Interparticle energy transfer between Europium doped and Terbium doped NaYF4 nanocrystals

Abstract

Development and improvement of white light emitting diodes (LEDs) demand new materials and approaches of its application. Conventional LEDs based on blue LED and YAG:Ce phosphor lack red light emission making them cold white light emitters, which are not desired. Because of this, a search for red emitting components to implant into the blue LED; YAG:Ce system has been fuelled. A promising candidate with a characteristic narrow-band emission in red region is trivalent europium (Eu3+). However, Eu3+ has low absorption in the blue light region. A new approach to increase the excitation probability is to use sensitizer ions, which will absorb blue light and transfer the energy to the Eu3+ ion and enhance its red light output. In this thesis, research was focused on the use of interparticle energy transfer between differently doped nanomaterials to obtain sensitized Eu3+. This separation was done to avoid the quenching effects the lanthanides used as sensitizer create when doped in the same material. Therefore, spherical hexagonal β-phase NaYF4 nanoparticles with a size of 10 nm doped with Eu, Tb or Ce were synthesized. The optical measurements of excitation and emission spectra were performed to prove the energy transfer between Ce3+ or Tb3+ and Eu3+ in different NaYF4 nanoparticles takes place. We did not observe any evidence for energy transfer between Ce and Eu, as Eu3+ emission was observed when Ce3+ was not present when excited at 255nm(Ce3+ f-d transition) and did not increase upon addition of Ce3+ doped particles. For the Tb and Eu couple we did observe energy transfer. When exciting terbium (λ = 485nm) the 5D0-7F4 emission line of europium, around 695nm, appears. As Eu3+ does not show emission when excited with 485nm light, this is strong evidence for interparticle energy transfer. The observed energy transfer peak has an intensity of 5% compared to the 5D4-7F5 transition in Tb, which we like to improve. The increase of energy transfer can be achieved by modifying the surface of the nanocrystal, which contain organic ligands. When these ligands are not present, particles are able to come closer together and have more efficient energy transfer, as the energy transfer efficiency scales with 1/r6. For this modification the following methods were used: treatment with (NH4)2S, treatment with base piranha and treatment with hydrochloric acid. These methods decrease the overall intensity of the emission but give an increase of the interparticle energy transfer. The (NH4)2S, base piranha and hydrochloric acid treatment give a 15%, 270% and 280% increase of the energy transfer respectively.