Origin of Two-Dimensional Vertical Ferroelectricity in WTe₂ Bilayer and Multilayer

Abstract

In a recent report, room-temperature vertical ferroelectricity was experimentally shown in WTe2 bilayer, while its mechanism of ferroelectric switching without vertical ion displacements remains unclarified. In this work, we reveal its origin by first-principles calculations that the polarization stems from uncompensated interlayer vertical charge transfer depending on in-plane translation, which can be switched upon interlayer sliding. The calculated results are consistent with experimental data, and a similar switching mechanism can be applied to a multilayer counterpart. Despite its small ferroelectric switching barrier and polarization, the in-plane rigidity of WTe2 layer gives rise to a high Curie temperature. A moire pattern of ferroelectric domain superlattice can be formed and tuned upon a small-angle twist of bilayer, which is unique compared with traditional ferroelectrics. Similar interlayer translational ferroelectricity may exist in a series of van der Waals bilayers or even bulk phases.

Nonvolatile Electrical Control and Heterointerface‐Induced Half‐Metallicity of 2D Ferromagnets

“…Note that ferroelectric materials are divided into in‐plane and out‐of‐plane ferroelectricity. Black phosphorus (BP)‐like group IV monochalcogenides such as GeSe and SnS are the typical in‐plane ferroelectric materials, while In 2 Se 3 , MXenes Sc 2 CO 2 , and Hf 2 VC 2 F 2 , 1T‐phase MoS 2 , bilayer BN, WTe 2 , etc. possess out‐of‐plane polarization.…”

Niobium oxide dihalides NbOX2: a new family of two-dimensional van der Waals layered materials with intrinsic ferroelectricity and antiferroelectricity

“…Moreover, the energy cost associated with ''collective'' reversal of whole column polarization is negligible compared to the ''isolated'' cation dipole switching energy (C y c C x , Table 2), thus T C of NbOX 2 monolayer is mainly determined by ''isolated'' rather than ''collective'' cation dipole switching events. 49 Therefore, T C is not simply proportional to the energy difference associated with ''collective'' polarization reversal (FE-to-AFE phase transition).…”

Review on ferroelectric/polar metals

“…7a, one can see that the polarization can be switched by moving the upper layer along the −y axis by a distance of 1 + 2, as evidenced by the inversion of the charge distribution between the upper and lower layers in Fig. 7b-c. Yang et al also showed that the switchability of multilayer WTe2 can also be described by this model by selectively sliding some specific layers 100) . This interlayer sliding model explains some important aspects in WTe2, but also left some critical questions unanswered.…”

“…The natural questions following the demonstration of the first ferroelectric metal are: what is the origin of the ferroelectricity in WTe2 and what is its switching mechanism? Following the suggestion by Fei et al that "the polarization could principally involve a relative motion of the electron cloud relative to the ion cores, rather than a lattice distortion", Yang et al 100) and Liu et al. 101) demonstrated that the polarization stems from uncompenstated interlayer charge transfer.…”

“…The electrodes are composed of two multilayer (≥ 3) WTe2 covered with a ferromagnetic metal, the middle channel structure is composed of a bilayer WTe2, and the gate and back electrodes allow the application of the required voltage to switch the dielectric polarization of the bilayer WTe2. Based on the model that the polarization and hence the electric potential between bilayer WTe2 can be switched by interlayer sliding, Yang et al proposed a bilayer WTe2-based nanogenerator for harvesting energy from human activities, ocean waves, mechanical vibration, etc 100,123) . It should be noted, however, despite these fascinating proposals, most of them are hypothetical at the moment and deserve more experimental evidence for future applications in fields of photovoltaics, electronics and spintronics.…”