【作者】 贺伟；

【导师】 杨金龙；

【作者基本信息】 中国科学技术大学 ， 化学物理， 2008， 博士

【摘要】 近年来,随着理论计算方法的发展和计算机水平的提高,基于密度泛函理论的第一性原理计算方法在凝聚态物理、量子化学以及材料科学中得到了广泛应用。由于纳米材料具有独特的结构和性质,纳米技术迅速渗透到材料的各个领域,成为当前世界科学研究的热点。其中,纳米管吸附分子对其电子结构性质的调节效应对于纳米电子器件的研究具有重要意义。同时,表面体系在吸附分子时也会得到各种奇特的表面结构和性质,这方面的研究对新型低维材料的研制和表面催化反应的理解及应用也非常重要。本论文的方向是通过密度泛函理论研究吸附分子的纳米体系以及表面体系的物理化学性质。在第一章中,我们简要介绍了密度泛函理论的基本框架及其近年来的发展和应用。人们在密度泛函理论的发展过程中一直致力于寻找合适的交换相关能量泛函。从最初的局域密度近似（LDA）、广义梯度近似（GGA）到现在的非局域泛函、自相互作用修正,多种泛函形式的出现使得密度泛函理论可以提供越来越精确的计算结果。近年来,密度泛函理论向动力学平均场和含时理论等方面的扩展也很活跃。这些发展使得密度泛函理论的应用范围不断拓宽。在本章最后,我们介绍了一些密度泛函理论的应用实例。在第二章中,我们研究了硼氮纳米管封装有机分子对其电子结构性质的调节。硼氮纳米管具有较为均一的几何和电子结构性质,同时有很好的热稳定性和抗氧化性,人们期待它在纳米电子器件等方面能发挥重要的作用。然而硼氮纳米管带隙宽度很大,约为5.5 eV,在实际应用中人们希望能对其能带结构进行修饰,得到窄带隙的n型或p型半导体。最近的实验表明,在碳纳米管中封装有机分子,可以调节其能带结构。因此我们用密度泛函理论研究了硼氮纳米管内封装有机分子后,其电子结构性质的变化。发现当封装具有亲电子性质的有机分子时,硼氮纳米管转变为p型半导体,而当封装具有亲核性质的有机分子时,却较难形成n型半导体。由于施加横向电场引起的巨Stark效应会改变纳米管的带隙宽度,我们还研究了对封装了有机分子的硼氮纳米管施加横向电场的影响。我们发现外加电场对体系的带隙宽度、电荷转移以及光学性质具有较大影响,同时硼氮纳米管显示出了对内部有机分子较好的静电屏蔽保护能力。在第三章中,我们研究了吸附在Ag（100）表面的C₆₀分子单层。由于碱金属掺杂的C₆₀分子晶体A₃C₆₀具有超导性质和很高的转变温度,人们对吸附在Ag（100）表面的C₆₀分子单层是否可能成为二维的超导很感兴趣。我们通过和香港中文大学肖旭东教授领导的实验组合作,结合STM实验以及第一性原理计算,对该体系的几何及电子结构性质等问题做了详细探讨。我们发现,C₆₀/Ag（100）STM图像中的C₆₀分子“亮暗”对比主要源自Ag（100）衬底的几何效应。进一步地,我们发现存在两种不同的“暗”（D）C₆₀分子:一种以单体形式存在,另一种则和相邻的另一个“暗”C₆₀分子形成二聚体（dimer）。此外我们还研究了C₆₀分子在Ag（100）表面的多种吸附形式的电子结构和电荷转移等性质,并对该体系是否可能存在超导态作了讨论。在第四章中,我们研究了金红石型TiO₂（110）表面羟基化后的表面缺陷态。TiO₂由于其在光催化、CO氧化以及太阳能电池等多领域的广泛应用而备受关注。近年来,人们发现通过和吸附水分子的作用,TiO₂表面在实际情况下很容易被羟基化,然而羟基化后OH基团引入的缺陷态性质却至今未被清楚地了解。我们和侯建国院士及王兵教授领导的实验组合作,通过STM实验和密度泛函理论计算,研究了金红石型TiO₂（110）表面吸附单个OH基团以及一对相邻OH基团（OH pair）时引入的缺陷态性质。实验STM图像给出了孤立的单个OH基团和OH pair的离域性空间分布。通过采用（1x12）的大单胞模型,我们计算得到了可以和导带底区分的缺陷态能带结构,并给出了在离域程度、对称性等主要特征上和实验一致的STM拟和图像。实验和理论计算一致给出的TiO2（110）羟基化表面离域的缺陷态分布,有利于我们更加深入理解真实环境下TiO2表面的各种进程,同时也提示我们可以重新考虑广义梯度近似对过渡金属氧化物TiO₂（110）表面的适用性。更多还原

【Abstract】 With the progress in computational methods and the enhancement of computational ability,density functional theory（DFT）based first-principles calculation has been widely used in condensed matter physics,quantum chemistry,and material science. Nowadays,due to their peculiar structures and properties,nanomaterials have attracted a broad research interest.Modifying the electronic structures of nanotubes by organic molecule encapsulation is very important to nano device applications.Meanwhile, molecule adsorption on surfaces leads to novel structures and properties.Studies on these systems are important for development of new low-dimensional materials and for understanding the surface catalysis.The dissertation is devoted to the study of physical and chemical properties of nanotubes and surfaces with molecule adsorptions from first-principles calculations.In the first chapter,we introduce the basic concept of DFT and review its recent progress.People have been making great efforts to find good approximation for the exchange-correlation functional in DFT.With the development of modern functionals, we can obtain more and more accurate results.In addition,extension of DFT to the time dependent region and combination of DFT with the dynamic mean field theory （DMFT）are also active topics.All these progresses in DFT lead to a broad range for application.This is illustrated by some examples at the end of this chapter.In the second chapter,we study the boron nitride nanotubes with organic molecule encapsulation.Boron nitride nanotubes have uniform geometries and electronic structures, and show pronounced resistance to oxidation with high thermal stability.Therefore, boron nitride nanotubes may play an important role in nano-device application. However,the band gap of boron nitride nanotubes is very large（about 5.5eV）.It is desirable to modify the band structure to obtain a metallic or a p-type/n-type semiconducting behavior.Recent experiments show that,the band structures of carbon nanotubes can be modified through organic molecule encapsulation.Here,we perform first-principles calculations to study the electronic structure properties of boron nitride nanotubes with organic molecules encapsulated.We fred that,we can obtain a p-type semiconductor by doping electrophilic molecules,while for typical nucleophilic organic molecule doped BNNTs,their gap is too large to be considered as a good n-type semiconductor.Since applying transverse electric field may change the band gap of nanotubes according to the giant stark effect,we also study the effect of a transverse electric field on these doped systems.It is shown that electric fields greatly modify the band structure,the charge transfer,and the optical properties.In addition,boron nitride nanotubes provide a relatively good electrostatic shielding for the inside organic molecular chain.In the third chapter,we study the C₆₀monolayer adsorbed on Ag（100）surface. The bulk fulleride A₃C₆₀（A=K,Cs）are confirmed to be superconductors with high transition temperatures.Thus it is very interesting to check whether there is possible two-dimensional superconducting state in the C₆₀monolayer on Ag（100）.Cooperating with the experiment group leaded by Prof.X.D.Xiao at Chinese University of Hong Kong,we study the geometries and electronic structure properties of this system via combination of STM/STS experiment and DFT calculations.Our results reveal that the bright-dim contrast has a definite geometric origin and there are two types of dim C₆₀molecules,one is a monomer and the other is a dimer.We make comprehensive investigation on the various electronic properties and charge transfers in the differently adsorbed C₆₀molecules,and discuss the possibility of superconducting states in this system.In the fourth chapter,we study the defect states at the hydroxylated TiO₂（110） surface.TiO₂ has been paid a great attention due to its broad applications on photocatalysis, heterogeneous catalysis,and solar energy conversion.Recently it is revealed that,the reduced TiO₂ surface can be easily hydroxylated by reacting with the adsorbed water molecule.However,the electronic property of the OH defects is only very poorly understood.Cooperating with the experiment group leaded by Prof.J.G.Hou and Prof.B.Wang,we study the defect stats introduced by single OH group or OH pair groups on TiO₂（110）via STM and DFT calculations.The electronic state of OH group defects is found to be delocalized on the surface through STM experiment.Our DFT calculation with a vary large superceU c（1x12）give reasonable band structures with de- fect state separated from the conduction band bottom,and successfully reproduce our STM experiment observations.The remarkable agreement between theoretical and experimental results in this work makes it necessary to reconsider the previously doubted applicability of the generalized gradient approximation（GGA）for describing the TiO₂ （110）system.更多还原