Abstract
Due to the timely confluence of basic sciences, chemistry, physics, and biology as well as development of powerful new tools, nanotechnology has been advancing at a dazzling speed in recent years. The new breakthrough is matters will change their ordinary properties into new and undiscovered properties in nanoscale or atomic scale. In such a small scale, materials will be totally different with any matters which we have ever experienced before. Thanks to the instrumental development, we can observe materials in atomic scale via high resolution electron microscope and to explain the unexpected phenomenon. Nanotechnology becomes a connecting path to break down the barriers among the traditional physics, chemistry, and biology research fields. The present research is focused on three parts: fabrication of Ag2S-Cu2S superlattice p-n heterojunction by cation exchange, dynamic observation of crystallization of CuCl2 and cation exchange, and the facet-dependent I-V behaviors on a single Cu2O nanoparticle. Fabrication of superlattice nanowires (NWs) with precisely controlled segments normally requires a sequential introduction of reagents to the growing wires at elevated temperatures and low pressure. Here we demonstrate a new approach to fabricating superlattice NWs possessing multiple p-n heterojunctions by converting the initially-formed CdS to Cu2S NWs first and then to segmented Cu2S–Ag2S NWs through the sequential cation exchange at low temperatures. In the formation of Cu2S NWs, twin boundaries generated along the NWs act as the preferred sites to initiate the nucleation and growth of Ag2S segments. Varying the immersion time of Cu2S NWs in a AgNO3 solution controls the Ag2S segment length. Adjacent Cu2S and Ag2S segments in a NW were found to display the typical electrical behavior of a p-n junction. For chemical reactions in liquid state, such as catalysis, understanding of dynamical changes is conducive to practical applications. Solvation of copper salts in aqueous solution has implications for life, the environment, and industry. In an ongoing research, the question arises that why the color of the aqueous CuCl2 solution changes with solution concentration? In this work, we have developed a convenient and efficient in situ surface enhanced Raman scattering technique to probe the presence of many intermediates, some of them are responsible for the color change, in crystallization of aqueous copper chloride solution. The versatility of the novel technique was confirmed in the identification of five intermediate states in the transition from CdS to MoS2 nanowires in solution. The facile in situ method is expected to be widely applicable in probing intermediate states in a variety of chemical reactions in solution. It is of interest to examine facet-dependent electrical properties of single Cu2O crystals, since such study greatly advances our understanding of various facet effects exhibited by semiconductors. We show a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is non-conductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets.