Electronic structures and elastic properties of monolayer and bilayer transition metal dichalcogenides MX_2(M= Mo,W;X= O,S,Se,Te):A comparative first-principles study

First-principle calculations with different exchange-correlation functionals, including LDA, PBE, and vd W-DF functional in the form of opt B88-vd W, have been performed to investigate the electronic and elastic properties of twodimensional transition metal dichalcogenides（TMDCs） with the formula of MX2（M = Mo, W; X = O, S, Se, Te） in both monolayer and bilayer structures. The calculated band structures show a direct band gap for monolayer TMDCs at the K point except for MoO2 and WO2. When the monolayers are stacked into a bilayer, the reduced indirect band gaps are found except for bilayer WTe2, in which the direct gap is still present at the K point. The calculated in-plane Young moduli are comparable to that of graphene, which promises possible application of TMDCs in future flexible and stretchable electronic devices. We also evaluated the performance of different functionals including LDA, PBE, and opt B88-vd W in describing elastic moduli of TMDCs and found that LDA seems to be the most qualified method. Moreover, our calculations suggest that the Young moduli for bilayers are insensitive to stacking orders and the mechanical coupling between monolayers seems to be negligible.

Layout design and application of 4D-printing bio-inspired structures with programmable actuators

Four-dimensional(4D)printing is an advanced form of three-dimensional(3D)printing with controllable and programmable shape transformation over time.Actuators are used as a controlling factor with multi-stage shape recovery,with emerging opportunities to customize the mechanical properties of bio-inspired structures.The print pattern of shape memory polymer(SMP)fbers strongly afects the achievable resolution,and consequently infuences several other physical and mechanical properties of fabricated actuators.However,the deformations of bio-inspired structures due to actuator layout are more complex because of the presence of the coupling of multi-directional strain.In this study,the initial structure was designed from closed-shell behavior and divided into a general unit and actuator unit,the latter responsible for driving the transformation.Mutual stress confrontation between the actuator and the general unit was considered in the layout thermodynamic model,in order to eliminate the transformation produced by the uncontrolled shape memory behavior of the general unit.Three critical and efective strategies for the layout design of actuators were proposed and then applied to achieve the desired accurate deformation of 3D-printed bilayer structures.Finally,the proposed approach was validated and adopted for fabricating a complex shell-like gripper structure.

Complicated deformation simulating on temperature-driven 4D printed bilayer structures based on reduced bilayer plate model

The four-dimensional(4D) printing technology, as a combination of additive manufacturing and smart materials, has attracted increasing research interest in recent years. The bilayer structures printed with smart materials using this technology can realize complicated deformation under some special stimuli due to the material properties.The deformation prediction of bilayer structures can make the design process more rapid and thus is of great importance. However, the previous works on deformation prediction of bilayer structures rarely study the complicated deformations or the influence of the printing process on deformation. Thus, this paper proposes a new method to predict the complicated deformations of temperature-sensitive 4D printed bilayer structures,in particular to the bilayer structures based on temperature-driven shape-memory polymers(SMPs) and fabricated using the fused deposition modeling(FDM) technology. The programming process to the material during printing is revealed and considered in the simulation model. Simulation results are compared with experiments to verify the validity of the method. The advantages of this method are stable convergence and high efficiency,as the three-dimensional(3D) problem is converted to a two-dimensional(2D) problem.The simulation parameters in the model can be further associated with the printing parameters, which shows good application prospect in 4D printed bilayer structure design.

Syntheses, Crystal Structures and Properties of Two Metal-organic Complexes Based on 5-(4-Pyridyl)-methoxyl Isophthalic Acid

Two new metal-organic frameworks, [Co3L2（NO2）2（H2O）2],, （1） and{[Ni（L）（H2O）5] （H2O）2.DMF}, （2, H2L = 5-（4-pyridyl）-methoxyl isophthalic acid）, have been synthesized by the hydrothermal method and characterized by elemental analysis, infrared spectroscopy, thermogravimetric analysis （TGA） and X-ray crystallography. Compound 1 crystallizes in the monoclinic system with space group P2/c and adopts a slightly distorted octahedral configuration. In compound 1, the 2D bilayered structures are linked by O-H...O and O-H...N hydrogen bonds to form a 3D framework. Compound 2 crystallizes in the monoclinic system with space group P2/n, and the central nickel atoms are octa-coordinated with five O atoms from coordinated water molecules and one N atom from one H2L ligand. The abundant O-H…O hydrogen bonds and π…π interactions link the molecules into a 3D framework. In addition, compounds 1 and 2 exhibit strong ultraviolet absorption in the solid state at room temperature.

Improving photoelectrochemical activity of dye sensitized solar cell by a bilayered electrode with an overlayer of mesoporous anatase TiO_2

For better performance of dye sensitized solar cells （DSSCs）, a bilayer structured electrode was constructed by employing a mesoporous anatase TiO2 overlayer above a commercial P25 TiO2 nanoparticles underlayer. The mesoporous anatase TiO2, prepared through a facile surfactant-assisted sol-gel process, possessed large pore size and well inter-connected network structure, both beneficial for dye adsorption and electron transfer. The dye adsorption capability of the mesoporous TiO2 was nearly twice that of the P25 counterpart. In the electrode, the mesoporous TiO2 film enhanced both dye adsorption and lightharvest, to increase photocurrent （Jsc） from 12.32 to 14.78 mA/cm^2. Compared to the single P25 TiO2 film, the synergy of the mesoporous TiO2 and the P25 TiO2 nanoparticle films in the electrode resulted in a 24% improvement in light-to-electricity conversion efficiency （η）. This bilayered electrode provides an alternative approach for further developing a photovoltaic device with better cell performance.

Self-sensing magnetic actuators of bilayer hydrogels

Hard magnetic soft robots have been widely used in biomedical engineering.In these applications,it is crucial to sense the movement of soft robots and their interaction with target objects.Here,we propose a strategy to fabricate a self-sensing bilayer actuator by combining magnetic and ionic conductive hydrogels.The magnetic hydrogel containing NdFeB particles exhibits rapid response to magnetic field and achieve bending deformation.Meanwhile,the polyacrylamide(PAAm)hydrogel with lithium chloride(LiCl)allows for the sensing of deformation.The bending behavior of the bilayer under magnetic field is well captured by theoretical and simulated models.Additionally,the bilayer strain sensor shows good sensitivity,stability and can endure a wide-range cyclic stretching(0-300%).These merits qualify the self-sensing actuator to monitor the motion signals,such as bending of fingers and grasping process of an intelligent gripper.When subject to an external magnetic field,the gripper can grab a cube and sense the resistance change simultaneously to detect the object size.This work may provide a versatile strategy to integrate actuating and self-sensing ability in soft robots.

Bilayer carbon-based structure with the promotion of homogenous nucleation for lithium metal anodes

Lithium metal anodes (LMAs) are considered as the promising alternatives for next-generation high energy density batteries,but are still hampered by the severe growth of uncontrollable lithium dendrites.The growth of lithium dendrites induces poor cycling lifespan and serious safety concerns,dragging lithium metal batteries out of practical applications.We designed a bilayer carbonbased structure covered with Co/C nanosheets and vertical graphene sheets (VGS).The enormous specific surface area and uniformly distributed Co nanoparticles of the CC@Co/C-VGS host are derived from its unique design,which can reduce local current density and nucleation overpotential,resulting in a dendrite-free morphology and exceptional cycling stability.Symmetric cells exhibit over 400 cycles (800 h) at a high current density/capacity of 10 mA cm^(-2)/10 mA h cm^(-2).Full cells using LiFePO_(4)as the cathode have an enhanced rate capability and a prolonged lifespan,reaching 90 mA h g^(-1)after 1000 cycles at 2 C with 73.5% capacity retention.This unique design sheds light on developing high-performance LMAs.

Terahertz broadband polarizer using bilayer subwavelength metal wire-grid structure on polyimide film

A tcrahcrtz （THz） broadband polarizer using bilayer subwavelength metal wire-grid structure on both sides of polyimide fihn is simulated by the finite-difference time-domain method. We amdyze tile effect of film thickness, material loss, and lateral shift between two metallic gratings on the performance of the THz polarizer. Bilayer wire-grid polarizers are fabricated by a simple way of laser induced and non-electrolytic plating with copper. The THz time-domain spectroscopy measurements show that in 0.2 1.6 THz frequency range, the extinction ratio is better than 45 dB, the average extinction ratio reaches 53 dB, and the transmittance exceeds 67%, which shows great advantage over conventional single wire-grid THz polarizer.

Bilayer Structured Nanowire-array and Nanotube-cluster TiO2 Photoanode for Efficient Dye-sensitized Solar Cells

Nanowire-array nanotube-cluster bilayer TiO2 films, consisting of nanowire-array at the bottom layer and nanotube-cluster at the top layer were prepared via a two-step hydrothermal method. One step was a growth process and the other was an etching process. Etching process was used to acquire nanowire-array with a small diameter of 20--25 nm of the nanowire and nanotube-cluster. The bilayer nanostructured films were then modified with TiCI4. Villous TiO2 attached to both the inside and outside wails of the nanotube cluster was obtained only by changing the traditional concentration of TiCla-treatment to 0.2 mol/L. The novel bilayer photoanode finally revealed a higher effi- ciency of 5.25% when used in dye-sensitized solar cells（DSSCs）.

Magnetic properties of bilayer nano-stanenelike structure with Ruderman-Kittel-Kasuya-Yoshida coupling

A bilayer nano-stanene-like structure with Ruderman-Kittel-Kasuya-Yoshida(RKKY)coupling described by the Ising model is proposed. The magnetic and thermodynamic properties are studied using the effective-field theory with correlations. The exchange coupling,longitudinal magnetic field, number of non-magnetic layers, and anisotropies had major influences on the magnetization, specific heat, and internal energy. Different saturation magnetizations are observed on the magnetization curve. The variation in the system blocking temperature is studied. The results provide theoretical guidance for the magnetic investigation of nanomaterials with RKKY coupling.