Formation of Si/SiO2 Luminescent Quantum Dots From Mesoporous Silicon by Sodium Tetraborate/Citric Acid Oxidation Treatmentстатья
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Дата последнего поиска статьи во внешних источниках: 21 мая 2019 г.
Аннотация:We propose a rapid, one-pot method to generate photoluminescent (PL) mesoporous
silicon nanoparticles (PSiNPs). Typically, mesoporous silicon (meso-PSi) films, obtained
by electrochemical etching of monocrystalline silicon substrates, do not display strong
PL because the silicon nanocrystals (nc-Si) in the skeleton are generally too large to
display quantum confinement effects. Here we describe an improved approach to form
photoluminescent PSiNPs from meso-PSi by partial oxidation in aqueous sodium borate
(borax) solutions. The borax solution acts to simultaneously oxidize the nc-Si surface and
to partially dissolve the oxide product. This results in reduction of the size of the nc-Si core
into the quantum confinement regime, and formation of an insulating silicon dioxide (SiO2)
shell. The shell serves to passivate the surface of the silicon nanocrystals more effectively
localizing excitons and increasing PL intensity. We show that the oxidation/dissolution
process can be terminated by addition of excess citric acid, which changes the pH of
the solution from alkaline to acidic. The process is monitored in situ by measurement of
the steady-state PL spectrum from the PSiNPs. The measured PL intensity increases
by 1.5- to 2-fold upon addition of citric acid, which we attribute to passivation of
non-radiative recombination centers in the oxide shell. The measured PL quantum yield
of the final product is up to 20%, the PL activation procedure takes <20 min, and the
resulting material remains stable in aqueous dispersion for at least 1 day. The proposed
phenomenological model explaining the process takes into account both pH changes
in the solution and the potential increase in solubility of silicic acid due to interaction
with sodium cations.