Fabrication And Photophysical Properties Of Rare Earth And Silicon-based Nanostructures

Nanostructures have received steadily growing interests as a result of their peculiar properties and applications in modern science and technology. The ability to generate such structures and their fascinating physical properties are essential to nanoscale science. Silicon plays a central role in the semiconductor industry due to its mature fabrication technology. Rare earth compounds have been extensively applied to high-performance magnets, luminescence devices, catalysts, and other functional materials. In this thesis, four material systems, i. e. Eu complex/SiO2 colloidal spheres, Y2O3:Eu nanotubes, Si/SiO2 core-shell nanowires, and highly concentrated Si1-xCx alloy, were fabricated by a variety of chemical methods such as Stober method, self-assembly, and thermal evaporation etc. Their structures and photophysical properties were investigated, and the main results are as follows.Monodisperse colloidal silica spheres embedded with europium complex were fabricated by a modified Stober method. The hybrid spheres display strong characteristic emission of Eu3+ ions. Two emission species are identified in both the pure complex and the hybrid sphere. Low-temperature laser selective spectroscopy confirms that the two species in the hybrid spheres remain the same molecular conformations to those in the pure complex. Furthermore, when the hybrid spheres are dispersed in the polymers with different refractive index, the spontaneous emission rate of the Eu3+ ions is modified due to the dielectric confinement effect. Y2O3:Eu nanotubes were synthesized by a surfactant assembly mechanism. TEM image confirms the formation of the tubular structures. Under the excitation of ultraviolet light, the nanotubes show luminescence properties different from that of Y2O3:Eu nanocrystallites. The results of laser selective excitation indicate that the emission centers near the surface exhibit inhomogenously broadened spectra while the twodifferent symmetry sites inside the nanotube-walls present legible spectral structures. Silicon nanowires were prepared by a thermal evaporation method. SEM, TEM, EDX, and XPS measurements were used to characterize the composition, morphology and structures of the nanowires. Each nanowire consists of a single crystalline core covered by an amorphous silicon oxide sheath. TEM and electron diffraction patterns show that there exist two major forms of nanowires possessing different morphologies and growth directions, which may indicate that different mechanisms predominate in the growth process. The growth mechanisms and luminescence properties are discussed. An ordered Si-C alloy was observed in the products resulting from thermal reduction of molybdenum disilicide heating rods. HRTEM studies indicate that the Si1-xCx alloy has a high carbon concentration and possesses an ordered superlattice structure. The superlattice periodicity occurs along the diamond [001] direction and corresponds to the quintupling of the primary (002) periodicity. The Mo element is likely to play a crucial role in the growth process of the Si1-xCx alloy, since it can both reduce the energy required for breaking up C clusters and contribute to surface modification, which are of great benefit to enhance the carbon concentration and induce an ordered structure. Furthermore, the superstructure identified in the material indicates that a new ordered phase has been observed in pure Si-C system: Physical properties depend strongly on the composition and structure, which means that novel optical or electrical functions may be explored in the Si1-xCx alloy.