  Silicon (Si) nanoparticles (NPs) were synthesized by transforming Si wafer
surface to ammonium silicon hexafluoride (ASH) or (NH4)2SiF6 under acid vapor
treatment. Si-NPs are embedded within the polycrystalline (ASH) layer formed on
the Si surface exhibit a strong green-orange photoluminescence (PL). Difference
measurements revealed a major double component spectra consisting of a broad
band associated with the ASH-Si wafer interfacial porous oxide layer and a high
energy band attributable to Si-NPs embedded in the ASH. The origin of the
latter emission can be explained in terms of quantum/spatial confinement
effects probably mediated by oxygen related defects in or around Si-NPs.
Although Si-NPs are derived from the interface they are much smaller in size
than those embedded within the interfacial porous oxide layer (SiOx, 1 < x <
2). Transmission electron microscopy (TEM) combined with Raman scattering and
Fourier transformed infrared (FTIR) analysis confirmed the presence of Si-NPs
and Si-O bondings pointing to the role of oxygen related defects. The presence
of oxygen of up to 4.5 at.% in the (NH4)2SiF6 layer was confirmed by energy
dispersive spectroscopy (EDS) analysis.
