Ferroelectric materials are characterized by polar symmetries that allow two or more stable and switchable polarization states, and are cornerstones of non-volatile memories, sensors, microwave devices, and transistors. The growing demand for non-volatile storage and neuromorphic computing in the post-Moore era has subjected many conventional three-dimensional ferroelectrics to severe size constraints due to the well-known critical size effect, prompting active searches for room-temperature ferroelectric materials that retain functionality at atomic-scale thicknesses. Two-dimensional van der Waals ferroelectrics have emerged as promising candidates owing to their high carrier mobility, suitable band structures, and ferroelectricity that can persist down to the atomic limit; however, only a few intrinsic polar two-dimensional layered materials, such as CuInP₂S₆, SnTe, In₂Se₃, and Bi₂SeO₈, have been experimentally confirmed to date.

Emerging two-dimensional sliding ferroelectrics have attracted widespread attention. Their ferroelectricity originates from specific stacking configurations that break local inversion symmetry, triggering interfacial interlayer charge redistribution and forming interfacial electric dipoles, thereby producing a net out-of-plane polarization that can be switched by in-plane interlayer sliding under a vertical electric field. However, the realization of existing sliding ferroelectrics typically relies on complex and precise stacking processes, with limited scalability and reproducibility, which seriously hinders their large-scale application. Meanwhile, direct atomic-level evidence for the correlation between artificially stacked configurations and polarization switching remains scarce. Therefore, exploring intrinsically polymorphic two-dimensional ferroelectrics that are naturally grown and free from artificial stacking, and revealing their structure–property relationships at the atomic level, is of great scientific significance.

To address the above issues, a joint team comprising East China Normal University, Huazhong University of Science and Technology, and the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences conducted in-depth research using ZEPTOOLS' in-situ TEM measurement system. The team systematically identified multiple intrinsic stacking configurations in naturally formed SnSe₂ van der Waals layered semiconductors, confirmed that non-centrosymmetric antiparallel interfaces can produce robust and switchable out-of-plane ferroelectricity, and directly visualized field-induced interlayer sliding and coupled reversible polarization switching via in-situ bias transmission electron microscopy, elucidating the physical mechanism of sliding-induced interfacial ferroelectricity from the atomic scale.

Title: Interfacial ferroelectricity at naturally-formed van der Waals interfaces 
Journal: Nature Communications 
URL:https://doi.org/10.1038/s41467-026-78336-9

This study first focuses on structural identification and symmetry characterization of different stacking configurations. SnSe₂ is a 1T-phase vdW layered semiconductor with an optical bandgap of approximately 1.67 eV, possessing a CdI₂-type Se–Sn–Se sandwich lattice, and due to its rich polymorphism, it can naturally form two types of interlayer stacking configurations: parallel (AA, AA') and antiparallel (AB', AB''). Through HAADF-STEM imaging, the team systematically identified four typical stacking modes: AAA (S₀), AAA...B'B'B'... (S₁), AAB' (S₂), and AAB'B' (S₃), all of which maintain a vdW gap of approximately 0.313 nm with completely identical intralayer structural motifs. Second harmonic generation (SHG) characterization showed that the centrosymmetric S₀ exhibits no SHG response, while the symmetry-broken S₁, S₂, and S₃ exhibit robust (distorted) six-fold symmetric SHG signals at 532 nm under 1064 nm excitation, providing clear evidence that antiparallel interfaces break spatial inversion symmetry and form vertical Sn–Se interfacial dipoles.

The study then systematically verified the intrinsic out-of-plane ferroelectricity of the non-centrosymmetric stacking configurations using piezoresponse force microscopy. Under a vertical external electric field, all three samples S₁, S₂, and S₃ exhibited typical butterfly-shaped amplitude loops and sharp phase hysteresis with approximately 180° switching, and applying opposite DC biases (Vdc = ±6 V) enabled reversible domain writing and reading. Voltage-dependent measurements confirmed that the piezoelectric signal disappears below the coercive field, ruling out electrostatic artifacts. The team further prepared cross-sectional samples from opposite domain regions using a focused ion beam; HAADF-STEM and intensity line profiles precisely demonstrated that after +6 V domain writing, the antiparallel interface AB' adopts a polarization-down state (P_dn), while under −6 V it adopts a polarization-up state (P_up). The polarization switching is precisely coupled with the in-plane interlayer displacement Δ, and the I–V curves from conductive atomic force microscopy also exhibit reversible hysteresis between high- and low-resistance states, collectively confirming the intrinsic sliding ferroelectricity of antiparallel-interface-stacked SnSe₂.

The study then focuses on the microscopic atomic mechanism of electric-field-driven polarization switching. Using ZEPTOOLS' transmission electron microscope in-situ sample holder, the team applied an external electric field in a spherical-aberration-corrected STEM system while simultaneously acquiring structural images. The FIB-prepared cross-sectional sample was attached to a copper grid as the grounded bottom electrode, and a movable tungsten probe served as the top electrode connected to the electrical measurement unit. The I–V curves recorded during cyclic voltage sweeping exhibit classic butterfly-shaped hysteresis in double-logarithmic coordinates, and quasi-in-situ bias experiments directly show that adjacent layers in AAB'' and AAB''B'-stacked SnSe₂ undergo significant in-plane lateral sliding after biasing, with intensity line profiles quantifying the displacement Δ between the pristine and biased states accompanied by polarization direction reversal. In the S₂ sample after ±2.5 V sweeping, regions far from the probe retain their original stacking and polarization orientation, while regions near the probe transition from the initial P_up state to the P_dn state, and an intermediate domain boundary configuration between the two polarization states was captured, providing direct atomic-level evidence for interlayer-sliding-induced interfacial polarization switching.

The study ultimately verified the feasibility of the sliding switching path through first-principles calculations. DFT calculations show that the spontaneous polarizations of S₁, S₂, and S₃ are 0.12, 0.26, and 0.18 μC·cm⁻², respectively, with S₂ possessing the largest spontaneous polarization. Based on the CI-NEB method, the calculated sliding energy barrier for AAB'' (or AB'') stacking is only 77.0 meV/u.c., and in the AAB'B' configuration, the cooperative double-layer sliding path (Path-1) has a barrier of 68.5 meV/u.c., while the single-layer sliding path (Path-2) has a barrier of 73.2 meV/u.c., both comparable to the switching barriers of representative two-dimensional ferroelectrics, indicating that polarization switching is highly energetically favorable. The study infers that in vdW layered materials with non-centrosymmetric antiparallel interfaces, intrinsic interfacial ferroelectricity originating from directional interlayer sliding may exist, thereby expanding the family of two-dimensional van der Waals ferroelectric semiconductors and providing a potential material foundation for large-scale integrated applications such as ferroelectric photovoltaics, ferroelectric tunnel junctions, and in-memory computing.

As a leading Chinese high-end precision instrument company, ZEPTOOLS is a first-class supplier of in-situ electron microscopy characterization solutions. Its PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for landmark research achievements by users both domestically and internationally. The image below shows the ZEPTOOLS in-situ TEM sample holder product used in this research: