Luminescence Properties Of Er³⁺/Yb³⁺ And Pr³⁺/Yb³⁺ Earth Fluoride Nanocrystals

For the past years, trivalent rare earth ions doped upconversion（UC） nanocrystals（NCs） have attracted much attention. UC NCs emit visible photons by absorbing two or more near-infrared（NIR） photons. Thanks to such a unique luminescence mechanism, the UC nanoparticles can be applied to bioimaging, photodynamic therapy, UC lasers, optical memory, and so on. In comparison to conventional downconversion fluorescent materials such as organic dyes and quantum dots, UC NCs have obvious advantages, such as sharp emission bands, long emission lifetimes, higher photochemical stability and low toxicity. Rare earth fluorides such as YF3, LuF₃, β-NaYF₄ and β-NaLuF₄ are acknowledged as the very efficient UC host materials because of their low photon energy.This thesis mainly focuses on the rare earth ions doped fluoride NCs, and the NCs were synthesized by a solvothermal process. We explored the origin of the UC enhancement in β-NaYF₄: 2% Er³⁺/20% Yb³⁺@β-NaYF₄ sandwiched structure NCs in detail; the efficient quantum cutting（QC） process was found in β-NaLuF₄: Pr³⁺/Yb³⁺, and a novel method has been presented to verify the energy transfer（ET） mechanism in QC. In addition, we investigated the influence of nanocrystalline phase, size, shape and luminescence properties on the behaviors of La³⁺ and Y³⁺ doping in β-NaLuF₄: 2% Er³⁺/20% Yb³⁺ NCs. Finally, we synthesized LuF₃:Yb³⁺/Er³⁺ NCs and investigated the UC properties of the NCs in detail. The main results are listed as following:1. Nanosized Yb³⁺ and Er³⁺ codoped β-NaYF₄ cores coated with multiple β-NaYF₄ shell layers were synthesized by a solvothermal process. The UC emission intensity is strongly enhanced as increasing the shell numbers up to three. The green emission and the red emission are enhanced by a factor of 40 and 34, respectively, for shell layer number of three. The decrease of the UC emission intensity for shell number of four can be explained as the reduction of core number per volume with increasing the shell thickness. The origin of the upconversion enhancement was studied based on photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure is mainly originated from the suppression of deexcitation of Yb³⁺ ions. We also explored the population of Er³⁺ 4F9/2 level. The results reveal that ET from the lower intermediate Er³⁺ 4I13/2 level is dominant for populating Er³⁺ 4F9/2 level in nanomaterials; however, in bulk materials, the contribution of the green emitting Er³⁺ 4S3/2 level for populating Er³⁺ 4F9/2 level, which mainly comes from the cross relaxation energy transfer from Er³⁺ ions to Yb³⁺ ions followed by energy back transfer within the same Er³⁺-Yb³⁺ pair, become more and more important.2. NaLuF₄: 2% Er³⁺/20% Yb³⁺ NCs doped with different concentration of La³⁺ were prepared by a solvothermal process at 300℃. On doping with the increased La³⁺ ions concentration, the hexagonal phase structure is gradually decreased and transformed into the cubic phase structure. The pure cubic NCs are obtained when the presence of La³⁺ ions concentration reaches 30 mol%. Further increasing the La³⁺ content, impurity phase La F3 appears. Compared to cubic phase α-Na Lu0.78Yb0.2Er0.02F4 NCs with the similar size prepared at normal temperature（280℃）, the number of the surface organic groups such as hydroxyl group（–OH） attached on Na Lu0.48La0.3Yb0.2Er0.02F4 NCs is reduced, which results to the decrease of the nonradiative transition rate of 4I11/2 â†' 4I13/2 of Er³⁺ions, manifesting as the increase of the intensity ratio of green to red（3.2 times）. Through analyzing the luminescence decay curves for the 4S3/2â†'4I15/2 transition and 4F9/2 â†'4I15/2 transition under 980 excitation wavelength, it is concluded that the population of 4F9/2 level originates from 4I13/2 level, not from 2H11/2/4S3/2 level in the two samples. For α-Na Lu0.78Yb0.2Er0.02F4 NCs and Na Lu0.48La0.3Yb0.2Er0.02F4 NCs, a three-photon red UC process occur at high pump power density, which can be described as 4G11/2(Er³⁺) + 2F7/2(Yb³⁺) â†' 4F9/2(Er³⁺) + 2F5/2(Yb³⁺).3. NaLuF₄: 2% Er³⁺/20% Yb³⁺ NCs doped with different concentration of Y³⁺ were prepared by a solvothermal process at 300℃. With the increasing of the Y³⁺ doping concentration, the NCs size is continuous decrease with no phase transition. Meanwhile, the UC intensity is firstly increased and then decreased when the Y³⁺ ions concentration over 30 mol%. The NCs（β-Na Lu0.48Y0.3Yb0.2Er0.02F4） with the optimum Y³⁺ ions doping concentration（30 mol%） for UC emission own a diameter of about 80 nm. Compared with β-Na Lu0.78Yb0.2Er0.02F4 and β-Na Y0.78Yb0.2Er0.02F4 prepared under the same condition, the green UC emission is enhanced by a factor of 1.8 and 16, respectively, for β-Na Lu0.48Y0.3Yb0.2Er0.02F4. The variation of the UC intensity with the increasing Y³⁺ ions concentration is attributed to the changing of the symmetry of the local crystal field induced by Y³⁺ ions doping, which has been proved by the structural probe Eu³⁺ ions. For β-Na Lu0.78Yb0.2Er0.02F4 and β-Na Lu0.48Y0.3Yb0.2Er0.02F4, a three-photon green UC process and a three-photon red UC process occur simultaneously at high pump power density, which can be described as 4G11/2(Er³⁺) + 4I15/2(Er³⁺) â†' 2H11/2/4S3/2(Er³⁺) + 4I13/2(Er³⁺) and 4G11/2(Er³⁺) + 2F7/2(Yb³⁺) â†' 4F9/2(Er³⁺) + 2F5/2(Yb³⁺), respectively.4. About 10 nm orthorhombic LuF3: Yb³⁺/Er³⁺ rectangular NCs were synthesized by a facile and effective solvothermal process. Compared with YF3 and α-NaYF₄ NCs, owning the similar size and the same doping levels of Yb³⁺ ions and Er³⁺ ions as LuF₃ NCs, the green UC emission of LuF₃ NCs is 18.7 times and 5.1times stronger than that of YF3 and α-NaYF₄ NCs respectively; the red UC emission of LuF₃ NCs is 13.2 times and 0.6 times stronger than that of YF3 and α-NaYF₄ NCs respectively. Excited by 980 nm wavelength, the decay curves of both 4S3/2 â†' 4I15/2 transition and 4F9/2 â†' 4I15/2 transition exhibit a single exponential function, caused by the high concentration of Yb³⁺ ions（20 mol%）. Moreover, both the green and red UC emissions are two-photon processes. However, the slopes of the linear plots between log（I） and log（P） for green UC and red UC are all less than 2, which is due to the thermal effect. In addition, utilizing the pump power dependence of the intensity ratio of 2H11/2 â†' 4I15/2 transition to 4S3/2 â†' 4I15/2 transition, we proved the existence of the thermal effect.5. The β-NaLuF₄ NCs with fixed Pr³⁺ concentration at 1 mol% and various Yb³⁺ concentration were synthesized by a solvothermal process. The results indicate β-NaLuF₄: Pr³⁺/Yb³⁺ NCs is a promising quantum cutting（QC） material. Moveover, a new physical method was proposed to verify the ET mechanism in QC process. According to the analysis of the dependence of the initial ET transfer rate upon Yb³⁺ ions concentration, it indicates that the ET from Pr³⁺ ions to Yb³⁺ ions is only by the two-step ET process when the Yb³⁺ concentration is very low; however, with the increase of the Yb³⁺ concentration, the cooperative ET process occurs and gradually increases; when the Yb³⁺ ions concentration increases to 20 mol%, the ET from Pr³⁺ ions to Yb³⁺ ions is only by the cooperative ET process. The theoretical quantum efficiency of β-NaLuF₄: 1% Pr³⁺/20% Yb³⁺ NCs is 192.2%, close to the limit of 200%.