Self-aggregation and assembly of size-tunable transition metal doped ZnO nanocrystals

Size tunable transition metal (Mn, Ni) doped ZnO nanocrystals have been synthesized through a soft chemical route. These nanocrystals self-aggregated themselves in a highly mesoporous spherical superstructure of broad size distribution. The formation of nanocrystals and porous spherical superstructures can be understood as a result of a combination of factors such as the presence of negatively charged carboxylate ions, van der Waals force, the polar nature of ZnO crystal surfaces, and oriented attachment. Monodispersed porous spheres were obtained by size selective separation and then assembled into a face-centered cubic (FCC) close packed superstructure with its (111) plane oriented parallel to the sample surface. The assembled structures exhibit the presence of the photonic stop band in the (111) direction, and the nanocrystals display ferromagnetic like behavior presumably due to defects.