Preparation and characterisation of luminescent alkylated-silicon quantum dots

A colloidal suspension of luminescent silicon particles is produced when porous silicon prepared at high current densities is refluxed in toluene solutions of alkenes under conditions which allow hydrosilation to occur. The alkyl monolayer on the surface of the silicon particles renders the colloid lyophilic and stable against flocculation over the longest times studied in this report (14 days). Luminescent colloids were prepared using 1-octene, 1-undecene and difunctional unsaturated molecules such as dimethoxytrityl-protected undecenol and 1,9-decadiene. The dry silicon powder can be re-suspended in solvents such as toluene, dichloromethane and trichloromethane and these suspensions are also similarly stable against flocculation. Fluorescence spectra of the luminescent colloid show a peak wavelength for the emission at 670 nm. The emission intensity increases monotonically with excitation energy, being weak close to the emission peak, but becomes intense for photon energies greater than ca. 3.3 eV. This is consistent with the luminescent species being an indirect gap semiconductor and the known properties of porous silicon. MALDI-TOF mass spectra of the colloid show a broad band for m/z between 1500 and 3000 Da. Assuming this feature is due to singly charged ions and the bulk density of silicon applies, it sets a lower bound on the silicon particle radius of about 0.7 nm. This value is consistent with the radius range (1.4–1.7 nm) for the silicon core deduced from the emission maximum of the photoluminescence spectrum and published particle size-bandgap correlations.