Luminescence Properties Of Tm³⁺/yb³⁺and Ho³⁺/yb³⁺codoped Calcium Scandate Oxide

The study on upconversion luminescence (UCL) of rare earth (RE) ions dopedmaterials began in1950s. The UCL properties of RE doped materials have beenstudied extensively over the past few decades because of the interesting physicsresearch as well as the potential applications in solid-state lasers, high resolutiondisplay, biological labeling, infrared imaging, and diagnosis and therapy of diseasesand so on. CaSc2O4is a kind of promising oxide hosts because of the low cutoffphonon frequency, strong crystal feld, near atom distance, as well as stable chemicaland thermal properties.This thesis mainly focuses on the RE ions activated CaSc2O4oxide material. Weresearch upconversion properties and dynamics study in Tm3+/Yb3+and Ho3+/Yb3+codoped CaSc2O4oxide material in detail. The efficient quantum cutting process isalso found in CaSc2O4: Tm3+/Yb3+material. Furthermore, we have synthesized theCaSc2O4: Tm3+/Yb3+submicro-rods in favor of application in biological assays andmedical image. The major results obtained are as following:1. We research the doped concentration dependence of spectral distribution anddiscuss upconversion dynamics in Tm3+and Yb3+codoped CaSc2O4material. In theinfrared emission spectra from1000nm to1750nm, Yb3+emission reaches themaximum when Yb3+concentration reaches to5%, and Tm3+1600nm emissiongradually enhances when Yb3+concentration increases from1%to15%. The distancesof Yb-Yb and Yb-Tm pairs decrease with Yb3+concentration increasing, then theenhanced energy migration among Yb3+ions speeds up energy transfer from Yb3+toTm3+. With Tm3+concentration increasing, Yb3+emission has amonotonic decline. Itcan be considered as the indication of effcient Yb3+Tm3+energy transfer. Theintensity ratio of Tm3+1600nm emission to Yb3+980nm emission satisfies aproportional relationship with the Tm3+concentration and exhibits the quadratic dependence on Yb3+concentration. The first step energy transfer rate W1is written asW1=C1n0(N0)2. In the UCL spectrum, for480nm and800nm emissions of Tm3+ions,the optimized Yb3+concentration and Tm3+concentration is10%and0.4%,respectively. The480nm to800nm intensity ratio exhibits the same trend with Yb3+980nm emission intensity. Compared with that in dopant concentration optimizedY2O3:0.004Tm3+/0.1Yb3+, furthermore, the UCL around800nm and480nm inCaSc2O4:0.004Tm3+/0.1Yb3+is enhanced by a factor of3.5and2.2, respectively. TheUCL enhancement is attributed to a large absorption cross section (3times as that inY2O3) at980nm of Yb3+and Yb3+â†'Tm3+first step energy transfer coefficient(9.29×10-17cm3s-1in CaSc2O4vs2.87×10-17cm3s-1in Y2O3). The Yb3+â†'Tm3+energytransfer efficiency in CaSc2O4:0.004Tm3+/0.1Yb3+is0.70quite larger than0.47inY2O3:0.004Tm3+/0.1Yb3+.2. We research the doped concentration dependence of spectral distribution anddiscuss upconversion dynamics in Ho3+and Yb3+codoped CaSc2O4material. TheCaSc2O4:0.2%Ho3+,10%Yb3+sample has been optimized for the strongest greenUCL. And it presents the perfect green monochromaticity as Sgr=0.85, which favoursthe simultaneous tracking of multiple fluorescent probes. The studies of spectraldistribution, power dependence, and lifetime measurement reveal the UCLmechanism involved in CaSc2O4: Ho3+/Yb3+material. The intensity ratio of green toNIR emission does not vary with the excitation wavelengths and doped concentrations.Both emissions have the same time evolutions and pump power dependences under980nm excitation. We conclude the green and NIR emissions come from the sameupper levels (5F4+5S2). The evolution of the green intensity by experiment is in goodagreement with the theoretical calculation basing on infrared spectral distributions,illustrating it is populated via Yb3+:2F5/2+Ho3+:5I6â†'Yb3+:2F7/2+Ho3+:(5F4+5S2)pathway. The red emitting is from Ho3+:5F5level. The5F5state is populated through5I6â†'5I7nonradiative relaxation, subsequent Yb3+:2F5/2+Ho3+:5I7â†'Yb3+:2F7/2+Ho3+:5F5energy transfer. Furthermore, we found a large enhancement of UCL inconcentration optimized CaSc2O4:0.2%Ho3+/10%Yb3+. UCL intensities around545nm and660nm are enhanced by a factor of2.6and1.6by comparison with that inconcentration also optimized Y2O3:0.2%Ho3+/10%Yb3+, respectively. The largerabsorption cross section at980nm of Yb3+and Yb3+â†'Ho3+efficient energy transferplay an important role for achievement of intense UCL in CaSc2O4phosphor. Themeasured energy transfer efficiency reaches up to50%for CaSc2O4:0.2%Ho3+, 10%Yb3+.3. CaSc2O4: Tm3+/Yb3+submicro-rods were synthesized using the mildsolvothermal and annealing technique. The phase structures, morphologies, and UCLproperties of bulk and submicro-rod CaSc2O4: Tm3+/Yb3+samples were measured andinvestigated. The synthesized CaSc2O4: Tm3+/Yb3+sample possesses thecharacteristics of small size, remarkable monodispersity, and low synthesistemperature, compared with that prepared using conventional solid state reactionmethod. It also exhibits the stronger UCL than solid state reaction sample. Theenhancement factor approaches to1.5for800nm emission.4. The single phase β-NaGdF4submicron crystal phosphors were synthesizedusing the mild hydrothermal technique by one-step procedure. The pH value andchelators (EDTA and citric acid) have a crucial effect on the morphology of β-NaGdF4sample. The UCL properties for the Tm3+/Yb3+or Ho3+/Yb3+codoped samplessynthesized were researched under980nm excitation. The hexagonal prisms thatmeaning high degree crystallinity demonstrated the strong UCL in comparison withother morphologies such as spindles and spheres. By the spectral distribution andpower dependence, UCL mechanism is revealed in β-NaGdF4: Tm3+/Yb3+material.5. The series of CaSc2O4:0.2%Tm3+/x%Yb3+powder samples were synthesizedby a solid state reaction. An efficient near infrared quantum cutting has beendemonstrated. Upon excitation of Tm3+:1G4level with a blue photon at466nm, Yb3+:2F5/2level can emit two NIR photons around1000nm through cooperative ET fromTm3+to Yb3+. The estimated maximum energy transfer efciency (ηETE) from Tm3+toYb3+is71%. The theoretical quantum efciency (ηTQE) reaches up to171%. The Tm3+:3H6â†'1G4absorption around466nm is observed by monitoring Yb3+:2F5/2â†'2F7/2emission. Under466nm excitation corresponding to Tm3+:3H6â†'1G4transition, astrong emission band around1000nm assigned to the Yb3+:2F5/2â†'2F7/2transition isobserved. The emission intensities of Tm3+ion have a decline and NIR emissionintensity of Yb3+ion increases rapidly with Yb3+concentration increasing. For thehigher Yb3+concentration over5%, Yb3+emission is reduced due to concentrationquenching. It indicates the presence of Tm3+â†'Yb3+efficient energy transfer.