A spectroscopic and thermometric comparison of α- and β-phase KYF4:Yb3+/Er3+ nanoparticles

We report absorption, fluorescence, and thermometric studies of both (α) cubic and (β) hexagonal phase KYF4:Yb/Er nanoparticles. The cubic phase of KYF4:Yb/Er nanoparticles was achieved using a hydrothermal approach, and the cubic to hexagonal phase transformation was successfully realized by controlling the annealing temperature at 500 °C. Both nanoparticle systems exhibit similar maxima in the Yb3+ absorption spectra at 10 268 cm−1 (974 nm). The 2H11/2, 4S3/2, and 4F9/2 → 4I15/2 transitions are recorded in the Er3+ upconversion fluorescence spectrum using 974 nm laser excitation. The green and red fluorescence of the hexagonal phase were approximately 100 and 2000 times more intense, respectively, than that of the cubic phase. The red to green ratio (R/G) was 50:1 and 2:1 for β-KYF4:Yb/Er and α-KYF4:Yb/Er nanoparticles, respectively. A three-photon process is involved in green upconversion for β-KYF4:Yb/Er, as described by 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 4G11/2 (Er3+). Using the fluorescence intensity ratio technique from thermally coupled Er3+ 2H11/2 and 4S3/2 levels, a very high thermal sensitivity of 1.76 and 1.49% K−1 at ambient temperatures was achieved for α- and β-phase materials, respectively.