Frictional Properties of Zinc Oxide Thin Films: Interplay of oxygen vacancies, ambient humidity and surface roughness*

Being able to manipulate the frictional properties of materials at the nanoscale is important for future development of nano-electro-mechanical systems (NEMS), where moving components are inevitably present. By means of atomic force microscopy (AFM), we investigated the frictional properties of ZnO thin films with nanoscale surface roughness. Photo-catalytic effect (PCE) is used to induce oxygen vacancies in the surface layer of ZnO, which can act as binding sites for ambient water molecules, further converting the ZnO surface from being hydrophobic to superhydrophilic. Due to the capillary effect, nanoscale water bridges can form between the surface asperities in the contact zone of AFM tip and superhydrophilic ZnO surface. Therefore, the measured friction exhibits a negative dependence on the logarithm of tip sliding velocity. Such dependence is found to be a strong function of both ambient humidity and the concentration of oxygen vacancies, and can be reversibly manipulated by the PCE. These results might find applications in future ZnO based NEMS.¹

1. Langmuir 33, 8362 (2017)