Synthesis And Investigation On Lanthanides Incorporated Tungstates And Vanadate Phosphors Materials

Upconversion luminescent materials have always been the most popular research in luminescence field.Different from the traditional fluorescent material,it can absorb near infrared light and convert low energy photon into high energy photon emitting visible or ultraviolet light.Comparing with the latter,it has better security,stronger photostablilty,and lower ultraviolet pollution.It is generally used in infrared imaging,biological probes,anti-counterfeiting identification,and cancer therapy,etc.Therefore,it is foreseeable that up-conversion luminescent materials will play an important role in the future application.In this work,we aim to develop a novel Lanthanides doped upconversion material,with excellent fluorescence property and potential commercial use.The two series of near-infrared excited UC phosphors were prepared by the traditional solid-state reaction method and their phase structures,luminescent properties were studied as follow.The main content will be detailed as follow:?1?In this work,we have successfully prepared Er³⁺,Yb³⁺,Nd³⁺co-doped LiGd?WO₄?₂?LGW?phosphor.The resultant XRD pattern shows that the samples are in a single tetragonal phase.Upon 980nm excitation system,the relation between Yb³⁺concentration and luminescent properties of Er³⁺/Yb³⁺co-doped phosphor were studied,and the optimal concentration of Yb³⁺is 0.2.Upon 808nm excitation system,we have studied the relation between concentration of dopant Yb³⁺,Nd³⁺and the luminescent properties of Er³⁺/Yb³⁺/Nd³⁺co-doped phosphor.The optimum concentratin of dopant Yb³⁺and Nd³⁺is0.2?0.035,respectively.?2?Besides,we have taken advantage of the phosphor LGW:Er³⁺,Yb³⁺,(Nd³⁺)for temperature-sensing sensors.In thermal equilibrium state,the distribution of electrons between the thermal-couple energy levels(²H_11/2 and ⁴S_3/2)follows the Boltman distribution law.The fluorescence intensity ratio?FIR?of the two spontaneous emission(²H_11/2?⁴I_15/2,⁴S_3/2?⁴I_15/2)is used to monitor the temperature change of the phosphor.The measured temperature of the sample is expressed by a function:lnFIR=C-ln??E/kT?.The abosolute sensitivity S is used to characterize the temperature sensing performance of the sample,according the formula S=dFIR/dT.In the temperature range of 202 K⁴27 K,LGW:Er³⁺,Yb³⁺phosphors were characterized to variable temperature spectroscopy(?_ex=980nm).The experimental results showed that the phosphor LGW:Er³⁺,Yb³⁺exhibited a strong temperature-sensing sensitivity in the range of 200⁷00K upon near-infrared excitation.The sensitivity of the LGW:Er³⁺,Yb³⁺(?_ex=980nm),its absolute sensitivity is maintained above0.006K^-1 in the range of 200⁷00K,with the maximum value of 0.0098 K^-1 at 427K.Meanwhile,under the excitation of 808nm,the temperature-varying spectra of the phosphors LGW:Er³⁺,Yb³⁺,Nd³⁺were measured,and the maximum sensitivity was found to be0.0057K^-1,whose value of absolute sensitivity maintains over 0.003K^-1 in range of 200 to700K.In conclusion,the Lanthanides doped LGW phosphor is an ideal material for optical temperature sensor.?3?Series phosphors of double tungstates crystal LiLn?WO₄?₂:Er³⁺,Yb³⁺,Nd³⁺have been prepared by high-temperature solid-state reaction method.In this work,the effect of different Ln cation in the host on the fluorescent intensity is studied.Through X-ray diffraction,diffuse reflection spectrum,photoluminescent spectrum and Raman spectrum,we have analysed the phase crystal,optical and luminescent properties and the distribution of vibrational peak of the samples.And the luminescence properties of these phosphors LiY?WO₄?₂:Er³⁺,Yb³⁺,Nd³⁺,LiLa?WO₄?₂:Er³⁺,Yb³⁺,Nd³⁺,LiGd?WO₄?₂:Er³⁺,Yb³⁺,Nd³⁺are compared.According to the photoluminescent spectrum,the intensity of the host materials is in the order of:I_Gd>I_La>I_Y.In this work,we calculate the integral area of the raman peak,and hence the relationship of phonon density is derived.To compare the value of the density of phonon states,the value of the integral area of Raman Peaks is in favor to explore the value of the density of phonon state.The intensity of the integral area of Raman Peaks of all samples is in the order:S_Y>S_La>S_Gd,corresponding to the phonon density:?_Y>?_La>?_Gd.It can be seen that even if the value of phonon energy of LGW,LLW,and LYW is appromimately equal,the luminsecent intensity of the three phosphors are different because of the different value of the density of phonon states.Based on the results,the density of phonon states decreases with the increase of the relative molecular mass of the cation Ln,which corresponds to the magnitude of luminescent intensity of I_Gd>I_La>I_Y.?4?Lanthanides doped GdVO₄?GVO?vanadate phosphors with different doping ratio of Tm³⁺/Ho³⁺were successfully prepared by high-temperature solid-state reaction method.The luminescence properties were characterized by photoluminescent spectrum,with excitation of 808nm or 980nm laser diode.Based on the spectroscopy spectrum upon near-infrared excitation,the photoluminescent spectrum exhibits a blue-green-red three-peak emission at482 nm,552 nm,and 656 nm.Upon 980nm excitation,the chromaticity coordinates of GVO:Tm³⁺,Ho³⁺,Yb³⁺can be adjusted by the change of doping ratio of Tm³⁺/Ho³⁺.When the doping ratio of Ho³⁺/Yb³⁺was 1:1,the white light emission intensity reaches the maximum value.Upon the 808 nm excitation,it is found that Nd³⁺/Yb³⁺can sensitize the upconversion luminescence intensity of Tm³⁺and Ho³⁺.Meanwhile,the Tm³⁺,Ho³⁺,Yb³⁺,Nd³⁺co-doped GVO phosphor can be used as a white phosphor.When the doping ratio of Tm³⁺/Ho³⁺/Nd³⁺is 0.5:0.8:1,the intensity of the white light emitting reaches the maximum value,and its color coordinates is approximately similar to the chromaticity coordinates of the ideal white emission.?5?A novel single phase Sr₂V₂O₇?SVO?:Ln(Ln=Eu³⁺,Dy³⁺,Sm³⁺,Tb³⁺)phosphors for white LEDs were prepared by the high-temperature solid-state reaction method under air atmosphere.The crystal structures,photoluminescence?PL?properties and energy transfer between the host and rare earth ions were investigated by X-ray diffraction?XRD?patterns and luminescence spectra.SVO phosphor is a self-activated phosphor under the excitation of near ultraviolet,emitting intense and broad yellow-greenish light upon the 350 nm excitation.It is concluded from the photoluminescence spectra that all samples doped with rare earth ion exhibited a broad near ultraviolet band due to charge transfer transition of the SVO host,indicating that there were energy transfer from the host SVO to Eu³⁺?Dy³⁺?Sm³⁺?Tb³⁺ions,respectively.The dopants Eu³⁺or Sm³⁺displayed characteristic narrow red emissions adjusting the CIE coordinates of SVO:Eu³⁺/Sm³⁺close to the white light region;SVO:Dy³⁺and SVO:Tb³⁺phosphors absorbed UV light effectively,indicating that the emission intensity was enhanced greatly by the incorporation of Dy³⁺or Tb³⁺into the SVO host.Furthermore,the energy transfers from VO3-4 to rare earth ions occurred mainly via dipole-dipole interactions according to Dexter's theory.The results shows that through doping rare-earth ions into SVO host,the luminescent intensity was enhanced,the CIE coordinates were adjusted and the correlated color temperature were reduced,demonstrating that phosphors SVO:Ln(Ln=Eu³⁺?Dy³⁺?Sm³⁺?Tb³⁺)phosphors are promising single phase converting phosphors for white LEDs with near UV chips.