Jiuxiang Dai

The recent discovery of sliding ferroelectricity, where polarization is linked to the polar stacking orders of originally non-polar monolayers, has significantly broadened the scope of 2D ferroelectrics. This review discusses the underlying mechanisms and experimental methods for probing stacking orders, the origin of sliding ferroelectricity, and the applications of metallic, insulating and semiconducting 2D sliding ferroelectrics.

Abstract

Ferroelectric materials, celebrated for their switchable polarization, have undergone significant evolution since their early discovery in Rochelle salt. Initial challenges, including water solubility and brittleness, are overcome with the development of perovskite ferroelectrics, which enable the creation of stable, high-quality thin films suitable for semiconductor applications. As the demand for miniaturization in nanoelectronics has increased, research has shifted toward low-dimensional materials. Traditional ferroelectrics often lose their properties at the nanoscale; however, 2D van der Waals (vdW) ferroelectrics, including CuInP₂S₆ and α-In₂Se₃, have emerged as promising alternatives. The recent discovery of sliding ferroelectricity, where polarization is linked to the polar stacking configuration of originally non-polar monolayers, has significantly broadened the scope of 2D ferroelectrics. This review offers a comprehensive examination of stacking orders in 2D vdW materials, stacking-order-linked ferroelectric polarization structures, and their manifestations in metallic, insulating and semiconducting 2D vdW materials. Additionally, it explores the applications of 2D vdW sliding ferroelectrics, and discusses the future prospects in nanotechnology.

Nature Communications, Published online: 23 January 2025; doi:10.1038/s41467-025-56386-9

This study experimentally demonstrated the ferroelasticity in 1T′ Molybdenum ditelluride (MoTe₂), which exhibits three equivalent states. The investigation into the ferroelastic properties of MoTe₂ involved obtaining reoriented domains through uniaxial strain. Moreover, the orientation is controlled based on the direction of the applied strain. The results show that multidirectional ferroelastic and anisotropic electrical properties of 1T′ MoTe₂ offer exciting opportunities for applications such as multilevel memory devices.

Abstract

Ferroelastic properties in two-dimensional materials have garnered significant attention due to their potential to control structural anisotropy, thereby influencing optical and electrical characteristics under external stress. Despite these potential applications, there remains a scarcity of reports on ferroelastic behavior in two-dimensional van der Waals materials. In this study, we investigate multidirectional ferroelastic reorientation in the two-dimensional van der Waals semimetallic phase of Molybdenum ditelluride (1T′ MoTe₂). Employing a polymer encapsulation method, ferroelastic switching in 1T′ MoTe₂ is examined via real-time polarized optical imaging under uniaxial strain. Polarized Raman spectroscopy confirms the orientation of Mo–Mo chains in original and reoriented domains, revealing 120° rotational switching in reconstructed domains. Analysis using selective area electron diffraction and scanning transmission electron microscopy elucidate the atomic arrangements of original and reconstructed domains, including domain wall structures. Additionally, it is demonstrated that external uniaxial strain can differentiate between the stabilities of three equivalent states in 1T′ MoTe₂, enabling manipulation of the orientation of Mo–Mo chains based on the direction of the applied strain. The findings highlight the capability for multidirectional ferroelastic reorientation in the two-dimensional semimetal MoTe₂ through the application of uniaxial strain.

This work realizes a 2-inch hexanary medium-entropy alloy monolayer (Re_aW_bMo_cIn_dS_xSe_y) on c-plane sapphire via the chemical vapor deposition method. The ME alloys show an ultrawide photo-response from the visible to near-infrared wavelengths with a responsivity of 100.2 A W⁻¹ under 520 nm laser illumination. Furthermore, the ME alloys exhibit ultra-low overpotential of 43.7 mV under 520 nm laser irradiation.

Abstract

Mixing entropy engineering is a promising strategy to tune the physical and chemical properties of materials. Although high-entropy in van der Waals bulk solids have been reported, entropy engineering in 2D monolayers remains unconquered. In this work, the epitaxial growth of a 2-inch 1T″ hexanary medium-entropy alloy monolayer (Re_aW_bMo_cIn_dS_xSe_y) is reported via the chemical vapor deposition method. The atomic structure and chemical composition are confirmed by X-ray photoelectron spectroscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy, illustrating the uniform distribution of the six elements. The hexanary medium-entropy alloy photodetectors show an ultrawide photo-response from visible to near-infrared wavelengths with a responsivity of 100.2 A W⁻¹ under 520 nm laser illumination. Meanwhile, the hexanary medium-entropy alloy monolayer exhibits excellent electrocatalytic hydrogen production with an overpotential of 176.6 mV in dark. Importantly, an overpotential of 43.7 mV at 10 mA cm⁻² with a lowered Tafel slope of 51.9 mV dec⁻¹ under 520 nm laser irradiation is obtained due to the excellent conductivity. The work opens a new way to design mixing = entropy alloys and enables the application of transition metal dichalcogenides in photo-enhanced electrocatalytic hydrogen production.

npj 2D Materials and Applications, Published online: 24 January 2025; doi:10.1038/s41699-025-00527-7

npj 2D Materials and Applications, Published online: 25 January 2025; doi:10.1038/s41699-024-00523-3

A class of ultrathin GdOX with a high dielectric constant and breakdown field strength is synthesized via van der Waals epitaxy. The fabricated MoS₂ transistor gated by monocrystalline dielectric exhibits an on/off current ratio exceeding 10⁹ and a near-Boltzmann-limit SS, indicating their tangible applications in 2D electronics.

Abstract

Van der Waals (vdW) dielectrics are extensively employed to enhance the performance of 2D electronic devices. However, current vdW dielectric materials still encounter challenges such as low dielectric constant (κ) and difficulties in synthesizing high-quality single crystals. 2D rare-earth oxyhalides (REOXs) with exceptional electrical properties present an opportunity for the exploration of novel high-κ dielectrics. In this study, for the first time, the synthesis of a series of van der Waals layered gadolinium oxyhalides with thicknesses down to monolayer through a space-confined vdW epitaxy approach and demonstrating their application as a single-crystalline gate dielectric is reported. It exhibits a remarkable relative dielectric constant exceeding 17 and an impressive breakdown field strength of 13.5 MV cm⁻¹. The 2D transistors directly gated by the REOXs layer exhibit enhanced electron mobility and a low interface trap density. An ultrahigh on/off current ratio of 10⁹ and a near-Boltzmann-limit subthreshold swing is achieved. The superior dielectric properties, combined with the universality and scalability of the production method (e.g., millimeter-scale films are achieved), demonstrate that 2D REOXs can serve as promising gate dielectrics for 2D electronics, thereby expanding the study of high-κ vdW materials and potentially providing new opportunities for the development of low-power electronic devices.

The 2D Cr₅Te₆ nanosheets are systematically synthesized with an in situ formed ≈2 nm-thick Te doped Cr₂O₃ layer (Te-Cr₂O₃) on the upper surface by chemical vapor deposition method. The strong and air stable EB effect, achieving a |H_EB/H_C| of up to 80% under an ultralow cooling field of 0.01 T, which is greater than that of the reported 2D magnetic heterostructures.

Abstract

The exchange bias (EB) effect is a fundamental magnetic phenomenon, in which the exchange bias field/coercive field ratio (|H_EB/H_C|) can improve the stability of spintronic devices. Two-dimensional (2D) magnetic heterostructures have the potential to construct low-power and high-density spintronic devices, while their typically air unstable and |H_EB/H_C| lesser, limiting the possibility of applications. Here, 2D Cr₅Te₆ nanosheets have been systematically synthesized with an in situ formed ≈2 nm-thick Te doped Cr₂O₃ layer (Te-Cr₂O₃) on the upper surface by chemical vapor deposition (CVD) method. The strong and air stable EB effect, achieving a |H_EB/H_C| of up to 80% under an ultralow cooling field of 0.01 T, which is greater than that of the reported 2D magnetic heterostructures. Meanwhile, the uniformity of thickness and chemical composition of the Te-Cr₂O₃ layer can be controlled by the growth conditions which are highly correlated with the EB effect of 2D Te-Cr₂O₃/Cr₅Te₆ heterostructures. First-principles calculations show that the Te-Cr₂O₃ can provide uncompensated spins in the Cr₂O₃, thus forming strong spin pinning effect. The systematical investigation of the EB effect in 2D Te-Cr₂O₃/Cr₅Te₆ heterostructures with high |H_EB/H_C| will open up exciting opportunities in low-power and high-stability 2D spintronic devices.

Nature Electronics, Published online: 27 January 2025; doi:10.1038/s41928-025-01344-y

Nature Electronics, Published online: 27 January 2025; doi:10.1038/s41928-025-01342-0

Nature Materials, Published online: 23 January 2025; doi:10.1038/s41563-024-02106-5

Nature Nanotechnology, Published online: 23 January 2025; doi:10.1038/s41565-024-01850-8

A latticed holographically steganographic array is constructed by incorporating a plasticizer (tri-o-cresyl phosphate, TOCP) into spirooxazine (SO) based polymers, thereby modulating the photochromic and UV-fluorescence kinetics between SO and merocyanine (MC). The display period of fluorescence array is widely modulated, corresponding to the conversion from spatial lattice coordinates to holographic display time. By using spatiotemporal dual-encryption keys, hidden 3D images can be easily retrieved from reconstructed holographic videos with a refresh rate of 1.125Hz.

Abstract

Independently-manipulated optical encryption has attracted great attention due to its high security and multiple application scenarios. Although photochromic molecules have become preferred holographic display units, the integration of dynamic modulation both in hologram and fluorescence for orthogonal encryption is still challenged. Herein, a latticed holographically steganographic array is constructed by incorporating a plasticizer (tri-o-cresyl phosphate, TOCP) into spirooxazine (SO) based polymers, thereby modulating the isomerization rate and the UV-fluorescence kinetics between SO and merocyanine (MC). The period to reach fluorescence saturated value for each information bit in the latticed device can be widely modulated between 60 and 1700 s. Based on the property, only through ultraviolet stimulation can array information gradually be converted from spatial lattice coordinates to holographic display time. Then, a hidden 3D image can be determined from rapidly updatable reconstructed hologram video with refresh rate 1.125 Hz by the above spatiotemporal dual-encryption keys. This work provides a bright path for the ultra-high safety steganography and adaptive stereoscopic imaging.

From oil and glass refining to high performance commodity polymers with enhanced thermomechanical and optical properties—the synthesis of a new high refractive index, highly transparent optical thermoset polymer is reported, termed, disulfide glass that affords a robust optical glass amenable to fabrication of precision optics and thin film photonic device constructs.

Abstract

The development of an organic optical glass, termed, disulfide glass (DSG), is reported as a new polymer for commodity plastic optics and thin film photonic applications. This low-cost thermoset polymer possesses excellent transparency across the visible and infrared spectrum comparable to the best optical plastic to date, poly(methyl methacrylate), while having superior refractive index (n ≈ 1.6). DSG can be fabricated into defect-free, thick optical glass by bulk addition polymerization of two commodity monomers (sulfur monochloride, 1,3,5-triallyl isocyanurate) via a new polymerization, sulfenyl chloride inverse vulcanization. The robust mechanical properties and optical clarity of DSG enable fabrication of precision optics (lenses, prisms) via diamond turn machining to demonstrate the manufacturability of DSG for commodity plastic optics. Finally, the synthetic modularity of DSG is demonstrated to form solution processable forms for the fabrication of thin film polymer photonic devices, negative tone polymer photoresists, and micropatterned arrays.

The in situ atomic scale transmission electron microscopy experiment reveals a non-polar phase-mediated ferroelectric-ferroelastic coupled switching pathway in fluorite oxides.

Abstract

HfO₂/ZrO₂-based ferroelectrics present tremendous potential for next-generation non-volatile memory due to their high scalability and compatibility with silicon technology. Unlike the continuous polar layers in perovskite ferroelectrics, HfO₂/ZrO₂-based ferroelectrics are composed of alternating polar layers with oxygen shifts and non-polar spacers, which leads to a distinct ferroelectric switching mechanism. However, directly observing the switching process has been a big challenge due to the polymorph feature of nanoscale fluorites and the difficulty in in situ imaging on light elements. Here, the ferroelectric-ferroelastic coupled switching process in freestanding ZrO₂ thin films is directly visualized by in situ imaging on oxygen motions. A multi-step 90-degree polarization switching mechanism is uncovered that challenges the conventional one-step 180-degree switching paradigms in fluorite oxides, which is highly consistent with the interlocked nature of ferroelectricity and ferroelasticity. A non-polar tetragonal (T) phase is discovered as a crucial intermediate state, lowering the energy barrier for polarization switching by 35%. More importantly, the T phase prevents irreversible transitions to the non-polar ground state and facilitates stable ferroelectric switching. These findings are fundamental to understanding nanoscale polarization switching mechanisms in fluorite ferroelectrics, paving the way for advanced high-durability devices.

MXenes, particularly Ti₃C₂T×, are a promising class of 2D materials with exceptional properties like high electrical conductivity and tunable work functions. This review highlights their potential in enhancing silicon-based optoelectronic devices, including solar cells and photodetectors. Key focus areas include MXene composition, synthesis, properties, and their impact on device performance, along with current challenges and future opportunities for their integration into the silicon-dominated semiconductor industry.

Abstract

MXenes, a rapidly emerging class of 2D transition metal carbides, nitrides, and carbonitrides, have attracted significant attention for their outstanding properties, including high electrical conductivity, tunable work function, and solution processability. These characteristics have made MXenes highly versatile and widely adopted in the next generation of optoelectronic devices, such as perovskite and organic solar cells. However, their integration into silicon-based optoelectronic devices remains relatively underexplored, despite silicon's dominance in the semiconductor industry. In this review, a timely summary of the recent progress in utilizing Ti-based MXenes, particularly Ti₃C₂T_x, in silicon-based optoelectronic devices is provided. The composition, synthesis methods, and key properties of MXenes that contribute to their potential for enhanced device performance are focused on. Furthermore, the latest advancements in MXene applications in silicon-based solar cells and photodetectors are discussed from fundamental and applied perspectives. Finally, the key challenges and future opportunities for the integration of MXenes in silicon-based optoelectronic devices are outlined.

Nature Synthesis, Published online: 17 January 2025; doi:10.1038/s44160-025-00736-4

A dual-template method was developed to synthesize MoS₂ catalysts with tunable layer numbers. Single-layer MoS₂ demonstrates superior activity in the RWGS reaction, outperforming Multi-layer MoS₂. Comprehensive characterization techniques combined with density functional theory calculations demonstrate that in-plane sulfur vacancies enhance C−O bond cleavage, exhibiting intrinsic activity 5.8 times higher than that of edge vacancies.

Abstract

The reduction of CO₂ to CO provides a promising approach to the production of valuable chemicals through CO₂ utilization. However, challenges persist with the rapid deactivation and insufficient activity of catalysts. Herein, we developed a soft-hard dual-template method to synthesize layered MoS₂ using inexpensive and scalable templates, enabling facile regulation of sulfur vacancies by controlling the number of layers. The concentration of in-plane vacancies keeps increasing with the reduction of MoS₂ layer number, contributing to 100 % CO selectivity over single-layer MoS₂ and a stable performance over 300-hour reaction at 600 °C. The space-time-yield of CO reached 35.7 g_CO g_cat ⁻¹ h⁻¹, outperforming most current catalysts. Multiple characterizations and theoretical calculations revealed that in-plane sulfur vacancy sites endowed enhanced production of CO via direct dissociation of CO₂, showing an intrinsic activity of above 5.8 times higher than that of edge sulfur vacancy sites. The rate-limiting step was shifted from C−O cleavage in edge to sulfur vacancy regeneration in plane with a lower energy barrier. Our findings exemplified the specified design and synthesis of MoS₂ for high-temperature CO₂ reduction through the effective manipulation of distinct vacancy sites, shedding light on their potential industrial application.