Non-volatile Tuning of 2D Excitons in Rhombohedral MoS₂ Through Sliding Ferroelectricity

The tunability in the stacking degree of freedom of 2D van der Waals materials provides a new and powerful approach to engineer their physical properties. Sliding ferroelectricity is one such example where an electric field can couple to a stacking-dependent out-of-plane polarization, driving one layer of atoms to move relative to its neighbors, as previously reported in artificially stacked boron nitrides and transition metal dichalcogenides. In this talk, I will show that such a hysteretic phenomenon can occur in chemically synthesized rhombohedral molybdenum disulfide (3R-MoS₂), a polytype in which each MoS₂ layer is stacked in parallel naturally. Besides the uniform crystal orientation, each layer in the 3R polytype is coherently shifted by one-third of the unit cell in the same direction, leading to a uniform polarization and an intrinsic bulk photovoltaic effect that can accumulate over multiple layers. On the other hand, sliding between layers can occur under shear force during sample preparation, creating a variety of domains of different stacking with a power-law size distribution. When the externally applied electric field overcomes the local pinning, some of these pre-existing domain walls can be released for propagation, switching the polarization in a large portion of the flake. Such atomic-scale motion can induce significant changes in the optical properties of atomically thin MoS₂ through strong excitonic resonances in the visible range, revealing interesting information about the switching pathway in sliding ferroelectricity. The large optical reflectance change associated with the stacking switch happens on an ultrafast timescale, making it promising to be implemented in non-volatile optical memories with high performance.