Semiconducting and piezoelectric oxide nanostructures induced by polar surfaces

Zinc oxide, an important semiconducting and piezosecond material, has three key characteristics. First, it is a semiconductor, with a direct bandgap of 3.37 eV and a large exicitation binding energy (60 meV), and exhibits near-UV emission and transport conductivity. Secondly, due to its non-centrosymmetric symmetry, it is piezoelectric, which is a key phenomenon in building electro-mechanical coupled sensors and transductors. Finally, ZnO is bio-safe and bio-compatible, and can be used for biomedical applications without coating. With these unique advantages ZnO is one of the most important nanomaterials for integration with microsystems and biotechnology. Structurally, due to the three types of fastest growth directions - <0001>, <0 (11) over bar0>, and <2 (11) over bar0> - as well as the +/-(0001) polar surfaces, a diverse group of ZnO nanostructures have been grown in our laboratory. These included nanocombs, nanosprings, nanorings, nanobows, and nanopropellors. This article reviews our recent progress in the synthesis and characterisation of polar-surface-induced ZnO nanostructures, their growth mechanisms, and possible applications as sensors, transducters, and resonators. It is suggested that ZnO could be the next most important nanomaterial after carbon nanotubes.