Tuning of the Valley Structures in Monolayer InSe/WSe Heterostructures via Ferroelectricity.

Recent findings of two-dimensional ferroelectric (FE) materials have enabled the integration of nonvolatile FE functions into device applications based on van der Waals (vdW) heterojunctions (HJs), resulting in versatile technological advances. In this paper, we report the results of direct probing of the electronic structures of InSe/WSe heterostructures at the single-layer limit, where monolayer (ML)-InSe was found to be either antiferroelectric (AFE, β') or ferroelectric () at sufficiently low temperatures. A general type-II band alignment was revealed for this heterostructure. Moreover, we observed significant modulations of the valley structures of WSe, and in situ transformations between the FE and AFE InSe phases demonstrated the dominant role of the polarizations in the top ML-InSe layer. The observed phenomena can be attributed to the combination of both the linear and quadratic Stark shifts from the out-of-plane electric field, which has only been previously theoretically explored for ML-transition metal dichalcogenides (TMDs).