(Y,Yb,Tm,Er)VO₄ nanoparticles for up conversion thermometry from cryogenic to high temperatures

Full text: Noncontact nano thermometers for small or low-accessibility objects are an immediate demand. Temperature measurements obtained by monitoring changes in the optical properties of luminescent materials are a promising way to meet those needs [1]. Thus, we propose to develop a highly-crystalline up conversion (UC) nanomaterial (Y_0.77Yb_0.20Tm_0.02Er_0.01VO₄) as a ratiometric luminescent sensor for a wide range of temperatures (77-673 K). The particles were prepared by colloidal precipitation followed by a crystallinity enhancement procedure, which consisted of a protected annealing in mesoporous SiO₂ and subsequent dissolution of the silica matrix by hydrofluoric acid [2]. The XRD patterns confirmed the crystallinity increase of the particles obtained after this procedure. TEM images showed that particles keep the same ellipsoid prolate morphology and nearly same size (from 60 to 73 nm) after annealing. The solid presented intense UC emissions (λ_exc=980 nm) between 450 and 830 nm. The intensity of the Er³⁺ (²H_11/2→4I_15/2) and Tm³⁺ (³F_2,3→³H₆) emissions grew exponentially with increasing temperatures, which were ratiometrically compared with other transitions to attain temperature correlations. This enabled us to obtain a self-referencing sensor with high relative thermal sensitivity (S_R>1% K^-1) over the range from 107 to 673 K covering from the blue to the near-infrared spectral region. A SR of ~2% K^-1 was achieved in the physiologic range (298-313 K), while the maximum SR was 7.3% K^-1 at 107 K. These results are better than SR values of other ceramic oxides. The system was reproducible after five heating-cooling cycles (267-673 K). Thus, the nanomaterial can be used for up conversion thermometry from cryogenic to high temperatures. (author)