A self-assembly mechanism for sol-gel derived ZnO thin films

ZnO thin films with c-axis orientation have been prepared by a sol-gel method on glass substrates. An ethanol solution mixed with zinc acetate dihydrate and monoethanolamine was used as the precursor for spin-coating. Preferentially oriented ZnO thin films were prepared by preheating the as-deposited films at 300 degrees C and sintering from 350 to 600 degrees C. Thermogravimetry-differential thermal analysis (TG-DTA), x-ray diffraction (XRD) and scanning tunneling microscopy (STM) were employed to investigate the growth and morphology of the films. The results reveal that the thin films consist of stacked, nearly spherical, nanocrystalline particles, and that 500 degrees C is optimal temperature for the crystallization of ZnO thin films. It is argued that the extremely preferential c-axis orientation is a result of self-assembly induced by dipole-dipole interaction between polar nanograins. Moreover, the orientation and continuity of ZnO thin film are affected by the number and kind of the hydroxyl groups of the complexing agent.