Silicon Radical Surface Chemistry

Over the past two decades, covalently attached self-assembled organic monolayers on silicon (Si) surfaces have gained much interest, because of their potential use in electronic devices. The formation of these layers is commonly accepted to proceed mainly via a radical chain mechanism at the Si surface. This article deals with relevant issues regarding the preparation and the functionalization of such monolayers on Si, with emphasis on the involved radical chemistry. The attachment modes of olefins, and mechanisms thereof, depend largely on the surface geometry. Section 1 deals with the different surface geometries and the required surface pretreatment prior to monolayer formation (etching steps). Monolayer formation finds itself in competition with oxidation of the surface. Both the oxidation and monolayer attachment are described by surface radical chain reactions. The hydrosilylation reaction at the surface is explained in detail in Section 2. Surface radicals were typically initiated by direct cleavage of the Si-H bonds at the Si surface (at high temperatures or under UV irradiation). However, as reaction conditions have become milder, the radical mechanism has also been shown to be in effect under mild initiation conditions that do not allow direct cleavage of Si-H bonds. Several mechanisms are discussed in Section 3, which range from initiation by adventitious oxygen and reactive species to exciton-mediated mechanisms. To support these theories, many theoretical and experimental modeling studies are reported in the literature. Those relevant to the initiation routes presented here are discussed in detail in Section 4. Monolayers on Si form an ideal scaffold for further surface functionalization and patterning. Section 5 gives an overview of the recent progress that has been made in expanding the versatility of ω-functionalizations (mostly after 2005). Finally, Section 6 presents perspectives for further research and new applications. Keywords: silicon; monolayers; functionalization; mechanism; radical