Polaritons-hybrid light-matter excitations-enable nanoscale control of light. Particularly large polariton field confinement and long lifetimes can be found in graphene and materials consisting of ...two-dimensional layers bound by weak van der Waals forces
(vdW materials). These polaritons can be tuned by electric fields
or by material thickness
, leading to applications including nanolasers
, tunable infrared and terahertz detectors
, and molecular sensors
. Polaritons with anisotropic propagation along the surface of vdW materials have been predicted, caused by in-plane anisotropic structural and electronic properties
. In such materials, elliptic and hyperbolic in-plane polariton dispersion can be expected (for example, plasmon polaritons in black phosphorus
), the latter leading to an enhanced density of optical states and ray-like directional propagation along the surface. However, observation of anisotropic polariton propagation in natural materials has so far remained elusive. Here we report anisotropic polariton propagation along the surface of α-MoO
, a natural vdW material. By infrared nano-imaging and nano-spectroscopy of semiconducting α-MoO
flakes and disks, we visualize and verify phonon polaritons with elliptic and hyperbolic in-plane dispersion, and with wavelengths (up to 60 times smaller than the corresponding photon wavelengths) comparable to those of graphene plasmon polaritons and boron nitride phonon polaritons
. From signal oscillations in real-space images we measure polariton amplitude lifetimes of 8 picoseconds, which is more than ten times larger than that of graphene plasmon polaritons at room temperature
. They are also a factor of about four larger than the best values so far reported for phonon polaritons in isotopically engineered boron nitride
and for graphene plasmon polaritons at low temperatures
. In-plane anisotropic and ultra-low-loss polaritons in vdW materials could enable directional and strong light-matter interactions, nanoscale directional energy transfer and integrated flat optics in applications ranging from bio-sensing to quantum nanophotonics.
In this work, tapered/etched multicore fiber (MCF) probes are spliced with multimode fiber (MMF) to fabricate the sensor structure. To improve sensitivity, gold nanoparticles (AuNPs) and molybdenum ...disulfide nanoparticles (MoS 2 -NPs) are used to immobilize both probes. Synthesized AuNPs and molybdenum disulfide (MoS 2 )-nanoparticles (NPs) have peak absorption wavelengths of 519 and 330 nm, respectively. High-resolution transmission electron microscopy is used to examine the morphology of the NPs. The scanning electron microscope (SEM) is used to characterize the NP-immobilized optical fiber sensor structures, and SEM-EDX is used to verify the NP-coating over fiber structure. The functionalization of the acetylcholinesterase enzyme over the NP-immobilized probe increases the specificity of the sensor later on. Finally, the developed sensor probes are tested by detecting various acetylcholine concentrations. In addition, performance analyses, such as reusability, reproducibility, and selectivity (in the presence of ascorbic acid, glucose, dopamine, and uric acid), are carried out, and proposed biosensors are experimentally evaluated. The developed tapered fiber sensor with a sensitivity of 0.062 nm/<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula> can detect even very low concentrations, such as 14.28 <inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula>, over a wide detection range of 0-1000 <inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula>.
•A MWCNTs-MoS2-MoO3 HER photoelectrocatalyst with low band gap was obtained.•The MWCNTs-MoS2-MoO3 exhibited NIR enhanced electrocatalytic HER performance.•NIR irradiation and MWCNTs could improve ...charge separation efficiency.•MoS2-MoO3 heterostructure and bridging S22− would improve the HER performance.
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As a new zero-carbon energy with high calorific value, hydrogen has attracted extensive attention in recent years. In this work, multiwalled carbon nanotubes modified molybdenum sulfide and molybdenum oxide heterostructure (MWCNTs-MoS2-MoO3) was obtained via a simple hydrothermal method. The low band gap of 0.02MWCNTs-MoS2-MoO3 (1.15 eV) endowed a good near-infrared (NIR) light response. The conduction band position of 0.02MWCNTs-MoS2-MoO3 (−0.63 V vs. reversible hydrogen electrode (RHE)) was negative enough to produce hydrogen. Compared with pure MoS2, the formation of MoS2-MoO3 heterostructure and introduction of MWCNTs generated better photoelectrochemical activity and NIR irradiation improved photoelectrocatalytic (PEC) hydrogen evolution performance in wide pH range. The 0.02MWCNTs-MoS2-MoO3 had superior overpotential (68 mV @ 10 mA cm−2) and Tafel slope (56 mV dec−1) in acidic media. The NIR improved PEC hydrogen evolution activity of 0.02MWCNTs-MoS2-MoO3 in acidic media could be explained by Volmer-Heyrovsky mechanism. The presence of large amount of bridging S22− in 1 T/2 H phase MoS2 and the synergistic effect between MWCNTs and MoS2-MoO3 heterostructure made a joint effect on the photoelectrocatalytic hydrogen evolution reaction (HER). NIR irradiation could increase the charge transport rate. The introduction of MWCNTs and heterostructure formation could increase the charge transport rate and improve the electron and hole separation ability of 0.02MWCNTs-MoS2-MoO3 as well.
Single-layered molybdenum disulphide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for the next generation of nanoelectronics. Here, we report the controlled ...vapour phase synthesis of molybdenum disulphide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area, single- and few-layered films. Using high-resolution electron microscopy imaging, the atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulphide atomic layers are examined, and the primary mechanisms for grain boundary formation are evaluated. Grain boundaries consisting of 5- and 7- member rings are directly observed with atomic resolution, and their energy landscape is investigated via first-principles calculations. The uniformity in thickness, large grain sizes, and excellent electrical performance signify the high quality and scalable synthesis of the molybdenum disulphide atomic layers.
Ni(HCO3)2 working for the first time as an adsorption site for hydroxyl species for a MoS2-based catalyst has efficiently facilitated the alkaline HER kinetics, as demonstrated by experimental and ...DFT calculation results. The ultra-small Ni(HCO3)2 is vital to achieving excellent HER activity of Ni(HCO3)2/MoS2/CC in alkaline media, which transcended the performance of Pt/C at high current density.
Aqueous zinc‐ion batteries (ZIBs) have attracted significant attention due to their intrinsic safety, cost‐effectiveness, and environmental friendliness. However, the common zinc metal anode suffers ...from zinc dendrite formation, self‐corrosion, and surface passivation, which impede the further application of aqueous ZIBs. Herein, carbon‐inserted molybdenum dioxide (MoO2) materials with laminated structure are designed as novel intercalation‐type anodes for ZIBs by combination of interlayer engineering and in situ carbonization of aniline guest in molybdenum trioxide interlayers. The uniform dispersion of carbon layers in laminated MoO2 not only provide fast transportation paths for electron but also strengthen the framework of MoO2, leading to high structural integration during high‐rate cycling. Benefiting from the unique structural design, the carbon‐inserted MoO2 electrode exhibits high initial Coulombic efficiency, excellent cycling stability, and outstanding rate capability. Multiple ex situ characterizations reveal its excellent electrochemical stability is derived from reversible intercalation mechanism and ultrastable structural framework. Furthermore, the rocking‐chair zinc‐ion full battery assembled with the zinc pre‐intercalated Na3V2(PO4)2O2F cathode presents excellent stability and ultralong lifespan with a high capacity retention of 91% over 8000 cycles.
Carbon‐inserted molybdenum dioxide (MoO2) with a laminated structure is designed as a novel intercalation‐type anode for zinc‐ion batteries by the combination of interlayer engineering and in situ carbonization of aniline guest in molybdenum trioxide interlayers. The uniform dispersion of carbon layers in laminated MoO2 not only provides fast transportation path for electron but also strengthen the framework, causing high structural integration during high‐rate cycling.
We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory. The latter connects ab initio calculations of the ...single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra. For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure. Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands. The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap. The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emission from the A-exciton transition and a visible B-peak even if the band gap is indirect. For above-band gap excitation, we predict a strongly reduced emission intensity at comparable carrier densities and the absence of B-exciton emission. The results agree well with PL measurements performed on monolayer MoS2 at excitation wavelengths of 405 nm (above) and 532 nm (below the band gap).
Memristors are two-terminal passive circuit elements that have been developed for use in non-volatile resistive random-access memory and may also be useful in neuromorphic computing. Memristors have ...higher endurance and faster read/write times than flash memory and can provide multi-bit data storage. However, although two-terminal memristors have demonstrated capacity for basic neural functions, synapses in the human brain outnumber neurons by more than a thousandfold, which implies that multi-terminal memristors are needed to perform complex functions such as heterosynaptic plasticity. Previous attempts to move beyond two-terminal memristors, such as the three-terminal Widrow-Hoff memristor and field-effect transistors with nanoionic gates or floating gates, did not achieve memristive switching in the transistor. Here we report the experimental realization of a multi-terminal hybrid memristor and transistor (that is, a memtransistor) using polycrystalline monolayer molybdenum disulfide (MoS
) in a scalable fabrication process. The two-dimensional MoS
memtransistors show gate tunability in individual resistance states by four orders of magnitude, as well as large switching ratios, high cycling endurance and long-term retention of states. In addition to conventional neural learning behaviour of long-term potentiation/depression, six-terminal MoS
memtransistors have gate-tunable heterosynaptic functionality, which is not achievable using two-terminal memristors. For example, the conductance between a pair of floating electrodes (pre- and post-synaptic neurons) is varied by a factor of about ten by applying voltage pulses to modulatory terminals. In situ scanning probe microscopy, cryogenic charge transport measurements and device modelling reveal that the bias-induced motion of MoS
defects drives resistive switching by dynamically varying Schottky barrier heights. Overall, the seamless integration of a memristor and transistor into one multi-terminal device could enable complex neuromorphic learning and the study of the physics of defect kinetics in two-dimensional materials.
Large-scale implementation of electrochemical hydrogen production requires several fundamental issues to be solved, including understanding the mechanism and developing inexpensive electrocatalysts ...that work well at high current densities. Here we address these challenges by exploring the roles of morphology and surface chemistry, and develop inexpensive and efficient electrocatalysts for hydrogen evolution. Three model electrocatalysts are flat platinum foil, molybdenum disulfide microspheres, and molybdenum disulfide microspheres modified by molybdenum carbide nanoparticles. The last catalyst is highly active for hydrogen evolution independent of pH, with low overpotentials of 227 mV in acidic medium and 220 mV in alkaline medium at a high current density of 1000 mA cm
, because of enhanced transfer of mass (reactants and hydrogen bubbles) and fast reaction kinetics due to surface oxygen groups formed on molybdenum carbide during hydrogen evolution. Our work may guide rational design of electrocatalysts that work well at high current densities.
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