Er3+-activated Na0.5La0.5MoO4 multifunctional molybdate phosphors for optical thermometers and white LEDs

Abstract

In this report, we have successfully prepared a series of Er3+-activated Na0.5La0.5MoO4 green molybdate phosphors by solid-state reaction. The X-Ray diffraction patterns confirmed that all diffraction peaks of Na0.5La0.5MoO4:Er3+ phosphors matched well with the standard data card with orthorhombic phase and the Rietveld refinement results indicated that the space group of the crystal structure was I41/a. Under 379 nm excitation, Na0.5La0.5MoO4:3%Er3+ phosphor exhibited strong green emission, with two strong emission peaks at 530 nm, 551 nm and a weak one centered at 656 nm, corresponding to the transitions of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. The experimental results showed that the optimal doping concentration of Er3+ was x = 3%, and electric dipole-dipole interaction was the major mechanism for the concentration quenching according to Dexter theory. Surprisingly, the optimal Na0.5La0.5MoO4:3%Er3+ phosphor presented thermal enhancement to the ratio of 2H11/2 → 4I15/2 transition to 4S3/2 → 4I15/2 transition in the elevated temperature range of 300–475 K. The maximum Sa and Sr were 1.79 × 10−2 K−1 and 1.094% K−1 respectively, which were applied to optical thermometers based on fluorescence-intensity-ratio technique. Furthermore, Na0.5La0.5MoO4:3%Er3+ phosphor had excellent thermal quenching performance (I425K/I300K = 93.47%) and high activation energy (Ea = 0.366 eV). Finally, the prepared Na0.5La0.5MoO4:3%Er3+ green phosphor, commercial BaMgAl10O17:Eu2+ blue phosphor and CaAlSiN3:Eu2+ red phosphor were combined with the 365 nm near-UV chip to produce a warm white LED device with high color index (Ra = 86.4) and low correlation color temperature (CCT = 4328 K). The above results suggested that the Er3+-activated Na0.5La0.5MoO4 multifunctional phosphors had considerable potential in the field of optical temperature measurement and phosphor-converted white light emitting diodes.