Jiuxiang Dai

A comprehensive structure-property study revealing the disordered structural characteristics of widely-debated Bi-rich bismuth oxyhalide photocatalysts, through a combination of electron-, X-ray- and neutron scattering analyses. The link between these structural characteristics and the charge carrier dynamics and photocatalytic performance is considered via time-resolved spectroscopy and photocatalytic screenings.

Abstract

Advancing the field of photocatalysis requires the elucidation of structural properties that underpin the photocatalytic properties of promising materials. The focus of the present study is layered, Bi-rich bismuth oxyhalides, which are widely studied for photocatalytic applications yet poorly structurally understood, due to high levels of disorder, nano-sized domains, and the large number of structurally similar compounds. By connecting insights from multiple scattering techniques, utilizing electron-, X-ray- and neutron probes, the crystal phase of the synthesized materials is allocated as layered Bi₂₄O₃₁X₁₀ (X = Cl, Br), albeit with significant deviation from the reported 3D crystalline model. The materials comprise anisotropic platelet-shaped crystalline domains, exhibiting significant in-plane ordering in two dimensions but disorder and an ultra-thin morphology in the layer stacking direction. Increased synthesis pH tailored larger, more ordered crystalline domains, leading to longer excited state lifetimes determined via femtosecond transient absorption spectroscopy (fs-TAS). Although this likely contributes to improved photocatalytic properties, assessed via the photooxidation of benzylamine, increasing the overall surface area facilitated the most significant improvement in photocatalytic performance. This study, therefore, enabled both phase allocation and a nuanced discussion of the structure-property relationship for complicated, ultra-thin photocatalysts.

The article explores the significant impact of magnetic ordering in FePS₃ on WSe₂’s optical properties, revealing a giant linear polarization degree of 5.1 in exciton emission. It highlights a phase transition in FePS₃ that enhances interlayer interactions in FePS₃/WSe₂ heterostructures, enabling precise manipulation of anisotropic optical behaviors in transition-metal dichalcogenides (TMDs).

Abstract

Magnetic 2D materials offer a promising platform for manipulating quantum states at the nanoscale. Recent studies have underscored the significant influence of 2D magnetic materials on the optical behaviors of transition-metal dichalcogenides (TMDs), revealing phenomena such as interlayer exciton-magnon interactions, magnetization-dependent valley polarization, and an enhanced Zeeman effect. However, the controlled manipulation of anisotropic optical properties in TMDs via magnetism remains challenging. Here, the magnetic ordering in FePS₃ profoundly impacts the optical characteristics of WSe₂, achieving a giant linear polarization degree of 5.1 in exciton emission is demonstrated. This is supported by a detailed analysis of low-temperature photoluminescence (PL) and Raman spectra from nL-FePS₃/WSe₂ heterostructures. These findings indicate that a phase transition in FePS₃ from paramagnetic to antiferromagnetic enhances interlayer Coulomb interactions, inducing a transition from non-polar to polar behavior in the heterostructures. Additionally, valley-polarized PL spectra under magnetic fields from −9 to 9 T reveal the influence of FePS₃ on valley polarization and Zeeman splitting of excitons in monolayer WSe₂. These results present a novel strategy for tailoring the optoelectronic properties of 2D magnetic van der Waals heterostructures, paving the way for advancements in nanoscale device design.

This paper examines the crucial role of 2D material-based photodetectors in advancing optoelectronics from UV to THz spectra. It discusses optical interactions, device mechanisms, and fabrication, addressing challenges like material integration and performance optimization. The study underscores the need for novel synthesis techniques, comprehensive characterization, and improved heterostructure design to unlock their full commercial potential.

Abstract

Photodetectors are one of the most critical components for future optoelectronic systems and it undergoes significant advancements to meet the growing demands of diverse applications spanning the spectrum from ultraviolet (UV) to terahertz (THz). 2D materials are very attractive for photodetector applications because of their distinct optical and electrical properties. The atomic-thin structure, high carrier mobility, low van der Waals (vdWs) interaction between layers, relatively narrower bandgap engineered through engineering, and significant absorption coefficient significantly benefit the chip-scale production and integration of 2D materials-based photodetectors. The extremely sensitive detection at ambient temperature with ultra-fast capabilities is made possible with the adaptability of 2D materials. Here, the recent progress of photodetectors based on 2D materials, covering the spectrum from UV to THz is reported. In this report, the interaction of light with 2D materials is first deliberated on in terms of optical physics. Then, various mechanisms on which detectors work, important performance parameters, important and fruitful fabrication methods, fundamental optical properties of 2D materials, various types of 2D materials-based detectors, different strategies to improve performance, and important applications of photodetectors are discussed.

The In₂O₃ and In₂S₃ composite reduces desulfurization and deoxygenation under applied potential, stabilizing the high-valence state of indium, particularly in In₂O₃. This stable structure lowers the energy barrier of the reaction pathway by boosting CO₂ adsorption, thus facilitating formate production. Importantly, it maintains consistent formate generation at an industrial-level current density over the long term, demonstrating its practical feasibility.

Abstract

Electrocatalysts based on high-valent indium are promising for formate production via CO₂ electroreduction. However, reconstruction often occurs during the reaction progress, resulting in a decline in catalytic performance. Here, a composite of In₂O₃/In₂S₃ is developed, and its catalytic performance exceeds that of either individual phase, particularly in stability. Analysis of morphology, valence state, and in situ Raman spectroscopy reveals that In₂O₃ is well preserved during the reaction. Theoretical calculations suggest that the desorption energy of lattice oxygen on In₂O₃ can be strengthened due to In₂O₃-In₂S₃ bonding within the composite. This reinforcement facilitates the formation of more active sites and promotes CO₂ adsorption, further decreasing the energy barrier of formate production to only 0.12 eV. As a result, the composite exhibits a formate selectivity over 95.05% at –1.13 V vs reversible hydrogen electrode accompanied by a partial current density of 434.4 mA cm^–2. Notably, the selectivity of formate maintains over 95% even after 50 h at an industrial-level current density of 200 mA cm^–2, 17 times longer than the individual phase. Furthermore, 18.33% solar-to-formate and 19.49% solar-to-fuel are obtained when coupled with III-V solar cells, demonstrating its feasibility.

Abstract

Moiré superlattices based on twisted transition metal dichalcogenide (TMD) heterostructures have recently emerged as a promising platform for probing novel and distinctive electronic phenomena in two-dimensional (2D) materials. By stacking TMD monolayers with a small twist angle, these superlattices create a periodic modulation of the electronic density of states, leading to the formation of mini bands. These mini bands can exhibit intriguing properties such as flat bands, correlated electron behavior, and unconventional superconductivity. This review provides a comprehensive overview of recent progress in Moiré superlattices formed from twisted TMD heterostructures. It covers the theoretical principles and experimental techniques for creating and studying these superlattices, and explores their potential applications in optoelectronics, quantum computing, and energy harvesting. The review also addresses key challenges, such as improving the scalability and reproducibility of the fabrication process, emphasizing the exciting opportunities and ongoing hurdles in this rapidly evolving field.

A classic material incorporates three distinct magnetic transitions: domain wall pinning, spin reorientation, and unstable rare-earth magnetic moments, allowing for the simultaneous appearance of two inverse MCEs and one direct MCE within the same compound. Revealing the MCE mechanism at different scales, including macroscopic magnetism, micrometer-scale magnetic domains, atomic magnetic moments, and electronic structure, is of great significance for the study of magnetic refrigeration.

Abstract

Advancements and utilization of magnetic refrigeration technology hinge on the ongoing enhancement and optimization of magnetic refrigeration material properties. Nevertheless, the intricacy of the magnetocaloric effect (MCE) mechanism has emerged as a bottleneck, constraining the progress and refinement of magnetic refrigeration materials. In this study, a classic magnetic system is chosen to investigate the mechanism of MCE across four different scales–macroscopic magnetism, micrometer-scale magnetic domains, atomic magnetic moments, and electronic structure. It simultaneously exhibits two inverse MCEs and one direct MCE, with a working temperature span as wide as 125 K (most are <50 K) for the direct MCE. The measurements of the vibrating sample magnetometer, in situ Lorentz electron microscopy and variable-temperature neutron powder diffraction directly reveal that the complex magnetic entropy changes arise from the magnetic domain wall pinning, the instability of Ho magnetic moments, and the spin rotation. First-principles calculations elucidate the crucial role of strong hybridization between localized Ho and itinerant Co electrons in the spin reorientation of HoCo₄Al. This study contributes significantly to comprehending the induction mechanism of the MCE and holds vital reference value for exploring new magnetic refrigeration materials and enhancing MCE performance.

This review presents the status and future development of the perovskite devices. The fundamental properties of perovskites related to their effective device applications are summarized. The perovskite solar cell as a robust candidate for next-generation solar energy harvesting are considered. The selected perovskite photodetector structures, including perovskite quantum dot photodetectors are discussed. Their performance is compared with the commercial photodetectors.

Abstract

The perovskite materials are broadly incorporated into optoelectronic devices due to a number of advantages. Their rapid technological progress is related to the relatively simple fabrication process, low production cost and high efficiency. Significant improvement is made in the light emitting, detection performance and device design especially operating in the visible and near-infrared regions. This review presents the status and possible future development of the perovskite devices such as solar cells, photodetectors, and light-emitting diodes. The fundamental properties of perovskite materials related to their effective device applications are summarized. Since the development of the perovskite technology is mainly driven by the revolutionary evolution of the semiconductor perovskite solar cell as a robust candidate for next-generation solar energy harvesting, this topic is considered first. The device engineering of various perovskite photodetector structures, including perovskite quantum dot photodetectors, is then discussed in detail. Their performance is compared with the current commercial photodetectors available on the global market together with their challenges. Finally, the considerable progress in the fabrication of the perovskite light-emitting diodes with external quantum efficiency exceeding 20% is presented. The paper is completed in an attempt to determine the development of perovskite optoelectronic devices in the future.

Nature Synthesis, Published online: 06 September 2024; doi:10.1038/s44160-024-00643-0

Synthesis of metastable materials away from thermodynamic equilibrium has been a challenge in materials chemistry, but thin-film methods often struggle to yield ground-state structures. Now, a synthesis pathway to thin films of stable layered ternary nitrides is revealed, and the tendency for metastable intermediate formation is discussed.

Light: Science & Applications, Published online: 05 September 2024; doi:10.1038/s41377-024-01576-1

Ultra-low loss silicon nitride becomes even cooler

Nature Communications, Published online: 06 September 2024; doi:10.1038/s41467-024-52158-z

Artificial photocatalytic systems based on graphitic carbon nitride are improving, but production of high-value products at electron and hole sites is challenging. Here, the authors report a system using graphite-phase carbon nitride modified with potassium iodide/triiodide eutectic salt to generate H2O2 and benzaldehyde.

The study introduces a van der Waals (vdW) metal mask for 2D material engineering, employing a probe tip-assisted transfer method. This approach enables residue-free, all-solid treatments without the need for lithography, ensuring ultra-clean interfaces and robust performance under various conditions. This technique enhances device fabrication and research in 2D materials with sub-1 µm resolution using an optimal Ag/Au film, offering a cost-effective approach.

Abstract

The van der Waals (vdW) contact, characterized by its bondless interactions, opens up exciting possibilities in cutting-edge mask technology. It enables incredibly close proximity to samples at the atomic level while facilitating non-destructive engineering. In this study, the concept of a vdW metal mask using the template striped ultra-flat Ag/Au film is introduced. The probe tip-assisted metal film transfer under an optical microscope is employed to showcase all-solid and residue-free engineering on 2D materials. The robust nature of the vdW metal mask allows for various treatments, including gas, liquid, solid, plasma, and light, making it a universal tool for fabricating 2D material-based devices and samples with sub-1 µm resolution, all without the need for lithography technologies. With the superiority in simple sample fabrication, ultra-clean surfaces, and robustness under harsh conditions, the technique is believed to flourish in the 2D material research field.

This review discusses the formation of atomic defects and their effects on the properties of 2D transition metal dichalcogenides (TMDCs). Then, techniques for characterizing atomic defects are systematically summarized. Further, recent progress on defect suppression during growth and defect repair after growth is reviewed. The challenges and opportunities in this important field are also presented.

Abstract

Preparing high-quality 2D semiconductors represented by transition metal dichalcogenides (TMDCs) is of great importance for the next-generation devices. However, currently available 2D TMDCs contain many atomic defects, which greatly affect their electronic and optical properties. This review starts with the formation of atomic defects and their effects on the properties of 2D TMDCs. Then, techniques for characterizing atomic defects are systematically summarized, including atomic-resolution imaging and spectroscopy measurements. Further, recent progress on defect suppression during growth and defect repair after growth is reviewed. Finally, challenges and opportunities in this important field are discussed.

A new kagome-lattice semimetal Co₃In₂S₂ single crystal is successfully synthesized via a polycrystal-source CVT approach, possessing a high carrier mobility reaching 10⁴ cm²V⁻¹s⁻¹ and a canted antiferromagnetic order below 5 K. Intriguingly, the Co₃In₂S₂ single crystal exhibits a field-induced butterfly-like anisotropic magnetoresistance (AMR) with a superposition of the two-fold, four-fold and six-fold AMR signals.

Abstract

With the interplay between magnetism and topological bands, magnetic kagome semimetals provide promising platforms for exploring exotic correlated electronic states and quantum phenomena such as anomalous Hall effect, quantum spin liquid, and unconventional magnetoresistance, as well as driving advances in electronic and spintronic applications. Here, a field-induced butterfly-like anomalous anisotropic magnetoresistance (AMR) effect in an intriguing kagome semimetal Co₃In₂S₂ is reported. The kagome-lattice Co₃In₂S₂ single crystals are synthesized via a polycrystal-source chemical vapor transport approach, possessing a high carrier mobility reaching 10⁴ cm²V⁻¹s⁻¹. The Co₃In₂S₂ single crystal exhibits a canted antiferromagnetic state below 5 K, but intriguingly, it is easily transformed into a ferromagnetic state under a small external magnetic field. Furthermore, the planar Hall effect (PHE) is detected, stemming from the complex contribution of field-induced ferromagnetism and orbital magnetoresistance. Remarkably, as the magnetic field increases, the low-temperature magnetoresistance behavior of the Co₃In₂S₂ reveals a butterfly-like AMR effect with a maximum value of 850%, exhibiting a superposition of the two-, four-, and six-fold AMR terms. Band structure calculations suggest that such a field-induced butterfly-like AMR effect may originate from the modulations of the electronic structure near the Fermi level by the magnetic moment. The findings offer a valuable platform for understanding the anomalous AMR effect and for the development of advanced spintronic devices.

A self-separation technique for generating thick GaN films on a 2D material substrate is demonstrated for the first time. The reusability of mica substrates by repeatedly growing thick GaN films on the same substrate is shown. Moreover, ultraviolet light-emitting diodes on self-separated thick GaN films, demonstrating the device quality of these thick GaN films are fabricated.

Abstract

Free-standing gallium nitride has been prepared using various methods; however, the removal of the original substrate is still challenging with low success rates. In this work, 2-inch free-standing GaN films are obtained by direct growth on a fluoro phlogopite mica by hydride vapor-phase epitaxy. Depending on the van der Waals (vdW) interaction between GaN and mica, the effect of the significant lattice mismatch is effectively reduced; thus, enabling the production of a high-quality wafer-scale GaN film on mica. The vdW-induced cracks at GaN–mica interface are found to be initiated near the interface so that GaN can easily separate from mica during rapid cooling. Owing to the hydrophilic nature of mica, the residual GaN on the mica can be lifted off by following deionized water treatment, and the mica substrate can be repeatedly used to grow free-standing GaN films. The self-separated GaN films grown on both pristine and used mica substrates are single crystallinity and strain-free. Additionally, a fully functional ultraviolet light-emitting diode is demonstrated to show that the self-separated GaN films are of device quality. The proposed approach achieves epitaxial growth of wafer-scale single-crystalline GaN on 2D materials and provides a new substrate option in the technology of III-V materials.

An anti-self-oxidation a concept of Eu²⁺ in Li₂SrSiO₄ substrate by adding trivalent rare-earth ions for highly efficient and stable orange–yellow light emission have been proposed. The designed phosphors present great potential in WLED and display fields.

Abstract

Li₂SrSiO₄:Eu²⁺ is a promising substitute for traditional Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) owing to its strong orange–yellow emission of 4f-5d transition originating from Eu²⁺ dopant, covering the more red-light region. However, its inevitable luminescence thermal quenching at high temperatures and the self-oxidation of Eu²⁺ strongly impede their applications. Their remediation remains highly challenging. Herein, an anti-self-oxidation(ASO) concept of Eu²⁺ in Li₂SrSiO₄ substrate by adding trivalent rare-earth ions (A³⁺: A = La, Gd, Y, Lu) for highly efficient and stable orange–yellow light emission have been proposed. A significantly increased orange–yellow emission (202% improvement) from Li₂Sr_0.95A_0.05SiO₄:Eu²⁺ with a wide range near-zero thermal quenching is obtained, superior to other Eu²⁺ activated phosphors. The presence of A³⁺ ions with various radii modifies the ASO degree of Eu²⁺ ions, achieving the tunable chemical state, composition, electronic configuration, crystal-field strength, and luminescent characteristics of the developed phosphors. For the proof of the concept, a W-LED device and a PDMS (Polydimethylsiloxane) luminescent film are fabricated, endowing excellent luminescence performance and thermal stability and the huge application prospects of Li₂SrSiO₄:Eu²⁺ in lighting and display fields.

The Te NSs@rGO is synthesized through a self-assembled process. Lithiophilic Te nanosheets can induce uniform Li nucleation and deposition, while the resulting product Li₂Te can reduce the energy barrier for anion decomposition and promote the generation of LiF in SEI. This work provides new insights for constructing LiF-rich SEI.

Abstract

The practical application of lithium-metal batteries (LMBs) remains impeded by uncontrollable Li dendrite growth and unstable solid-state electrolyte interphase (SEI) on lithium-metal anodes. Constructing the inorganic-rich SEI is considered as an effective strategy to realize the dense Li deposition and inhibit interfacial side reactions, thereby improving the lifespans of LMBs. Herein, an anion-reduction-catalysis mechanism is proposed to design a LiF-rich SEI utilizing 2D tellurium (Te) nanosheets as catalysts, which are homogenously implanted on the substrate. Lithiophilic Te nanosheets can induce uniform Li nucleation and deposition through in situ lithiation reactions, while the resulting product Li₂Te can reduce the energy barrier for anion decomposition and promote the generation of LiF in the SEI. Consequently, Li dendrite growth and interfacial side reactions are effectively suppressed, enabling long-cycle-life LMBs. The Te-modified electrode in half-cells delivers superior cycle life exceeding 500 cycles and a high average Coulombic efficiency of 97.8% at 5 mAh cm⁻². The high-energy-density (405 Wh kg⁻¹) pouch cells pairing the Te-modified Li anodes with high-mass-loading LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90) cathodes exhibit stable cycling performance with a high average Coulombic efficiency of 99.3% in carbonate electrolytes. This work provides a promising anion catalyst design for LiF-rich SEI and paves the way for developing high-energy-density LMBs.

Abstract

Chemical vapor deposition (CVD) has shown great promise for the large-scale production of high-quality graphene films for industrial applications. Atomic-scale theoretical studies can help experiments to deeply understand the graphene growth mechanism, and serve as theoretical guides for further experimental designs. Here, by using density functional theory calculations, ab-initio molecular dynamics simulations, and microkinetic analysis, we systematically investigated the kinetics of hydrogen constrained graphene growth on Cu substrate. The results reveal that the actual hydrogen-rich environment of CVD results in CH as the dominating carbon species and graphene H-terminated edges. CH participated island sp² nucleation avoids chain cyclization process, thereby improving the nucleation and preventing the formation of non-hexameric ring defects. The graphene growth is not simply C-atomic activity, rather, involves three main processes: CH species attachment at the growth edge, leading to a localized sp³ hybridized carbon at the connecting site; excess H transfer from the sp³ carbon to the newly attached CH; and finally dehydrogenation to achieve the sp² reconstruction of the newly grown edge. The threshold reaction barriers for the growth of graphene zigzag (ZZ) and armchair (AC) edges were calculated as 2.46 and 2.16 eV, respectively, thus the AC edge grows faster than the ZZ one. Our theory successfully explained why the circumference of a graphene island grown on Cu substrates is generally dominated by ZZ edges, which is a commonly observed phenomenon in experiments. In addition, the growth rate of graphene on Cu substrates is calculated and matches well with existing experimental observations.

Coherence tomography with extreme ultraviolet light (EUV) is applied to different van der Waals heterostructures, which enables a 3D sample reconstruction of encapsulated interfaces with nanoscopic axial resolution. This sets the basis for a new spectroscopy tool that, thanks to the temporal profile of laser-driven EUV sources, can become an ideal probe of ultrafast processes occurring in opto-electronic devices.

Abstract

New experimental methods with high out-of-plane spatial sensitivity combined with ultrafast temporal resolution can revolutionize the understanding of charge- and heat-transfer dynamics occurring at interfaces. In this work, a step forward is taken in this direction by applying coherence tomography with extreme ultraviolet (EUV) light to different van der Waals heterostructures, which enables a 3D sample reconstruction with nanoscopic axial resolution. Furthermore, the measurements and, more in general, the approach is confirmed by ab initio calculations of the refractive index of layered materials that we compare to existing databases of empirical data. The EUV coherence tomography contrast is estimated in a broad spectral range (photon energy 65 –100 eV). This work sets the basis for the development of a new spectroscopy tool that, thanks to the temporal profile of EUV light sources and the high axial resolution of coherence tomography, can become the ideal probe of ultrafast processes occurring in van der Waals heterostructures and buried nanoscale opto-electronic devices.

Nature Materials, Published online: 03 September 2024; doi:10.1038/s41563-024-01998-7

While transition metal nitrides are promising low-cost electrocatalysts for the oxygen reduction reaction in alkaline media, a fundamental understanding of their activity is still lacking. Here MnN nanocuboids with well-defined surface structures are investigated, providing atomistic insight and mechanistic understanding.

Nature Materials, Published online: 03 September 2024; doi:10.1038/s41563-024-01989-8

The practical application of 2D transition metal dichalcogenides (TMDs) requires robust and scalable synthesis of these atomically thin materials and their heterostructures. This Review discusses the key challenges, current progress and opportunities in the controllable synthesis of TMD-based heterostructures, superlattices and moiré superlattices.

This study investigates the adaptation of multifunctional behaviors within a single memristor to enhance its effectiveness for future applications. By utilizing the uniform analog gradual switching functions of the Ti/NbO_x/Pt memristor, it demonstrates the implementation of artificial synapses and nociceptors, emulation of synapse firing, synapse arrays, Pavlovian associative learning, and edge and reservoir computing within a single device.

Abstract

To achieve cost-effectiveness, researchers are exploring various memristors for their adaptation in neuromorphic computing. Recent studies have focused on developing versatile functioning singular memristors, such as those involved in on-receptor computing, which integrates sensory functions into current neuromorphic computing paradigms. Additionally, adaptations like reservoir computing are being investigated for computing systems. In this study, a memristor composed of a stack of Ti/NbO_x/Pt layers is fabricated to explore multifunctional behaviors within a single memristor. By applying bias toward the top Ti electrode, gradual current changes with volatile features are demonstrated, revealing an ion-migration-based nonfilamentary switching memristor. Leveraging this volatile functionality, an artificial nociceptor is first implemented, demonstrating key functions of biological nociceptors including thresholding, relaxation, no-adaptation, and sensitization. Subsequently, synapse emulation akin to the biological brain is achieved through easy conductance potentiation and depression with diverse synapse functions, enabling the memristor to mimic learning activities with spike firing. Lastly, computational applications are explored by adapting edge computing and multi-bit reservoir computing, expanding the memristor's applications across diverse fields with versatile behaviors.