Low dimensional systems such as nanotubes and nanowires have fascinating, and technologically useful, optical and electrical properties. Studies on these systems advance our knowledge on the science at the nanoscale, while simultaneously provides the possibility for developing miniaturized electronics and optoelectronics. The material system attracting increasing attention is zinc oxide (ZnO),1-6which is a II-VIcompound semiconductor with a wide and direct banc gap of 3.37 eV at room temperature. ZnO has demonstrated unique properties and potential applications in manifold fields, such as transparent electronics, ultraviolet (UV) light emitter, surface acoustic wave devices, gas sensors and magnetoelectronics. It is shown to have wurtzite structure with lattice constant a = 3.249 Α, c = 5.207 Α. Its large exciton binding energy (60 meV), which is much greater than the thermal energy at room temperature, makes it a promising candidate for applications in blue-UV light emission and room temperature UV lasing7. Furthermore, its high piezoelectric constant (d33=246) makes it a highly valuable material for fabricating mechanical devices, such as acoustic transducers, sensors and actuators.