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.     

Strain-dependent electronic and magnetic of Co-doped monolayer of WSe2

We perform first-principles calculation to investigate electronic and magnetic properties of Co-doped WSe₂ monolayer with strains from −10% to 10%. We find that Co can induce magnetic moment about 0.894 μ_B, the Co-doped WSe₂ monolayer is a magnetic semiconductor material without strain. The doped system shows half-metallic properties under tensile strain, and the largest half-metal gap is 0.147 eV at 8% strain. The magnetic moment (0.894 μ_B) increases slightly from 0% to 6%, and jumps into about 3 μ_B at 8% and 10%, which presents high-spin state configurations. When we applied compressive strain, the doped system shows a half-metallic feature at −2% strain, and the magnetic moment jumps into 1.623 μ_B at −4% strain, almost two times as the original moment 0.894 μ_B at 0% strain. The magnetic moment vanishes at −7% strain. The Co-doped WSe₂ can endure strain from −6% to 10%. Strain changes the redistribution of charges and magnetic moment. Our calculation results show that the Co-doped WSe₂ monolayer can transform from magnetic semiconductor to half-metallic material under strain.     

Doping level and environment dependence of structural stability and magnetic properties in Mn-doped WS2 bilayer in first principles

We carried out first-principles electronic structure calculation to study the structural stability and magnetic properties of Mn-doped WS₂ ultra-thin films within the density functional theory. Adopting various configurations of Mn doping into WS₂ bilayer, we find that the magnetic phase can be manipulated among the ferromagnetic, antiferromagnetic, or ferrimagnetic phases by altering doping level and growth environment. Magnetic phase and strength are determined by magnetic coupling of Mn dopants 3d electrons which can be attributed crucially to the exchange interaction mediated by neighboring S atoms 3p electrons. Accompanying to the magnetic phase transition, the electronic structure reveals that transport properties switch from semiconducting with various bandgap to half-metallic states. This result implicates possible way to develop magnetic semiconductors based on Mn doped 2D WS₂ ultra-thin films for spintronics applications.     

Band gap modulation of transition-metal dichalcogenide MX2 nanosheets by in-plane strain

The electronic properties of quasi-two-dimensional honeycomb structures of MX₂ nanosheets (M=Mo, W and X=S, Se) subjected to in-plane biaxial strain have been investigated using first-principles calculations. We demonstrate that the band gap of MX₂ nanosheets can be widely tuned by applying tensile or compressive strain, and these ultrathin materials undergo a universal reversible semiconductor-metal transition at a critical strain. Compared to WX₂, MoX₂ need a smaller critical tensile strain for the band gap close, and MSe₂ need a smaller critical compressive strain than MS₂. Taking bilayer MoS₂ as an example, the variation of the band structures was studied and the semiconductor-metal transition involves a slightly different physical mechanism between tensile and compressive strain. The ability to tune the band gap of MX₂ nanosheets in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of application.     

Co对单层MoS_2电子结构与磁性的影响

为了研究Co对单层MoS_2电子结构和磁性的影响,本文基于第一性原理,采用数值基组的方法计算了Co吸附式掺杂、Co替代式掺杂单层MoS_2的能带结构、态密度以及分析了其结构的稳定性.结果发现:Co替换式掺杂体系的形成能较低,实验上容易实现;Co在Mo位吸附的稳定性强于在S位吸附;Mo位吸附体系的总磁矩为0.999μB,其磁矩的主要来源于Co原子的吸附所贡献的0.984μB,Co原子的掺杂体系总磁矩为1.029μB,其磁矩的主要由Co原子替代掉一个Mo原子所贡献的磁矩为0.9444μB,相比于吸附体系,Co原子对磁矩的贡献率有所降低;无论是Co吸附在单层MoS_2表面还是Co直接替代掉Mo原子的掺杂体系,Co原子3d轨道的引入是引起单层MoS_2体系磁性的主要原因.     

二维InSe/SnSe2范德华异质结的电子结构和光学特性研究

本文构建了三种不同堆叠形式下的二维InSe/SnSe₂范德华异质结模型,利用基于密度泛函理论的第一性原理方法综合考察了二维InSe/SnSe₂三种不同堆叠情况下的几何构型及稳定性,在此基础上选取具有最稳定性能的构型.该异质结呈现出Ⅱ型能带对齐特征,带隙值为1.118 eV,可以实现电子-空穴的有效分离.另外,相比与单层二维InSe/SnSe₂范德华异质结的光吸收能力达到明显提升,在紫外光范围内吸收系数达到10~6 cm^-1.研究结果将为相关物理实验及机理研究提供理论基础,对二维InSe/SnSe₂范德华异质结在光电器件中的应用具有重要的物理意义.     

Electronic and thermodynamic properties of B2-FeSi from first principles

The electronic and thermodynamic properties of B2-FeSi have been investigated using the first-principles method based on the plane-wave basis set. The calculated equilibrium lattice constant is in good agreement with available experimental and theoretical data. Our results have shown that B2-FeSi was a narrow gap semiconductor of above 0.055 eV and exhibited metallic characteristics. The density of states (DOS) can also describe orbital mixing. Using the quasi-harmonic Debye model, the thermodynamic properties of B2-FeSi have been analyzed. Variations of the Debye temperature Θ_D, thermal expansion α, heat capacity C_v, entropy S and the Grüneisen parameter γ on temperature T and pressure P were obtained successfully in the ranges of 0-2400 K and 0-140 GPa.     

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%.     

First principles lattice dynamical calculation of semiboride Be2B and its ternary alloys XBeB (X=Na, Mg, Al)

We present results of first principles total energy calculations of the structure, electronic and lattice dynamics for beryllium semiboride and its three ternary alloys using generalized gradient and local density approximations under the framework of density functional theory. The generalized gradient approximation is used for all compounds except MgBeB using the Perdew-Burke-Ernzehorf exchange correlation functional while local density approximations use the Perdew-Zunger ultrasoft exchange correlation functional. The calculated ground state structural parameters are in good agreement with those of experimental and previous theoretical studies. The electronic band structure calculations show that Be₂B may transform to a semiconductor after Al substitution. A linear response approach to density functional theory is used to calculate phonon dispersion curves and vibrational density of states. The phonon dispersion curves of Be₂B and AlBeB are positive indicating a dynamical stablility of the structure for these compounds. The phonon dispersion curves of NaBeB and MgBeB show the imaginary phonons throughout the Brillouin zone, which confirms dynamical instability as indicated in band structures for these alloys. We also present the partial phonon density of states for different species of Be₂B and AlBeB to bring out the details of the participation of different atoms in the total phonon density of state, particularly the role played by Al atom. The first time calculated phonon properties are clearly able to bring out the significant effect of isoelectronic substitution in Be₂B.     

Ti和Al共掺杂ZnS的电子结构和光学性质

基于密度泛函理论的第一性原理研究Ti和Al单掺杂和(Ti,Al)共掺杂ZnS的能带结构、电子态密度分布、介电函数、光学吸收系数,分析了掺杂后电子结构与光学性质的变化.计算结果表明:掺杂后禁带中引入了新的杂质能级,费米能级进入导带.掺杂改变了ZnS晶体的导电特性,使它表现出金属特性,导电性能增强;与纯净ZnS相比,Ti单掺杂和(Ti,Al)共掺杂ZnS的吸收边均出现明显的红移,且在1.79eV左右出现了一个新峰;而Al单掺杂ZnS的吸收边则发生明显的蓝移,且不产生新的吸收峰.     

Theoretically exploring the possible configurations,the electronic and transport properties of MoS2-OH bilayer

Inspired by MoS₂-OH bilayer framework (Zhu et al. 2019 [19]), first principles calculations are applied to explore its possible configurations as well as their electronic and transport properties. The calculated results indicate O-MoS₂ and OH…O-MoS₂ are two primary configuration in MoS₂-OH bilayer. It shows negligible difference in electronic structure between O-MoS₂ and pure MoS₂, but a flat band arise at the Fermi level in OH…O-MoS₂. Their contact characteristics show larger binding energy with selected metals and smaller contact barrier with Pt electrode. Besides, the currents of both O-MoS₂ and OH…O-MoS₂ are enlarged compared with that of pure MoS₂ in finite bias, indicating MoS₂-OH bilayer may be potential candidate for future electron device applications.     

Density functional theoretical study of the structural, electronic and lattice dynamical properties of platinum pernitride

In the framework of density functional theory, the structural, electronic and lattice dynamical properties of platinum pernitride have been investigated using the plane wave pseudopotential within the GGA and LDA functional for treating the effects of exchange correlation implemented in PWSCF and ABINIT packages. The computed lattice constant and bulk modulus agree well with the experiment and other theoretical calculations. Both packages and correlation functional agree well on the lattice constant within the deviation of about 1.6%. The bulk modulus has been quite successfully predicted by LDA. The electronic structure and DOS of platinum pernitride show a narrow gap and confirms semiconducting nature of this compound. The lattice dynamical calculation shows that the platinum nitride in pyrite structure (platinum pernitride) is dynamically stable. The zone center phonon frequencies particularly the Raman active phonons agree well with the experimental Raman data in the case of GGA implemented in PWSCF. The pressure variation of Raman active modes shows a linear variation; however, at higher pressure the variation is fast.     

基于密度泛函理论下O和Na掺杂单层h-BN的电子结构性质和光学效应的分析

基于密度泛函理论的第一性原理方法研究了O、Na单掺杂及O和Na共掺杂单层h-BN的形成能、电子结构和光学性质.结果表明:单掺杂体系中,O掺杂N位置、Na掺杂B位置时,掺杂形成能最低;共掺杂体系中,O和Na邻位掺杂,掺杂形成能最低.与单层h-BN相比,引入杂质原子后的体系禁带宽度均减小,其中O掺杂为n型掺杂,Na掺杂为p型掺杂,而O和Na共掺h-BN体系为直接带隙材料,有利于提高载流子的迁移率.在光学性质方面,Na掺杂h-BN体系与O和Na共掺h-BN的静介电常数均增大,在低能区介电虚部和光吸收峰均发生红移,其中Na掺杂体系红移最为显著,极化能力最强.因此Na单掺和O和Na共掺有望增强单层h-BN的光催化能力,可扩展其在催化材料、光电器件等领域的应用.     

WSe2光电性质的第一性原理研究

本文采用基于非平衡态格林函数-密度泛函理论的第一性原理方法,计算了单层WSe₂的光电性质.计算结果表明：在小偏压下,几乎整个可见光范围内都能产生较强的光响应,且光响应与偏振角θ呈现完美的余弦关系,与唯象理论相符合.锯齿型和扶手椅型WSe₂纳米器件均在光子能量为2.8 eV(443 nm,对应于可见光)时,能产生较大的光响应;利用能带结构和态密度分析了产生较大光响应的原因,其主要来自第一布里渊区高对称点X处的电子受激跃迁.此外,WSe₂纳米器件还具有较强的各向异性和较高的偏振灵敏度;这些结果可为WSe₂在光电子器件中的应用提供重要的理论参考.     

Chirality effect of mechanical and electronic properties of monolayer MoS2 with vacancies

We present first principles theory calculations about the chirality and vacancy effects of the mechanical and electronic properties of monolayer MoS₂. In the uni-axial tensile tests, chirality effect of the mechanical properties is negligible at zero strain and becomes significant with the increasing strain, regardless of vacancies. The existence of vacancies decreases the Young's modulus and ultimate strength of the MoS₂ structure. During the uni-axial tensile tests, the band gap decreases with the increasing strain, regardless of chirality and vacancies. The band gap is reduced with the intermediate state brought by the existence of vacancies. No chirality effect can be observed on the band gap variations of perfect MoS₂. Chirality effect appears to the band gap variation of defected MoS₂ due to the local lattice relaxation near the vacancies.     

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.     

Investigation on the structural,electronic, elastic and mechanical properties of Ti2Al(C1-xOx) solid solutions: First-principles calculations

In the present work, the structural, electronic, elastic and mechanical properties of Ti₂AlC and Ti₂Al(C_1-xO_x) solid solutions were investigated using first-principles calculations for varied O content incorporation (x = 0, 0.125, 0.25, 0.375, 0.5). According to the calculation results, all Ti₂Al(C_1-xO_x) solid solutions with various x values are stable, and the bonding strength of the Ti–Al bond increases with the doping of O element. In addition, the shear modulus G and C₄₄ elastic constant of Ti₂Al(C_1-xO_x) solid solutions are both lower than the bulk modulus B, indicating that the phase has good damage tolerance. Not only that, compared with Ti₂AlC, the plasticity and toughness of Ti₂Al(C_1-xO_x) solid solutions are improved with the increase of O atom doping and doping ratio. Simultaneously, the doping of O atom is also beneficial to reduce the generalized stacking fault energy of Ti₂AlC, making the Ti₂Al(C_1-xO_x) solid solutions more prone to shear deformation, thereby further enhancing plasticity.     

Electronic structure of MgB2

Results of ab initio electronic structure calculations on the compound MgB₂ using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (E _F) and the experimental specific heat data. The T _c is found to be 24.7 K.     

Stress and strain effects on the electronic structure and optical properties of ScN monolayer

Based on the density functional theory, electronic and optical properties of a monolayer scandium nitride structure have been studied under different strain conditions. Our results indicate that both biaxial compressive and tensile strain effects lead to change the band gap of this structure with different rates. Also, optical absorption spectrum peaks experience an obvious red and blue shifts with the exerting of tensile and compressive strains, respectively. Our results express that ScN monolayer can be the promising candidate for the future nano-base electrical and optical devices.     

First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers

Using first principles calculations, we investigate the structural, vibrational and electronic structures of the monolayer graphene-like transition-metal dichalcogenide (MX₂) sheets. We find the lattice parameters and stabilities of the MX₂ sheets are mainly determined by the chalcogen atoms, while the electronic properties depend on the metal atoms. The NbS₂ and TaS₂ sheets have comparable energetic stabilities to the synthesized MoS₂ and WS₂ ones. The molybdenum and tungsten dichalcogenide (MoX₂ and WX₂) sheets have similar lattice parameters, vibrational modes, and electronic structures. These analogies also exist between the niobium and tantalum dichalcogenide (NbX₂ and TaX₂) sheets. However, the NbX₂ and TaX₂ sheets are metals, while the MoX₂ and WX₂ ones are semiconductors with direct-band gaps. When the Nb and Ta atoms are doped into the MoS₂ and WS₂ sheets, a semiconductor-to-metal transition occurs. Comparing to the bulk compounds, these monolayer sheets have similar structural parameters and properties, but their vibrational and electronic properties are varied and have special characteristics. Our results suggest that the graphene-like MX₂ sheets have potential applications in nano-electronics and nano-devices.