Use of chemically modified AFM tips as a powerful tool for the determination of surface energy of functionalised surfaces

Abstract
Atomic Force Microscopy (AFM) has been used to determine the surface energy of chemically modified surfaces at a local scale. To achieve this goal, it was necessary to graft the AFM tip with the same chemical functional groups as our analysed surface. Two different chemical functionalities were used: hydrophilic and hydrophobic. The grafting of tip is well developed in the literature [1-5], it consists in depositing a layer of titanium from 3 to 4 nm followed by a 30 nm layer of Au. The thickness of this layer deposited on the tip is of the same order of magnitude than the radius of the AFM tip. On the other hand, the deposition of Ti increases the gold surface roughness. To avoid the use of Ti and to decrease the thickness of the gold layer, we developed a new way of grafting by using organic molecules such as the (3 mercaptopropyl)-triethoxysilane (MPS) as coupling agent [6]. Then we checked this way of grafting and finally we carried out Deflection-Distance curves (DD) with grafted tips in static mode by using AFM. Calibration of the various parts of the apparatus [7] and especially of the cantilever (spring constant and radius) was primordial to reach quantitative data. Finally, by applying a suitable theory of contact [8,9], we were able to determine the surface energy of our system.