【作者】 李继玲；

【导师】 夏曰源；

【作者基本信息】 山东大学 ， 理论物理， 2008， 博士

【摘要】 近年来,纳米材料作为一种新型的材料结构,以其独特的性能和诱人的应用前景,引起了科学工作者的极大的兴趣和关注,成为材料科学研究领域一个新兴热点。1985年,幻数为60的富勒烯C₆₀结构被首次提出,随后其大规模合成取得成功,之后的短短二十年来,富勒烯所表现出的独特的结构性质,电子学特性,光学性能,磁学性能等受到各国研究人员的极大关注,并取得了一系列有意义的研究成果。其中,通过掺杂来改变富勒烯的力学性能和电子学特性,进而以富勒烯为基材料来合成具有更加优良的物理化学性能的新型纳米材料是当前人们关于富勒烯研究中的一个热点。理论上研究富勒烯的掺杂及以富勒烯为基材料合成新型材料的微观结构和性能对于探讨其形成机理,开发其应用前景具有非常重要的意义。计算机模拟在今天的科学研究中起着非常重要的作用,成为理论分析和实验研究外,研究物质世界的第三大工具,是沟通理论和实验的桥梁,不仅可以辅助实验,指导实验,还可以得到一些实验上无法测量到的结果,并能深入揭示所研究系统的内在行为机制。在研究纳米材料时,通常有两种计算方法:第一种称为从头算法或第一原理计算法,主要包括Hartree-Fock自洽场方法和密度泛函方法;第二种称为经验法或半经验法,比较常用的就是经典分子动力学模拟。从头算法是从量子力学第一原理出发,通过自洽迭代求解薛定谔方程或者Kohn-Sham方程,预测材料的各种微观性质。经验算法是根据已有的相互作用势函数解析形式计算材料的某些性质。本论文中,我们采用密度泛函理论计算和经典分子动力学模拟相结合的方法,研究了Si原子替代掺杂富勒烯C₆₀及其衍生物Si-C异质富勒烯的聚合,形成一维Si-C异质富勒烯聚合纳米管,并对其进行功能化的研究,并研究了非碳类富勒烯笼状团簇的聚合等问题。论文主要包括两部分,第一部分介绍了我们进行研究工作的理论基础,第二部分介绍了作者本人在攻读博士学位期间所做的主要研究工作,分别介绍如下:一、研究工作的理论基?第二章描写从头算法和分子动力学模拟的理论基础本章中,我们对从头算法的基本原理、密度泛函计算、经典分子动力学、以及第一原理分子动力学的基本原理进行了简要地介绍。密度泛函方法是当前最为重要的一种基于第一原理计算的研究方法,由于其不依赖于任何经验的参数而具有非常广泛的应用,可以计算结构性质、机械性质、电学性质、光学性质、磁学性质等多方面的性能。但是,由于其计算量非常巨大,在当前的计算条件下,一般只能处理几百个原子的体系。经典分子动力学模拟方法以其程序简单、计算速度快,能处理大的体系而成为研究工作者从事材料设计、物性研究的有力工具。但是,经典分子动力学方法一般只能用来研究物质的结构和能量以及由此推导出来的其他性质,比如机械性质。当研究物质的电子学、光学等特性时,则只能求助于第一原理计算。因此,在实际应用中,需要把两种方法结合起来,才能研究更为广泛的课题。二、攻读博士学位期间所做的主要工作第三章研究Si替代掺杂富勒烯C₆₀及其衍生物Si-C异质富勒烯的聚合。近年来,随着人们以富勒烯为基材料来合成具有特殊的物理化学性能的新型纳米结构研究的深入,富勒烯C₆₀的修饰衍生物逐渐引起了科学家们的越来越多的关注。对富勒烯的修饰可分为三种方式:外部吸附、内部吸附和替代掺杂。其中,替代掺杂是指用外来原子来替代富勒烯C₆₀表面上的C原子。较之于外部和内部修饰,替代掺杂能够更大程度上改变富勒烯分子的反应活性和电子结构,因而,是当前修饰富勒烯的主要方式。Si原子与C原子位于元素周期表中的同一主族,具有相同的外层电子数,因而成为对富勒烯C₆₀上C原子进行替代掺杂的首选元素。在本章中,我们采用密度泛函理论计算方法研究多个Si原子对富勒烯C₆₀的替代掺杂及其衍生物Si-C异质富勒烯的聚合。我们的研究结果表明,Si原子在富勒烯C₆₀表面进行替代掺杂时,异质富勒烯表面上Si原子与C原子处于相分离的分布方式时,体系比较稳定。通过考察Si-C异质富勒烯的聚合产物的稳定情况,我们发现,以Si-C异质富勒烯为结构单元的聚合产物的稳定性随着体系的增大而增强。另外,电子特性计算表明,Si原子对富勒烯C₆₀的替代掺杂可以明显地改变体系的电子特性。随着体系的增大,Si-C异质富勒烯为结构单元的聚合产物的能隙逐渐变窄。以异质富勒烯为结构单元的一维Si-C纳米管表现出非常规则的“哑铃”状构型,且为禁带宽度非常窄的直接带隙半导体,这种独特的电子特性使其在光电子器件方面具有潜在的应用价值。第四章研究用H原子对Si-C异质富勒烯聚合形成的纳米管的侧壁功能化。纳米管的功能化修饰是指人们按照特定的需要,通过引入外来分子或者基团对纳米管进行管外或者管内壁吸附修饰来改变体系固有的物理化学性质,从而为其应用开辟更为广阔的空间。在上一章中,我们通过对Si原子替代掺杂富勒烯C₆₀及其掺杂产物聚合反应的研究,预言了具有窄禁带直接带隙半导体特性的一维Si-C纳米管合成的可能性。考虑到其特殊的能带结构和电子特性,本章中我们采用密度泛函方法研究H原子对以异质富勒烯为结构单元的Si-C纳米管进行侧壁修饰功能化。我们的研究结果表明,通过H原子的侧壁修饰功能化,我们发现,修饰位置的Si（C）原子向外突出,并且,修饰位置均发生了较为明显的局部结构形变。在H原子向纳米管侧壁靠近的过程中,没有出现反应势垒,因而,可以很容易地与Si-C纳米管的侧壁发生反应。同时,电子特性计算表明,以异质富勒烯为结构单元的Si-C聚合纳米管的电子特性发生了明显的变化。对于不同位置的修饰,跨越费米能级均出现一条半填充的杂质能带,因而,室温下表现金属性。H原子侧壁修饰对Si-C聚合纳米管的电子特性的可调控性对其在电子器件上的应用具有重要意义。第五章研究类富勒烯笼状团簇（SiC）₁₂的聚合反应对非碳元素的纳米管和类富勒烯笼状团簇的研究,近年来引起了科学家们广泛的关注。最近,R.X.Wang等人采用第一原理计算方法系统研究了碳化硅异质富勒烯（SiC）_n（n=6-36）的结构和稳定性。他们的研究结果表明,在异质富勒烯（SiC）_n（n=6-36）中,n=12时,体系最稳定,即（SiC）₁₂是最稳定构型。在人们早期关于类富勒烯笼（BN）_n和（AlN）_n的研究中,（BN）₁₂与（AlN）₁₂也同样被证明为最稳定构型。n=12时异质类富勒烯笼所具有的特殊稳定性,使得我们对n=12的异质类富勒烯笼状团簇的进一步研究产生了很大的兴趣。在本章中,我们采用第一原理计算方法研究类富勒烯笼（SiC）₁₂,其聚合衍生二聚体（SiC）₁₂-（SiC）₁₂及以类富勒烯笼状团簇（SiC）₁₂为结构单元的一维SiC纳米线的几何结构、稳定性和电子结构。我们的计算结果表明,类富勒烯笼（SiC）₁₂可以形成稳定的以（SiC）₁₂为结构单元的碳化硅纳米结构,包括聚合衍生二聚体和以类富勒烯笼（SiC）₁₂为结构单元的一维SiC纳米线,且稳定性与类富勒烯笼（SiC）₁₂相比增强。优化后的以类富勒烯笼（SiC）₁₂为结构单元的一维SiC纳米线呈规则的“哑铃”状结构。电子结构计算表明,类富勒烯笼（SiC）₁₂的聚合产物具有宽的能隙,其电子特性与类富勒烯笼（SiC）₁₂及传统的碳化硅纳米材料相比没有明显的改变。这些研究结果对于实验上合成新型碳化硅纳米材料具有重要的指导意义。第六章给出类富勒烯笼（AlN）₁₂的理论预测一般来说,Ⅲ-Ⅴ族化合物,尤其是Ⅲ族氮化物所表现出的宽带隙电子、光电子特性,对纳米器件在科技上的实际应用具有非常重要的意义。目前,氮化铝纳米材料以其优良的热传导性、高硬度、可靠的电绝缘性、低的介电常数和介电损耗、无毒以及与硅相匹配的热膨胀系数等一系列的优良特性,受到了国内外研究者的广泛关注。在众多关于氮化铝材料的研究中,2003年H.S.Wu等人通过第一原理对氮化铝类富勒烯团簇（AiN）_n的研究引起了我们的关注。他们的研究结果表明,对于异质类富勒烯团簇（AiN）_n（n=2-41）,n=12时,团簇的稳定性最高,即（AiN）₁₂为最稳定构型。受上一章关于（SiC）₁₂研究的启发,本章中,我们将采用第一原理计算方法研究类富勒烯笼（AlN）₁₂,其聚合二聚体（AlN）₁₂-（AlN）₁₂及一维以类富勒烯笼（AlN）₁₂为结构单元的氮化铝纳米线的结构、稳定性和电子结构。计算结果表明,聚合后的类富勒烯笼（AlN）₁₂二聚体和一维氮化铝纳米线具有规则的“哑铃”状结构,且与（AlN）₁₂相比,稳定性增强。电子特性计算表明,类富勒烯笼（AlN）₁₂的聚合衍生物与传统的氮化铝材料电子特性相似,具有宽的带隙。作为新型的氮化铝纳米结构,必将具有潜在的应用价值。更多还原

【Abstract】 In recent years,nanomaterials,as new type material structures,have attracted considerable attention because of their unique properties and potential applications. The fullerenes,since the discovery of C₆₀in 1985 and successful synthesis of it in 1990,have attracted a great deal of interest in the community of physics,chemistry and material engineering due to their remarkable structural,electronic,optical and magnetic properties.One of the most important attempts is to change their electronic and mechanical properties by doping and synthesizing novel fullerene-based materials with special physical and chemical properties.The research on the structures and properties of the doped fullerenes and fullerene-based nanomaterials is essential for better understanding of forming mechanism of the materials and exploring their potential applications.Computational simulation,which is the third extremely powerful tool to study physical world following experiments and theories,plays a very important role in science and technology.It provides a bridge between theory and experiment and helps us not only to understand and interpret the experiments at the microscopic level,but also to study regions which are not accessible experimentally,or too expensive to approach experimentally,e.g.extremely high pressure and high temperature conditions to be required.There are two kinds of computational methods to study nanoscale materials.One is empirical or semi-empirical method;the other is ab initio or first-principle calculations,such as density functional theory（DFT）calculation. Empirical method can predict some properties of materials through analytic potential functions,while ab initio calculation starts with first-principle quantum mechanics and solves Schrodinger or Kohn-Sham equations self-consistently.In the present work,by using DFT calculations and molecular dynamics（MD） simulations,we have studied the doping of fullerene C₆₀with Si atoms,the polymerization of the Si-C hetero fullerene to form nanotubes,the functionalization of polymerized Si-C hetero fullerene-based nanotubes（Si-CHFBNT）,and the polymerization of the noncarbon fullerene-like cage clusters.This dissertation includes two parts.The first part introduces the theoretical fundamentals that we used in our research work.The second part introduces the main work done during my Ph.D. degree studies.The following gives a brief outline of the main contents of this dissertation.Ⅰ.The Theoretical Fundamentals Used in Our Research WorkChapterⅡgives a brief introduction of fundamental aspects for the ab initio calculations and MD simulations used in this work.In this chapter,the theoretical fundamentals of the first principle calculations,the molecular dynamics（MD）simulations and ab initio MD simulations are described briefly.At present,DFT calculation is one of the most important investigation methods that can describe the structural,mechanical,electronic,magnetic,and optic properties of small systems,without need of any empirical parameters.However,due to its large computational demands,DFT calculation can only deal with the small systems which contain no more than several hundreds of atoms.Classical MD simulation is an effective tool for material design and the studies of the material properties.But only the structures,energies of the materials and some related properties,such as mechanical properties,can be obtained.We have to use the first principle calculations to study the electronic properties and optoelectronic properties of the materials.So in the practical calculations,we are trying to combine the two methods together to obtain as much information as possible concerning the system under study.Ⅱ.The Main Work Done during My Ph.D.Degree StudiesChapterⅢSubstitutionally doping C₆₀with Si atoms and polymerization of the Si-C hetero fullerene derivativesIn recent years,doped fullerenes with various elements have attracted considerable research efforts due to the interest in synthesizing fullerene-based nanomaterials with special physical and chemical properties.In addition to endohedrally and exohedrally adsorption of foreign species on the wall of fullerenes, the substitutionally doped fullerenes,which have the C atoms substituted by the foreign atoms,are also intriguing for their novel geometric and electronic structures. Silicon should be an ideal candidate to substitute the C atoms on the fullerene cages due to the similarity of their valent electron configurations.In this chapter,we use DFT calculations to investigate the energetics,structural and electronic properties of the Si-C hetero fullerene-based materials obtained by doping C₆₀with different numbers of Si atoms.The results indicate that,among the different Si-C hetero fullerene isomers obtained,the one with the C atoms and the Si atoms located in separated region,i.e.,with a phase-separated structure is more stable.The energy gaps of these Si-C hetero fullerene-based materials demonstrate that the HOMO-LUMO gaps are greatly modified and show a decreasing trend with increasing the size of the clusters.The structures of the fully optimized Si-C hetero fullerene-based nanotubes （SiCFBNT）are especially regular and exhibits interesting dumbbell-shaped chain structures.The Si-C hetero fullerene-based nanotubes have narrow and direct energy band gaps,implying that it is a narrow gap semiconductor and may be a promising candidate for optoelectronic devices.ChapterⅣFunctionalization of the polymerized one-dimensional Si-C hetero fullerene-based nanotube with H adsorption on the sidewallNanotubes are always functionalized via different approaches to tune their electronic structures and thus the relevant electronic and optoelectronic properties by using the methods,such as adsorption of foreign atoms or molecules on the interior or exterior-wall of the nanotube.It provides a great potential of tailoring the properties of one-dimensional nanomaterials to meet the needs of application in nanotechnology. In view of the very narrow band gap of the one-dimensional Si-C hetero fullerene-based nanotube（Si-CHFBNT）predicted in the previous chapter,in this chapter,we investigate hydrogen atom being adsorbed on the Si-C hetero fullerene-based one-dimensional nanotube using density functional theory（DFT）.The results indicate that,the electronic structures of the Si-CHFBNT can be drastically changed by the H atom adsorbed on the exterior-wall.The functionalized Si-CHFBNT generated conducting properties at room temperature independent of the adsorption sites of H atom on the Si-CHFBNT.This result may be utilized for band structure engineering.ChapterⅤPolymerization of the fullerene-like cage（SiC）₁₂to novel silicon carbide nanowiresIn recent years,the exploration of possible fullerene-like structures or nanotubes composed of noncarbon elements has attracted much attention.Very recently, theoretical investigations addressing the stability of SiC nanostructures based on ab initio calculations using density functional theory（DFT）were reported.The structures and stability of fullerene-like cages（SiC）_n（n=6-36）were studied and it was suggested that the fullerene-like cage（SiC）₁₂was energetically the most stable cluster among those cage structures and would be possibly synthesized under certain condition.In the previous theoretical studies on the fullerene-like cages（BN）_n and （AlN）_n,the fullerene-like cages（BN）₁₂and（AlN）₁₂were also predicted to be the most stable ones.Therefore,the fullerene-like cage structure（XY）_n may be a magic cluster when n is equal to 12.Considering the promising applications of the silicon carbide nanomaterials in optoelectronic devices,in this chapter,we performed ab initio studies on the stabilities and structural,as well as,electronic properties of the fullerene-like cage（SiC）₁₂,（SiC）₁₂-（SiC）₁₂dimers and（SiC）₁₂-based SiC nanowires obtained from the（SiC）₁₂clusters.The results indicate that the stable fullerene-like cage（SiC）₁₂can form a new family of SiC nanomaterials,including（SiC）₁₂-based dimers and nanowires,which are energetically more stable than the（SiC）₁₂cluster. The fully optimized structures of the two（SiC）₁₂-based nanowires are regular and exhibit interesting dumbbell-shaped chain structures.The calculated HOMO-LUMO gaps of all these configurations of（SiC）₁₂-based materials are not recognized to have a remarkable difference from that of the（SiC）₁₂cluster.They have the similar semiconducting electronic properties as SiC nanotubes.We hope this finding could motivate further studies on the（SiC）₁₂-based materials,for instance,on the synthesis methods,the applications and functionalizations of this novel material family.ChapterⅥTheoretical prediction of the（AlN）₁₂fullerene-like cage based nanomaterialsGenerally,Ⅲ-Ⅴcompounds,especially the groupⅢnitrides,are found to be important source of nanoscale materials for their direct band gaps affording optical and electrooptical properties that are of considerable importance to technology applications.Among these,aluminum nitride semiconducting nanostructures are especially promising materials and have attracted considerable attention due to their large band gap,low electron affinity,and excellent physical and chemical properties, e.g.,high thermal-conductivity,low thermal-expansion coefficient,and chemical inertness.Recently,the theoretical investigations addressing the stability of aluminum nitride nanostructures based on ab initio calculations were reported and it was suggested that the fullerene-like cage（AlN）₁₂is energetically the most stable cluster in the family of（AlN）_n（n=2-41）.Based on the study in the former chapter,in this chapter,we performed ab initio calculations on the stability and structural and electronic properties of the fullerene-like cage（AlN）₁₂and the polymerized dimers and nanowires obtained from it.The results indicat that the（AlN）₁₂-（AlN）₁₂dimers and aluminum nitride（AlN）₁₂-based nanowires polymerized from the（AlN）₁₂cage are more stable than the（AlN）₁₂.The optimized configurations of the nanowires are especially regular and exhibit an interesting dumbbell-shaped chain structure.We also calculated the electronic structures of all the constructed nanostructures.The two novel（AlN）₁₂-based aluminum nitride nanowires have band gaps of 2.844eV and 3.085eV,respectively,implying that they are both wide-gap semiconductors and may be promising candidates for nanotechnology as novel AlN nanomaterials.更多还原