Design Of CAG-Based Multimodal Long Afterglow Materials And Their Anti-Counterfeiting Applications

Long afterglow luminescent materials have been attracting much attention in the anti-counterfeiting field by virtue of their excellent optical properties（diversified luminescent modes,adjustable colors and long luminescent lifetime,etc.）,such as the printing of securities,banknotes and bank checks,etc.,all of which contain logos composed of some afterglow materials.However,unfortunately,most of the anti-counterfeiting luminescent materials reported so far are of simple unimodal type（with fixed excitation modes,single output color and static luminescence）,which leads to a low level of anti-counterfeiting and easy to counterfeit,and fails to meet the higher anti-counterfeiting demands at this stage.Therefore,the development of advanced multimodal afterglow luminescent materials is of great significance for anti-counterfeiting applications.In this thesis,Ca₃Al₂Ge₃O₁₂,a garnet structure with abundant sites,is selected as the matrix,and a series of new multifunctional multimodal and multicolor long afterglow anti-counterfeiting materials are designed and prepared with the research idea of obtaining the features of color tunable,multimodal luminescence（photoluminescence,up-conversion luminescence,afterglow,photo-excited luminescence,and thermoluminescence）and dynamic luminescence,which demonstrates the advantages in information encryption,industrial anti-counterfeiting detection,fingerprint identification,information storage and advanced coding.The research content of this thesis mainly includes the following five parts:（1）A series of Ca₃Al₂Ge₃O₁₂:Cr³⁺phosphors with deep red long afterglow luminescence have been designed,and the occupancy of Cr³⁺ions in the matrix material Ca₃Al₂Ge₃O₁₂ has been analyzed in detail by XRD property characterization and DFT first-principle calculations.A deep red emission with spectral coverage of 650-860 nm peak located at 722 nm can be obtained under the excitation of 408 nm light,and deep red luminescence with different afterglow intensities and durations can be obtained by adjusting the Cr³⁺doping concentration from 2%to 6%.The material defect information was systematically analyzed and the afterglow luminescence mechanism was explained in detail by testing the pyroelectric spectra under different conditions.A series of demonstration experiments of information encryption design were carried out,providing a new way for multifunctional anti-counterfeit luminescent materials.（2）Conventional anti-counterfeit fluorescent materials usually show single-peak output under single wavelength excitation,which cannot guarantee the security of authentication information.Therefore the realization of multimodal luminescent properties with multi-color and up-down conversion still needs to be explored.With the idea of changing the excitation wavelength to achieve color-tunable luminescence,the co-doped Ca₃Al₂Ge₃O₁₂:Yb³⁺,Er³⁺multimodal afterglow luminescent material was prepared,and the luminescence color was mainly red under 980 nm laser irradiation,and the luminescence color was mainly green under 490 nm laser irradiation,and the samples showed excellent afterglow performance at559 nm,with the best afterglow duration exceeding 6 hours.The samples showed excellent afterglow performance at 559 nm,with the best afterglow duration of more than 6 hours.The upconversion luminescence mechanism of the phosphor under 980 nm irradiation was investigated.Fluorescent security patterns and multifunctional fingerprint afterglow imaging experiments were designed to validate their potential for anti-counterfeiting and information storage applications.（3）The single color and short duration limit the higher level of anti-counterfeiting and information encryption.Rare earth ions(Re³⁺=Tb³⁺,Pr³⁺,Dy³⁺)were selected to be doped into Ca₃Al₂Ge₃O₁₂,and a three-modal,multi-color long afterglow anti-counterfeiting material was prepared.The afterglow duration of Ca₃Al₂Ge₃O₁₂:Tb³⁺varied from 4 to 24 hours when the concentration of Tb³⁺was adjusted.With the change of Pr³⁺concentration,the long afterglow color of Ca₃Al₂Ge₃O₁₂:Pr³⁺changes from red to green due to the cross-relaxation process in the low-phonon energy host.When doped with different concentrations of Dy³⁺ions,Ca₃Al₂Ge₃O₁₂:Dy³⁺has different afterglow decay lifetimes and the color can change from blue to cyan.By adjusting the concentration of dopant ions,the afterglow color and lifetime can be changed to form different trap types,trap concentrations,and trap depths.Three kinds of ions correspond to three different afterglow anti-counterfeiting modes,which increases the difficulty of imitation.In addition,the afterglow will still be displayed under high temperature conditions.A series of anti-counterfeiting demonstration experiments on encryption of fingerprint information in virtual military scenarios and passport identification in customs were conducted,realizing multi-modal anti-counterfeiting applications.（4）Long afterglow phosphors face challenges in achieving multicolor anticounterfeiting and visualization（affected by ambient light）,and a multicolor,multimodal anticounterfeiting strategy for Ca₃Al₂Ge₃O₁₂:Bi³⁺,Re³⁺（Re=Tb,Eu,Sm,Dy）phosphors has been devised,and the hidden information can be observed with a UV device.As the concentration of Re³⁺rare earth ions increases,the samples can be transformed from UV to blue emission to green emission(Ca₃Al₂Ge₃O₁₂:Bi³⁺,Tb³⁺),red emission(Ca₃Al₂Ge₃O₁₂:Bi³⁺,Eu³⁺),yellow-green emission(Ca₃Al₂Ge₃O₁₂:Bi³⁺,Dy³⁺)and orange-red emission(Ca₃Al₂Ge₃O₁₂:Bi³⁺,Sm³⁺),and successfully realized afterglow emission in the UV band with an afterglow time greater than 20 hours.The combination of multicolor afterglow and UV-band long afterglow phosphor realizes the identification of fingerprints,advanced and secure Mos code,QR code,ASCll code and other anti-counterfeiting applications,which further strengthens the information security and opens up new perspectives for anti-counterfeiting and optical storage data systems.（5）In order to explore and improve the afterglow properties and colour diversity of the materials,new highlights and application potential for multifunctional anti-counterfeiting applications were provided Ca₃Al₂Ge₃O₁₂:Mn²⁺,Cr³⁺multimode dynamic afterglow materials were prepared.In the garnet Ca₃Al₂Ge₃O₁₂ structure,the Mn²⁺and Cr³⁺ions occupy the dodecahedral position（Ca site）and the hexahedral position（Al site）,respectively,and emit green peaks(⁴T₁（G）→⁶A₁（S）for Mn²⁺)and deep red peaks(²E→⁴A₂ for Cr³⁺)at 503nm and 708 nm,respectively,under the excitation of 265 nm UV light.As the concentration of Cr³⁺increases,it makes the green colour diminish and the crimson colour enhance,due to the energy transfer of Mn²⁺→Cr³⁺.After cessation of UV excitation,the afterglow colours can be dynamically tuned to show green,red and long afterglow in the NIR by increasing external physical stimuli such as temperature（300 K-473 K）.The continuous luminescence of the tunable colours with temperature is attributed to the high temperature thermal burst of the Mn emission peak at 568 nm in high temperature environments,which leads to tunable emission colours from green to yellow and orange-red.Based on the multimode dynamic luminescence of Cr³⁺concentration,excitation wavelength,temperature and time thermochromism,a series of advanced multilevel anti-counterfeiting patterns were designed and fabricated for the detection of chips and tunnel tires in a dark environment,which provides a new idea for multifunctional anti-counterfeiting applications.