2014 Volume 87 Issue 7 Pages 765-772
Monolithic transparent silica glasses containing LaF3 nanocrystals were prepared by a cosolvent-free sol–gel method based on the pyrolysis of lanthanum trifluoroacetate. The two-step mixing of alkoxide and water, consisting of partial hydrolysis under acidic conditions and subsequent neutralization to catalyze the polycondensation of hydrophobic silica oligomers, makes it possible to shorten the processing time while eliminating the need for cosolvents. The size of the LaF3 nanocrystals decreases with an increase in the volume fraction of LaF3. This suppresses Rayleigh scattering and improves transparency at LaF3-rich compositions. Green and red upconversion photoluminescence (PL) bands, mainly caused by energy-transfer upconversion (ETU), are observed in Er-doped samples under excitation at 980 nm. The PL intensity drastically increases with an increase in the volume fraction of the LaF3 phase as a result of improved crystallinity of LaF3. The quantum yields recorded for the green and red upconversion PL bands were ca. 0.08% and ca. 0.04%, respectively.
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