Synthesis And Luminescence Properties Of Rare Earth Ions Doped SrLu₂O₄ Upconversion Phosphors

It is well known that upconversion luminescence?UCL?,via which the long-wavelength?near-infrared or infrared?pump sources are converted to short-wavelength?ultraviolet or visible?radiation lights by means of energy-transfer?ET?processes and multiphoton absorption,has attracted much attention.The oxide and complex oxide have been widely investigated due to their good physical and chemical stability,high mechanical strength and laser resistance threshold.The SrLu₂O₄,belonges to alkaline lutetium,has many advantages,such as relative lower phonon energy,good physical and chemical stability and simplicity in preparation process.Therefore,it is a kind of potential materials on the UCL.In addition,the UCL of SrLu₂O₄ as a matrix has not been reported yet.In this paper,SrLu₂O₄ was used as the matrix material,Er³⁺/Tm³⁺/Ho³⁺as the activator and Yb³⁺as the sensitizer ion,studied the UCL of various RE³⁺doped or co-doped SrLu₂O₄ matrix for the first time.By simple Li⁺doping,the UCL intensity of the material is promoted and enhancement mechanism is discussed.Based on the investigation of the change of Red/Green ratio and ET mechanism,controlling the the light output of upconversion emission,finally,obtaining the white UCL in a single matrix.The main work and results are as follows:?1?A series of SrLu_2-x-yO₄:xHo³⁺,yYb³⁺upconversion phosphors were synthesized by solid-state reaction at different sintering temperatures.Upon the excitation of 980 nm LD,the samples exhibit bright green emission?541 nm?and weak red emission?673 nm?,which were assigned to the ⁵F₄,⁵S₂?⁵I₈ and ⁵F₅?⁵I₈ of Ho³⁺ion,respectively.The UCL processes can be ascribed to two-photon absorption process according to the pump-power dependence.The doping concentrations and sintering temperature are optimized to be x=0.01,y=0.5 and T=1400?.At high Ho³⁺concentrations?x>0.01?,the CR process[⁵F₄,⁵S₂(Ho³⁺)+⁵I₇(Ho³⁺)?⁵F₅(Ho³⁺)+⁵I₆(Ho³⁺)]acts as an important role.The BET[⁵F₄,⁵S₂(Ho³⁺)+²F_7/2(Yb³⁺)?⁵I₆(Ho³⁺)+²F_5/2(Yb³⁺)]and ET processes[⁵I₇(Ho³⁺)+²F_5/2(Yb³⁺)?⁵F₅(Ho³⁺)+²F_7/2(Yb³⁺)]become dominant with increasing the Yb³⁺concentrations.Under different pump-power excitation,the UCL can be finely tuned from yellow-green?chromaticity coordinate?x=0.3945,y=0.5923??to green light?x=0.3283,y=0.6627?.?2?Based on the optimum concentration SrLu_1.49O₄:0.01Ho³⁺,0.5Yb³⁺phosphor,a series of SrLu_1.49-xO₄:0.01Ho³⁺,0.5Yb³⁺,xLi⁺?x=0⁰.12?phosphors were synthesized by Li⁺doping.The samples maintain the primary orthorhombic structure by co-doping Li⁺ions.The Li⁺doping concentrations are optimized to be x=0.04,the upconversion green and red emission intensities were drastically increased by a factor of 7.7 and 7.2,respectively.At relatively lower Li⁺doping content,the Li⁺ions could be introduced into the host lattice by substituting the Lu³⁺sites,increasing the doing concentration,the extra Li⁺could occupy the interstitial sites between Ho³⁺,Yb³⁺and Lu³⁺ions.It is noteworthy that both the substitutional doping and interstitial doping would modify the local crystal field around Ho³⁺ions and reduce crystal lattice symmetry in the host lattice,thereby giving rise to increase UCL intensity.Meantime,due to the charge non-equivalence substitution between Li⁺and Lu³⁺ions,the introduction of Li⁺ions would naturally result in the creation of defective structure of oxygen vacancies to keep charge neutrally.In general,the oxygen vacancies can act as the upconversion sensitizer and leading to the enhancement of UCL efficiency.Compared with commercial NaYF₄:Er³⁺/Yb³⁺and common Y₂O₃:Ho³⁺/Yb³⁺as well as SrY₂O₄:Ho³⁺/Yb³⁺phosphors,the Ho³⁺/Yb³⁺/Li⁺co-doped SrLu₂O₄ samples show good color purity and high UCL intensity.?3?A series of SrLu_2-x-y-zO₄:xEr³⁺,yTm³⁺,zYb³⁺upconversion phosphors were synthesized by simple solid-state reaction method.Under 980 nm excitation,a bright white UCL of optimum sample is obtained.The red,green and blue?RGB?components of the white light come from the ⁴F_9/2?⁴I_15/2 transitions and ²H_11/2/⁴S_3/2?⁴I_15/2 transitions of Er³⁺ions as well as the ¹G₄?³H₆ transition of Tm³⁺ions,respectively.The intensity of the RGB UCL and the corresponding color coordinates can be precisely modulated by changing the Yb³⁺doping concentration and the pump power.The calculated color coordinate of SrLu₂O₄:0.5%Er³⁺,0.5%Tm³⁺,20%Yb³⁺sample under various pump power fall exactly within the white region,in which?0.3336,0.3244?under 630 mW is very close to the standard for equal energy white light illumination?0.333,0.333?.Based on the analysis of pump power dependence and luminescence decay,the energy transfer processes of Yb³⁺?Er³⁺,Yb³⁺?Tm³⁺and Tm³⁺?Er³⁺are proven to work simultaneously in Er³⁺/Tm³⁺/Yb³⁺tri-doped system and produce the RGB UCL by means of two-or three-photon processes.Due to the color controllable and bright white UCL,it is expected to apply Yb³⁺,Er³⁺and Tm³⁺tri-doped SrLu₂O₄ phosphor in the fields of displays,backlights and white light sources.