Achieving blue emission via f→d transition from pyrochlore Eu2+ and Ce3+-doped La2Zr2O7 nanoparticles

Abstract

Solid state lighting has been fetching the world attention in order to solve the global problem of energy crisis. In this context, phosphor-converted light-emitting diodes (pc-LEDs) are in high demand owing to their energy efficiency, longevity, and non-toxicity. In this work, we have taken an initiative of designing pyrochlore-based blue nanophosphors by doping La2Zr2O7 nanoparticles (NPs) with divalent Eu2+ and trivalent Ce3+ ions. The structure, size, morphology, and optical properties of La2Zr2O7:Eu2+ (LZOE) and La2Zr2O7:Ce3+ (LZOC) NPs have been characterized using X-ray diffraction, Raman spectroscopy, field emission scanning electron microscope, and photoluminescence spectroscopy. Both LZOE and LZOC NPs have ordered pyrochlore structure with La3+ and Zr4+ sites occupied by the dopants to form emission centers. Due to the occupancy of Eu2+ dopant at La3+ and Zr4+ sites with different crystalline fields, the LZOE NPs display (i) dual excitation bands at 310 and 265 nm, (ii) dual blue emission bands at 420 nm (weak peak due to Eu2+@Zr4+ as Eu2 site) and at 485 nm (intense peak due to Eu2+@La3+ as Eu1 site), and (iii) double decay times of 2.32 and 1.56 μs at Eu1 and Eu2 centers. On the other hand, the LZOC NPs display (i) excitation bands at 267 and 274 nm triggered by crystal field splitting of Ce3+ 5d orbitals from the ground state and (ii) emission doublet due to the transition of electrons from 5d level to 2F5/2 and 2F7/2 states of the 4f1 ground state of Ce3+ dopant. The color coordination diagrams of the LZOE and LZOC NPs suggest that they are potential blue emitting phosphors for pc-LED applications.