α-NaYb(Mn)F4:Er3+/Tm3+@NaYF4 UCNPs as “Band-Shape” Luminescent Nanothermometers over a Wide Temperature Range

Novel flower-like α-NaYb(Mn)F4:Er3+/Tm3+@NaYF4 upconversion nanoparticles (UCNPs) as luminescent nanothermometers have been developed by combining liquid–solid solution hydrothermal strategy with thermal decomposition strategy. Under 980 nm excitation, they exhibit intense upconversion luminescence and temperature-dependent upconversion luminescence over a wide temperature range. The influence of temperature on “band-shape” upconversion luminescence (UCL) spectra and the intensity of emission bands are analyzed and discussed in detail. We further successfully test and verify that intensity ratios REr of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 and RTm of 1G4 → 3H5 and 3H4 → 3H6 are sensitive to temperature, and the population of active ions follows Boltzmann-type population distribution very well. These luminescent nanothermometers could be applied over a wide temperature range from 123 to 423 K with high sensitivity, which enable them to be excellent candidates for temperature sensors.