Xingxing Zhang

One‐dimensional defect line patterns are formed in monolayer VSe₂ upon high‐temperature annealing. X‐ray magnetic circular dichroism and magnetic force microscopy confirm that such Se‐deficient reconstructed defects cause the onset of ferromagnetism in pristine VSe₂. This study provides a way to engineer the magnetism via controlling the atomic structures of surface defects in 2D crystals.

Abstract

There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe₂. Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe₂) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q‐plus atomic force microscopy techniques, the 1D line features are determined to be 8‐member‐ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe₂ monolayer with Se‐deficient line defects displays room‐temperature ferromagnetism under X‐ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe₂, and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications.

In article number https://doi.org/10.1002/adma.2019062381906238, Kai Xiao and co‐workers develop a novel chemical vapor deposition method to successfully synthesize a pentagonal 2D material, PdSe₂, which exhibits strong optical anisotropy and high carrier mobility. The growth of high‐quality, anisotropic 2D crystals is essential for the rapid exploration and development of electronic devices with unique optoelectronic properties.

2D metal‐organic frameworks (MOFs) show great advantages for optical applications, such as multiple light‐emitting sites, wide emission wavelength ranges, and facile functional modifications, which are of great significance for the future development of advanced chemical sensors, photonics and optoelectronic devices. The strategies for the design and construction of 2D MOFs and their exfoliating methods are reviewed and applications are highlighted.

Abstract

2D metal‐organic frameworks (MOFs) have attracted broad research interest in recent years owing to their unique dimension‐related properties for widespread applications in catalysis, energy storage, conductivity, and optoelectronic devices. In this review, first the strategies for the rational design and precise construction of 2D MOFs are introduced. Then, the synthesis of 2D MOFs and their nanosheets by using top‐down and bottom‐up methods are summarized. Subsequently, the recent advances in optical/photonic applications of these 2D MOFs are highlighted, with special focus on lighting and display devices, nonlinear optics, as well as the luminescent sensing and biomedicine applications. Finally, the future potentials and challenges for the construction of 2D MOFs for optical materials are outlooked.

The layered franckeite, one of the natural van der Waals heterostructures composed from individual 2D crystals, is demonstrated to exhibit large Kerr nonlinearity, broadband nonlinear absorption, and ultrafast optical response. In addition, the all‐optical modulator and switcher function are achieved based on the strong light–matter interaction in the layered franckeite.

Abstract

Van der Waals heterostructures (vdWH) composed from individual 2D crystals offer a platform to obtain unprecedented functionalities that are not accessible in other heterostructures. The research to date has been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here, an experimental study of the broadband nonlinear optical performance of a naturally occurring vdWH franckeite is presented, which is composed of alternating SnS₂‐like and PbS‐like layers stacked on top of each other. Few‐layer franckeite is prepared via liquid‐phase exfoliation method and its broadband ultrafast third‐order nonlinear optical behavior is characterized experimentally. It is found that the layered franckeite exhibits broadband nonlinear optical response and the ultrafast carrier dynamics with the intraband carrier recovery time of ≈16 ps and the interband carrier recovery time of ≈300 ps. With the large optical nonlinearity of layered franckeite, the all‐optical modulator and switcher have been demonstrated based on the spatial phase modulation effect. The experimental results provide a fundamental understanding of the ultrafast nonlinear optical response in a complex naturally occurring vdWH, and may pave an avenue toward developing novel broadband optoelectronic devices.

Recent progress on 2D superlattices prepared by solution‐phase strategies using diverse genuine unilamellar nanosheets as building blocks is summarized. A flocculation strategy is considered an efficient method for large‐scale synthesis of these 2D superlattices. High‐performance devices enabled by these 2D superlattices for energy storage and conversion are presented together with a discussion on challenges and perspectives.

Abstract

2D genuine unilamellar nanosheets, that are, the elementary building blocks of their layered parent crystals, have gained increasing attention, owing to their unique physical and chemical properties, and 2D features. In parallel with the great efforts to isolate these atomic‐thin crystals, a unique strategy to integrate them into 2D vertically stacked heterostuctures has enabled many functional applications. In particular, such 2D heterostructures have recently exhibited numerous exciting electrochemical performances for energy storage and conversion, especially the molecular‐scale heteroassembled superlattices using diverse 2D unilamellar nanosheets as building blocks. Herein, the research progress in scalable synthesis of 2D superlattices with an emphasis on a facile solution‐phase flocculation method is summarized. A particular focus is brought to the advantages of these 2D superlattices in applications of supercapacitors, rechargeable batteries, and water‐splitting catalysis. The challenges and perspectives on this promising field are also outlined.

Mixed‐dimensional van der Waals heterostructures (VDWHs) are fabricated based on 1D graphene nanoribbons onto 2D MoS₂, showing a significantly suppressed persistent photoconductivity effect of MoS₂. Photomodulation of the charge transport of the obtained VDWHs field‐effect transistor is realized by interfacing with photochromic molecules, demonstrating its great potential for multilevel memories, which are promising for future development of ultrathin multifunctional optoelectronics.

Abstract

Van der Waals heterostructures (VDWHs), obtained via the controlled assembly of 2D atomically thin crystals, exhibit unique physicochemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom‐up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS₂ are fabricated. Field‐effect transistors (FETs) based on such VDWHs show an efficient suppression of the persistent photoconductivity typical of MoS₂, resulting from the interfacial charge transfer process. The MoS₂‐GNR FETs exhibit drastically reduced hysteresis and more stable behavior in the transfer characteristics, which is a prerequisite for the further photomodulation of charge transport behavior within the MoS₂‐GNR VDWHs. The physisorption of photochromic molecules onto the MoS₂‐GNR VDWHs enables reversible light‐driven control over charge transport. In particular, the drain current of the MoS₂‐GNR FET can be photomodulated by 52%, without displaying significant fatigue over at least 10 cycles. Moreover, four distinguishable output current levels can be achieved, demonstrating the great potential of MoS₂‐GNR VDWHs for multilevel memory devices.

Negatively polarized BaTiO₃ (BTO)@MoS₂, positively polarized BTO@MoS₂, and a hydrogel serve as the cathode, anode, and electrolyte for a humidity digester. The hydrogel harvests humidity from humid air and transfers the collected water to surface of the electrodes. Under the well‐tuned polarization, the vector of the built‐in electric field in the ferroelectric can be controlled, and therefore drive enhanced atmospheric humidity splitting for dehumidification and fuel production.

Abstract

Unlike traditional water splitting in an aqueous medium, direct decomposition of atmospheric water is a promising way to simultaneously dehumidify the living space and generate power. Here, a tailored superhygroscopic hydrogel, a catalyst, and a solar cell are integrated into a humidity digester that can break down ambient moisture into hydrogen and oxygen, creating an efficient electrochemical cell. The function of the hydrogel is to harvest moisture from ambient humidity and transfer the collected water to the catalyst. Barium titanate and vertical 2D MoS₂ nanosheets are integrated as the catalyst: the negatively polarized cathode can enhance the electron transport and attract H⁺ to the MoS₂ surface for water reduction, while water oxidation takes place at the positively polarized anode. By employing this mechanism, it is possible to maintain the relative humidity in a medium‐sized room at <60% without any additional energy input, and a stable current of 12.5 mA cm⁻² is generated by the humidity digester when exposed to ambient light.

The effects of material thickness on optical nonlinearity are studied as an important subject recently. Here, thickness dependent nonlinear absorption properties of 2D materials in fiber lasers are presented. Those thickness‐dependent photonic devices are successfully applied in fiber lasers to achieve Q‐switched and mode‐locked operation. Experiments prove that fiber lasers based on those devices have excellent performance in ultrafast optics.

Abstract

The explosive success of graphene opens a new era of ultrathin 2D materials. It has been realized that the van der Waals layered materials with atomic and less atomic thickness can not only exist stably, but also exhibit unique and technically useful properties including small size effect, surface effect, macro quantum tunnel effect, and quantum effect. With the extensive research and revealing of the basic optical properties and new photophysical properties of 2D materials, a series of potential applications in optical devices have been continuously demonstrated and realized, which immediately roused an upsurge of study in the academic circle. Therefore, the application of 2D materials as broadband, efficient, convenient, and versatile saturable absorbers in ultrafast lasers is a potential and promising field. Herein, the main preparation methods of 2D materials are reviewed and technical guidelines for identifying and characterizing layered 2D materials are provided. After investigating the characteristics of 2D materials thoroughly in nonlinear optics, their performances in fiber lasers are comprehensively summarized according to the types of materials. Finally, some developmental challenges, potential prospects, and future research directions are summarized and presented for such promising materials.

A method to precisely repair macroscopic damage to few‐layer 2H‐MoTe₂ films at the atomic scale is demonstrated, and its mechanism is elucidated. The repaired 2H‐MoTe₂ inherits the lattice orientation of the adjacent original 2H‐MoTe₂ and seamlessly contacts to it, thereby forming an atomically perfect lattice. In addition the repaired MoTe₂ shows the same electrical quality as the original one.

Abstract

2D semiconductors have emerged as promising candidates for post‐silicon nanoelectronics, owing to their unique properties and atomic thickness. However, in the handling of 2D material, various forms of macroscopic damage, such as cracks, wrinkles, and scratches, etc., are usually introduced, which cause adverse effects on the material properties and device performance. Repairing such macroscopic damage is crucial for improving device performance and reliability, especially for large‐scale 2D device arrays. Here, a method is demonstrated repair damage to few‐layer 2H‐MoTe₂ films with atomic precision, and its mechanism is elucidated. The repaired 2H‐MoTe₂ inherits the lattice orientation of the adjacent original 2H‐MoTe₂, thereby forming an atomically perfect lattice at the repaired interface. The time‐evolution experiments show that the interface between the 2H‐ and early formed 1T'‐MoTe₂ plays an important role in the subsequent phase transition and recrystallization. Electrical measurements on the original MoTe₂, repaired MoTe₂, and cross‐interface regions show unobservable differences, indicating that the repaired MoTe₂ has the same electrical quality as the original one and the interface does not introduce extra scattering centers for carrier transport. The findings provide an effective strategy for macroscopic damage repair of few‐layer 2H‐MoTe₂, which paves the way for its practical application in advanced electronics and optoelectronics.