Up-Conversion Luminescence Processes in NaLaF₄ Doped with Tm³⁺ and Yb³⁺ and Dependence on Tm³⁺ Concentration and Temperature

Abstract

In this work, luminescence processes in polycrystalline NaLaF4:Tm3+ and NaLaF4:Tm3+,Yb3+ materials were studied. Luminescence spectra and decay kinetics measurements were performed for NaLaF4 doped with various Tm3+ concentrations (0.01, 0.1, 0.5, 1, and 2 mol%) under direct excitation to 3P0, 1D2, 1G4, and 3H4 states. It was found that some of the Tm3+ excited states are more affected by Tm3+ concentration than other states. Under infrared excitation of Yb3+, energy transfer to Tm3+ occurred and intensive ultraviolet and blue up-conversion luminescence was observed. Possible up-conversion mechanisms are discussed. Spectroscopic measurements show that long-duration excitation radiation reduces ultraviolet up-conversion luminescence intensity, and this intensity reduction is related to sample heating due to high excitation radiation density and a poor heat sink from samples. It was found that sample configuration for spectroscopic measurements is crucial to correctly describe measured up-conversion luminescence spectra.

Controllable and tuneable growth of NaYbF4:Tm(0.5%)Fe(5%)@Na(Yb/Y)F4-core@shell structures and the effect of their geometry on upconversion luminescence

“…Overall, the n-values of the 4 bands are much lower than the theoretical value (n = 5 for 345 nm and n = 4 for 362 nm, 450 nm, and 475 nm) that could be strongly ascribed to the saturation effect, which has been seen previously by other groups. [56][57][58][59][60]…”

Thermal Effects in Up-Conversion Luminescence NaYF4: Tm3+, Yb3+ Core-Shell Nanoparticles in the Temperature Range of 18–312 K

“…15 There are several energy transfer processes responsible for the excitation of Tm 3+ from Yb 3+ : a subsequent energy transfer step for several excited Yb 3+ to Tm 3+ , cooperative sensitization from two Yb 3+ to single Tm 3+ , and crossrelaxation processes in Tm 3+ . 12 The explanation of these variations in Tm 3+ UC luminescence spectra has to be of complex nature: it should consider the temperature effect on f-f transitions processes in Tm 3+ itself, and the temperature effect on the energy transfer process from excited Yb 3+ to Tm 3+ . For a better understanding of these processes, UC luminescence spectra were acquired at different temperatures, keeping constant excitation wavelength (976 nm) and power (23 W/cm 2 ).…”

“…15 There are several energy transfer processes responsible for the excitation of Tm 3+ from Yb 3+ : a subsequent energy transfer step for several excited Yb 3+ to Tm 3+ , cooperative sensitization from two Yb 3+ to single Tm 3+ , and cross-relaxation processes in Tm 3+ . 12…”

“…The UC luminescence spectra measurements were performed using a short exposition time (around 10 s) to avoid sample heating, since heating affects the spectroscopic properties of the samples. 12 All spectral measurements were corrected according to the system spectral response.…”