Impact of host composition and dopant ion concentration on the thermometric properties of a Eu3+ activated fluoride-based single-band ratiometric luminescent thermometer

Highlights: • Novel SBR luminescent thermometers based on Eu³⁺ are presented. • The beneficial influence of Eu³⁺ concentration on the sensitivity was presented. • The ultra-high relative sensitivity S_R= 16.9%/K at 163 K for the NaGdF₄:50%Eu³⁺. -- Abstract: Luminescence thermometry (LT) is a remote technique of temperature monitoring, which shows immense potential and has become very popular among researchers in recent years. LT can be realized by several different measurement methods, but one of the most recently proposed and least studied relies on a single-band ratiometric (SBR) approach. In the SBR approach, the temperature readout is achieved from a single emission band being photoinduced by two different excitation conditions, i.e. ground (GSA) and excitated (ESA) state absorption. However, the intensity of emission resulting from ESA excitation is dependent on many factors such as host material properties, type and concentration of optically active dopant ions, surface effects, and others, so an in-depth understanding of photophysical processes is essential for the intentional development of novel highly sensitive luminescent thermometer operating on the SBR principle. In these studies, the impact of host and interionic interactions occurring through cross-relaxation on the thermometric properties of a single-band ratiometric thermometer based on nanosized NaYF₄ and NaGdF₄ fluorides doped with Eu³⁺ ions are investigated and discussed. The maximum relative temperature sensitivity was equal S_R= 16.9%/K at 163 K for the NaGdF₄:50%Eu³⁺ nanocrystals and was close to 1%/K over the entire higher temperature range analyzed.