Silyl Radical Abstraction in the Functionalization of Plasma-Synthesized Silicon Nanocrystals

Many silicon nanostructures have exhibited favorable optical properties following surface functionalization with molecular groups through a silicon-carbon bond. Here, we show the mechanism of functionalization of silicon nanocrystals synthesized in a nonthermal radiofrequency plasma is fundamentally different than in other silicon systems. In contrast to hydrosilylation, where homolytic cleavage of Si-H surface bonds is typically a prerequisite to functionalization, we demonstrate the dominant initiation step for plasma-synthesized silicon nanocrystals is abstraction of a silyl radical, ·SiH₃, and generation of radical at the silicon nanocrystal surface. We experimentally trap the abstracted silyl radical and show this initiation mechanism occurs for both radical- and thermally-initiated reactions of alkenes using complementary FTIR and ¹H NMR spectroscopies. These data additionally indicate that silylsilylation, addition of a Si-SiH₃ group across an unsaturated hydrocarbon, competes with hydrosilylation. We also present a new empirical sizing curve as a convenient method to determine Si NC size from photoluminescence peak energy.