We report the bulk preparation of nanocrystalline Si−SiO2 (nc-Si/SiO2) composites via straightforward
reductive thermal annealing of a well-defined molecular precursor, hydrogen silsesquioxane. The presented
method affords quantitative yields of composite powders in large quantities. Freestanding, hydride-surface-terminated silicon nanocrystals that photoluminesce throughout the visible spectrum are readily liberated
from nc-Si/SiO2 composite powders upon etching in ethanol−water solutions of hydrofluoric acid.
Composites and freestanding particles were characterized using transmission electron microscopy (TEM),
selected area electron diffraction (SAED), X-ray powder diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), photoluminescence (PL) spectroscopy, Fourier transform infrared spectroscopy (FT−IR), and thermogravimetric analysis (TGA).
Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1-5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of "green" silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts.
We report the preparation of SiO2-embedded silicon nanocrystals (Si-NCs) from the thermal processing of sol−gel polymers derived from trichlorosilane (HSiCl3). Straightforward addition of water to HSiCl3 generates a cross-linked (HSiO1.5)
n
sol−gel polymer suitable for the generation of bulk quantities of SiO2-embedded Si-NCs. It is shown that structural differences between the present (HSiO1.5)
n
polymer and hydrogen silsesquioxane (HSQ) result in controllable differences in the resulting oxide-embedded Si-NCs produced from these precursors. A polymer structure/NC size relationship is further delineated through the preparation and evaluation of methyl-modified (HSiO1.5)
n
(CH3SiO1.5)
m
(m ≪ n, m + n = 1) sol−gel copolymers, in which a low concentration of methyl groups acts as a polymer network modifier and influences the formation of Si-NCs during thermal processing. Si-NC size is readily tailored by controlled variations to peak processing temperature for (HSiO1.5)
n
and composition (n and m) for (HSiO1.5)
n
(CH3SiO1.5)
m
. Furthermore, the present Si-NCs exhibit size-dependent photoluminescence (PL) in accordance with the principles of quantum confinement. Freestanding Si-NCs are obtained through chemical etching of the oxide matrix and exhibit tunable PL throughout the visible spectrum.
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