Impact of crystal structure on optical properties and temperature sensing behavior of NaYF 4 :Yb 3+ /Er 3+ 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 4 :Yb 3+ /Er 3+ nanoparticles. The α-NaYF 4 :Yb 3+ /Er 3+ and β-NaYF 4 :Yb 3+ /Er 3+ 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 4 :Yb 3+ /Er 3+ and β-NaYF 4 :Yb 3+ /Er 3+ was 9.2 nm and 29 nm, respectively. The Raman spectra reveal five sharp peaks at 253 cm -1 , 307 cm -1 , 359 cm -1 , 485 cm -1 , and 628 cm -1 for β-NaYF 4 :Yb 3+ /Er 3+ , whereas α-NaYF 4 :Yb 3+ /Er 3+ shows two broad peaks centred at 272 cm -1 and 721 cm -1 . The optical property measurements show that α- and β-NaYF 4 :Yb 3+ /Er 3+ phases have distinct upconversion emission and temperature sensing behavior. The upconversion emission measurements show that β-NaYF 4 :Yb 3+ /Er 3+ has higher overall emission intensities and green/red emission intensity ratio. The temperature-dependent upconversion emission measurements show that α-NaYF 4 :Yb 3+ /Er 3+ has higher energy separation between 2 H 11/2 and 4 S 3/2 energy states. The temperature sensing performed utilizing these thermally coupled energy levels shows a maximum sensitivity of 0.0069 K -1 at 543 K and 0.016 K -1 at 422 K for β-NaYF 4 :Yb 3+ /Er 3+ and α-NaYF 4 :Yb 3+ /Er 3+ , respectively.