The Research On Synthesis And Spectroscopic Properties Of Rare Earth Ions Doped And Cr³⁺ Doped Luminescent Materials

Inorganic luminescent materials are widely used in various fields of our life,such as solid-state lasers,scintillators,WLEDs and biomedical,etc.Among them,inorganic luminescent materials doped with rare earth ions and NIR long persistent luminescent materials doped with Cr³⁺ion are particularly prominent.Rare earth ions have abundant energy levels and three electron transition ways including f-f,f-d and charge transfer in the crystal,therefore,luminescent materials activated by rare earth ions have excitation and emission spectra covered with ultraviolet,visible,infrared light and near infrared light,and according to their different luminescent mechanism,they can be divided into three kinds:down-conversion luminescence materials,up-conversion luminescence materials and long persistent luminescence materials.Meanwhile,due to the rapid development of life science in recent years,Cr³⁺ion-doped NIR long persistent luminescence materials have been widely applied in the field of biological imaging due to their unique near-infrared luminescence that can realize imaging or detection of deep tissue.The lumi nescence properties of rare earth ion doped luminescent materials and Cr³⁺ion-doped NIR long persistent luminescence materials not only depend highly on the structure and composition of matrix lattice,but also depend on the type of doped ions and the type of introduced homogeneous or heterogeneous ions.In this paper,we designed four kinds luminescent materials:down-conversion red phosphor Sr₂CeO₄:Eu³⁺excited by blue light,up-conversion phosphor based on NaBiF₄,long persistent nanomaterial of stronti um aluminate and NIR persistent material based on Cr³⁺doped gallium germanium system.Their structures and spectroscopical properties are characterized and related luminescence mechanism is deeply analyzed.The whole paper mainly contains the following fi ve parts.The first part mainly introduces the overview of luminescent materials,three categories of luminescent materials:the basic concepts of down-conversion luminescent materials,up-conversion luminescent materials and long-persistent luminescent materials,the luminescent mechanism and the main problems existing in the current research process,and summarizes the current conventional synthesis methods of luminescent materials and the main application fields.Combined with the problems existing in the development and application within the three kinds of luminescent materials,the design ideas and main research contents of this paper are proposed.The second part is the preparation and spectroscopical properties of Eu³⁺doped Sr₂CeO₄ red phosphor.In this chapter,with different Eu³⁺ion concentration,a series of red phosphors Sr₂CeO₄:xEu³⁺(x=0.15,1,2,5 mol%)were successfully prepared by traditional co-precipitation&hydrothermal method,and the excitation spectrum was regulated to visible region by co-doping with different bivalent metal ions M²⁺(M²⁺:Mg,Ca,Ba).All the samples were pure Sr₂CeO₄ phase,with regular morphology in rectangular shape.Absorption spectra show that there is an overlap between Ce⁴⁺-O^2-charge transfer energy band and Eu³⁺ion absorption band,which leads to energy transfer from matrix to Eu³⁺ion.The fluorescence intensity is the highest when the concentration of Eu³⁺is 2 mol%,and the energy transfer mechanism of Eu³⁺ion is dipole-dipole interaction.When the concentration of Eu³⁺ion is at 2 mol%,the introduction of metal ions M²⁺will interfere the matrix lattice field anisotropic to produce defects,promote the energy transfer between Ce⁴⁺-O^2-charge transition band and the Eu³⁺ion,thus enhancing the red fluorescence.The fluorescence intensity of the sample Sr₂CeO₄:2%Eu³⁺,2%Mg²⁺is the best.The CIE chroma indicates that the prepared Sr₂CeO₄:2%Eu³⁺,2%Mg²⁺phosphors are red warm light.The successfully prepared red phosphors Sr₂CeO₄:2%Eu³⁺,2%Mg²⁺provides a good choice for blue light-excited red phosphors that are scarce in the WLEDs field.The third part studies the different doping concentrations of Ce³⁺(0,2%,4%)regulate the fluorescence properties of two groups up-conversion luminescent materials NaBiF₄:Yb³⁺/Ho³⁺and NaBiF₄:Yb³⁺/Er³⁺.A series of Yb³⁺/Ho³⁺(Er³⁺)/Ce³⁺three-doped NaBiF₄ up-conversion luminescence materials were synthesized by one-step precipitation method at room temperature.The effect of Ce³⁺ion on crystal structure and upconversion luminescence properties were studied.XRD and IR spectra show that all samples form NaBiF₄ hexagonal phase succefully,which is uniformly dispersed as micron ball.XPS results show that all the rare earth ions,Yb³⁺,Ho³⁺,Er³⁺and Ce³⁺,all exist in the matrix materials at their stable valence states.Combined with their luminescence spectra,the possible emission mechanism and energy transfer process of up-conversion phosphors NaBiF₄:Yb³⁺/Ho³⁺(Er³⁺)/Ce³⁺were speculated,and the up-conversion luminescence process was further verified by the luminescent lifetime curve and the relationship between up-conversion luminescence intensity and pump power.Under the excitation of 980nm,the phosphor NaBiF₄:Yb³⁺/Ho³⁺/Ce³⁺is mainly performed the characteristic emission of the Ho³⁺ion:541nm and 647nm.The introduction of Ce³⁺does not change the emission spectra profile.With the increasing of Ce³⁺ion concentration,the corresponding red-green fluorescence ratio gradually increases.Since the ²F_7/2?²F_5/2 energy level transition of Ce³⁺ion promotes the ⁵F₅energy level state of Ho³⁺ion,the emission of red conversion fluorescence of Ho³⁺ion at 647nm is enhanced.The characteristic emission peaks of phosphor NaBiF₄:Yb³⁺/Er³⁺/Ce³⁺are mainly located at 523nm,543nm,and 668nm,corresponding to the Er³⁺ions energy level transition process of ²H_11/2?⁴I_15/2,⁴S_3/2?⁴I_15/2 and ⁴F_9/2?⁴I_15/2.Ce³⁺ion doping can promote the ⁴F_9/2 energy level state of Er³⁺and enhance the red up-conversion luminescence of Er³⁺ion.However,with the introduction of Ce³⁺ions,the green fluorescence lifetime and red fluorescence lifetime of the two groups of phosphors gradually decreased,indicating that the introduction of Ce³⁺ions influenced the upconversion efficiency and finally were released in the form of heat.In total,the introduction of Ce³⁺ions can effectively enhance the red luminescence of the NaBiF₄:Yb³⁺/Ho³⁺(Er³⁺)system,and the red upconversion phosphor NaBiF₄:Yb³⁺/Ho³⁺(Er³⁺)/Ce³⁺as a biological imaging probe has a potential application prospect.The fourth part introduce the optical properties of lon g afterglow nanomaterial Sr Al₂O₄:Eu²⁺,Dy³⁺prepared by pulsed laser liquid ablation method.In this chapter,high crystallized long persistent materials Sr Al₂O₄:Eu²⁺,Dy³⁺were firstly prepared by high temperature solid reaction method,and then strontium aluminate nanoparticles with long persistent luminescence,different morphology and size were obtained in different laser wavelengths(355nm,1064nm)and different reaction solvents(ethanol,acetone).The laser wavelength and the reaction solvent have n o effect on the crystal phase of the target materials,and both are Sr Al₂O₄ crystal phase.The prepared particles have two size distributions:micro-size and nano-size,which are mainly related to the crystal growth process when the laser reacted between t he material surface and the solvent interface during the laser liquid ablation process.In the same reaction solvent,the size of the target material increases with the increasing of the laser wavelength,the aggregation phenomenon happens under 1064nm,a nd when laser wavelength is the same,due to the polarity,viscosity and chain length of the anhydrous ethanol is higher than that of acetone,and eventually in acetone solution,the sample with much smaller size than that in anhydrous ethanol solution.Take the nanoparticles under 355nm laser wavelength for optical performance analysis,as a result of the Eu²⁺ion occupy two Sr²⁺(I,II)lattice,so under low temperature(20 K),the fluorescence spectra are located at two peaks:445nm and 520nm,while u nder high temperature(300 K),the radiation energy transition occur between two Sr(I,II)lattice,so only shows the peak at 520nm.Long persistent nanomaterials Sr Al ₂O₄:Eu²⁺,Dy³⁺also show certain afterglow performance,but their initial brightness is much weaker than that of traditional bulk materials.At the same time,the corresponding afterglow performance of sample prepared in acetone is also better than that in ethanol.It is speculated that the possible reason is that the small size samples in a cetone have more surface defect sites and high effective defect density,which make their afterglow storage capacity better.The nanomaterials Sr Al₂O₄:Eu²⁺,Dy³⁺with good long afterglow performance were successfully prepared by pulsed laser liquid ablation method,which provided favorable condition for the expansion of strontium aluminate long afterglow luminescent materials in the field of photoelectric devices or biomedical.The fifth part is to study the preparation and properties of the low energy excit ed NIR persistent phosphors Zn Ga₂O₄:Cr³⁺/Bi³⁺.In this chapter,NIR persistent phosphors Zn Ga₂O₄:Cr³⁺/Bi³⁺could be excited by low excitation energy through Sn⁴⁺ion doping,and are prepared by high temperature solid reaction method.The crystal structure shows that when the doping concentration of Sn⁴⁺increases from 0.03 to 0.30,part of Zn₂Sn O₄ phase appears in the crystal phase,indicating that Zn Ga ₂O₄-Zn₂Sn O₄composite spinel structure is formed after the introduction of Sn⁴⁺ions into Zn Ga₂O₄:Cr³⁺.At the same time,the excitation spectra have a red-shift.According to the excitation spectra,the Tanabe-Sugano map of d³ electron configuration of Cr³⁺in Zn Ga₂O₄-Zn₂Sn O₄ is obtained,and inferred that the introduction of Sn⁴⁺would cause variation in the crystal field strength.The curve of long afterglow excitation spectrum shows that the introduction of Sn⁴⁺ions promote the red shift of excitation spectrum in both low energy region and high energy region,and the red shift of excitation spectrum in low energy region indicates that this kind long afterglow luminescent materials can be excited by visible light.The emission spectra of the series NIR phosphors show the characteristic R band and N₂ band emission peaks of Cr³⁺ion,and the luminescence in the infrared region has a certain red shift.In addition,with the increasing of Sn⁴⁺ion concentration,the corresponding fluorescence intensity enhancement but the afterglow decay rate increases.Through the thermoluminescence curve and calculated the corresponding trap depth,the introduction of Sn⁴⁺ions change the crystal field strength or that can change the local electric field through the band gap and Cr³⁺ion ground state detrapping efficiency prompted thermoluminescence curve move to the low temperature area,at the same time the trap depth declined slightly,but the total storage capacity is increased,thereby increasing the long afterglow fluorescence decay rate.By adjusting the concentration of Sn⁴⁺ion,the NIR long afterglow phosphors could b e excited by visible light(low energy spectrum)and emit near-infrared or even deep red spectra.The successful preparation of low-energy excited NIR long afterglow luminescent materials could expand its further application in the field of biological imaging.