Electroluminescent properties of a device based on terbium-doped ZnS nanocrystals

Rare earth terbium (Tb)-doped zinc sulfide (ZnS) nanocrystals (NCs) sized between 3 and 4 nm were synthesized via a co-precipitation reaction of precursors, zinc acetate (Zn(CH3COO)2), terbium chloride (TbCl3 * 6H2O), lithium fluoride (LiF) and thiocarbamide in a methacrylic acid/citric acid/methanol mixing solution. The NCs were characterized by means of x-ray powder diffraction, a transmission electron microscope and a fluorescence spectrophotometer. Electroluminescent (EL) properties of the device having a hybrid organic/inorganic multilayer structure with ITO/(poly(3, 4-ethylene dioxythiophene):poly(styrene sulfonate (PEDOT-PSS)(70 nm)/poly(vinylcobarzale)(PVK)(100 nm)/ZnS:Tb NCs(120 nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP)(30 nm)/LiF(1 nm)/Al(100 nm) were studied. Injecting electrons and holes were recombined to form excitons which were confined within the ZnS:Tb NC host and then the exciton energy was transferred to the Tb3+ centre, and finally the green emission of the Tb3+ ion was observed. The four emissions which peaked at 430, 491, 546 and 577 nm were attributed to the electronic transitions of the ZnS host and the 5D4 - 7F6, 5D4 - 7F5 and 5D4 - 7F4 of the Tb3+ centre, respectively, when the EL device was driven at 10 V. The maximum luminance of the ZnS:Tb NCs-based device is about 15 cd m-2 at 25 V driving voltage.