Luminescence nanothermometry using self-assembled Er3+, Yb3+ doped Y2O3 nanodiscs: Might the upconversion mechanism condition their use as primary thermometers?

Self-assembled Er3+, Yb3+ doped Y2O3 colloidal nanodiscs were synthesized via a digestive ripening process using oleic acid and oleylamine as organic surfactants, and NaCl as the structure-directing agent. X-ray powder diffraction confirms the formation of cubic yttrium oxide, regardless of the doping concentration, although this affects to the crystallinity of the samples. Transmission electron microscopy (TEM) and high-resolution transimission electron microscopy (HRTEM) images reveal that these nanodiscs tend to self-assemble in fiber-like structures on the grids, with an average diameter of 20 nm, thicknesses down to the unit cell, and lengths of several micrometers. The Er3+, Yb3+ doped Y2O3 nanodiscs were tested as luminescent nanothermometers operating in the visible region. Upon excitation at 980 nm, three emission bands were generated at 525 nm, 550 nm, and 650–690 nm, assigned to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 electronic transitions of Er3+ ions, respectively. A relative thermal sensitivity of 1.11% K−1 and a temperature uncertainty of 0.44 K at room temperature were determined for these thermometers. However, the possibility of using them as primary luminescent thermometers was ruled out by the important amount of heat released by the four-photons upconversion mechanism for the generation of the green light in the 4 mol% Er3+ and 4 mol% Yb3+ doped Y2O3 nanodiscs.