Visible and ultraviolet upconversion and near infrared downconversion luminescence from lanthanide doped La2Zr2O7 nanoparticles

Single materials which have a multitude of photophysical processes by selective doping are in high demand in the community of material science due to their multifunctional applications. Upconversion (UC) and downconversion (DC) luminescence properties of lanthanide doped nanoparticles (NPs) are imperative for their broad application potentials. Here, we describe two series of either doubly or triply doped La2Zr2O7 NPs synthesized by a molten salt method. For the former, La2Zr2O7:Yb,Er NPs display bright red and moderate green UC (VUC) and NIR-B DC (NDC) at around 1550 nm, which are highly desirable for in-vivo bioimaging applications. For the latter, La2Zr2O7:Gd,Yb,Tm NPs demonstrate ultraviolet UC (UVUC) which can be exploited for water purification. In addition, we systematically investigated the effects of sensitizer doping level on the morphology, crystal structure and UC and DC emission intensity of these NPs. The processes involved in the VUC, NDC and UVUC emissions are evaluated in detail by pump power dependence studies which reveal that the VUC emissions are two-photon processes whereas the UVUC emission is a five-photon absorption process. The mechanisms of all these three luminescence processes have been extensively explained based on energy transfer and f-f transition processes. The idea provided in this work extends the knowledge on doping induced UC and DC luminescence in pyrochlore NPs which show multifunctionalities in light emission properties.