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

Two-dimensional (2D) molybdenum disulfide (MoS), one of the most extensively studied van der Waals (vdW) materials, is a significant candidate for electronic materials in the post-Moore era. MoS exhibits various phases, among which the 1T‴ phase possesses noncentrosymmetry. 1T‴-MoS 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‴-MoS crystals and experimentally confirmed the room-temperature out-of-plane ferroelectricity. The noncentrosymmetric crystal structure in 2D 1T‴-MoS was convinced by atomically resolved transmission electron microscopic imaging and second harmonic generation (SHG) measurements. Further, the ferroelectric polarization states in 2D 1T‴-MoS 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‴-MoS originates from the intralayer charge transfer of S atoms within the layer. The discovery of intrinsic ferroelectricity in the 1T‴ phase of MoS further enriches the properties of this important vdW material, providing more possibilities for its application in the field of next-generation electronic devices.