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

Hexagonal NaYF 4 : Er 3+ /Yb 3+  (β‐NaYF 4 ) 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‐NaYF 4 : Er 3+ /Yb 3+ (α‐NaYF 4 ) nanoparticles were transformed into β‐NaYF 4 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‐NaYF 4 microprisms was further improved via regulating the concentration of chelating agents and pH values in the precursor solution. β‐NaYF 4 microprisms displayed red and green color emissions for 980 nm excitation owing to the 4 F 9/2 and 2 H 11/2 / 4 S 3/2  →  4 I 15/2  transitions of Er 3+ respectively. The ratio ( 2 H 11/2 / 4 S 3/2  →  4 I 15/2 ) between green fluorescence intensities (FIR) of β‐NaYF 4 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‐NaYF 4 microprisms in measuring the laser‐induced heating effects.