双轴应变条件下单层MoSe2电子结构和拉曼光谱的第一性原理计算

二硒化钼的层间相互作用强,单层结构具有更低的带隙和更好的稳定性.由于独特的光学性质和优异的电学性能受到研究人员的广泛关注.本文基于密度泛函理论的第一原理,计算和分析了在双轴拉伸压缩应变条件下单层MoSe₂能带结构,拉曼光谱和声子谱的变化规律以及性质产生的原因.在拉伸压缩应变作用下,直接带隙转变为间接带隙.当拉伸应变达到12%时,材料发生半导体-金属相变.当压缩应变达到6%时,声子谱中开始出现虚频率,表明结构开始变得不稳定.     

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%.     

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.     

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 WSe_2 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 WSe_2 is modified by the applied biaxial strain. Furthermore, the doping dependence of the thermoelectric properties of n-and p-doped monolayer WSe_2 under biaxial strain is estimated. The obtained results show that the power factor of n-doped monolayer WSe_2 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.     

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 modulations of magnetism in Fe-doped InSe monolayer

First-principles calculations are performed to investigate the electronic and magnetic characteristics of Fe-doped two-dimensional (2D) InSe monolayer by applying biaxial compressive and tensile strains. Our studies show that Fe substituting indium atom can be realized easily under Se-rich experimental environments, and can induce the magnetic semiconducting characteristics. Interestingly, the magnetic moments are insensitive to the strain ~ −6% to 6% range. However, loading larger tensile strain can decrease the magnetic moments sharply. Moreover, the system still retains semiconducting characteristics under compressive strain, while a transition occurs from semiconductor to metal beyond the tensile strain 8%. These results provide the theoretical predications that Fe-doped 2D InSe material may be applied in the spintronic devices.     

Electronic and magnetic properties of Mn-doped WSe2 monolayer under strain

Electronic and magnetic properties of Mn-doped WSe₂ monolyer subject to isotropic strain are investigated using the first-principles methods based on the density functional theory. Our results indicate that Mn-doped WSe₂ monolayer is a magnetic semiconductor nanomaterial with strong spontaneous magnetism without strain and the total magnetic moment of Mn-doped system is 1.038μ_B. We applied strain to Mn-doped WSe₂ monolayer from -10% to 10%. The doped system transforms from magnetic semiconductor to half-metallic material from −10% to −2% compressive strain and from 2% to 6% tensile strain. The largest half-metallic gap is 0.450 eV at −2% compressive strain. The doped system shows metal property from 7% to 10%. Its maximum magnetic moment comes to 1.181μ_B at 6% tensile strain. However, the magnetic moment of system decreases to zero sharply when tensile strain arrived at 7%. Strain changes the redistribution of charges and arises to the magnetic effect. The coupling between the 3d orbital of Mn atom, 5d orbital of W atom and 4p orbital of Se atom is analyzed to explain the strong strain effect on the magnetic properties. Our studies predict Mn-doped WSe₂ monolayers under strain to be candidates for thin dilute magnetic semiconductors, which is important for application in semiconductor spintronics.     

Magnetic properties of a Na-doped WS<Subscript>2</Subscript> monolayer in the presence of an isotropic strain

The magnetic properties of Na-doped WS₂ monolayer under strain are investigated by ab initio methods. Without strain, the Na-doped WS2 monolayer is a magnetic nanomaterial and the total magnetic moment is about 1.07μ_B. We applied strain to Na-doped WS₂ monolayer from–10% to 10%. The magnetic properties are modified under different strain; the doped system gets a maximum value of at 2.01μ_B 10% tensile strain and a minimum value of at 0μ_B–10% compressive strain. The coupling between 3p states of S and 5d states of W is responsible for the strong strain effect on the magnetic properties. Our studies predict Na-doped WS₂ monolayer under strain to be candidates for application in spintronics.     

Effect of strain on electronic and magnetic properties of n-type Cr-doped WSe2 monolayer

Using first-principles calculations based on the density functional theory, we study the effect of strain on the electronic and magnetic properties of Cr-doped WSe₂ monolayer. The results show that no magnetic moment is induced in the Cr-doped WSe₂ monolayer without strain. For the Cr substitutions, the impurity states are close to the conduction bands, which indicate n-type doping occurs in this case. Then we applied strain (from −10% to 10%) to the doped system, and find that a little magnetic moment is induced with tensile strain from 6% to 9% and negligible. We find that the influence of strain on the magnetic properties is inappreciable in Cr-doped WSe₂. Moreover, the tensile strain appears to be more effective in reducing the band gap of Cr-doped WSe₂ monolayer than the compressive strain.     

Electronic structure and optical properties of CuAlO2 under biaxial strain

An ab initio calculation has been carried out to investigate the biaxial strain ( - 10.71% < ε < 9.13%) effect on elastic, electronic and optical properties of CuAlO(2). All the elastic constants (c(11), c(12), c(13), c(33)) except c(44) decrease (increase) during tensile (compressive) strain. The band gap is found to decrease in the presence of tensile as well as compressive strain. The relative decrease of the band gap is asymmetric with respect to the sign of the strain. Significant differences between the parallel and perpendicular components of the dielectric constant and the optical properties have been observed due to anisotropic crystal structure. It is further noticed that these properties are easily tunable by strain. Importantly, the collective oscillation of the valence electrons has been identified for light polarized perpendicular to the c-axis. From calculations, it is clear that the tensile strain can enhance the hole mobility as well as the transparency of CuAlO(2).     

Strain-tunable electronic and optical properties of h-BN/BC_3 heterostructure with enhanced electron mobility

By using first-principles calculation, we study the properties of h-BN/BC_3 heterostructure and the effects of external electric fields and strains on its electronic and optical properties. It is found that the semiconducting h-BN/BC_3 has good dynamical stability and ultrahigh stiffness, enhanced electron mobility, and well-preserved electronic band structure as the BC_3 monolayer. Meanwhile, its electronic band structure is slightly modified by an external electric field. In contrast,applying an external strain can mildly modulate the electronic band structure of h-BN/BC_3 and the optical property exhibits an apparent redshift under a compressive strain relative to the pristine one. These findings show that the h-BN/BC_3 hybrid can be designed as optoelectronic device with moderately strain-tunable electronic and optical properties.     

Effect of strain on electrochemical performance of Janus MoSSe monolayer anode material for Li-ion batteries: First-principles study

First-principles calculations are performed to investigate the effect of strain on the electrochemical performance of Janus MoSSe monolayer.The calculation focuses on the specific capacity,intercalation potential,electronic structure,and migration behavior of Li-ion under various strains by using the climbing-image nudged elastic band method.The result shows that the specific capacity is nearly unchanged under strain.But interestingly,the tensile strain can cause the intercalation potential and Li-ion migration energy barrier increase in MoSSe monolayer,whereas the compressive strain can lead to the intercalation potential and energy barrier decreasing.Thus,the rate performance of the MoSSe anode is improved.By analyzing the potential energy surface of MoSSe surface and equilibrium adsorption distance of Li-ion,we explain the physical origin of the change in the intercalation potential and migration energy barrier.The increase of MoSSe potential energy surface and the decrease of adsorption distance caused by tensile strain are the main reason that hinders Li-ion migration.     

Tuning electronic properties of the S_2/graphene heterojunction by strains from density functional theory

Based on the density functional calculations, the structural and electronic properties of the WS_2/graphene heterojunction under different strains are investigated. The calculated results show that unlike the free mono-layer WS_2, the monolayer WS_2 in the equilibrium WS_2/graphene heterojunctionis characterized by indirect band gap due to the weak van der Waals interaction. The height of the schottky barrier for the WS_2/graphene heterojunction is 0.13 eV, which is lower than the conventional metal/MoS_2 contact. Moreover, the band properties and height of schottky barrier for WS_2/graphene heterojunction can be tuned by strain. It is found that the height of the schottky barrier can be tuned to be near zero under an in-plane compressive strain, and the band gap of the WS_2 in the heterojunction is turned into a direct band gap from the indirect band gap with the increasing schottky barrier height under an in-plane tensile strain. Our calculation results may provide a potential guidance for designing and fabricating the WS_(2~-)based field effect transistors.     

Tight-binding parameterization of α-Sn quasiparticle band structure

The diamond structure of tin (α-Sn) can be stabilized in nanocrystals embedded in a suitable host. We developed highly accurate parameterizations for tight-binding simulation of such structures incorporating strain and spin-orbit interaction. Parameters are obtained by fitting to ab initio GW quasiparticle band structures of unstrained α-Sn as well as geometries under uniform compressive or tensile strain. The optical response calculated from this fit is in excellent agreement with experiments. As an application, confinement induced band gaps in strained and unstrained 3 nm nanocrystals are computed. It is found that compressive and tensile strain raises and lowers the gap, respectively.     

Effect of strain on geometric and electronic structures of graphene on Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations. Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately. Considering the strain induced by the lattice mismatch, we optimize the atomic structures and investigate the electronic properties of the graphene. Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N×N superlattice and the substrate. The strain does not induce significant changes in electronic structure. Furthermore, the results calculated in the local density approximation (LDA) are compared with those obtained in the generalized gradient approximation (GGA), showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.     

直流脉冲磁控反应溅射技术制备掺铝氧化锌薄膜的研究

文章采用直流脉冲磁控反应溅射(DCPsputtering)技术,在不同氧氩比(GFR)条件下玻璃衬底上制备了一系列掺铝氧化锌(AZO)薄膜,并利用X射线衍射、扫描电子显微镜和分光光度计从宏观应力和微观晶格畸变的角度研究了GFR对薄膜结构、表面形貌和光学特性的影响.制备的多晶AZO薄膜呈现了明显的ZnO-(103)择优取向,这归结于3小时薄膜沉积过程中伴随的退火引起的薄膜晶面能转变.随着GFR的增大,AZO薄膜内宏观拉应力先增大到最大值,随后宏观压应力随着GFR的继续增大而增大.薄膜中的宏观应力明显随着GFR从拉应力向压应力转变.这与晶格微观畸变诱导的微观应力的研究结果趋势恰恰相反.随着GFR的增加,薄膜在可见光区的平均透射率先增加后减小,薄膜晶粒尺寸诱导的晶界散射是影响薄膜透射率的主导机制.     

Electronic and optical properties of kesterite Cu2ZnSnS4 under in-plane biaxial strains: First-principles calculations

The electronic structures and optical properties of Cu₂ZnSnS₄ (CZTS) under in-plane biaxial strain were systematically investigated using first-principles calculations based on generalized gradient approximation and hybrid functional method, respectively. It is found that the fundamental bandgap at the Γ point decreases linearly with increasing tensile biaxial strain perpendicular to c-axis. However, a bandgap maximum occurs as the compressive biaxial strain is 1.5%. Further increase of compressive strain decreases the bandgap. In addition, the optical properties of CZTS under biaxial strain are also calculated, and the variation trend of optical bandgap with biaxial strain is consistent with the fundamental bandgap.     

Design of polarization independent InGaAs/InGaAlAs coupled quantum well structure in 1.55 μm wavelength region

A novel polarization independent InGaAs/InGaAlAs quantum well (QW) structure in the 1.55 m wavelength region is proposed. A coupled QW structure with tensile strain in the QW and/or barrier region is considered for the reduction of the optical gain difference between TE and TM modes in the wide spectral range. A triple-coupled QW structure with alternative strain (tensile/compressive/tensile) is found to be the most effective in reducing the polarization gain difference. This is because the transition strength difference of each polarization is reduced by energy states coupling. The optimized triple-coupled QW structure shows polarization independence for wide carrier density and wavelength range, which is suitable for polarization independent operation of QW based semiconductor devices, such as semiconductor optical amplifiers.     

Electronic structure and optic absorption of phosphorene under strain

We studied the electronic structure and optic absorption of phosphorene (monolayer of black phosphorus) under strain. Strain was found to be a powerful tool for the band structure engineering. The in-plane strain in armchair or zigzag direction changes the effective mass components along both directions, while the vertical strain only has significant effect on the effective mass in the armchair direction. The band gap is narrowed by compressive in-plane strain and tensile vertical strain. Under certain strain configurations, the gap is closed and the energy band evolves to the semi-Dirac type: the dispersion is linear in the armchair direction and is gapless quadratic in the zigzag direction. The band-edge optic absorption is completely polarized along the armchair direction, and the polarization rate is reduced when the photon energy increases. Strain not only changes the absorption edge (the smallest photon energy for electron transition), but also the absorption polarization.     

Raman Scattering Modification in Monolayer ReS_2 Controlled by Strain Engineering

Regulation of optical properties and electronic structure of two-dimensional layered ReS_2 materials has attracted much attention due to their potential in electronic devices.However,the identification of structure transformation of monolayer ReS_2 induced by strain is greatly lacking.In this work,the Raman spectra of monolayer ReS_2 with external strain are determined theoretically based on the density function theory.Due to the lower structural symmetry,deformation induced by external strain can only regulate the Raman mode intensity but cannot lead to Raman mode shifts.Our calculations suggest that structural deformation induced by external strain can be identified by Raman scattering.