La2O2S:Eu3+ stability as temperature sensor

The chemical and structural stability of the La₂O₂S:Eu³⁺ phosphor material was tested during different heat treatments in order to determine if it can be used as a possible temperature sensor. It was observed that the overall luminescence intensity of the La₂O₂S:Eu³⁺ increased with annealing time. X-ray diffraction results indicated a decrease in the strain of the lattice during annealing, which was due to the removal of defects or impurities in the crystal lattice. The reduction of hydroxide impurities was also identified using X-ray photoelectron spectroscopy. The increase in luminescence intensity was attributed to the reduction of the hydroxide impurities. Diffused reflectance spectroscopy was used to determine the optical band gap of the La₂O₂S:Eu³⁺ as 2.75 eV. Using the excitation spectra it was established that the S⁻² to Eu³⁺ charge transfer band absorbs ultraviolet radiation and transfers the excited electrons to the excited states of the Eu³⁺ ions from where emission could take place. The lifetime of the luminescence results showed that the higher excited states have a double exponential lifetime that resulted from the emission from both the conventional Eu³⁺ ions and Eu³⁺ ions that were in the vicinity of a defect or impurity group. The average emission decay constants of the ⁵D₂, ⁵D₁ and ⁵D₀ excited states were determined as 0.01 ms, 0.08 ms and 0.34 ms, respectively. A modified system was used to measure the emission of the La₂O₂S:Eu³⁺ phosphor material at different temperatures. The thermal quenching process was identified as the main process that influenced the emission intensity with temperature and the average activation energies for the emission from the ⁵D₂, ⁵D₁ and ⁵D₀ excited states were determined as 0.49 eV, 0.55 eV and 0.77 eV, respectively and the average pre-exponential constant was determined as 9.5 × 10⁷ s⁻¹. It was also shown that La₂O₂S:Eu³⁺ can be utilised as a temperature sensor by using the fluorescence intensity ratio of the emission from the ⁵D₁ and ⁵D₀ excited states. It was established that this material worked well as temperature sensor for the temperature range from 80 °C to 180 °C.