Modulation of Schottky barrier in XSi_(2)N_(4)/graphene(X=Mo and W)heterojunctions by biaxial strain

Reducing the Schottky barrier height(SBH)and even achieving the transition from Schottky contacts to Ohmic contacts are key challenges of achieving high energy efficiency and high-performance power devices.In this paper,the modulation effects of biaxial strain on the electronic properties and Schottky barrier of Mo Si_(2)N_(4)(MSN)/graphene and WSi_(2)N_(4)(WSN)/graphene heterojunctions are examined by using first principles calculations.After the construction of heterojunctions,the electronic structures of MSN,WSN,and graphene are well preserved.Herein,we show that by applying suitable external strain to a heterojunction stacked by MSN or WSN—an emerging two-dimensional(2D)semiconductor family with excellent mechanical properties—and graphene,the heterojunction can be transformed from Schottky ptype contacts into n-type contacts,even highly efficient Ohmic contacts,making it of critical importance to unleash the tremendous potentials of graphene-based van der Waals(vd W)heterojunctions.Not only are these findings invaluable for designing high-performance graphene-based electronic devices,but also they provide an effective route to realizing dynamic switching either between n-type and p-type Schottky contacts,or between Schottky contacts and Ohmic contacts.

Temperature dependence of biaxial strain and its influence on phonon and band gap of GaN thin film

Hexagonal GaN epilayer grown on sapphire substrate by metal organic chemical vapour deposition （MOCVD） is studied using Raman scattering and photoluminescence in a temperature range from 100K to 873 K. The model of strain （stress） induced by the different lattice parameters and thermal coefficients of epilayer and substrate as a function of temperature is set up. The frequency and the linewidth of E2^high mode in a GaN layer are modelled by a theory with considering the thermal expansion of the lattice, a symmetric decay of the optical phonons, and the strain （stress） in the layer. The temperature-dependent energy shift of free exeiton A is determined by using Varshni empirical relation, and the effect of strain （stress） is also investigated. We find that the strain in the film leads to a decreasing shift of the phonon frequency and an about 10meV-inereasing shift of the energy in a temperature range from 100 K to 823 K.

Tuning charge and orbital ordering in DyNiO_(3) by biaxial strain

The first-principles calculations were used to explore the tunable electronic structure in DyNiO_(3)(DNO)under the effects of the biaxial compressive and tensile strains.We explored how the biaxial strain tunes the orbital hybridization and influences the charge and orbital ordering states.We found that breathing mode and Jahn–Teller distortion play a primary role in charge ordering state and orbital ordering state,respectively.Additionally,the calculated results revealed that the biaxial strain has the ability to manipulate the phase competition between the two states.A phase transition point has been found under tensile train.If the biaxial train is larger than the point,the system favors orbital ordering state.If the strain is smaller than the point,the system is in charge ordering state favorably.

Theoretical study of M_(6)X_(2) and M_(6)XX'structure(M=Au,Ag;X,X'=S,Se):Electronic and optical properties,ability of photocatalytic water splitting,and tunable properties under biaxial strain

MoS_(2),a transition metal dichalcogenide(TMDC),has attracted significant amount of attention due to its direct bandgap,tunability and optical properties.Recently,a novel structure consisting of MoS_(2) and noble metal nanoclusters has been reported.Inspired by this,first principle calculations are implemented to predict the structures of M_(6)X_(2) and M_(6)XX'(M=Au,Ag;X,X'=S,Se).The calculated bandgap,band edge position,and optical absorption of these structures prove that the silver compounds(Ag_(6)X_(2) and Ag_(6)XX')have great potential for catalytic water splitting.In addition,biaxial strain(tensile strain and compressive strain)is applied to adjust the properties of these materials.The bandgap presents a quasi-linear trend with the increase of the applied strain.Moreover,the transition between the direct and indirect bandgap is found.The outstanding electronic and optical properties of these materials provide strong evidence for their application in microelectronic devices,photoelectric devices,and photocatalytic materials.

Biaxial strain-induced enhancement in the thermoelectric performance of monolayer WSe_2

The effects of biaxial strain on the electronic structure and thermoelectric properties of monolayer WSe2 have been investigated by using first-principles calculations and the semi-classical Boltzmann transport theory. The electronic band gap decreases under strain, and the band structure near the Fermi level of monolayer WSe2 is modified by the applied biaxial strain. Furthermore, the doping dependence of the thermoelectric properties of n-and p-doped monolayer WSe2 under biaxial strain is estimated. The obtained results show that the power factor of n-doped monolayer WSe2 can be increased by compressive strain while that of p-doping can be increased with tensile strain. Strain engineering thus provides a direct method to control the electronic and thermoelectric properties in these two-dimensional transition metal dichalcogenides materials.

A density functional theory study of the tunable structure, magnetism and metal-insulator phase transition in VS2 monolayers induced by in-plane biaxial strain

We report a density functional theory study of a phase transition of a VS2 monolayer that can be tuned by the in-plane biaxial strain. This results in both a metal-insulator transition and a low spin-high spin magnetic transition. At low temperature, the semiconducting H-phase is stable and large strain （〉3%） is required to provoke the transition. On the other hand, at room temperature （300 K）, only a small tensile strain of 2% is needed to induce the phase transition from the semiconducting H-phase to the metallic T-phase together with the magnetic transition from high spin to low spin. The phase diagram dependence on both strain and temperature is also discussed in order to provide a better understanding of the phase stability of VS2 monolayers.

Biaxial strained dual-phase palladium-copper bimetal boosts formic acid electrooxidation

Surface strain engineering is considered as an effective strategy to promote the electrocatalytic properties of noble metal nanocrystals.Herein,we construct a dual-phase palladium-copper(DP-PdCu)bimetallic electrocatalyst with remarkable biaxial strain via a one-pot wet-chemical approach for formic acid oxidation.The biaxial strain originates from the lattice mismatch between the disordered face-centered cubic(FCC)phase and ordered body-centered cubic(BCC)phase in each of DP-PdCu:nanoparticles.The proportion of FCC and BCC phases and size of PdCu nanoparticles are dependent on the addition amount of:capping agent,cetyltrimethylammonium bromide(CTAB).Density functional theory calculations reveal the downshift of d-band:center of Pd atoms due to the interfacial strain,which weakens the adsorption strength of undesired intermediates.These merit the,DP-PdCu catalyst with superior mass activity of 0.55 A·mg_(pd)^(-1) and specific activity of 1.91 mA·cm_(pd)^(-2) toward formic acid oxidation,outperforming the single FCC/EICC PdCu and commercial Pd/C catalysts.This will provide new insights into the structure design of high-performance electrocatalysts via strain engineering.

Effect of biaxial strain and hydrostatic pressure on the magnetic properties of bilayer CrI_(3)

Two-dimensional van der Waals magnetic materials are intriguing for applications in the future spintronics devices,so it is crucial to explore strategy to control the magnetic properties.Here,we carried out first-principles calculations and Monte Carlo simulations to investigate the effect of biaxial strain and hydrostatic pressure on the magnetic properties of the bilayer CrI_(3).We found that the magnetic anisotropy,intralayer and interlayer exchange interactions,and Curie temperature can be tuned by biaxial strain and hydrostatic pressure.Large compressive biaxial strain may induce a ferromagnetic-to-antiferromagnetic transition of both CrI_(3) layers.The hydrostatic pressure could enhance the intralayer exchange interaction significantly and hence largely boost the Curie temperature.The effect of the biaxial strain and hydrostatic pressure revealed in the bilayer CrI_(3) may be generalized to other two-dimensional magnetic materials.

Effects of Biaxial Tensile on the Deformation Behavior of DP590 High-strength Steel Sheet under High Strain Rate

The effects of biaxial tensile pre-strain on the forming limit of DP590 high-strength steel under high strain rate were investigated. The stress-strain curves of DP590 steel sheet under the biaxial tensile of different load ratios were obtained. The forming limit diagrams（FLDs） of DP590 steel sheet under biaxial tensile pre-strain and electromagnetic hybrid forming were established. Results showed that the biaxial tensile pre-strain had significant effects on the formability of DP590 steel. The pre-strain in a certain range improved the forming limit of DP590 steel under high rate, and the forming limit increased with the pre-strain. The prestrain in the same direction of high rate increased the forming limit of the final deformation under complex strain paths conditions, but the pre-strain in the vertical direction decreased the minor strain under the high strain rate.

Valence band structure and density of states effective mass model of biaxial tensile strained silicon based on k·p theory

After constructing a stress and strain model, the valence bands of in-plane biaxial tensile strained Si is calculated by k·p method. In the paper we calculate the accurate anisotropy valance bands and the splitting energy between light and heavy hole bands. The results show that the valance bands are highly distorted, and the anisotropy is more obvious. To obtain the density of states （DOS） effective mass, which is a very important parameter for device modeling, a DOS effective mass model of biaxial tensile strained Si is constructed based on the valance band calculation. This model can be directly used in the device model of metal-oxide semiconductor field effect transistor （MOSFET）. It also a provides valuable reference for biaxial tensile strained silicon MOSFET design.

双轴应变调控下单层双面神结构MoSSe拉曼光谱的理论研究

单层双面神结构过渡金属硫化物具有低维度、高迁移率以及奇特的电子结构性质,因此其在电子学和光电子学器件方面具有潜在的应用前景。由单层双面神结构过渡金属硫化物和基底材料制成的器件,很容易因基底材料和单层双面神结构过渡金属硫化物的晶格失配,导致单层双面神结构过渡金属硫化物受基底材料的应力,因此通过拉曼散射系统研究双轴应变对单层双面神结构MoSSe物性的影响具有重要意义。系统研究了双轴调控下单层双面神结构MoSSe的原子结构、电子结构、声子结构和拉曼散射特性。结果表明,在双轴应变调控下,单层双面神结构MoSSe的电子能带带隙出现了直接与间接的转换;随着压应变的减小和拉应变的增加,3个拉曼特征峰(E^(1)、E^(2)、A_(1)^(1))的频率都发生了单调红移,而A_(1)^(2)的拉曼特征峰在压应变减小的过程中出现了反常的蓝移;随着压应变的减小和拉应变的增加,双重简并模式(E^(1)、E^(2))的拉曼强度单调增加,单重简并模式的拉曼强度单调减小,而A_(1)^(1)的拉曼强度先减小再增加。针对这些具有普遍性和特殊性的应变效应,通过建模进行了研究。结果表明,基于特征峰频率和强度随应变的变化差异,可以通过特征峰之间的频率差和强度比,快速定量化双面神结构材料的应变类型和大小。

二维MoSi_(2)N_(4)/WSe_(2)异质结的第一性原理研究

实验上新合成的MoSi_(2)N_(4)(MSN)由于其独特的七原子层结构和电子特性引起了人们的广泛关注.本文搭建了一种由二维MSN与二维WSe_(2)(WS)垂直堆垛而成的二维MSN/WS异质结并基于第一性原理计算对其电子性质进行了计算,其表现出直接间隙半导体和I型能带排列的特性,具有1.46 eV的带隙.在异质结界面处存在一个由电荷耗尽层MSN指向电荷积累层WS微弱的内建电场.最后,通过施加双轴应变对二维MSN/WS异质结进行调控.发现在正双轴应变的作用下,MSN/WS异质结保持了原来直接带隙半导体和I型能带排列特性;在负双轴应变作用下,MSN/WS异质结由原来的直接带隙半导体转变为间接带隙半导体,当施加的负双轴应变达到-6%与-8%时,I型能带排列转变为Ⅱ型能带排列.

Strong anti-strain capacity of CoFeB/MgO interface on electronic structure and state coupling

Electronic structure and spin-related state coupling at ferromagnetic material （FM）/MgO （FM = Fe, CoFe, CoFeB） interfaces under biaxial strain are evaluated using the first-principles calculations. The CoFeB/MgO interface, which is su- perior to the Fe/MgO and CoFe/MgO interfaces, can markedly maintain stable and effective coupling channels for majority- spin A1 state under large biaxial strain. Bonding interactions between Fe, Co, and B atoms and the electron transfer between Bloch states are responsible for the redistribution of the majority-spin A1 state, directly influencing the coupling effect for the strained interfaces. Layer-projected wave function of the majority-spin Al state suggests slower decay rate and more stable transport property in the CoFeB/MgO interface, which is expected to maintain a higher tunneling magnetoresistance （TMR） value under large biaxial strain. This work reveals the internal mechanism for the state coupling al strained FM/MgO interfaces. This study may provide some references to the design and manufacturing of magnetic tunnel .junctions with high tunneling magnetoresistance effect.

Topological Transition in Monolayer Blue Phosphorene with Transition-Metal Adatom under Strain

We carried out first-principles calculations to investigate the electronic properties of the monolayer blue phosphorene(BlueP)decorated by the group-IVB transition-metal adatoms(Cr,Mo and W),and found that the Cr-decorated BlueP is a magnetic half metal,while the Mo-and W-decorated BlueP are semiconductors with band gaps smaller than 0.2 eV.Compressive biaxial strains make the band gaps close and reopen,and band inversions occur during this process,which induces topological transitions in the Mo-decorated BlueP(with strain of-5.75%)and W-decorated BlueP(with strain of-4.25%)from normal insulators to topological insulators(TIs).The TI gap is 94 meV for the Mo-decorated BlueP and218 me V for the W-decorated BlueP.Such large TI gaps demonstrate the possibility to engineer topological phases in the monolayer BlueP with transition-metal adatoms at high temperature.

镭射BOPP基膜循环拉伸力学性能的试验研究

目的为研究双向拉伸聚丙烯(Biaxially Oriented Polypropylene,BOPP)薄膜在重复拉伸使用后的弹性模量变化以及应变变化趋势。方法以26μm BOPP薄膜为研究对象,以1、1.5、2、2.5、3、3.5、4 N为最大加载力分别进行加载周期为15次的单轴循环拉伸试验,总结对比循环次数以及加载力对弹性模量及棘轮应变的影响,分析薄膜性能变化趋势。结果通过对比同一加载应力不同循环下的弹性模量以及棘轮应变,发现在预设加载力下随着循环次数增加,薄膜弹性模量呈增大趋势,且增大幅度逐渐减小,其棘轮应变与弹性模量变化趋势一致;通过取均值对比不同加载力下的弹性模量以及棘轮应变变化,发现薄膜弹性模量随加载力增大而先增大后减小,最大应变以及最小应变均随加载力的增加而增大。结论通过试验研究得出了BOPP薄膜在循环拉伸中弹性模量及棘轮应变随加载力、循环次数增大的变化趋势;对薄膜性能变化有了一定的预测,并为实际加工中的张力选择提供了指导性建议。

Further study of rubber-like elasticity: elastic potentials matching biaxial data

By virtue of the rational interpolation procedure and logarithmic strain, a direct approach is proposed to obtain elastic potentials that exactly match uniaxial data and shear data for elastomers. This approach reduces the determination of multi axial elastic potentials to that of two one-dimensional potentials, thus bypassing usual cumbersome procedures of identifying a number of unknown parameters. Predictions of the suggested potential are derived for a general biaxial stretch test and compared with the classical data given by Rivlin and Saunders （Rivlin, R. S. and Saunders, D. W. Large elastic deformation of isotropic materials. VII： experiments on the deformation of rubber. Phill. Trans. Royal Soc. London A, 243, 251-288 （1951））. Good agreement is achieved with these extensive data.

FAILURE MODE AND CONSTITUTIVE MODEL OF PLAIN HIGH-STRENGTH HIGH-PERFORMANCE CONCRETE UNDER BIAXIAL COMPRESSION AFTER EXPOSURE TO HIGH TEMPERATURES

An orthotropic constitutive relationship with temperature parameters for plain highstrength high-performance concrete （HSHPC） under biaxial compression is developed. It is based on the experiments performed for characterizing the strength and deformation behavior at two strength levels of HSHPC at 7 different stress ratios including a=σs ： σ3=0.00：-1,-0.20：-1,-0.30 ： -1,-0.40：-1,-0.50：-1,-0.75：-1,-1.00：-1, after the exposure to normal and high temperatures of 20, 200, 300, 400, 500 and 600℃, and using a large static-dynamic true triaxial machine. The biaxial tests were performed on 100 mm×100 mm×100 mm cubic specimens, and friction-reducing pads were used consisting of three layers of plastic membrane with glycerine in-between for the compressive loading plane. Based on the experimental results, failure modes of HSHPC specimens were described. The principal static compressive strengths, strains at the peak stress and stress-strain curves were measured; and the influence of the temperature and stress ratios on them was also analyzed. The experimental results showed that the uniaxial compressive strength of plain HSHPC after exposure to high temperatures does not decrease dramatically with the increase of temperature. The ratio of the biaxial to its uniaxial compressive strength depends on the stress ratios and brittleness-stiffness of HSHPC after exposure to different temperature levels. Comparison of the stress-strain results obtained from the theoretical model and the experimental data indicates good agreement.

混凝土动态双轴压缩强度准则细观研究

由于物理试验设备和条件限制,目前对混凝土动态双轴压缩强度准则的研究仅停留在低应变率范围(10^(−5) s^(−1)~10^(−2) s^(−1))。针对这些强度准则在更高应变率范围内是否适用,该研究建立了细观随机骨料模型,对边长100 mm的混凝土立方体试块开展了动态双轴压缩细观模拟分析。研究了应变率和侧应力比对混凝土动态双轴压缩破坏模式及压缩强度的影响,建立了适用于更高应变率的动态强度准则。研究结论:相同侧应力比下,随应变率增大,混凝土内部损伤区域增多,动态压缩强度增大;相同应变率下,随侧应力增大,混凝土破坏模式由柱状压裂变为片状劈裂,动态压缩强度先增大后减小。现有的混凝土动态双轴压缩强度准则在应变率为10^(−5) s^(−1)~1 s^(−1)时很难适用,而该研究建议的强度准则适用于更高的应变率范围,并且得到了不同物理试验的初步验证。

Stress—strain curves for different loading paths and yield loci of aluminum alloy sheets

To carry out biaxial tensile test in sheet metal, the biaxial tensile testing system was established. True stress—true strain curves of three kinds of aluminum alloy sheets for loading ratios of 4:1, 4:2, 4:3, 4:4, 3:4, 2:4 and 1:4 were obtained by conducting biaxial tensile test in the established testing systems. It shows that the loading path has a significant influence on the stress—strain curves and as the loading ratio increases from 4:1 to 4:4, the stress—strain curve becomes higher and n-value becomes larger. Experimental yield points for three aluminum alloy sheets from 0.2% to 2% plastic strain were determined based on the equivalent plastic work. And the geometry of the experimental yield loci were compared with the yield loci calculated from several existing yield criteria. The analytical result shows that the Barlat89 and Hosford yield criterion describe the general trends of the experimental yield loci of aluminum alloy sheets well, whereas the Mises yield criterion overestimates the yield stress in all the contours.

FATIGUE CRACK PROPAGATION FOR BIAXIAL STRESS CYCLING

Based on the experimental study of complex biaxial mode Ⅰ fatigue crack growth and the discussion on Von Mises'theory,a new approach is proposed for correlating crack propaga- tion rate under both in-phase and out-of-phase biaxial stress cycling.The results emphasize the contribution of plasticity to fatigue crack growth.