A library of atomically thin metal chalcogenides.

Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting physical phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional superconductivity 5 , suggesting potential applications for functional devices6-10. However, of the numerous compounds available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11-15, selenization16,17 and tellurization 18 of metals and metal compounds. Many TMCs are difficult to produce because of the high melting points of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temperature 19 and this approach 20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chemical vapour deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compounds, including 32 binary compounds (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compounds. We elaborate how the salt decreases the melting point of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of the synthesized materials in our library are useful, as supported by evidence of superconductivity in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.

Salt-Assisted MoS2 Growth: Molecular Mechanisms from the First Principles.

Two-dimensional transition metal dichalcogenides (TMD), such as molybdenum disulfide (MoS2), have aroused substantial research interest in recent years, motivating the quest for new synthetic strategies. Recently, halide salts have been reported to promote the chemical vapor deposition (CVD) growth of a wide range of TMD. Nevertheless, the underlying promoting mechanisms and reactions are largely unknown. Here, we employ first-principles calculations and ab initio molecular dynamics (AIMD) simulations in order to investigate the detailed molecular mechanisms during the salt-assisted CVD growth of MoS2 monolayers. The sulfurization of molybdenum oxyhalides MoO2X2 (X = F, Cl, Br, and I)─the form of Mo-feedstock dominating in salt-assisted synthesis─has been explored and displays much lower activation barriers than that of molybdenum oxide present during conventional "saltless" growth of MoS2. Furthermore, the rate-limiting barriers appear to depend linearly on the electronegativity of the halogen element, with oxyiodide having the lowest barrier. Our study reveals the promoting mechanisms of halides and allows growth parameter optimization to achieve even faster growth of MoS2 monolayers in the CVD synthesis.

Atomic Molybdenum for Synthesis of Ammonia with 50% Faradic Efficiency.

The electrochemical dinitrogen (N2 ) reduction reaction (NRR) under ambient conditions has gained significant interest as an environmentally friendly alternative to the traditional Haber-Bosch process for the synthesis of ammonia (NH3 ). However, up to now, most of the reported NRR electrocatalysts with satisfactory catalytic activities have been hindered by the large overpotential in N2 activation. The preparation of highly efficient Mo-based NRR electrocatalyst in acidic electrolytes under ambient conditions is demonstrated here, consisting of stabilized single Mo atoms anchored on holey nitrogen-doped graphene synthesized through a convenient potassium-salt-assisted activation method. At -0.05 V versus a reversible hydrogen electrode (RHE), an electrode consisting of the resultant electrocatalyst immobilized on carbon fiber paper can attain an exceptional Faradaic efficiency of 50.2% and a NH3 yield rate of 3.6 µg h-1 mgcat-1 with low overpotentials. Density functional theory calculations further unveil that compared to the original graphene without holes, the edge coordinated Mo atoms and the existence of vacancies on holey graphene lower the overpotential of N2 reduction, thereby promoting the NRR catalytic activity. This work could provide new guidelines for future designs in single-atom catalysis that would be beneficial to ambient N2 fixation, and replacement of classical synthesis processes that are very energy-intensive.

Gaussian-based quasiparticle self-consistent GW for periodic systems.

We present a quasiparticle self-consistent GW (QSGW) implementation for periodic systems based on crystalline Gaussian basis sets. Our QSGW approach is based on a full-frequency analytic continuation GW scheme with Brillouin zone sampling and employs the Gaussian density fitting technique. We benchmark our QSGW implementation on a set of weakly correlated semiconductors and insulators as well as strongly correlated transition metal oxides, including MnO, FeO, CoO, and NiO. The band gap, band structure, and density of states are evaluated using finite size corrected QSGW. We find that although QSGW systematically overestimates the bandgaps of the tested semiconductors and transition metal oxides, it completely removes the dependence on the choice of density functionals and provides a more consistent prediction of spectral properties than G0W0 across a wide range of solids. This work paves the way for utilizing QSGW in ab initio quantum embedding for solids.

Microwave-photonic low-coherence interferometry for dark zone free distributed optical fiber sensing.

A microwave-photonic low-coherence interferometry (MPLCI) system is proposed for fully distributed optical fiber sensing. Assisted by an unbalanced Michelson interferometer, a low-coherence laser source is used to interrogate cascaded Fabry-Perot interferometers along with an optical fiber for a dark zone free (or spatially continuous) distributed measurement. By combining the advantages of microwaves and photonics, the MPLCI system can synergistically achieve high sensitivity and high spatial resolution. Our tests have confirmed a strain resolution of 95 nε at the spatial resolution of 10 cm.

Information integrated glass module fabricated by integrated additive and subtractive manufacturing.

In this Letter, we report a novel integrated additive and subtractive manufacturing (IASM) method to fabricate an information integrated glass module. After a certain number of glass layers are 3D printed and sintered by direct ${{\rm CO}_2}$CO2 laser irradiation, a microchannel will be fabricated on top of the printed glass by integrated picosecond laser, for intrinsic Fabry-Perot interferometer (IFPI) optical fiber sensor embedment. Then, the glass 3D printing process continues for the realization of bonding between optical fiber and printed glass. Temperature sensing up to 1000°C was demonstrated using the fabricated information integrated module. In addition, the long-term stability of the glass module at 1000°C was conducted. Enhanced sensor structure robustness and harsh temperature sensing capability make this glass module attractive for harsh environment structural health monitoring.

Glass 3D printing of microfluidic pressure sensor interrogated by fiber-optic refractometry.

This letter reports a novel fused silica microfluidic device with pressure sensing capability that is fabricated by integrated additive and subtractive manufacturing (IASM) method. The sensor consists of a capillary and a 3D printed glass reservoir, where the reservoir volume change under pressure manifests liquid level deviation inside the capillary, thus realizing the conversion between small pressure change into large liquid level variation. Thanks to the design flexibility of this unique IASM method, the proposed microfluidic device is fabricated with liquid-in-glass thermometer configuration, where the reservoir is sealed following a novel 3D printing assisted glass bonding process. And liquid level is interrogated by a fiber-optic sensor based on multimode interference (MMI) effect. This proposed microfluidic device is attractive for chemical and biomedical sensing because it is flexible in design, and maintains good chemical and mechanical stability, and adjustable sensitivity and range.

The effect of laser sintering on the microstructure, relative density, and cracking of sol-gel-derived silica thin films.

Combining sol-gel processing and laser sintering is a promising way for fabricating functional ceramic deposition with high dimensional resolution. In this work, crack-free silica tracks on a silica substrate with a thickness from ~360 nm to ~950 nm, have been obtained by direct exposure to a CO2 laser beam. At a fixed scanning speed, the density and microstructures of the silica deposition can be precisely controlled by varying the laser output power. The porosity of the laser-sintered silica tracks ranged from close to 0% to ~60%. When the thickness of the silica deposition exceeded the critical thickness (eg, ~2.2 µm before firing), cracks occurred in both laser-sintered and furnace-sintered samples. Cracks propagated along the edge of the laser-sintered track, resulting in the crack-free track. However, for the furnace heat-treated counterpart, the cracks spread randomly. To understand the laser sintering effect, we established a finite element model (FEM) to calculate the temperature profile of the substrate during laser scanning, which agreed well with the one-dimensional analytical model. The FEM model confirmed that laser sintering was the main thermal effect and the calculated temperature profile can be used to predict the microstructure of the laser-sintered tracks. Combining these results, we were able to fabricate, predesigned patterned (Clemson tiger paw) silica films with high density using a galvo scanner.

Laser-assisted embedding of all-glass optical fiber sensors into bulk ceramics for high-temperature applications.

We develop a laser-assisted sensor embedding process to embed all-glass optical fiber sensors into bulk ceramics for high-temperature applications. A specially designed two-step microchannel was fabricated on an Al2O3 substrate for sensor embedment using a picosecond (ps) laser. An optical fiber Intrinsic Fabry-Perot Interferometer (IFPI) sensor was embedded at the bottom of the microchannel and covered by Al2O3 slurry which was subsequently sintered by a CO2 laser. The sensor spectrum was in-situ monitored during the laser sintering process to ensure the survival of the sensor and optimize the laser sintering parameters. By testing in furnace through high temperature, the embedded optical fiber shows improved stability after CO2 laser sealing, resulting in the linear temperature response of the embedded optical fiber IFPI sensor. To improve the embedded IFPI sensor for thermal strain measurement, a dummy fiber was co-embedded with the sensing fiber to improve the mechanical bonding between the sensing fiber and the ceramic substrate so that the thermal strain of the ceramic substrate can apply on the sensing fiber. The response sensitivity, measurement repeatability and high-temperature long-term stability of the embedded optical fiber IFPI sensor were evaluated in this work.

3D printing of all-glass fiber-optic pressure sensor for high temperature applications.

In this paper, we report a fiber-optic pressure sensor fabricated by three-dimensional (3D) printing of glass using direct laser melting method. An all-glass fiber-housing structure is 3D printed on top of a fused silica substrate, which also serves as the pressure sensing diaphragm. And an optical fiber can be inserted inside the fiber housing structure and brought in close proximity to the diaphragm to form a Fabry-Perot interferometer. The theoretical analysis and experimental verification of the pressure sensing capability are presented.

A photoacoustic imaging method for in-situ monitoring of laser assisted ceramic additive manufacturing.

We propose an in-situ monitoring method of laser assisted ceramic additive manufacturing (AM) fabrication process using photoacoustic (PA) imaging technique. This method is potentially very practical and of low cost, as it can be easily implemented with little modification to the original AM system. A major advantage of this method is that it does not require any complex or expensive component to be installed, just need a microphone to be setup near the work-piece and a data processing program to analyse the acoustic data. By collecting the photoacoustic wave that produced by the laser scanning during the AM process, a spectrogram of the acoustic signal can be generated using short-time Fourier transform (STFT). The PA signal can be extracted by specifying the modulation frequency of the laser in the spectrogram. Combining with the scanning position of the laser beam, the PA image of the layer under processing can be obtained. Detection of two kinds of defects (metal defect and dried paste defect) have been demonstrated. Although there are many other kinds of possible defects in additive manufacturing processes, this method could be a practical and low cost way to monitor most of them with proper choice of parameters of the laser and detection system. The parameters of the system that can influence the PA image quality have also been discussed.

Microwave-assisted frequency domain measurement of fiber-loop ring-down system.

A new frequency domain measurement method for a fiber-loop ring-down system is proposed in this Letter. Compared to traditional time domain measurement, this method uses a microwave modulated continuous wave (CW) laser as a light source, making full use of the duty cycle to achieve enhanced measurement efficiency. By measuring the amplitude modulation over a frequency span, this technique can be used to determine the ring-down time in the frequency domain, which will then be used to calculate the loss in the ring.

Micro-cantilever-based fiber optic hydrophone fabricated by a femtosecond laser.

We report an open cavity, cantilever-based fiber optic Fabry-Perot interferometer hydrophone. The hydrophone is made of fused silica material, and its micro-cantilever beam is directly fabricated by femtosecond (fs) laser micromachining. The theoretical analyses and experimental verifications of the frequency response of the sensor are presented.

Floating Fe Catalyst Formation and Effects of Hydrogen Environment in the Growth of Carbon Nanotubes.

Hydrocarbon conversion to advanced carbon nanomaterials with concurrent hydrogen production holds promise for clean energy technologies. This has been largely enabled by the floating catalyst chemical vapor deposition (FCCVD) growth of carbon nanotubes (CNTs), where commonly catalytic iron nanoparticles are formed from ferrocene decomposition. However, the catalyst formation mechanism and the effect of the chemical environment, especially hydrogen, remain elusive. Here, by employing atomistic simulations, we demonstrate how (i) hydrogen accelerates the ferrocene decomposition and (ii) prevents catalyst encapsulation. A subsequent catalytic dehydrogenation of methane on a liquid Fe nanoparticle showed that carbon dimers tend to be the dominant on-surface species. Such atomistic insights help us better understand the catalyst formation and CNT nucleation in the early stages of the FCCVD growth process and optimize it for potential scaleup.

Marijuana smoking and asthma: a protocol for a meta-analysis.

INTRODUCTION: Recent studies have raised the concern on the risk of asthma in marijuana smokers; however, the results remain controversial and warrant further investigation. With a growing number of marijuana smokers, examining the association between marijuana smoking and asthma and quantifying such association through meta-analysis have important implications for public health and clinical decision-making. In view of this, the present protocol aims to detail a comprehensive plan of meta-analysis on the association aforementioned. The findings are expected to strengthen the current knowledge base pertaining to the potential adverse effects of marijuana smoking on pulmonary health and to facilitate the development of prevention strategies for asthma. METHODS AND ANALYSIS: The MEDLINE/PubMed, Web of Science and EMBASE databases will be searched systematically from inception to 1 September 2021 to retrieve the relevant observational studies focusing on the association between marijuana smoking and asthma. Both unadjusted and adjusted effect sizes, such as OR, relative risk, HR and the corresponding 95% CIs will be extracted for pooled analyses. Heterogeneity and publication bias across the included studies will be examined. The Newcastle-Ottawa Quality Scale will be used to assess the quality and risk of bias. Statistical software Review Manager V.5.3 and Stata V.11.0 will be used for statistical analyses. ETHICS AND DISSEMINATION: Since no private and confidential patient data will be included in the reporting, approval from an ethics committee is not required. The results will be published in a peer-reviewed journal or disseminated in the relevant conferences. The study raises no ethical issue. OSF REGISTRATION NUMBER: 10.17605/OSF.IO/UPTXC.

A Ternary Seismic Metamaterial for Low Frequency Vibration Attenuation.

Structural vibration induced by low frequency elastic waves presents a great threat to infrastructure such as buildings, bridges, and nuclear structures. In order to reduce the damage of low frequency structural vibration, researchers proposed the structure of seismic metamaterial, which can be used to block the propagation of low frequency elastic wave by adjusting the frequency range of elastic wave propagation. In this study, based on the concept of phononic crystal, a ternary seismic metamaterial is proposed to attenuate low frequency vibration by generating band gaps. The proposed metamaterial structure is periodically arranged by cube units, which consist of rubber coating, steel scatter, and soft matrix (like soil). The finite element analysis shows that the proposed metamaterial structure has a low frequency band gap with 8.5 Hz bandwidth in the range of 0-20 Hz, which demonstrates that the metamaterial can block the elastic waves propagation in a fairly wide frequency range within 0-20 Hz. The frequency response analysis demonstrates that the proposed metamaterial can effectively attenuate the low frequency vibration. A simplified equivalent mass-spring model is further proposed to analyze the band gap range which agrees well with the finite element results. This model provides a more convenient method to calculate the band gap range. Combining the proposed equivalent mass-spring model with finite element analysis, the effect of material parameters and geometric parameters on the band gap characteristic is investigated. This study can provide new insights for low frequency vibration attenuation.

Gas-Phase "Prehistory" and Molecular Precursors in Monolayer Metal Dichalcogenides Synthesis: The Case of MoS2.

Two-dimensional MoS2 is one of the most promising materials for nanoelectronics due to its semiconducting nature and plethora of extraordinary properties. The main method for mass production of large-scale, high-quality MoS2 monolayers is chemical vapor deposition (CVD). Yet, the details of the chemistry occurring during the synthesis remain largely unknown, hindering process optimization. Combining ab initio molecular dynamics (AIMD) simulations and first-principles calculations allows us to explore the complete processes of MoS2 monolayer growth at the atomic level. We find that solid MoO3 precursor sublimates forming ringlike molecules, such as Mo3O9, which can later be regarded as gas-phase Mo-carrier reactants, undergoing sulfurization in three main stages: ring opening, chain breaking as the rate-limiting step, and further sulfurization. The fully sulfurized MoS6 molecule emerges as an immediate gas precursor to the crystal growth, as it reacts to join the MoS2-layer edge, with the release of a S4 molecule. Our comprehensive study provides detailed insights into the microscopic reaction mechanisms of MoS2 CVD growth and guidance for optimizing the synthesis parameters for transition metal dichalcogenides.

Tuning Metal Elements in Open Frameworks for Efficient Oxygen Evolution and Oxygen Reduction Reaction Catalysts.

Electrochemical methods are promising technical routes for future clean energy storage and conversion. Most of the electrochemical methods involve oxygen reactions. Unfavorable kinetics and sluggish reactions are the main challenges for these processes. We report here a facile synthesis of highly efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts. The catalysts are synthesized through the fine-tuning of metal ions (M, specifically Co, Ni, Zn, and Cu) in Prussian blue analogues (PBAs) and thus termed as M-PBAs. The CoNi-PBA-2 catalyst shows the highest activity toward OER with an onset potential at 280 mV and a Tafel slope of 63 mV dec-1. Zn-PBA catalysts demonstrate high selectivity in two-electron-transfer ORR. The H2O2 yield is as high as 88% at 0 V vs RHE. Density functional theory (DFT) calculations also confirm the high selectivity of Zn-PBA toward H2O2 in ORR.

Nickel particle-enabled width-controlled growth of bilayer molybdenum disulfide nanoribbons.

Transition metal dichalcogenides exhibit a variety of electronic behaviors depending on the number of layers and width. Therefore, developing facile methods for their controllable synthesis is of central importance. We found that nickel nanoparticles promote both heterogeneous nucleation of the first layer of molybdenum disulfide and simultaneously catalyzes homoepitaxial tip growth of a second layer via a vapor-liquid-solid (VLS) mechanism, resulting in bilayer nanoribbons with width controlled by the nanoparticle diameter. Simulations further confirm the VLS growth mechanism toward nanoribbons and its orders of magnitude higher growth speed compared to the conventional noncatalytic growth of flakes. Width-dependent Coulomb blockade oscillation observed in the transfer characteristics of the nanoribbons at temperatures up to 60 K evidences the value of this proposed synthesis strategy for future nanoelectronics.

Study on Large Deformation Behavior of Polyacrylamide Hydrogel Using Dissipative Particle Dynamics.

Meso-scale models for hydrogels are crucial to bridge the conformation change of polymer chains in micro-scale to the bulk deformation of hydrogel in macro-scale. In this study, we construct coarse-grain bead-spring models for polyacrylamide (PAAm) hydrogel and investigate the large deformation and fracture behavior by using Dissipative Particle Dynamics (DPD) to simulate the crosslinking process. The crosslinking simulations show that sufficiently large diffusion length of polymer beads is necessary for the formation of effective polymer. The constructed models show the reproducible realistic structure of PAAm hydrogel network, predict the reasonable crosslinking limit of water content and prove to be sufficiently large for statistical averaging. Incompressible uniaxial tension tests are performed in three different loading rates. From the nominal stress-stretch curves, it demonstrated that both the hyperelasticity and the viscoelasticity in our PAAm hydrogel models are reflected. The scattered large deformation behaviors of three PAAm hydrogel models with the same water content indicate that the mesoscale conformation of polymer network dominates the mechanical behavior in large stretch. This is because the effective chains with different initial length ratio stretch to straight at different time. We further propose a stretch criterion to measure the fracture stretch of PAAm hydrogel using the fracture stretch of C-C bonds. Using the stretch criterion, specific upper and lower limits of the fracture stretch are given for each PAAm hydrogel model. These ranges of fracture stretch agree quite well with experimental results. The study shows that our coarse-grain PAAm hydrogel models can be applied to numerous single network hydrogel systems.