Strain-tunable electronic,elastic, and optical properties of CaI2 monolayer: first-principles study

ABSTRACT

The effects of biaxial strain on the electronic structure and the elastic and optical properties of monolayer CaI₂ were studied using first-principles calculations. The two-dimensional (2D) equation of state for monolayer CaI₂ as fit in a relative area of 80–120% is more accurate. The band gap can be tuned under strain and reached a maximum at a tensile strain of 4%. Under compressive strains, the absorption spectrum showed a significant red shift at higher strains. The static reflectance and static refractive index decreased in the strain range of ?10% to 10%.     

Strain Effects on Properties of Phosphorene and Phosphorene Nanoribbons: a DFT and Tight Binding Study

We perform comprehensive density functional theory calculations of strain effect on electronic structure of black phosphorus(BP) and on BP nanoribbons. Both uniaxial and biaxial strain are applied, and the dramatic change of BP's band structure is observed. Under 0-8% uniaxial strain, the band gap can be modulated in the range of 0.55-1.06 eV, and a direct-indirect band gap transition causes strain over 4% in the y direction. Under 0-8% biaxial strain, the band gap can be modulated in the range of 0.35-1.09 eV, and the band gap maintains directly.Applying strain to BP nanoribbon, the band gap value reduces or enlarges markedly either zigzag nanoribbon or armchair nanoribbon. Analyzing the orbital composition and using a tight-binding model we ascribe this band gap behavior to the competition between effects of different bond lengths on band gap. These results would enhance our understanding on strain effects on properties of BP and phosphorene nanoribbon.     

Electronic properties of Z-shaped graphene nanoribbon under uniaxial strain

Based on tight-binding approximation and a generalized Green's function method, the effect of uniaxial strain on the electron transport properties of Z-shaped graphene nanoribbon (GNR) composed of an armchair GNR sandwiched between two semi-infinite metallic armchair GNR electrodes is numerically investigated. Our results show that the increase of uniaxial strain enhances the band gap and leads to a metal-to-semiconductor transition for Z-shaped GNR. Furthermore, in the Landauer–Büttiker formalism, the current–voltage characteristics, the noise power resulting from the current fluctuations and Fano factor of strained Z-shaped GNR are explored. It is found the threshold voltage for the current and the noise power increased so that with reinforcement of the uniaxial strain parameter strength, the noise power goes from the Poisson limit to sub-Poisson region at higher bias voltages.     

单轴应变对氮化铟电子结构及光学性质的影响

采用密度泛函理论框架下的第一性原理平面波赝势方法,计算单轴应变下闪锌矿氮化铟的电子结构及光学性质.结果表明：施加应变会使带隙变窄.对于拉应变,随着应变增大带隙减小程度增大;对于压应变,随应变增大带隙减小程度减弱;且拉、压应变对带隙调控都是线性的.在能量区间4 eV~12 eV范围内施加应变时,氮化铟的吸收光谱发生红移,随拉应变程度增加,吸收光谱的红移进一步加大;随压应变增加,吸收光谱红移减弱;在该范围内,氮化铟的折射率、反射率随拉应变的增大而增加,随压应变增加减小;施加拉应变时能量损失函数峰值增大,施加压应变后能量损失函数峰值减小.通过施加单轴应变能有效调节氮化铟材料的电结构及光学性质.     

Anisotropic elastic behaviour and one‐dimensional metal in phosphorene

Using first‐principles calculations, we investigate the mechanical and electronic properties of phosphorene nanosheets under tensile strains. It is found that phosphorene possesses a prominent anisotropic elasticity with the large anisotropic factor of 15.5. Along the armchair direction, the phosphorene sheet exhibits a high tensile ductility, characterized by a large elastic strain limit of 0.31. While in the zigzag direction, the critical strain of phosphorene is dictated by the phonon instability and the in‐plane soft mode occurs beyond the 0.22 strain. Under uniaxial strains, the band gaps of phosphorene can be modulated continuously, whose band features are also altered accordingly. A Dirac‐like band structure appears in phosphorene under adequate strains along the zigzag direction. More interestingly, these Dirac cones of phosphorene display evident anisotropy, which have high Fermi velocities up to (6 – 7) × 10⁵ m/s along the armchair direction but drop to zero along the zigzag direction. With such a characteristic, the strained phosphorene sheet acts as an intriguing one‐dimensional metal, which enables the system many potential applications in power‐efficient and ultrafast nanodevices. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

Uniaxial strain-modulated electronic structures of CdX (X = S,Se, Te) from first-principles calculations:A comparison between bulk and nanowires

Using first-principles calculations based on density functional theory, we systematically study the structural deformation and electronic properties of wurtzite CdX(X = S, Se, Te) bulk and nanowires(NWs) under uniaxial [0001] strain. Due to the intrinsic shrinking strain induced by surface contraction, large NWs with {10ˉ10} facets have heavy hole(HH)-like valence band maximum(VBM) states, while NWs with {11ˉ20} facets have crystal hole(CH)-like VBM states. The external uniaxial strain induces an HH–CH band crossing at a critical strain for both bulk and NWs, resulting in nonlinear variations in band gap and hole effective mass at VBM. Unlike the bulk phase, the critical strain of NWs highly depends on the character of the VBM state in the unstrained case, which is closely related to the size and facet of NWs. The critical strain of bulk is at compressive range, while the critical strain of NWs with HH-like and CH-like VBM appears at compressive and tensile strain, respectively. Due to the HH–CH band crossing, the charge distribution of the VBM state in NWs can also be tuned by the external uniaxial strain. Despite the complication of the VBM state, the electron effective mass at conduction band minimum(CBM) of NWs shows a linear relation with the CBM–HH energy difference, the same as the bulk material.     

Hole mobility enhancement in uniaxial stressed Ge dependence on stress and transport direction

Utilizing a six-band k.p valence band calculations that considered a strained perturbation Hamiltonian, uniaxial stress-induced valence band structure parameters for Ge such as band edge energy shift, split, and effective mass were quantitatively evaluated. Based on these valence band parameters, the dependence of hole mobility on uniaxial stress (direction, type, and magnitude) and hole transport direction was theoretical studied. The results show that the hole mobility had a strong dependence on the transport direction and uniaxial stress. The hole mobility enhancement can be found for all transport directions and uniaxial stess configurations, and the hole transport along the [110] direction under the uniaxial [110] compressive stress had the highest mobility compared to other transport directions and stress configurations.     

First-principles study on superconductive properties of compressive strain-engineered cryogenic superconducting heavy metal lead(Pb)

As one of the main materials in the practical application.of superconductor,lead(Pb)has been used to manufacture superconducting AC power cable,and some weak current fields.With the development of manufacturing,technology,more and more researchers focus on exploring the physical and chemical properties of cryogenic superconducting materials,instead of blindly pursuing the improvement of the superconducting transition temperature(T_c).In this paper,the structural properties and superconducting transition temperature under high pressure of Pb have been studied by first-principles calculations.It has shown that Pb can withstand the compressive strain up to 10%while the lattice structure remains stable,indicated by the calculations of phonon band structures.From 0%to 10%compressive strain,there is neither a band-gap nor changing of the band structure.The.changing of electronic DOS.at the Fermi level leads to a decreasing of T_c.Our calculations show that Pb is a stable elemental metallic superconductor even under high pressure,which explains the reason why it has been used in practical productions.     

Mechanodynamic penetration of helium atoms into aluminum and its alloys under strain in liquid helium

The specific features of helium penetration into aluminum and its alloys, V95 and D16T, at a temperature of 4.2 K under uniaxial tension, compression, and reversal of the sign of the load are investigated. The role played by serrated strain in the intensity of the effect under consideration and the influence of impurities on the number of helium atoms penetrating into strained samples are elucidated. It is shown that the condition of additivity of the effect observed under successive reversal of the sign of the load depends on the specific features of the tensile and compressive strains.     

Efficient band structure engineering and visible-light response in ZrS2/GaS heterobilayer by electrical field or external strains

Via first-principle methods, the electronic structures and optical properties of 2D ZrS₂/GaS van der Waals heterostructure (vdWH) are studied. It is found that the band alignment changes from type-II to type-I under negative electrical field, and compressive strains. The transition points are -0.2 V/Å and -1%, respectively. The band gap changes efficiently under positive electrical field and compressive strains. The tensile strains increase the optical adsorption coefficients in ultraviolet regions, while the compressive strains increase the optical adsorption coefficients in visible region significantly.     

Electronic properties of a zinc oxide nanotube under uniaxial tensile strain: a density functional theory study

In this article, density functional theory calculations were employed to investigate the electronic properties of (4,4) armchair zinc oxide single-walled nanotubes (ZNONTs) under uniaxial mechanical deformations. It was found that the highest-occupied molecular orbital and the lowest-unoccupied molecular orbital gap and the value of radial buckling will both decrease linearly with the increase of axial strain. The elongation of the ZNONT mainly originates from the decrease and increase of two characteristic bond angles rather than Zn–O ionic bond elongation. This mechanical behavior is very different from the uniaxial tensional processes of carbon nanotubes and silicon carbide nanotubes formed by covalent bonds. The partial densities of states of the Zn atom and O atom show that the unoccupied states are gradually left-shifted as ZNONT elongates from 0 to 15%. Neither Mulliken charge nor deformation density clearly changes with the different tension strains. Bond order analysis also indicates the bonding strength will decrease as the strain increases from 0 to 15%.     

（110）应变对立方相Ca2Si能带结构及光学性质的影响

采用第一性原理贋势平面波方法对(110)应变下立方相Ca2P0.25Si0.75的能带结构及光学性质进行模拟计算,全面分析了应变对Ca2P0.25Si0.75能带结构、光学性质的影响。计算结果表明：在92%~100%压应变范围内随着应变的逐渐增大导带向低能方向移动,价带向高能方向移动,带隙呈线性逐渐减小,但始终为直接带隙;在100%~102%张应变范围内随着应变的增加,带隙呈逐渐增大,应变达到102%直接带隙最大Eg=0.54378eV;在102%~104%应变范围内随着应变的增加,带隙逐渐减小;当应变大于104%带隙变为间接带隙且带隙随着应变增大而减小。施加应变Ca2P0.25Si0.75的介电常数、折射率均增大;施加压应变吸收系数增加,反射率减小;施加张应变吸收系数减小,反射率增加。综上所述,应变可以改变Ca2P0.25Si0.75的电子结构和光学常数,是调节Ca2P0.25Si0.75光电传输性能的有效手段。     

（110）应变对立方相Ca2Si能带结构及光学性质的影响

采用第一性原理贋势平面波方法对(110)应变下立方相Ca2P0.25Si0.75的能带结构及光学性质进行模拟计算,全面分析了应变对Ca2P0.25Si0.75能带结构、光学性质的影响。计算结果表明：在92%~100%压应变范围内随着应变的逐渐增大导带向低能方向移动,价带向高能方向移动,带隙呈线性逐渐减小,但始终为直接带隙;在100%~102%张应变范围内随着应变的增加,带隙呈逐渐增大,应变达到102%直接带隙最大Eg=0.54378eV;在102%~104%应变范围内随着应变的增加,带隙逐渐减小;当应变大于104%带隙变为间接带隙且带隙随着应变增大而减小。施加应变Ca2P0.25Si0.75的介电常数、折射率均增大;施加压应变吸收系数增加,反射率减小;施加张应变吸收系数减小,反射率增加。综上所述,应变可以改变Ca2P0.25Si0.75的电子结构和光学常数,是调节Ca2P0.25Si0.75光电传输性能的有效手段。     

Unraveling the effect of uniaxial strain on thermoelectric properties of Mg_2Si: A density functional theory study

In this work, the effect of uniaxial strain on electronic and thermoelectric properties of magnesium silicide using density functional theory(DFT) and Boltzmann transport equations has been studied. We have found that the value of band gap increases with tensile strain and decreases with compressive strain. The variations of electrical conductivity,Seebeck coefficient, electronic thermal conductivity, and power factor with temperatures have been calculated. The Seebeck coefficient and power factor are observed to be modified strongly with strain. The value of power factor is found to be higher in comparison with the unstrained structure at 2% tensile strain. We have also calculated phonon dispersion, phonon density of states, specific heat at constant volume, and lattice thermal conductivity of material under uniaxial strain. The phonon properties and lattice thermal conductivity of Mg_2Si under uniaxial strain have been explored first time in this report.     

Strain and high temperature superconductivity: unexpected results from direct electronic structure measurements in thin films

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.     

Uniaxial stress influence on lattice, band gap and optical properties of n-type ZnO:first-principles calculations

The lattice, the band gap and the optical properties of n-type ZnO under uniaxial stress are investigated by first-principles calculations. The results show that the lattice constants change linearly with stress. Band gaps are broadened linearly as the uniaxial compressive stress increases. The change of band gap for n-type ZnO comes mainly from the contribution of stress in the c-axis direction, and the reason for band gap of n-type ZnO changing with stress is also explained. The calculated results of optical properties reveal that the imaginary part of the dielectric function decreases with the increase of uniaxial compressive stress at low energy. However, when the energy is higher than 4.0 eV, the imaginary part of the dielectric function increases with the increase of stress and a blueshift appears. There are two peaks in the absorption spectrum in an energy range of 4.0-13.0 eV. The stress coefficient of the band gap of n-type ZnO is larger than that of pure ZnO, which supplies the theoretical reference value for the modulation of the band gap of doped ZnO.     

Electronic properties of monolayer copper selenide with one-dimensional moiré patterns

Strain engineering is a vital way to manipulate the electronic properties of two-dimensional (2D) materials. As a typical representative of transition metal mono-chalcogenides (TMMs), a honeycomb CuSe monolayer features with one-dimensional (1D) moiré patterns owing to the uniaxial strain along one of three equivalent orientations of Cu(111) substrates. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/S) experiments and density functional theory (DFT) calculations, we systematically investigate the electronic properties of the strained CuSe monolayer on the Cu(111) substrate. Our results show the semiconducting feature of CuSe monolayer with a band gap of 1.28 eV and the 1D periodical modulation of electronic properties by the 1D moiré patterns. Except for the uniaxially strained CuSe monolayer, we observed domain boundary and line defects in the CuSe monolayer, where the biaxial-strain and strain-free conditions can be investigated respectively. STS measurements for the three different strain regions show that the first peak in conduction band will move downward with the increasing strain. DFT calculations based on the three CuSe atomic models with different strain inside reproduced the peak movement. The present findings not only enrich the fundamental comprehension toward the influence of strain on electronic properties at 2D limit, but also offer the benchmark for the development of 2D semiconductor materials.     

生长在Si(001)衬底上应变合金层GexSi1-x的光学性质

采用经验的紧束缚方法对生长在Si(001)衬底上的应变合金Ge_xSi_1-x的光学常数进行了计算。应变对电子能带结构的影响,通过紧束缚参数随键角方向余弦的变化以及键长按经验的标度定则的变化而进行计算。其中标度指数根据对Ge和Si的畸变势常数的实验值进行拟合而确定。计算介电常数虚部时出现的动量矩阵元,根据对Ge和Si的介电常数虚部的实验曲线拟合而决定。列出了当x=0.2和1时的光学常数——介电常数虚部ε₂、折射率n、吸收系数α和反射率R的计算结     

Electronic and optical properties of ZnO thin film under in-plane biaxial strains: Ab initio calculation

The full regions of the electronic and optical properties of in-plane biaxial strained thin film ZnO are studied using pseudopotential plane-wave method. The fundamental band gap at the Γ point increase linearly with the increase of tensile strains, but decreased with the compressive ones. The strains affected the local tetrahedral symmetry, and so the splitting of crystal field energy. The band dispersion relation of the valence band maximum changes with the strains, which means the residual strains have effects on the effective hole mass, thus the transportation properties of the p-type ZnO. The changes tendency of optical properties up to full regions under strains have been shown and discussed.