Cumulative Polarization Coexisting with Conductivity at Interfacial Ferroelectrics

TL;DR

This paper investigates interfacial ferroelectricity in 2D layered materials, revealing multi-state polarization stability under doping and elucidating charge redistribution mechanisms through experiments and DFT calculations.

Contribution

It uncovers the coexistence of cumulative polarization with conductivity in 2D ferroelectric stacks and introduces multi-state 'ladder ferroelectrics' with high charge doping resilience.

Findings

Potential steps indicate highly confined interfacial electric fields

Internal polarization persists at high doping levels

Charge redistribution mechanisms identified via DFT calculations

Abstract

Ferroelectricity in atomically thin bilayer structures has been recently predicted1 and measured[2-4] in two-dimensional (2D) materials with hexagonal non-centrosymmetric unit-cells. Interestingly, the crystal symmetry translates lateral shifts between parallel 2D layers to a change of sign in their out-of-plane electric polarization, a mechanism referred to as "Slide-Tronics"[4]. These observations, however, have been restricted to switching between only two polarization states under low charge carrier densities[5-12], strongly limiting the practical application of the revealed phenomena[13]. To overcome these issues, one needs to explore the nature of the polarization that arises in multi-layered van der Waals (vdW) stacks, how it is governed by intra- and inter-layer charge redistribution, and to which extent it survives the introduction of mobile charge carriers, all of which are presently unknown14. To explore these questions, we conduct surface potential measurements of parallel WSe2 and MoS2 multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields, which provide means to design multi-state "ladder ferroelectrics". Furthermore, we find that the internal polarization remains significant upon electrostatic doping of a mobile charge carrier density as high as 1013 cm-2, with substantial in-plane conductivity. Using first-principles calculations based on density functional theory (DFT), we trace the extra charge redistribution in real and momentum space and identify an eventual doping-induced depolarization mechanism.