Lok Wing Wong

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.

Phase engineering of transition-metal dichalcogenides provides a powerful means to regulate atomic-scale catalysis, yet the role of interfacial electronic structures in controlling reaction kinetics remains unclear. Here, we demonstrate that phase-dependent Mo–S–Fe interfacial coupling in MoS₂ induces pronounced interfacial polarization, accelerating dual Fe/Mo redox cycles during peroxymonosulfate (PMS) activation. Atomic resolution differential phase contrast scanning transmission electron microscopy (DPCSTEM) directly visualizes isolated Fe single atoms and polarized charge redistribution at the Fe–S–Mo interface in metallic 1TMoS₂, whereas the 2H phase favors Fe₃O₄ nanoparticle formation. Density functional theory calculations reveal that strong interfacial coupling shortens the Fe–S bond, promotes charge delocalization, and lowers the kinetic barriers for Fe and Mo redox transitions. Consequently, Fe–1T MoS₂ exhibits greatly enhanced PMS activation, with reaction rate constants up to 0.62min⁻¹, exceeding those of 2H phase catalysts by over two orders of magnitude.

The conventional Lewis acid molten salt etching approach for synthesizing MXenes typically necessitates elevated temperatures exceeding 550 °C, in addition to the use of inert gas protection to prevent oxidation. Additionally, delamination of molten salt‐etched MXenes typically requires hazardous intercalating agents. Herein, a scalable and non‐toxic low‐temperature shielded salt (LSS) approach for synthesizing MXene in air is reported, with the use of only a small portion of salts and a low reaction temperature down to 150 °C. Especially, the synergistic effect of the increased diffusion rate by Li ⁺ ions and the phase change of magnesium chloride hexahydrate (MgCl ₂ ·6H ₂ O) enables a redox‐controlled A‐site etching of the parent MAX phase, which even facilitates the synthesis of hard‐to‐etch MXenes. As a proof‐of‐concept demonstration, several theoretically hard‐to‐synthesize MXenes including Cr ₂ CT _x and Nb ₂ CT _x are successfully prepared through the LSS technique, where Cr ₂ CT _x is not achieved by Lewis acidic molten salt yet. Compared to conventional techniques, this low‐temperature shielded salt etching method exhibits unconventional molten behavior while offering several advantages, including a non‐oxidizing environment, shorter processing time, and elimination of highly corrosive washing agents and organic intercalants. These advances render the LSS approach a promising route for synthesizing MXenes with applications toward diverse practical applications.

Material fractures are typically irreversible, marking a one-time event leading to failure. Great efforts have been made to enhance both strength and fracture toughness of bulk materials for engineering applications, such as by introducing self-recovery and secondary breaking behaviours. In low-dimensional structures, two-dimensional materials often exhibit exceptional strength but accompanied by extreme brittleness. Here we discover that the toughness of two-dimensional materials can be enhanced without sacrificing strength—by simply twisting the layers. Through in situ scanning transmission electron microscopy, supported by nanoindentation and theoretical analysis, we reveal that twisted bilayer structures enable sequential fracture events: initial cracks heal to form stable grain boundaries, which then shield subsequent fracture tips from stress concentration. This process consumes additional energy compared with conventional fracture, with toughness enhancement tunable through twist angle adjustment. The intrinsic toughening mechanism via twisting, along with the emerging electronic properties of twistronics that are currently attracting substantial attention, presents an exciting opportunity for future devices.

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.

The quest for electronic devices that offer flexibility, wearability, durability and high performance has spotlighted two-dimensional (2D) van der Waals materials as potential next-generation semiconductors. Especially noteworthy is indium selenide, which has demonstrated surprising ultra-high plasticity. To deepen our understanding of this unusual plasticity in 2D van der Waals materials and to explore inorganic plastic semiconductors, we have conducted in-depth experimental and theoretical investigations on metal monochalcogenides (MX) and transition metal dichalcogenides (MX₂). We have discovered a general plastic deformation mode in MX, which is facilitated by the synergetic effect of phase transitions, interlayer gliding and micro-cracks. This is in contrast to crystals with strong atomic bonding, such as metals and ceramics, where plasticity is primarily driven by dislocations, twinning or grain boundaries. The enhancement of gliding barriers prevents macroscopic fractures through a pinning effect after changes in stacking order. The discovery of ultra-high plasticity and the phase transition mechanism in 2D MX materials holds significant potential for the design and development of high-performance inorganic plastic semiconductors.

Electric‐field regulation of magnetic properties in perovskite manganites has attracted much attention for its potential in spintronics. For antiferromagnetic perovskite manganites, fewer studies are reported due to technological difficulties in probing their magnetic properties. Here, negative exchange bias (EB) is realized in epitaxial antiferromagnetic/ferromagnetic manganite bilayers with atomically flat interfaces. The low‐voltage pulse modulation of EB is demonstrated using the field‐effect device geometry with the ferroelectric copolymer, polyvinylidene fluoride with trifluoroethylene as a dielectric gating layer, antiferromagnetic La_0.35Sr_0.65MnO₃ (AF‐LSMO) as pinning layer, and ferromagnetic La_0.7Sr_0.3MnO₃ (FM‐LSMO) as conduction channel. Instead of using non‐volatile polarizations to control the EB, volatile polarizations in ferroelectric field effect transistors are suggested to be capable of modulating the EB. With high‐resolution electron microscopy and spectroscopy, the non‐volatile regulation of EB is attributed to the creation/annihilation of oxygen vacancies in the AF‐LSMO layer via low‐voltage pulses. This study reveals the effect of volatile electric polarizations in ferroelectric field effect devices and highlights the potential for low‐voltage pulse control of the physical properties in antiferromagnetic perovskite oxide insulators.

A black phosphorus (BP) ultrathin nanosheet has significant research values in broad fields ranging from nano-electronics/photonics to quantum physics. Here, a piezoelectric actuator is utilized to perform biaxial strain engineering for the investigation of anisotropic Raman response of the ultrathin BP transferred to the oxide dielectric substrate. Three characteristic peaks exhibit redshift when tensile strain is applied, while the peaks reveal blueshift under compressive strain. When applying compressive strain of −0.2%, the Raman shift rate of B_2g mode can reach up to 15.3 cm⁻¹/%. In contrast, with the application of 0.2% tensile strain, the B_2g mode is shifted by −12.2 cm⁻¹/%. Furthermore, we calculated the Grüneisen parameters to deduce the relationship between the tensile or compressive strain and phonon behavior of crystalline BP. The physical mechanism behind the observation of strained Raman response is discussed, which is related to the variations of bond angle and bond length in BP. Additionally, biaxial strain modulation may change the anisotropic dispersion of BP, revealing the significant potential of BP in innovative polarized light detection.