Jiuxiang Dai

A single crystal of two-dimensional (2D) layered α-MoO₂ using the physical vapor deposition (PVD) method is generated. Since there is no change in composition before and after PVD, the obtained nanosheets exhibit low defects, high purity, and large crystal domains, and ultimately exhibit conductivity of up to 5 × 10⁶ S m⁻¹, which shows a promising application as 2D conductors.

A single crystal of 2D layered α-MoO₂ using a simple technique of physical vapor deposition (PVD) method is generated. By adjusting the growth temperature, the thickness of α-MoO₂ single crystals can be tuned to sub ten nanometers. High-resolution transmission electron microscopy (HRTEM) images verified the atomic structural details of PVD grown orthorhombic MoO₂ (α-MoO₂) nanosheet with high quality. Additionally, the unsaturated linear magnetic range (LMR) in the grown nanosheets is up to 5 T. Also, α-MoO₂ nanosheet with low defect displays excellent conductivity that is comparable to the metal silver. Thus, this new nanomaterial can show a promising application as a potential 2D conductor.

The evaporation of Au or the sputtering of Cr on 2D layered hexagonal boron nitride (h-BN) produces local atomic defects in its structure, specially at its interfaces, which increases the leakage current across it. It is found that the deposition of metal on hexagonal boron nitride (h-BN) using inkjet printing does not introduce any defect, and maintains its layered structure free of defects.

Abstract

2D materials have many outstanding properties that make them attractive for the fabrication of electronic devices, such as high conductivity, flexibility, and transparency. However, integrating 2D materials in commercial devices and circuits is challenging because their structure and properties can be damaged during the fabrication process. Recent studies have demonstrated that standard metal deposition techniques (like electron beam evaporation and sputtering) significantly damage the atomic structure of 2D materials. Here it is shown that the deposition of metal via inkjet printing technology does not produce any observable damage in the atomic structure of ultrathin 2D materials, and it can keep a sharp interface. These conclusions are supported by abundant data obtained via atomistic simulations, transmission electron microscopy, nanochemical metrology, and device characterization in a probe station. The results are important for the understanding of inkjet printing technology applied to 2D materials, and they could contribute to the better design and optimization of electronic devices and circuits.

Hydration chemistry of inorganic salts is utilized to decrease the attack of MXene by free water and oxygen molecules in aqueous solution. This approach can prolong the MXene shelf life to up to 400 days with negligible loss of surface chemistry and bulk carrier properties.

Abstract

MXenes attract interest in diverse fields but suffer from fast structural degradation by attacking of dissolved oxygen and water molecules in aqueous solution. This drawback hinders the long-term storage, applications and understanding of the chemical nature of MXenes. Herein, we report a cost-effective and environmentally sustainable way for long-term storage of MXenes in aqueous solution by hydration chemistry of nontoxic inorganic salts. The attacking of MXene by free water and dissolved oxygen molecules is inhibited by decreasing the water activity, which simultaneously lowers the dissolved oxygen concentration, of saline solution. As a result, the storage life of MXene can be prolonged to up to 400 days at ambient conditions without loss of intrinsic surface chemistry and bulk carrier properties. Over 90 % of salt protectant can be recycled by simply evaporating the final waste liquor after fully extracting the MXene to minimize the waste discharge and processing cost. This work offers a commercializable approach with high cost-effectiveness, processing sustainability and environmental benefit for extending the shelf life of MXenes.

Nature Communications, Published online: 01 November 2021; doi:10.1038/s41467-021-26589-x

Carbon nanotubes and graphene have unique structures and properties, specifically the unusual thermal, electronic, and photonic effects, which make them highly promising for high performance terahertz detectors. This review covers the major advances in terahertz detectors based on carbon nanotubes and graphene, mainly including the structure characteristic and fundamental principle, as well as the challenges and outlook.

Abstract

Various applications of terahertz (THz) radiation and the importance to fundamental science make the development of THz technology one of the key areas in modern applied physics. THz detectors are a key component of THz technology, whose performance determines the application market. One feasible approach to fabricate high performance THz detectors is to utilize carbon nanomaterials, particularly, carbon nanotubes (CNTs), and graphene. Their novel thermal, optical, and electronic properties make them promising in the field of THz technology. In this review, based on a brief introduction of the unique properties of graphene and CNTs, the fundamental principles of thermal, electronic, and photonic effects in carbon-based THz detection are thoroughly discussed, together with the detailed summary of major progresses in the past decades. Finally, the challenges and opportunity of such THz detectors are presented from three aspects of precise preparation of carbon nanomaterials, mechanism of THz detectors and device technology.

To addressing the challenge of low radiation efficiency of current universal terahertz sources, a van der Waals ferromagnetic semiconductor-based efficient monochromatic terahertz radiation is developed here. It takes advantage of the bosonic quasi-particles, phonon-polaritons, that be coupled by exciting terahertz photons and inherent inter-layered phonons in Cr₂Ge₂Te₆. The radiation could also be effectively modulated by external magnetic fields.

Abstract

Searching multiple types of terahertz (THz) irradiation source is crucial for the THz technology. In addition to the conventional fermionic cases, bosonic quasi-/particles also promise energy-efficient THz wave emission. Here, by utilizing a 2D ferromagnetic Cr₂Ge₂Te₆ crystal, first a phonon-related magneto-tunable monochromatic THz irradiation source is demonstrated. With a low-photonic-energy broadband THz pump, a strong THz irradiation with frequency ≈0.9 THz and bandwidth ≈0.25 THz can be generated and its conversion efficiency could even reach 2.1% at 160 K. Moreover, it is intriguing to find that such monochromatic THz irradiation can be efficiently modulated by external magnetic field below 160 K. According to both experimental and theoretical analyses, the emergent THz irradiation is identified as the emission from the phonon-polariton and its temperature and magnetic field dependent behaviors confirm the large spin-lattice coupling in this 2D ferromagnetic crystal. These observations provide a new route for the creation of tunable monochromatic THz source which may have great practical interests in future applications in photonic and spintronic devices.

This is the first report of a destructive photon echo effect detected in a six-wave mixing signal from the atomically thin semiconductor MoSe₂. By developing a sound theoretical model, it is shown that the effect is caused by the exciton-exciton interaction in the material. It provides a new ultrafast spectroscopy tool to study the many-particle interaction in 2D semiconductors.

Abstract

Monolayers of transition metal dichalcogenides display a strong excitonic optical response. Additionally encapsulating the monolayer with hexagonal boron nitride allows to reach the limit of a purely homogeneously broadened exciton system. On such a MoSe₂-based system, ultrafast six-wave mixing spectroscopy is performed and a novel destructive photon echo effect is found. This process manifests as a characteristic depression of the nonlinear signal dynamics when scanning the delay between the applied laser pulses. By theoretically describing the process within a local field model, an excellent agreement with the experiment is reached. An effective Bloch vector representation is developed and thereby it is demonstrated that the destructive photon echo stems from a destructive interference of successive repetitions of the heterodyning experiment.

This review discusses the optical inspection techniques that are available to characterize various 2D materials. The current status and perspective of future trends for optical inspection of the structural properties of 2D materials are comprehensively reviewed, which will facilitate the development of next-generation 2D material-based devices.

Abstract

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

A comprehensive overview on the synthesis of 2D layered material alloys, as well as, the corresponding electronic and optoelectronic devices is provided. In addition, the ongoing challenges are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2D materials toward the next generation of electronic and optoelectronic devices.

Abstract

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano-electronics and nano-optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed-laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up-to-date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up-to-date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

The correlation of novel electrical properties and structural evolutions in van der Waals layered material is revealed by performing special high-pressure measurements. A superconducting model is proposed to explain the emergence of “stripe”-like phases and the evolution of critical temperature. Spatial modulation of interlayer Josephson coupling furthers the understanding of the complex relationships between structure, anisotropy, and superconductivity.

Abstract

The unique electronic structure and crystal structure driven by external pressure in transition metal tellurides (TMTs) can host unconventional quantum states. Here, the discovery of pressure-induced phase transition at ≈2 GPa, and dome-shaped superconducting phase emerged in van der Waals layered NbIrTe₄ is reported. The highest critical temperature (T _c) is ≈5.8 K at pressure of ≈16 GPa, where the interlayered Te–Te covalent bonds form simultaneously derived from the synchrotron diffraction data, indicating the hosting structure of superconducting evolved from low-pressure two-dimensional (2D) phase to three-dimensional (3D) structure with pressure higher than 30 GPa. Strikingly, the authors have found an anisotropic transport in the vicinity of the superconducting state, suggesting the emergence of a “stripe”-like phase. The dome-shaped superconducting phase and anisotropic transport are possibly due to the spatial modulation of interlayer Josephson coupling .