Multicolor Modulation And Anticounterfeiting Application Of Lanthanide-Doped Upconversion Emission Fluoride Nanocrystals

Benefiting from the abundant and characteristic excited state levels of lanthanide ions(Ln³⁺),Ln³⁺-activated upconversion(UC)nanocrystals(NCs)have attracted great attentions owing to their potential applications in diverse fields,such as imaging,optical multiplexing,temperature sensing,as well as anticounterfeiting and display.Up to date,a series of activators,such as Er³⁺,Ho³⁺,Tm³⁺,Tb³⁺,Eu³⁺,Sm³⁺,Dy³⁺and Ce³⁺ions,co-doped with sensitizer(Yb³⁺or/and Nd³⁺ion)have been widely employed to achieve strong multi-photon UC emissions ranging from ultraviolet to near infrared.The output emission color is highly related to the type of the selected lanthanide dopants as well as their local environment determined by the chemical composition and crystal structure.In this occasion,to obtain different emission colors,kinds of samples with different designed nanostructures need to be synthesized,which is clearly time consuming and laborious.In this regard,multifunctionality is expected for single elaborate UC nanostructure when different external stimuli are employed,which is actually highly demanded in many circumstances.Therefore,we did a lot of experiments and achieved some results as follows:1.Nanocrystals with the mean particle size of sub-10 nm usually exhibit low UC emission efficiency,which limits their practical application.It was reported that the UC emission intensity can be improved by introducing a thermal field and this phenomenon is highly related to the Yb³⁺ions.In this work,taking Na₃Zr F₇ as matrix,the effect of Yb³⁺and Ho³⁺doping content in the Yb/Ho:Na₃Zr F₇ core NCs and Yb³⁺content in the shell layer of Yb/Ho:Na₃Zr F₇@Yb:Na₃Zr F₇ NCs are systematically investigated.We found that with rising the temperature from 293 to 413 K,the total enhancement degree of Ho³⁺ions decrease with the increasing of Ho³⁺doping content from 2 to 6 mol%owing to the increased deleterious cross-relaxation between Ho³⁺ions.High Yb³⁺concentration benefit the population of the defect energy level and also induce back energy transfer from Ho³⁺to Yb³⁺ions.Hence,the thermal-induced UC enhancement degree can be further increased by separating the Yb³⁺ions in the core and shell.2.An integrated core/triple-shell nanostructure,where different activators are spatially separated,is applied for a dual-mode manipulation of UC emission color.Taking Na Er F₄@Na YF₄@Na Yb F₄:0.5Tm@Na YF₄ as example,the 3-photon dominant UC emission intensity of Tm³⁺ions enhances more than 2-photon counterpart of Er³⁺ions with increasing the excitation power,leading to the excitation power induced color variation.On the other hand,the UC emission intensity of Er³⁺activators in the core exhibit thermal quenching effect,while that of Tm³⁺ions in the second-shell show near-zero thermal quenching effect in the visible region,which contributes to the thermal dependent color change.Upon tuning the emission color by means of appropriate co-doping in the core,a series of power/thermal dual-mode responsive multi-color UC NCs are achieved.3.A pumping-controlled dual-mode anti-counterfeiting strategy is conceived by taking advantage of the combination of upconversion processes with different photon numbers.Er³⁺and Tm³⁺are spatially separated within a designed core/triple-shell nanoarchitecture,which is Na Er F₄:10Yb@Na Yb F₄:20Ca/1Tm@Na YF₄.Upon infrared excitation,the emission color of a designed pattern can be switched from red to purple by increasing the excitation power density from 5 to 11 W/cm²,while a bright luminescent trajectory including red,white and blue-green color with different length is observed when rotating the pattern above 600 rpm.In addition,the relative upconversion emission intensities of the Er³⁺and Tm³⁺ions can be manipulated through tailoring interfacial or inner defects in the core/triple-shell nanocrystals,which enables an ultrahigh sensitivity for the pumping-controlled emission color variation to be observed under excitation power well below 11 W/cm².