Photochromic Properties And Luminescent Modulation Of Rare Earth Modified Potassium Sodium Niobate Based Materials

Rare earth doped photochromic materials reveal great potential applications in luminescent switch and optical data storage devices due to their excellent reversible luminescence modulation behavior.However,insensitivity to visible light,low luminescence modulation rate and a single-waveband optical information readout are the common issues of photochromic materials with luminescence modulation behavior.Here,single rare earth ions Eu³⁺and Tb³⁺with characteristic red and green luminescence as activators were introduced into potassium sodium niobate（KNN）photochromic matrix to achieve effective luminescence modulation and significantly improve luminescence modulation rate.Meanwhile,the tunable multi-band luminescence emission is successfully realized by Tb³⁺/Eu³⁺co-doping,which further broaden the controllable multicolor luminescence readout in the process of optical information readout.The mechanism of photochromic and luminescence modulation was deeper explored by adjusting the ratio of K/Na in the KNN matrix.Finally,the optimal composition of photochromic ceramic is selected to prepare flexible and transparent photochromic thin films realizing the prototype optical storage devices with integrated functions,which laid a foundation for the research and development of high-power optical information storage devices in the future.In order to realize the luminescence modulation irradiated by visible light and improve the luminescence modulation rate,Eu³⁺and Tb³⁺were introduced into the KNN photochromic matrix respectively.KNN:x Eu³⁺ceramics can achieve strong red light emission with good color purity effectively excited by the blue light with the wavelength of 465nm,so it can well compatible with commercial blue light LED chips.The obvious photochromic phenomenon（from Light green to dark gray）and luminescence quenching occur,after the irradiation of 407nm visible light,phenomena.Under 10 min of thermal stimulation at 230℃,the color and luminescence intensity of ceramics almost recover to the initial state,showing reversible luminescence modulation behavior,and the maximum luminescence modulation rate reached 84%.KNN:x Tb³⁺ceramics has high optical transmittance.The maximum optical transmittance is 67%in visible light.Concurrently,the addition of Tb³⁺endows KNN ceramics excellent luminescent properties,which can achieve characteristic green light emission.Compared with KNN:x Eu³⁺ceramics,KNN:x Tb³⁺photochromic ceramics have higher luminescence modulation rate and faster light response rate.The photochromic phenomenon is obviously observed,after the irradiation of 407nm visible light for only 20 s,the color of the ceramics changed from light yellow to dark gray,and the maximum luminescence modulation rate is up to 92.67%.After thermal treatment at 150℃for 10s,the color of the ceramics can be quickly restored to the original state.The luminescence modulation mechanism based on the photochromic reaction is related to the defect related vacancy:（1）The inevitable alkali element volatilization after high temperature sintering and the A-site substitution of rare earth ions induce the the cation vacancies(V_Na’ and V_K’)and the corresponding oxygen vacancies(V_O^··).These vacancies will capture some photogenerated electrons under visible light irradiation to form the color centers;（2）When electrons in rare earth ions are excited from the ground state to the excited state,phonon relaxation is prohibited due to the existence of the color centers,the transition of electrons from the excited state to the ground state is effectively restricted,and electron energy is absorbed by the color centers,resulting in a significant decrease in luminescence emission intensity.Based on the principle of energy transfer,the photochromic ceramics of KNN:Tb/x Eu are prepared by Eu³⁺and Tb³⁺co-doping,which realized multi-color emission and multi-mode luminescence modulation.Under the excitation of blue light with a wavelength of 488 nm,strong emission peaks appear in the green region at 546nm and the red region at 615 nm,and the multicolor luminescence can be regulated by changing the doping concentration of Eu³⁺and the excitation wavelength.Under the irradiation of 488 nm,the color of the light emitted by ceramics changes from green to yellow,finally turned orange-red with the increase of Eu³⁺concentration.Under the irradiation of visible light with the wavelength of 407nm,the color of ceramics changes obviously from white to dark gray.The emission intensity of red and green decreases to different degrees caused by photochromic reaction.When x=0.016,luminescence modulation rate of red and green areΔR_Igreen=66.88%and R_Ired=69.77,respectively.KNN:Tb/x Eu ceramics exhibit excellent optical transparent（48%in visible light region and61%in IR region）.The luminescence modulation ratio shows strong dependence on excitation wavelength and Eu³⁺contents,and optical transparent can also realize an effective reversible modulation based on photochromic reaction.According to the principle of energy transfer,the energy transfer from Tb³⁺to Eu³⁺forming two luminescent centers,then part of the energy is absorbed by color centers,which is the source of the multicolor emission and multimode luminescent modulation.In order to explore the photochromic mechanism in depth,a series of K_xNa_1-xNb O₃:0.03Eu³⁺ceramics are prepared.The effects of different K/Na ratios on the luminescent modulation of K_xNa_1-xNb O₃:0.03Eu³⁺ceramics are studied.With increasing of K⁺concentration,the luminescence intensity of K_xNa_1-xNb O₃:0.03Eu³⁺ceramics is significantly reduced.The concentration of oxygen vacancy in K_xNa_1-xNb O₃:0.03Eu³⁺ceramics increase,and the luminescent modulation rates exhibit an increasing trend,indicating that the volatilization of K⁺plays a major role in the photochromic reaction process of rare earth doped KNN material.In order to meet the requirements of practical application,based on the photochromic mechanism of ceramic materials,a new type of KNN:Eu³⁺photochromic thin film was prepared by radio frequency（RF）magnetron sputtering.The KNN:Eu³⁺film deposited on mica substrate grows uniformly,forming a uniform perovskite structure.The thickness of thin film is about946 nm.KNN:Eu³⁺thin film has excellent luminescence properties and reversible luminescence modulation behavior,which is similar to KNN:Eu³⁺ceramics.The thin film has characteristic red emission exited by blue light with a wavelength of 465nm,which makes it a potential application for W-LEDs because their excitation bands can match well with commercial blue LED chips.Under light irradiation with a wavelength of 407nm,the luminescence intensity decreased significantly,and the luminescence modulation rate reached 37.65%.After the mechanical bend test with the tensile bending radius of about 5 mm for KNN:Eu³⁺film,the luminescence modulation rate have not been decreased significantly,illustrating that the film has good stability and bending resistance.It has a potential application in flexible and foldable optical memory storage devices.