Efeitos da concentração do sensibilizador e da temperatura na luminescência de nanocristais fluoretos co-dopados com Y b3+/Tm3+ sob excitação anti-Stokes

Abstract

In this work, we investigated the effects of Y b3+ concentration and temperature on the structural and optical properties of nanocrystals (NCs) based on fluorides (CaF2, LaF3 and N aY F4) co-doped with Y b3+/Tm3+ under anti-Stokes excitation at 1064 nm. For that, sets of samples were synthesized varying the Y b3+ concentration (2, 10, 20 and 40 mol%) and keeping that of Tm3+ fixed at 0.5 mol%. We initially analyzed the structural properties by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). Subsequently, the Upconversion (UC) processes in the 400 to 900 nm spectral region were investigated, where we observed the effect of the Y b3+ concentration under anti-Stokes excitation. Comparatively, the UC emission spectra of the CaF2 and N aY F4 samples had similar behaviors with the Y b3+ concentration increasing, with the samples doped with 10 and 20 mol% being the best for UC, i.e., these NCs showed concentration quenching, which did not happen with the LaF3 NC. An important observation was the increase in the UC emissions with temperature for all concentrations, which enhances applications such as fluorescent and thermal imaging. Finally, we investigate the potential of the investigated NCs as thermal nanosensors based on optical properties, using the fluorescence intensity ratio (FIR) approach, in the temperature range from 313 to 473 K (40 ∘C to 200 ∘C), under anti-Stokes excitation. Regarding the results of nanothermometry, all emission bands increased with temperature, and this increase in the UC was attributed to the phonon-assisted anti-Stokes excitation process, which is strongly dependent on temperature. FIRs of several bands were investigated with the purpose of obtaining the best thermal sensor and, finally, no major differences were found in the relative thermal sensitivity (Sr) of the optical nanothermometers, with the increase in the Y b3+ concentration, but obtained different evolutions with the temperature. We believe that these results are due to the fact that these NCs have very similar phonon energies (they are all fluorides), although they present very different fluorescent yields.