The Research In New Structure Of Nanocrystals To Enhance Upconversion Luminescence And Physical Properties

In recent years, the nanotechnology has been developed so fast that become thefrontier domains. The combination of nanotechnology and biology makes it possibleto use biomacromolecule and its complex to detect and simulate. It is well knownthat nanoparticals has a large ratio of surface to volume leading to the rapid increaseof the surface atoms, surface energy and surface tension with the size ofnanoparticals decreasing resulting the special properties of heat, light, magnetism,high sensitivity. The nanopartical is much smaller than the size of cells generally andcan circulate in the blood so that they can detect pathological changes and are usedas specific biomarkers. The development and applications of nanotechnology inbiomedicine will overcome complicated disorders incurable before.The Rare earth （RE） elements are composed of the lanthanide（Ln） series （fromlanthanum to lutetium）, yttrium, and scandium.With abundant and unique energylevel structures arisingfrom4fninner shell configurations, Ln³⁺ions can exhibitsharp fluorescent emission via intra-4f or4f–5d transitions. The luminescencefeatures of Ln³⁺ions include narrow bandwidth, long-lived emission, large Stokesshifts, ligand-dependent luminescence sensitization which have received startlinginterest because of the continuously expanding need for luminescent materialsmeeting the stringent requirements of telecommunication, lighting, electroluminescent devices,（bio-） analytical sensors, bioimaging set-ups and solarcells. While the lower of upconversion luminescence efficiency attributed to themultiphoton absorbed process limits its applications in luminescent device,bioimaging and photodynamics therapy （PDT）, so how to enhance the luminescenceefficiency and intensity is an important frontier research direction which hasattracted significant research interest. This thesis carried on the synthesis andluminescent properties of NaYF₄：Yb³⁺，Tm³⁺nanocrystals in order to solve theefficiency and get new approaches to enhance the luminescence efficiency:（1） NaYF₄： Yb³⁺， Tm³⁺nanocrystals were synthesized by thermaldecomposition. We changed the concentrations of Yb³⁺and Tm³⁺ions and got theoptimization condition. Under the optimized conditions, the nanocrystals was coateda homogeneous shell of NaYF₄; the effect of different thickness on thecharacteristics of luminescence of Tm³⁺was also investigated.（2） NaYF₄:Yb³⁺, Tm³⁺nanocrystals doped with Li⁺ions of various concentrationswere studied in detail. The significant enhancement of the UC emission inNaYF₄:Yb³⁺, Tm³⁺nanocrystals by introducing Li⁺ions without changes in phaseand morphology were observed. In contrast to lithium-free NaYF₄:Yb³⁺, Tm³⁺, theUC emission intensitys of452nm and479nm of the NPs codiped with7ml%Li⁺ions were increased by about8and5times respectively. The mechanisms for theenhancement of upconversion emission were also discussd which can be attributedto the improvement of the NPs’crystallinity and the lower local crystal fieldsymmetry around the Tm³⁺.（3） The upconversion luminescence of core NaYF₄:Yb³⁺,Er³⁺and core/shellconstruction NaYF₄:Yb³⁺,Er³⁺@NaYF₄nanocrystals was studied at varioustemperatures from15K to300K. Both the luminescence intensity and peak energywere found to depend on temperature. A nonradiative process plays a key role in theupconversion luminescence, which results in the increase and then decrease in theintensity when temperature is increased.