Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been considered as promising candidates for next generation nanoelectronics. Because of their atomically-thin structure and high surface to volume ratio, the interfaces involved in TMDC-based devices play a predominant role in determining the device performance, such as charge injection/collection at the metal/TMDC interface, and charge carrier trapping at the dielectric/TMDC interface. On the other hand, the crystalline structures of TMDCs are enriched by a variety of intrinsic defects, including vacancies, adatoms, grain boundaries, and substitutional impurities. Customized design and engineering of the interfaces and defects provides an effective way to modulate the properties of TMDCs and finally enhance the device performance. Herein, we summarize and highlight recent advances and state-of-the-art investigations on the interface and defect engineering of TMDCs and their corresponding applications in electronic and optoelectronic devices. Various interface engineering approaches for TMDCs are overviewed, including surface charge transfer doping, TMDC/metal contact engineering, and TMDC/dielectric interface engineering. Subsequently, different types of structural defects in TMDCs are introduced. Defect engineering strategies utilized to modulate the optical and electronic properties of TMDCs, as well as the developed high-performance and functional devices are summarized. Finally, we highlight the challenges and opportunities for interface and defect engineering in TMDC materials for electronics and optoelectronics.

The ultrafast growth of high-quality uniform monolayer WSe is reported with a growth rate of ≈26 µm s by chemical vapor deposition on reusable Au substrate, which is ≈2-3 orders of magnitude faster than those of most 2D transition metal dichalcogenides grown on nonmetal substrates. Such ultrafast growth allows for the fabrication of millimeter-size single-crystal WSe domains in ≈30 s and large-area continuous films in ≈60 s. Importantly, the ultrafast grown WSe shows excellent crystal quality and extraordinary electrical performance comparable to those of the mechanically exfoliated samples, with a high mobility up to ≈143 cm V s and ON/OFF ratio up to 9 × 10 at room temperature. Density functional theory calculations reveal that the ultrafast growth of WSe is due to the small energy barriers and exothermic characteristic for the diffusion and attachment of W and Se on the edges of WSe on Au substrate.

The electrical performance of two dimensional transitional metal dichalcogenides (TMDs) is strongly influenced by the amount of structural defects inside. In this work, we provide an optical spectroscopic characterization approach to correlate the amount of structural defects and the electrical performance of WSe 2 devices. Low temperature photoluminescence (PL) spectra of electron beam lithography (EBL) processed WSe 2 presents a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an e-beam-free transfer-electrode technique, we are able to prepare backgated WSe 2 device with limited amount of defects. A maximum hole-mobility of about 200 cm 2 /Vs was achieved due to reduced scattering sources, which is the highest reported value among its type.This work would not only provide a versatile and nondestructive method to monitor the defects in TMDs, but also a new route to approach the room temperature phonon-limited mobility in high performance TMDs devices.

Two-dimensional (2D) materials have been extensively studied in recent years due to their unique properties and great potential for applications. Different types of structural defects could present in 2D materials and have strong influence on their properties. Optical spectroscopic techniques, e.g. Raman and photoluminescence (PL) spectroscopy, have been widely used for defect characterization in 2D materials. In this review, we briefly introduce different types of defects and discuss their effects on the mechanical, electrical, optical, thermal, and magnetic properties of 2D materials. Then, we review the recent progress on Raman and PL spectroscopic investigation of defects in 2D materials, i.e. identifying of the nature of defects and also quantifying the numbers of defects. Finally, we highlight perspectives on defect characterization and engineering in 2D materials.

Metallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas-phase methods and requires high-temperature condition. As an alternative to the gas-phase synthetic approach, lower temperature eutectic liquid-phase synthesis presents a very promising approach with the potential for larger-scale and controllable growth of high-quality thin metallic TMD single crystals. Here, the first realization of low-temperature eutectic liquid-phase synthesis of type-II Dirac semimetal PtTe 2 single crystals with thickness ranging from 2 to 200 nm is presented. The electrical measurement of synthesized PtTe 2 reveals a record-high conductivity of as high as 3.3 × 10 6 S m −1 at room temperature. Besides, the weak antilocalization behavior is identified experimentally in the type-II Dirac semimetal PtTe 2 for the first time. Furthermore, a simple and general strategy is developed to obtain atomically thin PtTe 2 crystal by thinning as-synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electrical conductivity. The results of controllable and scalable low-temperature eutectic liquid-phase synthesis and layer-by-layer thinning of high-quality thin PtTe 2 single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high meltingpoint transition metal.

Excitons in two-dimensional (2D) materials are tightly bound and exhibit rich physics. So far, the optical excitations in 2D semiconductors are dominated by Wannier-Mott excitons, but molecular systems can host Frenkel excitons (FE) with unique properties. Here, we report a strong optical response in a class of monolayer molecular J-aggregates. The exciton exhibits giant oscillator strength and absorption (over 30% for monolayer) at resonance, as well as photoluminescence quantum yield in the range of 60–100%. We observe evidence of superradiance (including increased oscillator strength, bathochromic shift, reduced linewidth and lifetime) at room-temperature and more progressively towards low temperature. These unique properties only exist in monolayer owing to the large unscreened dipole interactions and suppression of charge-transfer processes. Finally, we demonstrate light-emitting devices with the monolayer J-aggregate. The intrinsic device speed could be beyond 30 GHz, which is promising for next-generation ultrafast on-chip optical communications.

Atomically thin hexagonal boron nitride ( h -BN) is often regarded as an elastic film that is impermeable to gases. The high stabilities in thermal and chemical properties allow h -BN to serve as a gas barrier under extreme conditions. Here, we demonstrate the isolation of hydrogen in bubbles of h -BN via plasma treatment. Detailed characterizations reveal that the substrates do not show chemical change after treatment. The bubbles are found to withstand thermal treatment in air, even at 800 °C. Scanning transmission electron microscopy investigation shows that the h -BN multilayer has a unique aligned porous stacking nature, which is essential for the character of being transparent to atomic hydrogen but impermeable to hydrogen molecules. In addition, we successfully demonstrated the extraction of hydrogen gases from gaseous compounds or mixtures containing hydrogen element. The successful production of hydrogen bubbles on h -BN flakes has potential for further application in nano/micro-electromechanical systems and hydrogen storage.

Defects in transition metal dichalcogenides (TMDs) play an important role in tailoring electrical and optical properties.Here we employ Ar + plasma to controllably generate active defects in WSe 2 monolayers to tune their optical properties. Two defect-activated PL emission peaks are emerging in the low temperature PL spectra of WSe 2 monolayer treated with Ar + plasma. These emissions are attributed to the recombination of excitons bound to different types of structural defects. The shallow level emission originates from the recombination of excitons at chalcogen vacancies, while the deep level emission might arise from other types of defects, such as transition metal vacancies, cluster of vacancies, rotational defects, or antisite defects. Our results demonstrate that Ar + plasma treatment is an effective approach to induce desirable defects in TMDs monolayers and PL spectroscopy is an efficient method to investigate these defects.