Scanning force microscopic study of surface structure and properties of (alkylsilane/ fluoroalkylsilane) mixed monolayers

(Alkylsilane/fluoroalkylsilane) mixed monolayers were immobilized covalently on a silicon wafer surface with stable surface structure. Atomic force microscopic observation of the n-octadecyltrichlorosilane (OTS)/[2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) mixed monolayer revealed that the crystalline OTS circular domains of ca. 1–2μm in diameter were surrounded by a sealike amorphous FOETS matrix, even though the molar fraction of OTS was above 75%. Also, the phaseseparated monolayer can be prepared from FOETS, and a non-polymerizable and crystallizable amphiphile such as lignoceric acid (LA). The phase separation of the (alkylsilane/fluoroalkylsilane) mixed monolayer might be attributed to both faster spreading of FOETS molecules on the water surface and the crystallizable characteristics of alkylsilane molecules. The mixed monolayer of crystalline alkylsilane (OTS) and amorphous alkylsilane (n-dodecyltrichlorosilane, DDTS) formed a phase-separated structure on the water surface because of the crystallizable characteristics of OTS. Lateral force microscopic (LFM) observation revealed that the order of the magnitude of lateral force generated against the silicon nitride tip was: n-triacontyltrichlorosilane (TATS) domain with longer alkyl chain > amorphous FOETS matrix > crystalline OTS domain. On the other hand, scanning viscoelasticity microscopic observation revealed that the order of the magnitude of modulus was: Si substrate > crystalline OTS domain > amorphous FOETS matrix.