Deep-red to near-infrared luminescence from Eu-trapped exciton states in YSiON.

The valence state of Eu ions doped in inorganic compounds is easily influenced by the synthesizing conditions. In this study, X-ray absorption spectroscopy revealed that almost half of Eu ions incorporated in the YSiON host were reduced into the divalent state through the sintering process at 1600 °C under a N gas atmosphere without any annealing processes. The prepared Eu-doped YSiON sample showed anomalous deep-red to near-infrared luminescence below 300 K under violet light illumination, whose luminescent properties are discussed through detailed spectroscopic analyses. In the photoluminescence spectra at 4 K, the broad luminescence band ranging from 550 to 1100 nm with a large Stokes shift of 5677 cm was observed, assigned to the recombination emission related to the Eu-trapped exciton state. The temperature dependence of luminescence lifetime suggests that the thermal quenching of Eu-trapped exciton luminescence takes place through complicated processes in addition to thermal ionization. The energy diagrams based on the spectroscopic results indicate that Eu-trapped exciton luminescence in the YSiON:Eu sample was observed because all the Eu: 5d excited levels are degenerated with the host conduction band, and the relatively stable Eu-trapped exciton state in the Y sites is formed just below the conduction band bottom. A comprehensive discussion on the deep-red to near-infrared luminescence in the YSiON host could give new insights into the mechanism of Eu-trapped exciton luminescence in Y sites, which has potential in near-infrared emitting devices.