Impact of crystal structure on optical properties and temperature sensing behavior of NaYF:Yb/Er nanoparticles.

We report a comprehensive study of the structural, morphological, and optical properties, and UC-based ratiometric temperature sensing behavior of (α) cubic and (β) hexagonal phases of NaYF:Yb/Er nanoparticles. The α-NaYF:Yb/Er and β-NaYF:Yb/Er nanoparticles were synthesized using co-precipitation and hydrothermal methods, respectively. Powder X-ray diffraction studies confirmed the phase purity of the samples. The morphological studies show uniform particle sizes of both phases; the average particle size of α-NaYF:Yb/Er and β-NaYF:Yb/Er was 9.2 nm and 29 nm, respectively. The Raman spectra reveal five sharp peaks at 253 cm, 307 cm, 359 cm, 485 cm, and 628 cm for β-NaYF:Yb/Er, whereas α-NaYF:Yb/Er shows two broad peaks centred at 272 cm and 721 cm. The optical property measurements show that α- and β-NaYF:Yb/Er phases have distinct upconversion emission and temperature sensing behavior. The upconversion emission measurements show that β-NaYF:Yb/Er has higher overall emission intensities and green/red emission intensity ratio. The temperature-dependent upconversion emission measurements show that α-NaYF:Yb/Er has higher energy separation between H and S energy states. The temperature sensing performed utilizing these thermally coupled energy levels shows a maximum sensitivity of 0.0069 K at 543 K and 0.016 K at 422 K for β-NaYF:Yb/Er and α-NaYF:Yb/Er, respectively.