一些二维材料的第一性原理计算与设计

发布时间：2018-09-13 12:15

【摘要】：材料在我们日常生产生活中扮演着非常重要的作用,并且其还推动着人类文明科学的进展。人类的文明经历了从石器时代,青铜器时代,以及铁器时代等的演化,这些发展的过程都有一个相同的点,那就是人类对材料进行发展和改良。现在,人们又对技术展开了新的革新,比如半导体材料的发现,以及大规模的投入生产应用,使得人类社会又进入了更先进的微电子时代,大大的丰富和改进了人类的生产生活。但是,由于自然界常规的材料非常有限,早已无法满足人类日益增长地对材料的需求。这就使得人类必须在现有的材料基础上,通过已掌握的科学技术设计出具有特定功能的新型材料。可是,如果用常规的实验手段,对已有的材料进行简单的组合,无疑需要投入大量的人力,物力和财力,这是很不现实的方法,而且效率也相对很低。另一方面,随着量子力学,量子化学的发展和改进,借助高性能的计算设备,我们可以用数值的方法求解一些复杂体系的薛定谔方程,进而理论上得到一些体系的物理化学性质。这样我们就能初步的对材料进行筛选,然后再进行实验的验证,这就将大大的提高实验工作的效率,节省不必要的开支。本论文的目的即在于介绍我们通过使用第一性原理计算,对现如今非常热门的一类新型材料——二维材料进行研究,希望通过对其进行改良和设计,理论上得到我们需要的功能材料。通过对这些功能材料的研究,希望能解决目前人类所面临的能源紧缺,电子器件的改良和提升等方面的需求。本论文由如下六章组成。第一章首先简要介绍计算量子化学的基础和理论框架,以及密度泛函理论。其中包含通常所使用的薛定谔方程,然后对这个基础方程进行的一些列处理简化,比如绝热、单电子近似,Hohenberg-Kohn定理,Kohn-Sham方程,然后在此基础上发展起来的适合诸多体系的各种交换关联泛函。通过量子化学的方法,我们可以对体系的许多性质进行描述,也可以研究很多类型的体系,在此基础上人们还开发出了很多的方法计算体系的各种性质。最后再简单介绍几种常用的基于密度泛函理论发展起来的计算软件包。第二章主要介绍一些目前非常热门的二维纳米材料。其中,由于石墨烯是二维材料中的”明星“级材料,所以我们以其和其衍生物为开端介绍了二维材料的发展和研究状况。在此之外,我们还介绍了六方氮化硼的合成和基本的物理化学性质；过渡金属硫化物目前的研究状况,和在实际当中的应用；黑磷作为目前新发现的一种二维材料,一经发现就引起了巨大的轰动,被人称作带隙最合适的二维材料,大有超过石墨烯的趋势,我们也简要的对其性质和特点进行了介绍。第三章研究了一种新型的二维材料——锗烷(Germanane, GeH)。锗烷是最近实验上才新合成出来的一种新型二维材料,其有着很好的物理化学性质,在实际的应用中具有很大的潜力。实验和理论的工作都显示,这个材料具有1.56 eV的直接带隙,说明其在光解水方面能有所应用；另一方面,其迁移率是锗体材料的5倍,说明在电子器件方面也能有所发展。而我们希望从理论上对这个材料进行化学修饰,如取代的方法,继续改进这个材料的性质,使其具有更好的应用前景。通常情况下,实验上常用氟元素进行掺杂或取代,所以这里我们也选择氟元素对我们的体系进行取代,由于不同的掺杂比例,以及掺杂位置都可能对体系的性质有影响。所以我们通过第一性原理计算的方法,搭建结构,考虑不同的取代浓度和取代位置,计算得到材料的电子结构方面的性质。主要有能带的宽度,能级的位置等等。通过这些性质,我们可以改良这个材料的性质,得到理论上适合做光解水的材料,为实验工作提供了一个可行的方案。第四章我们主要介绍一下由清华大学帅志刚教授小组开发的,利用第一性原理计算结合玻尔兹曼输运方程和弛豫时间近似理论来预测一些碳和有机材料的载流子迁移率。并且成功的计算出了比如石墨烯单层,石墨烯纳米条带的载流子迁移率,跟实验上测量的结果符合得非常的好。我们主要学习了他们这一方法,并且将其运用到我们所关心的二维材料的计算当中,预测出了一个新型的二维材料C5N的载流子迁移率。第五章介绍了一种利用双层二维材料之间的弱相互作用——范德瓦尔斯(van der Waals, vdW)相互作用,去完成纳米尺度的高精度测量。在纳米尺度的高精度测量是一个非常具有价值的研究课题。目前几乎所有的测量都集中在微米尺度(micro-scale)上,然而现在很多材料都已经是纳米尺度,所以我们急需发展一种新的手段去测量纳米量级的物质。常规的测量方法有光学,压电等方面。这些方法或多或少都具有缺陷和不足,所以我们需要发展一种新的方法去取代这些常规的方法。因此我们设计了一种新的方法,基于vdW相互作用的纳米尺度测量方法。我们设想,通过vdW作用构建一个双层二维材料,固定其中一层,移动另一层,如果移动的过程中,这个材料的其他性质,比如带隙的变化很大,这样我们就能通过带隙的变化来反映这个微小的位移,从而达到对纳米尺度的测量。因为,vdW相互作用是一个很弱的相互作用,移动其中一层,并不需要很大的能量或者力,所以这也可以作为测量微小作用下位移的方法。基于以上的构想,我们通过第一性原理设计和寻找,发现双层蓝磷(blue phosphorus)非常符合我们对这种材料的要求。并且也对比了其他双层二维材料,得到了一个较为普适的规律,去寻找满足这个条件的材料。第六章我们从理论上搜寻到了一种二维材料,而这种材料是基态反铁磁的金属,其具有非常好的物理化学性质,有可能作为很优秀的自旋电子学器件。首先,低维材料在现代的纳米科学和纳米技术中起着非常重要的作用。其中,石墨烯的发现和制备成为二维材料发展的里程碑。近些年来,其他的二维材料也有了长足的发展,其中硅烯,锗烯也表现出了跟石墨烯类似的优良的物理化学性质。另一方面,自旋电子学在近些年也有着非常显著的发展,通常我们使用的都是铁磁材料作为自旋电子学的主要材料。而最近,反铁磁自旋电子学却吸引了越来越多的研究者的注意,主要在于反铁磁有很多铁磁所不具有的优良的性质,这些性质只要我们加以利用,可以表现出相比于铁磁更好的特性。因此,我们希望找到一些低维的(如二维材料),具有优良性质的反铁磁材料。这样的材料之前并不多见,能在较常规的情况下正常工作的,那更是几乎没有。所以我们希望通过第一性原理,配合全局搜索的方法,找到一种具有这些优良性质的材料。这不仅仅是理论的需要,更可以为实验上提供合成的方向。第七章我们研究了石墨烯/二碲化钨异质结材料的磁阻效应。磁阻效应存在于一些金属和半导体当中,具体是指这些材料的电阻在外加磁场的作用下会有一定的相应。如果一个材料具有较大的磁阻效应,即对外加磁场的相应非常明显,则这个材料在电子学和磁学领域将有可能有很重要的应用前景,比如可以用来作为磁传感器,磁存储设备等。最近,实验上合成出了一种新的晶体材料—二碲化钨,这种材料具有很大的磁阻效应,远高于之前所发现的其他磁阻材料,引起了巨大的轰动。这里,我们设计了一个石墨烯/二碲化钨组成的异质结结构,两个单层之间通过范德瓦尔斯相互作用结合在一起。我们用第一性原理计算,研究了这个材料的几何结构,电子结构,以及磁学方面的性质。我们发现这个异质结材料,相比于二维的二碲化钨,磁阻效应上有很大的提升；另一方面,对比纯净的石墨烯,这个材料的载流子浓度有一定的增加,说明在实际的应用中,导电性比纯净的石墨烯更加的优秀。整体看来,异质结在某些方面的性质是要优于个单组分的,这也达到了我们对材料设计的初衷。
[Abstract]:Material plays a very important role in our daily production and life, and it also promotes the progress of human civilization and science. Human civilization has undergone the evolution from the Stone Age, Bronze Age, and Iron Age, and so on. All these developments have the same point, that is, human development and improvement of materials. The discovery of semiconductor materials and large-scale production and application have brought the human society into a more advanced microelectronics era, greatly enriching and improving human production and life. Increasing demand for materials makes it impossible for humans to design new materials with specific functions on the basis of existing materials through the science and technology already mastered. However, it is unrealistic to invest a lot of manpower, material and financial resources in simple combinations of existing materials by conventional experimental means. On the other hand, with the development and improvement of quantum mechanics and quantum chemistry, and with the help of high-performance computing equipment, we can solve some complex Schrodinger equations by numerical methods, and then get some physical and chemical properties of the systems theoretically. So we can improve the materials preliminarily. The purpose of this paper is to introduce a new kind of material, two-dimensional material, which is very popular nowadays, by using the first-principles calculation, to study and improve it. This paper consists of six chapters. The first chapter briefly introduces the basic and theoretical framework of computational quantum chemistry, as well as the density. Functional theory. It contains the commonly used Schrodinger equation, and then simplifies some of the column treatments of this basic equation, such as adiabatic, single electron approximation, Hohenberg-Kohn theorem, Kohn-Sham equation, and then develops various exchange-related functions suitable for many systems on this basis. Many properties of the system can be described, and many types of systems can be studied. On this basis, many methods have been developed to calculate the properties of the system. Finally, several commonly used density functional theory-based computational software packages are briefly introduced. Two-dimensional nanomaterials, in which graphene is the "star" grade material in two-dimensional materials, we introduced the development and research status of two-dimensional materials at the beginning of graphene and its derivatives. In addition, we also introduced the synthesis and basic physical and chemical properties of hexagonal boron nitride; the current research status of transition metal sulfides As a new two-dimensional material, black phosphorus has caused a great sensation. It is called the most suitable two-dimensional material for band gap and has a tendency to exceed graphene. We also briefly introduce its properties and characteristics. In the third chapter, a new two-dimensional material is studied. Germanane (GeH) is a new type of two-dimensional material which has been synthesized recently. It has good physical and chemical properties and has great potential in practical application. On the other hand, its mobility is five times higher than that of germanium-based materials, indicating that it can also be developed in electronic devices. We hope that this material can be chemically modified theoretically, such as substitution method, and the properties of this material can be further improved so as to have a better application prospect. So here we also choose fluorine to replace our system, because different doping ratio and doping position may have an impact on the properties of the system. So we use the first-principles calculation method, build the structure, consider different substitution concentration and substitution position, calculate the electronic structure of the material. The properties of the surface, such as the width of the energy band, the position of the energy level and so on. Through these properties, we can improve the properties of this material, get a material suitable for photolysis water in theory, and provide a feasible scheme for the experimental work. The carrier mobility of some carbon and organic materials is predicted by using the Boltzmann transport equation and relaxation time approximation theory. The carrier mobility of graphene monolayer and graphene nanoribbon is calculated successfully, which is in good agreement with the experimental results. A new two-dimensional material C5N is predicted by applying this method to the calculation of two-dimensional materials of interest. In the fifth chapter, a novel two-dimensional material C5N with high precision at nanometer scale is introduced by using the van der Waals (vdW) interaction, which is a weak interaction between two-layer two-dimensional materials. Degree measurement. High precision measurement at nanoscale is a very valuable research topic. At present almost all the measurements are focused on micro-scale. However, many materials are already nano-scale, so we need to develop a new method to measure nano-scale materials. There are optical, piezoelectric, and so on. These methods are more or less defective and inadequate, so we need to develop a new method to replace these conventional methods. Determine one layer, move the other, and if other properties of the material, such as the band gap, change dramatically during the movement, then we can reflect this tiny shift through the band gap change to achieve nanoscale measurements. It requires a lot of energy or force, so it can also be used as a method to measure the displacement under small action. Based on the above idea, we design and find the double-layer blue phosphorus, which is very suitable for this material, and compared with other two-layer two-dimensional materials, we get a better one. In Chapter 6, we have theoretically searched for a two-dimensional material, which is a ground-state antiferromagnetic metal. It has very good physical and chemical properties and may be used as an excellent spintronic device. First, low-dimensional materials are used in modern nanoscience and nanotechnology. The discovery and preparation of graphene has become a milestone in the development of two-dimensional materials. In recent years, other two-dimensional materials have made considerable progress. Among them, silicone and germanium also exhibit excellent physical and chemical properties similar to graphene. On the other hand, spintronics also has non-spintronics in recent years. Recently, antiferromagnetic spintronics has attracted more and more researchers'attention. The main reason is that antiferromagnetism has many excellent properties that ferromagnetism does not have. These properties can show phase if we use them. Therefore, we hope to find some low-dimensional (such as two-dimensional materials), with good properties of anti-ferromagnetic materials. Such materials are rare before, can work under more conventional circumstances, it is almost No. So we hope to use the first principle, with the global search method, to find one. In Chapter 7, we study the magnetoresistance effect of graphene/tungsten telluride heterojunction materials. The magnetoresistance effect exists in some metals and semiconductors, specifically the effect of the resistance of these materials on the applied magnetic field. If a material has a relatively large magnetoresistive effect, i.e. the corresponding effect on the applied magnetic field is very obvious, then this material may have important application prospects in the fields of electronics and magnetism, for example, it can be used as magnetic sensors, magnetic storage devices and so on. Recently, a new crystal material has been synthesized experimentally. Material-tungsten telluride, which has a very large magnetoresistive effect, is much higher than other magnetoresistive materials previously discovered, causing a huge stir. Here, we design a heterojunction structure consisting of graphene/tungsten telluride. The two monolayers are bonded by van der Waals interaction. We use first-principles calculations. We find that the magnetoresistance effect of this heterojunction material is much higher than that of two-dimensional tungsten telluride. On the other hand, compared with pure graphene, the carrier concentration of this material increases to a certain extent, indicating that in practical applications, Conductivity is better than pure graphene. Overall, heterojunctions are superior in some respects to a single component, which is what we intended for material design.
【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】：2016
【分类号】：TB383;O413.1

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