Frictional Properties of UV illuminated Zinc Oxide Thin Films Grown by Pulsed Laser Deposition
Huan-Pu Chang1*, Yu-Ting Yeh1, Fang-Yu Lo1, Hsiang-Chih Chiu1
1Department of Physics, National Taiwan Normal University, Taipei, Taiwan
* presenting author:Huan-Pu Chang, email:toy8842@gmail.com
Zinc Oxide (ZnO) have potential applications in nano-electro-mechanical systems (NEMS) because of their exceptional physical properties. In addition, ZnO is also an excellent lubricant and hence a promising material for protective coatings in NEMS. Here, we investigate the frictional properties of pulsed laser deposited ZnO thin films by using atomic force microscopy, while the surface wettability of ZnO is changed by UV illumination via the photo-catalyst effect. The surface of ZnO is changed from being more hydrophobic to hydrophilic after 2-hour UV exposure. We found that at high relative humidity (> 60%), the friction force measured on a hydrophilic ZnO decreases with a logarithmic increase of the tip sliding velocity. On the other hand, the friction force on hydrophobic ZnO shows a reverse dependence. This is explained by the nanoscale water capillary bridge forming between tip-ZnO contacts due to surface roughness. Our findings might offer a possibility to control the frictional properties of ZnO surfaces.


Keywords: wettability, zinc oxide, atomic force microscopy, friction, photo-catalyst effect