石墨烯（炔）材料电荷输运性质的第一性原理研究

发布时间：2018-06-07 15:56

本文选题：石墨烯 + 石墨炔　；参考：《山东大学》2016年博士论文

【摘要】：随着电子器件小型化的不断发展,传统的硅基材料集成电路的尺寸在不断地缩小,根据著名的摩尔定律的预测,硅基器件的尺寸将进一步的缩小到1～2nm左右,这个尺寸的大小已经进入到分子和原子领域,因此能否利用单个分子来形成纳米回路中元器件的这个想法很自然的成为人们非常关注的问题。令人们感到高兴的事,单个分子确实可以小型化传统的硅基微电子器件,并且还可以表现出传统硅基器件所具有的功能。因此,我们把研究利用单个原子或者利用单个分子来构建电子回路中的功能性元件的方法叫做分子电子学。一直以来,人们都在设计寻找并且制备分子尺寸的电子器件。1959年,美国的物理学家费曼在物理学会年会上发表了著名的演讲"Plenty of Room at the Bottom ",提出了在分子和原子的尺寸上建立电子器件然后构成集成电路的想法,这个想法在当时引起了深远的影响。传统的想法是建立好宏观系统和元件,然后将他们的尺寸缩小,而费曼的想法是直接从原子尺度上来建立电子器件,最后利用这个电子器件组合成电路。相比于传统的硅基半导体材料来说,应用分子作为电子器件的元件具有许多巨大的优势,例如：①大小尺寸方面：分子的体积小(1-10nm)并且具有非常高的集成密度,在能源消耗及利用效率方面具有极大的优势；②运算速度方面：电子输运性质较好的分子线可以减小晶体管的渡越时间,减少运算时间；③组装和识别方面：可以利用特殊的分子间相互作用形成纳米尺度自组装结构。分子识别可以用来调节电子行为,提供单分子尺度上的开关和感知功能；④合成可修正性：选择不同的组合和结构就能改变分子电子输运性质；⑤具有新的功能：某些特殊种类的分子,存在不同的稳定结构或者同分异构体,这在传统固态材料中是不可能实现的。随着实验技术手段的发展,多种分子电子器件都可以制备出来,这都要归功于这些先进的设备仪器,这里我们简单的介绍几种。①扫描电子显微镜技术(STM),是目前最通用的制备原子尺度接触的方法之一。实验方法是将含有目标分子的溶液放在金属的衬底上,利用STM探头在金属衬底表面做伸缩运动,从而形成了探头、目标分子和金属衬底的三明治电极结构,从而测得电导数值。此外还有②原子力显微镜技术,③透射电镜技术(TEM)④力学可控破缺结等等。随着实验手段的发展,理论方法也在不断地创新,用来更精确的表述分子电子器件中的电荷或自旋输运问题。就目前来说,结合密度泛函理论和非平衡格林函数方法是处理分子电子输运的首选方法,并且已经广泛应用于相关的研究中,例如单分子团簇,分子线,石墨烯以及纳米管等。自从2004年,英国曼彻斯特大学的Novoselov和Geim首次在实验上分离出单层石墨烯以来,石墨烯的相关研究就受到了人们的广泛关注。也就是在这个时期,二维的平面材料成为人们研究的热点。基于单层的石墨烯,沿着两个不同的方向进行裁剪,可以得到两种不同种类的石墨烯纳米条带。通常按照边缘的样式,可以将石墨烯纳米条带区分为锯齿形石墨烯纳米条带(ZGNRs)和扶手椅形石墨烯纳米条带(AGNRs)。其中,关于ZGNRs的研究更是非常的兴盛,先前的研究人员的工作重点是研究ZGNRs的电子输运特性,并且是将ZGNRs的一部分链接到ZGNRs电极上,在这种情况下,就会忽略掉ZGNRs片段与ZGNRs电极的接触位置效应。再者,ZGNRs具有边缘态,能够在Fermi能级处产生较强的电子密度分布,这种奇特电子密度分布可能会对基于ZGNRs电子器件产生特殊的输运特性。因此基于ZGNRs的研究的非常重要的。人们在研究sp2杂化的石墨烯的同时,也在不断地探索具有不同杂化形式新的碳的同素异形体。由于石墨炔具有独特的sp2和sp杂化特性,近几年石墨炔方面的研究同样受到广泛关注,Hirsch认为对石墨炔研究的新时代即将到来。尽管科学家在有关石墨炔结构、合成、特性等方面开展了许多研究工作,但是有关石墨炔体系电子输运性质方面的研究却鲜有报道。由于石墨炔中组成π电子的碳原子轨道成份不同,导致以往关于石墨烯结构的电子输运性质的结论不能完全适用于石墨炔体系。因此,研究石墨炔体系的电子输运性质,探索适用于研究sp2和sp杂化体系的电子输运机制的理论具有非常重要的理论和应用价值。本论文中的三个工作就是基于石墨烯和石墨炔这两种材料展开研究的。1. ZGNRs电极与石墨一炔子结构的电子输运特性考虑到石墨一炔结构中包含sp2和sp两种不同的杂化模式,它的电子输运特性以及相关的物理机制问题研究的较少,因此探索适用于研究sp2和sp杂化体系的电子输运机制的理论具有非常重要的理论和应用价值。考虑到电极与中心分子的接触电阻问题,我们选取了ZGNRs作为电极,这是由于它们都是二维的平面材料,并且ZGNRs具有边缘态,能够在Fermi能级处产生较强的电子密度分布,这种奇特电子密度分布可能会对基于ZGNRs电子器件产生特殊的输运特性。我们利用密度泛函理论和非平衡格林函数相结合的方法,研究了石墨一炔子结构与锯齿形石墨烯电极构成的三明治结构的电荷输运特性。主要考虑石墨一炔与电极链接不同位置以及电极的不同宽度对该体系电荷输运特性的影响。我们发现：①无论石墨一炔与ZGNRs电极的链接位置如何,体系都会表现出半导体的电荷输运特性,这是由石墨一炔体系的HOMO-LUMO gap决定的。②当石墨一炔链接在ZGNRs电极的边缘位置处的电流要比石墨一炔链接在ZGNRs电极中心位置处的电流要大,并且随着链接位置越来越靠近ZGNRs中心处,电流的大小会越来越小。③当整个体系关于xz中垂面镜面对称时,宇称限制隧穿效应可以完全破坏掉体系的电子输运特性,增大体系的开启偏压。在本研究工作中,我们着重对这三个现象的物理机制进行了系统性的探讨。2.锯齿形石墨一炔(ZGYRs)与ZGNRs异质结的整流特性研究传统的整流观点是要保证体系具有不对称性,这样就可能会产生整流现象,从而制造分子整流器件。我们利用密度泛函理论和非平衡格林函数相结合的方法计算了两种不同的体系ZGYRs和ZGNRs构成异质结的整流特性,发现不对称的体系不一定能够产生整流现象。为了更深入的研究这个问题,我们对ZGYRs进行了氧原子的替换,我们发现未受氧原子替换的ZGYRs与ZGNRs形成的异质结没有出现整流现象,这主要归因于这两个体系的能带结构是关于Fermi能级对称的,在正负偏压的影响下,电流都是对称性分布的。而当ZGYRs被氧原子替换后,体系的整流方法发生变化,替换不同位置,整流方向是不相同的。更重要的是,如果氧原子是对称性替换掉ZGYRs的话,体系会表现出偏压诱导的整流反转现象。3.局域应力下的ZGNRs的电子输运特性先前的工作者对ZGNRs的电荷输运性质的研究发现,偶数宽度的ZGNRs具有镜面对称性,因此它的电流大小会受到宇称限制隧穿效应的影响,从而表现出电流抑制效应。我们设想如果偶数宽的ZGNRs的对称性被部分的打破,是否还会出现电流抑制效应?基于这一点,我们利用密度泛函理论和非平衡格林函数相结合的方法,研究了ZGNRs在外加局域应力后的电荷输运特性,主要考虑应力作用的不同范围以及应力作用的大小对该体系电荷输运性质的影响。研究发现,在较小的局域应力作用下,体系的电子输运特性基本上没有发生变化,表现出与未受应力的体系相同的电流曲线特征-电流抑制现象。继续增大应力后,一些体系在应力的作用下诱导产生了局域态,并且我们发现局域态在零偏压下阻碍的电子的传输。然而在有限偏压下,应力诱导的局域态却可以促进ZGNRs的电荷输运行为,原因在于偏压下的局域态破坏了ZGNRs电子密度分布的对称性,从而打破了电流抑制效应。进一步研究发现,应力作用的范围不同也会明显影响体系的电子输运特性,应力作用在边缘原子上对体系的电子输运的改变小于应力作用在中心区域,并且体系原本的电子密度分布也对应力的响应不同,也会对体系电子输运性质产生影响。
[Abstract]:With the development of electronic devices miniaturization, the size of traditional silicon based integrated circuits is shrinking. According to the prediction of the famous Moore's law, the size of silicon based devices will be further reduced to about 1 ~ 2nm. The size of this size has already entered the field of molecules and atoms, and can be formed by a single molecule. This idea is a natural concern in nanoscale circuits. It makes people feel happy that a single molecule can actually miniaturized the traditional silicon based microelectronic devices and can also show the functions of traditional silicon based devices. So, we use a single atom or use a single point. The way to build functional components in electronic circuits is called molecular electronics. All the time, people have been designing electronic devices to find and prepare molecular sizes for.1959. The American physicist Feynman published a famous speech "Plenty of Room at the Bottom" at the annual meeting of the Physics Society, which was put forward in molecules and atoms. The idea of building an electronic device in size and forming an integrated circuit had a profound effect at the time. The traditional idea was to establish macro systems and components and reduce their size, and Feynman's idea was to establish electronic devices directly from the atomic scale and then combine this electronic device into a circuit. Compared to the traditional silicon based semiconductor materials, the elements used as electronic devices have many great advantages, such as: (1) size dimension: small size of the molecule (1-10nm) and very high integration density, which has great advantages in energy consumption and utilization efficiency mask; Molecular lines with better transport properties can reduce the transition time of transistors and reduce operation time; (3) assembly and recognition: a nano scale self-assembly structure can be formed by special intermolecular interaction. Molecular recognition can be used to regulate electronic behavior, provide switching and perceptual functions on a single sub scale; (4) synthesis Modifiability: the selection of different combinations and structures can change the molecular transport properties of molecules; (5) new functions: some special types of molecules have different stable structures or isomers, which can not be realized in traditional solid materials. With the development of experimental techniques, various molecular electronic devices are available. In order to prepare it, this is due to these advanced equipment. Here we simply introduce several kinds. (1) the scanning electron microscope (STM) is one of the most common methods for preparing the atomic scale contact. The experimental method is to put the solution containing the target molecules on the metal substrate and use the STM probe on the surface of the metal substrate. With the expansion motion, the probe, the sandwich electrode structure of the target molecule and the metal substrate are formed, and the conductance values are measured. In addition, the atomic force microscope technique, the transmission electron microscope (TEM), mechanical and controllable breaking and so on. With the development of the experimental means, the theoretical method is constantly innovating and used for more precise expression. Charge or spin transport in molecular electronic devices. At present, the combination of density functional theory and nonequilibrium Green function method is the first choice to deal with molecular electron transport, and has been widely used in related studies, such as single molecular clusters, molecular lines, graphene, and nanotubes, etc. since 2004, Manchester United The study of graphene has attracted wide attention since the first single graphene was separated from Novoselov and Geim in the University. In this period, two-dimensional plane materials have become the hot spots. Based on the single graphene, two different directions are cut, and two kinds of materials can be obtained. The same kind of graphene nanoscale bands. Usually, the graphene nanometers can be divided into serrated graphene nanometers (ZGNRs) and armchair graphene nanoscale ribbons (AGNRs) according to the edge patterns. The research on ZGNRs is more prosperous. The previous researchers focused on the study of the electron transport of ZGNRs. In this case, a part of ZGNRs is linked to the ZGNRs electrode. In this case, the contact position effect of the ZGNRs fragment and the ZGNRs electrode is ignored. Furthermore, the ZGNRs has the edge state, which can produce a strong electron density distribution at the Fermi level. This peculiar electron density distribution may produce a special electronic density distribution based on the ZGNRs electronic devices. Therefore, the research based on ZGNRs is very important. In the study of SP2 hybrid graphene, people continue to explore the Homo heteromorphs with different kinds of hybrid forms of carbon. Because of the unique SP2 and SP hybrid properties of graphite alkynes, the research of graphite and acetylene in recent years has also been widely concerned, Hirsch It is believed that the new era of graphite acetylene is coming. Although scientists have done a lot of research on the structure, synthesis and properties of graphite alkynes, there are few reports about the electronic transport properties of graphite alkynes. The conclusion that the electronic transport properties of the graphene structure can not be fully applied to the graphite alkyne system. Therefore, it is very important to study the electronic transport properties of the graphite acetylene system and explore the theory and application value for the study of the electronic transport mechanism of SP2 and SP hybrid systems. The three work in this paper is based on graphene and stone. The electronic transport properties of.1. ZGNRs electrodes and graphite monyne structures studied by the two kinds of graphite alkynes are considered as two different hybrid modes of SP2 and SP in the graphite monyne structure. The electron transport properties and related physical mechanism problems are less studied. Therefore, the probe is suitable for the study of the electrons of the SP2 and SP hybrid systems. The theory of transport mechanism has a very important theoretical and practical value. Considering the contact resistance between the electrodes and the central molecules, we choose ZGNRs as the electrode, which is because they are two-dimensional plane materials, and ZGNRs has the edge state, which can produce strong electron density distribution at the Fermi level, this strange electron. Density distribution may produce special transport properties for ZGNRs based electronic devices. Using the method of density functional theory and nonequilibrium Green function, we study the charge transport properties of the sandwich structure composed of the graphite monyne structure and the sawtooth graphene electrode. The main consideration is that the graphite one acetylene is linked to the electrode. The influence of the different widths of the electrode and the electrode on the charge transport characteristics of the system. We found that: (1) the charge transport properties of the semiconductor, regardless of the location of the link between the graphite one and the ZGNRs electrode, are determined by the HOMO-LUMO gap of the Shi Moyi acetylene system. The current at the position of the set is larger than the graphite one at the center of the ZGNRs electrode. And as the link position gets closer to the center of the ZGNRs, the size of the current will become smaller and smaller. In this research work, we have focused on the physical mechanism of the three phenomena systematically to study the rectifying characteristics of the.2. sawtooth graphite one acetylene (ZGYRs) and the ZGNRs heterojunction. The traditional rectification viewpoint is to ensure that the system has asymmetry, which may produce rectifying phenomena and thus make the molecules. Rectifying devices. We use the density functional theory and the nonequilibrium Green function method to calculate the rectifying characteristics of two different systems, ZGYRs and ZGNRs, and find that the asymmetric system does not necessarily produce rectifying phenomena. In order to further study this problem, we have replaced the oxygen atom in ZGYRs. We found that the heterostructure formed by the unoxygen atom substitution of ZGYRs and ZGNRs has no rectifying phenomenon, which is mainly attributed to the band structure of the two systems is about the symmetry of the Fermi energy level, and the current is symmetrical under the influence of positive and negative bias. When the ZGYRs is replaced by oxygen atom, the rectification method of the system is changed and replaced. In different positions, the rectification direction is different. More importantly, if the oxygen atom is symmetric and replace the ZGYRs, the system will show the bias induced rectifying and reversal phenomenon, the electron transport properties of the ZGNRs under the local stress of.3., the previous workers on the charge transport quality of the ZGNRs found that the even number of ZGNRs has a mirror surface. We assume that if the symmetry of the even wide ZGNRs is partially broken, will there be a current suppression effect? Based on this, we use the combination of the density functional theory and the nonequilibrium Green function. The charge transport characteristics of ZGNRs after external stress are studied. The effect of the different range of stress and the size of stress on the charge transport properties of the system is mainly considered. It is found that the electronic transport properties of the system have not changed under the effect of small local stress, and show that the system is not subjected to the stress. The same current curve characteristic current suppression. After the stress continues to increase, some systems induce local states under the action of stress, and we find the transmission of electrons hindered by the local state under the zero bias. However, under the finite bias, the local state induced by the stress can promote the charge transport behavior of ZGNRs. It is that the local state under partial pressure destroys the symmetry of the distribution of ZGNRs electron density, thus breaking the effect of current suppression. Further studies have found that the range of the stress action will obviously affect the electronic transport properties of the system, and the change of the electron transport of the stress on the edge atom on the system is less than the stress in the central region, Moreover, the original electron density distribution also responds to the force response, which will also affect the electron transport properties of the system.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

【相似文献】