Jiuxiang Dai

Nature Materials, Published online: 11 April 2024; doi:10.1038/s41563-024-01856-6

A wide range of large-area 2D monolayers or few layers, including semiconductors, superconductors, and magnets, with high production yield are successfully exfoliated by a gold-template-assisted mechanical exfoliation method. Various moiré superlattices, including twisted homobilayers, twisted heterobilayers, and twisted multilayer superlattices, are efficiently and precisely constructed by picking-up and stacking these 2D layers using a dry-transfer method.

Abstract

Moiré superlattices, consisting of rotationally aligned 2D atomically thin layers, provide a highly novel platform for the study of correlated quantum phenomena. However, reliable and efficient construction of moiré superlattices is challenging because of difficulties to accurately angle-align small exfoliated 2D layers and the need to shun wet-transfer processes. Here, efficient and precise construction of various moiré superlattices is demonstrated by picking up and stacking large-area 2D mono- or few-layer crystals with predetermined crystal axes, made possible by a gold-template-assisted mechanical exfoliation method. The exfoliated 2D layers are semiconductors, superconductors, or magnets and their high quality is confirmed by photoluminescence and Raman spectra and by electrical transport measurements of fabricated field-effect transistors and Hall devices. Twisted homobilayers with angle-twisting accuracy of ≈0.3°, twisted heterobilayers with sub-degree angle-alignment accuracy, and multilayer superlattices are precisely constructed and characterized by their moiré patterns, interlayer excitons, and second harmonic generation. The present study paves the way for exploring emergent phenomena in moiré superlattices.

Nature Reviews Chemistry, Published online: 11 April 2024; doi:10.1038/s41570-024-00599-x

The synergistic effect between lanthanide ions and perovskite matrix for electroluminescent devices is studied. The luminescent mechanism and synthesis methods for lanthanide-ion-doped perovskite (LIDP) nanocrystals are summarized and current challenges and potential strategies in LIDP-based electroluminescent devices are analyzed. This discussion provides guidance for future LIDP-based device development, drawing insights from organic/perovskite light-emitting diodes to accelerate progress.

Abstract

Lanthanide ions doped in perovskite (LIDP) nanocrystals (NCs) provide an effective way to utilize the emission of lanthanide metals in a solution-processable way, combining the theoretical photoluminance quantum yield (PLQY) of ≈200%. To utilize advantages, LIDP-NCs have inspired studies exploring the fundamental physics of energy transfer, including the up-conversion or down-conversion process, and the optoelectronic applications of solar cells and white light-emitting didoes. This review broadens the scope of LIDP nanocrystal matrix semiconductors in electroluminescence devices in the near-infrared (NIR) range (>900 nm). A research is summarized on the synergistic effect of lanthanide ions and perovskite matrix in the near-infrared region, and discuss from the perspective of fabrication of lanthanide-based electroluminescent devices using perovskite materials as the matrix. The multiple optical transitions, bandgap tunability, and quantum-cutting effect to provide a tutorial on understanding LIDP-NCs are started. The details of synthesizing LIDP materials and aim to lay the foundation for preparing NIR electroluminescent devices with high efficiency and application value are then illustrated. The scientific issues that limit the performance of LIDP NCs-based electroluminescent devices and discuss the potential strategies for the future development of LIDP material are focused on.

In this work, the controllable synthesis of 2D Fe_1+yTe nanoflakes are realized with tunable Fe content by CVD method. Among them, the Fe_1.13±0.06Te crystal exhibits a sharp superconducting transition (ΔT _c = 1 K) at B = 0 T, and the transition temperature T _c is measured to be 10.2 K at B = 12 T. Additionally, the high-Fe content Fe_1.43±0.07Te crystal shows a relative broad superconducting transition (ΔT _c = 2.6 K) at B = 0 T, and T _c is suppressed to 3.8 K at B = 12 T, which is related to the 3D-to-2D vortex liquid transition. This research will pave the way for understanding the intrinsic mechanisms of high-temperature superconductivity.

Abstract

2D materials provide an ideal platform to explore novel superconducting behavior including Ising superconductivity, topological superconductivity and Majorana bound states in different 2D stoichiometric Ta-, Nb-, and Fe-based crystals. However, tuning the element content in 2D compounds for regulating their superconductivity has not been realized. In this work, the synthesis of high quality Fe_1+yTe with tunable Fe content by chemical vapor deposition (CVD) is reported. The quality and composition of Fe_1+yTe are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM). The superconducting behavior of Fe_1+yTe crystals with varying Fe contents is observed. The superconducting transition of selected Fe_1.13±0.06Te sample is sharp (ΔT _c = 1 K), while Fe_1.43±0.07Te with a high-Fe content shows a relative broad superconducting transition (ΔT _c = 2.6 K) at zero magnetic field. Significantly, the conspicuous vortex flow and a transition from a 3D vortex liquid state to a 2D vortex liquid state is observed in Fe_1.43±0.07Te sample. This work highlights the tunability of the superconducting properties of Fe_1+yTe and sheds light on the vortex dynamics in Fe-based superconductors, which facilitates them to understand the intrinsic mechanisms of high-temperature superconductivity.

The synthesis of Bi_2–xSb_xSe₃ nanoflakes on a mica substrate with a thickness tunable to 2 nm through a BiOCl-aidedchemical vapor deposition (CVD) process is reported. Electronic transportstudies reveal that the Field-effect transistor (FET) devices built from Bi_2–xSb_xSe₃ exhibit ambipolar properties, with a carrier density down to 1.6 × 1012 cm^–2 and field-effect mobility and Hall mobility up to 3411 and 6462 cm²V^–1 s^–1, respectively.

Abstract

Highcarrier concentration and low mobility in Bi₂Se₃ hide thetopological surface states (TSS). In the 2D ternary topological insulator (TI) Bi_2–xSb_xSe₃,compensatory Sb doping regulates the carrier concentration and mobility withambipolar performance, together with the ultrathin thickness; these factorsmake the TSS in the 2D ternary TI Bi_2–xSb_xSe₃ more observable. Here, a chemical vapor deposition (CVD) method is provided for synthesizing ultrathin Sb-doped Bi₂Se₃ nanoplates with dimensions of 2–126 nm in thickness, 3–100 µm in lateral size, and an average Sb doping ranging from 0.15 ≤ x ≤ 0.75. Bi_2–xSb_xSe₃ field effect transistors and Hall devices are manufactured to determine the carrier concentration and mobility of the obtained Bi_2–xSb_xSe₃ nanoplates. These findings demonstrate that the 2D carrier concentration for Bi_2–xSb_xSe₃ nanoplates can decrease up to 1.6 × 10¹² cm^–2. Furthermore, field-effect mobility and Hall mobility of up to 3411 cm² V^–1s^–1 and 6462 cm² V^–1 s^–1, respectively, are realized. A strong ambipolar field effect is found in low-carrier-density Bi_2–xSb_xSe₃ nanoplates, proving that these nanostructures may be freely controlled in terms of carrier type and concentration. The synthesis of high-quality Bi_2–xSb_xSe₃ nanoplates with low-carrier concentration and high-mobility provides a platform for investigating TI characteristics more clearly.

Nature Reviews Materials, Published online: 08 April 2024; doi:10.1038/s41578-024-00666-1

Nature, Published online: 10 April 2024; doi:10.1038/s41586-024-07360-w

Publication date: June 2024

Source: Materials Today, Volume 75

Author(s): Jiawei Li, Xuesong Li, Hongwei Zhu

Nature, Published online: 10 April 2024; doi:10.1038/s41586-024-07175-9

This study reports that conjugated diynes and oxygenic functional groups tune the proton–electron conductivity in graphdiyne. In addition, a wet-chemistry lithography technique for uniform preparation of graphdiyne on flexible substrates is provided. As a proof-of-concept, a breath–machine interface for sentence-based communication and self-nursing tasks with an accuracy of 98% is designed.

Abstract

Mixed conducting materials with both ionic and electronic conductivities have gained prominence in emerging applications. However, exploring material with on-demand ionic and electronic conductivities remains challenging, primarily due to the lack of correlating macroscopic conductivity with atom-scale structure. Here, the correlation of proton–electron conductivity and atom-scale structure in graphdiyne is explored. Precisely adjusting the conjugated diynes and oxygenic functional groups in graphdiyne yields a tunable proton–electron conductivity on the order of 10³. In addition, a wet-chemistry lithography technique for uniform preparation of graphdiyne on flexible substrates is provided. Utilizing the proton–electron conductivity and mechanical tolerance of graphdiyne, bimodal flexible devices serving as capacitive switches and resistive sensors are created. As a proof-of-concept, a breath–machine interface for sentence-based communication and self-nursing tasks with an accuracy of 98% is designed. This work represents an important step toward understanding the atom-scale structure–conductivity relationship and extending the applications of mixed conducting materials to assistive technology.

Nature Nanotechnology, Published online: 08 April 2024; doi:10.1038/s41565-024-01639-9

The large-scale two-dimensional inorganic molecular crystals (2DIMCs) and heterostructures are successfully synthesized, demonstrating superior stability over 80 days in air. The universal growth of 2DIMCs on various substrates enables the construction of 2D BiBr₃/MoS₂ heterostructure with significantly enhanced photoluminescence. This work provides a mature strategy for synthesizing 2DIMC materials toward practical application in electronics and optoelectronics.

Abstract

In contrast to conventional atomic crystals, two-dimensional inorganic molecular crystals (2DIMCs) possess a unique crystal structure composed of small molecules held together by van der Waals force. Such distinctive structure grants them specific chemical and physical properties highly suitable for electronic and optoelectronic applications. However, the synthesis of 2DIMCs has posed significant challenges due to the inherent issues such as uncontrollable growth orientation stemming from their natural crystalline isotropy and poor stability resulting from the weak intermolecular connections. Addressing these obstacles, the preparation of large-scale and highly-stable 2DIMC BiBr_3, and its heterostructure are reported here by developing crystallization orientation engineering strategies. This approach has successfully yielded centimeter-scale 2DIMC BiBr₃ with a clean and dense surface, showcasing exceptional long-term air-stability exceeding 80 days. Furthermore, the universal growth of large-scale 2DIMC BiBr₃ is demonstrated on diverse substrates, including SiO₂, Al₂O₃, and MoS₂ monolayers. Notably, the resultant 2D BiBr₃/MoS₂ heterostructure exhibits remarkable photoluminescence enhancement. This contribution paves a universal avenue for the mature synthesis of large-scale 2DIMCs with superior stability, holding great promise for prompting their integration in practical electronic and optoelectronic devices.

Fractal engineering opens new avenues to fabricate highly efficient 2D laser protection materials, with Fractal NbSe₂ demonstrating giant and broadband laser attenuation behaviors at a huge nonlinear absorption coefficient of 9.7 × 10⁻⁴ m W⁻¹ and an ultralow starting threshold of 5 mJ cm⁻² at 532 nm.

Abstract

The compelling demand for laser protection both for civil and defense use calls for new-generation nonlinear optical materials. Chemical vapor deposition (CVD) techniques provide extra tricks to modulate crystal optical nonlinearity through fractal growth. The synthesized 2D NbSe₂ and its fractal structures exhibit giant, broadband laser attenuation behaviors extending into the near Infrared (NIR) range. Particularly, the optical limiting performance generally correlates with the fractal dimension, where Fractal NbSe₂ demonstrates enhanced third-order optical nonlinearity at a huge nonlinear absorption coefficient of 9.7 × 10⁻⁴ m W⁻¹ and an ultralow starting threshold of 5 mJ cm⁻² at 532 nm. Various techniques include femtosecond spectroscopy, Density functional theory (DFT) calculation and Kelvin probe force microscopy disclose the origin of the strong nonlinearity of the 2D NbSe₂ crystals, and suggest the formation of edge states and overall faster carrier dynamics, larger surface contact potential difference NbSe₂ fractals contribute to their even augmented nonlinear responses. Fractal engineering thus opens new avenues to fabricate highly efficient laser protection materials, and the blocking of intense beam (a large attenuation factor over 13.3 dB at 532 nm) while allowing transmission of weak one (a high linear optical transmittance over 80%) fulfills the much desired “smart” defense.

A near-field transient nanoscopy is developed to probe exciton dynamics in 2D transition metal dichalcogenides beyond the diffraction limit. The investigation of local exciton recombination and exciton–exciton annihilation processes with nanoscale resolution is demonstrated.

Abstract

The electronic and optical properties of 2D transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic behaviors remains challenging. Here, a near-field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton–exciton annihilation processes in monolayer and bilayer MoS₂ are studied as the proof-of-concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer-bilayer interface and at the MoS₂ nano-wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices.

Leveraging Gd’s magnetocaloric effect to absorb frictional heat enables the triboelectric nanogenerator to sustain output performance in long-term operation. Harnessing the magnetization effect of Gd to concentrate the magnetic field results in a significant enhancement in the electromagnetic generator’s outputs. The interaction of the internal components achieves a synergistic effect where 1 + 1 > 2, providing a general solution for self-reinforcing hybrid generators.

Abstract

Triboelectric-electromagnetic hybrid nanogenerator (TEHG) has emerged as a promising technology for distributed energy harvesting. However, currently reported hybrid generators are straightforward combinations of two functional components. Moreover, inevitable heat from friction intensifies material abrasion and degrades the performance of polymer-based triboelectric nanogenerators (TENGs). Here, a self-reinforcing TEHG (SR-TEHG) that harnesses the magnetocaloric and magnetization effects of gadolinium (Gd), is proposed. The synergy between TENG and electromagnetic generator (EMG) renders them an indivisible unit. Leveraging Gd's magnetocaloric effect, an efficient heat transfer mechanism is constructed to cool the tribolayer and strengthen the device's electrical stability. After 80 h of continuous operation, the optimized TENG occupies a charge decay rate of only 0.32% per hour, significantly outperforming most existing TENGs. Additionally, Gd's magnetization effect boosts the power of EMG by ≈80.84%. This work provides a universal solution in hybrid generators where internal components reinforce each other, achieving a synergistic effect of 1 + 1 > 2.

4D printing of azobenzene-containing liquid crystal elastomers (LCEs) actuating at low temperatures close to room temperature is demonstrated. Elastomer irradiation with moderate-intensity ultraviolet light leads to fast photodeformation underwater indicating predominant photochemical response. Periodic illumination of a 4D printed biomimetic four-lapped ephyra-like LCE swimmer, with moderate-intensity UV and green light, covering the entire sample, propels the swimmer away the light source.

Abstract

Underwater organisms exhibit sophisticated propulsion mechanisms, enabling them to navigate fluid environments with exceptional dexterity. Recently, substantial efforts have focused on integrating these movements into soft robots using smart shape-changing materials, particularly by using light for their propulsion and control. Nonetheless, challenges persist, including slow response times and the need of powerful light beams to actuate the robot. This last can result in unintended sample heating and potentially necessitate tracking specific actuation spots on the swimmer. To tackle these challenges, new azobenzene-containing photopolymerizable inks are introduced, which can be processed by extrusion printing into liquid crystalline elastomer (LCE) elements of precise shape and morphology. These LCEs exhibit rapid and significant photomechanical response underwater, driven by moderate-intensity ultraviolet (UV) and green light, being the actuation mechanism predominantly photochemical. Inspired by nature, a biomimetic four-lapped ephyra-like LCE swimmer is printed. The periodically illumination of the entire swimmer with moderate-intensity UV and green light, induces synchronous lappet bending toward the light source and swimmer propulsion away from the light. The platform eliminates the need of localized laser beams and tracking systems to monitor the swimmer's motion through the fluid, making it a versatile tool for creating light-fueled robotic LCE free-swimmers.

Nature Synthesis, Published online: 09 April 2024; doi:10.1038/s44160-024-00502-y

With phase-tailored synthetic strategies, wafer-scale production of tin selenides in the 2D limit is achieved via a low-temperature metal-organic chemical vapor deposition (MOCVD) process. Directly grown 2D-SnSe₂ exhibits outstanding crystallinity and tunable thickness, and SnSe, which has intrinsic limitations for 2D film growth, can be prepared via a phase transition, thereby retaining all of the advantages in the MOCVD-grown product.

Abstract

Following an initial nucleation stage at the flake level, atomically thin film growth of a van der Waals material is promoted by ultrafast lateral growth and prohibited vertical growth. To produce these highly anisotropic films, synthetic or post-synthetic modifications are required, or even a combination of both, to ensure large-area, pure-phase, and low-temperature deposition. A set of synthetic strategies is hereby presented to selectively produce wafer-scale tin selenides, SnSe _x (both x = 1 and 2), in the 2D forms. The 2D-SnSe₂ films with tuneable thicknesses are directly grown via metal–organic chemical vapor deposition (MOCVD) at 200 °C, and they exhibit outstanding crystallinities and phase homogeneities and consistent film thickness across the entire wafer. This is enabled by excellent control of the volatile metal–organic precursors and decoupled dual-temperature regimes for high-temperature ligand cracking and low-temperature growth. In contrast, SnSe, which intrinsically inhibited from 2D growth, is indirectly prepared by a thermally driven phase transition of an as-grown 2D-SnSe₂ film with all the benefits of the MOCVD technique. It is accompanied by the electronic n-type to p-type crossover at the wafer scale. These tailor-made synthetic routes will accelerate the low-thermal-budget production of multiphase 2D materials in a reliable and scalable fashion.

A broad-band photodetector with polarization sensitivity is successfully demonstrated through an engineered coupling of Te with PdSe₂. Specifically, synergistic effects of multiple physical mechanisms are employed to enhance polarization ratio in the near-infrared (NIR) region.

Abstract

Polarization-sensitive broadband optoelectronic detection is crucial for future sensing, imaging, and communication technologies. Narrow bandgap 2D materials, such as Te and PdSe₂, show promise for these applications, yet their polarization performance is limited by inherent structural anisotropies. In this work, a self-powered, broadband photodetector utilizing a Te/PdSe₂ van der Waals (vdWs) heterojunction, with orientations meticulously tailored is introduced through polarized Raman optical spectra and tensor calculations to enhance linear polarization sensitivity. The device exhibits anisotropy ratios of 1.48 at 405 nm, 3.56 at 1550 nm, and 1.62 at 4 µm, surpassing previously-reported photodetectors based on pristine Te and PdSe₂. Additionally, it exhibits high responsivity (617 mA W⁻¹ at 1550 nm), specific detectivity (5.27 × 10¹⁰ Jones), fast response (≈4.5 µs), and an extended spectral range beyond 4 µm. The findings highlight the significance of orientation-engineered heterostructures in enhancing polarization-sensitive photodetectors and advancing optoelectronic technology.

2D FeX (X = S, Se) sheets with one-unit-cell thickness are intrinsic altermagnetic semiconductors with observable magnitude of spin splitting, high magnetic transition temperature, large out-of-plane magnetic anisotropy energy and considerable metal-like anomalous Hall conductivity. Moreover, an inverse magnetocaloric effect with positive ∆S _m values of 1.7–8.2 mJ kg⁻¹ K⁻¹ near T _AM is observed.

Abstract

The concept of altermagnet (AM), which is neither ferromagnet nor antiferromagnet, has recently been proposed as compensated collinear magnet. Among 2D non-van der Waals crystals, one of the thinnest altermagnets is identified and their interesting physical phenomenon at 2D limit is investigated. According to symmetry analysis and first-principles calculations, 2D FeX (X = S, Se) sheets with one-unit-cell thickness are intrinsic altermagnetic semiconductors with observable magnitude of spin splitting (103−193 meV), high magnetic transition temperature (T _AM = 400 and 190 K), large out-of-plane magnetic anisotropy energy (0.24 and 0.74 meV per atom) and considerable metal-like anomalous Hall conductivity (143 and 249 Ω⁻¹ cm⁻¹). Moreover, the spin lattices of these 2D FeX ultrathin films are formed by parallel diamond-like chains with spin frustrated interaction through spin exchange along the b direction. Consequently, an inverse magnetocaloric effect with positive ∆S _m values of 1.7–8.2 mJ kg⁻¹ K⁻¹ near T _AM is observed. These results enrich the database of altermagnets and provide deep insights into the spin frustration effect in 2D magnets, which paves the avenue for the next-generation spintronics.

An ultrasensitive UV-responsive 2D Ca₂Nb₃O₁₀ optoelectronic synapses detect UV-C at an ultralow intensity of 70 nW cm⁻², a record low for 2D UV synapses, showing applications in image distinction, learning processes, and encrypted information extraction with spectral selectivity, ultrahigh sensitivity, and low-energy consumption.

Abstract

Neuromorphic device that percepts ultraviolet (UV) radiations is essential for supplementing human visual perception. However, UV-responsive synapses with high sensitivity and wavelength selectivity have not yet been developed so far. Herein, 2D optoelectronic synapses that are able to distinguish between UV-A, UV-,B and UV-C radiations using a single material are demonstrated. The synapses use Ca₂Nb₃O₁₀ nanosheets with wide bandgap and persistent photoconductivity effect (PPC) as UV-responsive layer. The devices can detect UV-C rays at ultralow intensity of 70 nW cm⁻², which is the lowest record among 2D UV synaptic devices. Moreover, such a device demonstrates UV image vision, recognition, and memorization functions based on its diverse synaptic behaviors. The atomically thin UV synapses provide implications for enhancing human visual perception capabilities with spectral selectivity, ultrahigh sensitivity, and low-energy consumption.