Organic-free direct crystallization of t-LaVO4:Eu nanocrystals with favorable luminescence for LED lighting and optical thermometry

Abstract Tetragonal structured LaVO4 (t-LaVO4) is superior to its thermodynamically stable monoclinic polymorph for luminescence, but is difficult to synthesize without using chelating molecules/ions. With Ln2(OH)4SO4·2H2O layered hydroxide as an original template for phase conversion, well-crystallized t-(La0.95Eu0.05)VO4 nanocrystals (∼40–80 nm) of favorable dispersion and size uniformity were directly produced via hydrothermal reaction at 200 °C for 24 h without using any organic additive. Luminescence analysis showed that the as-made nanocrystals have an absolute quantum yield of ∼38.2%, fluorescence lifetime of ∼1.52 ms and color coordinates of around (0.33, 0.65) under 304 nm UV excitation at room temperature (RT), and the 620 nm main emission can retain as high as ∼66% of its RT intensity at 150 °C. The activation energies for thermal quenching were analyzed to be ∼0.321, 0.286 and 0.291 eV for the 537 nm (5D1 → 7F1 transition), 593 nm (5D0 → 7F1) and 620 nm (5D0 → 7F2) emissions of Eu3+, respectively. Application of the nanocrystals in LED lighting found electric-field enhanced 5D0 → 7F4 emission of Eu3+ (∼697 nm) and increasing brightness but relatively stable color correlated temperature (∼3270–3400 K) at a higher driving current (30–100 mA) for the lamp. The nanocrystals were also demonstrated to be well capable of temperature sensing with the I537/I593 fluorescence intensity ratio, whose maximum absolute sensitivity, maximum relative sensitivity and minimum temperature resolution are ∼0.27% K−1, 2.01% K−1, and 0.015 K at 298 K, respectively.