Preparation And Anti-counterfeiting Application Of Rare Earth Doped Multimodal Luminescence Materials

Rare earth luminescent materials play an important role in anti-counterfeiting field due to their excellent optical properties（such as color-tunable,diverse luminescence modes and long luminescence life,etc.）.For example,the printing of currencies,securities and bank checks,etc.all contain a series of rare earth luminescent materials.Unfortunately,many reported rare earth luminescent materials so far exhibit fixed excitation modes and single-color output,which leads to poor anti-counterfeiting effect and easy to be counterfeited.Therefore,it is of great significance to develop advanced rare earth luminescent materials for anti-counterfeiting applications.In view of the problems of rare earth luminescent materials in the field of anti-counterfeiting,we have designed and prepared three types of anti-counterfeit materials with different functions based on the characteristics of adjustable color,variable excitation mode,multimodal luminescence（photoluminescence,upconversion luminescence,photoexitation luminescence,afterglow,mechanical luminescence）and dynamic luminescence in this paper,and achieved five innovative achievements,successfully solving the shortcomings of rare earth light-emitting materials in the anti-counterfeit field.Specific research contents are as follows:（1）A series of multicolor long afterglow materials Na Ca₂Ge O₄F:x Tb³⁺（x=0.1%～0.8%）were prepared via high temperature solid phase method.the afterglow color of the material can change from blue to cyan and bright green with the difference of doping Tb³⁺ions concentration,and the longest afterglow time is 5.62 h,8.52 h and 7.14 h when the doping concentration is 0.1%,0.4%and 0.8%,respectively.Further studies show that this multicolor afterglow is not only related to the trapping depth and concentration in Na Ca₂Ge O₄F:Tb³⁺material,but also to the cross-relaxation phenomenon between the characteristic emission peaks of Tb³⁺ions.Finally,on the basis of multicolor afterglow performance,we fabricated the anti-counterfeiting pattern by screen printing technology,and realized the application of multicolor anti-counterfeiting.（2）Based on the idea of changing excitation wavelength to realize color tunable luminescence,we prepared a multimodal luminescence material La₄Ge O₈:Eu^2+/3+,Er³⁺by means of ion co-doping strategy to regulate the internal energy levels of La₄Ge O₈matrix.The material can be changed from red to purple,light blue and green under the excitation wavelength of 250 nm～380 nm.At the same time,the phosphor showed green upconversion emission when irradiated by a NIR laser（980 nm and 808 nm）.Finally,inspired by the multimodal luminescence feature,we designed an anti-counterfeiting application that depends on the excitation source to present different luminescence.The results show that the different luminescence colors can be easily detected using portable UV lamps and NIR（980 nm and 808 nm）lasers,and multimodal anti-counterfeiting applications are successfully realized.（3）We prepared a trimodal luminescence material Ba₂Ga₂Ge O₇:Pr³⁺,Er³⁺by introducing afterglow on the basis of color adjustable and upconversion luminescence.Firstly,Pr³⁺ions were introduced into Ba₂Ga₂Ge O₇ matrix for trap and energy level regulation to achieve photoluminescence and afterglow,and then Er³⁺ions were co-doped to regulate energy level in Ba₂Ga₂Ge O₇ matrix again to obtain color-tunable luminescence and upconversion luminescence.Therefore,the material can be changed from white to green when illuminated by portable ultraviolet lamps（254 nm and 365nm）,and a white afterglow can be produced after 254 nm UV illumination.In addition,the material produces green upconversion luminescence when illuminated by a 980 nm NIR laser.Finally,based on the trimodal luminescence characteristics,the anti-counterfeiting application was designed.The results show that the color change of the anti-counterfeiting pattern can be easily observed under the irradiation of a portable UV lamp and 980 nm laser,and the multimode anti-counterfeiting application is successfully realized.（4）On the basis of the third work,we continue to utilize rare earth ions to regulate energy levels and traps in La Ca Al₃O₇ matrix,and synthesize a highly integrated four-mode luminescence material La Ca Al₃O₇:Eu^2+/3+,Yb³⁺,Er³⁺.The material can change from purplish red to blue under the excitation light source of 250 nm～380 nm.When the excitation source is turned off,the material presents a blue afterglow.Moreover,it exhibits green upconversion luminescence when irradiated by a 980 nm NIR laser.In addition,when combined with PDMS,the material produces a waterproof,stretchable and flexible elastic film,and emits a blue mechanoluminescence when stretched or rubbed against a glass rod.Finally,inspired by the multimodal luminescence performance,the anti-counterfeiting application was designed.The results indicate that the color change of the anti-counterfeiting pattern can be easily observed by means of portable UV lamp,NIR laser irradiation and glass rod friction,thereby realize multimodal anti-counterfeiting application.（5）Guided by the design of dynamic luminescence,we prepared the multimodal dynamic afterglow material Li Ga O₂:Ce³⁺.Since Li Ga O₂ matrix can produce NIR luminescence and afterglow,we introduced Ce³⁺ions for trap regulation,so that the material can produce orange photoluminescence under 254 nm UV irradiation.When the excitation source was turned off,the material presented a dynamic afterglow（orange yellow→white）,and then after the removal of excitation light source,the NIR afterglow can be seen when the 650 nm filter was used to observe.Moreover,the material was preirradiated with a 254 nm UV light source and left for a period of time,an orange-yellow photostimulated afterglow was produced after irradiated with a 980nm NIR laser.In addition,the elastic film made by combining the material with PDMS has the mechanoluminescence property.Finally,the anti-counterfeiting pattern was designed based on the unique luminescence characteristics.The color change of the anti-counterfeiting pattern can be observed by portable UV lamp and 980 nm NIR laser.Multimodal dynamic anti-counterfeit application is realized.