Uniaxial Strain Research Articles

Enhancement of remnant polarization in ferroelectric HfO2 thin films induced by mechanical uniaxial tensile strain after the crystallization process

The effect of strain on the ferroelectricity of HfO2 thin films after crystallization was investigated by applying uniaxial mechanical strains to Au/HfO2/TiN metal–ferroelectric–metal (MFM) capacitors. The remnant polarization (2P r) of MFM capacitors increased when tensile strain was applied during polarization switching. This phenomenon should not be attributed to phase transformation from the non-ferroelectric to the ferroelectric phase, taking account of the fast relaxation of 2P r after removal of the mechanical strain and the fact that the crystal structure of HfO2 thin films evaluated by grazing incidence X-ray diffraction measurement was not changed by the tensile strain.

Cyclic strain alters the transcriptional and migratory response of scleral fibroblasts to TGFβ

In glaucoma, scleral fibroblasts are exposed to IOP-associated mechanical strain and elevated TGFβ levels. These stimuli, in turn, lead to scleral remodeling. Here, we examine the scleral fibroblast migratory and transcriptional response to these stimuli to better understand mechanisms of glaucomatous scleral remodeling. Human peripapillary scleral (PPS) fibroblasts were cultured on parallel grooves, treated with TGFβ (2 ng/ml) in the presence of vehicle or TGFβ signaling inhibitors, and exposed to uniaxial strain (1 Hz, 5%, 12–24 h). Axis of cellular orientation was determined at baseline, immediately following strain, and 24 h after strain cessation with 0° being completely aligned with grooves and 90° being perpendicular. Fibroblasts migration in-line and across grooves was assessed using a scratch assay. Transcriptional profiling of TGFβ-treated fibroblasts with or without strain was performed by RT-qPCR and pERK, pSMAD2, and pSMAD3 levels were measured by immunoblot. Pre-strain alignment of TGFβ-treated cells with grooves (6.2 ± 1.5°) was reduced after strain (21.7 ± 5.3°, p < 0.0001) and restored 24 h after strain cessation (9.5 ± 2.6°). ERK, FAK, and ALK5 inhibition prevented this reduction; however, ROCK, YAP, or SMAD3 inhibition did not. TGFβ-induced myofibroblast markers were reduced by strain (αSMA, POSTN, ASPN, MLCK1). While TGFβ-induced phosphorylation of ERK and SMAD2 was unaffected by cyclic strain, SMAD3 phosphorylation was reduced (p = 0.0004). Wound healing across grooves was enhanced by ROCK and SMAD3 inhibition but not ERK or ALK5 inhibition. These results provide insight into the mechanisms by which mechanical strain alters the cellular response to TGFβ and the potential signaling pathways that underlie scleral remodeling.

Predicting maximum strain hardening factor in elongational flow of branched pom-pom polymers from polymer architecture

We present a model-driven predictive scheme for the uniaxial extensional viscosity and strain hardening of branched polymer melts, specifically for the pom-pom architecture, using the small amplitude oscillatory shear mastercurve and the polymer architecture. A pom-pom shaped polymer is the simplest architecture with at least two branching points, needed to induce strain hardening. It consists of two stars, each with q\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$q$$\\end{document} arms of the molecular weight Mw,a\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{w,a}$$\\end{document}, connected by a backbone of Mw,b\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{w,b}$$\\end{document}. Despite the pom-pom constitutive model, experimental data of systematic investigations lack due to synthetic complexity. With an optimized approach, we synthesized polystyrene pom-pom model systems with systematically varied Mw,a\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{w,a}$$\\end{document} and Mw,b\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{w,b}$$\\end{document}. Experimentally, we identify four characteristic strain rate dependent regimes of the extensional viscosity, which can be predicted from the rheological mastercurve. Furthermore, we find that the industrially important maximum strain hardening factor depends only on the arm number by [q2/ln(3q)]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[{q}^{2}/{{{{{\\mathrm{ln}}}}}}({\\sqrt{3}}q)]$$\\end{document}. This framework offers a model-based design of branched polymers with predictable melt flow behavior.

Electride pure α-Zr: interstitial electrons induced type-II nodal line

Electrides have attracted significant attention in the fields of physics, materials science, and chemistry due to their distinctive electron properties characterized by weak nuclear binding. In this study, based on first-principles calculations and symmetry analysis, we report that the pure zirconium with alpha-phase (α-Zr) is expected to be the electrically neutral electride with topological nodal loop. Furthermore, the nodal loop located at the kz = 0 plane exhibits a clear drumhead-like surface state. The energy levels of the topological nodal loop can be regulated by applying uniaxial strain, resulting in the topological nodal loop being closer to the Fermi level. Remarkably, the work function of the electride Zr shows a significant anisotropy along the (001), (100), and (110) directions, particularly with a low work function of 3.14 eV along the (110) surface. Therefore, we predict that α-Zr provides a promising platform for future research on topological electrides.

Epigenetic marks uniquely tune the material properties of HP1α condensates

Biomolecular condensates have emerged as a powerful new paradigm in cell biology with broad implications to human health and disease, particularly in the nucleus where phase separation is thought to underly elements of chromatin organization and regulation. Specifically, it has been recently reported that phase separation of heterochromatin protein 1alpha (HP1α) with DNA contributes to the formation of condensed chromatin states. HP1α localization to heterochromatic regions is mediated by its binding to specific repressive marks on the tail of histone H3, such as trimethylated lysine 9 on histone H3 (H3K9me3). However, whether epigenetic marks play an active role in modulating the material properties of HP1α and dictating emergent functions of its condensates remains to be understood. Here, we leverage a reductionist system, composed of modified and unmodified histone H3 peptides, HP1α, and DNA, to examine the contribution of specific epigenetic marks to phase behavior of HP1α. We show that the presence of histone peptides bearing the repressive H3K9me3 is compatible with HP1α condensates, whereas peptides containing unmodified residues or bearing the transcriptional activation mark H3K4me3 are incompatible with HP1α phase separation. Using fluorescence microscopy and rheological approaches, we further demonstrate that H3K9me3 histone peptides modulate the dynamics and viscoelastic network properties of HP1α condensates in a concentration-dependent manner. Additionally, in cells exposed to uniaxial strain, we find there to be a decreased ratio of nuclear H3K9me3 to HP1α. These data suggest that HP1α-DNA condensates are viscoelastic materials, whose properties may provide an explanation for the dynamic behavior of heterochromatin in cells and in response to mechanostimulation.

Feeling the strain: Enhancing the thermodynamics characteristics of magnesium nickel hydride Mg2NiH4 for hydrogen storage applications through strain engineering

This work is based on density functional theory (DFT) and studies the structural, electronic, thermodynamic properties and dehydrogenation kinetics of the hydride Mg2NiH4 under different uniaxial and biaxial strain applied to Mg2Ni or Mg2NiH4. structural properties are first studied based on generalized gradient (GGA) and local density (LDA). Thermodynamic and electronic properties and dehydrogenation kinetics are then studied using the GGA method based on the Perdew-Burke-Ernzerhor (PPE) approximation. The results obtained show that uniaxial/biaxial tensile and compressive strains are capable of inducing structural deformation by increasing the value of the strain step on both Mg2Ni and Mg2NiH4, and that these strains applied to Mg2NiH4 are capable of reducing the stability of the system more than strains applied to Mg2Ni, due to the contribution of strain energy. More specifically, it lowers the enthalpy of formation to −40 kJ/mol.H2 under compressive strain ε = −7.5% or biaxial tensile strain ε = +7.8%, which corresponds exactly to the ideal value suggested by the US Department of Energy (DOE) for materials suitable for hydrogen storage. And the decomposition temperature fell within the fuel cell operating range (PEM) of 289–393 K. On the other hand, under uniaxial and biaxial strains, the activation energy of hydrogen atom diffusion in Mg2NiH4 varied between 0.40 eV and 0.22 eV, resulting in a remarkable improvement in dehydrogenation properties. Analysis of the total (TDOS) and partial (PDOS) density curves shows that the Mg2NiH4 system has a metallic characteristic, and that this characteristic is maintained by increasing the strain pitch, with a variation in the energy width of the valence band.

Strain tunable optical and transport properties of intrinsic ferromagnetic semiconductor CrOCl monolayer

Based on density functional theory, the electronic structure, transport, and optical properties of monolayer CrOCl are studied theoretically. The electronic structure analysis shows that monolayer CrOCl is a ferromagnetic semiconductor with an indirect band gap of 2.45 eV, and the electronic properties of monolayer CrOCl can be changed significantly by applying external strain. When 12 % uniaxial tensile strain is applied along the a-axis, the band gap of monolayer CrOCl decreases to 1.94 eV, the hole mobility can reach 1520.11 cm2 V−1·s−1, and the mobility difference between holes and electrons increase to 1471.53 cm2 V−1·s−1. Furthermore, the applied strain, regardless of tensile or compressive, can cause the redshift of the optical absorption peak of monolayer CrOCl, thereby improving the absorption performance in the visible light region. In addition, it was found that the in-plane absorption of bilayer CrOCl increased to nearly twice as compared with the monolayer one. These results indicate that single and few layers of CrOCl have the potential application prospect in the field of optoelectronic devices.

Mechanical Control of Quantum Transport in Graphene.

2D materials (2DMs) are fundamentally electro-mechanical systems. Their environment unavoidably strains them and modifies their quantum transport properties. For instance, a simple uniaxial strain can completely turn off the conductance of ballistic graphene or switch on/off the superconducting phase of magic-angle bilayer graphene. This article reports measurements of quantum transport in strained graphene transistors which agree quantitatively with models based on mechanically-induced gauge potentials. A scalar potential is mechanically induced in situ to modify graphene's work function by up to 25 meV. Mechanically generated vector potentials suppress the ballistic conductance of graphene by up to 30% and control its quantum interferences. The data are measured with a custom experimental platform able to precisely tune both the mechanics and electrostatics of suspended graphene transistors at low-temperature over a broad range of strain (up to 2.6%). This work opens many opportunities to harness quantitative strain effects in 2DM quantum transport andtechnologies.

Exceptional Alignment in a Donor–Acceptor Conjugated Polymer via a Previously Unobserved Liquid Crystal Mesophase

AbstractOrienting polymer semiconductors is desirable to optimize device characteristics, provide insight into microstructure, and magnify subtle phase behavior. Here, a combination of uniaxial strain and subsequent heating of the donor–acceptor (DA) polymer PBnDT‐FTAZ is discovered to lead to exceptional optical dichroic ratios of up to 38 (and close to 50 near the polymer's absorption edge). This alignment is achieved due to the existence of a previously undetected thermotropic liquid crystal (LC) mesophase. The LC transition, not discernable through calorimetry, is uncovered through a combination of in situ UV–vis spectroscopy, X‐ray scattering, and dynamic mechanical analysis (DMA). Comparing PBnDT‐FTAZ to the non‐fluorinated PBnDT‐HTAZ and the homo polymer PBnDT, all of which show similar thermal transitions, reveals that exceptional alignment is only found in PBnDT‐FTAZ. This is attributed to the PBnDT‐FTAZ film having two distinct liquid crystal populations, and the polymer templating to a highly aligned, high‐clearing temperature population when heated. The DMA thermal relaxation behavior observed here is also seen in other DA conjugated polymers suggesting that such thermotropic LC mesophases may be common in these materials. These findings demonstrate a polymer semiconductor with remarkable alignment and uncover phase behavior with broad implications for process‐structure‐property relationships in polymer semiconductors.

Softened membrane model for shear of ultra-high-performance concrete (SMM-UHPC)

Understanding the shear behavior of Ultra-High-Performance (UHPC) is crucial to prevent undesired responses that could be induced by diagonal shear cracks. Hence, the ability of analytical models to predict shear responses of UHPC members is of paramount importance, which is addressed in this paper by the Softened Membrane Model (SMM). Since the SMM is a two-dimensional shear modeling framework developed for reinforced concrete, a parametric investigation was conducted to assess its suitability for UHPC, namely SMM-UHPC. Through this assessment, a softening coefficient for the biaxial behavior of UHPC was developed and new constitutive laws were integrated in SMM. Comparisons with shear panel tests were used to evaluate the accuracy of SMM-UHPC and quantify the significance of key model parameters. Sensitivity analyses showed that the decomposition between biaxial and uniaxial strain fields, which is used in SMM to capture the Poisson effect, has a negligible influence on UHPC shear. Combining experimental and analytical data, the tensile localization strain was identified to be of most significance for predicting the shear stress-strain response of UHPC.

Electronic phase transformations and energy gap variations in uniaxial and biaxial strained monolayer VS[formula omitted] TMDs: A comprehensive DFT and beyond-DFT study

In the rapidly evolving field of 2D materials, transition metal dichalcogenides (TMDs) have emerged as compelling candidates for electronic applications. This study investigates the electronic structure of the H-phase monolayer VS2 belonging to TMD family and the influence of strain on its band structure through Density Functional Theory (DFT). We employ two different pseudopotential approximations and a suite of computational methods including DFT+U, GAUPBE, G0W0, and self-GW to provide a nuanced understanding of its electronic band structure. A highlight of the study is its focus on how both uniaxial and biaxial strains, ranging from -5% to +5%, affect the electronic properties of the H-phase monolayer VS2. Our comprehensive analysis reveals that these tensile strains significantly widen the energy gap, with uniaxial strains having a more pronounced effect than their biaxial counterparts. Additionally, we identify an intriguing phase transition from a semiconducting to a metallic state under compressive strains, this transition is attributed to both symmetry breaking and bond length variation in the uniaxial case, the bond length in biaxial. These key findings not only enrich our understanding of the intricate electronic behavior of monolayer VS2 under different strains but also pave the way for the design of innovative electronic devices using strain engineering.

Anisotropic metal–insulator transition in strained VO2(B) single crystal

Mechanical strain can induce noteworthy structural and electronic changes in vanadium dioxide, imparting substantial scientific importance to both the exploration of phase transitions and the development of potential technological applications. Unlike the traditional rutile (R) phase, bronze-phase vanadium dioxide [VO2(B)] exhibits an in-plane anisotropic structure. When subjected to stretching along distinct crystallographic axes, VO2(B) may further manifest the axial dependence in lattice–electron interactions, which is beneficial for gaining insights into the anisotropy of electronic transport. Here, we report an anisotropic room-temperature metal–insulator transition in single-crystal VO2(B) by applying in-situ uniaxial tensile strain. This material exhibits significantly different electromechanical responses along two anisotropic axes. We reveal that such an anisotropic electromechanical response mainly arises from the preferential arrangement of a strain-induced unidirectional stripe state in the conductive channel. This insulating stripe state could be attributed to the in-plane dimerization within the distorted zigzag chains of vanadium atoms, evidenced by strain-modulated Raman spectra. Our work may open up a promising avenue for exploiting the anisotropy of metal–insulator transition in vanadium dioxide for potential technological applications.

Inverse finite element method with energy-based regularization for deformation reconstruction and structural health monitoring

Distributed Fiber Optical Sensing (DFOS) can be used to perform compact, low-invasive and large-scale strain measurements, providing an important tool for strain-based deformation reconstruction and Structural Health Monitoring (SHM). However, most current deformation reconstruction methods are difficult to apply to the sparse uniaxial strain measurement conditions associated with DFOS, especially for structures with complex geometries. This work introduces an energy-based regularization method, i.e., a priori assumption that the actual deformation state is close to the state with the minimum deformation energy under the premise of satisfying the strain constraints, into inverse finite element method to perform load-independently deformation reconstruction under the condition of sparse uniaxial strain measurement. The Inverse Finite Element Method with Energy-based Regularization (iFEM-EBR) can naturally construct the DFOS-based deformation reconstruction model with quasi-forward finite element analysis without introducing additional mathematical processing. The sensitivity of this method to structural characteristics, such as material parameters, makes it a unique prospect in SHM.

Chiral Modulations in Non-Heisenberg Models of Non-Centrosymmetric Magnets Near the Ordering Temperatures

The structure of skyrmion and spiral solutions, investigated within the phenomenological Dzyaloshinskii model of chiral magnets near the ordering temperatures, is characterized by the strong interplay between longitudinal and angular order parameters, which may be responsible for experimentally observed precursor effects. Within the precursor regions, additional effects, such as pressure, electric fields, chemical doping, uniaxial strains and/or magnetocrystalline anisotropies, modify the energetic landscape and may even lead to the stability of such exotic phases as a square staggered lattice of half-skyrmions, the internal structure of which employs the concept of the “soft” modulus and contains points with zero modulus value. Here, we additionally alter the stiffness of the magnetization modulus to favor one- and two-dimensional modulated states with large modulations of the order parameter magnitude. The computed phase diagram, which omits any additional effects, exhibits stability pockets with a square half-skyrmion lattice, a hexagonal skyrmion lattice with the magnetization in the center of the cells parallel to the applied magnetic field, and helicoids with propagation transverse to the field, i.e., those phases in which the notion of localized defects is replaced by the picture of a smooth but more complex tiling of space. We note that the results can be adapted to metallic glasses, in which the energy contributions are the same and originate from the inherent frustration in the models, and chiral liquid crystals with a different ratio of elastic constants.

Site-selective cobalt substitution in La–Co co-substituted magnetoplumbite-type ferrites: 59Co-NMR and DFT calculation study

The La–Co co-substituted magnetoplumbite-type (M-type) ferrites AFe12O19 (A = Ca, Sr and Ba, ion sizes Ca2+ < Sr2+ < Ba2+) with Co compositions around 0.2 have been subjected to 59Co-NMR. The results show that Co occupies the 4f1, 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f1 minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped AFe12O19 and a supercell ( 2×2×1 of the unit cell) in which 1/96 of Fe3+ is replaced by Co2+ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A, Co is most stable when it occupies the 4f1 site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f2 sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f1 site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c-axis associated with the difference in A. However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La–Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f1 site in tetrahedral coordination.

The strain regulated physical properties of PbI2/g-C3N4 for potential optoelectronic device

The van der Waals (vdW) heterostructures of Z-scheme PbI2/g-C3N4 with an indirect bandgap have gained much attention in recent years due to their unique properties and potential applications in various fields. However, the optoelectronic characteristics and strain-modulated effects are not yet fully understood. By considering this, six stacking models of PbI2/g-C3N4 are proposed and the stablest structure is selected for further investigation. The uniaxial and biaxial strains (−10%–10%) regulated band arrangement, charge distribution, optical absorption in the framework of density functional theory are systematically explored. The compressive uniaxial strain of −8.55% changes the band type from II→I, and the biaxial strains of −7.12%, −5.25%, 8.91% change the band type in a way of II→I→II→I, acting like the ‘band-pass filter’. The uniaxial strains except −10% compressive strain, and the −6%, −4%, 2%, 4%, 10% biaxial strains will enhance the light absorption of PbI2/g-C3N4. The exerted strains on PbI2/g-C3N4 generate different power conversion efficiency ( ηPCE ) values ranging from 3.64% to 25.61%, and the maximum ηPCE is generated by −6% biaxial strain. The results of this study will pave the way for the development of new electronic and optoelectronic materials with customized properties in photocatalytic field and optoelectronic devices.

Effect of strain on the hysteresis loop of PbTiO3 nanoporous

Spontaneous polarisation of ferroelectric materials has been widely applied in electronic components such as random access memory, sensor, and transducer. Among ferroelectric materials, PbTiO3 (PTO) is one of the dominant materials because it has a large spontaneous polarisation value and can operate stably at high temperatures. PTO has been studied in many different structures such as thin film, nanowire, nanodot, nanodisk, and nanotube. In order to clarify the strain effect on the hysteresis loop of PTO nanoporous, the author investigated the effect of mechanical deformation on porous structure PTO materials using calculations based on the core-shell model. The obtained results show that the hysteresis loop is expanded under the compression of the biaxial strain (x, y) and the tensile of the uniaxial strain (z). On the contrary, the loop is shrunk under the tensile of biaxial strain (x, y) and the compression of uniaxial strain (z). Besides, in the study, spontaneous polarisation distribution and regression function of hysteresis loop area - strain of PbTiO3 nanoporous is discussed as well.

Impact of strain on the SOT-driven dynamics of thin film Mn3Sn

Mn 3 Sn, a metallic antiferromagnet with an anti-chiral 120° spin structure, generates intriguing magneto-transport signatures such as a large anomalous Hall effect, spin-polarized current with novel symmetries, anomalous Nernst effect, and magneto-optic Kerr effect. When grown epitaxially as MgO(110)[001]∥Mn3Sn(01¯1¯0)[0001], Mn3Sn experiences a uniaxial tensile strain, which changes the bulk sixfold anisotropy to a twofold perpendicular magnetic anisotropy (PMA). Here, we investigate the field-assisted spin–orbit-torque (SOT)-driven dynamics in single-domain Mn3Sn with PMA. We find that for non-zero external magnetic fields, the magnetic octupole moment of Mn3Sn can be switched between the two stable states if the input current is between two field-dependent critical currents. Below the lower critical current, the magnetic octupole moment exhibits a stationary state in the vicinity of the initial stable state. On the other hand, above the higher critical current, the magnetic octupole moment shows oscillatory dynamics which could, in principle, be tuned from the 100s of megahertz to the terahertz range. We obtain approximate analytic expressions of the two critical currents that agree very well with the numerical simulations for experimentally relevant magnetic fields. We also obtain a unified functional form of the switching time vs the input current for different magnetic fields. Finally, we show that for lower values of Gilbert damping (α≲2×10−3), the critical currents and the final steady states depend significantly on α. The numerical and analytic results presented in our work can be used by both theorists and experimentalists to understand the SOT-driven order dynamics in PMA Mn3Sn and design future experiments and devices.

Strain-induced superstructure evolution and Te vacancy accumulation in AgTe monolayer grown on Ag(111) substrate

Two-dimensional (2D) materials have attracted increasing attention for frontier science and industrial applications in the past few decades. As a 2D materials with honeycomb lattice, AgTe monolayer grown on Ag (111) substrate belongs to the transition metal monochalcogenides (TMMs) family possessing various superstructures and exotic electronic structure. In this report, the AgTe monolayer superstructure is mainly studied using scanning tunneling microscope/spectroscopy (STM/S) measurements. With the increase of Te deposition, the planar structure of the AgTe monolayer becomes the one-dimensional (1D) moiré pattern structure, which transformation is related to the change of strain combing with theoretical analysis. When there is an isotropic strain on the surface, the AgTe monolayer is a flat honeycomb structure. In contrast, the lattice structural strain is released under a uniaxial strain as the AgTe monolayer is transformed into a 1D moiré pattern structure. In addition, Te vacancies in the AgTe monolayer are also affected by the synergistic effect of strain and Ag (111) substrate, and the concentration of Te vacancy changes significantly with the strain transformation. Moreover, both experimental statistics and theoretical calculations verify that Te vacancy tend to appear in the bright ridge (BR) region of the AgTe monolayer with a 1D moiré pattern structure, which is also the result of strain modulation. Our findings open up a new thought for strain transformation control of 2D materials driven by elemental concentration.

Engineering interfacial polarization switching in van der Waals multilayers.

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here we show that deliberate interlayer rotations in multilayer van der Waals heterostructures modulate both the spatial ordering and switching dynamics of polar domains. The engendered tunability is unparalleled in conventional bulk ferroelectrics or polar bilayers. By means of operando transmission electron microscopy we show how alterations of the relative rotations of three WSe2 layers produce structural polytypes with distinct arrangements of polar domains with either a global or localized switching response. Furthermore, the presence of uniaxial strain generates structural anisotropy that yields a range of switching behaviours, coercivities and even tunable biased responses. We also provide evidence of mechanical coupling between the two interfaces of the trilayer, a key consideration for the control of switching dynamics in polar multilayer structures more broadly.