Precisely controlling the twist angle of epitaxial MoS_(2)/graphene heterostructure by AFM tip manipulation

Two-dimensional(2D)moirématerials have attracted a lot of attention and opened a new research frontier of twistronics due to their novel physical properties.Although great progress has been achieved,the inability to precisely and reproducibly manipulate the twist angle hinders the further development of twistronics.Here,we demonstrated an atomic force microscope(AFM)tip manipulation method to control the interlayer twist angle of epitaxial MoS_(2)/graphene heterostructure with an ultra-high accuracy better than 0.1°.Furthermore,conductive AFM and spectroscopic characterizations were conducted to show the effects of the twist angle on moirépattern wavelength,phonons and excitons.Our work provides a technique to precisely control the twist angle of 2D moirématerials,enabling the possibility to establish the phase diagrams of moiréphysics with twist angle.

Twisted Integration of Complex Oxide Magnetoelectric Heterostructures via Water‑Etching and Transfer Process

Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering.In recent years,lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations.Moreover,twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures.A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process,which could provide some insight in to the physical phenomena.In this work,the La_(0.67)Sr_(0.33)MnO_(3)(001)/0.7Pb(Mg_(1/3)Nb_(2/3))O_(3)-0.3PbTiO_(3)(011)(LSMO/PMN-PT)heterostructures with 45.and 0.twist angles were assembled via water-etching and transfer process.The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO<110>.A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO[110]easy axis is observed for the 45°Sample by applying a 7.2 kV cm^(−1)electrical field,significantly different from a uniaxial anisotropy with LSMO[100]easy axis for the 0°Sample.The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45°twist angle causes different lattice distortion of LSMO,thereby enhancing both the fourfold and uniaxial anisotropy.This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures.

Twist-angle two-dimensional superlattices and their application in(opto)electronics

Twist-angle two-dimensional systems,such as twisted bilayer graphene,twisted bilayer transition metal dichalcogenides,twisted bilayer phosphorene and their multilayer van der Waals heterostructures,exhibit novel and tunable properties due to the formation of Moirésuperlattice and modulated Moirébands.The review presents a brief venation on the development of"twistronics"and subsequent applications based on band engineering by twisting.Theoretical predictions followed by experimental realization of magic-angle bilayer graphene ignited the flame of investigation on the new freedom degree,twistangle,to adjust(opto)electrical behaviors.Then,the merging of Dirac cones and the presence of flat bands gave rise to enhanced light-matter interaction and gate-dependent electrical phases,respectively,leading to applications in photodetectors and superconductor electronic devices.At the same time,the increasing amount of theoretical simulation on extended twisted 2D materials like TMDs and BPs called for further experimental verification.Finally,recently discovered properties in twisted bilayer h-BN evidenced h-BN could be an ideal candidate for dielectric and ferroelectric devices.Hence,both the predictions and confirmed properties imply twist-angle two-dimensional superlattice is a group of promising candidates for next-generation(opto)electronics.

Analysis on the Characterization of the Twist Insertion Level and Its Effect of the Compact and Ring Spun Yarns

In this paper, the difference of the twist insertion levels of the compact and ring spun yarns of the same count is analyzed based on the relationship of the yarn twist factor to the twist angle and the bulk density. It is proposed that the relationship of the twist angle, twist level, and the yarn diameter should be synthetically considered when evaluating the twist insertion level of the compact yarn. The twist angle of the compact yarn is smaller than that of the ring yarn with the same twist insertion level. This results from the ordered fiber arrangement and compact structure of the compact yarn. Experiment was conducted to verify the conclusions. It is also discovered that the selection of the twist factor of the compact yarn, which can be usually lower than that of the ring yarn by 10%-15%, can be determined based on the yarn tensile strength.

Vibration and wave propagation analysis of twisted micro-beam using strain gradient theory

In this research, vibration and wave propagation analysis of a twisted micro- beam on Pasternak foundation is investigated. The strain-displacement relations （kine-matic equations） are calculated by the displacement fields of the twisted micro-beam. The strain gradient theory （SGT） is used to implement the size dependent effect at micro-scale. Finally, using an energy method and Hamilton＇s principle, the governing equations of motion for the twisted micro-beam are derived. Natural frequencies and the wave prop- agation speed of the twisted micro-beam are calculated with an analytical method. Also, the natural frequency, the phase speed, the cut-off frequency, and the wave number of the twisted micro-beam are obtained by considering three material length scale parameters, the rate of twist angle, the thickness, the length of twisted micro-beam, and the elastic medium. The results of this work indicate that the phase speed in a twisted micro-beam increases with an increase in the rate of twist angle. Moreover, the wave number is in- versely related with the thickness of micro-beam. Meanwhile, it is directly related to the wave propagation frequency. Increasing the rate of twist angle causes the increase in the natural frequency especially with higher thickness. The effect of the twist angle rate on the group velocity is observed at a lower wave propagation frequency.

Novel cell parameter determination of a twisted-nematic liquid crystal display

In this paper a novel method is proposed to determine the cell parameters including the twist angle, optic retardation and rubbing direction of twisted-nematic liquid crystal displays （TNLCD） by rotating the TNLCD. It is a single-wavelength method. Because using subtraction equation of transmittance as curve fitting equation, the influence of the light from environment and the absorption by polarizer, the sample of TNLCD and analyser on the transmittance is eliminated. Accurate results can also be obtained in imperfect darkness. By large numbers of experiments, we found that not only the experimental setup is quite simple and can be easily adopted to be carried out, but also the results are accurate.

Geometric effects of cross sections on equilibrium of helical and twisted ribbon

In the framework of elastic rod model, the Euler-Lagrange equations characterizing the equilibrium configuration of the polymer chain are derived from a free energy functional associated with the curvature, torsion, twisting angle, and its derivative with respect to the arc-length. The configurations of the helical ribbons with different cross-sectional shapes are given. The effects of the elastic properties, the cross-sectional shapes, and the intrinsic twisting on the helical ribbons are discussed. The results show that the pitch angle of the helical ribbon decreases with the increase in the ratio of the twisting rigidity to the bending rigidity and approaches the intrinsic twisting. If the bending rigidity is much greater than the twisting rigidity, the bending and twisting of the helical ribbon always appear simultaneously.

Interface engineering in two-dimensional heterostructures towards novel emitters

Two-dimensional(2D) semiconductors have captured broad interest as light emitters, due to their unique excitonic effects. These layer-blocks can be integrated through van der Waals assembly, i.e., fabricating homo-or heterojunctions, which show novel emission properties caused by interface engineering. In this review, we will first give an overview of the basic strategies that have been employed in interface engineering, including changing components, adjusting interlayer gap, and tuning twist angle. By modifying the interfacial factors, novel emission properties of emerging excitons are unveiled and discussed. Generally, well-tailored interfacial energy transfer and charge transfer within a 2D heterostructure cause static modulation of the brightness of intralayer excitons. As a special case, dynamically correlated dual-color emission in weakly-coupled bilayers will be introduced, which originates from intermittent interlayer charge transfer. For homobilayers and type Ⅱ heterobilayers, interlayer excitons with electrons and holes residing in neighboring layers are another important topic in this review. Moreover, the overlap of two crystal lattices forms moiré patterns with a relatively large period, taking effect on intralayer and interlayer excitons. Particularly, theoretical and experimental progresses on spatially modulated moiré excitons with ultra-sharp linewidth and quantum emission properties will be highlighted. Moiré quantum emitter provides uniform and integratable arrays of single photon emitters that are previously inaccessible, which is essential in quantum many-body simulation and quantum information processing. Benefiting from the optically addressable spin and valley indices, 2D heterostructures have become an indispensable platform for investigating exciton physics, designing and integrating novel concept emitters.

Thermal transport in twisted few-layer graphene

Twisted graphene possesses unique electronic properties and applications, which have been studied extensively. Recently, the phonon properties of twisted graphene have received a great deal of attention. To the best of our knowledge,thermal transports in twisted graphene have been investigated little to date. Here, we study perpendicular and parallel transports in twisted few-layer graphene（T-FLG）. It is found that perpendicular and parallel transports are both sensitive to the rotation angle θ between layers. When θ increases from 0° to 60°, perpendicular thermal conductivity κ（｜｜） first decreases and then increases, and the transition angle is θ = 30°. For the parallel transport, the relation between thermal conductivity κand θ is complicated, because intra-layer thermal transport is more sensitive to the edge of layer than their stacking forms. However, the dependence of interlayer scattering on θ is similar to that of κ⊥. In addition, the effect of layer number on the thermal transport is discussed. Our results may provide references for designing the devices of thermal insulation and thermal management based on graphene.

Forward hot extrusion forming process of 4-lobe aluminum alloy helical surface rotor

The 4-lobe aluminum alloy helical surface rotors are widely applied in industry,such as superchargers.Generally,the conventional manufacturing processes of aluminum alloy helical surface are time consuming and costly.To make the manufacturing processes more flexible and economical,the forward hot extrusion process is proposed to form the 4-lobe aluminum alloy helical surface rotors.In this work,we implement both simulations and experiments to the forming process of the helical surface,of which the material is 6063 aluminum alloy.The forward hot extrusion process is simulated with finite element method in DEFORM-3D.Based on the simulation method,the influences of different extrusion parameters,such as extrusion temperature,extrusion speed and extrusion ratio,on the extrusion process are studied.According to the numerical simulation results,the optimal case is chosen to carry out the experiment.Furthermore,the experimental results show that the surface is smooth;the toothed fill is full;the twist angle in the length direction is evenly distributed;the value of twist angle is roughly in line with the design angle,which is mainly due to the modified die structure,having a positive and significant effect on the increment of twist angle.Therefore,the twist angle has an increase of about 76%,which verifies the modified die structure.

Synthesis, Crystal Structure, Physical Properties, and Application of a Series of Functional Dibenzo[d,f][1,3]dioxepine Derivatives

The twisted aromatics, functional dibenzo[d,f][1,3]dioxepine derivatives were synthesized in high yields from reactions of 5,5＇-dibromo-2,2＇-biphenol with corresponding ketal or ketone compounds under acid catalysis. The structures of these compounds were characterized by ^1H NMR, elemental analysis, UV-Vis absorption spectrum and X-ray diffraction analysis. The conformation of O--C--O bridged biphenyl derivatives with varied substitute groups on 6,6＇-position was studied by X-ray crystallography and force-field simulation. The result of calculations by UNIVERSAL 1.02 force field in Cerius2 package（4.6） indicates that dibenzo[d,f][1,3]dioxepine derivatives show twisted conformations and the twisted angle between the phenyl rings is about 40°, which is accordant with the result from crystal structure determination, though the obtained angles in the crystal of dibenzo[d,f][1,3]dioxepine derivatives with the varied substitute groups on 6,6＇-position are shown to be slightly shifted to 40° owing to intermolecular interactions in crystal stacking. DSC studies exhibit that the substitute groups on 6,6＇-position can induce a large variation of endothermic peaks ranging from 80 to 135 ℃. The conjugated polymers based on dibenzo[d,f][1,3]dioxepine derivatives, which have ultraviolet emitting with a quantum efficiency of 10%, were obtained by Yamamoto coupling.

Shear deformable finite beam elements for composite box beams

The shear deformable thin-walled composite beams with closed cross-sections have been developed for coupled flexural, torsional, and buckling analyses. A theoretical model applicable to the thin-walled laminated composite box beams is presented by taking into account all the structural couplings coming from the material anisotropy and the shear deformation effects. The current composite beam includes the transverse shear and the restrained warping induced shear deformation by using the first-order shear deformation beam theory. Seven governing equations are derived for the coupled axial-flexural-torsional-shearing buckling based on the principle of minimum total potential energy. Based on the present analytical model, three different types of finite composite beam elements, namely, linear, quadratic and cubic elements are developed to analyze the flexural, torsional, and buckling problems. In order to demonstrate the accuracy and superiority of the beam theory and the finite beam elements developed by this study,numerical solutions are presented and compared with the results obtained by other researchers and the detailed threedimensional analysis results using the shell elements of ABAQUS. Especially, the influences of the modulus ratio and the simplified assumptions in stress-strain relations on the deflection, twisting angle, and critical buckling loads of composite box beams are investigated.

Entanglement-free determination of pretilt angles of twisted nematic liquid-crystal cells by phase measurement

Because of low cost, fast response time, and high light transmittance, thin-film-transistor （TFT） driven twisted nematic （TN） liquid-crystal displays （LCDs） have been widely used in calculators, computer screens, and cell phones. The pretilt angle of the TN medium within the TFT-TN panel affects not only its response times and view- ing angles but also the light-leakage positions of fringed- field-induced disclination lines within pixels of theTFT-TN panel.

Moirésuperlattice effects on interfacial mechanical behavior:A concise review

The moirésuperlattice,arising from the interface of mismatched single crystals,intricately regulates the physical and mechanical properties of materials,giving rise to phenomena such as superconductivity and superlubricity.This study delves into the profound impact of moirésuperlattices on the interfacial mechanical behavior of van der Waals(vdW)layered materials,with a particular focus on tribological properties.A comprehensive review of continuum modeling approaches for vdW layered materials is presented,accentuating the incorporation of moirésuperlattice effects in theoretical models to unravel their distinctive interfacial frictional behavior and thermodynamic properties.The exploration of moirésuperlattices has significantly advanced our fundamental understanding of interface phenomena in vdW layered materials.This progress provides crucial theoretical insights that can inform the design of multifunctional devices based on the unique properties of twisted layered materials.

Raman scattering investigation of twisted WS_(2)/MoS_(2) heterostructures: interlayer mechanical coupling versus charge transfer

Twisted van der Waals homo-and hetero-structures have aroused great attentions due to their unique physical properties,providing a new platform to explore the novel two-dimensional(2D)condensed matter physics.The robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide(TMD)homo-structures.However,the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough attention.Here,we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS_(2)/MoS_(2) heterostructures.The shifts and linewidths of E_(2g)(Γ)and A_(1g)(Γ)phonon modes are found to be twist angle dependent.In particular,analogous to that of the twisted TMD homostructures,the frequency separations between E_(2g)(Γ)and A_(1g)(Γ)modes of MoS_(2) and WS_(2) in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling,essentially originating from the spacing-related repulsion between sulfur atoms.Moreover,the opposite shift behaviors and broadening of A_(1g)(Γ)modes caused by charge transfer are also observed in the twisted heterostructures.The calculated interlayer distances and band alignment of twisted WS_(2)/MoS_(2) through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman features.Such understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.

Controlling exciton-exciton annihilation in WSe_(2) bilayers via interlayer twist

The twist angle between two van der Waals coupled monolayers has emerged as a new and powerful degree of freedom for engineering physical properties of semiconductor homo-and hetero-bilayers.While the interlayer twist has shown prominent effect on electronic and optical properties of transition metal dichalcogenide(TMD)bilayers,it remains unclear how it could be used to manipulate the exciton dynamics,especially exciton-exciton annihilation(EEA)process which is the dominant energy loss channel in TMDs under moderate to high exciton density due to strong Coulomb interaction.Herein,we show that the twist angle in TMD bilayers can act as an effective knob to control the EEA process.Specifically,EEA rate constant increases from 1° twisted WSe_(2) bilayers(0.026 cm^(2)/s)by more than twice to 32° twisted bilayers(0.053 cm2/s)and then drops again in 60° twisted bilayers(0.019 cm^(2)/s).This twist-angle dependence can be attributed to the energy difference between indirect and direct excitons arising from the interlayer interaction.Our work opens up the possibility of artificially managing the exciton dynamics in TMD materials for optoelectronic applications via interlayer twist angle.

Fatigue crack propagation across grain boundary of Al-Cu-Mg bicrystal based on crystal plasticity XFEM and cohesive zone model

In this paper,a methodology integrating crystal plasticity(CP),the eXtended finite element method(XFEM)and the cohesive zone model(CZM)is developed for an Al-Cu-Mg alloy to predict fatigue crack propagation(FCP)across grain boundary(GB)of Al-Cu-Mg alloy during stageІІ.One GB model is incor-porated into FCP constitutive law to describe grain interaction at GB.A bicrystal containing GB is built up to simulate FCP behavior through L participated GBs.Modelling features including GB characteristic,cumulative plastic strain(CPS)distribution and crystal slipping evidence can be identified.The numer-ical results are compared with published experimental data to check the accuracy of model.This work demonstrates that the combination of CP containing GB constitutive laws,XFEM and CZM is a promising methodology in predicting twist angle-controlled crack deflection through GBs.