Xingxing Zhang

Here, recent advances in exfoliation and modification of layered two‐dimensional (2D) materials in various supercritical fluids (SCFs) are highlighted. The motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed.

Abstract

Since the first intercalation of layered silicates by using supercritical CO₂ as a processing medium, considerable efforts have been dedicated to intercalating and exfoliating layered two‐dimensional (2D) materials in various supercritical fluids (SCFs) to yield single‐ and few‐layer nanosheets. Here, recent work in this area is highlighted. Motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed. Further, advances in formation and application of 2D material–based composites with assistance from SCFs are presented. These discussions address chemical transformations accompanying SCF processing such as doping, covalent surface modification, and heterostructure formation. Promising features, challenges, and routes to expanding SCF processing techniques are described.

Technologies to enhance the stability of quantum dots (QDs), quantum dot films, and quantum dot light‐emitting diodes for display applications are summarized and suggested. Degradation mechanisms of QDs are discussed in aspects of water, oxygen, and thermal energy. Various technologies to maintain the quantum yield of QDs, the photoluminescence intensity of QD films, and the lifetime of quantum dot light‐emitting diodes are discussed.

Abstract

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color‐conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light‐emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.

The latest research advances in the chemical vapor deposition (CVD) synthesis of 2D transition metal dichalcogenides and related heterostructures/superlattices are comprehensively summarized. The controlled growth behavior, preparation strategies, and breakthroughs regarding their synthesis are also discussed. Finally, recent progress on the application of CVD‐grown 2D materials is presented with emphasis on the future prospects of these materials.

Abstract

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD‐grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.

2D MOFs often exhibit higher catalytic activity compared to their respective 3D solids due to the absence of diffusion limitation to reach active sites, easy accessibility to active sites, and high population of active sites. The use of these 2D MOFs as catalysts, electrocatalysts, and photocatalysts is reviewed, illustrating their advantages compared to the analogous 3D MOFs.

Abstract

Metal–organic frameworks (MOFs) are composed of particles with 3D geometry and are currently among the most widely studied heterogeneous catalysts. To further increase their activity, one of the recent trends is to develop related 2D materials with a high aspect ratio derived from a large lateral size and a small thickness. Here, the use of these 2D MOFs as catalysts, electrocatalysts, and photocatalysts is summarized, illustrating the advantages of these 2D materials compared to analogous 3D MOFs. The state of the art is summarized in tables and, when possible, pertinent turnover number (TON) and frequency (TOF) values. This enhanced activity of 2D MOFs derives from the accessibility of the active sites, the presence of a higher density of defects, and exchangeable coordination positions around the MOFs, as well as from their ability to form thin films on electrodes or surfaces. The importance of providing convincing evidence of the stability of 2D MOFs under reaction conditions and general characterization data of the used 2D material after catalysis is highlighted. In the last part, views regarding challenges in the field and new developments that can be expected are presented.

Controllable synthesis of graphene with low-cost, simple procedure, and outstanding reliability, is the foundation for basic researches and practical applications. Chemical vapor deposition (CVD) is a controllable, scalable, and promising way for graphene industry, but suffers from high resource consumption and limited productivity. The design of carbon sources is critical as it is strongly bound up with the growth strategy and features of graphene, contributing to optimize the cost, conditions, and efficiency of current technologies. Of late years, carbonaceous feedstocks, ranging from widely used methane to tailored monomers, have been used to grow graphene with a range of different structures and properties, triggering great progresses on the synthesis of nanoribbons, heteroatom-doped graphene (HG), and transfer-free graphene. Here, the diverse precursors with various features are systematically summarized by presenting corresponding advances and strategies. The growth mechan...

The growth of WSe (2− x ) Te x alloys by molecular beam epitaxy has been demonstrated for the first time to investigate the phase transition from the semiconducting 2H phase to the semi-metallic 1T′ phase as a function of Te concentration. Up to 14% Te incorporation, stable alloys in the semiconducting 2H phase are achieved while above 79% Te incorporation, stable alloys in the semi-metallic 1T′ phase are obtained. Our results indicate the MBE-grown WSe (2− x ) Te x alloys exhibit a miscibility gap from 14% to 79% Te concentrations at a growth temperature of 250 °C, a temperature compatible with direct vertical back-end-of-line integration. This miscibility gap results in phase separation of two different alloys, both with different composition and crystal structure. While the alloying of small Te concentrations does indeed result in a desired reduction of the semiconducting bandgap, the phase separation above 14%...

Robust strain gauges are fabricated by growing PtSe 2 layers directly on top of flexible polyimide foils. These PtSe 2 layers are grown by low-temperature, thermally-assisted conversion of predeposited Pt layers. Under applied flexure the PtSe 2 layers show a decrease in electrical resistance signifying a negative gauge factor. The influence of the growth temperature and film thickness on the electromechanical properties of the PtSe 2 layers is investigated. The best-performing strain gauges fabricated have a superior gauge factor to that of commercial metal-based strain gauges. Notably, the strain gauges offer good cyclability and are very robust, surviving repeated peel tests and immersion in water. Furthermore, preliminary results indicate that the stain gauges also show potential for high-frequency operation. This host of advantageous properties, combined with the possibility of further optimization and channel patterning, indicate tha...

The exceptional properties of graphene have inspired widespread efforts to integrate this two-dimensional (2D) material in functional applications in recent years. Within the broad spectrum of graphene processing frameworks, low-cost methods such as scalable, liquid-phase patterning are typically considered among the most straightforward routes for device integration. These ink-based printing methods require parallel research in graphene dispersion engineering, print process optimization, and post-processing methods to enhance target functional properties for a wide range of device applications in flexible electronics, energy storage, display technologies, and sensing. This review examines the current state-of-the-art and future prospects for integrating graphene and graphene composites with these versatile printing techniques to accelerate the development of scalable and low-cost graphene-based devices.

Lattice defects (mainly chalcogen vacancies) drastically affect the optoelectronic properties of monolayer transition metal dichalcogenides (TMDs) grown by chemical vapor deposition (CVD). They can be passivated through charge-transfer doping by laser irradiation in air. Here we perform a systematic in situ study to elucidate the passivation mechanism upon laser irradiation and show a way to controllably n-dope CVD-grown monolayer MoS 2 on SiO 2 substrates. By combining resonance Raman and photoluminescence (PL) spectroscopy we show that an increase in defect density correlates with a redshifted PL emission and hence an increase in electron density. Density functional theory (DFT) calculations identify chalcogen vacancies to be facilitators (not the source) of n-doping, and population of mid-gap levels upon doping lowers the activation barrier for O 2 adsorption from 0.3 to 0.03 eV. Laser irradiation aids in the oxygen-passivation of chalcoge...

Combining kinetic, thermodynamic, and microscopic measurements of crystallization kinetics in the classic phase‐change material Ge₂Sb₂Te₅, it is demonstrated that Ge₂Sb₂Te₅ crystallizes from the glassy phase at heating rates up to 10 000 K s⁻¹ and from the undercooled liquid at higher rates. Due to the concurrence of emerging glass transition and crystallization, the activation energy of crystallization drops by more than fourfold.

Abstract

Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 T _m to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge₂Sb₂Te₅ with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature T _g. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C below T _g, where the atomic mobility should be vanishingly small.

The latest research advances in the chemical vapor deposition (CVD) synthesis of 2D transition metal dichalcogenides and related heterostructures/superlattices are comprehensively summarized. The controlled growth behavior, preparation strategies, and breakthroughs regarding their synthesis are also discussed. Finally, recent progress on the application of CVD‐grown 2D materials is presented with emphasis on the future prospects of these materials.

Abstract

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD‐grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.

Nature Nanotechnology, Published online: 10 June 2019; doi:10.1038/s41565-019-0466-2

Strain-induced phase change in MoTe2 enables reversible channel conductivity switching in a field-effect transistor geometry.

We carried out point contact (PC) investigation of WTe 2 single crystals. We measured Yanson PC spectra ( d 2 V / dI 2 ) of the electron–phonon interaction (EPI) in WTe 2 . The PC spectra demonstrate a main phonon peak around 8 meV and a shallow second maximum near 16 meV. Their position is in line with the calculation of the EPI spectra of WTe 2 in the literature, albeit phonons with higher energy are not resolved in our PC spectra. An additional contribution to the spectra is present above the phonon energy, what may be connected with the peculiar electronic band structure and need to be clarified. We detected tiny superconducting features in d 2 V / dI 2 close to zero bias, which broadens by increasing temperature and blurs above 6 K. Thus, (surface) superconductivity may exist in WTe 2 with a topologically nontrivial state. We found a broad maximum in dV

Two-dimensional (2D) layered materials have attracted intensive interests in the past decade. Their unique electronic, magnetic, optical, and mechanical properties render broad applications in various fields. Stability of these ultrathin materials has to be delicately considered because their structures and properties are subject to ambient conditions. In this work, we review the chemical and structural stabilities of versatile 2D layered materials, and summarize the ways to modify the materials for the enhancement of their stabilities. Our review not only provides deep understandings of the stability of 2D materials, but may inspire new ideas to improve the reliability and durability of related devices.

Nature Nanotechnology, Published online: 17 June 2019; doi:10.1038/s41565-019-0467-1

Magneto-transport measurements on thin metallic crystals of the transition metal dichalcogenide PtSe2 show signatures of ferro- and antiferromagnetic order depending on the number of layers and first-principles calculations suggest Pt vacancies at the surface as a plausible cause.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have very versatile chemical, electrical and optical properties. In particular, they exhibit rich and highly tunable electronic properties, with a bandgap that spans from semi-metallic up to 2 eV depending on the crystal phase, material composition, number of layers and even external stimulus. This paper provides an overview of the electronic devices and circuits based on 2D TMDs, such as Esaki diodes, resonant tunneling diodes (RTDs), logic and RF transistors, tunneling field-effect transistors (TFETs), static random access memories (SRAMs), dynamic RAM (DRAMs), flash memory, ferroelectric memories, resistitive memories and phase-change memories. We address the basic device principles, the advantages and limitations of these 2D electronic devices, and our perspectives on future developments.

A topological layer convertor and near‐unity interlayer transmission are demonstrated. The structure is based on interplays of conventional and layer‐polarized valley‐Hall phases, two topologically distinct photonic valley‐Hall phases emerging from different breakings of mirror symmetries. Their interplays also lead to robust layer‐selected delay lines. They may serve as a paradigm in compact electromagnetic and photonic devices.

Abstract

Valley‐Hall phases, first proposed in 2D materials, originate from nontrivial topologies around valleys which denote local extrema in momentum space. Since they have been extended into classical systems, their designs draw inspirations from existing quantum counterparts, and their transports show similar topological protections. In contrast, it has been recently established in acoustics that layer pseudospins in valley‐Hall phases can give rise to special valley‐Hall edge states with fundamentally different transport behaviors at the interfaces compared with various 2D materials. Their realization in other classical systems, such as photonics, would allow to design topological insulators beyond quantum inspirations. Here, it is shown that layer pseudospins exist in photonic valley‐Hall phases, using vertically coupled designer surface plasmon crystals, a nonradiative system in open environment supporting tightly confined propagating modes. The negligible thermal and radiative losses in the structure pave the way for the direct observations of the layer pseudospins and associated topological phenomena stem from them in both real and reciprocal spaces. Photonic devices that manipulate the signals based on the layer pseudospins of the topological phases, such as layer convertors and layer‐selected delay lines, are experimentally demonstrated, confirming the potential applications of the layer pseudospins as a new degree of freedom carrying information.

2D materials generally exhibit less than ideal absorption characteristics due to their atomically thin thickness. An absorption enhancement is essential to achieve high power conversion efficiency in building 2D‐material‐based optoelectronic devices. Various designs, such as distributed Bragg reflector microcavities, metallic reflectors, photonic crystal nanocavities, and plasmonic nanostructures, provide promising approaches for the enhancement of light absorption in 2D materials.

Abstract

2D materials are promising but remain to be further explored, with respect to their usage in various optoelectronic devices. Generally, 2D materials exhibit far less than ideal absorption due to their thickness, limiting their deployment in practical optoelectronic applications. To address this challenge, extensive research has been performed utilizing different designs, such as distributed Bragg reflector microcavities, metallic reflectors, photonic crystal nanocavities, and plasmonic nanostructures, to confine light within 2D materials and increase light absorption. Recent progresses in enhancing light absorption in graphene and other 2D materials such as transition metal dichalcogenides and phosphorene are reviewed. Some physical mechanisms that realize enhanced absorption in 2D materials, as well as their potential applications are also discussed.