Structural and optical properties in Tm3+/Tm3+–Yb3+ doped NaLuF4 glass-ceramics

Transparent NaLuF4 glass-ceramics
(GCs) doped with Tm3+ and Tm3+/Yb3+ have
been prepared by melting-quenching
followed by thermal treatment at temperatures
near the glass transition temperature. The crystallization process has been
studied using X-ray
diffraction (XRD) and high-resolution
transmission electron
microscopy (HRTEM). NaLuF4 nanocrystals (NCs) ranging 9–30
nm in size are
the only crystalline phase, the crystal size increasing with the dopant concentration.
Energy dispersive X-ray
(EDX) measurements confirm the Tm3+ and Yb3+
incorporation in the NCs. Optical characterization included the analysis of up-conversion
(UC) as well as the Near-infrared
(NIR) luminescence. NIR emission
spectra of Tm3+ and Yb3+ in co-doped
samples confirmed an efficient energy
transfer between both ions. No UC emissions are observed in Tm3+ single-doped
glass and GCs. Yb3+ incorporation favors the Tm3+-Tm3+
UC processes resulting
in Tm3+ blue, yellowish-red
and NIR UC emissions after excitation at 975 nm.
Blue UC emission is also observed in the codoped samples after Tm3+ excitation
at 791 nm. These effects were more evident for the GCs compared to the base
glasses, confirming the RE ions incorporation in the NCs. As a result, these GCs
can be used to tune the UC emission from NIR to blue by selective excitation.