Characteristics And Mechanisms Of Upconversion Luminescence In Tm³⁺-doped Nanomaterials

Recently,trivalent lanthanide(Ln3+)ions doped upconvesion(UC)materials have been extensively exploited in various fields such as multicolor display,biological imaging,temperature sensing,and lasers due to their unique optical properties,including optical stability and narrow emissions.However,because the transitions between energy states in Ln3+ ions are forbidden and the energy states of Ln3+ ions are abundant,UC mechanisms are very complicated,which makes it a challenge to exclusively verify and quantitatively evaluate the dominant process.Blue light is irreplaceable in illumination,imaging and display.And Tm3+ ion can emit intense blue fluorescence.So it is important to investigate UC processes of Tm3+ ion.In this thesis,we lainly focuse on the mechanism of UC emissions and depletion luminescence in Tm3+ doped nanoparticles.The results are as follows:1.Because of the abundant energy states of Ln3+ ions,UC mechanisms are very complicated.We mainly investigated the UC properties and mechanisms of single Tm3+ doped ?-NaYF4 upon 800 nm laser excitation.The typical blue UC emissions,including 451 nm(1D2)and 481 nm(1G4),were observed.We investigated the excitation laser power dependent properties of the upconversion emissions,which showed that a three photon process was involved in the 1D2 emission while a two photon process was involved in the 1G4 emission.Besides,on the basis of the quantum transition principle,the mechanisms of the blue emission of NaYF4:Tm3+UCNPs under 800 nm CW laser excitation were determined through Tm3+ ions concentration dependence,different hosts with various doping and utilizing simultaneous two-wavelength excitation systems.These results indicate that the ESA pathway of 3H6?3H4?3H5-?1G4 is the important role in the 1G4 emissions(481 nm and 651 nm emissions),and that two kinds of ETU pathways,uniformly expressed as 1G4+3H4?1D2+3F4,play the primary roles in the 456 nm emission band.The results are helpful for us to further study and understand the UC mechanisms of complicated doped systems.In addition,the enhanced UC emissions indicates that simultaneous multiwavelength excitation can provide us with alter-native ways to obtain a substantial gain in the overall or partial UC performance by introducing rational excitation wavelengths.2.We investigated the UC properties and mechanism of NaYF4:Yb3+,Tm3+nanoparticles under simultaneous excitation at 980 nm and 1550 nm lasers.The typical bluish violet UC emissions were observed when samples were excited by a 980 nm laser.However,the intensities of UC emissions in UCNPs upon simultaneous excitation at 980 nn and 1550 nm are drastically depleted.The depletion efficiencies of 1D2,1G4,3F3,and 3H4 emissions are about 90%,61%,50%,and 40%respectively.The influences of 1550 nm laser on the ionic interaction between Yb3+ and Tm3+,cross relaxation,thermal effect and STED process are all evaluated.The unexpected depletion of UC emissions is attributed to the STED processes.These results offer potential applications on fine color modulation and STED nanoscopy.