Out-of-Plane Ferroelectricity in Two-Dimensional 1T‴-MoS₂ Above Room Temperature

Two-dimensional (2D) molybdenum disulfide (MoS₂), one of the most extensively studied van der Waals (vdW) materials, is a significant candidate for electronic materials in the post-Moore era. MoS₂ exhibits various phases, among which the 1T‴ phase possesses noncentrosymmetry. 1T‴-MoS₂ was theoretically predicted to be ferroelectric a decade ago, but this has not been experimentally confirmed until now. Here, we have prepared high-purity 2D 1T‴-MoS₂ crystals and experimentally confirmed the room-temperature out-of-plane ferroelectricity. The noncentrosymmetric crystal structure in 2D 1T‴-MoS₂ was convinced by atomically resolved transmission electron microscopic imaging and second harmonic generation (SHG) measurements. Further, the ferroelectric polarization states in 2D 1T‴-MoS₂ can be switched using piezoresponse force microscopy (PFM) and electrical gating in field-effect transistors (FETs). The ferroelectric-to-paraelectric transition temperature is measured to be about 350 K. Theoretical calculations have revealed that the ferroelectricity of 2D 1T‴-MoS₂ originates from the intralayer charge transfer of S atoms within the layer. The discovery of intrinsic ferroelectricity in the 1T‴ phase of MoS₂ further enriches the properties of this important vdW material, providing more possibilities for its application in the field of next-generation electronic devices.