聚偏二氟乙烯晶体的电子结构和光学性质

利用含Tkatchenko-Scheffler(TS)色散修正的密度泛函理论的第一性原理方法对九种聚偏二氟乙烯(PVDF)晶相的电子结构和光学性质进行了计算. 结果表明,PVDF晶体作为一种绝缘体,能带具有密集且平直等特征,其带隙值在6.05-7.34 eV之间,且和实验值接近. 价带主要是F原子的2s和2p态起主要贡献,导带主要由C原子的2p态和H原子的1s态共同参与构成. 在0-35 eV光子能量范围内,介电函数、吸收率、反射率和折射率等光学性质发生变化主要在深紫外区域. 根据介电函数等光学参数的谱特点,可以将九种PVDF的晶相划分为{Ⅰ_p},{Ⅱ_pu},{Ⅱ_au,Ⅱ_ad,Ⅱ_pd,Ⅲ_pu},{Ⅲ_au,Ⅲ_ad,Ⅲ_pd}等四类,每一类都具有相似的光学参数特点.     

钛铁矿型六方相ZnTiO3的电子结构和光学性质

分别采用基于密度泛函理论(DFT)的局域密度近似(LDA)和广义梯度近似(GGA)方法对钛铁矿型六方相ZnTiO₃的电子结构进行了第一性原理计算, 并在局域密度近似下计算了六方相ZnTiO₃的光学性质, 并将计算结果与实验数据进行了对比. 结果表明, 在局域密度近似下计算得到的结构参数更接近实验数据. 理论预测六方相ZnTiO₃属于直接带隙半导体材料, 其禁带宽度(布里渊区Z 点)为3.11 eV. 电子态密度和Mulliken 电荷布居分析表明Zn―O键是典型的离子键而Ti―O键是类似于钙钛矿型ATiO₃ (A=Sr, Pb, Ba)的Ti―O共价键. 在50 eV的能量范围内研究了ZnTiO₃的介电函数、吸收光谱和折射率等光学性质, 并基于电子能带结构和态密度对光学性质进行了解释.     

一种具有二次谐波发生和适中双折射的紫外透过稀土金属甲酸盐

本研究工作报道了一例采用溶液挥发法合成的稀土金属甲酸盐，分子式为(C(NH₂)₃)[Er(HCOO)₄]。该化合物结晶于非中心对称空间群。光学性质研究表明：该化合物具有较大光学带隙(4.76 eV)、适中的双折射率(0.066@546 nm)；在1 064 nm处，其粉末倍频效应为KH₂PO₄(KDP)的0.2倍，并且可实现相位匹配。第一性原理计算和单晶结构分析揭示了线性和非线性光学效应来源于C (NH₂)₃⁺、[ErO₈]和HCOO^-单元的协同作用。     

一种具有二次谐波发生和适中双折射的紫外透过稀土金属甲酸盐

本研究工作报道了一例采用溶液挥发法合成的稀土金属甲酸盐,分子式为(C (NH₂)₃)[Er (HCOO)₄]。该化合物结晶于非中心对称空间群。光学性质研究表明：该化合物具有较大光学带隙(4.76 eV)、适中的双折射率(0.066@546 nm);在1 064 nm处,其粉末倍频效应为KH₂PO₄(KDP)的0.20倍,并且可实现相位匹配。第一性原理计算和单晶结构分析揭示了线性和非线性光学效应来源于C (NH₂)₃⁺、[ErO₈]和HCOO^-单元的协同作用。     

一种超硬新材料BeP2N4的电子结构和力学性质及本征硬度

基于密度泛函理论,采用局域密度近似(LDA)和广义梯度近似(GGA)泛函研究了硅铍石、尖晶石结构的 BeP₂N₄ 材料的晶格参数、能带结构、态密度、分态密度、Mulliken布居值和弹性性质, 计算结果与已有的实验值和理论值符合很好. 能带结构和态密度表明两种结构的BeP₂N₄材料是宽的直接带隙的绝缘体材料. 尖晶石结构BeP₂N₄的体弹性模量、剪切模量和弹性模量比硅铍石结构的相应的力学量大得多. 利用Sung等提出的硬度经验判据和Gao等提出的基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法, 预测了两种结构的本征硬度值. 计算结果表明硅铍石结构BeP₂N₄虽然体弹模量小, 但是它并不是一种软的材料, 而是一种易脆的硬度较硬的材料, 随着压力增加硅铍石结构BeP₂N₄的脆性逐渐过渡到延性. 尖晶石结构BeP₂N₄是一种易脆的超硬材料. 采用GGA计算得到的硅铍石BeP₂N₄向尖晶石相转变压力为14 GPa, 与理论预测值(24 GPa)相比偏小.     

Ru掺杂SnO2半导体固溶体的电子结构研究

运用基于密度泛函理论的第一性原理方法,建立了SnO₂以及不同比例Ru掺杂的SnO₂超胞模型,在对其进行几何优化后计算了Sn_1-xRu_xO₂(x=0,1/16,1/12,1/8,1/6,1/4,1/2)半导体的电子结构,并讨论了其晶格参数、电荷密度、能带结构和态密度(包括分态密度)等性质。结果表明,掺杂后,晶格参数随掺杂量的增加线性减小,与实验值的偏差在4%以内;掺杂后,在费米能级处可以提供更多的填充电子,使得电子跃迁至导带更容易,固溶体的导电性增强。为Sn_1-xRu_xO₂固溶体电极材料的发展和应用提供了理论基础。     

Ru掺杂SnO2半导体固溶体的电子结构研究

运用基于密度泛函理论的第一性原理方法,建立了SnO₂以及不同比例Ru掺杂的SnO₂超胞模型,在对其进行几何优化后计算了Sn_1-xRu_xO₂(x=0,1/16,1/12,1/8,1/6,1/4,1/2)半导体的电子结构,并讨论了其晶格参数、电荷密度、能带结构和态密度(包括分态密度)等性质。结果表明,掺杂后,晶格参数随掺杂量的增加线性减小,与实验值的偏差在4%以内;掺杂后,在费米能级处可以提供更多的填充电子,使得电子跃迁至导带更容易,固溶体的导电性增强。为Sn_1-xRu_xO₂固溶体电极材料的发展和应用提供了理论基础。     

Cu2ZnSnS4(CZTS)纳米微球的制备及表征

为研究PVP含量对CZTS颗粒形貌以及分散性的影响,本文采用溶剂热法,以CuCl₂·2H₂O、Zn(Ac)₂·2H₂O、SnCl₄·5H₂O作金属源,硫脲作硫源,乙二醇为溶剂,在体系中加入不同含量的PVP,成功制备了CZTS微球。通过XRD、Raman、SEM、TEM、UV-Vis等方法检测分析CZTS纳米微球的物相、结构、形貌以及光学性能。结果表明:所得CZTS纳米颗粒具有锌黄锡矿结构;当体系中PVP含量为0.2g时,颗粒分散性较好,制备的颗粒形貌为表面嵌有纳米薄片的微球,纳米片较在体系中加入0.1gPVP更致密;光学带隙约为1.47eV,与太阳能电池所需的最佳带隙接近。最后,对表面嵌有纳米薄片的CZTS微球可能的形成机理进行了推测。     

6,12-二乙炔基茚并[1,2-b]芴系列衍生物的非线性光学性质

采用密度泛函理论(DFT) CAM-B3LYP方法对6,12-二乙炔基茚并[1,2-b]芴系列衍生物的极化率(α_s)和第二超极化率(γ_s)进行研究. 结果表明, 此类分子具有较大的γ_s值. 用乙炔基硅烷基和氧原子取代茚并[1,2-b]芴分子6,12位的氢原子后, 分子的几何构型发生改变, 进而影响其非线性光学(NLO)性质. 连接乙炔基硅烷基的分子α_s值和γ_s值均增大, 而连接氧原子的分子α_s值和γ_s值均减小. 茚并[1,2-b]芴环2,8位取代基R(R=H, F, CH₃)的不同, 对分子的γ_s值也有一定的影响, R为CH₃时分子的α_s值和γ_s值均较大. 由含时密度泛函理论(TD-DFT)方法计算的吸收光谱分析可知, 与茚并[1,2-b]芴系列分子相比, 引入乙炔基硅烷基的分子共轭性增强, 最大吸收波长红移; 引入氧原子的分子几何结构扭曲, 共轭性降低, 最大吸收波长蓝移.     

乙醇-水分子团簇C2H5OH(H2O)n (n=1-9)稳定结构的量子化学研究

采用密度泛函理论B3LYP方法, 在B3LYP/6-311++G(2d,2p)//B3LYP/6-311++G(d,p)基组水平上对乙醇-水分子团簇(C₂H₅OH(H₂O)_n (n=1-9))的各种性质进行研究, 如: 优化的几何构型、结构参数、氢键、结合能、平均氢键强度、自然键轨道(NBO)电荷分布、团簇的生长规律等. 结果表明, 从二维(2-D)环状结构到三维(3-D)笼状结构的过渡出现在n=5的乙醇-水分子团簇中. 此外, 利用团簇结合能的二阶差分、形成能、能隙等性质, 发现在n=6时乙醇-水分子团簇的最低能量结构稳定性较好, 可能为幻数结构. 最后, 为了进一步探讨氢键本质, 将C₂H₅OH(H₂O)_n (n=2-9)最低能量结构的各种性质与纯水分子团簇(H₂O)_n (n=3-10)比较, 结果表明前者与后者中的水分子之间氢键相似.     

固溶体Bi_2Mo_(1-x)W_xO_6的水热合成及光催化性能

采用水热法,在较低温度下合成了系列Bi2Mo1-xWxO6固溶体。结果表明,W的替代抑制了固溶体的晶粒生长,导致了较小的晶粒尺寸。随着x的增加,红外光谱中840cm-1处M-O键的振动频率νM-O有规律地向低频率方向移动,表明Mo6+离子逐步被W6+替代,生成了无限互溶的固溶体。光吸收性能研究表明,随着W6+逐步替代Mo6+,带隙出现了先降后升的趋势,x=0.4时带隙最小。而固溶体的光催化性能随着x的增加,出现了先增后减的趋势,x=0.4时光催化活性最高。此外,含W样品的光催化活性高于Bi2MoO6。这与固溶体的带隙、带结构和晶粒尺寸变化有关。     

Synthesis of Metal Chalcogenide Semiconductors by Thermal Decomposition of Organosulfur and Organoselenium Compounds

Metal chalcogenides – because of their excellent optical and electrical properties – are important semiconductor materials for optical devices, such as solar cells, sensors, and photocatalysts. The challenges associated with metal chalcogenides are the complexity of the conventional synthesis methods and the stringent synthesis conditions. In this study, the synthesis conditions were simplified in a solvent-free synthesis method using cadmium precursor, thiourea and selenium to synthesize metal chalcogenides, such as CdS and CdSe, which have particularly suitable band gaps for the optical devices. CdS_xSe_1-x solid solution was successfully synthesized under molten thiourea as the reactive reaction medium at relatively low temperatures, even at 180 °C, with residual melamine derivatives in the solid phase. The luminescence properties of CdS_xSe_1-x and the products in the gas and solid phases were investigated. Optimization of the synthesis conditions for solid solutions of CdS_xSe_1-x and the role of organic compounds in the formation of metal chalcogenides are discussed.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Thermoelectric properties of Ag-doped n-type (Bi2Te3)0.9-(Bi2−xAgxSe3)0.1 (x=0-0.4) alloys prepared by spark plasma sintering

Ag-doped n-type (Bi₂Te₃)_0.9-(Bi_2−xAg_xSe₃)_0.1 (x=0-0.4) alloys were prepared by spark plasma sintering and their physical properties evaluated. When at low Ag content (x=0.05), the temperature dependence of the lattice thermal conductivity follows the trend of (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1; while at higher Ag content, a relatively rapid reduction above 400 K can be observed due possibly to the enhancement of scattering of phonons by the increased defects. The Seebeck coefficient increases with Ag content, with some loss of electrical conductivity, but the maximum dimensionless figure of merit ZT can be obtained to be 0.86 for the alloy with x=0.4 at 505 K, about 0.2 higher than that of the alloy (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1 without Ag-doping.     

Vibrational and optical properties of CZTS derivatives for photovoltaic applications: Synthesis of Cu2ZnSn1-xGexS2Se2 compounds

Herein, we report the vibrational and optical properties of new Cu₂ZnSn_1-xGe_xS₂Se₂ solid solutions prepared by isomorphic substitution of a fraction of Se by S in Cu₂ZnSn_1-xGe_xSe₄ (CZTGeSe) through ceramic method. The Raman spectra and x-ray diffraction analysis on samples confirms that they crystallize in Kesterite-type structure. The Raman peaks were analyzed by fitting of the spectra allowing identification the vibrational modes by comparison with experimental and theoretical data from CZTGeSe and CZTGeS end-members. The dependency between the amount of cation and chalcogen substituent in optical characterization shows band gap around of 1.36 eV, close to the optimum value for solar cells with high efficiency.     

Transport and optical properties of heavily hole-doped semiconductors BaCu2Se2 and BaCu2Te2

Experimental and theoretical studies of the electronic and optical properties of orthorhombic BaCu₂Se₂ and BaCu₂Te₂ are reported. Experimental data include the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, and lattice constants for , and optical transmission and diffuse reflectance data at room temperature. Nominally stoichiometric, polycrystalline samples form with hole concentrations inferred from Hall measurements of 2×10¹⁸ and 5×10¹⁹ cm⁻³ near room temperature for the selenide and telluride, respectively. The corresponding mobilities are near 15 cm² V⁻¹ s⁻¹ for both materials. Optical measurements reveal a transition near 1.8 eV in BaCu₂Se₂, while no similar feature was observed for BaCu₂Te₂. First principles calculations indicate both materials are direct or nearly direct gap semiconductors with calculated gaps near 1.0 eV and 1.3 eV for the telluride and selenide, respectively, and predict weak absorption below about 2 eV. Transport properties calculated from the electronic structure are also presented.     

Synthesis, optical properties and electronic structures of polyoxometalates K3P(Mo1−xWx)12O40 (0?x?1)

Various compositions of solid solutions K₃P(Mo_1−xW_x)₁₂O₄₀ (0?x?1) were prepared using two solid state synthetic routes. The crystallite size was determined by linewidth refinements of X-ray diffraction patterns using the Warren-Averbach method, and the grain size distribution by laser scattering experiments. Optical properties were determined by diffuse reflectance measurements in the UV-visible range. The optical gap E_g was found to increase exponentially from ∼2.5 to ∼3.30 eV with increasing x, and is systematically shifted to a higher energy when the grain size decreases. The relation between E_g and x was analyzed by calculating the HOMO-LUMO gaps of the [P(Mo_1−xW_x)₁₂O₄₀]³⁻ anions on the basis of tight-binding electronic structure calculations.     

Transport Properties of Bi2−xInxSe3 Single Crystals

The paper reports on the temperature dependence of the electrical and thermal conductivity, Hall constant, and Seebeck coefficient of Bi_2−xIn_xSe₃ (x=0, 0.2, 0.4) single crystals measured over the temperature range from 2 to 300 K. One single-valley conduction band model is used to interpret relations among transport coefficients. The data analysis relies on the use of a mixed carrier scattering mechanism consisting of acoustic scattering and scattering on ionized impurities. The effect of In incorporation into the Bi₂Se₃ crystal lattice on the individual components of thermal conductivity is evaluated and discussed.