Monoclinic to cubic structural transformation, local electronic structure, and luminescence properties of Eu-doped HfO2

Abstract

We report the photoluminescence (PL) properties of Eu3+ doped HfO2 nanoparticles prepared using co-precipitation method and annealed at 600 degrees C. X-ray diffraction results revealed the monoclinic phase in undoped HfO2 and show mixed phase formation at lower concentration and a dominant cubic phase achieved at 5 mol% doping of Eu in HfO2. The phase transition anticipated by the density functional theory is in excellent agreement with experimental findings. The oxygen K-edge XANES spectra clearly depicts the diverse hybridization of O 2p orbitals in M-O7 (for monoclinic) and M-O8 (for cubic) polyhedra of HfO2. Hf L-edge XANES confirms Hf4+ ions in cubic and monoclinic structured HfO2. The Eu3+ ions are dominantly present in the Eu-doped HfO2 nanoparticles. PL study demonstrates the emission in red region with high color purity under different excitation wavelengths from near UV to blue light. PL emission spectra show four emission bands at 594 nm, 609 nm, 650 nm, and 716 nm corresponding to 4f-4f transitions of Eu3+ under excitation wavelengths of 361 nm, 383 nm, 394 nm and 465 nm. The reddish PL emission with high color purity under different excitation wavelengths from near-UV to blue region may be exploited in solid state lighting-based applications.