(458h) Assembly of Photoluminescent Silicon Nanocrystals with Reversible Covalent Bonds

Colloidal photoluminescent silicon (Si) nanocrystals are protected from oxidation by attaching organic ligands to the nanocrystal surface through hydrosilylation reactions of alkenes. Ligands with a wide variety of exposed functional groups can be tethered to the nanocrystals; thus, providing the opportunity for applications, such as contrast agents for biological imaging or as photosensitizers to drive molecularly-targeted redox reactions. Typically, Si nanocrystals are passivated by hydrosilylation reactions using high temperature or free radical initiators. We have shown that, in contrast to extended Si surfaces, hydrosilylation of Si nanocrystals smaller than about 5 nm in diameter can occur at room temperature, and that the yield of ligand-passivated Si nanocrystals is highly dependent on size. We also have used AIBN, a free radical initiator, to drive low temperature hydrosilylation of alkenes with aldehyde functional groups. The aldehyde has the ability to form reversible covalent bonds with hydrazide functional groups, which allows the creation of gels—networks of nanocrystals that can be assembled and disassembled depending on the reaction equilibrium. Here, we show the successful passivation of Si nanocrystals with alkenes with aldehyde functional groups at room temperature using AIBN. Hydrogen-terminated Si nanocrystals were first synthesized by thermal decomposition of hydrogen silsequioxane (HSQ), followed by HF etching. The aldehyde concentration on the nanocrystal surface could be adjusted by co-passivation with undecenoic acid and varying the mole ratio of aldehyde to undecenoic acid in the reaction mixture, as confirmed by characterization of the capping ligand layer composition using 1H NMR spectroscopy. Diffusion ordered spectroscopy (DOSY) NMR further confirmed that ligands with both acid and the aldehyde functional groups were bonded to the nanocrystal surface.