Photon avalanche assisted upconversion via customizing the green emission.

The developed SnWO4 phosphors incorporated with Ho3+, Yb3+ and Mn4+ ions have been explored under 980 nm laser irradiation. The molar concentration of dopants has been optimized to 0.5 Ho3+, 3.0 Yb3+ and 5.0 Mn4+ in SnWO4 phosphors. The upconversion (UC) emission from the codoped SnWO4 phosphors has been substantially amplified up to 13 times and described based on the energy transfer and charge compensation. On incorporating the Mn4+ ions in the Ho3+/Yb3+ codoped system the sharp green luminescence shifted to reddish broadband emission due to the photon avalanche mechanism. The processes accountable for the concentration quenching have been described based on critical distance. The interaction responsible for the concentration quenching in Yb3+ sensitized Ho3+ and Ho3+/Mn4+:SnWO4 phosphors is considered to be dipole-quadrupole and exchange interaction type, respectively. The activation energy 0.19 eV has been determined, and the phenomenon of thermal quenching is discussed using a configuration coordinate diagram.

Strategies for designing low thermal quenching upconverting temperature sensors.

The Er3+/Yb3+:NaGd(WO4)2 phosphors and the phosphor-in-glass (PIG) have been synthesized employing a typical approach to investigate their structural, morphological and optical properties. Several PIG samples containing different amounts of NaGd(WO4)2 phosphor have been manufactured by sintering the phosphor and glass [TeO2-WO3-ZnO-TiO2] frit together at 550 °C, and its impact on the luminescence characteristics has been extensively studied. It has been noticed that the upconversion (UC) emission spectra of PIG under 980 nm excitation display similar characteristic emission peaks to the phosphors. The maximum absolute sensitivity of the phosphor and PIG is 17.3 × 10-3 K-1 @ 473 K and the maximum value of relative sensitivity is 10.0 × 10-3 K-1 @ 296 K and 10.7 × 10-3 K-1 @ 298 K, respectively. However, thermal resolution at room temperature has been improved in the case of PIG as compared to the NaGd(WO4)2 phosphor. As compared to the Er3+/Yb3+ codoped phosphor and glass, the less thermal quenching of luminescence has been observed in PIG.

Simultaneous effects of synthesis temperature and dopants on MgWO4UC phosphors.

A sequence of coactivated divalent-metal tungstate Er3+/Yb3+/Mn4+: MgWO4phosphors have been successfully developed to study the effect of synthesis temperature on the crystal structure, surface morphology, fluorescence, temperature sensing and the dynamics involved in the processes. Upconversion (UC) intensity of the Er3+/Yb3+: MgWO4phosphors increased by ∼109 and ∼778 times on increasing the synthesis temperature from 800 °C to 1000 °C and 1200 °C. UC intensity of the Er3+/Yb3+/Mn4+: MgWO4phosphors has been significantly improved up to ∼90 times via charge compensation. The incorporation of Mn4+in the Er3+/Yb3+codoped crystal system shifted the UC spectra from sharp green peaks to broadband emission along with amended sensing abilities. The ratiometric techniques of thermally coupled stark sublevels of the Er3+have been used to achieve a wide temperature range (300-623 K). The prepared nanophosphors show maximum absolute & relative sensitivities ∼25.86 × 10-3K-1@453 K and ∼10.39 × 10-3K-1@303 K respectively with an accuracy of ±0.42 K@303 K.