| A novel synthesis for gram quantities of luminescent, oxide-embedded and freestanding silicon nanocrystals is presented. The reductive thermal processing of hydrogen silsesquioxane (HSQ) yields size-controlled oxide-embedded silicon nanocrystals, and their subsequent liberation through straight-forward hydrofluoric (HF) acid etching procedures allows tailoring of their optical response throughout the visible region of the electromagnetic spectrum.;The thermally-induced transformation of HSQ to oxide-embedded silicon nanocrystals occurs via a multi-step process, involving cage rearrangement, network decomposition to form silane-like species, in-situ silane decomposition to form elemental silicon clusters, and subsequent nucleation, growth and crystallization to form silicon nanocrystals in an SiO2 matrix. This formation process was confirmed using detailed spectroscopic and microscopic characterization.;The luminescence characteristics of oxide-embedded and freestanding silicon nanocrystals were determined using laser-based photoluminescence and X-ray excited optical luminescence spectroscopy. Oxide-embedded systems were found to exhibit two emission bands in the visible region of the electromagnetic spectrum: a high energy emission that is independent of nanocrystal size, and a size-dependent low energy emission. HF etching was employed to liberate the silicon nanocrystals from the encapsulating oxide in order to assign the origin of these emission bands. Oxide removal was accompanied by a loss of the green emission, indicating that it is of silicon oxide origin. Progressively decreasing the nanocrystal size with extended HF treatment was accompanied by a blue-shift in the emission energy and a narrowing of the emission profile, indicative of an increase in the bandgap energy and a narrowing of the density of states, respectively. This behavior establishes that the low energy emission band is a result of quantum confinement effects in sub-5nm silicon nanocrystals.;It was found that HSQ is an ideal precursor for conformal coating of both flat and not-flat surfaces with oxide-embedded silicon nanocrystals, highlighting its device application potential. Patterning of luminescent sub-10 nm structures was also achieved by electron beam writing and subsequent thermal processing.;The ability to produce large quantities of size-controlled oxide-embedded and freestanding silicon nanocrystals in bulk and thin film architectures demonstrates that HSQ is currently the most versatile silicon nanocrystal precursor available. |
