Synthesis And Intense Ultraviolet Emission Of β-NaREF₄:Yb,Tm Luminescent Nanomaterials

In recent years, with the rise of Nano Science and Technology, rare earth dopedluminescent nanomaterials have been widely used in many applications, such as truethree-dimensional displays, infrared photocatalysis, solar cell, bioimaging andphotodynamic therapy. Among these rare earth doped luminescent nanomaterials,rare earth fluorides have received great attention due to their low photo energy andhigh chemical stability. It is well known that controlled synthesis and surfacemodification of rare earth doped luminescent nanomaterials have to do with theirluminescent properties. Therefore, in the past few decades, researchers used differentmethods to achieve controlled synthesis and surface modification of materials.Recently, rare earth doped nanophosphors has been shown great value in cellimaging, photodynamic therapy, immunoassay and detection of nucleic acid. In orderto meet the needs of biomedical field, researchers have focus on upconversionluminescence materials, which have small size, monodispersity, water-solubility,strong luminescence, long life and biocompatibility. Surrounding the rare earthdoped fluoride upconcersion nanomaterials, this dissertation presented a systematicresearch about controlled synthesis and intense ultraviolet upconversion emissionunder infrared light excitation, and then obtained some original results from ourexperiments, and the main results were outlined as followings:(1) Different size, monodisperse and uniform β-NaYF₄nanocrystals have beensynthesized via high temperature thermal decomposition method and studied onphase transition temperature and mechanism of β-NaYF₄nanocrystals formationprocess. At the same time, through analysis of the formation mechanism of β-NaYF₄nanocrystals, illustrated that phases of crystalline seeds, reaction temperature,complete separation of nucleation and growth and surfactant played an importantrole in formation of β-NaYF₄nanocrystals. Under980nm near-infrared excitation,intense5-photon upconversion fluorescence from the1I6level of Tm³⁺ions indifferent sizes β-NaYF₄nanocrystals is much stronger than4-photon and3-photonupconversion fluorescence. Compared with the different sizes of the β-NaYF₄:20%Yb,0.5%Tm nanocrystals by dynamical analysis, we first found thesize-denpendent effect on the lifetime of1I6level of Tm³⁺ions, and illustrated thereasons for enhanced ultraviolet upconversion luminescence.(2) Monodisperse, pure hexagonal NaLuF₄nanocrystals with uniform size hasbeen successfully synthesized by a solution-based high temperature method.Through analysis of the formation mechanism of β-NaLuF₄nanocrystals, illustratedthat phases of crystalline seeds, reaction temperature, complete separation ofnucleation and growth and surfactant played a key role in formation of β-NaLuF₄nanocrystals. At the same time, under980nm near-infrared excitation, we first foundintense5-photon upconversion fluorescence (the intense5-photon upconversionfluorescence from the1I6level of Tm³⁺ions is much stronger than4-photon and3-photon upconversion fluorescence), and proved that population of the states1I6,¹D₂and¹G₄came from5-photon,4-photon, and3-photon upconversion processes,through investigated the pumping power dependence of the fluorescent intensity. Atlast, compared β-NaLuF₄:18%Yb,0.5%Tm nanocrystals to the same sizeβ-NaYF₄:18%Yb,0.5%Tm nanocrystals by dynamical analysis on the lifetime of1I6,¹D₂and¹G₄level of Tm³⁺ions, firstly proved that β-NaLuF₄is the best host.(3) Monodisperse and uniform size β-NaYbF₄:0.5%Tm nanocrystals wassynthesized via high temperature thermal decomposition method using oleic acid assurfactant, and ultraviolet to near-infrared upconversion luminescence wereinvestigated. Under980nm near-infrared excitation, intense ultraviolet upconversionluminescence of β-NaYbF₄:0.5%Tm nanocrystals were observed. Particularly,5-photon upconversion fluorescence of Tm³⁺ions is stronger than4-photon and3-photon upconversion fluorescence. From point view of upconversionluminescence mechanism and high excitation power density, illustrated the reasonsfor enhanced ultraviolet upconversion luminescence. Compared β-NaYbF₄:0.5%Tmnanocrystals to the similar size β-NaYF₄:18%Yb,0.5%Tm nanocrystals bydynamical analysis on the lifetime of1I6,¹D₂and¹G₄level of Tm³⁺ions, we provedthat β-NaYbF₄is also host beneficial to enhanced ultraviolet upconversionluminescence. (4) Firstly, small size β-NaYF₄:18%Yb,0.5%Tm nanocrystals was synthesizedusing a solvothermal approach, and coated with a thin layer of SiO₂on the surface ofβ-NaYF₄:18%Yb,0.5%Tm nanocrystals by modified St ber method, successfullycontrolled the SiO₂layer thickness³nm and completed the core-shell structuresynthesis of the particle size less than50nm. Under980nm near-infrared excitation,intense ultraviolet and blue upconversion luminescence ofβ-NaYF₄:18%Yb,0.5%Tm and β-NaYF₄:18%Yb,0.5%Tm@SiO₂nanocrystals wereobserved. After being modification with a layer of SiO₂, the core-shell structurenanocrystals exhibited a slight increase in upconversion luminescence. We attributedenhanced upconversion luminescence to the surface modification, which eliminatedsome unexpected surface defects and avoided some fluorescence quenching whichcaused by surface functional groups of nanocrystals directly contacting thesurrounding water.