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 center dot 6H(2)O), 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)(70nm)/poly(vinylcobarzale)(PVK)(100nm)/ZnS:Tb NCs(120nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)(30nm)/LiF(1nm)/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 D-5(4)-F-7(6), D-5(4)-F-7(5) and D-5(4)-F-7(4) of the Tb3+ centre, respectively, when the EL device was driven at 10V. The maximum luminance of the ZnS: Tb NCs-based device is about 15 cd m(-2) at 25V driving voltage.