Yb³⁺-doped Upconversion Micro-and Nanocrystals With Modulated Structure, Morphology And Optical Properties

Lanthanide-doped upconversion nanoparticles (UCNPs) display myriad advantages, such as low toxicity, high chemical and optical stability, sharp emission peaks, long emission lifetimes, etc. Inaddition, they are able to be excited by continuous-wave (cw) near infrared (NIR) diode lasers, e.g., at~980nm, prvoding practical advantages, such as deep light penetration in biological tissues, reduced photodamage to living organisms, and absence of autofluoreseence background, etc. All these favorable properties have promised the use of UCNPs as an emerging class of biological luminescent labels. However, there still exists some unresolved key issues on the way to their clinical translation including, the tuning of UC multicolor fluorescence, the enhancement of UC efficiency, the controlled synthesis of morphology and size of upconversion material, etc. To solve these problems, we have prepared a variety of rare earth doped materials though either a hydro(solvo)thermal or a thermal decomposition method. In essence, we focused on the utilization of Yb3+doping to manipulate the emission UC colors, the UC efficiency, and the structure and morphology of the resulting UC materials. The growth mechanism and UC mechanism were investigated and discussed.Doping Yb3+produce an sensitization effect in the BaYF5and NaYF4UC host matrix. We doped Yb3+/Er3+, Yb3+/Tm3+, or Yb3+/Ho3+ion pairs in the BaYF5system, in which Yb3+ions were utilized to sensitize activators ions (Er3+, Tm3+, or Ho3+) to produce intense red, blue and green UC emissions, respectively. The associated UC mechanisms were investigated and discussed. In the NaYF4system, the Nd3+and Yb3+ions are both utilized as sensitizers, whereby Nd3+(the first senstizer) was used to senstize Yb3+(the second sensitizer) which then sensitize Er3+ions. Optimizing Yb3+concentrations were utilized to enhance the efficiency of this cascade sensitizing, leading the UC luminescence to be enhanced by~13times.We first reported on the synthesis of ultrasmall YF3nanocrystals doped with various type of lanthanide ions via a thermal decomposition method; these resulting nanoparticles exhibit good thermal stabilities and are highly efficient in UC luminescence. By varying the concentration of Yb3+ions, we are able to precisely regulate the UC color output in these lanthanide doped YF3nanocrystals, acquiring a desired emission colors. For example, in Er3+/Yb3+-doped YF3nanocrystals, the main emission peak is the green emission band when the low concentration of Yb3+were employed. When the concentration of Yb3+increased, the red emission band gradually increased, tuning the UC color from from green to yellow, then finally to red. In the Tm3+/Yb3+-doped YF3nanocrystals, when elevating the Yb3+ concentration, the emission output was shifted from the magenta to the blue. This is because the intensity ratio of the blue emission band to the near infrared emission band increased with an increase of the Yb3+concentration. In Yb3+/Er3+/Tm3+-doped YF3nanocrystals, the output color was manifested to gradually change from pale yellow to pink, and then to a pure red when increasing Yb3+concentration. It is noting that the luminescent efficiencies of these multicolor UC can be enhanced by4~10folds by employing a core-shell structure to treat surface-related quenching mechanisms.Doping high concentration of Yb3+enhance UV UC emission Yb3+/Tm3+-codoped YF3rhombic nanodisks. The intensity of UC UV emissions gradually increased with an increase of Yb3+concentration. The strongest UV output was acquired for90mol%Yb3+, which was nearly6-fold higher than that of YF3nanoparticles containing20mol%Yb3+. Moreover, the intensity of UC UV remained nearly unchanged after a unique phase transfer process to render them hydrophilic. The corresponding upconversion luminescence and UV-enhanced mechanism were investigated and discussed. It was also found that doping Yb3+concentration higher than92mol%could cause the UC UV intensity to decrease, being ascribed to the increased surface quenching probability produced by Yb3+at a high concentration regime.Synthesis of hexagonal core β-NaYbF4and core-shell β-NaYbF4:0.5%Tm3+@NaYF4UCNPs, in which the Y3+ion in the well-known NaYF4host matrix were replaced in full by the Yb3+ion. UC fluorescence efficiency was enhanced by~50fold at800nm and~100fold at475nm in core-shell structure. We have also shown that the upconversion luminescence of typically used NaYF4:30%Yb3+/0.5%Tm3+nanoparticles can be enhanced by~100times through a hierarchical active core/active shell/inert shell (NaYF4:30%Yb3+/0.5%Tm3+)/NaYbF4/NaYF4design which inhibited surface-related quenching mechanisms of the active core and directed energy migration in the second active shell layer. Pertinent UC fluorescence quenching and enhancement mechanisms were investigated.We have established a simple method to tune the size and shape of colloidal CeO2nanocrystals through cationic doping of Yb3+ions. Differing from previous results on decreasing fluoride nanoparticle size through lanthanide doping, we found that cationic doping of Yb3+ions with lower valence can increase the nanoparticle size. The sphere-like shape of CeO2nanoparticles was converted to nanocube with (100) terminated surface through Yb3+ions doping. A similar phenomenon was also observed in the LiYF4microcrystals due to the doping of Yb3+. It is revealed that the exposed crystal surface of LiYF4microcrystals can be easily tailored through modulation of Yb3+doping concentration.