Theoretical Studies On Structures And Properties Of Atomic-sized Nanowires

With the increasing miniaturization of electronic and mechanical devices, nanowires have attracted extensive interest since they represent a family of ideal one-dimensional systems. The quantum confinement effect and the surface effect result in fascinating phenomena in their physical and chemical properties, especially in atomic-sized nanowires with diameter below 3 nm.High-resolution transmission electron microscopy experiments have revealed the existence of some novel structures of nanowires. Meanwhile, the conductance of nanowire is measured in mechanically controllable break junction experiments, in which electronic and atomic shell filling effects have been observed in the appearance of stable nanowires for alkali metals and noble metals. Both experimental and theoretical investigations have suggested that the atomic arrangements of nanowires are essential to understand their physical behaviors. However, an unambiguous identification of the atomic arrangements within present experimental accuracy is not possible. Furthermore, The interplay between physical properties, shell effects, and the formation mechanism of metal nanowires also needs to be fully understood.In this thesis, we have systematically studied the structures, stability and electronic properties of sodium, silver and zinc oxide nanowires by using empirical genetic algorithm (GA) combined with density functional theory. Our obtained results are summarized as follows:The structural and transport properties of sodium nanowires were systematically studied using unbiased genetic algorithm optimizations combined with density functional theory. Two competing structural series, i.e., crystalline and helical, are obtained for sodium nanowires. Helical structures are energetically more favorable for thinner wires and crystalline structures become dominant as the nanowire diameter become larger. With increasing nanowire radius, two alternative formation mechanisms of nanowires, i.e., layer-by-layer and facet-based are observed. The quantum conductance of these nanowires is computed from their one-dimensional band structures. Analyses show a crossover from electronic shell to atomic shell. Our theoretical simulations provide a unified picture of shell structures for sodium nanowires and explain the related physical properties, which were originally revealed by experiments.The structural formation process and physical properties of silver nanowires were investigated via an unbiased genetic algorithm search using empirical potential combined with density-functional theory calculations. Some unexpected structural behaviors resulting from the intrinsic properties of silver were revealed. Two kinds of atomic arrangements, i.e., normal and abnormal configurations, appear alternately during the growth of wire, from which a (111) facet-based formation mechanism is observed. The excellent agreements between theoretical results and experimental observations on the structural motif, Young's modulus, and shell effects of Ag nanowires indicate the importance of objective and precise atomistic descriptions in the study of nanosystems.The modification of the electronic structure of [0001] ZnO nanowires for different sizes and different surface coverage (free surfaces, half-hydrogenated surfaces, fully hydrogenated surfaces, fully fluorinated surfaces) has been systematically studied using gradient-corrected density-functional theory. We first pointed out metal-semiconductor transition of ZnO nanowires resulted from hydrogenated surfaces. Our resuls indicate that when the dangling bonds of the side surfaces are completely free or saturated by H/F atoms, the nanowires are insulating, with band gaps much higher than that of bulk solid. In contrast, the ZnO nanowires with only O atoms saturated by atomic H on the side surfaces are metallic. The surface chemistry is shown to have strong effects, comparable to that of quantum confinement, on the electronic structure of ZnO nanowires.