ZnS结构相变、电子结构和光学性质的研究

运用第一性原理平面波赝势和广义梯度近似方法, 对闪锌矿结构(ZB)和氯化钠结构(RS) ZnS的状态方程及其在高压下的相变进行计算研究, 分析相变点附近的电子态密度、能带结构和光学性质的变化机理. 结果表明: 通过状态方程得到ZB相到RS相的相变压强值为18.1 GPa, 而利用焓相等原理得到的相变压强值为18.0 GPa; 在结构相变过程中, sp³轨道杂化现象并未消失, RS相ZnS的金属性明显增强; 与ZB相ZnS相比, RS相ZnS的介电常数主峰明显增强, 并向低能方向出现了明显偏移, 使得介电峰向低能方向拓展, 在低能区电子跃迁大大增强. 关键词： 硫化锌 相变 电子结构 光学性质     

Ab initio study of structural,electronic and optical properties of ternary CdOl-xSex alloys using special quasi-random structures

The structural, electronic, and optical properties of binary CdO, CdSe, and their ternary CdOi xSex alloys （0 〈 x _〈 1） in the rock salt and zinc blend phases have been studied by the special quasi-random structure （SQS） method. All the calculations are performed using full-potential linearized augmented plane wave plus local orbital＇s （FP-LAPW＋Io） method within the framework of density function theory （DFT）. We use Wu-Cohen （WC） generalized gradient approximation （GGA） to calculate structural parameters, whereas both Wu-Cohen and Engel-Vosko （EV） GGA have been applied to calculate electronic structure of the materials. Our predicted results of lattice constant and bulk modulus show only a slight deviation from Vegard＇s law for the whole concentrations. The obtained band structure indicates that for the rock-salt phase, the ternary alloys present semi-metallic behavior, while for the zinc blend phase, semiconductor behavior with direct bandgap is observed with decreasing order of x except for CdSe. Finally, by incorporating the basic optical properties, we discuss the dielectric function, refractive index, optical reflectivity, the absorption coefficient, and optical conductivity in terms of incident photon energy up to 14 eV. The calculated results of both binaries are in agreement with existing experimental and theoretical values.     

First-principles study of structural,electronic and optical properties of ZnF_2

The structural, electronic, and optical properties of rutile-, CaC12-, and PdF2-ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile-, CaCl2-, and PdF2-ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.     

Ti-O共掺杂石墨烯的电子结构和光学性质的理论研究

本文利用基于密度泛函理论的第一性原理方法研究了本征石墨烯和不同掺杂浓度下Ti-O共掺杂石墨烯的电子结构和光学性质,并讨论了其内部的微观机制.研究结果表明:本征石墨烯是一种零带隙材料,狄拉克点在费米能级面上,其在紫外光区的光吸收强度较强.Ti-O共掺杂石墨烯可以很好的打开石墨烯的带隙和提高石墨烯的光催化强度,Ti18-O18@G模型费米能级附近的态密度主要由C-p轨道、Ti-d轨道和O-p轨道杂化而成.Ti18-O18@G模型在可见光区的吸收谱强度最大,主要归因于其内部晶格畸变、带隙被打开和杂质能带的出现,这些因素可以促进电子空穴对的产生和分离,从而使石墨烯在可见光区的光催化能力得到增强.本研究结果可为开发高催化活性的石墨烯提供理论依据.     

Theoretical study of structural, optical and electrical properties of zirconium-doped zinc oxide

The structural, optical and electrical properties of zirconium-doped zinc oxide have been investigated by first principle calculations. Three possible structures including substitutional Zr for Zn (Zr_Zn), interstitial Zr (Zr_i) and substitutional Zr for O (Zr_O) are considered. The results show that the formation energy of Zr_Zn defect is the lowest, which indicates that Zr_Zn defect forms easier and its concentration may be the highest in the samples. It is also found that as the proportion of Zr increases, the lattice constants increase while the optical band gap first becomes larger and then smaller, which are consistent with our recently experimental results. The electronic structure calculations display that as Zr_Zn defect is introduced into ZnO, the Fermi-level shifts to the conduction band, and there are excess electrons in the conduction band, which may be a possible reason of the good conductivity of Zr doped ZnO film.     

First-principles study of structural,electronic and optical properties of orthorhombic SrZrO3

We have investigated the structural parameters, electronic structure and optical properties of orthorhombic SrZrO₃ using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). Our calculated structural parameters are in good agreement with the previous theoretical and experimental data. Band structure, density of states and chemical bonding have been systematically studied. Furthermore, the complex dielectric function, refractive index, extinction coefficient, optical reflectivity, absorption coefficient, loss function and optical conductivity are calculated, which show an optical anisotropy in the components of polarization directions (100), (010) and (001).     

First-principles study of structural,electronic, and optical properties of ZnSnO3

The structural, electronic, and optical properties of ZnSnO₃ were investigated using density functional theory within the generalized gradient approximation. The structure parameters obtained agree well with the experimental results. The electronic structures indicate that ZnSnO₃ is a semiconductor with a direct band gap of 1.0 eV. The calculated optical spectra can be assigned to contributions of the interband transitions from valence band O 2p levels to conduction band Sn 5s levels or higher conduction band Zn 3d levels in the low-energy region, and from O 2p to Sn 5p or Zn 4p conduction band in the high-energy region.     

Ce掺杂CrSi2电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及Ce掺杂CrSi2的电子结构和光学性质进行理论计算。计算结果表明,未掺杂CrSi2是间接带隙半导体,其禁带宽度为0.392 eV,掺杂Ce元素,仍然是间接半导体,带隙宽度下降为0.031eV。未掺杂CrSi2在费米能级附近主要由Cr-5d、Si-3p态贡献。Ce掺杂后在费米能级附近主要由Cr-5d轨道,Ce-4f轨道,C-2p,Si-3p轨道贡献,掺杂后电导率提高。未掺杂CrSi2有两个介电峰,掺杂后,只有一个介电峰。未掺杂CrSi2,在能量为6.008处吸收系数达到最大值,掺杂后在能量为5.009eV处,吸收系数达到最大值。     

高压下LiNbO_3晶体电子结构与光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理超软赝势平面方法研究了外界压强对LiNbO_3晶体态密度,能带结构,电荷密度以及光学性质的影响.能带结构计算表明,价带顶主要由O-2p和Nb-4d态电子贡献,导带底主要由Nb-4d态电子贡献,且带隙随着压强的增加而线性增大.利用复介电函数计算了LiNbO_3晶体在不同压强下光学性质的折射率、反射率、吸收函数,能量损失函数以及光电导率.研究发现:外界压强大于10GPa时,静态折射率保持不变,随外界压强的增加,反射率、吸收函数以及光电导率区间有一定程度的拓宽,损失函数峰发生"蓝移".研究表明,外界高压可以有效调控LiNbO_3晶体的电子结构和光学性质,为LiNbO_3晶体的高压应用提供了有益的理论依据.     

高压下LiNbO3晶体电子结构与光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理超软赝势平面方法研究了外界压强对LiNbO3晶体波态密度,能带结构,电荷密度以及光学性质的影响.能带结构计算表明,价带顶主要由O-2p和Nb-4d态电子贡献,导带底主要由Nb-4d态电子贡献,且带隙随着压强的增加而线性增大.利用复介电函数计算了LiNbO3晶体在不同压强下光学性质的折射率、反射率、吸收系数,能量损失函数以及光电导率. 研究发现：外界压强大于10Gpa时,静态折射率保持不变,随外界压强的增加,反射率、吸收函数以及光电导率区间有一定程度的拓宽,损失函数峰发生“蓝移”.研究表明,高压可以有效调控LiNbO3晶体的电子结构和光学性质,为LiNbO3晶体的高压应用提供了有益的理论依据.     

BaZrO3和CaZrO3能带和光学性质的第一性原理研究

采用基于密度泛函理论基础上的CASTEP软件包,计算了BaZrO3和CaZrO3的能带以及光学性质.计算得到BaZrO3直接带隙和间接带隙分别为3.49 eV和3.23eV,CaZrO3直接带隙和间接带隙分别为3.73 eV和3.38 eV.对这两种材料的介电函数、吸收系数、反射系数、折射系数、湮灭系数和能量损失系数等光学系数进行了计算,并基于电子能带对光学性质进行了解释.得出,光学特性的异同是由于其内部微观结构上的异同所引起的.     

Effective optical constants and effective optical properties of ultrathin trilayer structures

This work presents an extension of the characteristic effective medium approximation (CEMA) to ultrathin trilayer systems. The extension has been carried out analytically and is supported by corresponding calculations of the effective optical constants of Cu-Au-Cu and Ag-SiO-Ag trilayer systems using the CEMA approximation. This work is in essence a generalization of the characteristic effective medium approximation introduced earlier for ultrathin bilayer structures. This method is used to derive the effective optical constants of a trilayer system, consisting of three thin layers with each constituent layer of thickness much less than the wavelength of the incident radiation. Within this regime a trilayer system is viewed as one effective layer referred to as an effective stack (ES) with well defined effective optical constants, which can be used to calculate the optical properties of the trilayer stack within a specified wavelength range. The CEMA based calculations of the effective optical constants are applied to two trilayer systems with a total of five stacks. Three are Cu-Au-Cu and two are Ag-SiO-Ag stacks. The thicknesses of the parent layers in the Cu-Au-Cu stack range from 3 to 30 nm for Cu and 4 to 40 nm for Au; in the Ag-SiO-Ag stack the constituent layers are 6 nm for Ag, but range from 5 to 10 nm for SiO. This study is for normal or near normal incidence spectroscopy in a wavelength range that extends from visible to near infrared. The agreement between CEMA based ES stack results and those of the standard CMT technique is very satisfactory.     

Exploring magnetic properties of ultrathin epitaxial magnetic structures using magneto-optical techniques

We describe magneto-optic Kerr effect studies of ultrathin Fe and Ni films on single crystal surfaces of Ag and Cu. Monolayer Fe films on Ag(100) exhibit the theoretically predicted spin-orbit anisotropy, but also yield some interesting discrepancies between behavior predicted by Kerr effect and by spin-polarized photoemission experiments. Layer-dependent studies of the magnetic moment of Ni on Ag(111) and Ag(100) suggest sp-d hybridization effects quench the first layer magnetic moment on Ag(111) but not on Ag(100). Temperature dependent studies of thin film magnetization obtained from Kerr effect measurements yield thickness dependent Curie temperatures, and critical exponents for several thin film systems.     

First-principle study of the structural,electronic,and optical properties of SiC nanowires

We preform first-principle calculations for the geometric, electronic structures and optical properties of SiC nanowires(NWs). The dielectric functions dominated by electronic interband transitions are investigated in terms of the calculated optical response functions. The calculated results reveal that the SiC NW is an indirect band-gap semiconductor material except at a minimum SiC NW(n = 12) diameter, showing that the NW(n = 12) is metallic. Charge density indicates that the Si–C bond of SiC NW has mixed ionic and covalent characteristics: the covalent character is stronger than the ionic character, and shows strong s–p hybrid orbit characteristics. Moreover, the band gap increases as the SiC NW diameter increases. This shows a significant quantum size and surface effect. The optical properties indicate that the obvious dielectric absorption peaks shift towards the high energy, and that there is a blue shift phenomenon in the ultraviolet region. These results show that SiC NW is a promising optoelectronic material for the potential applications in ultraviolet photoelectron devices.     

Co掺杂MgF2电子结构和光学特性的第一性原理研究

基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法,计算了Co掺杂MgF2晶体的几何结构、电子结构和光学性质.结果表明,Co掺杂导致MgF2晶体结构畸变,可能发生一种类四方和斜方型结构相变.由于Co原子的加入,体系的禁带宽度减小,可观察到半导体-金属性转变.计算也表明,Co掺杂对静态介电常数和光吸收系数有重要调制作用,所得结果与最近实验测量很好相符,揭示了Co:MgF2体系在光学元器件方面的潜在应用.     

La掺杂3C-SiC电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及稀土材料La掺杂3C-SiC的电子结构和光学性质进行理论计算.计算结果表明,La掺杂引起3C-SiC晶格体积增大,掺杂体系能量更小,掺杂体系的结构更稳定;未掺杂3C-SiC是直接带隙半导体,其禁带宽度为1.406 eV,La掺杂后带隙宽度下降为1.161 eV,La掺杂3C-SiC引入了3条杂质能级,能量较高的1条杂质能级与费米能级发生交叠,另外2条杂质能级都在费米能级以下价带顶之上,La掺杂引起3C-SiC吸收谱往低能区移动,未掺杂3C-SiC的静态介电常数为2.66,La掺杂引起静态介电常数增加为406.01,La掺杂3C-SiC是负介电半导体材料.     

Ce掺杂6H-SiC电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及Ce掺杂6H-SiC的电子结构和光学性质进行理论计算.计算结果表明,未掺杂6H-SiC是间接带隙半导体,其禁带宽度为2.045 eV,掺杂Ce元素,带隙宽度下降为0.812 eV.未掺杂6H-SiC在价带的低能区,Si-3s、C-2s电子轨道对态密度的贡献较大,在价带的高能区,主要是由Si-3p、Si-3s、C-2p态组成.掺杂后Ce原子的4f轨道主要贡献在导带部分,掺杂后电导率提高.未掺杂时,只有一个介电峰,是价带电子跃迁到导带电子所致,掺杂后有两个介电峰,第一个介电峰是由于导带电子跃迁到Ce原子4f轨道上产生,第二个峰是价带电子向导带电子跃迁产生.未掺杂6H-SiC,在能量为10.31 eV处吸收系数达到最大值,掺杂后在能量为6.57 eV处,吸收系数达到最大值.     

La单掺4H-SiC电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及La掺杂4H-SiC的电子结构和光学性质进行理论计算。计算结果表明,未掺杂4C-SiC其禁带宽度为2.257 eV。La掺杂后带隙宽度下降为1.1143eV,导带最低点为G点,价带最高点为F点,是P型间接半导体。掺杂La原子在价带的低能区间贡献比较大,而对价带的高能区和导带的贡献比较小。未掺杂4H-SiC在光子能量为6.25 eV时,出现一个介电峰,这是由于价带电子向导带电子跃迁产生。而La掺杂后,出现3个介电峰,分别对应的光子能量为0.47eV、2.67eV、6.21eV,前两个介电峰是由于价带电子向杂质能级跃迁产生,第三个介电峰是由于价带电子向导带电子跃迁产生。La掺杂后4H-SiC变成负介电半导体材料。未掺杂4h-SiC的静态介电常数为2.01,La掺杂的静态常数为12.01。