Emergence of Ferroelectricity at a Metal-Semiconductor Transition in a 1T Monolayer of MoS2

Using a combination of Landau theoretical analysis and first-principles calculations, we establish a spontaneous symmetry breaking of the metallic state of the 1T monolayer of MoS2 that opens up a band gap and leads to an unexpected yet robust ferroelectricity with ordering of electric dipoles perpendicular to its plane. Central to the properties of this thinnest known ferroelectric is a strong coupling of conducting states with valley phonons that induce an effective electric field. The current in a semiconducting 1T-MoS2 channel can, thus, be controlled independently by changing its ferroelectric dipolar structure with a gate field, opening up a possibility of a class of nanoscale dipolectronic devices. Our analysis applies equally well to MoSe2, WS2, and WSe2, giving tunability in design of such devices based on two-dimensional chalcogenides.