Recent Research Developments

September 5, 2001
 

Molecular functions on the Silicon Oxide Surface

The chemical and physical properties of a solid surface are often determined only by the top most atomic or molecular layer. Thus, the ability to easily form monolayer assemblies on solid surface is highly desirable because this allows one to tailor the surface properties on the molecular level. In the upcoming issue of Langmuir, graduate student Ryan Major and chemistry professor Xiaoyang Zhu report a new strategy for the formation of densely packed monolayers on oxide surfaces. It involved a simple two-step procedure in which the oxide surface was initially chlorinated and then exposed to alcohol molecules for monolayer formation via Si-O linkages (see scheme). This approach was demonstrated for the formation of octadecanol (C18) monolayers on the oxide terminated Si(111) surface. Characterization of the resulting C₁₈H₃₇O- monolayer by a combination of techniques revealed a densely packed film with thickness of 24 Å, which was nearly equal to molecular length in all-trans configuration. A more recent collaboration with Sandia National Laboratory showed the remarkable mechanical property of this film. The friction coefficient of the C₁₈H₃₇O- monolayer on silicon oxide was as low as that of octadecane thiol self-assembled monolayer (SAM) on Au. The latter was considered the best and most ordered SAM system known.

The advantage of this approach is that the attachment of organic molecules to the solid surface is a selective reaction, leading exclusively to monolayer formation; there is no competition between surface attachment and intermolecular interactions. This approach is also sufficiently general for the functionalization of a variety of oxide surfaces. A number of applications are being explored. For example, one of the major obstacles in the fabrication and operation of silicon based micromachines, called microelectromechanical systems (MEMS), is adhesion and friction between micro-components. This is a result of scaling law: the surface area-to-volume ratio increases with the decrease in dimension. Surface forces dominate at dimensions less than 1 mm. Such an adhesion/friction problem may be alleviated by coating the high energy silicon oxide surface with organic monolayers. Other applications include surface functionalization in biochip arrays and soft-lithography. These applications are being explored at the present time.