Xiaodong Zheng

Two-dimensional (2D) ferroelectric semiconductors offer unique opportunities for ultrathin non-volatile memory and neuromorphic devices, yet the role of stacking order in intrinsic ferroelectric systems remains largely unexplored. Here we report the stacking-controlled growth of large-area 2D β′- and α-In₂Se₃ with distinct 2H and 3R configurations through chemical vapour deposition by regulating selenium kinetics and strain relaxation. By combining atomic-resolution microscopy, non-linear optical characterization and first-principles calculations, we uncover a strong correlation between stacking order and ferroelectric behaviour: β′-In₂Se₃ undergoes an antiferroelectric-to-paraelectric transition when switching from 2H to 3R stacking, whereas in α-In₂Se₃, in-plane ferroelectric polarization vanishes during the 3R-to-2H transition. We further achieve the controlled synthesis of 2H/3R ferroelectric homojunctions and directly visualize polarization-dependent interfacial potential barriers using differential phase-contrast scanning transmission electron microscopy. The engineered homojunctions exhibit switchable band alignment and tunable carrier transport, yielding memristors with significantly reduced switching voltages and an on/off ratio of ∼2.6 × 10⁴, approximately one order of magnitude higher than devices based on single-stacking α-In₂Se₃. These findings establish stacking order as a powerful structural parameter for tailoring ferroelectricity in 2D materials and introduce stacking-engineered polymorphic heterogeneity as a versatile platform for high-performance ferroelectric electronics and memory technologies.

We report the observation of an electric field in twisted-bilayer molybdenum disulfide (MoS ₂ ) and elucidate its correlation with local polar domains using four-dimensional scanning transmission electron microscopy (4D-STEM) and first-principles calculations. We reveal the emergence of in-plane topological vortices within the periodic moiré patterns for both commensurate structures at small twist angles and the incommensurate quasicrystal structure that occurs at a 30° twist. The large-angle twist leads to mosaic chiral vortex patterns with tunable characteristics. A twisted quasicrystal bilayer, characterized by its 12-fold rotational symmetry, hosts complex vortex patterns and can be manipulated by picometer-scale interlayer displacement. Our findings highlight that twisting 2D bilayers is a versatile strategy for tailoring local electric polar vortices.

Phase transitions in two-dimensional (2D) materials promise reversible modulation of material physical and chemical properties in a wide range of applications. 2D van der Waals layered In ₂ Se ₃ with bistable out-of-plane ferroelectric (FE) α phase and antiferroelectric (AFE) β′ phase is particularly attractive for its electronic applications. However, reversible phase transition in 2D In ₂ Se ₃ remains challenging. Here, we introduce two factors, dimension (thickness) and strain, which can effectively modulate the phases of 2D In ₂ Se ₃ . We achieve reversible AFE and out-of-plane FE phase transition in 2D In ₂ Se ₃ by delicate strain control inside a transmission electron microscope. In addition, the polarizations in 2D FE In ₂ Se ₃ can also be manipulated in situ at the nanometer-sized contacts, rendering remarkable memristive behavior. Our in situ transmission electron microscopy (TEM) work paves a previously unidentified way for manipulating the correlated FE phases and highlights the great potentials of 2D ferroelectrics for nanoelectromechanical and memory device applications.