Scanning Force Microscopy of Surface Structure and Surface Mechanical Properties of Organotrichlorosilane Monolayers Prepared by Langmuir Method

Organochlorosilane monolayers were polymerized on the water surface and immobilized onto the silicon wafer surface by Langmuir method. The electron diffraction (ED) pattern of the n-octadecyltrichlorosilane(OTS) monolayer revealed that OTS molecules were regularly arranged in a hexagonal array with the (10) spacing of ca. 0.42 nm. On the other hand, ED study revealed that the [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) monolayer was amorphous state at room temperature. The high-resolution atomic force microscopic (AFM) image of the OTS monolayer in a scan area of 10×10 nm² exhibited the individual methyl groups of which packing was a hexagonal array in a similar molecular arrangement concluded by the ED study. AFM observation of the (OTS/FOETS) mixed monolayer revealed that the crystalline OTS circular domains of ca. 1-2 μm in diameter were surrounded by a sea-like amorphous FOETS matrix, even though the molar fraction of OTS was above 75%. Phase separation was observed for monolayer prepared from FOETS and non-polymerizable and crystallizable amphiphile such as lignoceric acid (LA). The phase separation of the (alkylsilane/fluoroalkylsilane) mixed monolayer might be attributed to both the crystallizable characteristics of alkylsilane molecules and faster spreading of FOETS molecules on the water surface. The mixed monolayer of crystalline alkylsilane (OTS) and amorphous alkylsilane (n-dodecyltrichlorosilane, DDTS) formed 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 crystalline Si substrate >crystalline n-triacontyltrichlorosilane (TATS) domain> crystalline n-dococyltrichlorosilane (DOTS) domain> amorphous FOETS matrix > crystalline OTS domain>amorphous DDTS matrix. 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.