NMR study of the synthesis of alkyl-terminated silicon nanoparticles from the reaction of SiCl4 with the zintl salt, NaSi

The synthesis of silicon nanoclusters and their characterization by multinuclear solid-state nuclear magnetic resonance (NMR) is presented. A combination of Na-23, Si-29, and C-13 magic angle spinning with and without cross polarization to H-1 nuclei have been used to investigate the reaction of sodium silicide (NaSi) with silicon tetrachloride (SiCl4) followed by varying degrees of surface passivation. The Na-23 and Si-29 NMR spectra of NaSi distinguish the two crystallographically inequivalent sites for each, consistent with the crystal structure. This compound exhibits extreme diamagnetic chemical shifts for Si-29 of -361 and -366 ppm. NaSi is reacted with SiCl4 in refluxing ethylene glycol dimethyl ether to produce both amorphous and crystalline Si nanoparticles with surfaces capped by chlorine. This reaction produces new Si-29 resonances that survive subsequent capping and oxidation reactions. The Si-29 NMR spectrum shows that the product is incompletely passivated with butyl groups and gives several peaks lying between -67 and -81 ppm that can be attributed to surface silicon atoms. Further reaction of this product with water produces a new NMR spectrum consistent with further termination of the surface by -OH groups.