Jiuxiang Dai

Nature Communications, Published online: 08 February 2025; doi:10.1038/s41467-025-56709-w

This review summarizes the recent progress in the direct syntheses of 2D noble transition metal dichalcogenides (nTMDCs) domains, continuous films, and their heterostructures, mainly focusing on the thermally assisted conversion and chemical vapor deposition routes, as well as their applications in electronics, optoelectronics, catalysts, etc. A prospective outlook for the future development of 2D nTMDCs is also outlined.

Abstract

2D noble transition metal dichalcogenides (nTMDCs, PdX₂ and PtX₂, where X═S, Se, Te) have emerged as a new class of 2D materials, owing to their unique puckered pentagonal structure in 2D PdS₂ and PdSe₂, largely tunable band structures or band gaps with decreasing the layer thickness at the 2D limit, strong interlayer interactions, superior optoelectronic properties, high edge catalytic properties, etc. Directly synthesizing 2D nTMDCs domains or thin films with large-area uniformity, tunable thickness, and high crystalline quality is the premise for exploring these salient properties and developing a wide range of applications. Hereby, this review summarizes recent progress in the direct syntheses and characterizations of 2D nTMDCs, mainly focusing on the thermally assisted conversion (TAC) and chemical vapor deposition (CVD) methods, by using various metal and chalcogen-contained precursors. Meanwhile, the applications of directly synthesized 2D nTMDCs in various fields, such as high-performance field effect transistors (FETs), broadband photodetectors, superior catalysts in hydrogen evolution reactions, and ultra-sensitive piezo resistance sensors, are also discussed. Finally, challenges and prospects regarding the direct syntheses of high-quality 2D nTMDCs and their applications in next-generation electronic and optoelectronic devices, as well as novel catalysts beyond noble metals are overviewed.

This review offers the first comprehensive analysis of electrochemical exfoliation (ECE) for layered non-van der Waals (L-NvdW) materials, compares exfoliation methods with L-vdW materials, and introduces novel techniques for selective layer extraction in MAX phases, Zintl phases CaSi _x Ge _y , and metal oxides, highlighting both the potential and challenges of L-NvdW materials for future advancements.

Abstract

2D materials have rapidly gained attention due to their exceptional properties like high surface area, flexibility, and tunable electronic characteristics. These attributes make them highly versatile for applications in energy storage, electronics, and biomedicine. Inspired by graphene’s success, researchers are exploring other 2D materials from bulk crystals. Electrochemical exfoliation (ECE) is an efficient method for producing these materials, offering more sustainable mild conditions, quick processing, simple equipment, and high yields. While substantial progress has been made in the ECE of layered van der Waals (L-vdW) crystals, the exploration of layered non-van der Waals (L-NvdW) materials remains in its early stages. This review delves into using ECE to create 2D nanoplatelets from L-NvdW crystals. A comparative analysis of exfoliation techniques is provided for L-vdW and L-NvdW materials, followed by a comprehensive overview of recent advances in ECE methods applied to L-NvdW crystals. The discussion is organized around key categories, including the selective extraction of “M” and “A” layers respectively from MAX phases, decalcification of Zintl phases, and oxide delocalization from metal oxides. It is concluded by highlighting the potential applications of these 2D materials and discussing the challenges and future directions in this evolving field.

The lamellar rare-earth oxychlorides (REOCl) are innovatively used as promoters for ruthenium (Ru) as alkaline hydrogen evolution reaction electrocatalysts. The [RE₂O₂] and [Cl] layers act as the negative and positive charge transfer channels, respectively, which endows Ru surface with a high density of electrons, thus accelerating the hydroxyl peeling process.

Abstract

Developing efficient electrocatalysts for hydrogen evolution reaction (HER) in alkaline environments is vital for hydrogen production, owing to the extra water dissociation and hydroxyl desorption steps. Here, rare-earth oxychlorides (REOCl) are proposed as innovative promoters for ruthenium as HER electrocatalyst in alkali. The lamellar structure of REOCl with weakly bond [Cl] layers can facilitate the formation of an internal electric field that enhances interphase charge transfer. Taking ruthenium/ neodymium oxychloride (Ru/NdOCl) composites as a case study, sub ≈4 nm Ru nanoparticles are successfully embedded into NdOCl crystals through a rapid self-exothermic process, and the highly-coupled Ru−Cl/O−Nd interfaces are observed as metallic Ru particles with the edge of the NdOCl lamellar layers, where the [Nd₂O₂] and [Cl] layers act as the negative and positive charge transfer channels, respectively. The enhanced charge transfer between REOCl and Ru makes the highly-coupled Ru/REOCl catalysts show better electrocatalytic activity than both the benchmark Pt and Ru catalysts in alkaline electrolyte. This work will encourage more novel promoters for electrocatalysis and other emerging technologies.

The first crown ether-based chiral ferroelectrics show a ferroelectric phase transition at around 336 K from the chiral-polar point group 2 to chiral-nonpolar 422, which enables the notable switching of second-harmonic generation circular dichroism (SHG-CD) response from SHG-CD active (SHG-CD on) to inactive (SHG-CD off) states. Such an on/off switchable SHG-CD effect is unprecedented.

Abstract

Chiral ferroelectrics have recently received considerable interest due to their unique chiroptical properties. They can adopt Kleinman symmetry second-harmonic generation (SHG)-active chiral-polar point groups in the ferroelectric phase while Kleinman symmetry SHG-inactive chiral-nonpolar point groups in the paraelectric phase, providing a great opportunity to realize on/off switching of SHG circular dichroism (SHG-CD) response. However, the SHG-CD effect was rarely explored in chiral ferroelectrics, and the on/off switchable SHG-CD has never been reported. Herein, we report the first crown ether-based chiral ferroelectrics (R/S-CS)Ca(18-crown-6) (CS=camphor-10-sulfonic acid), which undergo a 422F2 type ferroelectric phase transition at around 336 K from Kleinman symmetry SHG-active point group 2 to Kleinman symmetry SHG-inactive point group 422. Notably, they exhibit obvious SHG-CD responses with an anisotropy factor of up to 0.31. More importantly, the SHG-CD response can be switched between SHG-CD active (SHG-CD on) and inactive (SHG-CD off) states during the ferroelectric phase transition, which is unprecedented. To the best of our knowledge, this is the first example of Kleinman-type SHG-CD on/off switchable ferroelectric. Our findings open up a new way to switch SHG-CD response based on chiral ferroelectrics, which would greatly inspire the further exploration of switchable SHG-CD effects in chiral ferroelectrics.

Nature Electronics, Published online: 07 February 2025; doi:10.1038/s41928-024-01335-5

Nature Communications, Published online: 08 February 2025; doi:10.1038/s41467-025-56709-w

An ad-hoc engineered metallic surface is employed to perform enhanced terahertz spectroscopy of a monolayer transition metal dichalcogenide (TMD). Thanks to a local absorption boost of 10⁴, this technique allows for the extraction of the phonon resonance features and effective permittivity of the 2D material, paving the way for the rational design of phonon polariton devices exploiting monolayer TMDs.

Abstract

2D materials, including transition metal dichalcogenides, are attractive for a variety of applications in electronics as well as photonics and have recently been envisioned as an appealing platform for phonon polaritonics. However, their direct characterization in the terahertz spectral region, of interest for retrieving, e.g., their phonon response, represents a major challenge, due to the limited sensitivity of typical terahertz spectroscopic tools and the weak interaction of such long-wavelength radiation with sub-nanometer systems. In this work, by exploiting an ad-hoc engineered metallic surface enabling a ten-thousand-fold local absorption boost, enhanced terahertz spectroscopy of a monolayer transition metal dichalcogenide (tungsten diselenide) is performed and its dipole-active phonon resonance features are extracted. In addition, these data are used to obtain the monolayer effective permittivity around its phonon resonance. Via the direct terahertz characterization of the phonon response of such 2D systems, this method opens the path to the rational design of phonon polariton devices exploiting monolayer transition metal dichalcogenides.

This review provides an overview of the structures of 2D molecular crystals (2D MCs) and strategies to modify their morphology and properties. Next, it summarizes preparation methods for large-scale 2D MCs by solution-based processes or vapor deposition. Finally, it highlights the applications of 2D MCs in electronic and optoelectronic devices with the advantages of tunable properties and scalable preparation methods.

Abstract

2D molecular crystals (2D MCs) are an emerging family of 2D materials formed by organic or inorganic molecules held together entirely by weak intermolecular forces. 2D MCs are gaining attention in electronics and optoelectronics due to their structural diversity, scalability, and strong light–matter interactions. This review provides a comprehensive overview of 2D MCs and their potential in electronic and optoelectronic applications. It begins by highlighting the structural features and properties of key 2D MCs discovered to date, focusing on three strategies to manipulate intermolecular forces for better control over crystal morphology and properties. Then various methods are explored for fabricating large-area, highly-oriented 2D MCs, with an emphasis on vapor-phase and liquid-phase techniques. Last, their applications are reviewed in electronic and optoelectronic devices, such as channel materials, photosensitive components, and dielectrics. It is concluded by discussing future challenges and opportunities in the field, offering insights into scalable production and industrial applications of 2D MCs.

This study presents the first synthesis of 2D hexagonal Eu₂SO₂ and tetragonal Eu₂SO₆ with tunable dielectric properties. Eu₂SO₂ offers high dielectric performance, while Eu₂SO₆ provides a wider bandgap. Integrated into MoS₂ field-effect transistors, these materials demonstrate excellent performance, highlighting their potential as multifunctional dielectrics for next-generation low-power electronics.

Abstract

Advancing next-generation electronics necessitates precise control of dielectric properties in 2D materials. Here, the first synthesis of novel 2D quasi-van der Waals (vdW) europium oxysulfur (Eu₂SO_x) compounds, comprising hexagonal Eu₂SO₂ and tetragonal Eu₂SO₆ phases, with composition-tunable dielectric properties, is presented. Using a homodiffusive-controlled epitaxial growth method, materials are achieved with complementary characteristics: the hexagonal Eu₂SO₂ phase exhibits a high dielectric constant (≈30) paired with a moderate bandgap (≈4.56 eV), while the tetragonal Eu₂SO₆ phase offers a wider bandgap (≈5.62 eV) but a lower dielectric constant (≈20). The potential of these materials is demonstrated by integrating ultrathin Eu₂SO₂ nanoplates with molybdenum disulfide (MoS₂) field-effect transistors (FETs) via vdW forces. The resulting devices achieve a near-ideal I _on/I _off ratio (≈10⁸), minimal hysteresis (≈5.3 mV), a low subthreshold slope (≈63.5 mV dec⁻¹), and ultralow leakage current (≈10⁻¹⁴ A). These results highlight the capacity of europium oxysulfur compounds to address the trade-off between dielectric constant and bandgap, offering tailored solutions for diverse 2D electronic applications. This work underscores the potential of composition engineering to expand the family of rare-earth oxysulfur compounds for nanoelectronics, paving the way for innovative gate dielectrics in next-generation devices.

Nature Materials, Published online: 03 February 2025; doi:10.1038/s41563-025-02131-y

Nature Nanotechnology, Published online: 03 February 2025; doi:10.1038/s41565-025-01862-y

This review introduced the recent progress on the self-intercalated 2D transition metal chalcogenides with the focus on the group-V, VI, and VIII metals, including their controllable syntheses, atomic-scale characterizations, and property explorations. Challenges and prospects are also proposed for developing new self-intercalated 2D materials and their heterostructures, and exploring their unique properties and applications.

Abstract

2D transition metal dichalcogenides (2D TMDCs) have attracted intensive interest in physics and materials science-related fields, due to their exotic properties (e.g., superconductivity, charge density wave (CDW) phase transition, magnetism, electrocatalytic property). Intercalation of native metal atoms in the layered 2D TMDCs (e.g., from VS₂ to V₅S₈ by V intercalation) can afford new stoichiometric ratios, phase states, and thus rich properties. This review hereby summarizes the recent progress in the controllable syntheses, structure characterizations, and property explorations of self-intercalated 2D transition metal chalcogenides (TMCs), with the metal elements focusing on group-V, VI, and VIII metals. The self-intercalation-related synthetic strategies will be introduced via chemical vapor deposition (CVD) and molecule beam epitaxy (MBE), especially by tuning the chemical potentials of intercalated metal elements, growth promoters, substrates, etc. Additionally, the structure/phase identifications of the self-intercalated 2D TMCs through various characterization techniques will be overviewed. More significantly, the intriguing properties in such 2D TMCs will be thoroughly discussed, such as the thickness- or composition-dependent magnetism, CDW phase transition, electrocatalytic property, etc. Finally, challenges and prospects are proposed for developing new self-intercalated 2D materials and their heterostructures and exploring their unique properties and applications.

This work demonstrates how the topologically protected conduction in Sb₂Te₃ thin films can be extended from few-mm² samples’ area up to larger areas of several cm² Si(111) wafers. The reported findings represent a breakthrough for the future technology scale-up of topological insulators for applications such as spintronics, thermoelectrics, and quantum computing.

Abstract

Recently, metal-organic chemical vapor deposition (MOCVD) has been proven successful to grow topological insulators such as antimony telluride (Sb₂Te₃), with their use as efficient spin-charge converters at room temperature also being reported. On the other hand, a wafer-scale synthesis of Sb₂Te₃ thin films showing clear-cut electrical conduction driven by topologically protected surface states is still missing. Within this work, the growth of Sb₂Te₃ thin films with variable thicknesses over 4-inch (4″) wafer-scale Si(111) substrates as conducted via MOCVD is reported. By performing magnetoconductance measurements, weak antilocalization phenomena are detected over the whole 4″ area, thus proving the possibility to produce wafer-scale Sb₂Te₃ topological insulator thin films. Furthermore, comprehensive information on the variability of the functional properties of Sb₂Te₃ thin films with their morphological, chemical, and structural properties, as probed by scanning electron microscopy, X-ray diffraction/reflectivity, atomic force microscopy, Raman spectroscopy, time-of-flight secondary ion mass spectrometry, and energy-dispersive X-ray analyses is reported. This work provides a breakthrough for the technology scale-up of these novel materials to be employed in future spintronic devices as well as applications in nanoelectronics, thermoelectrics, and quantum computing.

Recent high-performance self-powered photodetectors for infrared bands are mostly made of 2D/3D heterostructures. Here, a 2D/2D van der Waals heterostructure PdSe₂/MoTe₂ is proposed to be a outstanding infrared self-powered detector. It exhibits a self-powered broadband detection from ultraviolet to mid-infrared. Moreover, it possesses a good environmental stability and an infrared imaging capability.

Abstract

Self-powered photodetection is an effective way to resolve the issue of high dark current in infrared photodetectors under a bias voltage. To date, high-performance infrared self-powered photodetectors (ISPDs) are mostly based on heterostructures consisting of 2D and 3D materials, while those based on 2D/2D heterostructures are rare. This will hinder the development of infrared devices toward miniaturization and energy-saving. By exploring some 2D/2D van der Waals (vdWs) heterostructures, constructed by typical 2D transition metal dichalcogenides (TMDs), it is found that the heterostructure PdSe₂/MoTe₂ is a high-performance ISPD. It exhibits a good capability of self-powered broadband detection from 300 to 1550 nm, even extending to 4050 nm. Especially, under near-infrared illumination of 980 nm, its responsivity and detectivity can approach 395 mA W⁻¹ and 1.92 × 10¹¹ Jones, respectively, which can be comparable with the high-performance 2D/3D ISPDs. The heterostructure also possesses good environmental stability and infrared imaging capability. In addition, Three necessary conditions are proposed to construct 2D/2D high-performance ISPD, i.e., a large difference of work function, high infrared absorption, and a type-II band alignment. This work will guide a way to search for excellent ISPDs.

A universal method for synthesizing nanostructured transition metal oxides (NTMOs) through induced solidification of microdroplets enables rapid production in air within a minute. This method allows precise control of alignment for various applications, including gas sensors and PUFs, and supports doping, reduction, and chalcogenization while preserving morphology.

Abstract

Nanostructured transition metal oxides (NTMOs) have consistently piqued scientific interest for several decades due to their remarkable versatility across various fields. More recently, they have gained significant attention as materials employed for energy storage/harvesting devices as well as electronic devices. However, mass production of high-quality NTMOs in a well-controlled manner still remains challenging. Here, a universal, ultrafast, and solvent-free method is presented for producing highly crystalline NTMOs directly onto target substrates. The findings reveal that the growth mechanism involves the solidification of condensed liquid-phase TMO microdroplets onto the substrate under an oxygen-rich ambient condition. This enables a continuous process under ambient air conditions, allowing for processing within just a few tens of seconds per sample. Finally, it is confirmed that the method can be extended to the synthesis of various NTMOs and their related compounds.

Nature Reviews Methods Primers, Published online: 06 February 2025; doi:10.1038/s43586-024-00379-3