Local homoepitaxy and optical properties of well-ordered ZnO nanowires

Well-aligned diameter-modulated ZnO nanowires were grown on a Si (1 0 0) substrate via a free-catalyst chemical vapor deposition method. X-ray diffraction pattern indicates that they have a hexagonal wurtzite structure and a preferred (0 0 0 2) orientation. Each individual nanowire consists of so-called “stem” and “top”. The average diameter of “stem” is about 20 nm and its length is in a range of several micrometers while the average diameter of “top” is about 50 nm, its length is in a range of up to 100 nm or more. Selective area electron diffraction and HRTEM results confirm that the “stems” and “tops” are single crystalline and along the [0 0 0 1] growth direction. We speculate that the axial growth of the nanowires is attributed to the vapor-solid mechanism while the formation of the “top” is due to local homoepitaxy on the corresponding surface, which is likely due to the presence of Mg in the reactant. On the other hand, optical properties of the nanowires were also investigated. A strong room-temperature photoluminescence at 377 nm (3.29 eV) was observed.