Molecular Assembly of Organosilanes
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Ultrathin films of organosilane have been identified as promising nanocoating for micro- and nanoscale technologies such as electronic devices and micromachines. As the silanol groups of organosilane monolayer prepared from organotrichlorosilane or organotrialkoxysilane strongly interact with the substrate surface, the monolayer is thermally and chemically robust compared with conventional amphiphilic monolayers. Because the chain length of organosilane is approximately 1–3 nm, the organosilane forms a uniform ultrathin film on the substrate surface.
Organosilane monolayers, which have surfaces terminated by various functional groups, are useful for manipulation of the physicochemical properties of solid surfaces such as wettability, nanotribology, and protein adsorption behavior. A key to fabricating functional organosilane monolayers is controlling the distribution of surface functional groups. Fabricating micro- and nanodevices using a bottom-up approach requires building blocks with a precisely controlled and tunable chemical composition, morphology, and size that can be fabricated virtually at will. The organosilane monolayer is a candidate for such a building block because of its stability and ease of fabrication. Patterned microfeatures of organosilane monolayers can be fabricated on the substrate, allowing surface physicochemical properties to be area-selectively controlled. Two methods will be discussed in this article. One of them utilizes crystallization of organosilane of the binary component monolayer at the air/water interface. Because the diffusion of organosilane molecules at the air/water interface is slow, macroscopic phase separation is inhibited, even with the alkylsilane and fluoroalkylsilane mixed monolayers. The phase-separated monolayer is transferred to the Si-wafer substrate by the LB method. Another method utilizes photolithography by a vacuum ultraviolet (VUV) ray source. In the case of a VUV source with λ = 172 nm, photodecomposition of the organic moiety occurs because of the higher photon energy of the VUV ray compared with the bond energy of a typical C–C linkage. Using photolithography, one can prepare a micropatterned surface with various organosilane monolayers by repeating the photodecomposition and chemisorption processes. By changing the shape and area ratio of the patterns of the photomask, this technique enables one to control the area ratio and the wettability gap of different organosilane monolayers.