Synthesis and design of NaYF 4 microprisms via a microwave‐assisted approach for highly sensitive optical thermometry applications

Abstract

Hexagonal NaYF4: Er3+/Yb3+ (β-NaYF4) microprisms with uniform particle sizes were synthesized via a microwave-assisted hydrothermal method (MWHM). Nucleation and crystal growth were significantly surpassed owing to the microwave irradiation. Cubic-NaYF4: Er3+/Yb3+ (α-NaYF4) nanoparticles were transformed into β-NaYF4 microprisms with reaction time in between 10 and 120 minutes at 180°C. The shape of the particles was enhanced via controlling the nucleation process with optimized irradiation duration and heating rates. The size distribution of β-NaYF4 microprisms was further improved via regulating the concentration of chelating agents and pH values in the precursor solution. β-NaYF4 microprisms displayed red and green color emissions for 980 nm excitation owing to the 4F9/2 and 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ respectively. The ratio (2H11/2/4S3/2 → 4I15/2) between green fluorescence intensities (FIR) of β-NaYF4 microprisms was recorded with the variation in temperature from 303 K to 550 K. The recorded FIR values obeyed Boltzmann distribution and the distribution was used to evaluate the sensitivity. The optimum absolute sensitivity was achieved as 0.0044 K−1 at 490 K with promising resolution, reversibility, and stability. Furthermore, the power-dependent variations in FIR values implied the suitability of β-NaYF4 microprisms in measuring the laser-induced heating effects.