Vertical van der Waals heterostructures are formed when different 2D crystals are stacked on top of each other. Improved optical properties arise in semiconducting transition metal dichalcogenide ...(TMD) 2D materials, such as MoS2, when they are stacked onto the insulating 2D hexagonal boron nitride (h-BN). Most work to date has required mechanical exfoliation of at least one of the TMDs or h-BN materials to form these semiconductor:insulator structures. Here, we report a direct all-CVD process for the fabrication of high-quality monolayer MoS2:h-BN vertical heterostructured films with isolated MoS2 domains distributed across 1 cm. This is enabled by the use of few-layer h-BN films that are more robust against decomposition than monolayer h-BN during the MoS2 growth process. The MoS2 domains exhibit different growth dynamics on the h-BN surfaces compared to bare SiO2, confirming that there is strong interaction between the MoS2 and underlying h-BN. Raman and photoluminescence spectroscopies of CVD-grown MoS2 are compared to transferred MoS2 on both types of substrates, and our results show directly grown MoS2 on h-BN films have smaller lattice strain, lower doping level, cleaner and sharper interfaces, and high-quality interlayer contact.
A power distribution grid exhibits the characteristics of a weak grid owing to the existence of scattered high-power distributed power-generation devices. The grid impedance affects the robust ...stability of grid-connected inverters, leading to harmonic resonance, or even instability in the system. Therefore, a study of the stability of grid-connected inverters with high grid impedance based on impedance analysis is presented in this paper. The output impedance modeling of an LCL-type single-phase grid-connected inverter is derived, where the effects of the PLL loop and the digital control delays on the output impedance characteristics have been taken into account. To enhance the stability of grid-connected inverters with different grid impedance, a novel impedance-phased compensation control strategy is proposed by increasing the phase margin of the grid-connected inverters. Specifically, a detailed implementation and parameter design of the impedance-phased compensation control method is depicted. Finally, an impedance-phased dynamic control scheme combined with online grid impedance measurement is introduced and also verified by the experiment results.
Layered transition metal dichalcogenides (TMDs) offer monolayer 2D systems with diverse properties that extend beyond what graphene alone can achieve. The properties of TMDs are heavily influenced by ...the atomic structure and in particular imperfects in the crystallinity in the form of vacancy defects, grain boundaries, cracks, impurity dopants, ripples and edge terminations. This review will cover the current knowledge of the detailed structural forms of some of the most intensively studied 2D TMDs, such as MoS
2
, WSe
2
, MoTe
2
, WTe
2
, NbSe
2
, PtSe
2
, and also covers MXenes. The review will utilize results achieved using state-of-the-art aberration corrected transmission electron microscopy, including annular dark-field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS), showing how elemental discrimination can be achieved to understand structure at a deep level. The review will also cover the impact of single atom substitutional dopants, such as Cr, V and Mn, and electron energy loss spectroscopy used to understand the local bonding configuration. It is expected that this review will provide an atomic level understanding of 2D TMDs with a connection to imperfections that can arise from chemical vapour deposition synthesis, intentional doping, rips and tears, dislocations, strain, polycrystallinity and confinement to nanoribbons.
Transmission electron microscopy can directly image the detailed atomic structure of layered transition metal dichalcogenides, revealing defects and dopants.
Split-step quintic B-spline collocation (SS5BC) methods are constructed for nonlinear Schrödinger equations in one, two and three dimensions in this paper. For high dimensions, new notations are ...introduced, which make the schemes more concise and achievable. The solvability, conservation and linear stability are discussed for the proposed methods. Numerical tests are carried out, and the present schemes are numerically verified to be convergent with second-order in time and fourth-order in space. The conserved quantity is also computed which agrees with the exact one. And solitary waves in one, two and three dimensions are simulated numerically which coincide with the exact ones. The SS5BC scheme is compared with the split-step cubic B-spline collocation (SS3BC) method in the numerical tests, and the former scheme is more efficient than the later one. Finally, the SS5BC scheme is also applied to compute Bose-Einstein condensates.
Abstract
Ionic conductive hydrogels prepared from naturally abundant cellulose are ideal candidates for constructing flexible electronics from the perspective of commercialization and environmental ...sustainability. However, cellulosic hydrogels featuring both high mechanical strength and ionic conductivity remain extremely challenging to achieve because the ionic charge carriers tend to destroy the hydrogen-bonding network among cellulose. Here we propose a supramolecular engineering strategy to boost the mechanical performance and ionic conductivity of cellulosic hydrogels by incorporating bentonite (BT) via the strong cellulose-BT coordination interaction and the ion regulation capability of the nanoconfined cellulose-BT intercalated nanostructure. A strong (compressive strength up to 3.2 MPa), tough (fracture energy up to 0.45 MJ m
−3
), yet highly ionic conductive and freezing tolerant (high ionic conductivities of 89.9 and 25.8 mS cm
−1
at 25 and −20 °C, respectively) all-natural cellulose-BT hydrogel is successfully realized. These findings open up new perspectives for the design of cellulosic hydrogels and beyond.
The effects of natural variability, especially El Niño-Southern Oscillation (ENSO) effects, have been the focus of several recent studies on the change of drought patterns with climate change. The ...interannual relationship between ENSO and the global climate is not stationary and can be modulated by the Pacific Decadal Oscillation (PDO). However, the global land distribution of the dry-wet changes associated with the combination of ENSO and the PDO remains unclear. In the present study, this is investigated using a revised Palmer Drought Severity Index dataset (sc_PDSI_pm). We find that the effect of ENSO on dry-wet changes varies with the PDO phase. When in phase with the PDO, ENSO-induced dry-wet changes are magnified with respect to the canonical pattern. When out of phase, these dry-wet variations weaken or even disappear. This remarkable contrast in ENSO's influence between the two phases of the PDO highlights exciting new avenues for obtaining improved global climate predictions. In recent decades, the PDO has turned negative with more La Niña events, implying more rain and flooding over land. La Niña-induced wet areas become wetter and the dry areas become drier and smaller due to the effects of the cold PDO phase.
Owing to the development of electronic devices moving toward high power density, miniaturization, and multifunction, research on thermal interface materials (TIMs) is become increasingly significant. ...Graphene is regarded as the most promising thermal management material owing to its ultrahigh in‐plane thermal conductivity. However, the fabrication of high‐quality vertical graphene (VG) arrays and their utilization in TIMs still remains a big challenge. In this study, a rational approach is developed for growing VG arrays using an alcohol‐based electric‐field‐assisted plasma enhanced chemical vapor deposition. Alcohol‐based carbon sources are used to produce hydroxyl radicals to increase the growth rate and reduce the formation of defects. A vertical electric field is used to align the graphene sheets. Using this method, high‐quality and vertically aligned graphene with a height of 18.7 µm is obtained under an electric field of 30 V cm−1. TIMs constructed with the VG arrays exhibit a high vertical thermal conductivity of 53.5 W m−1 K−1 and a low contact thermal resistance of 11.8 K mm2 W−1, indicating their significant potential for applications in heat dissipation technologies.
An alcohol‐based electric‐field‐assisted plasma enhanced chemical vapor deposition method is developed to grow vertical graphene (VG) arrays with high thermal conductivity. Using this method, high‐quality and vertically aligned graphene sheets at a height of 18.7 µm are obtained. Thermal interface materials constructed with these VG arrays exhibit excellent thermal properties for the heat dissipation of electrical devices.
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•The content of antibiotics and ARGs on PE was decreased with salinity.•PE MPs can selectively enrich antibiotics, ARGs and microbe in various waters.•Bacterial community on PE MPs ...differed from surrounding water by antibiotics.•ARGs and bacterial diversity on PE MPs increased in river water.•Bacteria on PE MPs were more susceptible to antibiotics in marine water.
The partition of antibiotics and antibiotic resistant genes (ARGs) between the microplastics (MPs) and the surrounding water with various salinity are still unclear. In this study, we hypothesized that adsorption of antibiotics on MPs might cause a significant change of the structure of microbial communities, diversity and abundance of ARGs on MPs and this might be further affected by change of salinity. In this study, we investigated adsorption of four common antibiotics (sulfamerazine, tetracycline, chloramphenicol and tylosin) to polyethylene (PE) MPs in river, estuary and marine waters, and the differences of antibiotic resistant genes (ARGs) and bacterial communities on MPs and in the three waters. The results showed that MPs can enrich antibiotics, ARGs and microbes from the surrounding water. Elevated salinity could reduce adsorption of antibiotics to MPs and the abundance of ARGs. For example, MPs can concentrate more antibiotics and ARGs in the fresh river water than in the estuary and the marine waters. In addition, ARGs and bacterial communities on MPs at various salinity were significantly different under the pressure of four antibiotics. On MPs, sul1, sulA/folP-01, tetA, tetC, tetX and ermE increased significantly but a few new ARGs such as sulA/folP-01 and tetA appeared. The structure of the bacterial communities on MPs was different from the surrounding water since some bacteria species found on MPs were barely detected in the surrounding water while some genera on MPs vanished after exposure to antibiotics. As the antibiotics adsorbed and the ARGs on MPs decreased with the water salinity, the structure of the communities on MPs thus varied with salinity change. These findings are important to understand the effects of MPs on the transport, fate and ecological risk of antibiotics and ARGs in different aquatic environments.
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•Lemon seed residue was utilized as a raw material for CNCs extraction.•CNCs were isolated using acid hydrolysis, APS oxidation and TEMPO oxidation.•Extraction method can influence ...the structure and properties of CNCs.•CNCs by acid hydrolysis and APS oxidation can stabilize Pickering emulsions better.
In this study, the lemon (Citrus limon) seeds as typical agricultural processing wastes were utilized to extract cellulose nanocrystals (CNCs) by sulfuric acid hydrolysis (S-LSCNC), ammonium persulfate oxidation (A-LSCNC) and TEMPO oxidation (T-LSCNC). The properties of CNCs were comparatively investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TG), and atomic force microscope (AFM), and the application in Pickering emulsions was also preliminarily studied. The results showed that all CNCs maintained cellulose Iβ structure and had a good dispersion regardless of extraction methods. Differently, T-LSCNC had a higher yield, larger size and lower CrI than A-LSCNC and S-LSCNC. Comparatively, A-LSCNC showed the highest CrI and S-LSCNC showed the lowest size. For the application of Pickering emulsions, S-LSCNC and A-LSCNC showed a better ability as Pickering stabilizers than T-LSCNC. This study is beneficial for developing the potential utilization of CNCs from lemon by-products.
In recent years, the number and variety of malicious mobile apps have increased drastically, especially on Android platform, which brings insurmountable challenges for malicious app detection. ...Researchers endeavor to discover the traces of malicious apps using network traffic analysis. In this study, we combine network traffic analysis with machine learning methods to identify malicious network behavior, and eventually to detect malicious apps. However, most network traffic generated by malicious apps is benign, while only a small portion of traffic is malicious, leading to an imbalanced data problem when the traffic model skews towards modeling the benign traffic. To address this problem, we introduce imbalanced classification methods, including the synthetic minority oversampling technique (SMOTE) + support vector machine (SVM), SVM cost-sensitive (SVMCS), and C4.5 cost-sensitive (C4.5CS) methods. However, when the imbalance rate reaches a certain threshold, the performance of common imbalanced classification algorithms degrades significantly. To avoid performance degradation, we propose to use the imbalanced data gravitation-based classification (IDGC) algorithm to classify imbalanced data. Moreover, we develop a simplex imbalanced data gravitation classification (S-IDGC) model to further reduce the time costs of IDGC without sacrificing the classification performance. In addition, we propose a machine learning based comparative benchmark prototype system, which provides users with substantial autonomy, such as multiple choices of the desired classifiers or traffic features. Using this prototype system, users can compare the detection performance of different classification algorithms on the same data set, as well as the performance of a specific classification algorithm on multiple data sets.
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