三元化合物的热电、光学、拓扑性质的第一性原理研究

发布时间：2018-05-29 16:47

本文选题：第一性原理 + 三元哈斯勒化合物　；参考：《河南师范大学》2017年博士论文

【摘要】：在过去的几十年里,信息与能源材料一直是凝聚态物理领域的研究热点。信息技术随着自旋电子技术的飞速发展而不断提高,同时也促使人们去研究新型自旋电子材料以满足新的技术要求。能源问题也一直是全人类关注的焦点,特别是近年来随着传统石化能源的枯竭和对环境污染的日益突出。这要求人们去开发新型的清洁、可再生、低成本的能源材料。半哈斯勒化合物ABX和钙钛矿结构化合物ABX_3等三元化合物由于其具有特殊的能带结构、拓扑和光电性质,在信息技术和清洁能源领域有非常好的应用前景。三元半哈斯勒化合物不但可以作为半金属材料和拓扑绝缘体材料,而且作为热电材料在理论和实验上已经被广泛的研究。热电材料可以把热能转变为电能,而不对环境产生污染。ABX_3型钙钛矿太阳能电池由于具有清洁无污染、可再生、便利的优点,有望成为未来高效太阳能吸收材料。本文主要研究三元半哈斯勒化合物的热电、拓扑性质,ABX_3型钙钛矿的光学性质,研究内容主要包括以下几个方面:(1)基于第一性原理的计算方法,我们通过对哈斯勒半金属HfIrX(X=As,Sb,Bi)进行了晶格畸变和原子替代,研究了材料拓扑相的转变。在平衡晶格结构并且未考虑自旋轨道耦合的条件下,HfIrAs和HfIrBi是拓扑非平庸的狄拉克半金属(s能带Г_1低于p能带Г5),而HfIrSb是平庸的拓扑半导体。HfIrSb拓扑的不同主要是因为“内部压力”提升了由s轨道贡献的Г_1能带,而相对降低了由p轨道贡献的Г5能带,从而成为正常能带序。考虑自旋轨道耦合效应后,HfIrAs和HfIrSb分别变为拓扑绝缘体、一般绝缘体,而HfIrBi则变成了拓扑半金属。对HfIrBi进行了晶格畸变,即压缩a-b面内晶格常数的同时又拉伸c轴晶格常数,HfIrSb变成了Weyl半金属,八个Weyl点的坐标分别为(0,±K_x,±K_z),(0,±K_y,±K_z),K_x=K_y=0.023?~(-1),K_z=0.108?~(-1).(2)采用TB-mBJ交换关联势的第一性原理的计算方法,学习了三元半哈斯勒化合物ZrIrX(X=As,Sb,Bi)的电子结构、热电性质以及拓扑性质在静水压作用下的影响。在平衡晶格下,三种化合物都是平庸的半导体和好的热电材料,塞贝克系数和功率因子最高值分别为1180(μV/K)和41(10~(10)Wm~(-1)K~(-2)s~(-1))。当对三种化合物施加压力(晶格常数变小)时,带隙增加了,施加拉力(晶格常数变大)时,带隙减小。出现这种现象主要是因为Г点的s-轨道对外界压力比较敏感造成的。带隙发生了改变,因此光吸收系数在压应力向拉应力作用的变化过程中逐渐增大。当力从拉应力到压应力的变化过程中,ZrIrAs和ZrIrBi的热电性质:p型掺杂的塞贝克系数和功率因子逐渐增加,而对于n型掺杂,在直接(间接)带隙转换的特殊值处,塞贝克系数以及功率因子出现了极大值,这个现象可以用态密度的有效质量来解释。三种化合物在立方相中,拉力的作用会在不同的特殊值下变为狄拉克半金属,并且Г6处的s-轨道低于Г8处的p-轨道。对ZrIrAs进行晶格畸变,即压缩ab面内的晶格常数,同时拉伸c轴晶格常数,从而打破了晶格的空间反演对称性,材料由狄拉克半金属变成了Weyl半金属,八个Weyl点的位置坐标分别为(±K_x,0,±K_z),(0,±K_y,±K_z),K_x=K_y=0.008?~(-1),K_z=0.043?~(-1)。(3)基于第一性原理和玻尔兹曼输运理论,我们研究了三元化合物TaIrGe的电子结构、光学性质与热电性质。自旋轨道耦合(SOC)使费米能级附近的价带顶发生了劈裂,从而能带的简并度减小,由此导致材料p型的塞贝克系数和功率因子减小。考虑静水压,从压应力到拉应力的变化过程中,带隙逐渐增加。光吸收系数呈现出了明显的红移现象。电子结构的改变使费米能级附近导带与价带的有效质量逐渐增加,热电性质出现了明显的影响,塞贝克系数与功率因子逐渐增加。拉力提高了材料的热电性能。(4)运用mBJ+SOC的方法研究了一种新型的太阳能电池吸收材料CsSn_xPb_(1-x)I_3的电子结构和光学性质。对于混合金属钙钛矿CsSn_xPb_(1-x)I_3,随着Sn原子在材料中浓度的增加,带隙呈非线性的减小,从CsPbI_3的0.96eV到CsSnI3的0.16eV,光吸收系数随着Sn浓度的增加而逐渐呈现出红移现象,并且吸收边缘增宽,吸收系数与理想功率吸收效率(IPAC)随着Sn原子替代Pb原子而增加。虽然CsSnI3有很高的IPAC,但是Sn2+在空气中很不稳定,很容易被氧化成Sn4+,因此我们的研究表明:混合金属的钙钛矿CsSn_xPb_(1-x)I_3将是未来很好地太阳能电池吸收材料。(5)系统的研究了混合卤化物钙钛矿CsAX_2X'(A=Ge,Sn,Pb;X,X'=Cl,Br,I)的晶体结构、电子结构和光学性质。采取了mBJ+SOC的计算方法,得到的能带结构说明这些材料是直接带隙的半导体,带隙范围从0.32eV到1.97eV。通过改变钙钛矿CsAX2X'中卤族元素(Cl,Br,I)和磷元素(Ge,Sn,Pb),我们找到了调整太阳能吸收材料带隙的方法。并且从计算结果我们预测CsGeI_2Br,CsGeI_2Cl,CsGeBr_3,CsGeI_3,CsSnI_2Br和CsSnI3有望成为环保的太阳能吸收材料,理想功率吸收效率最高达到了1052 W/m~2。
[Abstract]:In the past few decades, information and energy materials have been the focus of research in the field of condensed matter physics. With the rapid development of spintronics technology, information technology has also promoted people to study new spintronic materials to meet new technical requirements. Energy issues have always been the focus of all human concerns, especially In recent years, the depletion of traditional petrochemical energy and environmental pollution are becoming increasingly prominent. This requires people to develop new clean, renewable, low-cost energy materials. Semi Hassler compound ABX and perovskite structure compound ABX_3 and other three element compounds have special band structure, topological and photoelectric properties, in information technology. It has a very good application prospect in the field of clean energy. Three yuan and a half Hassler compounds can not only be used as semi metal materials and topological insulators, but also have been widely studied in theory and experiment as thermoelectric materials. Thermoelectric materials can transform thermal energy into electric energy and not pollute the environment of.ABX_3 perovskite sun. Because of the advantages of clean, pollution-free, renewable and convenient, the energy battery is expected to become a high-efficiency solar absorption material in the future. This paper mainly studies the thermoelectric, topological properties and optical properties of ABX_3 type perovskite of three yuan and a half compounds. The main contents of this study include the following aspects: (1) we are based on the first principle of the calculation method. By the lattice distortion and atomic substitution of Hassler semi metal HfIrX (X=As, Sb, Bi), the transformation of the topological phase of the material is studied. Under the equilibrium lattice structure and without considering the spin orbit coupling, HfIrAs and HfIrBi are topological non mediocre Dirac semi metals (s energy _1 is lower than p band 5), and HfIrSb is a mediocre topological semidem. The difference of the body.HfIrSb topology is mainly because the "internal pressure" improves the _1 energy band which is contributed by the s orbit, and reduces the 5 energy band which is contributed by the P orbit, and thus becomes the normal energy band. After considering the spin orbit coupling effect, HfIrAs and HfIrSb become topological insulators, general insulators, and HfIrBi become topological semi gold. The lattice distortion of HfIrBi, that is, to compress the lattice constant in the A-B surface and stretch the lattice constant of the c axis while HfIrSb becomes Weyl semi metal, the coordinates of the eight Weyl points are (0, + K_x, + K_z), (0, + K_y, + K_z), K_x=K_y=0.023? ~ (-1), (2) the first principle of the exchange of correlation potential. The electronic structure, thermoelectric properties and topological properties of three yuan and half Hassler compound ZrIrX (X=As, Sb, Bi) are affected by hydrostatic pressure. Under the equilibrium lattice, the three compounds are mediocre semiconductors and good thermoelectric materials, and the highest values of Sebek and power factors are 1180 (mu V/K) and 41 (10~ (10) Wm~ (-1) K~ (-2) s~ (-1)). When the pressure is applied to the three compounds (the lattice constant becomes smaller), the band gap increases and the band gap decreases when the tension is applied (the lattice constant becomes larger). This phenomenon is mainly due to the sensitivity to the external pressure of the s- orbit. The band gap changes, so the optical absorption coefficient is in the process of pressure stress to tensile stress. The thermoelectric properties of ZrIrAs and ZrIrBi: the Sebek coefficient and the power factor of the P type doping increase gradually during the change of the force from the tensile stress to the compressive stress, while for the N type doping, the Sebek coefficient and the power factor are maximized at the special value of the direct (indirect) band gap conversion, and this phenomenon can be used in the density of states. The effective mass is explained. In the cubic phase, the effect of the three compounds will change into Dirac semi metal at different special values, and the s- orbit at 6 is lower than the p- orbit of 8. The lattice distortion of ZrIrAs, that is, the lattice constant in the compression of the AB surface and the C axis lattice constant at the same time, thus break the space inversion symmetry of the lattice. The material is transformed from Dirac semi metal into Weyl semi metal, and the position coordinates of the eight Weyl points are (+ K_x, 0, + + K_z), (0, + K_y, + K_z), K_x=K_y=0.008? ~ (-1), K_z=0.043? ~ (-1). (3) based on the first principle and Boltzmann transport theory, we studied the electronic structure, optical properties and thermoelectric properties of the three element compound. The orbital coupling (SOC) cleavages the top of the valence band near the Fermi level, thus reducing the degeneracy of the energy band, which leads to the decrease of the cop coefficient and power factor of the material P. Considering the hydrostatic pressure, the band gap gradually increases from the pressure stress to the tensile stress. The effective mass of the conduction band and the valence band near the Fermi level is gradually increased, the thermoelectric property has an obvious influence, the cothermal coefficient and the power factor gradually increase. The tensile force increases the thermoelectric properties of the material. (4) the electronic structure of a new type of solar energy cell absorption material CsSn_xPb_ (1-x) I_3 is studied by the method of mBJ+SOC. And optical properties. For the mixed metal perovskite CsSn_xPb_ (1-x) I_3, the band gap decreases with the increase of the concentration of Sn atoms in the material. From 0.96eV of CsPbI_3 to 0.16eV of CsSnI3, the light absorption coefficient gradually shows a red shift with the increase of Sn concentration, and the absorption edge is broadened, the absorption coefficient and the ideal power absorption efficiency. The rate (IPAC) increases with the substitution of the Pb atom with the Sn atom. Although CsSnI3 has a very high IPAC, Sn2+ is very unstable in the air and is easily oxidized to Sn4+. Therefore, our study shows that the perovskite CsSn_xPb_ (1-x) I_3 of the mixed metal will be a good absorbing material for the future solar cell. (5) the system has studied the mixed halide calcium and titanium. The crystal structure, electronic structure and optical properties of the ore CsAX_2X'(A=Ge, Sn, Pb; X, X'=Cl, Br, I). A mBJ+SOC calculation method is adopted. The band structure shows that these materials are direct bandgap semiconductors, and the band gap ranges from 0.32eV to 1.97eV. by changing the halogen elements in the perovskite CsAX2X'. The method of adjusting the band gap of solar energy absorption is made. And from the calculation results we predict that CsGeI_2Br, CsGeI_2Cl, CsGeBr_3, CsGeI_3, CsSnI_2Br and CsSnI3 are expected to be environmentally friendly solar absorbing materials, and the ideal power absorption efficiency is up to 1052 W/m~2.
【学位授予单位】：河南师范大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

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