Synthesis And Photoluminescence, Field Emission Properties Of One-dimensional ZnO Nanowires

In this thesis, we briefly investigate the synthesis and crystal structure, photoluminescence and field emission properties of one-dimensional ZnO nanowires.Zinc oxide (ZnO), as wide band gap semiconductor material, has good thermal and chemical stability, and its excellent luminescence, transparent conductive properties have attracted much attention. Recently, there has been increasing interest in one-dimensional nanomaterials due to their unique properties.The latest researches on ZnO nanowires are briefly introduced in our paper. ZnO nanostructures have been grown on p-type Si (100) and glass substrates by thermal evaporating pure zinc powder without any metal catalysts at low temperature. The microstructures of the samples under SEM show quasi-aligned needle-like nanowires, well-aligned nanoscrews, hexagonal nanowires, nanobelts and feathers-like morphologies. XRD reveals all kinds of grown nanostructural samples are hexagonal ZnO crystal. Well-aligned nanoscrews are single crystals grown along the c-axis, with 18 sides on their heads but six concaves on their stems, while the whole is six-fold symmetry. This kind of ZnO nanoscrews had not been reported before. Temperature and substrate are critical experimental parameters for the formation of different morphologies of ZnO nanostructures. The growth mechanism of synthesized nanowires is discussed.Photoluminescence and field emission properties are investigated on needle-like nanowires and well-aligned nanoscrews. Room-temperature photoluminescence spectrum of needle-like nanowires has two emission peaks, centering at 380 nm (UV) and 500 nm (green). The UV peak originates from the free exciton recombination and the green emission is due to the transition between the antisite defect (O_Zn) and the conductive band. PL spectrum of the well-aligned nanoscrews shows a blue emission band at 420 nm and 444 nm. The blue luminescence is due to the electrons transition from the shallow donor levels of oxygen vacancies to the valence band. The electron field emission properties show that the needle-like nanowires has a low turn-on field at 6.8 V/μm, an emission current density of 1mA/cm² is achieved at 8.2 V/μm owing to better high aspect ratio. For the nanoscrews sample, the field to obtain a current density of 1 mA/cm² is higher (at 11.2 V/μm) than that of the nanowires sample due to its bigger diameter, but its turn-on field is lower (at 3.6 V/μm) owing to its better alignment. Both the ZnO nanowires and nanoscrews have good field emission stability. Results indicate that well-aligned ZnO nanowires arrays with sharp tips are likely to be good field emission cathode.