White light emission from co-doped La2Hf2O7 nanoparticles with suppressed host → Eu3+ energy transfer via a U6+ co-dopant

Abstract

Controlled energy transfer has been found to be one of the most effective ways of designing tunable and white photoluminescent phosphors. Utilizing host emission to achieve the same would lead to a new dimension in the design strategy for novel luminescent materials in solid state lighting and display devices. In this work, we have achieved controlled energy transfer by suppressing the host to dopant energy transfer in La₂Hf₂O₇:Eu³⁺ nanoparticles (NPs) by co-doping with uranium ions. Uranium acts as a barrier between the oxygen vacancies of the La₂Hf₂O₇ host and Eu³⁺ doping ions to increase their separation and reduce the non-radiative energy transfer between them. Density functional theory (DFT) calculations of defect formation energy showed that the Eu³⁺ dopant occupies the La³⁺ site and the uranium ion occupies the Hf⁴⁺ site. Co-doping the La₂Hf₂O₇:Eu³⁺ NPs with uranium ions creates negatively charged lanthanum and hafnium vacancies making the system highly electron rich. Formation of cation vacancies is expected to compensate the excess charge in the U and Eu co-doped La₂Hf₂O₇ NPs suppressing the formation of oxygen vacancies. This work shows how one can utilize the full color gamut in the La₂Hf₂O₇:Eu³⁺,U⁶⁺ NPs with blue, green and red emissions from the host, uranium and europium, respectively, to produce near perfect white light emission.