Cosolvent-Free Sol–Gel Synthesis and Optical Characterization of Silica Glasses Containing LaF3 and (La,Er)F3 Nanocrystals

Abstract

Monolithic transparent silica glasses containing LaF 3 nanocrystals were prepared by a cosolvent-free solgel 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 LaF 3 nanocrystals decreases with an increase in the volume fraction of LaF 3 . This suppresses Rayleigh scattering and improves transparency at LaF 3 -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 LaF 3 phase as a result of improved crystallinity of LaF 3 . The quantum yields recorded for the green and red upconversion PL bands were ca. 0.08% and ca. 0.04%, respectively.Transparent glass-ceramics containing rare-earth (RE) fluoride crystals are important for optical applications such as phosphor-based devices, scintillators, laser media, and wavelength conversion devices, because the low phonon energies of fluorides are suitable for suppressing the nonradiative decay of doped RE ions via multiphonon relaxation. 17 Silica glass is an attractive matrix for fluoride nanocrystals because of its excellent transparency in the infrared (IR) to vacuumultraviolet (UV) spectral range, good radiation hardness, chemical stability, and mechanical strength.Several research groups have reported solgel synthesis of monolithic glass-ceramics in silicametal fluoride binary systems, 813 mostly using trifluoroacetic acid (CF 3 COOH) as a fluorine source.812 These methods require alcohols as cosolvents to form homogeneous precursor solutions. In addition, the synthesis usually takes several weeks to prevent fracture during drying and sintering, and finding ways to shorten the processing time is important for the application of these materials. The fractures originate from the capillary forces that develop during the removal of liquid from the pores. They can be suppressed by increasing the pore size, which will also shorten the processing time. Recently, we reported the synthesis of monolithic silica glasses from tetraethoxysilane (TEOS) water binary systems, where macroporous gels, which can be dried and sintered relatively easily, are prepared utilizing macroscopic phase separation in parallel with gelation. 1417 Furthermore, this method does not require alcohols, which are used almost routinely as cosolvents to obtain homogeneous precursor solutions of water and alkoxides. Such reduced use of reagents is important for the synthesis of functional materials in the future.In this paper, we present a cosolvent-free solgel synthesis of monolithic silica glasses containing LaF 3 nanocrystals that utilizes the pyrolysis of RE trifluoroacetate...