Dependence of photoluminescence on doping concentration of Ho³⁺ in nanocrystalline La(OH)₃

Pure and Ho³⁺ doped La(OH)₃ phosphor nanoparticles have been synthesized through auto-combustion method. XRD patterns exhibited sharp peaks indicating the formation of highly crystalline hexagonal lattice with cell constants a = 6.5218 Ǻ and c = 3.8460 Ǻ. The dissolution of Ho³⁺ ions within La(OH)₃ host lattice resulted increase in lattice parameters and unit cell volume. Optical properties have been altered with Ho³⁺ doping; the band gap increases from 4.25 eV for pure up to 5.48 eV for 10 wt% Ho doped samples. Green emission around 495, 506, 520, 535 and 544 nm corresponded to (5F4, 5S2) → 5I8 transition, in which the excited electrons nonradioactively transferred to the level 5F5 and then radiate the energy on transferring to the ground state. Observed intense green emission is due to participation of more number of Ho³⁺ ions in the excitation process and the populations of the ions in the excited states. The CIE coordinate values confirm that Ho-doped La(OH)₃ phosphors are relatively useful for green emission photonic devices. The enhancement in the lifetime of 5F4/5S2 level of Ho³⁺ ion is a clear evidence for an increase in the emission intensity. The improvement in the crystallinity has resulted in an increase in the lifetime of the emitting levels, and thereby an enhancement of the quick excitation, energy transfer and radiative transitions.