【作者】 任浩；

【导师】 杨金龙；

【作者基本信息】 中国科学技术大学 ， 化学物理， 2010， 博士

【摘要】 近年来,纳米和表面体系方面的研究取得了非常大的进展,这些进展不仅丰富了传统的凝聚态物理、量子化学、新型功能材料等学科领域的研究内容,也对信息技术、洁净能源、环境科学等多个方面产生了影响。另一方面,随着研究体系复杂性的日益增加,以及纳米体系不可忽视的量子效应,第一性原理计算已经成为不可或缺的研究手段。在这些前沿领域的研究中,理论计算不仅用来解释观测到的现象,揭示其背后的物理本质,还可以用来指导实验方面的研究,甚至可以通过搭建合适的模型,进行理想实验。本论文主要论述了三个方面的工作：利用密度泛函理论研究了边界化学修饰对扶手椅(armchair)型石墨烯纳米条带电子结构性质的影响；利用非平衡格林函数技术结合密度泛函理论的方法研究了石墨烯纳米条带体系的电子输运性质,从理论上设计了两种纳米器件；发展了基于第一性原理计算的表面吸附体系非弹性电子隧穿谱的计算方法,并讨论了在几个实际体系中的具体应用。论文起始,第一章简要介绍了第一性原理电子结构计算的理论框架和目前研究中常用的近似与方法。自从量子理论诞生,人们就开始了应用量子理论描述实际体系的征途。而整个电子结构理论,就是人们在用量子力学的基本原理描述真实体系的过程中得到的一种既可以负担得起,又具有可接受的近似程度的方法体系。在这一章中,我们遵循着近似程度由低到高的脉络,首先简单介绍了目前电子结构计算中经常用到的三个基本近似,分别为：非相对论近似、玻恩-奥本海默近似和独立电子近似；然后在此基础上,叙述了Hartree-Fock近似方法和自洽场方法,这是现阶段电子结构计算的基石;随后,简要叙述了在研究中广泛应用的密度泛函理论,以及采用非平衡格林函数技术和密度泛函理论相结合的计算电子输运性质的方法；最后,简单介绍了本论文涉及到的工作中用到的几个软件包。纳米科学和表面科学作为新兴的交叉学科展现出了蓬勃的生机,在第二章中,我们简要叙述了近年来该领域的研究进展和现状。首先,从实验技术来看,扫描探针技术的发展,不仅使人们的目光达到化学极限,更重要的是作为一种实验手段,可以搭建、操控各种微观实验体系,甚至可以操控化学反应；近年来发展的分子结技术也是一个巨大的进步,使得人们可以对单个原子、分子量级的体系进行电子输运性质的测量,对下一代微电子电路的发展具有深远的影响。在新型的实验体系方面,石墨烯引发了当前的研究热潮,无论是从其基本物理性质还是在各个领域内的潜在应用来看,石墨烯都是一种令人激动的材料;表面吸附体系则是一个相对传统的研究范畴,但由于其在异相催化、晶体外延生长、生命科学以及分子电子学等领域都具有重要的应用,仍存在许多问题亟待解决。石墨烯相关材料的一个非常有吸引力的应用是作为下一代微电子器件。石墨烯纳米条带,作为一种石墨烯的衍生材料,其物理性质受边界手性、宽度、及化学环境等很多因素的影响,边界碳原子被氢饱和的扶手椅型石墨烯纳米带(AGNR)均为半导体。在第三章中,我们用各种不同原子或者基团对AGNR边界碳原子进行化学修饰,发现不仅可以改变AGNR体系的电子结构性质,比如改变带隙、引起半导体-金属转变等,边界化学修饰还可以改变AGNR体系带隙随宽度的变化规律。此外由于AGNR体系一个可能的重要应用就是分子器件,我们研究发现用常见的锚接基团NO2和NH2对AGNR进行边界化学修饰时,体系的电子结构性质变化不大,这就意味着可以比较容易地将具有特殊电子性质的AGNR单元模块化组装于分子电路中。分子电子学的研究主要有两个方面,一是通过实验或者理论的方法,研究分子或纳米体系的电子输运性质,以期找到具有特定电子学功能的器件,比如分子整流器件、分子开关、量子点、负微分电阻器件等；此外便是利用体系的电子输运性质,通过某些手段得到体系其他性质,比如化学识别。在第四章中,我们在理论上设计了基于石墨烯纳米带的量子点器件和负微分电阻器件,这两种纳米器件都具有良好的稳定性,有望在实际中应用。在第五章中,给出了基于第一性原理电子结构计算进行表面吸附体系非弹性电子隧穿谱的模拟方法。我们采用Tersoff-Hamann近似和有限差分方法,实现了STM实验体系中非弹性电子隧穿谱的计算程序,在两个广泛研究的体系：CO分子吸附在Cu(100)表面和乙炔分子吸附在Cu(100)表面的测试表明,我们的程序健壮性较好,能够重现实验结果,跟以前的理论计算结果也一致。随后我们将程序用于顺-2-丁烯分子吸附在Pd(100)表面和三聚氰胺分子吸附在Cu(100)表面两个体系的研究,体现了利用非弹性电子隧穿谱进行表面化学识别的优越性。非弹性电子隧穿谱在表面科学的研究中是一种强有力的实验工具。更多还原

【Abstract】 In the recent decades, we witnessed tremendous progresses in nanoscience and surface science. All these developments not only extended the scope of conventional subjects such as condensed matter physics, quantum chemistry, and functional ma-terial, but also possess great impacts on information technology, clear energy, envi-ronmental science and so on. On the other hand, with the increasing complexity and the considerable quantum effects in nano-scale systems, first-principles calculations became more and more important in research activities. Theoretical calculations can not only be employed to explain the experimental observation, to reveal the underly-ing physics, but also to guide of experiments, even to perform ideal experiments via constructing appropriate models.All the works in this thesis can be divided into three parts:the study of the effects of chemical modification of edged carbon atoms in armchair edged graphene nanorib-bons (AGNRs); the study of electronic transport properties of graphene nanoribbons (GNR) based nano-systems by using the non-equilibrium Green’s function technique combined with density functional theory; the development of computational proce-dures to simulate inelastic electron tunneling spectra in surface-adsorbate systems.At the beginning, we briefly introduced the background of the first-principles electronic structure calculation and some methods/approximations widely used in the nowadays research. People started the journey of describing real systems by using quantum mechanics since the birth of quantum theory in the beginning of twentieth century, from this point of view, the whole electronic structure theory is an outcome of the process of people looking for a method affordable and in the mean time can give acceptable results to describe the real world. In Chapter 1, along with the thread of approximations from the most rudimental level to the advanced, we first introduced the three basic approximations widely adopted in electronic structure calculations, i.e. non-relativistic approximation, Born-Oppenheimer approximation, and the inde-pendent particle approximation; Hartree-Fock approximation and self-consistent field method as the cornerstones of the modern electronic structure theory are then intro-duced; Next, we gave a brief description on density functional theory and the non-equilibrium Green’s function technique combined with density functional theory; In the end, a short introduction for software packages used in this thesis is presented.In chapter 2, we depicted the rapid developing interdisciplinary subjects named nanoscience and surface science. On the one hand, the development of scanning probe techniques enables us to observe objects at the chemical limit, such as single atom or single molecule, and more important, scanning probe techniques allow us to con-struct and control various microscopic systems, even allow us to control chemical re-actions; Molecular junction is another great progress that allows us to perform single atomic/molecular electric measurements, and has great potential in the future micro-circuits. On the other hand, the emerging of new materials provides platforms for peculiar physics, for instance, the rise of graphene attracted tremendous research inter-ests both in basic physics and various applied subjects. Due to the requirement of the detailed physics of surface-adsorbate systems in the research of heterogenous cataly-sis, epitaxial growth, life science and molecular electronics, there are many challenging open questions in surface science.Graphene related materials are expected to have great potential in the next gen-eration micro electric devices. As a kind of derived material, GNRs can be prepared from graphene. The electronic properties of GNRs are sensitive to its chirality, width, chemical environment, and many other issues. All the hydrogen saturated AGNRs are semiconductors. In chapter 3, we studied the effect on the electronic properties of edge chemical modification of AGNRs. We found that chemical modification not only changes the electronic structures of AGNRs, such as energy gap engineering and semiconductor-metal transition, but also changes the family behavior in gap-width re-lation. Furthermore, AGNR is expected to be a kind of molecular device, our study on the electronic structures of AGNRs upon chemical modification by NO2 and NH2, which is widely used in molecular electronics as anchoring groups, shows that the modification does not alter the electronic structure qualitatively, which implies that we can integrate AGNR functional units into molecular circuits without focusing on much extra work. There are mainly two kinds of research in the area of molecular electronics. First is the direct investigation of the electron transport properties of molecular or nano systems, which can be achieved via various experimental or theoretical approaches. Activities such as device design can also be classified into this field, indeed, many kinds of molecular devices such as molecular rectifier, molecular switch, quantum dot, and negative differential resistance (NDR) device have been designed or proposed the-oretically. The other is the application of electron transport properties to realize other purpose, such as chemical identification. In chapter 4, we designed a quantum dot and a NDR device based on GNRs by using theoretical calculations, the functionality are robust for both devices.In chapter 5, we discussed our own implementation for the simulation of inelastic electron tunneling spectra (IETS) in surface-adsorbate systems. Our method is based on Tersoff-Hamann aprroximation and finite difference method; We benchmarked our code in two test systems:CO adsorbed on Cu(100) and acetylene adsorbed on Cu(100), both cases give reliable/robust results and excellent performance. Then we applied it in two practical programs:cis-2-butene adsorbed on Pd(110) surface and melamine tautomers adsorbed on Cu(100) surface, all these test cases and practical applications are remarkable demonstrations of the fact that IETS is a power tool in surface sci-ence, especially in the chemical identification of adsorbate species, configuration, and conformation.更多还原