One Phosphor, Many\nPossibilities: A Nonlanthanoid\nHost for Optical Thermometry, Plasmon-Enhanced Upconversion Luminescence,\nand Stable Photoswitching Performance

Engineering a single phosphor for multiple objectives\nis challenging\nand essential for innovative technology, enriching productivity and\nflexibility, lowering cost, minimizing power intake, etc. In this\nstudy, a Yb<sup>3+</sup>,Er<sup>3+</sup>-activated NaBiF<sub>4</sub> nanophosphor synthesized via a thermolysis approach provided significant\nopportunity to explore the potential of single phosphor in diverse\nfields including contactless thermometry, plasmon-enhanced upconversion\nluminescence (UCL), solid-state lighting, and photoswitching probes.\nThe phosphor has been characterized using several standard techniques\nto unveil the phase, crystal structure, associated functional groups,\nmorphology, crystalline nature, and optical properties. A 5-fold enhancement\nin UCL intensity was observed from Yb<sup>3+</sup>,Er<sup>3+</sup>:NaBiF<sub>4</sub>@SiO<sub>2</sub>@Ag. Upconversion luminescence\nthermometry of Yb<sup>3+</sup>,Er<sup>3+</sup>:NaBiF<sub>4</sub> based\non thermally coupled energy levels (TCELs) <sup>2</sup>H<sub>11/2</sub> and <sup>4</sup>S<sub>3/2</sub> was studied via the fluorescence\nintensity ratio (FIR) technique. Thermometric parameters based on\nthe ratio of Er<sup>3+</sup> transitions (<sup>2</sup>H<sub>11/2</sub> → <sup>4</sup>I<sub>15/2</sub> and <sup>4</sup>S<sub>3/2</sub> → <sup>4</sup>I<sub>15/2</sub>) results in ln <i>I</i><sub>520</sub>/<i>I</i><sub>540</sub> = 3.88–1086.83/<i>T</i> and Δ<i>E</i>∼ 755.34 cm<sup>–1</sup>. Results revealed Δ<i>T</i> = 0.4 K at 297 K, and\nmaximum relative sensitivity (<i>S</i><sub>r</sub>) and\nabsolute sensitivity (<i>S</i><sub>a</sub>) values are 1.23%\nK<sup>–1</sup> (at 297K) and 0.173% K<sup>–1</sup> (at\n397K), respectively. The energy transfer and thermal quenching process\nwere demonstrated mechanistically. The Commission Internationale de\nl’Éclairage (CIE) chromaticity diagram confirms the\nsuitability of this material as a green light emitter. The unprecedented\nphotostability and photoswitching behavior has been addressed by a\nphotoelectrochemical cell experiment under light and dark conditions.\nResults suggest that Yb<sup>3+</sup>,Er<sup>3+</sup>-activated NaBiF<sub>4</sub> is the most promising high-quality luminescent nanophosphor,\nreflecting the realm of scientific evolution and providing applications\nin luminescent thermometers, plasmon-enhanced UC luminescence, solid-state\nlighting, and fast switching photodetectors.