Room-Temperature Out-Of-Plane Ferroelectricity in 1T'/1H MoS₂ Heterophase Bilayer

The emergence of heterophase 2D materials, distinguished by their unique structures, has led to the discovery of a multitude of intriguing physical properties and a broad range of potential applications. Here, out-of-plane ferroelectricity is uncovered in a heterophase structure of 1T'/1H MoS₂, which is synthesized via chemical vapor deposition (CVD) by tuning the formation energies for MoS₂ with varied phases. The atomically resolved structures of the obtained 1T'/1H MoS₂ bilayers are captured using scanning transmission electron microscopy (STEM) and are confirmed to be non-centrosymmetric using second-harmonic generation (SHG) characterizations. The intrinsic out-of-plane polarization is visualized by piezoresponse force microscopy (PFM), which reveals that ferroelectric domains can be manipulated under an applied electric field. Ferroelectric tunnel junction (FTJ) devices fabricated on these bilayers exhibit reversible switching between a high resistance state (HRS) and a low resistance state (LRS). Density functional theory (DFT) calculations elucidate that the intrinsic ferroelectricity in 1T'/1H bilayers is attributed to interlayer sliding and lattice mismatch. The findings not only expand the scope of 2D ferroelectrics to include vertically stacked heterophase bilayers but also open avenues for exploring the coupling effect between ferroelectricity and other phenomena such as magnetism, superconductivity, and photocatalysis in 2D heterophase TMDCs.