Gas-phase hydrosilylation of plasma-synthesized silicon nanocrystals with short- and long-chain alkynes

Surface passivation of Si nanocrystals (NCs) is necessary to enable their utilization in novel photovoltaic and optoelectronic devices. Herein, we report the surface passivation of plasma-synthesized, H-terminated Si NCs via gas-phase hydrosilylation using a combination of short- and long-chain alkynes. Specifically, using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that a sequential exposure of the Si NC surface to acetylene and phenylacetylene results in a surface alkenyl coverage of ∼58%, which is close to the theoretical maximum of ∼55% and ∼60% predicted for alkyl- and alkenyl-terminated Si(111) surfaces, respectively. We attribute this unprecedented high surface hydrocarbon coverage to the combination of short- and long-chain alkynes that reduce the steric hindrance on the surface, higher reactivity of 1-alkynes versus 1-alkenes of the same chain length, and the smaller van der Waals radius of the alkenyl groups compared to the alkyl groups. Unlike 1-alkenes, 1-alkynes also react with the surface to form the 1,1- and 1,2-bridge structures via the bis-hydrosilylation reaction. However, our data clearly show that this reaction pathway cannot account for the enhanced surface coverage in the sequential exposure experiments, since exposure of the surface to just acetylene or phenylacetylene results in an almost identical surface coverage due to the 1,1- and 1,2-bridge sites.