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 dynamics 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 F-4(9/2) -> H-6(15/2) transition of Dy3+ ions, a yellowish-green (575 nm) emission from the F-4(9/2) -> H-6(13/2) transition of Dy3+ ions and a red (612 nm) emission from the D-5(0) -> F-7(2) 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 H-4(9/2) 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.