All Inorganic Zinc Halides As Efficient Phosphors For White LEDs

Metal halides,a new type of luminescent material,have attracted much interest because of their superior luminescence performance,simple preparation,and low cost.They are expected to be a new type of fluorescent material for white LEDs.Among them,the materials containing Pb have been extensively studied.However,due to the toxicity of lead,its commercial application has been greatly hindered.Therefore,lead-free halides have gradually attracted much attention,such as the Cu,Bi,Sb,Mn,Zn and other elements based compounds.So far,compared with other lead-free halides,the luminescence efficiency of Zn halides is relatively low（the maximum～69%）,which has attracted little attention.However,the unique advantages of Zn,such as its non-toxicity and high abundance,suggest that it is worth exploring this family.This study carried out the synthesis and properties of Cu⁺doped all-inorganic Cs₃ZnX₅（X=Cl,I）compounds.More details are included following:（1）Solid-state synthesis and luminescence properties of Cs₃Zn_1-xCu_xCl_5-x（x=0–0.15）:A series of Cu⁺doped Cs₃ZnCl₅ samples were synthesized by high-temperature solid-state reaction at 700℃.The purity,microstructure,elemental valence states,and luminescence properties of the materials were characterized by X-ray powder diffraction（XRD）,scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,and fluorescence spectrometry（PL）.When x=0.03,strong green emission with the highest photoluminescence quantum yield（PLQY）of 90.2%was achieved,which is the highest quantum efficiency among all reported Zn halides.The corresponding emission center is located at 480 nm,with an excitation center at 265 nm,and a Stokes shift of 215 cm^-1 is discovered.This indicates a self-trapped emission mechanism.In addition,the luminescence spectrum under X-ray excitation was measured,and the results show that the material is suitable as a scintillation material for high-resolution X-ray imaging.（2）Solid-state synthesis and luminescence properties of Cs₃Zn_1-xCu_xI_5-x（x=0–0.15）:A series of Cu⁺-doped Cs₃Zn I₅ samples were also synthesized at 700℃ using the high-temperature solid-state reaction method,and the purity,microscopic morphology,elemental valence state,and luminescence properties of the material were also characterized.When x=0.03,the corresponding luminescence center is located at 450 nm,the excitation center is at 310nm,and the Stokes shift is 140 cm^-1.Meanwhile,the luminescence spectrum under X-ray excitation is also tested,and the result shows that it is also suitable as a scintillation material for high-resolution X-ray imaging.Subsequently,by adjusting the ratio of synthesized Cs₃Zn_0.97Cu_0.03Cl_4.97 and Cs₃Zn_0.97Cu_0.03I_4.97 fluorescent powders and commercial red fluorescent powders(CaAlSiN₃:Eu²⁺),lead-free fluorescent powders with white luminescence were successfully obtained.The mixed fluorescent powders were uniformly coated on a 280nm UV chip,followed by packaging and curing.Finally,a white LED device with a high color rendering index（Ra=92）and low color temperature（CCT=4236 K）was successfully fabricated.The above results indicate that Zn halides,as a new type of lead-free halides,could also have excellent optoelectronic properties,which enables their applications in optoelectronic fields.They can meet the lighting needs in daily life and have promising applications in fields such as X-ray imaging and LED lighting.Zn halides are a family of lead-free halides which deserves more attention.