Excitation wavelength dependent photoluminescence and photochromic studies of Yb3+/Er3+ doped β−KYF4 and β−NaYF4 nanoparticles

We report both resonant and off-resonant laser excitation of Er3+ upconversion fluorescence and colour tunability in Er3+/Yb3+ co-doped KYF4 and NaYF4 nanoparticles. The nanoparticles were prepared using a hydrothermal method with particle sizes of 74 ± 20 nm and 345 ± 91 nm for β−KYF4 and β−NaYF4, respectively. The Yb3+ 2F7/2 → 2F5/2 absorption spectra exhibit absorption maxima at 10237 cm−1 (977 nm) for β−NaYF4 and 10267 cm−1 (974 nm) for β−KYF4 nanoparticles. The Er3+ upconversion fluorescence spectra consist of the 2H11/2, 4S3/2, and 4F9/2 → 4I15/2 transitions for either 974, 977 or 980 nm laser excitation in both materials. We observed an enhancement in the upconversion intensity by a factor of up to 20 for β−KYF4 and 1.5 fold for β−NaYF4 under resonant excitation compared with off-resonant excitation at 980 nm. Tuneable upconversion fluorescence wavelength was achieved in β−KYF4:Yb/Er nanoparticles by adjusting the excitation wavelength. The CIE chromaticity coordinates are (0.6836, 0.3151) with a highest red colour purity of 99.70% for β−KYF4 (4700 K) and (0.3953, 0.5915) with a colour purity of 96.84% for β−NaYF4 (4533 K) nanoparticles. The respective absolute upconversion quantum yields are 1.18% and 2.34% under low power density (1.65 W⋅cm−2).