First Principles Study On Cobalt Cluster, Gold Cluster And Silicon Nanowire

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.