ZnO twin-spheres exposed in ±(001) facets: stepwise self-assembly growth and anisotropic blue emission.

ZnO twin-spheres topologically exposed in ±(001) polar facets have been successfully produced on a large scale. The fragmentary and hexagonal ±(001) facets of ZnO tilt and assemble gradually for 8-12 generations to form supercrystals. The surfactant effect on the formation of ZnO supercrystals reveals that their structure stepwise evolves from prisms to dumbbells to twin-spheres exposed in ±(001) facets and eventually to twin-spheres covered with dots. A hollow ring around a prism, which connects two hemispheres of the supercrystals, is finally sealed inside each of the twin-spheres. Based on the experimental observations, a stepwise self-assembly mechanism is proposed to understand the formation of the supercrystals. It is also observed that the ZnO twin-spheres exhibit anisotropic blue emission in intensity attributed to their special surfaces exposed in ±(001) facets. Novel devices could be designed and fabricated through carefully tailoring the microstructure of ZnO supercrystals.