UV-treated ZnO films for liquid crystal alignment

Abstract

The surface wettability of ZnO films prepared by a sol-gel process is altered from hydrophobicity to hydrophilicity via ultraviolet (UV) light irradiation. The results indicate that hydrophilicity of the ZnO films is enhanced along with the increase of UV irradiation energy. The measurements of X-ray photoelectron spectroscopy and photoluminescence show that the amount of oxygen vacancies in ZnO films is increased with the UV irradiation. The results of UV-vis absorption spectroscopy and X-ray diffraction also show that the crystalline quality of the ZnO films is slightly changed after UV treatment. The increase in oxygen vacancies means that water molecules can easily coordinate into the oxygen vacancy sites, leading to the increase of surface wettability. It is observed that liquid crystal (LC) molecules can be aligned on the UV-treated ZnO films, and the orientation of LCs on the UV-treated ZnO films can be tuned from homeotropic to homogeneous alignment by controlling the surface wettability of ZnO films. Our results show that the pretilt angle of LCs on ZnO films depends on their surface wettability, and it can be successfully adjusted over a wide range from 89.5 degrees to 0.5 degrees as the contact angle of water on ZnO films changes from 97 degrees to 60 degrees.