The mechanism behind the visible luminescence of ZnO is still a question of debate. To find out the mechanism

the emission properties of nanocrystalline ZnO colloids with different particle sizes were studied in the present paper. The preparation procedure of nanocrystalline ZnO colloids consists of two major steps:(1) preparation of precursor and (2) hydrolysis of the precursor to form the colloid. Zinc acetate and methanol were used to prepare the precursor without further purification

and NaOH was used to hydrolyze the precursor

respectively. The sizes of ZnO particles increase with the reaction time. The emission spectra were taken at different times during the growth of ZnO particles in methanol at room temperature. In this way one can study nanocrystalline ZnO particles with different sizes. Two emission bands were observed

one being an exciton emission and the other being visible emission. The energetic positions of the maxima of both emission bands depend on the size of the ZnO particles. We found that the two emission bands shift to lower energies positions upon particle growth. This is a quantum size effect and can be understood in terms of confinement of charge carriers. Contrary to the nanocrystalline ZnO powder

the emission intensities of the nanocrystalline ZnO colloids increases as the sizes of ZnO particles are decreased. Studies on the variation of the energetic positions of the maxima of both the UV and visible emission band of ZnO as a function of particle size provide novel information on the nature of the visible emission transition

which is valid for ZnO in general. The energetic positions of both band edges can be calculated as a function of particle size. A linear relationship between the maximum energetic positions of the two emission bands was given both from our experiments and from our calculation in this paper. Accordingly we attribute the visible emission to the transition of an electron from the conduction band to a deep trap.