Enhancing Inverse-thermal Quenching Effect In Upconversion Nanocrystals Via Heterovalence Ions Doping

The crystal structure of the matrix can be tailored by ion doping,thereby,the coordination field and crystal field environment around the rare earth ions could be influenced,affecting the luminescence of rare earth ions.Moreover,the phenomenon in which the luminescence intensity increases with increasing temperature,that is,the inverse-thermal quenching effect.In this paper,the effect of heterovalence ion doping on inverse-thermal quenching effect of upconversion luminescent nanocrystals was investigated using fluoride nanocrystals as matrix.Fluoride core nanocrystals and core-shell nanocrystals with low phonon energy were synthesized by co-precipitation method.The correlation between heterovalent ion doping and microstructure was studied by XRD and TEM.Under temperature dependent optical spectra and fluorescence lifetime spectra,the variation of the kinetics of rare earth ions with temperature by the microstructure of nanocrystals were investigated.The following are results obtained:(1)The NaGdF₄ core nanocryastals and NaGdF₄@Ca/Yb/Er:NaGdF₄ core-shell nanocrystals with good homogeneity and disperibility were successfully prepared by co-precipitation method.By heterogeneous ion doping,vacancy defects are introduced into the fluoride matrix lattice,so that the defect state captures a lot of electrons at normal temperature.As the temperature increases,these electrons are gradually released to the excited state of the rare earth and produce luminescence;thereby,realizing inverse-thermal quenching effect of activated ions.By rising the temperature from 293 K to 413 K,the integral UC emission intensity of 20Yb/2Er:NaGdF₄ only increases for?2.2 times,while that of NaGdF₄@20Ca/20Yb/2Er:NaGdF₄ core-shell NCs with similar particle size enhances for?10.9 times,indicating the enhancement of the inverse-thermal quenching effect of Er³⁺ions by doping heterogeneous Ca²⁺ions.(2)The Yb:NaGdF₄@Yb/Er:NaGdF₄ active-core/active-shell structure nanocrystals with good homogeneity and dispersibility were synthesized by co-precipitation method,and the upconversion emission inverse-thermal quenching effect under high sensitizer ion doping was realized.On the one hand,nanocrystals with the high Yb³⁺ions doping content greatly enhance the absorption intensity of Er³⁺ions.On the other hand,the Er³⁺activators,confined in the two-dimensional shell layer,enable the energy migration process from activators to surface defects.In this scenario,when increase temperature,the suppression degree of this energy migration process is enlarged,followed by an improved inverse-thermal quenching effect.Specifically,by rising temperature from 293 K to 413 K,the integral upconversion emission intensity of 40Yb:NaGdF₄@60Yb/2Er:NaGdF₄ NCs increases?8.24 times,while that of NaGdF₄@20Yb/2Er:NaGdF₄ NCs with a similar mean particle size only enhances?3.44 times,suggesting the effect of high senitizer ion concentration doping on enhancing the upconversion inverse-thermal quenching effect.(3)On the basis of good inverse-thermal quenching effect of the prepared fluoride nanocrystals,temperature detection is acheived by measuring the fluorescence intensity ratio(FIR),and the best value of calculated maximum relative temperature sensitivity(S_r)is 1.2%K^-1.In addition,since the emission intensity continuously increases with the increasing temperature,the fluoride nanocrystals could be applied to the anti-counterfeiting feilds.In this paper,NaGdF₄@20Yb/2Er:NaGdF₄ NCs and NaGdF₄@20Yb/2Er:NaGdF₄-Tm:Na Er F₄ hybrid nanocrystals were successfully prepared into two sets of anti-counterfeit labels,respectively.Under the excitation of 980 nm laser,various changes in color intensity or chromaticity were obtained.