White Light Emission and Luminescence Dynamics in Eu³⁺/Dy³⁺ Codoped ZnO Nanocrystals.

In order to expand the use of ZnO in advanced display and lighting device applications, such as distinguishable emissive flat panel displays and liquid crystal display backlights, Eu³⁺/Dy³⁺-codoped ZnO nanocrystals were synthesized using a low temperature wet chemical doping technique and chemical surface modification. X-ray diffraction patterns revealed that co-doping Eu³⁺ and Dy³⁺ does not change the wurtzite structure of ZnO. A high-resolution TEM image showing obvious lattice fringes confirmed the high crystallinity of the nanosized sample. The luminescence and dynam- ics of Eu³⁺/Dy³⁺-codoped ZnO nanocrystals of various doping concentrations were studied under ultraviolet excitation. Excitation into the ZnO conduction band was also studied. ZnO doped with Eu³⁺ and Dy³⁺ ions exhibited a strong blue (483 nm) emission from the ⁴F₉/₂ --> ⁶H₁₅/₂ transition of Dy³⁺ ions, a yellowish-green (575 nm) emission from the ⁴F₉/₂ --> ⁶H₁₃/₂ transition of Dy³⁺ ions and a red (612 nm) emission from the ⁵D₀ --> ⁷F₂ transition of Eu³⁺ ions, without a defect background. Undoped ZnO emitted a broadband green light, demonstrating an efficient energy transfer from the ZnO host to the Eu³⁺ and Dy³⁺ ions. Moreover, energy transfer from the Eu³⁺ ions to the Dy³⁺ ions in the ZnO host was also observed by analyzing luminescence decay curves. The luminescence dynamics of the Eu³⁺/Dy³⁺-codped ZnO sample indicate that as the Eu³⁺ concentration increased, both the rise and the decay time constants of the ⁴H₉/₂ level of the Dy³⁺ ions became longer, while the decay time constants of the ⁵D₀ level of the Eu³⁺ ions became shorter, suggesting an energy transfer from the Eu³⁺ ions to the Dy³⁺ ions in the ZnO host. Furthermore, by adjusting the doping concentration ratio of Eu³⁺ and Dy³⁺ ions, the Eu³⁺/Dy³⁺-codoped ZnO phosphors emitted strong white luminescence with a high color purity and high color rendering index. The results indicate that the Eu³⁺/Dy³⁺-codoped ZnO phosphors are promising light-conversion materials, and have the potential to be used in field emission display devices and LCD backlights.