Thermally-induced Upconversion Luminescence Enhancement Of NaYF₄-based Nanocrystals And Anti-counterfeiting Applications

Upconversion nanocrystals can emit visible light under near-infrared excitation through a multiphoton absorption process,and have great application prospects in life sciences,anti-cou nterfeiting and other fields.Rare earth-doped hexagonal phase NaYF₄-based upconversion na-nocrystals are currently the most efficient upconversion system.The luminescence intensity of upconversion nanocrystals is usually reduced by thermal quenching,while the Yb³⁺sensitized small-size upconversion nanocrystals exhibited thermally enhanced luminescence.In recent y ears,researchers still have a lot of controversy about the explanation of this abnormal lumines-cence behavior,and the similarities and differences of the two types of important upconversio n nanocrystals sensitized by Yb³⁺and Nd³⁺still need to be further explored and analyzed.Usi-ng the luminescence thermal enhancement properties of upconversion nanocrystals,it is possi-ble to design and synthesize thermochromic upconversion nanocomposite systems and constru-ct safer anti-counterfeiting patterns.However,it is still a difficult problem to coordinately opt-imize upconversion luminescence and thermochromic performance.In addition,the luminous efficiency of upconversion nanocrystals is still low,and exploring more efficient upconversio-n luminescent nanosystems is still an important research direction.Therefore,this article takes?-NaYF₄-based upconversion nanocrystals with higher lumen-ous efficiency as the research object.The details are as follows:(1)NaYF₄:Yb³⁺,Ho³⁺nanocrystal were prepared by co-precipitation method,and NaYF₄:Yb³⁺,Ho³⁺@NaYF₄:Yb³⁺and NaYF₄:Yb³⁺,Ho³⁺@NaYF₄:Nd³⁺two types of core-shell nanocr-ystals were prepared by layer-by-layer method.The upconversion luminescence performance and luminescence thermal behavior of these two core-shell nanocrystals were studied.It was f-ound that two types of core-shell structured nanocrystals exhibit completely different upconv-ersion luminescence thermal behaviors:Yb³⁺sensitized NaYF₄:Yb³⁺,Ho³⁺@NaYF₄:Yb³⁺core-shell structured nanocrystals show thermally enhanced luminescence,while similar structure Nd³⁺sensitized NaYF₄:Yb³⁺,Ho³⁺@NaYF₄:Nd³⁺nanocrystals show typical thermal quenchin-g.(2)The reasons for the different light-emitting thermal behaviors of the two core-shell na-nocrystals are clarified.Thermal cycling experiments in different atmospheres further prove t-hat the thermally enhanced luminescence of Yb³⁺-sensitized core-shell nanocrystals is derive d from the quenching effect of adsorbed water molecules on the surface.When the temperatur-e is raised,the water molecules gradually separate from the surface of the nanocrystals,there by enhancing the upconversion luminescence.Through the temperature dependence of the lum-inescence lifetime in the air and the thermal cycle luminescence performance characterization under different environmental atmospheres,it was proved that the Nd³⁺sensitized nanocrystal s are still affected by the quenching of surface water molecules,but at high temperature,the in-trinsic thermal quenching of Nd³⁺is dominant.Therefore,the upconversion luminescence int-ensity of Nd³⁺sensitized nanocrystals decreases with increasing temperature.(3)Using the different luminescence thermal behaviors of Yb³⁺and Nd³⁺sensitized core-shell nanocrystals,a new type of luminescence thermochromic upconversion nanocomposite p-owder material was designed.The higher luminous efficiency NaYF₄:Yb³⁺,Ho³⁺@NaYF₄:Yb³⁺(thermal enhancement)and NaYF₄:Yb³⁺,Tm³⁺@NaYF₄:Nd³⁺(thermal quenching)nanocrys-tals are uniformly mixed.The composite nano-powder material constitutes a single green or b-lue emission under the excitation of 980 nm or 808 nm laser,respectively.However,under th-e excitation of the 980 nm/808 nm dual laser,it shows a significant blue?white?green lumi-nous color change(?(8=0.07)with the increase of temperature,which can be used to constru-ct a safer up-conversion anti-counterfeiting pattern.(4)The Y-site multi-element doped Na(Y_0.28Gd_0.25Lu_0.25Yb_0.2Er_0.02)F₄nanocrystals were successfully prepared by the coprecipitation method.the formation temperature,particle size a-nd luminous performance of the Y-site multi-element doping on the upconversion nanocrysta ls were studied.Na(Y_0.28Gd_0.25Lu_0.25Yb_0.2Er_0.02)F₄ nanocrystals have a particle size of about 21nm,presenting a hexagonal phase structure.The results show that multi-element doping at the Y site will lower the formation temperature of the pure hexagonal phase and reduce the partic-le size of the final product.In addition,compared with the luminescence performance of NaY F₄:Yb³⁺,Er³⁺nanocrystals under the same particle size,the upconversion luminescence intensi-ty of Y-site multi-element Na(Y_0.28Gd_0.25Lu_0.25Yb_0.2Er_0.02)F₄ nanocrystals is significantly imp-roved.The reason may be that the Y-site multi-element doping increases the absorption rate o-f the sensitizer Yb³⁺,thereby increasing the energy transfer efficiency of Yb³⁺?Er³⁺.Finally,the thermal stability of Na(Y_0.28Gd_0.25Lu_0.25Yb_0.2Er_0.02)F₄ nanocrystals was explored,and it w as found that its luminescence showed typical thermal quenching characteristics.