NiBr2电子结构和光学性质的第一性原理研究

Study on the Electronic Structure and Optical Properties of NiBr2 Using First-Principles Method

摘要:

采用基于密度泛函理论的第一性原理方法，研究单层和块体NiBr₂的电子结构及光学性质。研究结果表明：自旋向下的单层和块体NiBr₂的导带底和价带顶分别在不同的波矢上，说明他们为间接带隙半导体；自旋向上的单层和块体NiBr₂均是直接带隙半导体，带隙分别为4.25和 3.97 eV。由于单层NiBr₂的自旋向上和自旋向下的能带结构不同，由此可知其存在磁性。结合态密度分析可知，块体和单层NiBr₂的价带主要由s态电子贡献，导带主要是由s态和p态电子贡献，而d态电子贡献较低。但是，由于单层NiBr₂未受到层间范德华力的束缚，电子跃迁更活跃，导致在导带中自旋向下的态密度向高能方向移动，从而使带隙增加。当NiBr₂由块体变成单层时，激发能量得到提升，可有效地调控能带结构。光学性质研究结果表明，随着入射光能量的增大，块体NiBr₂位移电流对磁场的贡献率及传导电流对磁场的贡献率整体趋势大于单层NiBr₂。由此可知，单层NiBr₂的电子跃迁概率大于块体NiBr₂。但是，当入射光能量增大到接近10 eV时，位移电流对磁场的贡献率发生转变，并且随着入射光能量的增大，块体NiBr₂的反射率、折射率及吸收率都大于单层NiBr₂。在可见光范围内，块体NiBr₂对光子的吸收能力强于单层NiBr₂，块体NiBr₂和单层NiBr₂分别在能量为12.66和12.55 eV处存在最强吸收峰，峰值分别为2.99×10⁵和1.84×10⁵。表明，降低NiBr₂的维度有利于光学性能的提高。

Abstract:

The electronic structure and optical properties of monolayer and bulk NiBr₂ are studied by first-principles methods which based on density functional theory. The result show that the conduction band edge and valence band of spin-down monolayer and bulk NiBr₂ are indirect bandgap semiconductors. In contrast， both the spin-up monolayer and bulk NiBr₂ are direct bandgap semiconductors with bandgaps of 4.25 eV and 3.97 eV， respectively. Moreover， the spin-down band structure is different from spin-up band structure， it shows that monolayer NiBr₂ is magnetic. State density analysis shows that the valence band of bulk NiBr₂ is mainly contributed by s-state electrons， and the conduction band is mainly contributed by s-state and p-state electrons， the d-state electron contribution is least， however because monolayer NiBr₂ is not bound by the interlayer Van der Waals forces， bring about in the conduction band， the spin-down state density moves towards the high energy， resulting an increased band gaps. Indicating that when NiBr₂ changes from a bulk to a monolayer， the excitation energy is improved and the band structure is effectively regulated. The optical properties show that： With the increase of incident light energy， the contribution rate of the bulk NiBr₂ displacement current to the magnetic field and the contribution rate of the conduction current to the magnetic field as a whole tend to be larger than that of the monolayer NiBr₂， so the electron transition probability of the monolayer NiBr₂ is larger than that of the bulk NiBr₂， however， when the incident light energy increases to nearly 10eV， the contribution of the displacement current to the magnetic field changes. With the increase of incident light energy， the reflectivity， refractive index and absorptivity of bulk NiBr₂ are higher than those of monolayer NiBr₂， the absorption capacity of bulk NiBr₂ on photons is stronger than that of monolayer NiBr₂ in the visible light range. Bulk NiBr₂ and monolayer NiBr₂ have the strongest absorption peaks at energies of 12.66 eV and 12.55 eV ， with peaks of 2.99×10⁵ and 1.84×10⁵， it shows that reducing the dimensionality of NiBr₂ is beneficial to improve its optical properties.