共轭低聚物中光生分子间电荷转移态的第一性原理研究（英文）

本文基于第一性原理中的Heyd-Scuseria-Ernzerh方法研究了单层In_(1-x)Ga_xN的电子结构和光学性质.计算得到单层In_(1-x)Ga_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-x)Ga_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-x)Ga_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.     

Ga_(1–x)Cr_xSb(x=0.25,0.50,0.75)磁学和光学性质的第一性原理研究

随着高性能电子器件需求的日益增加,自旋电子学材料在材料研究和电子元件中具有重要地位,因为自旋电子学器件相比于传统半导体电子元件具有非易失性,低功耗和高集成度的优点.本文研究了Cr离子注入GaSb的电子性质、磁学和光学性质,采用基于密度泛函理论框架下的缀加投影平面波方法,利用广义梯度近似下电子交换关联泛函,考虑了Heyd-Scuseria-Ernzerhof(HSE06)杂化泛函计算进行电子能带带隙修正,首先对不同浓度Cr离子注入下的闪锌矿结构半导体GaSb形成的Ga_(1-x)Cr_xb(x=0.25,0.50,0.75)进行结构优化,计算了无磁、铁磁及反铁磁的基态能量,对比发现它们的基态均为铁磁态.对电子能带结构图分析发现,它们自旋向上的电子能带穿过费米面,而自旋向下的电子能带存在直接带隙,呈现铁磁半金属性质.同时在平衡晶格常数下具有整数倍玻尔磁矩,并在一定晶格变化范围内保持磁矩不变.同时发现它们在红外波段具有良好的吸收能力,这使得Ga_(1-x)Cr_xSb在自旋电子学器件和红外光电器件中拥有潜在的应用前景.     

Zn1-xMgxO电子结构及光学性质的第一性原理GGA+U方法研究

Mg掺杂ZnO形成的固溶体Zn_1-xMg_xO（ZMO）（0 ≤ x ≤ 0.25）是一种带隙较宽、电子学性质可调控的半导体材料,在薄膜太阳电池及光电设备的透明电极等方面具有重要的应用价值。基于密度泛函理论下的第一性原理超软赝势方法,采用GGA+U计算了ZMO的电子结构和光学性质。计算结果表明,随着x值的增加,ZMO的禁带宽度由x=0时的3.32 eV增加到x=0.25时的3.78 eV;光吸收边及反射谱和能量损失谱均发生明显蓝移,峰值存在于紫外光区。计算结果与实验结论相符合。     

基于锡组分和双轴张应力调控的临界带隙应变Ge_(1-x)Sn_x能带特性与迁移率计算

能带工程通过改变材料的能带结构可以显著提升其电学和光学性质,已广泛应用于半导体材料的改性研究.双轴张应力和Sn组分共同作用下的Ge_(1-x)Sn_x合金,不仅可以解决直接带隙转变所需高Sn组分带来的工艺难题,而且载流子迁移率会显著提升,在单片光电集成领域有很好的应用前景.根据形变势理论,分析了(001)面双轴张应变Ge_(1-x)Sn_x的带隙转变条件,并给出了在带隙转变临界点Sn组分和双轴张应力的关系;采用8κ·p方法,得到了临界带隙双轴张应变Ge_(1-x)Sn_x在布里渊区中心点附近的能带结构,进而计算得到电子与空穴有效质量;基于载流子散射模型,计算了电子与空穴迁移率.计算结果表明:较低Sn组分和双轴张应力的组合即可得到直接带隙Ge_(1-x)Sn_x合金,且直接带隙宽度随着应力的增大而减小;临界带隙双轴张应变Ge_(1-x)Sn_x具有极高的电子迁移率,空穴迁移率在较小应力作用下即可显著提升.考虑工艺实现难度和材料性能两个方面,可以选择4%Sn组分与1.2 GPa双轴张应力或3%Sn组分与1.5 GPa双轴张应力的组合用于高速器件和光电器件的设计.     

石墨烯光学性质及其应用研究进展

石墨烯因其优异的光学和电学性能,及其与硅基半导体工艺的兼容性,而备受学术界和工业界的广泛关注。作为一种独特的二维原子晶体材料,石墨烯有着优异的机械性能、超高的热导率和载流子迁移率、超宽带的光学响应谱及极强的非线性光学特性,使其在新型光学和光电器件领域具有得天独厚的优势。一系列基于石墨烯的新型光电器件先后被研制出,已显示出优异的性能和良好的应用前景。本文将介绍石墨烯光学性质、与光的相互作用以及提高方法,并给出其在光子和光电子器件领域的应用,分析了这些器件所使用的结构及特点,重点阐述了在全内反射结构下,石墨烯与光相互作用的增强及其偏振依赖性质,及其利用该偏振依赖性质在光学传感、光存储等方面的应用,以及在细胞传感方面的重要发现。最后对石墨烯光学性质及其应用的现状进行了总结和展望。     

Cr含量对ZB型半金属铁磁体Cr_xZn_(1-x)Se磁电性能的影响

使用基于密度泛函理论的第一性原理方法,优化了闪锌矿结构的Cr_xZn_(1-x)Se(x=0.000,0.125,0.250和0.375)的2×2×2超胞的几何结构,计算了其自旋极化的态密度和能带结构、离子磁矩、电荷分布等磁电性能,详细分析了Cr含量对Cr_xZn_(1-x)Se磁电性能的影响.结果表明,Cr掺杂后ZB型ZnSe具有明显的半金属特性;当x=0.125,0.250和0.375时,Cr_xZn_(1-x)Se均有较宽的半金属带隙,从而可能具有较高的居里温度;当x=0.125时,Cr_xZn_(1-x)Se的半金属性最稳定;x=0.125,0.250和0.375时,Cr_xZn_(1-x)Se的超胞磁矩分别为整数磁矩4.0,8.0和12.0μ_B,而具有整数磁矩是半金属铁磁体非常重要的特征之一.Cr_xZn_(1-x)Se的半金属性和磁性主要来源于Cr离子的自旋极化,Cr离子的电子结构为Cr e_g~2↑e_g~1↓t_(2g)~3↑.     

卤化硅烯结构、电子和光学性质的第一性原理研究

二维硅烯的商业用途通常受到其零带隙的抑制,限制了其在纳米电子和光电器件中的应用.利用基于密度泛函理论的第一性原理计算,单层硅烯的带隙通过卤原子的化学官能化被成功打开了,并综合分析了卤化对单层硅烯的结构,电子和光学性质的影响.研究结果表明卤化使结构变得扭曲,但保持了良好的稳定性.通过HSE06泛函,全功能化赋予硅烯1.390至2.123 eV的直接带隙.键合机理分析表明,卤原子与主体硅原子之间的键合主要是离子键.最后,光学性质计算表明,I-Si-I单层在光子频率为10.9 eV时达到最大光吸收,吸收值为122000 cm^-1,使其成为设计新型纳米电子和光电器件的有希望的候选材料.     

非层状二维CdSe的制备及厚度对带隙的影响

二维半导体材料的天然带隙有望弥补石墨烯的零带隙缺陷,打破后者在场效应晶体管、开关器件、逻辑电路等领域的应用瓶颈.相较于层状半导体材料,非层状半导体材料多以较强的离子键/共价键结合,且各向同性,因此要获取其二维结构存在一定挑战.本论文通过化学气相沉积法实现了非层状Cd Se在云母衬底上的二维各向异性生长.详细表征了二维Cd Se的微观形貌、晶体结构和光学特性等.结果表明,样品具有显著的光致发光(PL)效应,说明厚度减薄至纳米级时不会破坏Cd Se的直接带隙属性.此外,随着厚度减小,样品的PL峰逐渐蓝移.为了进一步解释该现象,采用基于密度泛函理论的第一性原理计算方法,研究了不同厚度的Cd Se的能带结构,结果显示均为直接带隙,且随厚度减小,带隙值增大,与实验现象吻合.由此可知,通过生长参数调控二维Cd Se的厚度,即可实现对其带隙的有效调控,这对相关光电器件的性能提升具有指导意义.     

立方晶相HfO2电子结构与光学性质的第一性原理计算

利用密度泛函理论框架下的第一性原理平面波超软赝势方法计算了立方晶相二氧化铪(c-HfO2)的电子结构,得到了c-HfO2的总念密度、分波态密度和能带结构.经带隙校正后,计算了c-HfO2的光学线性响应函数随光子能最的变化关系,包括复介电函数,反射率、复折射率以及光学吸收系数,并从理论上给出了c-HfO2材料光学性 质与电子结构的关系.经比较发现,对c-HfO2的电子结构和光学性质的计算结果与已有的实验数据和其它理论研究吻合得较好,从而为c-HfO2光电材料的设计与应用提供了理论依据.同时,计算结果也表明采用密度泛函理论的广义梯度近似来计算和预测c-HfO2材料的电子结构和光学性质是比较可靠的.     

多层结构下石墨烯的可见性研究

石墨烯-氮化硅光电器件在光电集成方面具良好的性能,但在氮化硅上转移石墨烯的可见度通常不佳,导致难以判断转移石墨烯的效果,因此研究石墨烯在氮化硅上的可见性具有重要意义。基于菲涅耳公式,推导出石墨烯在多层结构下的光学反射率和对比度理论计算公式,使用MATLAB仿真并分析石墨烯/氮化硅/二氧化硅/硅多层结构下不同二氧化硅厚度对光学对比度的影响。仿真结果表明,在特定的二氧化硅层厚度下,石墨烯的光学对比度能显著增加,从而使石墨烯在显微镜下清晰可见,能更好地判断石墨烯的转移效果,当二氧化硅厚度为2547 nm时,使用500 nm的青绿色光源作为背景光,石墨烯与周围介质的对比度能够超过0.3。最后对二氧化硅厚度为2330 nm和2690 nm的石墨烯/氮化硅/二氧化硅/硅多层结构的样品进行对比度实验,实验结果与理论预测有一定差异,分析表明主要原因为材料沉积厚度和光源波长存在偏差,这对后续改进器件制作工艺以及完善测试方式具有指导意义。     

Effect of Ni substitution on the formability and magnetic properties of Gd_(50)Co_(50) amorphous alloy

A small amount of Ni was added into the binary Gd_(50)Co_(50) amorphous alloy to replace Gd in order to obtain ternary Co_(50)Gd_(50-x)Ni_x(x = 1, 2, and 3) amorphous alloys. Compared to the binary Gd_(50)Co_(50) amorphous alloy, the Co_(50)Gd_(50-x)Ni_x amorphous alloys show an enhanced Curie temperature(T_C) with a weakened formability. The maximum magnetic entropy change(-?S_m~(peak)) of the Co_(50)Gd_(50-x)Ni_x amorphous alloys is found to decrease with the increasing T_C.The adiabatic temperature rise(?T_(ad)) of the Co_(50)Gd_(47)Ni_3 amorphous alloy is superior to that of the Fe-based metallic glasses at room temperature. The variation of the T_C and -?S_~(peak) of the Gd_(50)Co_(50) amorphous alloy with Ni addition, and the mechanism involved, were discussed.     

Random alloy-like local structure of Fe(Se, S)(1-x)Te(x) superconductors revealed by extended x-ray absorption fine structure

The local structure of Fe(Se, S)(1-x)Te(x) ternary (11-type) chalcogenides has been studied by temperature dependent Fe K-edge extended x-ray absorption fine structure measurements. We find that the Fe-Se and Fe-Te distances in ternary FeSe(1-x)Te(x) are closer to the respective distances in the binary systems, revealing significant divergence of the local structure from the average one. The mean square relative displacements show a systematic change with Te content, consistent with bond relaxation in the inhomogeneous ternary phases. Also, the Fe-Te and Fe-S distances in the FeS(0.2)Te(0.8) ternary system are found to be different in the crystallographically homogeneous structure. The observed features are characteristic of ternary random alloys, suggesting that a proper consideration should be given to the atomic distribution for describing the complex electronic structure of these multi-band Fe-based chalcogenides.     

新型红色荧光粉NaLa_(0.7)(MoO_4)_(2-x)(WO_4)_x∶0.3Eu~(3+)的制备及发光性质研究

采用高温固相法合成具有余辉性能的发光材料NaLa_(0.7)(MoO_4)_(2-x)(WO_4)_x∶0.3Eu~(3+)(x=0,0.5,1,1.5,2)。用X射线衍射(XRD)和荧光光谱对样品的晶体结构和发光特性进行表征。测试结果表明,在900℃下烧结8 h所合成的NaLa_(0.7)(MoO_4)_(2-x)(WO_4)_x∶0.3Eu~(3+)样品为纯相Na La(Mo O_4)_2,样品可被近紫外光393nm和蓝光462 nm有效激发,其发射主峰位于615 nm处,属于Eu3+的5D0-7F2跃迁。Na La_(0.7)(Mo O_4)_(2-x)-(WO_4)_x∶0.3Eu~(3+)的发光强度随着W6+浓度的增加而增大,当W6+掺杂量x=1时发光最强,而后随W6+掺杂浓度的增加出现浓度猝灭现象。通过计算得到样品在393 nm和462 nm激发下的色坐标,当W6+的掺杂量x=1时,样品的红光色纯度最好。     

Theoretical optoelectronic analysis of intermediate-band photovoltaic material based on ZnY1-xOx （Y = S,Se, Te） semiconductors by first-principles calculations

The structural, energetic, and electronic properties of lattice highly mismatched ZnY1-xOx （Y = S, Se, Te） ternary alloys with dilute O concentrations are calculated from first principles within the density functional theory. We demonstrate the formation of an isolated intermediate electronic band structure through diluted O-substitute in zinc-blende ZnY （Y = S, Se, Te） at octahedral sites in a semiconductor by the calculations of density of states （DOS）, leading to a significant absorption below the band gap of the parent semiconductor and an enhancement of the optical absorption in the whole energy range of the solar spectrum. It is found that the intermediate band states should be described as a result of the coupling between impurity O 2p states with the conduction band states. Moreover, the intermediate bands （IBs） in ZnTeO show high stabilization with the change of O concentration resulting from the largest electronegativity difference between O and Te compared with in the other ZnSO and ZnSeO.     

Site Preference of Se and Te in Bi_2Se_(3-x)Te_x Thin Films

The ternary topological insulators Bi_2Se_(3-x)Te_x have attracted a great deal of attention due to their exotic physical and chemical properties.While most of the studies focus on the properties of these ternary TIs,limited research was performed to investigate the dynamic atomic stack of its crystal structure.We prepared highquality Bi_2Se_(3-x)Te_x thin films on Ga As(111)B substrates using molecular beam epitaxy,characterized with Raman spectroscopy,x-ray diffraction and photoelectron spectroscopy.It is found that when Se is replaced by Te,the preferred substituting sites are the middle layer at 0x1,and this is also valid for Se substituting Te at 2x3.In the middle region,the substituting atoms prefer to go to the first and the fifth layer.     

The structural,elastic,and electronic properties of Zr_x Nb_(1-x)C alloys from first principle calculations

The structural,elastic,electronic,and thermodynamic properties of Zr x Nb1-x C alloys are investigated using the first principles method based on the density functional theory.The results show that the structural properties of Zr x Nb1-x C alloys vary continuously with the increase of Zr composition.The alloy possesses both the highest shear modulus(215 GPa)and a higher bulk modulus(294 GPa),with a Zr composition of 0.21.Meanwhile,the Zr0.21Nb0.79C alloy shows metallic conductivity based on the analysis of the density of states.In addition,the thermodynamic stability of the designed alloys is estimated using the calculated enthalpy of mixing.     

Structural and Electrical Properties of Be_xZn_(1-x)O Alloys under High Pressure

We conduct extensive research into the structures of Be_xZn_1-xOO ternary alloys in a pressure range of 0-60GPa,using the ab initio total energy evolutionary algorithm and total energy calculations,finding several metastable structures.Our pressure-composition phase diagram is constructed using the enthalpy results.In addition,we calculate the electronic structures of the Be_xZn_1-xOO structures and investigate the bandgap values at varying pressures and Be content.The calculated results show that the bandgap of the Be_xZn_1-xOO ternary alloys increases with an increase in Be content at the same pressure.Moreover,the bandgap of the Be_xZn_1-xOO ternary alloys increases with the increasing pressure with fixed Be content.At the same Be content,the formation enthalpy of the Be_xZn_1-xOO ternary alloys first decreases,then increases with the increasing pressure.     

(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)的荧光以及长余辉发光性能

采用高温固相法制备了一系列(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)(0≤x≤0.25)绿色荧光粉,并研究了Mg离子对(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)的结构、荧光以及长余辉发光性能的影响。Mg离子取代Zn进入Zn_2GeO_4晶格,形成(Zn_(1-x),Mg_x)_2GeO_4固溶体,并产生了晶格畸变。光谱分析结果表明,样品中位于533 nm的绿色荧光源于Mn~(2+)的~4T_1(~4G)→~6A_1(~6S)跃迁。随着Mg离子浓度的增加,(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)样品的激发光谱出现了蓝移现象,说明Mg离子进入到Zn_2GeO_4晶格中对其晶格结构产生了影响,导致(Zn_(1-x),Mg_x)_2GeO_4的带宽发生改变。发射光谱则表明Mg离子进入Zn_2GeO_4晶格引起Mn~(2+)的~4T_1(~4G)→~6A_1(~6S)跃迁绿色荧光发光强度的增强。Zn_2GeO_4基质中的氧空位缺陷陷阱深度由于基质带宽的变化而变深,样品具有良好的长余辉发光效果。通过热释光谱分析研究了材料中缺陷陷阱的特征,进一步证实了(Zn_(1-x),Mg_x)_2GeO_4中缺陷陷阱深度发生改变。根据光谱分析结果给出了(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)中荧光与余辉发光的产生机理。     

Increasing the equilibrium solubility of dopants in semiconductor multilayers and alloys

We have theoretically studied the possibility to control the equilibrium solubility of dopants in semiconductor alloys, by strategic tuning of the alloy concentration. From the modeled cases of C(0) in Si(x)Ge(1-x), Zn(-) and Cd(-) in Ga(x)In(1-x)P it is seen that under certain conditions the dopant solubility can be orders of magnitude higher in an alloy or multilayer than in either of the elements of the alloy. This is found to be due to the solubility's strong dependence on the lattice constant for size mismatched dopants. The equilibrium doping concentration in alloys or multilayers could therefore be increased significantly. More specifically, Zn- in a Ga(x)In(1-x)P multilayer is found to have a maximum solubility for x = 0.9, which is 5 orders of magnitude larger than that of pure InP.     

Mg-Al-Ca合金中三元Laves相的稳定性和电子结构

采用基于密度泛函理论的第一性原理方法,应用VASP (Vienna Ab-initio Simulation Package) 计算软件,研究了Mg-Al-Ca合金中三元Laves相,即Ca(Mg1-x,Alx)2和Al2(Ca1-x,Mgx) (x=0, 0.25, 0.50, 0.75, 1)在不同形态结构(C14, C15和C36)下的相稳定性及电子结构。计算所得的晶格常数和实验值吻合很好,形成能和相关能的计算用来研究三元Laves相的合金化能力和稳定性,结果表明：C14-Ca(Mg0.25,Al0.75)2具有很好的合金化能力,而C15- CaAl2具有很好的结构稳定性。态密度和电荷密度的计算用来研究Mg-Al-Ca合金中三元Laves相稳定性的内在微观机制。