Study on single-bond interaction between amino-terminated organosilane self-assembled monolayers by atomic force microscopy

Chemically modified atomic force microscope (AFM) tips and modified silicon wafers with self-assembled monolayers (SAMs) were produced using 3-aminopropyltriethoxysilane (APTES) molecules. Prior to modification, the silicon wafers were thoroughly cleaned by improved RCA method, producing hydrophilic and smooth surfaces. The bare and modified silicon substrates were characterized by contact angle measurement, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). In this paper, a statistical analysis method, which makes use of the properties of the Poisson distribution, was applied to investigate the interaction between the tips and silicon surfaces covered with SAM terminating in amino groups. The single-bond force between a single amino group pair in water was obtained from the total adhesion forces measured by AFM and was found to be 200.0±43.6 pN. This value was assigned to the result of mainly hydrogen bond interaction between a single NH2–NH2 pair. Comparisons between our results and those measured by means of Johnson–Kendall–Roberts (JKR) theory and other methods were also made.