Characterization of perfluorodecanoate self-assembled monolayers on aluminum and comparison of stability with phosphonate and siloxy self-assembled monolayers

The long term performance of micro-/nano-electromechanical systems (MEMS/NEMS) depends on their stability to operating conditions, contact friction, adhesion, and wear. Aluminum (Al) substrates have been chemically reacted with perfluorodecanoic acid (PFDA) to form a self-assembled monolayer (SAM) and their surface properties (chemical composition, roughness, friction, and adhesion) characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurement (CAM), and atomic force microscopy (AFM). In addition, stability of the PFDA/Al SAM films have been tested via exposure to corrosive conditions (aqueous solutions of low pH and different temperatures) for various periods of time. The sessile drop static contact angle of pure water demonstrates that PFDA/Al is extremely hydrophobic, giving values typically >125° compared to that of <10° for unmodified Al. A systematic comparison of stability between SAM films formed by reaction of Al with perfluorodecylphosphonic acid (PFDP), octadecylphosphonic acid (ODP), and perfluorodecyldimethylchlorosilane (PFMS) shows PFDA/Al to be less stable than PFDP/Al and ODP/Al, but more stable than PFMS/Al.