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.