Highly-oriented ZnO thin films prepared by using low-cost technique such as sol-gel technique are of much importance to ZnO-based white light-emitting diodes. The chose of proper preheating temperature in sol-gel technique is still critical for highly-oriented ZnO thin film so far. The mechanisms for the preheating in the formation of ZnO thin film and for the reactions involved in the sol solution have not been clearly stated yet. Thus, in this work, the highly-oriented ZnO thin films were prepared on glass substrates by using sol-gel spincoating technique. The sol solution was prepared by using the two-step method rather than usual one-step method, which facilitates the understanding of the mechanism for the reactions involved in the sol solution. The effect of the preheating temperature on the microstructure and the optical properties of the films were in particular investigated. The mechanisms for the preheating in the formation of the films and for the reactions involved in the sol solution prepared by the two-step method were also proposed in terms of the experimental results. The preheating not only enhances the volatilization of the solvent 2-methoxyethanol and the decomposition of the residual organic species, but also results into the formation of small number of ZnO particles. The preheating temperature of 300• C is most favorable for the highly-oriented ZnO thin film. Increasing the preheating temperature results into the blue shift of the absorption edges of the films. This can be explained by using the quantum-size effect. The photoluminescence spectra of the films show an UV emission at the near-band edge and a broad green-yellow emission at 470-620 nm. The former is closely related to the excitons, while the latter is to the intrinsic defect species in the film.

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