Flower-shaped ZnO nanostructures obtained by cyclic feeding chemical vapour deposition: structural and optical properties.

Flower-shaped ZnO nanostructures were grown on Si(100) and Si(111) substrates by cyclic feeding chemical vapour deposition without the use of a metal catalyst. The structures obtained on the Si(100) substrate exhibited triangle-shaped leaves with lengths and diameters in the ranges 300-400 and 90-130 nm, respectively. Uniformly shaped leaves with hexagonal facets originating from one centre were observed on the flower-shaped structures grown on the Si(111) substrate. Transmission electron microscopy and selected area electron diffraction patterns showed that these structures are highly crystalline, with the wurtzite hexagonal phase, and preferentially oriented in the c-axis direction. Unlike star-shaped ZnO nanostructures grown on Au-coated silicon substrates, the flower-shaped nanostructures showed sharp and strong UV emission at 378 nm and broad and weak green emission at 520 nm, indicating a good crystal quality and few structural defects.