Jiuxiang Dai

It is reported that visible-light-driven fluorescence turn-on photoswitches with near quantitative photocyclization yield. The ortho and para isomers exhibit high ring closing yield of up to 94% upon 405 nm light irradiation, however, that of meta isomer is low to 22%. Moreover, the applications in anti-counterfeiting, information encryption, and photo rewritable patterns have been demonstrated.

Abstract

Photoswitchable fluorescent materials have gained significant attention for their potential in advanced information encryption and anti-counterfeiting applications. However, the common use of UV light to trigger the isomerization processes leads to photobleaching and poor fatigue resistance. Visible-light-driven fluorescent photoswitches are highly desirable, but achieving high cyclization yield remains challenging. Herein, it is reported that all visible-light-driven turn-on fluorescence in dimethoxyphenyl functionalized diarylethene isomers. The open-ring form of o-DMPB and p-DMPB exhibits near quantitative conversion yields (up to 94%) under 405 nm visible light, attributed to the strong electron-donating character. In contrast, the meta isomer m-DMPB shows limited response to visible light, with a ring-closing yield of 22%. Furthermore, all photoswitches display good thermal stability, photostability, and fatigue resistance. Notably, o-DMPB demonstrates promising applications in anti-counterfeiting, information encryption, and photorewritable patterns. This work provides a valuable strategy for the development of high-performance fluorescent photoswitches.

npj 2D Materials and Applications, Published online: 14 January 2025; doi:10.1038/s41699-025-00524-w

Defect-assisted reversible phase transition in mono- and few-layer ReS₂

Author(s): Henry Mittenzwey, Abhijeet M. Kumar, Raghav Dhingra, Kenji Watanabe, Takashi Taniguchi, Cornelius Gahl, Kirill I. Bolotin, Malte Selig, and Andreas Knorr

We investigate spin relaxation dynamics of interlayer excitons in a MoSe2/MoS2 heterostructure induced by the Rashba effect. In such a system, Rashba interactions arise from an out-of-plane electric field due to photogenerated interlayer excitons inducing a phonon-assisted intravalley spin relaxatio…

[Phys. Rev. Lett. 134, 026901] Published Tue Jan 14, 2025

Nature Nanotechnology, Published online: 15 January 2025; doi:10.1038/s41565-024-01846-4

Sliding ferroelectricity with finite conductance can yield ferroelectric hysteresis with concomitant Coulomb screening in monolayer graphene containing van der Waals heterostructures of various compositions.

Highlights

• Two-dimensional (2D) materials are highlighted for their exceptional mechanical, electrical, optical, and chemical properties, making them ideal for fabricating high-performance wearable biodevices.

• The review categorizes cutting-edge wearable biodevices by their interactions with physical, electrophysiological, and biochemical signals, showcasing how 2D materials enhance these devices' functionality, mainly including self-powering and human-machine interaction.

• 2D materials enable multifunctional, high-performance biodevices, integrating self-powered systems, treatment platforms, and human-machine interactions, though challenges remain in practical applications.

Nature Reviews Methods Primers, Published online: 16 January 2025; doi:10.1038/s43586-024-00373-9

Metal halide perovskite solar cells are emerging as next-generation photovoltaics, offering an alternative to silicon-based cells. This Primer gives an overview of how to fabricate the photoactive layer, electrodes and charge transport layers in perovskite solar cells, including assembly into devices and scale-up for future commercial viability.

Researchers have experimentally unveiled a perfect interface between the monoclinic (β)-phase and orthorhombic (κ)-phase of Ga₂O₃, establishing a “phase heterojunction” within the ultrawide bandgap Ga₂O₃ material system. This phase heterojunction exhibits a type-II band alignment with significant band offset, highlighting a large interfacial electrical field, which is verified through the separation of photogenerated electron–hole pairs, even without external bias.

Abstract

Ultrawide-bandgap gallium oxide (Ga₂O₃) holds immense potential for crucial applications such as solar-blind photonics and high-power electronics. Although several Ga₂O₃ polymorphs, i.e., α, β, γ, δ, ε, and κ phases, have been identified, the band alignments between these phases have been largely overlooked due to epitaxy challenges and inadvertent neglect. Despite having similar stoichiometry, heterojunctions involving different phases may exhibit band offsets. Here, β-Ga₂O₃/κ-Ga₂O₃-stacked “phase heterojunction” is demonstrated experimentally. This phase heterojunction has a sharp and well-defined interface, and subsequent measurements reveal an unbeknown type-II band alignment with significant valence/conduction band offsets of ≈0.65 eV/0.71 eV. This alignment is promising for self-powered deep ultraviolet (DUV) signal detection, necessitating an internal electric field near the junction and matching the absorption properties for effective electron–hole separation. The fabricated phase heterojunction photodetector displays a responsivity of three orders of magnitude higher at 17.8 mA W⁻¹, with improved response times (rise time ≈0.21 s, decay time ≈0.53 s) under DUV illumination and without external bias in comparison to the bare β-Ga₂O₃ and κ-Ga₂O₃ photodetectors, confirming the strong interfacial electrical field. This study provides profound insight into Ga₂O₃/Ga₂O₃ heterojunction interfaces with different polymorphs, allowing the use of phase heterojunctions to advance electronic device applications.

Centimeter-size, uniform 2D ErOCl film, and diverse 2D rare-earth materials are grown using self-encapsulation strategy. 2D ErOCl possesses high crystalline quality, excellent ambient and thermal stability. 2D ErOCl exhibits outstanding optical properties, featuring sub-meV narrow emissions at the telecom C-band. These emissions are tunable with external magnetic fields, highlighting their potential for advanced optical applications.

Abstract

Van der Waals (vdWs) materials are promising candidates for hetero-integration with silicon photonics toward miniaturization and integration. VdWs materials like molybdenum telluride and black phosphorus, despite being prominent, exhibit air sensitivity, and their room temperature emissions can be significantly broadened by tens of meV. Here, a self-encapsulation strategy is developed to scalably synthesize robust 2D vdWs ErOCl with sub-meV narrow emissions at the telecom C-band. Diverse 2D rare earth materials are also grown via chemical vapor deposition (TmOCl, YbOCl, HoOCl, DyOCl, SmOCl, NdOCl, TbOCl, GdOCl, EuOCl, and PrOCl), demonstrating the strategy's generalizability. The as-grown ErOCl exhibits high crystalline quality and excellent ambient and thermal stability (300 °C). Photoluminescence analysis reveals a series of narrow emissions across the visible to near-infrared spectrum. The ErOCl's emission at the telecom band is narrowest among 2D luminescent materials, and suitable for integrating with photonic chips. Temperature-dependent photoluminescence spectra facilitate the understanding of emission mechanisms, analyzed using a crystal field perturbation model. Moreover, these emissions can be tuned by external magnetic fields. This research not only pioneers a novel strategy for synthesizing 2D rare earth materials but also paves the way for innovative building blocks in the realm of on-chip optical communications.

This review paper presents a comprehensive collection of approaches to improve gas sensor performance, addressing selectivity challenges in 2D material-based sensors. The strategies are categorized into Material Approaches (surface functionalization, physical barriers, electronic modification, heterostructures), Engineering Approaches (sensor arrays, external condition modification), and Machine Learning Approaches (supervised and unsupervised learning).

Abstract

Two-dimensional (2D) materials have emerged as promising candidates for gas sensing applications due to their exceptional electrical, structural, and chemical properties, which enable high sensitivity and rapid response to gas molecules. However, despite their potential, 2D material-based gas sensors face a significant challenge in achieving adequate selectivity, as many sensors respond similarly to multiple gases, leading to cross-sensitivity and inaccurate detection. This review provides a comprehensive overview of the recent advancements for improving the selectivity of 2D gas sensors. It explores material modification strategies, such as functionalizing the sensing components and tuning adsorption dynamics, to enhance selective gas interactions. Engineering approaches, including field-effect modulation and sensor array design, are also discussed as effective methods to fine-tune sensor performance. Additionally, the integration of machine learning (ML) algorithms is highlighted for their potential to differentiate among multiple analytes. Prospects for further improving selectivity through material optimization, sensor calibration, and drift compensation are explored, along with the incorporation of smart sensing systems into the Internet of Things (IoT). This review outlines key objectives and strategies that pave the way for next-generation gas sensors with enhanced selectivity, reliability, and versatility, poised to impact a wide range of applications from environmental monitoring to industrial safety.

Passive X-ray dosimeter is proposed by leveraging the X-ray-induced conductivity change of amorphous Ga₂O₃ channel. Increasingly negatively shifted threshold voltage is recorded together with almost unchanged subthreshold swing and field-effect mobility. The slow neutralization rate of ionized oxygen vacancy imparts an offline working capability to the dosimeter. Through an annealing process in air condition, the functional recovery is successfully realized.

Abstract

As most commonly used miniature solid-state dosimeter, metal-oxide-semiconductor field effect transistors (MOSFETs) have been facing unavoidable gate leakage and robustness problems due to the double-sided role of traps in oxide insulators. Herein, a new solution for X-ray dosimeter has been proposed which leverages the conductivity change of amorphous Ga₂O₃ (a-Ga₂O₃) channel instead of charge trapping in oxide insulators. Increasingly negatively-shifted threshold voltage (V_th) of a-Ga₂O₃ thin-film transistor is recorded together with almost unchanged subthreshold swing (SS) and field-effect mobility (µ_FE) after X-ray irradiations. X-ray-induced-variation of oxygen vacancy (V_O) related defects in a-Ga₂O₃ is revealed after a combined investigation of X-ray photoelectron spectroscopy (XPS) and neutron reflectivity (NR) measurements, contributing to the indicative V_th shift related with X-ray dosage. A functional recovery is realized through an annealing process in air condition, showing the high reliability of a-Ga₂O₃ semiconductor. Moreover, the unique merit of no need for power supply during X-ray irradiations, beneficial from the slow neutralization rate of ionized V_O related defects, imparts an offline working capability to the dosimeter. This work provides a potential strategy to monitor X-ray dosage via utilizing the X-ray-induced change of amorphous oxide semiconductor conductivity, hence addressing the reliability and repeatability issues in present MOSFET devices.

Nature Materials, Published online: 13 January 2025; doi:10.1038/s41563-024-02074-w

The authors report on their observation of magnetic polymorphs in CrSBr using phase-sensitive second harmonic generation.

Nature Communications, Published online: 10 January 2025; doi:10.1038/s41467-024-55224-8

The proposed edge detection based on ferroelectric field effect transistor does not rely on conventional convolution operation, realizing no-accuracy-loss, low-power (~10 fJ/per operation) and analogue-to-digital converter (ADC)-free edge computing.

Nature Synthesis, Published online: 10 January 2025; doi:10.1038/s44160-024-00714-2

The observation of superconductivity in infinite-layer nickelates has been limited to epitaxial thin films, which has restricted their experimental investigation. Now, a technique is reported for the release of millimetre-scale freestanding superconducting nickelate membranes. This geometry enables a range of studies, including the response of superconductivity to strain.

Nature Reviews Chemistry, Published online: 10 January 2025; doi:10.1038/s41570-024-00680-5

As their applications grow, it is vital to understand how 2D materials degrade in the environment and biological systems. Current knowledge remains limited, and the available methodologies are specific and challenging to carry out. This article aims to identify opportunities for us to better understand the end-of-life characteristics of advanced materials.

In this article, an etching-and-growth coexistence technique is introduced for the direct synthesis of high-quality, highly symmetric MoS₂ bilayer with diverse morphologies through CVD. The growth mechanism is elucidated through analyzing the carrier Ar perturbation associated with the variation of the precursor concentration, revealing four growth stages: the growth-priority, the local-etching, the equilibrium of etching and growth, and the etching-priority.

Abstract

The etch-engineering is a feasible avenue to tailor the layer number and morphology of 2D layered materials during the chemical vapor deposition (CVD) growth. However, less reports strengthen the etch-engineering used in the fabrication of high-quality transition metal dichalcogenide (TMD) materials with tunable layers and desirable morphologies to improve their prominent performance in electronic and optoelectronic devices. Here, an etching-and-growth coexistence method is reported to directly synthesize high-quality, high-symmetric MoS₂ bilayers with versatile morphologies via CVD. The growth mechanism is intensively elucidated through analyzing the carrier Ar perturbation associated with the precursor concentration variations, revealing four growth stages including the growth-priority, local-etching, equilibrium of etching and growth, and etching-priority. The as-grown polygonal bilayer MoS₂ exhibits a uniform characteristic, attributed to the formation of the high-quality single crystal bilayer MoS₂ owing to the limitation of the multigrain generation. The work not only enriches the understanding of the growth mechanism of the direct fabrication of TMD materials, but also offers a controllable protocol to engineer their morphologies and the shapes, which can benefit their applications in the electronic and optoelectronic devices.

This article demonstrates low-birefringence photonic integrated circuits based on lithium-tantalate-on-insulator, enabling high-performance passive photonic devices as well as electro-optic (EO) modulators for large-scale photonic chips. Various representative passive photonic devices are demonstrated with impressive performances. An optical transmitter with a data rate of 1.6 Tbps is further realized by monolithically integrating 8 EO modulators and an 8-channel arrayed waveguide grating.

Abstract

Photonic manipulation of large-capacity data with the advantages of high speed and low power consumption is a promising solution for explosive growth demands in the era of post-Moore. A well-developed lithium-niobate-on-insulator (LNOI) platform has been widely explored for high-performance electro-optic (EO) modulators to bridge electrical and optical signals. However, the photonic waveguides on the x-cut LNOI platform suffer serious polarization-mode conversion/coupling issues because of strong birefringence, making it hard to realize large-scale integration. Here, low-birefringence photonic integrated circuits (PICs) based on lithium-tantalate-on-insulator (LTOI) are proposed and demonstrated, which enables high-performance passive photonic devices as well as EO modulators, showing great potential for large-scale photonic chips. Analysis of mode conversion and evolution behaviors with both low- and high-birefringence shows undesired mode hybridizations can be effectively suppressed. A simple and universal fabrication process is developed and various representative passive photonic devices are demonstrated with impressive performances. Finally, a wavelength-division-multiplexed optical transmitter is developed with a data rate of 1.6 Tbps by monolithically integrating 8 EO modulators and an 8-channel arrayed waveguide grating. Therefore, the demonstrated low-birefringence LTOI platform shows strong ability in both passively and actively controlling photon behaviors on a chip, indicating great potential for ultrafast processing and communicating large-capacity data.

Nanometer-sized hierarchical interface heterostructures with controlled interface size and composition are synthesized. The visualized relaxation process and high-density magnetic coupling at the nanoscale demonstrate exceptional magnetic and electric interactions, contributing to the high-performance electromagnetic absorption. This work paves a novel pathway for controllably constructing low-dimensional hierarchical heterointerfaces, which possesses significant potential to drive innovations across various fields.

Abstract

Quantum size effects and interfacial dimensional interactions enable nanometer-scale hierarchical heterostructures to adjust band structures by energy level discretization, impurity level formation, and band inversion, allowing for controlled carrier localization and directional relaxation. These unique characteristics show great potential for applications in ferroelectrics, optoelectronics, capacitors, and sensors. Yet, optimizing performance by fine-tuning the dimensional properties of nanoscale systems, especially size and composition, remains a considerable challenge. Here a dimensionally confined controlled synthesis of hierarchical heterostructures is reported through a pyrolysis-based metal-organic framework-on-metal-organic framework (MOF-on-MOF) strategy, resulting in continuous metal-carbon and carbon-oxide interfaces below 50 nm. Off-axis electron holography and theoretical calculations are utilized to visualize the dynamic conversion between localized and free electrons, as well as the relaxation processes and high-density magnetic coupling at the nanoscale. These phenomena are rarely observed in micron-scale or non-hierarchical heterostructures. These improvements lead to significantly enhanced magnetic and dielectric properties, allowing for efficient interaction with high-frequency electromagnetic (EM) fields, as indicated by a loss of bandwidth covering the full C-band. Future work will explore constructing these interfaces with targeted materials to examine new properties, such as topological behavior, ferrimagnetism, and giant magnetoresistance, with applications in sustainability and optoelectronic technology.

Nature Materials, Published online: 10 January 2025; doi:10.1038/s41563-024-02100-x

The two-dimensional Czochralski growth method enables the rapid production of large-area single-crystal MoS2, effectively alleviating the issues related to defect density and scalability for devices based on two-dimensional materials.

Nature Materials, Published online: 10 January 2025; doi:10.1038/s41563-024-02069-7

A 2D Czochralski method is introduced for rapidly growing centimetre-scale single-crystal MoS2 domains with low defect density and impressive electrical performance. This method shows potential for fabricating high-quality and scalable 2D semiconductor materials and devices.

Although 2D metal and semiconductor materials provide a promising solution to realize ohmic contacts, the additional van der Waals (vdW) gap inevitably induces a large tunneling barrier, significantly restraining the charge transport. By effectively replacing the vdW bond with the covalent bond at 2D interfaces, weakening the tunneling barrier and realizing Ohmic contacts can be achieved simultaneously.

Abstract

2D metal and semiconductor materials provide a promising solution to realize Ohmic contacts by suppressing the strong Fermi level pinning (FLP) effect due to without dangling bonds. However, the 2D metal-semiconductor Van der Waals (vdW) interfaces induce an inevitable tunnel barrier, significantly restraining the injection of charge carriers into the conduction channel. Herein, by replacing the vdW bond with the covalent bond in interfaces, the Ohmic and tunneling-barrier-inhibition contacts are realized simultaneously based on the 2D XSi₂N₄ (X = Cr, Hf, Mo, Ti, V, Zr) semiconductor and the 2D Mxene metal family. Taking 60 2D Mxene-XSi₂N₄ contacts as examples, although the vdW-type contacts exhibit Ohmic contacts, the tunneling probability (P_TB) can be as low as 0.4%, while the P_TB can increase to 88.09% by removing the Mxene terminations at the adjacent interface to form the covalent bond. The weak FLP and Ohmic contacts are retained at covalent bond interfaces since the outlying Si─N sublayer protects the band-edge electronic states of XSi₂N₄ semiconductors. This work provides a straightforward strategy for advancing high-performance and energy-efficient 2D electronic nanodevices.

Twisting and stacking are key for engineering twisted 2D magnets, as shown here by stacking pristine monolayers and bilayers of CrSBr with a twist-angle of 90°. By creating symmetric (monolayer/monolayer and bilayer/bilayer) and asymmetric (monolayer/bilayer) twisted heterostructures, the switch between volatile and non-volatile magnetic memory at zero-field and the appearance of abrupt magnetic reversal processes and hysteresis are controlled on demand.

Abstract

Twisting 2D van der Waals magnets allows the formation and control of different spin-textures, as skyrmions or magnetic domains. Beyond the rotation angle, different spin reversal processes can be engineered by increasing the number of magnetic layers forming the twisted van der Waals heterostructure. Here, pristine monolayers and bilayers of the A-type antiferromagnet CrSBr are considered as building blocks. By rotating 90 degrees these units, symmetric (monolayer/monolayer and bilayer/bilayer) and asymmetric (monolayer/bilayer) heterostructures are fabricated. The magneto-transport properties reveal the appearance of magnetic hysteresis, which is highly dependent upon the magnitude and direction of the applied magnetic field and is determined not only by the twist-angle but also by the number of layers forming the stack. This high tunability allows switching between volatile and non-volatile magnetic memory at zero-field and controlling the appearance of abrupt magnetic reversal processes at either negative or positive field values on demand. The phenomenology is rationalized based on the different spin-switching processes occurring in the layers, as supported by micromagnetic simulations. The results highlight the combination between twist-angle and number of layers as key elements for engineering spin-switching reversals in twisted magnets, of interest toward the miniaturization of spintronic devices and realizing novel spin textures.

2D nonlayered CuInSe₂ (CIS) nanoflakes with well-defined facet exposure and ordered Cu vacancies within the noncentrosymmetric lattice structure are synthesized via a molecular sieve-assisted chemical vapor deposition process. The exceptional optoelectronic properties of the ionic-electronic coupled CIS materials, combined with the intrinsic Cu vacancies, enable outstanding broadband photodetection performance.

Abstract

The properties and device applications of 2D semiconductors are highly sensitive to intrinsic structural defects due to their ultrathin nature. CuInSe₂ (CIS) materials own excellent optoelectronic properties and ordered copper vacancies, making them widely applicable in photovoltaic and photodetection fields. However, the synthesis of 2D CIS nanoflakes remains challenging due to the nonlayered structure, multielement composition, and the competitive growth of various by-products, which further hinders the exploration of vacancy-related optoelectronic devices. Here, 2D CIS nanoflakes are successfully synthesized using a molecular sieve-assisted chemical vapor deposition process. The anisotropic van der Waals growth with well-defined exposed facets is closely associated with the presence of the molecular sieve. Electron microscopy techniques reveal the ordered copper vacancies within the as-grown 2D crystals, extending the optical absorption to the near-infrared region. Consequently, 2D CIS nanoflake-based photodetectors exhibit broadband photodetection capabilities (470 to 1550 nm) and exceptional performance, such as a high responsivity of 11 A W⁻¹, an external quantum efficiency (EQE) of 2143%, and a fast response speed of ≈46.5 ms under an incident wavelength of 637 nm, highlighting their promising potential in next-generation optoelectronics.

Highlights

• Methods for creating the local deformation in two-dimensional transition metal dichalcogenides (2D TMDCs) are introduced.

• Modulations of local strain on their optical properties and excitonic behaviors are discussed.

• Quantum emitters based on strained 2D TMDCs and other applications are presented.