Study On Mechanism And Application Of Thermo-enhanced Lanthanides Upconversion Luminescence

Lanthanides-doped luminescent materials usually suffer from the luminescence thermal quenching because of the aggravation of various nonradiation relaxations with elevated temperature.However,recent discovery on the abnormal thermal-enhanced upconversion luminescence has triggered extensive attentions.Although considerable efforts have been spent to explore the underlying mechanism of the luminescence thermal enhancement behavior,significant disagreements and controversies remain for different proposed mechanisms.Therefore,researchers are committed to explore a reasonable mechanism to account for the abnormal thermal enhancement of upconversion luminescence.In addition,the luminescence thermal enhancement behavior can provide a new perspective for the functional application of lanthanides-doped upconversion materials,which has gradually become a research hotspot.In this dissertation,we investigated the abnormal thermal enhancement behavior in the specific core/shell structure and the mechanism behind it is explored.In addition,we also used the core-shell upconversion nanoparticles(UCNPs)to control the temperature-dependent fluorescence characteristics,and expanded its application research in luminescence thermometry.In summary,the main results are listed as following:1.Herein,inert-core/active-shell UCNPs,where both sensitizer and activator are located in the shell area near the nanoparticle surface,has been designed and synthesized for the temperature-dependent upconversion luminescence(UCL)behavior.The results show that the inert-core/active-shell UCNPs exhibit a stronger luminescence thermal enhancement tendency when compared with the active-core UCNPs(?17-fold).Therefore,in order to further study the influence of inert-core on the abnormal thermal enhancement fluorescence effect,we synthesized inert-core/active-shell UCNPs with different inert-core sizes.Specifically,the luminescence thermal enhancement behavior of inert-core/active-shell UCNPs appears core-size dependent,which cannot be explained by either surface-phonon-assisted mechanism or surface moisture release mechanism.Based on the temperature-dependent luminescence and the corresponding decay performance as well as temperature-dependent XRD measurements,a relationship between the size-dependent luminescence and size-dependent lattice expansion coefficient can be built up,which suggests the lattice thermal expansion induced alleviation of surface quenching is responsible for the present luminescence thermal behavior of the inert-core/active-shell UCNPs.2.Taking into account the relationship between the core/shell structure and the fluorescence thermal quenching/enhancement characteristics,we attempt to regulate the temperature-dependent upconversion fluorescence characteristics by converting the core/shell UCNPs.On the one hand,through coating a certain thickness of inert layer,the inner sensitizer and activator(Yb³⁺and Tm³⁺)are not affected by the surface,so that Tm³⁺exhibits thermal quenching characteristics.On the other hand,by controlling the thickness of the active-shell layer,the Er³⁺in the outer layer exhibits abnormal thermal enhancement of upconversion luminescence characteristics.Combining these two-opposite temperature-dependence luminescence characteristics,in the temperature range of 303-423 K,we have obtained a dual-emission ratiometric thermometer with good sensitivity(S_a(max)=0.0157 K^-1,S_r(max)=3.55%·K^-1).This work breaks the limitation of the traditional ratiometric thermometers based on the electrically-coupled system,and provides a new aspect for the design of fluorescent thermometers.