摘要
石墨烯因为其独特的性质,吸引了众多的理论和实验研究。相关研究也取得了许多进展,特别是对掺杂、有限尺寸约束、外加电场、以及各种化学吸附条件下的电子结构和磁性不稳定性等方面的研究取得了许多引人注目的成果。
本论文一共分为五章,第一章介绍了石墨烯的发现、实验制备以及独特的输运性质;第二章介绍了密度泛函理论;第三章讨论了锯齿型石墨烯纳米带电子结构调控的两种方法;第四章研究了少层锯齿型石墨烯纳米带的结构和磁性不稳定性问题。
第一章中首先介绍了石墨烯的发现,重点介绍石墨烯的电子结构和一些独特的输运性质,例如:奇异的量子霍尔效应、反常的弱局域化等。其次介绍了实验制备的常用方法,之后介绍不同边界的石墨烯纳米带各自的特点,为下文研究奠定基础。
第二章中介绍了第一性原理的理论—密度泛函理论及常用近似LDA和GGA。密度泛函的基础是一个假定:假定一个多粒子体系的任何性质都是基态电子密度的函数。用无相互作用系统代替多体相互作用,并把一切与多体相关的相互作用包含在交换能中。找到合适的交换能量泛函是密度泛函的主要内容之一。最后,对本论文使用的软件包做了简单的介绍。
第三章中讨论了两种调节锯齿型石墨烯纳米带电子结构的方法:施加横向电场和对边界进行化学修饰。锯齿型石墨烯纳米带因其边界态的存在,要通过自旋极化降低自身能量。用LDA关联我们证实了施加横向电场可以使该体系产生半金属性。借鉴横向电场的原理效果,我们研究了对纳米带两个边界修饰不同基团进而调节电子结构的方法,我们发现一边用H、另一边用CH3可以使体系出现自旋极化,费米面附近出现某一自旋,而另一自旋离费米能级较远。
第四章中,通过对双层、三层锯齿型石墨烯纳米带稳态几何结构、稳态电子结构和磁性的研究,得到以下结论:无色散边界带的存在导致了结构、磁性中一个或者两者的不稳定性;边界结构的弯曲直接导致结构的不稳定;有磁性的基态将出现磁性不稳定性。从结构稳定性来看:结构不稳定仅出现在α堆叠的少层锯齿型石墨烯纳米带中,β堆叠的少层锯齿型石墨烯纳米带结构通常保持平整并且具有有磁性的基态。观察磁性的稳定性可以得到:对于两层和三层,无论层数和边界堆叠方式如何,只要基态是有磁性的,层内原子均满足反铁磁序排列;并且随着层数的增加,基态磁性变的复杂且倾向非共线性。
第五章是对全文工作的总结和展望。
Due to its novel properties, graphene has been widely studied in recent years. Progress has been achieved on electronic characteristics and magnetic instability manipulated by doping, size confinement, electric field, chemical adsorption.
This paper is divided into five chapters. The first chapter gives a simple introduction of the discovery, experimental preparation and novel properties of graphene. The second chapter presents the Density Function Theory. The third chapter discusses two methods to manipulate the electronic properties of zigzag graphene nanoribbon. The fourth chapter studies the structural and magnetic instability in layered zigzag graphene nanoribbon.
In chapter 1, first we introduce the discovery of graphene, mainly introduce the novel electronic properties and transport characteristics, such as anomalous quantum hall effect, unusual weak localization. Then we present the experimental techniques to produce graphene. Last we describe their characteristics of the nanoribbons with different edge-shapes.
In chapter 2, we present the Density Function Theory (DFT), base of the first-principle, and two important approximations:LDA and GGA. The basic idea of DFT is an ansatz, which assumes that any properties of many-body system can be determined by the ground state density. Many-body problem can be replaced by an auxiliary independent-particle problem, while many-body interaction can be included in the exchange-correlation energy. Finding good approximation of exchange-correlation functional is one of the main targets in DFT. At the end of this chapter, we introduce some packages used in this work.
In chapter 3, we introduce two methods to control the electronic properties: applying transverse electric field and modifying by chemical group. In zigzag graphene nanoribbon, two localized states occur the both edges, which are ferromagnetically ordered, and antiferromagnetically coupled each other. Our calculated results with LDA show that the zigzag graphene nanoribbon can be converted to half metal. Then we alter the electronic property through modifying different chemical groups. The ribbon maintain a spin-polarized ground state, when modified by H and CH3, one of the spin appears near the Fermi level, the other far from the Fermi level.
In chapter 4, we investigate both structural and magnetic instability of bilayer-zigzag graphene nanoribbons (B-ZGNRs) and trilayer-zigzag graphene nanoribbons (T-ZGNRs) with different edge alignments and get the following conclusions:(1) the edge states around the Fermi energy form flat bands in flat ZGNRs, they lead to structural instability or magnetic instability, or both, in layered ZGNRs; structural instability results in structural bending of edges; while magnetic instability results in magnetic ground state; (2) structural instability only happens in layered ZGNRs with a-alignment edges; (3) layered ZGNRs with P-alignment edges are always flat and have magnetic ground states; (4) the inlayer magnetic order in a layered ZGNR is always antiferromegnetic regardless of edge, alignment and number of layers, as long as its ground state is magnetic; (5) with the increasing of the number of layers, the magnetic order of the ground states can be complex and non-collinear. Though our conclusions are only based on the studies of B-ZGNRs and T-ZGNRs, they should be still valid when studying layered ZGNRs with more than four layers.
Conclusions and outlook are given in chapter 5.
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