exfoliated a centrosymmetric AA’ stacked h-BN crystal into monolayers and reassembled the monolayers to AB stacking using the tear-and-stack method 14. Very recently, this concept was experimentally proved by several groups. This flips the spontaneous polarization in the material and gives rise to “sliding ferroelectricity”. When the atomic layers slide against each other to reverse their stacking sequence, the charge transfer between them is also reversed. predicted that specific stacking sequences of atomic layers in non-centrosymmetric van der Waals (vdW) materials lead to charge transfers between the atomic layers and give rise to out-of-plane spontaneous polarizations 26. However, as the pre-requisite for ferroelectricity, i.e., non-centrosymmetric atomic structure, excludes a large proportion of the known materials, the number of 2D ferroelectric materials reported to date is woefully limited. Since then, more 2D ferroelectric materials such as MoTe 2, WTe 2, α-In 2Se 3, β-InSe, and ReS 2 have been reported 8, 9, 10, 11, 12, 25. However, even though a lot of theoretical works have earlier predicted the existence of 2D ferroelectric materials, the experimental study of ultrathin layered ferroelectric material CuInP 2S 6 has only been conducted recently in 2016 13. As the intersect of ferroelectrics and 2D materials, 2D ferroelectrics have attracted considerable attention in the research community in recent years 4, 5, 22, 23, 24. With an even shorter but equally successful history, two-dimensional (2D) materials have also garnered extensive interest for their superior chemical and physical properties 15, 16, 17, 18, 19, 20 since the first successful exfoliation of graphene in 2004 21. These materials have shown tremendous industrial potential in applications such as non-volatile memory, actuators, and negative capacitance field-effect transistors 8, 9, 10, 11, 12, 13, 14. The family of ferroelectric materials, whose spontaneous polarizations can be switched under electric fields, has since grown significantly to include perovskite oxides, hybrid perovskites, organic compounds, among many others 6, 7. Since the discovery of Rochelle salt in 1920, the study of ferroelectric materials has come a long way in its relatively short history of just one century 1, 2, 3, 4, 5. This work reveals the critical roles layer number and interlayer dipole coupling play in sliding ferroelectricity and presents a new strategy for the design of novel sliding ferroelectric devices. Using results from ab initio density functional theory calculations, we propose a generalized model to describe the ferroelectric switching process in multilayer 3 R MoS 2 and to explain the formation of these intermediate polarization states. By fabricating 3 R MoS 2 of various thicknesses into dual-gate field-effect transistors, we obtain anomalous intermediate polarization states in multilayer (more than bilayer) 3 R MoS 2. Here, we present layer dependence as a new dimension to control sliding ferroelectricity. This phenomenon is known as sliding ferroelectricity and it is markedly different from conventional ferroelectric switching mechanisms relying on ion displacement. When the atomic layers in a non-centrosymmetric van der Waals structure slide against each other, the interfacial charge transfer results in a reversal of the structure’s spontaneous polarization.
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