Upconversion Luminescence Thermal Enhancement Mechanism And Property Of Ho³⁺/Yb³⁺ Doped NaYF₄ Nanocrystals

The rare earth doped upconversion nanocrystals,which can convert the absorbed low-energy photons into high-energy photon emission,have broad application prospects in many fields,such as biological imaging,fluorescence anti-counterfeiting,fluorescence sensing and so on.Recently,the abnormal upconversion luminescence thermal enhancement phenomenon found in upconversion nanocrystals has attracted extensive attention,because it can make up for the short board of fluorescence quenching in high temperature section of traditional fluorescence thermometer technology and its advantages in fluorescence anti-counterfeiting application.However,the mechanism of upconversion luminescence thermal enhancement is still under debate,which causes the lack of effective methods to modulate the effect of upconversion luminescence thermal enhancement and hinders its further application.Therefore,it is necessary to study and determine the enhancement mechanism and explore the method of effectively modulating the thermal enhancement effect.In this paper,fluoride nanomaterials doped with rare earth ions is studied.The synthetic method of high temperature coprecipitation and high temperature thermal decomposition were used to synthesize rare earth doped nanocrystals.Then the mechanism of upconversion luminescence thermal enhancement in Ho3+/Yb3+doped NaYF4 nanocrystals are systematically studied,and a new method of vacuum-induced stronger thermal enhancement of upconversion luminescence is proposed.The energy loss pathways of activator and sensitizer in rare earth doped nanocrystals during heating process are studied in detail.It is found that the doped Ho3+can be long-range quenched by water molecules adsorbed on the surface and the excitation energy of Yb3+needs to be transferred to the surface lattice of Yb3+before it can be consumed by adsorbed water.In addition,according to the different energy consumption modes of activator and sensitizer,the upconversion luminescence thermal enhancement effect of nanocrystals is successfully controlled through the design of core-shell structure,which provides a reference for the regulation of upconversion luminescence thermal enhancement in different applications.The main results are listed as following:1.The NaYF4:2%Ho,20%Yb@NaYF4:40%Yb active-core/active-shell nanocrystals were synthesized by high temperature coprecipitation method and seed mediated austenite ripening coating method.By comparing the upconversion luminescence thermal enhancement effect of the sample in vacuum and air,it is found that the vacuum environment can promote the stronger upconversion luminescence thermal enhancement effect,especially,the enhancement amplitude of upconversion green emission is about twice than that of the maximum enhancement effect in air when the sample goes through a heating-cooling process in vacuum and returns to room temperature.Then the mechanism of upconversion luminescence enhancement caused by heating or vacuum is attributed to desorption-induced alleviation of quenching of doped rare earth ions by analysis of temperature-dependent spectra.2.The inertcore/activeshell/activshell upconversion nanocrystals NaYF4@NaYF4:40%Yb@NaYF4:2%Ho,20%Yb is designed and synthesized,which greatly improves the upconversion luminescence thermal enhancement effect under 980nm excitation,such as there is a～140 fold thermal enhancement of upconversion green emission intensity at 210℃.In addition,the upconversion luminescence intensity of the sample excited in vacuum environment will continue to increase with the accumulation of irradiation time,for example,irradiation for 50s can increase the upconversion green emission intensity by about 30 times.Through spectral analysis,it is found that the mechanism of vacuum assisted upconversion luminescence enhancement is that the vacuum-induced weakening of heat dissipation accelerates the heat accumulation of sample caused by a photothermal conversion process which resulting in the thermal enhancement of upconversion luminescence.The upconversion nanocrystals sensitive to negative pressure would be expected to be used as a gas pressure sensor.3.In the synthesized NaYF4:2%Ho,20%Yb nanocrystals,it is found that the activator Ho3+ has a direct interaction with the water molecules adsorbed on the surface,which causing a strong thermal enhancement effect of the emission intensity of 5I6 level in the heating process.The interaction between Ho3+ and adsorbed water molecules on surface can induce a maximum 5-folds thermal enhancement of upconversion green emission during heating process.Then,through the analysis of a series of synthesized Ho3+single doped NaYF4 samples,it is proved that there is a long-range resonance energy transfer effect between Ho3+and surface adsorbed water molecules,which caused by highly matched energy between the 5I6→5I7 transition of Ho3+and the stretching vibration energy level of hydroxyl in water.Then according to the long-range interaction between Ho3+and adsorbed water molecules,the average distance between Ho3+and surface adsorbed water molecules is adjusted through the size and core-shell structure design of nanocrystals,which successfully controls the growth rate of 5I6 energy level population of Ho3+in heating process.Furthermore,this method can be used to regulate the thermal enhancement effect of upconversion luminescence.4.According to the temperature-dependent spectral and luminescence lifetime analysis of a series of synthesized Yb3+single doped NaYF4 nanocrystals with different doped concentrations and structures,it is determined that the energy transfer between doped Yb3+in nanocrystals and surface adsorbed water molecules only occurs at the Yb3+lattice on the sample surface.The excitation energy of the other Yb3+in in the nucleus needs to be transferred to the Yb3+lattice on the sample surface by multi-step energy transfer between ytterbium ions,and then transfers to the adsorbed water molecules.According to the analysis of the energy transfer properties of Yb3+,the NaYF4:Ho,Yb@NaYF4:Yb active-core/actives-shell nanocrystals is designed and synthesized,which can regulate the migration rate of excitation energy in the core to the surface by changing the doped concentration of Yb3+in the active-shell,and then realize the regulation of the upconversion luminescence thermal enhancement effect of the sample.