Jiangtan Yuan

Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction

A facile chemical vapor deposition method to prepare single-crystalline VS2 nanosheets for the hydrogen evolution reaction is reported. The electrocatalytic hydrogen evolution reaction (HER) activities of VS2 show an extremely low overpotential of -68 mV at 10 mA cm(-2), small Tafel slopes of ≈34 mV decade(-1), as well as high stability, demonstrating its potential as a candidate non-noble-metal catalyst for the HER.

Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS2 and WS2

A range of semiconductors can host both spin and valley polarizations. Optical experiments on single layers of transition metal dichalcogenides now show that inter-valley scattering can accelerate spin relaxation. The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics1,2,3. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments4,5,6,7,8, PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination9,10. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage11,12,13. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS2 and WS2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (two to three orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.

Photoluminescence Quenching and Charge Transfer in Artificial Heterostacks of Monolayer Transition Metal Dichalcogenides and Few-Layer Black Phosphorus

Transition metal dichalcogenides monolayers and black phosphorus thin crystals are emerging two-dimensional materials that demonstrated extraordinary optoelectronic properties. Exotic properties and physics may arise when atomic layers of different materials are stacked together to form van der Waals solids. Understanding the important interlayer couplings in such heterostructures could provide avenues for control and creation of characteristics in these artificial stacks. Here we systematically investigate the optical and optoelectronic properties of artificial stacks of molybdenum disulfide, tungsten disulfide, and black phosphorus atomic layers. An anomalous photoluminescence quenching was observed in tungsten disulfide-molybdenum disulfide stacks. This was attributed to a direct to indirect band gap transition of tungsten disulfide in such stacks while molybdenum disulfide maintains its monolayer properties by first-principles calculations. On the other hand, due to the strong build-in electric fields in tungsten disulfide-black phosphorus or molybdenum disulfide-black phosphorus stacks, the excitons can be efficiently splitted despite both the component layers having a direct band gap in these stacks. We further examine optoelectronic properties of tungsten disulfide-molybdenum disulfide artificial stacks and demonstrate their great potentials in future optoelectronic applications.

Synthesis and defect investigation of two-dimensional molybdenum disulfide atomic layers.

CONSPECTUS: The unique physical properties of two-dimensional (2D) molybdenum disulfide (MoS2) and its promising applications in future optoelectronics have motivated an extensive study of its physical properties. However, a major limiting factor in investigation of 2D MoS2 is its large area and high quality preparation. The existence of various types of defects in MoS2 also makes the characterization of defect types and the understanding of their roles in the physical properties of this material of critical importance. In this Account, we review the progress in the development of synthetic approaches for preparation of 2D MoS2 and the understanding of the role of defects in its electronic and optical properties. We first examine our research efforts in understanding exfoliation, direct sulfurization, and chemical vapor deposition (CVD) of MoS2 monolayers as main approaches for preparation of such atomic layers. Recognizing that a natural consequence of the synthetic approaches is the addition of sources of defects, we initially focus on identifying these imperfections with intrinsic and extrinsic origins in CVD MoS2. We reveal the predominant types of point and grain boundary defects in the crystal structure of polycrystalline MoS2 using transmission electron microscopy (TEM) and understand how they modify the electronic band structure of this material using first-principles-calculations. Our observations and calculations reveal the main types of vacancy defects, substitutional defects, and dislocation cores at the grain boundaries (GBs) of MoS2. Since the sources of defects in two-dimensional atomic layers can, in principle, be controlled and studied with more precision compared with their bulk counterparts, understanding their roles in the physical properties of this material may provide opportunities for changing their properties. Therefore, we next examine the general electronic properties of single-crystalline 2D MoS2 and study the role of GBs in the electrical transport and photoluminescence properties of its polycrystalline counterparts. These results reveal the important role played by point defects and GBs in affecting charge carrier mobility and excitonic properties of these atomic layers. In addition to the intrinsic defects, growth process induced substrate impurities and strain induced band structure perturbations are revealed as major sources of disorder in CVD grown 2D MoS2. We further explore substrate defects for modification and control of electronic and optical properties of 2D MoS2 through interface engineering. Self-assembled monolayer based interface modification, as a versatile technique adaptable to different conventional and flexible substrates, is used to promote significant tunability in the key MoS2 field-effect device parameters. This approach provides a powerful tool for modification of native substrate defect characteristics and allows for a wide range of property modulations. Our results signify the role of intrinsic and extrinsic defects in the physical properties of MoS2 and unveil strategies that can utilize these characteristics.

Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures

Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Excitonic Resonant Emission–Absorption of Surface Plasmons in Transition Metal Dichalcogenides for Chip-Level Electronic–Photonic Integrated Circuits

The monolithic integration of electronics and photonics has attracted enormous attention due to its potential applications. A major challenge to this integration is the identification of suitable materials that can emit and absorb light at the same wavelength. In this paper we utilize unique excitonic transitions in WS2 monolayers and show that WS2 exhibits a perfect overlap between its absorption and photoluminescence spectra. By coupling WS2 to Ag nanowires, we then show that WS2 monolayers are able to excite and absorb surface plasmons of Ag nanowires at the same wavelength of exciton photoluminescence. This resonant absorption by WS2 is distinguished from that of the ohmic propagation loss of silver nanowires, resulting in a short propagation length of surface plasmons. Our demonstration of resonant optical generation and detection of surface plasmons enables nanoscale optical communication and paves the way for on-chip electronic–photonic integrated circuits.

Thickness-Dependent and Magnetic-Field-Driven Suppression of Antiferromagnetic Order in Thin V5S8 Single Crystals.

With materials approaching the 2D limit yielding many exciting systems with intriguing physical properties and promising technological functionalities, understanding and engineering magnetic order in nanoscale, layered materials is generating keen interest. One such material is V5S8, a metal with an antiferromagnetic ground state below the Néel temperature TN ∼ 32 K and a prominent spin-flop signature in the magnetoresistance (MR) when H∥c ∼ 4.2 T. Here we study nanoscale-thickness single crystals of V5S8, focusing on temperatures close to TN and the evolution of material properties in response to systematic reduction in crystal thickness. Transport measurements just below TN reveal magnetic hysteresis that we ascribe to a metamagnetic transition, the first-order magnetic-field-driven breakdown of the ordered state. The reduction of crystal thickness to ∼10 nm coincides with systematic changes in the magnetic response: TN falls, implying that antiferromagnetism is suppressed; and while the spin-flop signature remains, the hysteresis disappears, implying that the metamagnetic transition becomes second order as the thickness approaches the 2D limit. This work demonstrates that single crystals of magnetic materials with nanometer thicknesses are promising systems for future studies of magnetism in reduced dimensionality and quantum phase transitions.

Direct Assessment of the Toxicity of Molybdenum Disulfide Atomically Thin Film and Microparticles via Cytotoxicity and Patch Testing

The low toxicity of molybdenum disulfide (MoS2 ) atomically thin film and microparticles is confirmed via cytotoxicity and patch testing in this report. The toxicity of MoS2 thin film and microparticles is extensively studied but is still inconclusive due to potential organic contamination in the preparations of samples. Such contamination is avoided here through preparing MoS2 atomically thin film via direct sulfurization of molybdenum thin film on quartz plate, which permits a direct assessment of its toxicity without any contamination. Six different types of cells, including normal, cancer, and immortal cells, are cultured in the media containing MoS2 thin film on quartz plates or dispersed MoS2 microparticles and their viability is evaluated with respect to the concentrations of samples. Detached thin films from the quartz plates are also investigated to estimate the toxicity of dispersed MoS2 in biological media. Allergy testing on skin of guinea pigs is also conducted to understand their effect on animal skins. By avoiding possible organic contamination, the low toxicity of MoS2 atomically thin film and microparticles to cells and animal skins paves the way for its applications in flexible biosensing/bioimaging devices and biocompatible coatings.

Author Correction: Ultrafast probes of electron–hole transitions between two atomic layers

The original version of this Article omitted an affiliation of Xiewen Wen: ‘College of Chemistry and Molecular Engineering, Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871, China’. This has been corrected in both the PDF and HTML versions of the Article.

Photoluminescence quenching in hybrid gold/MoSe 2 nanosheets

Transition Metal Dichalcogenide (TMD) materials have increasingly gained attention, due to their unique optical, spintronic, and electronic properties [1]. These properties at the monolayer limit are by part a result of the ultimate confinement imposed on their excitonic transitions that originate a direct band-gap and a lack of inversion symmetry in their crystallographic structure [2]. Many promising recent efforts in control of excitonic transitions in these materials have been devoted to study of their coupling with plasmonic nanoresonators. Plasmonic nanoresonators are known for their ability to control and modify the optical response of materials in their proximity [3]. These findings have motivated the study of emergent phenomena associated with the plasmon exciton interaction in these hybrid systems [4-5], which include enhancement [6] and quenching of the TMDs photoluminescence [7].