Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构，态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据；而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加，决定带隙宽度的CBM的位置下降，同时VBM的位置上升，从而导致了带隙的变窄，出现了红移现象. 此外，Cd掺杂会使晶胞发生膨胀，这种张应变也是导致Cd 关键词： 密度泛函理论 电子结构 Cd掺杂ZnO     

In掺杂ZnO电子结构的第一性原理研究

采用第一性原理平面波超软赝势,计算了纤锌矿ZnO和不同掺杂量下In掺杂ZnO晶体的能带结构、态密度和分波态密度.计算表明,In的掺杂导致ZnO禁带宽度变窄.随着掺杂量的增大,In_xZn_1－xO的导带底和价带顶同时下降,但是导带底比价带顶下降得多,这导致了带隙的变窄.此外,In掺杂使晶胞晶格常数增大,这对带隙的变窄也有一定作用.     

纤锌矿BeO掺Cd的电子结构与能带特性研究

采用基于密度泛函理论平面波赝势方法, 对纤锌矿BeO掺Cd的Be_1-xCd_xO合金进行电子结构与能带特性研究. 结果表明: Be_1-xCd_xO的价带顶始终由O 2p电子态决定, 而导带底由Be 2s和Cd 5s的电子态决定.随着Be_1-xCd_xO合金的Cd掺杂量增加, Cd 4d与O 2p的排斥效应逐渐加强, 同时Be_1-xCd_xO的带隙逐渐变小, 出现"直接-间接-直接"的带隙转变. 为了使理论值与实验值相一致, 对Be_1-xCd_xO带隙进行修正, 并分析了纤锌矿BeO-ZnO-CdO三元合金的带隙和弯曲系数与晶格常数的关系.     

Be掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对Be掺杂导致ZnO禁带宽度增加的机理进行了研究.通过对掺杂前后电子能带结构、总态密度以及分态密度的计算和比较，发现导带底(CBM)是由Be 2s电子与Zn 4s电子共同控制；而Be_xZn_1-xO价带顶 (VBM)始终由O 2p电子占据.随着掺杂量的增加，决定带隙宽度的CBM的位置上升，同时VBM的位置下降，从而导致了带隙的变宽，出现了蓝移现象.此外，Be掺杂会使晶胞发生压缩，这种压应变也是导致Be 关键词： 密度泛函理论 电子结构 Be掺杂ZnO     

共轭低聚物中光生分子间电荷转移态的第一性原理研究

本文基于第一性原理中的Heyd-Scuseria-Ernzerh方法研究了单层In_1-xGa_xN的电子结构和光学性质.计算得到单层In_1-xGa_xN的能带结构和态密度（DOS），发现随着掺杂比例的变化，体系带隙的变化范围是1.8~3.8 eV，表明通过Ga的掺杂可以实现体系带隙值的调节.并且还研究了单层In_1-xGa_xN的介电函数，折射率和吸收系数等光学性质，结果表明随着Ga掺杂浓度的增加，介电函数谱的主峰和吸收谱发生了显著的蓝移.此外，基于能带结构和态密度图谱，对单层In_1-xGa_xN的光学性质进行分析，预测这种材料独特的光学性质在纳米电子学和光学器件中会有广泛的应用.     

铂掺杂扶手椅型石墨烯纳米带的电学特性研究

本文采用基于密度泛函理论(DFT)的第一性原理计算了铂原子填充扶手椅型石墨烯纳米带(AGNR)中双空位结构的电学性能.计算结果表明: 通过控制铂原子的掺杂位置, 可以实现纳米带循环经历小带隙半导体—金属—大带隙半导体的相变过程; 纳米带边缘位置是铂原子掺杂的最稳定位置, 边缘掺杂纳米带的带隙值随宽度的变化与本征AGNR一样可用三簇曲线表示, 但在较大宽度时简并成两条曲线, 一定程度上抑制了带隙值的振荡; 并且铂原子边缘掺杂导致宽度系数N_a = 3p和3p + 1(p是一个整数)的几个较窄纳米带的带隙中出现杂质能级, 有效地降低了其过大的带隙值. 此外, 铂掺杂AGNR的能带结构对掺杂浓度不是很敏感, 从而降低了对实验精度的挑战. 本文的计算有利于推动石墨烯纳米带在纳米电子学方面的应用.     

N掺杂Cu2O薄膜的光学性质及第一性原理分析

采用射频磁控溅射技术, 在不同温度下制备了N掺杂Cu₂O薄膜．透射光谱分析发现, N掺杂导致Cu₂O成为允许的带隙直接跃迁半导体, 并使Cu₂O的光学禁带宽度增加．不同温度下沉积的薄膜光学禁带宽度E_g=2.52± 0.03 eV．第一性原理计算表明, N掺杂导致Cu₂O的禁带宽度增加了约25%, 主要与价带顶下移和导带底上移有关, 与实验报道基本符合．N的2p电子态分布不同于O原子, 在价带顶附近具有较大的态密度是N掺杂Cu₂O变成允许的带隙直接跃迁半导体的根本原因．     

铁基Heusler合金Fe2Co1-xCrxSi的结构、磁性和输运性质的研究

基于多种实验手段和能带计算的方法, 对四元合金Fe₂Co_1-xCr_xSi的晶体结构、 磁性、输运性质及能带结构进行了研究. 研究发现, 随着Cr的增加, 合金Fe₂Co_1-xCr_xSi保持了高度有序结构, 逐渐从Hg₂CuTi结构的Fe₂CoSi 过渡到L2₁结构的Fe₂CrSi; 由于次晶格网络的破坏, 居里温度逐渐下降; 系列合金的分子磁矩呈现线性下降, 符合半金属特性; 剩余电阻比率与原子占位有序程度密切相关, 呈现两端大、 中间小的特点. 在Cr替代Co的过程中, 材料半金属能隙逐渐打开, 表现半金属特征. 同时费米能级从Fe₂CoSi半金属能隙的价带顶上移至Fe₂CrSi能隙的导带底. 最大的能隙宽度出现在x= 0.75处, 这表明四元合金有可能成为具有更高自旋极化率和更强抗干扰能力的自旋电子学材料.     

Mn掺杂LiFePO4的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 计算了不同Mn掺杂浓度LiFe_1-xMn_xPO₄ (x=0,0.25,0.50,0.75) 的电子结构. 同时采用流变相辅助高温固相碳热还原法制备了LiFe_1-xMn_xPO₄ (x= 0,0.25,0.50,0.75) 材料. 理论计算表明: LiFePO₄具有E_g = 0.725 eV的带隙宽度, 为半导体材料. 通过Fe位掺杂25%的Mn离子可最大程度地 减小材料带隙宽度、降低Fe---O键及Li---O键键能, 进而提高材料的电子电导率及锂离子扩散速率. 实验结果亦表明, 当Mn掺杂量x=0.25时, 材料具有最优的电化学性能, 其具有约为158 mAh· g^-1的放电比容量以及551 Wh· kg^-1的能量密度. 理论计算与实验结果非常符合.     

Fe掺杂对溶胶凝胶法制备的ZnO: Ni薄膜结构及发光特性的影响

采用溶胶凝胶法在玻璃基片上制备了ZnO及Ni, Fe共掺杂的Zn_0.95-xNi_0.05Fe_xO (x=0, 0.005, 0.01, 0.03, 0.05) 薄膜. 通过扫描电镜(SEM) 和X射线衍射(XRD) 研究了薄膜样品的表面形貌和晶体结构. 结果表明所有样品都具有(002) 择优取向, Fe掺杂导致ZnO: Ni薄膜的晶体质量变差, 晶粒尺寸减小, 但适当的Fe掺杂有利于获得致密、 均匀的薄膜. XPS测试结果表明样品中Ni离子的价态为+2价, Fe离子的价态为+2价和+3价.室温光致发光(PL) 测量表明, 所有样品均观察到较强的紫外发光峰, 蓝光双峰和绿光发光峰. ZnO: Ni薄膜的发光强度可以通过Fe掺杂进行有效调节. 进而我们讨论了Ni, Fe共掺杂ZnO样品的发光机理.     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

Exploring the electronic structure of Pb2+ ions containing material Pb16(OH)16(NO3)16

A theoretical band structure calculation for lead nitrate hydroxide Pb₁₆(OH)₁₆(NO₃)₁₆ single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70 eV in close agreement to the measured one (3.78 eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbO_n ployhedra and N–O of [NO₃]⁻ triangle. The calculated bond lengths and angles show good agreement with the experimental data.     

A model for the band gap energy of the N-rich GaN1−xAsx (0<x≤0.07) and the As-rich GaN1−xAsx (0.95≤x≤1)

In this paper, a model for the band gap energy of the N-rich GaNAs (0<x≤0.07) and the As-rich GaNAs (0.95<x≤1) is developed. For the N-rich GaNAs, The parameters describing the variation of the CBM and the VBM in the N-rich GaNAs are obtained by fitting the experimental data. For the As-rich GaNAs, the parameters in the model are obtained by fitting the experimental data of the band gap energy. It is found that the band gap evolution of the N-rich energy is different from that of the As-rich band gap energy. The model may be used to calculate the band gap energy of other dilute group III-N–V nitrides.     

First-principles energy band calculation for undoped and N-doped InTaO4 with layered wolframite-type structure

The electronic structures of undoped and N-doped InTaO₄ with optimized structures are calculated within the framework of the density functional theory. Calculated lattice constants are in excellent agreement with experimental values, within a difference of 2%. The valence band maximum (VBM) is located near the middle point on the ZD line and the conduction band minimum (CBM) near the middle point on the DX line. This means that InTaO₄ is an indirect-gap material and a minimum theoretical gap between VBM and CBM is ca. 3.7 eV. The valence band in the range from −6.0 to 0 eV mainly consists of O 2p orbitals, where In 4d5s5p and Ta 5d orbitals are slightly hybridized with O 2p orbitals. On the other hand, the conduction band below 5.5 eV is mainly composed of the Ta 5d orbitals and the contributions of In and O orbitals are small. The band gap of N-doped InTaO₄ decreases by 0.3 eV than that of undoped InTaO₄, because new gap states originating from N 2p orbitals appear near the top of the valence band. This result indicates that doping of N atoms into metal oxides is a useful method to develop photocatalysts sensitive to visible light.     

Electronic structure of half-metallic ferromagnet Co<Subscript>2</Subscript>MnSi at high-pressure

In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co₂MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (σ-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L2₁ structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around ~70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at Γ-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.     

Electronic structures and effective masses of photogenerated carriers of CaZrTi2O7 photocatalyst: First-principles calculations

The band structures, density of states and effective masses of photogenerated carriers for CaZrTi₂O₇ photocatalyst were performed using first principles method with the virtual crystal approximation. The results indicated that CaZrTi₂O₇ has an indirect band gap of about 3.25 eV. The upper valence bands of CaZrTi₂O₇ are formed by O 2p states mixed with Ti 3d states, Zr 4d, 4p and 5s states, while the conduction bands are dominated by Ti 3d states, Zr 4d states and O 2p states. The calculated valence bands maximum (VBM) potential is located at 2.60 V (vs. normal hydrogen electrode (NHE)), while the conduction bands minimum (CBM) potential at ?0.65 V. Therefore, CaZrTi2O7 has the ability to split water to hydrogen and oxygen under UV light irradiation. The calculated minimum effective mass of electron in CBM is about 1.35 m₀, and the minimum effective mass of hole in VBM is about 1.23 m₀. The lighter effective masses facilitate the migration of photogenerated carriers and improve photocatalytic performance.     

First-principles study of atomic and electronic structures of kaolinite in soft rock

Kaolinite is a kind of clay mineral which often causes large deformations in soft-rock tunnel engineering and thus causes safety issues. To deal with these engineering safety issues, the physical/chemical properties of the kaolinite should be studied from basic viewpoints. By using the density-functional theory, in this paper, the atomic and the electronic structures of the kaolinite are studied within the local-density approximation (LDA). It is found that the kaolinite has a large indirect band gap with the conduction band minimum (CBM) and the valence band maximum (VBM) being at the Γ and the B points, respectively. The chemical bonding between the cation and the oxygen anion in kaolinite is mainly ionic, accompanied by a minor covalent component. It is pointed that the VBM and the CBM of kaolinite consist of oxygen 2p and cation s states, respectively. The bond lengths between different cations and anions, as well as of the different OH groups, are also compared.