Mechanisms of the shape evolution of CdSe nanocrystals

The temporal shape evolution of CdSe quantum confined nanorods (quantum rod s) in nonaqueous solvents with organometallic precursors was studied quanti tatively and systematically. The experimental results revealed three distin guishable stages in the shape evolution. At high monomer concentrations, na nocrystals grow exclusively along the c-axis of the wurtzite structure, mak ing this axis the long axis of the rods. At intermediate concentrations, na nocrystals grow simultaneously in three dimensions. At low monomer concentr ations, the aspect ratio decreases in a process controlled by intraparticle diffusion on the surface of the nanocrystals. This intraparticle ripening stage;is different from normal Ostwald ripening, which-occurs at lower mono mer concentrations and is by monomer migration from small to larger ones., Addition of hexylphosphonic acid or tetradecylphosphonic acid, strong cadmi um ligands, is important mainly because it enables the high monomer concent rations needed for the growth of quantum rods. A simple model is proposed t o explain the growth of faceted CdSe nanocrystals on the basis of diffusion control.