Mechanic studies of monolayer formation on H-Si(111) surfaces

Covalently attached organic monolayers on silicon surfaces form thermally and chemically stable platforms for (bio)functionalization of the surface. Recent advances in monolayer formation – yielding increases in monolayer quality and the complete exclusion of oxygen at modified surfaces – have paved the way for the future development of biosensors, photovoltaics, and molecular electronic devices. Despite these successful innovations in monolayer formation (including patterning and functionalization) over the last two decades, the actual knowledge of the processes at the silicon surface still lags behind. Yet, a good understanding of the mechanisms of initiation and propagation will help in finding new tunable parameters for further speed-up, and new strategies for attaching interesting biomolecules to the silicon surface. In the search for even faster synthetic methods to build monolayers of even higher quality, this thesis presented a combined systematic experimental and theoretical study of the mechanisms underlying monolayer formation. A detailed overview of the current knowledge regarding the mechanisms that underlie monolayer formation onto hydrogen-terminated silicon (H-Si) is presented in Chapter 2. The focus of this chapter is mainly directed to H-Si(111) and H-Si(100) surfaces, where silyl radicals play a key role in the formation of Si-C bonds that link the monolayers to the surface. These radicals also readily react with oxygen leading to oxidation of the surface. Several initiation mechanisms that induce the formation of these radicals are discussed, along with supporting theoretical and experimental modeling studies. The radical cation initiation mechanism is studied in more detail in Chapter 3. This chapter describes how radical cations of low molecular-weight silicon model compounds, which were synthesized to represent the H-Si surface, were obtained by photo-induced electron transfer. The stability and the nature of the radical cations were investigated with lifetime and secondary electron transfer studies. This chapter shows that radical cation initiation at the silicon surface is feasible. However, given the differences in reactivity between oxygen-centered and carbon-centered nucleophiles, this reaction is likely to only play a significant role in the initiation steps of monolayer formation, and not in the propagation of the Si-C bond formation. Chapter 4, describes the experimental and theoretical study of the radical chain mechanism, and in particular the effect of stabilization of the b-carbon radical. The radical reactivity was studied by performing competition reactions of precursors (alkenes, alkynes, etc.) with the tris(trimethylsilyl)silyl radical, and via high-level theoretical calculations on a theoretical Si4-model to obtain the activation barrier and overall free energy changes. Based on the insights obtained in the research described in Chapters 3 and 4, Chapter 5 describes in a combined experimental and theoretical study a significant improvement of surface coverage and speed-up of monolayer formation on H-Si.