Self-assembled monolayers of perfluoroalkylsilane on plasma-hydroxylated silicon substrates

Abstract

In this study, a novel kind of fluoroalkylsilane monomers with different fluoroalkyl chain lengths was synthesized via three steps method and characterized by Fourier transform infrared (FT-IR) spectroscopy, 1H and 19F nuclear magnetic resonance (1H NMR and 19F NMR), and mass spectra (MS). Fluoroalkyl-terminated self-assembled monolayers (SAMs) on silanol-terminated silicon substrates (O2 plasma treatment) were chemically fabricated via –Si–O– covalent bonds using the liquid phase deposition method (LPD). The wetabilities of the SAMs were characterized by water contact angles (CA), surface free energies and adhesive force (AF) measurements. 3-(1H,1H,2H,2H-perfluorooctyloxycarbonyl)-propionamidepropyl-triethoxysilane (PFOPT) assembled monolayer was chosen for in-depth investigation as its CA was higher than the others. Attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to validate the attachment of PFOPT on the silicon substrate, together with the chemical composition and structure of the SAMs. The surface morphologies and roughness of the monolayers were obtained and calculated through atomic force microscopy (AFM). The migration of fluoroalkyl groups to the outermost surface of the SAMs was confirmed by AFM, XPS, CA, and AF measurements. It was found that the silicon substrate assembled with fluoroalkylsilane after heat treatment has higher contact angle, lower surface energy and adhesive force compared to that without heat treatment, and the outermost surface was rich in fluorine due to the surface microphase separation.