【作者】 董传鼎；

【导师】 龚新高；

【作者基本信息】 复旦大学 ， 理论物理学， 2010， 博士

【摘要】 本文采用第一性原理计算的方法研究了钴团簇、Au团簇的结构和硅纳米线的掺杂,探讨了纳米结构和电子行为的关系。具体内容包括以下部分：第一章介绍了第一性原理计算方法。包括密度泛函的理论基础、赝势方法以及VASP程序中求解Kohn-Sham方程使用的平面波方法和变分使用的两种优化方法。第二章研究了Co13-Co23的结构。我们提出一种具有块体堆积方式的层状结构类型。相比以前被广泛接受的基于正二十面体的结构,层状结构具有更高的稳定性,也由于较低的密堆程度而有更强的自发磁化。层状结构的结构转变和绝热离化能都与光电子谱实验的信息吻合。这表明Co团簇在尺寸很小的时候就已经形成了块体结构。计算表明,磁性是层状结构获得较高稳定性的原因。第三章研究了含有50多个原子的Au团簇的结构。我们发现,较大尺寸Au团簇的结构依然受到相对论效应的强烈影响,表现为sd杂化引起的表面键长的收缩。这一效应导致了Au55及更大尺寸的团簇不能保持对称性较高的结构,而变为无序结构。其中Au58的近球形无序结构是一个特别稳定的结构,成为后续结构生长的基础。这得到了光电子谱实验和第一性原理计算的证实。利用相对论效应引起的表面收缩,我们改变团簇内部和表面的原子数量,发现了Au58的更加稳定的双空心壳层结构。基于这种结构我们讨论了Au团簇结构向块体的演化。第四章研究了Li原子在Si纳米线中的掺杂,表明了尺寸效应对掺杂趋势的影响并探讨了其机理。我们发现较大尺寸的Si纳米线可以稳定的容纳Li原子,而较小尺寸的纳米线不利于掺杂。在Si纳米线中,Li引入的电子基本上落在导带底附近,小尺寸纳米线限制了其空间分布是其能量上不利的来源,其本质为能隙的变宽提高了Li的施主态的能量。掺杂的临界尺寸与Li引入的施主态的扩展半径基本一致。更多还原

【Abstract】 This thesis is devoted to the study of atomic structure of cobalt cluster and gold cluster, as well as the doping of silicon nanowire, with a emphasis on the interplay between the structure energetics and electronic behavior. The research works addressed herein include:Chapter One:The method of first principles electronic structure calculation. We give a brief introduction of the framework of density functional theory, followed by the pseudo potential method. We further interpret the plane wave scheme and the two variation optimization methods used in VASP code.Chapter Two:The structures of Co13-Co23. We propose a layer-like structure with fcc or hcp stacking for these small Co clusters. Compared with the widely recognized icosahedral-type structures, the layer-like structures are more favorable in energy with also enhanced magnetization due to the low coordina-tion. The structural transition in this size range and the adiabatic detachment energy of layer-like structures are in good coincidence with the variation of pho-toelectron spectrum. This confirms the existence of present structures, which in-dicates that the Co clusters with only a few dozens of atoms have already shown bulk-like structures. Our calculations indicate that the enhanced magnetization plays a key role in stabilization of layer-like structures.Chapter Three:The structures of Au55-Au64. The structures of clus-ters in this size range are still strongly modified by the relativistic effect, which manifests itself as the surface contraction. Driven by the surface contraction, the Au55 and larger clusters fail to maintain the closely-packed structure and trans-form into amorphous core-shell configuration. The combination of photoelectron spectrum and density functional calculation demonstrate that the amorphous Au58 is uniquely stable and can be seen as the base of Au59-Au64. Due to the relativistic effect induced surface contraction, the stability of core-shell Au clus-ter depends dramatically on the relative size of inner core and outer shell. By altering the relative core-shell size we find a highly robust double shell structure of Au58 with hollow center. Based on this novel structure type we discussed the growth pathway of gold cluster into bulk structure.Chapter Four:The size effect on Li-doping in silicon nanowire and the underlying mechanism. We demonstrate that doping Li into silicon nanowire tends to be energetically unfavorable when the size of wire is reduced. When the Li atom is inside the nanowire, basically its outer electron falls to the conduction band minimum. This donor state is confined by the small diameter of wire, resulting a high energy cost to enter the nanowire because essentially the widen energy gap of smaller nanowire uplift the donor state to a higher energy. The critical size of doping is in coincidence with the expansion scope of the donor state in the cross section.更多还原