Synthesis And Luminescence Properties Of ALa?MoO₄?₂?A=Li,Na,K? Luminescent Materials

In recent years,white light-emitting diodes?white LEDs?have attracted a lot of attention because of their high luminescence efficiency,long lifetime,environmental-friendly,energy savings and high reliability,which are considered as the next generation of light source.Nowadays,there are two methods for obtaining white LEDs,One of the methods is to combine blue chips?GaN?with yellow phosphors(YAG:Ce³⁺).However,the white LED based on YAG:Ce³⁺phosphor possesses high thermal quenching and low color rendering index?CRI?owing to lacking a red component.Another way for obtaining white LED is to use ultraviolet?UV?-LED chips to excite tricolor?red,green,and blue?phosphors.At present,the commercially used red phosphor(Y2O2S:Eu³⁺)is chemically unstable,and can not satisfy the requirements.Therefore,it is significant to search for a more stable red-emitting phosphor.Double molybdates ARE?MoO₄?₂?A=an alkali mental ion;RE=a trivalent rare-earth ion?,which share a scheelite-type structure,are promising host materials due to their excellent luminescence properties and physical-chemical stability.In this paper,we synthesized ALa?MoO₄?₂ phosphors by hydrothermal method and solid-state reaction.X-ray diffractometer?XRD?,Scanning electronic microscope?SEM?,Fluorescence spectrophotometer?PL?,Fourier Transform Infrared spectrometer?FT-IR?,and double beam spectrophotometer were employed to characterize the crystal structure and photoluminescence properties of the as-prepared samples.Main results are as follows:?1?KLa?MoO₄?₂:Eu³⁺ red emitting phosphors were prepared by hydrothermal process.The morphology of the phosphors were depend on the concentration of Eu³⁺.The photoluminescence results showed that the samples could be excited by 288 nm,394 nm,and 465 nm,respectively,and show a strong red emission at about 618 nm.The optimal doping concentration of Eu³⁺ was 15 mol.%.According to Dexter's theory,the mechanism of concentration quenching was discussed.?2?A series of Bi³⁺-Eu³⁺ co-doped KLa?MoO₄?₂ red emitting phosphors were synthesized by hydrothermal method.The UV-vis absorption spectra,PLE and PL spectra demonstrated that the Bi³⁺ could absorbed the energy and transfer it to Eu³⁺.That is,the red light was significantly enhanced by adding Bi³⁺,which was attributed to the sensitization of Eu³⁺ by Bi³⁺.The optimal doping concentration of Bi³⁺ was 0.3 mol.%.The energy transfer mechanism in KLa?MoO₄?₂:Eu³⁺,Bi³⁺ was discussed.Besides,the lifetime and chromaticity coordinates of single-doped Eu³⁺ and Eu³⁺-Bi³⁺ co-doped products were compared.The results suggested that KLa?MoO₄?₂:Eu³⁺,Bi³⁺ is a promising phosphor for white LEDs.?3?A series of Dy³⁺/Eu³⁺ co-doped KLa?MoO₄?₂ phosphors were synthesized by a hydrothermal method.Photoluminescence results suggested that KLa?MoO₄?₂:Dy³⁺ exhibits yellow peaks at 576 nm.The optimal doping concentration of Dy³⁺ were about 7 mol.%.The excitation spectra obtained by monitoring the emission of Dy³⁺ at 576 nm,and the emission of Eu³⁺ at 618 nm,respectively,suggested that the Dy³⁺ could transfer energy to Eu³⁺.KLa?MoO₄?₂:Dy³⁺,Eu³⁺ samples showed emission peaks of both Dy³⁺ and Eu³⁺under 389 nm excitation.Furthermore,according to Dexter's theory,the mechanism for energy transfer between Dy³⁺ and Eu³⁺ was discussed.Finally,the multicolor luminescence properties were discussed.?4?KLa?MoO₄?₂:Eu³⁺ red emitting phosphors were synthesized by an EDTA-assisted hydrothermal method.XRD and SEM results demonstrated that the amounts of EDTA and the pH values of precursor solution have crucial influences on the structure,morphology and size of the obtained samples,respectively.The FT-IR results suggested that there exist residual EDTA in as-prepared samples,and it will cause the negative influence to the luminescence properties of samples.The emission intensities were enhanced upon calcination.The photoluminescence results showed that under 465 nm excitation,KLa?MoO₄?₂:Eu³⁺ samples show red emissions at 618 nm.All results indicated that the as-annealed samples may have promising applications in WLEDs.?5?Eu³⁺-doped NaLa?MoO₄?₂ red phosphors were prepared via a hydrothermal process.The SEM results suggested that the morphology was mainly depend on Eu³⁺ doping concentration.The as-prepared phosphor could be effectively excited by 280 nm,395nm and 465 nm and exhibited strongluminescence emissions of Eu³⁺ at 617 nm.The optimum Eu³⁺ doping concentration for obtaining maximum emission intensity was confirmed to be 15 mol.%.The mechanism of concentration quenching was discussed.?6?LiLa?MoO₄?₂:Eu³⁺ samples were synthesized by a traditional solid-state method.The obtained phosphors prepared with different doping concentrations were pure tetragonal phase of LiLa?MoO₄?₂.In order to obtain pure phase,the calcinations temperature must be controlled higher than 700 ?.The optimal calcinations temperature was 800 ? for obtaining excellent luminescence properties.Under 465 nm excitation,as-prepared samples showed red emissions at 618 nm,and the optimal doping concentration was 17 mol.%.