Novel Ferroelectric and Optoelectronic Responses in 2D Semiconducting Transition Metal Dichalcogenides by Defect and Strain Engineering

Ferroelectric materials are important components for modern electronic applications, particularly in high-density data storage, microwave devices, pyroelectric sensors, non-volatile memories. Recently, ferroelectricity has been observed in a two-layer van der Waals (vdW) interfaces using marginally twisted two-dimensional (2D) materials that lack polar point groups in their parent lattices, such as hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs). For 2H-TMDs in the monolayer limit, the D_3h point group preserves the centrosymmetry so that there is no out-of-plane polarization nor ferroelectricity. On the other hand, if mirror symmetry breaking is introduced, such as in the distorted-1T phase, emergence of ferroelectricity may occur. However, due to domains of randomly orientated spontaneous polarization, a tip-induced flexoelectric training field would be needed to achieve polarization alignment before ferroelectricity could be detected by scanning probe microscopy. To date, robust ferroelectric responses of monolayer 2H-TMDs on device scales have not been reported.<br/><br/>Here we present our new discovery of giant magnetic field-induced ferroelectric responses and novel optoelectronic responses in monolayer MoS₂-based field effect transistors (FETs). We attribute the physical origin for the giant field-induced ferroelectric responses to combined effects of sulfur vacancies-induced centrosymmetric breaking and excess electric/magnetic dipole moments as well as asymmetric thermal relaxations between the top and bottom sulfur layers on a substrate at low temperatures (&lt; 20 K). We further demonstrate that nanoscale strain-engineering of monolayer MoS₂ can induce significant centrosymmetric breaking regardless of the presence of sulfur vacancies, which can enhance the ferroelectric responses. We also observe sensitive responses of the ferroelectric signals from the monolayer MoS₂-FETs to light excitations as a function of the photon frequency, polarization, and orbital angular momentum, suggesting strong interactions of excitonic polaritons with carriers in the 2D-TMDs. The physical mechanisms for these novel findings and potential ferroelectric and optoelectronic applications based on monolayer TMD-FETs will be discussed.<br/><br/>This work was jointly supported by the National Science Foundation in the US, and the National Science and Technology Council and the Ministry of Education in Taiwan.