自旋电子学材料和光解水催化材料的第一性原理计算与设计
发布时间：2018-07-26 13:31
【摘要】：材料与人们的生活息息相关,并推动着人类社会的发展。历史上每一次重大的经济和社会变革几乎都能在背后找到材料的影子。例如,半导体材料的发现和大规模应用,导致了人类社会步入微电子时代,至今使人受益。然而自然界存在的材料有限,已无法满足人们日益增长的物质需求。这就需要人们基于现有的材料,利用各种物理或化学手段设计出具有特定功能的新型材料。在实验学家看来,这是一个不断尝试不断修正的艰巨任务,在这一过程中,需要投入大量的时间和精力,而且不可避免的会导致实验资源的浪费。 计算量子化学的发展为这一困境带来了福音。利用第一性原理计算对材料的性质进行预测,根据预测的性质对材料进行初步筛选,然后再进行实验验证,将极大地提高实验学家工作的效率,缩短材料设计的周期。本论文的目的在于介绍我们基于第一性原理计算,对自旋电子学材料和光解水催化材料进行功能导向设计的工作。这两个领域看似毫无关联,实际都是日益严重的能源和环境危机促使的。前者的目的在于以更低的能量成本获得更高的速度,后者则是寻找洁净的可持续能源,即太阳能制取氢能,替代现有的化石能源。 自旋电子学基于电子自旋进行信息的传递、处理与存储,具有目前传统半导体电子器件无法比拟的优势,比如说运行速度更快,集成度更高,耗能更低,因而成为近年来人们研究的热点。然而,自旋电子学面临着三大挑战：自旋的产生和注入,自旋的长程输运,以及自旋的调控和探测。这些问题的解决将主要依赖于寻找具有特定性质的自旋电子学材料,例如磁性半导体材料,半金属材料等。尽管已经有不少自旋电子学材料被相继提出,但是他们距离实际应用还存在较大的距离。其中原因包括自旋热翻转导致半金属性被破坏,磁有序温度低于室温,合成困难或不易控制等。我们着眼于通过第一性原理计算设计具有特殊功能的新型自旋电子学材料,以及寻找在室温环境下可用的自旋电子学材料,为自旋电子学器件的合成和应用铺平道路。 另一方面,用太阳光分解水制氢,为人类提供清洁燃料,被视为化学的圣杯。光解水的核心在于寻找能够高效吸收太阳光的半导体催化剂。可惜的是,传统的金属氧化物催化剂带隙较大,仅能够吸收太阳光的紫外部分,而紫外光仅占太阳光能量的7%左右,导致太阳光利用效率很低。其它一些金属化合物虽然能够吸收可见光,但是本身稳定性太差,容易分解,或者催化活性低,量子产率难以满足实际要求。为此,我们通过第一性原理计算探索能够有效利用可见光,甚至红外光,并且稳定性良好的半导体催化剂,为实现高效光解水制氢指明一条方向。 本论文共分为三章。第一章介绍材料设计的理论基础,即计算量子化学。根据基本变量选择的不同,量子化学可分为两种不同的表达途径：从波函数出发和从电子密度出发。基于波函数的量子化学的优点是精度可以达到很高,缺点就是费时。依目前的计算机硬件条件,仅适合处理几个到几十个原子的分子或团簇体系。而且对化学家来说,波函数距离直观感觉太远：它是3N维函数(N为电子数),难以想象它的具体形状。相反地,电子密度只是三维空间函数,是一个很直观的量。因此,基于电子密度的量子化学,即密度泛函理论,得到化学家的偏爱。同时,引入电子密度作为基本变量后,薛定谔方程更加容易求解,因此计算速度很快而且结果的精度基本能达到化学家的要求,这就使得处理上百甚至上千个原子的体系成为可能。故而,密度泛函理论被广泛地应用于大体系和固态周期体系的模拟。本论文的工作就是基于密度泛函理论,对材料的性质进行计算和设计。在本章中,我们较为详细地介绍了密度泛函理论的起源和发展,以及具体的理论框架。 第二章中我们介绍自旋电子学材料的理论设计。首先,为解决在自旋电子学中如何用电场调控载流子自旋取向这一关键性的科学问题,我们在概念上提出了一种新型的自旋电子学材料,即双极磁性半导体(英文全称为bipolar magnetic semiconductors,缩写为BMS)。此类材料具有特殊的能带构造,通过它的电流不仅可以达到完全的自旋极化,而且载流子的自旋取向可以简单地通过加门电压的方法直接进行调制。基于这一概念,我们相继设计了多种双极磁性半导体体系,包括一维化学修饰的碳纳米管,准二维La(Mn,Zn)AsO合金,MnPSe3纳米片,表层掺杂Sic纳米薄膜,三维四元Heusler合金FeVXSi (X=Ti, Zr)等。其次,为解决磁性半导体的磁有序温度普遍低于室温这一难题,我们在概念上提出了一种新的解决方案,即非对称反铁磁半导体(英文全称为asymmetric antiferromagnetic semiconductors,缩写为AAMS)。在这类半导体中,磁矩来源于不同的磁性原子,各相邻磁矩以反铁磁排列耦合在一起。由于很强的反铁磁超交换作用,磁耦合温度很容易超过室温。同时,半导体的价带和导带是高度自旋极化的,这是不同磁性原子之间磁轨道能级相互交错所导致的。因此非对称反铁磁半导体同时具备室温磁序以及大自旋极化两个性质。基于这一设计想法,我们在钙钛矿型A2CrMO6(A=Ca,Sr,Ba; M=Ru, Os)系列体系实现了该类材料,从而验证了所提概念的可行性。再次,我们设计了一种新型二维铁磁半导体,即CrXTe3(X=Si, Ge)纳米片,其价带和导带具有相同的自旋极化方向,能够用作纳米尺度的自旋产生源。最后,我们介绍对另一类自旋电子学材料,即半金属磁性材料的设计工作。为了构造能在常温下工作的自旋电子器件,半金属必须具有高于室温的铁磁居里温度,较宽的半金属能隙,以及显著的磁各向异性能。不幸的是,人们至今还没有找到同时满足这些条件的材料。基于前面提出的与“1111”型铁基超导体同构的层状La(Mn,Zn)AsO合金,我们对其进行元素替代掺杂,使该材料从反铁磁半导体转变成铁磁半金属。理论预测半金属的居里温度高于室温,半金属能隙高达0.74eV。同时,体系内禀的准二维结构赋予了半金属材料极高的磁各向异性能,其理论预测值比目前已获得的半金属材料高一至两个数量级。 在第三章,我们介绍光解水催化剂的理论设计。一方面,基于单层氮化硼纳米片,我们通过化学修饰的方法设计了一种新型非金属光解水催化剂,它能够有效地吸收可见光催化分解水制氢。另一方面,我们提出了一种光解水制氢的新机制,可以把太阳光的红外段也有效地利用起来。在传统理论中,光催化剂的能隙至少要大于水分解反应吸热(1.23eV),因而占太阳光能量近一半的红外光无法被吸收用来分解水制氢。我们首次提出利用具有内禀电偶极矩的二维纳米催化剂,可突破传统理论对催化剂能隙的限制。这种催化剂存在偶极内电场,吸附在催化剂两个表面上的水分子会感受到不同的静电势,从而导致两个表面上水的氧化还原电势变得不再相同。如果氧化和还原分别发生在不同的表面,催化剂受到的能隙限制原则上将不再存在。在这一新的光解水机制中,不仅紫外光和可见光,红外光也可以用来促使水分解产生氢气。另外,这种催化剂的光激发是一个电荷转移过程,电子和空穴分别产生在两个不同的表面,催化剂固有偶极电场有效地促进了光生电子空穴对的空间分离,并做功帮助水分解产生氢气。基于这一机制,我们设计了一种双层氮化硼纳米体系,其两个表面分别用氢和氟修饰。理论计算与模拟表明这是一种有效的红外光催化分解水体系。
[Abstract]:Material is closely related to people's life and promotes the development of human society. Every major economic and social change in history can almost find the shadow of material. For example, the discovery and large-scale application of semiconductors have led to the human society entering the microelectronic age, and so far it has benefited. However, the nature has existed. The limited material has been unable to meet the growing material needs of people. This requires people to design new materials with specific functions, based on existing materials, using various physical or chemical means. In the view of the experimenter, this is a constantly trying and arduous task that needs to be put in a lot of time in this process. And energy will inevitably lead to waste of experimental resources.
The development of computational quantum chemistry has brought the gospel to this dilemma. Using the first principle to predict the properties of the material, the preliminary screening of the material according to the nature of the prediction, and then the experimental verification, will greatly improve the efficiency of the experimenter's work and shorten the cycle of material design. The purpose of this paper is to introduce the purpose of this paper. Based on the first principle, we work on the functional design of spintronic materials and photolysis of water catalyzed materials. These two areas seem unrelated, and are actually caused by the increasingly serious energy and environmental crisis. The former is aimed at getting higher speed with lower energy, and the latter looking for cleanliness. The sustainable energy sources, that is, solar energy, is the substitute of fossil energy.
Spintronics has become a hot spot in recent years. However, spintronics is facing three major challenges: spin generation and injection. The long range transport of spin and the regulation and detection of spin will depend mainly on the search for spintronic materials with specific properties, such as magnetic semiconductors, semi metal materials, etc. Although many spintronics materials have been proposed in succession, they are still larger than the actual applications. The reasons include the spin heat turnover resulting in the destruction of semi metal, the magnetic ordering temperature below room temperature, the synthesis difficulty or the difficult control. We focus on the design of new spintronics materials with special functions through the first principle, and the search for spintronic materials available in the room temperature ring, for spin electricity. The synthesis and application of the subsystems are paved.
On the other hand, using sunlight to break water to produce hydrogen and provide clean fuel for human beings, it is regarded as the Holy Grail of chemistry. The core of the photolysis of water is to find a semiconductor catalyst that can absorb the sun's light efficiently. Unfortunately, the traditional metal oxide catalyst has a large band gap and can only absorb the ultraviolet part of the sun light, while the ultraviolet light only accounts for the sun's light. About 7% of the energy, resulting in low solar efficiency. Although some other metal compounds can absorb visible light, but their stability is too poor, easy to decompose, or low catalytic activity, the quantum yield is difficult to meet the actual requirements. Therefore, we can use the first principle to explore the effective use of visible light, even infrared light. Moreover, the semiconductor catalyst with good stability indicates a direction for the production of hydrogen by efficient photolysis of water.
This paper is divided into three chapters. The first chapter introduces the theoretical basis of material design, which is the calculation of quantum chemistry. According to the selection of basic variables, quantum chemistry can be divided into two different ways of expression: starting from the wave function and starting from the electron density. The advantage of the quantum chemistry based on the wave function is that the precision can be high and the disadvantage is that Time consuming. According to the current computer hardware conditions, it is only suitable for dealing with several molecules or clusters of several dozens of atoms. And for chemists, the distance of the wave function is far too far away: it is a 3N dimensional function (N is an electron number), and it is difficult to imagine its specific shape. On the contrary, the electrical subdensity is only a three-dimensional space function, and it is a very intuitive one. Therefore, the electron density based quantum chemistry, the density functional theory, gets the preference of the chemist. At the same time, after introducing the electron density as the basic variable, the Schrodinger equation is easier to solve, so the calculation speed is very fast and the accuracy of the result can reach the requirement of the chemist, which makes the processing hundreds of thousands of atoms. Therefore, the density functional theory is widely used in the simulation of large systems and solid state periodic systems. The work of this thesis is based on the density functional theory and the calculation and design of the properties of the material. In this chapter, we introduce the origin and development of the density function theory, as well as the specific theory in this chapter. Frame.
In the second chapter, we introduce the theoretical design of spintronics materials. First, in order to solve the key scientific problem of how to use electric field to regulate the carrier spin orientation in spintronics, we have proposed a new kind of spintronic material, that is, bipolar magnetic semiconductor (the full name of bipolar magnetic semicon in English. Ductors, abbreviated as BMS). This kind of material has a special band structure, which can not only achieve full spin polarization through its current, but also the spin orientation of the carrier can be directly modulated by the method of adding the gate voltage. Based on this concept, we have set up a variety of bipolar magnetic semiconductor systems, including one dimension. Chemically modified carbon nanotubes, quasi two-dimensional La (Mn, Zn) AsO alloy, MnPSe3 nanoscale, surface doped Sic nanometers, three dimensional four element Heusler alloy FeVXSi (X=Ti, Zr), etc. Secondly, to solve the problem that magnetic ordered temperature of magnetic semiconductors is generally lower than room temperature, we have proposed a new solution, that is, asymmetric antiferromagnetism. The semiconductor (in English is called asymmetric antiferromagnetic semiconductors, abbreviated as AAMS). In this type of semiconductors, the magnetic moments are derived from different magnetic atoms, and the adjacent magnetic moments are coupled in the antiferromagnetic arrangement. The magnetic coupling temperature is easy to exceed the room temperature because of the strong antiferromagnetic exchange. The conduction band is highly spin polarized, which is caused by the interlacing of the magnetic orbital energy levels between different magnetic atoms. Therefore, the asymmetric antiferromagnetic semiconductor has two properties at the same time at room temperature magnetic order and large spin polarization. Based on this design idea, we have implemented this kind of A2CrMO6 (A= Ca, Sr, Ba; M=Ru, Os) system. Again, we have designed a new two-dimensional ferromagnetic semiconductor, CrXTe3 (X=Si, Ge) nanoscale. The valence band and the guide band have the same spin polarization direction and can be used as the nanoscale spintronic source. Finally, we introduce to another kind of spintronics material, the semi metal magnetic material. Design work. In order to build a spintronic device capable of working at room temperature, semi metal must have a ferromagnetic Curie temperature above room temperature, a wider half metal gap, and significant magnetic anisotropy. Unfortunately, people have not yet found the material to meet these conditions at the same time. Based on the previous and "1111" type A layered La (Mn, Zn) AsO alloy with an isomorphic iron base superconductor is doped to make the material change from antiferromagnetic semiconductor to ferromagnetic semi metal. It is predicted that the Curie temperature of semi metal is higher than room temperature, the half metal gap is as high as 0.74eV., and the intrinsic quasi two-dimensional structure endows semi metal materials with very high magnetic properties. Its theoretical prediction value is about one to two orders of magnitude higher than that of half metallic materials currently obtained.
In the third chapter, we introduce the theoretical design of the photodissociation water catalyst. On the one hand, based on the monolayer boron nitride nanoscale, we designed a novel non-metallic photodissociation water catalyst by chemically modified method. It can effectively absorb the visible light catalytic decomposition of water for hydrogen production. On the other hand, we have proposed a new mechanism for the photodissociation of hydrogen. In the traditional theory, the energy gap of the photocatalyst is at least greater than that of the water decomposition reaction (1.23eV), so that the infrared light, which accounts for nearly half of the solar energy, can not be absorbed into the decomposition of water for hydrogen production. It breaks through the limitation of the traditional theory on the energy gap of the catalyst. This catalyst has the dipole internal electric field, and the water molecules adsorbed on the two surface of the catalyst will feel the different electrostatic potential, resulting in the redox potential of the water on the two surfaces. The energy gap limit will not exist in principle. In this new photolysis mechanism, not only ultraviolet and visible light, but also infrared light can also be used to induce water to produce hydrogen. In addition, the light excitation of the catalyst is a charge transfer process, and the electrons and holes are produced on two different surfaces, and the intrinsic dipole electric field of the catalyst is effective. Based on this mechanism, we designed a two-layer boron nitride nano system, and the two surfaces were modified with hydrogen and fluorine respectively. The theoretical calculation and simulation show that this is an effective infrared photocatalytic decomposition water system.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB39;O643.36
本文编号：2146167
[Abstract]:Material is closely related to people's life and promotes the development of human society. Every major economic and social change in history can almost find the shadow of material. For example, the discovery and large-scale application of semiconductors have led to the human society entering the microelectronic age, and so far it has benefited. However, the nature has existed. The limited material has been unable to meet the growing material needs of people. This requires people to design new materials with specific functions, based on existing materials, using various physical or chemical means. In the view of the experimenter, this is a constantly trying and arduous task that needs to be put in a lot of time in this process. And energy will inevitably lead to waste of experimental resources.
The development of computational quantum chemistry has brought the gospel to this dilemma. Using the first principle to predict the properties of the material, the preliminary screening of the material according to the nature of the prediction, and then the experimental verification, will greatly improve the efficiency of the experimenter's work and shorten the cycle of material design. The purpose of this paper is to introduce the purpose of this paper. Based on the first principle, we work on the functional design of spintronic materials and photolysis of water catalyzed materials. These two areas seem unrelated, and are actually caused by the increasingly serious energy and environmental crisis. The former is aimed at getting higher speed with lower energy, and the latter looking for cleanliness. The sustainable energy sources, that is, solar energy, is the substitute of fossil energy.
Spintronics has become a hot spot in recent years. However, spintronics is facing three major challenges: spin generation and injection. The long range transport of spin and the regulation and detection of spin will depend mainly on the search for spintronic materials with specific properties, such as magnetic semiconductors, semi metal materials, etc. Although many spintronics materials have been proposed in succession, they are still larger than the actual applications. The reasons include the spin heat turnover resulting in the destruction of semi metal, the magnetic ordering temperature below room temperature, the synthesis difficulty or the difficult control. We focus on the design of new spintronics materials with special functions through the first principle, and the search for spintronic materials available in the room temperature ring, for spin electricity. The synthesis and application of the subsystems are paved.
On the other hand, using sunlight to break water to produce hydrogen and provide clean fuel for human beings, it is regarded as the Holy Grail of chemistry. The core of the photolysis of water is to find a semiconductor catalyst that can absorb the sun's light efficiently. Unfortunately, the traditional metal oxide catalyst has a large band gap and can only absorb the ultraviolet part of the sun light, while the ultraviolet light only accounts for the sun's light. About 7% of the energy, resulting in low solar efficiency. Although some other metal compounds can absorb visible light, but their stability is too poor, easy to decompose, or low catalytic activity, the quantum yield is difficult to meet the actual requirements. Therefore, we can use the first principle to explore the effective use of visible light, even infrared light. Moreover, the semiconductor catalyst with good stability indicates a direction for the production of hydrogen by efficient photolysis of water.
This paper is divided into three chapters. The first chapter introduces the theoretical basis of material design, which is the calculation of quantum chemistry. According to the selection of basic variables, quantum chemistry can be divided into two different ways of expression: starting from the wave function and starting from the electron density. The advantage of the quantum chemistry based on the wave function is that the precision can be high and the disadvantage is that Time consuming. According to the current computer hardware conditions, it is only suitable for dealing with several molecules or clusters of several dozens of atoms. And for chemists, the distance of the wave function is far too far away: it is a 3N dimensional function (N is an electron number), and it is difficult to imagine its specific shape. On the contrary, the electrical subdensity is only a three-dimensional space function, and it is a very intuitive one. Therefore, the electron density based quantum chemistry, the density functional theory, gets the preference of the chemist. At the same time, after introducing the electron density as the basic variable, the Schrodinger equation is easier to solve, so the calculation speed is very fast and the accuracy of the result can reach the requirement of the chemist, which makes the processing hundreds of thousands of atoms. Therefore, the density functional theory is widely used in the simulation of large systems and solid state periodic systems. The work of this thesis is based on the density functional theory and the calculation and design of the properties of the material. In this chapter, we introduce the origin and development of the density function theory, as well as the specific theory in this chapter. Frame.
In the second chapter, we introduce the theoretical design of spintronics materials. First, in order to solve the key scientific problem of how to use electric field to regulate the carrier spin orientation in spintronics, we have proposed a new kind of spintronic material, that is, bipolar magnetic semiconductor (the full name of bipolar magnetic semicon in English. Ductors, abbreviated as BMS). This kind of material has a special band structure, which can not only achieve full spin polarization through its current, but also the spin orientation of the carrier can be directly modulated by the method of adding the gate voltage. Based on this concept, we have set up a variety of bipolar magnetic semiconductor systems, including one dimension. Chemically modified carbon nanotubes, quasi two-dimensional La (Mn, Zn) AsO alloy, MnPSe3 nanoscale, surface doped Sic nanometers, three dimensional four element Heusler alloy FeVXSi (X=Ti, Zr), etc. Secondly, to solve the problem that magnetic ordered temperature of magnetic semiconductors is generally lower than room temperature, we have proposed a new solution, that is, asymmetric antiferromagnetism. The semiconductor (in English is called asymmetric antiferromagnetic semiconductors, abbreviated as AAMS). In this type of semiconductors, the magnetic moments are derived from different magnetic atoms, and the adjacent magnetic moments are coupled in the antiferromagnetic arrangement. The magnetic coupling temperature is easy to exceed the room temperature because of the strong antiferromagnetic exchange. The conduction band is highly spin polarized, which is caused by the interlacing of the magnetic orbital energy levels between different magnetic atoms. Therefore, the asymmetric antiferromagnetic semiconductor has two properties at the same time at room temperature magnetic order and large spin polarization. Based on this design idea, we have implemented this kind of A2CrMO6 (A= Ca, Sr, Ba; M=Ru, Os) system. Again, we have designed a new two-dimensional ferromagnetic semiconductor, CrXTe3 (X=Si, Ge) nanoscale. The valence band and the guide band have the same spin polarization direction and can be used as the nanoscale spintronic source. Finally, we introduce to another kind of spintronics material, the semi metal magnetic material. Design work. In order to build a spintronic device capable of working at room temperature, semi metal must have a ferromagnetic Curie temperature above room temperature, a wider half metal gap, and significant magnetic anisotropy. Unfortunately, people have not yet found the material to meet these conditions at the same time. Based on the previous and "1111" type A layered La (Mn, Zn) AsO alloy with an isomorphic iron base superconductor is doped to make the material change from antiferromagnetic semiconductor to ferromagnetic semi metal. It is predicted that the Curie temperature of semi metal is higher than room temperature, the half metal gap is as high as 0.74eV., and the intrinsic quasi two-dimensional structure endows semi metal materials with very high magnetic properties. Its theoretical prediction value is about one to two orders of magnitude higher than that of half metallic materials currently obtained.
In the third chapter, we introduce the theoretical design of the photodissociation water catalyst. On the one hand, based on the monolayer boron nitride nanoscale, we designed a novel non-metallic photodissociation water catalyst by chemically modified method. It can effectively absorb the visible light catalytic decomposition of water for hydrogen production. On the other hand, we have proposed a new mechanism for the photodissociation of hydrogen. In the traditional theory, the energy gap of the photocatalyst is at least greater than that of the water decomposition reaction (1.23eV), so that the infrared light, which accounts for nearly half of the solar energy, can not be absorbed into the decomposition of water for hydrogen production. It breaks through the limitation of the traditional theory on the energy gap of the catalyst. This catalyst has the dipole internal electric field, and the water molecules adsorbed on the two surface of the catalyst will feel the different electrostatic potential, resulting in the redox potential of the water on the two surfaces. The energy gap limit will not exist in principle. In this new photolysis mechanism, not only ultraviolet and visible light, but also infrared light can also be used to induce water to produce hydrogen. In addition, the light excitation of the catalyst is a charge transfer process, and the electrons and holes are produced on two different surfaces, and the intrinsic dipole electric field of the catalyst is effective. Based on this mechanism, we designed a two-layer boron nitride nano system, and the two surfaces were modified with hydrogen and fluorine respectively. The theoretical calculation and simulation show that this is an effective infrared photocatalytic decomposition water system.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB39;O643.36
【参考文献】
相关期刊论文 前1条
1 CHEN Peng;ZHANG GuangYu;;Carbon-based spintronics[J];Science China(Physics,Mechanics & Astronomy);2013年01期
，本文编号：2146167
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