The two polytypes of ZnSe nanocrystals (NCs), the wurtzite (WZ) and zincblende (ZB) phases, were synthesized in hot organic solutions using zinc acetate as the zinc monomer with oleic acid and oleylamine as ligands. The growth rate, number and size of NCs during the growth stage were evaluated based on the X-ray diffraction profiles, high-resolution transmission electron microscope observations and optical absorption spectra. Experimental results suggested that in these systems, the phase that appeared was not determined during the growth stage but during the nucleation stage. High supersaturation during the nucleation stage resulted in the thermodynamically stable ZB phase, while low supersaturation resulted in the metastable WZ phase. The reason why supersaturation during nucleation determined the phase was discussed on the basis of the size of the critical nucleus influenced by the total free energy change in the system during nucleation.
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