Shape transition in ZnO nanostructures and its effect on blue-green photoluminescence

We report that ZnO nanostructures synthesized by a chemical route undergo a shape transition at ~20 nm from spherical to hexagonal morphology thereby changing the spectral components of the blue-green emission. Spherically shaped nanocrystals (size range 11-18 nm) show emission in the range of 555-564 nm and the emission shifts to the longer wavelength as the size increases. On the other hand, rods and hexagonal platelets (size range 20-85 nm), which are the equilibrium morphology after the shape transition, show an emission near 465-500 nm which shifts to shorter wavelength as the size increases. The shape transition also leads to relaxation of microstrain in the system. Our analysis shows that the visible emission originates from a defect layer on the nanostructure surface which is affected by the shape transition. The change in the spectral component of the blue-green emission on change of shape has been explained as arising from band bending due to a depletion layer in smaller spherical particles which is absent in the larger particles with flat faces.