Enhanced multimodal behaviour of Tm3+/Yb3+ co-doped YTaO4 ceramic material through Bi3+ activation and sensitization: Application as a spectral converter

Abstract

The multimodal [Upconversion (UC), Downshifting (DS) and Quantum cutting (QC)] behaviour of Tm3+/Yb3+ co-doped YTaO4 ceramic material is investigated extensively. The samples are prepared by solid-state reaction method at two different temperatures (at 1573 and 1723 K) and their structural and optical properties are compared with each other. The effect of synthesis temperature as well as doping, on crystal phase was analysed via X-ray diffraction measurements. Particles shape and size are investigated through scanning electron microscopy images. The structural behaviour is further verified through vibrational structural study of material via FTIR measurements. This shows similar structural changes as observed in XRD patterns of the samples prepared at two different temperatures. The optical band gap of YTaO4 is nearly 4.8 eV at both the temperatures. Intense blue (476 nm), red (648 nm) and NIR (802 nm) peaks are observed in UC emission spectra of Tm3+/Yb3+ co-doped YTaO4 ceramic on excitation with 980 nm. The blue UC emission is suitable for display devices. The UC emission intensity increases significantly when sample is prepared at higher temperature (1723 K). Addition of Bi3+ ion further enhances the overall UC emission intensity due to change in environment around the activator ion. On excitation with UV radiation (λExc = 284 nm), YTaO4: 1 Tm3+ produces intense DS emission at 454 nm (blue region) due to energy transfer from charge transfer band of [TaO4]3- to Tm3+ ion. The sample synthesized at 1723 K shows better DS emission. One interesting result is that addition of Bi3+ ion enhances the DS emission due to formation of metal-metal charge transfer band. We have also observed the quantum cutting phenomenon in which self-activated blue photon is converted into two or more NIR photons. The efficiency of quantum cutting emission increases significantly in presence of Bi3+ ion and shows strong, intense NIR emission through CET. Thus, the multimodal behaviour of Tm3+/Yb3+/Bi3+ doped YTaO4 makes them a suitable candidate for a spectral converter that can be used in display and solar cell devices.