NIR photon downshifting via host sensitization in Yb3+-doped La2Ti2O7 and BaTi(PO4)2

Abstract

La2-xYbxTi2O7 (x = 0.0, 0.01, 0.02, 0.04, 0.06 and 0.08) and Ba1-yYbyTi(PO4)2 (y = 0.0, 0.01, 0.02, 0.04, 0.06 and 0.08) ceramic powders were synthesized by the polymeric complex and solid state methods, respectively. The crystal structure and optical properties of the powders were investigated by X-ray diffraction (XRD), diffuse reflectance, and photoluminescence spectroscopies. From the XRD results it was possible to observe that all the samples exhibit a monoclinic structure. One the other hand, the diffuse reflectance spectra show a broadband below 400 nm ascribed to the ligand-to-metal charge transfer (LMCT) O2− → Ti4+ fundamental state. Near infrared (NIR) photon downshifting via host sensitization in both Yb3+-doped solid solutions was observed. The La2Ti2O7 and BaTi(PO4)2 were chosen by their highest self-trapped exciton, which could increase the sensitization of the Yb3+ emission. However, the localization of the exciton does not play an important role in this process, since the level of localization is similar, but the integrated emission intensity is quite different for Yb3+ in both solid solutions. The undoped compounds can be excited by UV light and they present a broad visible-NIR emission (centered at 850 nm) which may be attributed to a lower electronic energy state of the exciton (reverse LMCT O2− → Ti4+ transition), this emission is reduced with the increasing of the Yb3+ amount. It provides evidence about the energy transfer process from the lower electronic energy state of the exciton to the Yb3+ ion. Photoluminescence emission and diffuse reflectance spectra in the visible and NIR, reveal an emission band that overlaps with the absorption band of the Yb3+, explaining the energy transfer process from the host to Yb3+.