Poisson-Schrödinger calculations.

Findings

Polarization saturates beyond a critical thickness in multilayered MoS2.

Electronic redistribution causes bandgap closure and free surface charge emergence.

Free carriers reduce the saturation polarization and critical thickness.

Abstract

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.