Upconversion Emission Design In Rareearth Doped Oxide And Fluoride Nanocrystals

Contiuous-wave-induced photon upconversion has prosmising applications in many fields, such as biomedical detection and encoding, photodynamic therapy of human diseases, continous-wave short-wavelength lasers, volumetric color imaging and displays, etc. Unfortunately, upconversion materials currently have several serious spectrocscopic problems, including low upconversion efficiency, unsatisfied emission spectra without design, and scarce realization of ultraviolet upconversion emissions, etc. These problems seriously limit their applications in photonics and biomedicine. Based on rare-earth-ion-doped oxide and fluoride materials, we have performed systematic investigations on upconversion emission spectral design, upconversion emission enhancement, ultraviolet upconversion emission realization, as well as practical applications of designed nanocrystals in biomedicine.Green, red and white color upconversion emissions were designed in rare-earth-ion-doped oxide nanocrystals. Single green and red colors were obtained in oxide nanocrystals through controlling the doped Yb3+ ion concentrations under the excitation of 980 nm diode laser. Their corresponding mechanisms were prosposed and verified based on systematic analyses of pump power dependence, fluorescence spectroscopy, and theoretical calculations. The parameter of"suppression ratio"was introduced, which can quantitatively define the monochromonity of upconversion biolabels. White color output was achieved in oxide nanocrystals by controlling appropriate ion concentrations of Er3+,Tm3+ and Yb3+. The mechanism for white upconversion was proposed based on experimental observations and theortical analysis.A general strategy for upconversion emission enhancement in oxide nanocrystals is proposed. Upconversion emissions in rare-earth-ion-doped oxide nanocrystal were increased by two orders of magnitude via introducing monovalent Li+ ions in the host lattice. Mechanisms for the enhancement were proposed and verified based on substantial experimental observations and theoretical analyses. Introducing Li+ ions in the host lattice can modify rare-earth ions'local crystal filed, lengthen the lifetime of energy levels involved, and finally lead to the significant increase in upconversion radiations. The proposed strategy not only can apply to nanocrystals of different oxide host lattices, but also can apply to diverse codoping and tridoping rare-earth ions. Hence, the proposed strategy has general validity in rare-earth-ion-doped oxide nanocrystals.Ultraviolet upconversion emissions were investigated. Ultraviolet upconversion emissions around 300 nm were observed in Yb3+/Tm3+-codoped oxide nanocrystals under diode laser excitation of 980 nm. Pump power dependence investigations indicate that they arise from five- and six-photon processes, which can be well explained by using the proposed upconversion mechanisms. Upconversion mechanisms for blue and ultraviolet emissions in Yb3+/Er3+ -codoped oxide nanocrystals were proposed and demonstrated. Ultraviolet upconversion emissions of 240-490 nm were observed in Yb3+/Ho3+-codoped fluoride materials. Their generation mechanisms and saturation effects were explained. Ho3+-sensitized ultraviolet upconversion of Gd3+ ions were designed in NaGdF4:Yb3+/Ho3+ nanocrystals. Near vaccum ultraviolet upconversion emission of Er3+ and Er3+-sensitized Gd3+ ions as well as their"super saturation effect"were experimentally observed in fluoride materials, which can be well explained by the proposed upconversion mechanisms.Synthesis, upconversion emission design, and biomedical applications of fluoride nanocrystals were performed. Size- and morphology-controllable fluoride nanocrystals and core/shell fluoride nanocrystals were prepared by wet chemical methods. Single red upconversion in fluoride nanocrystals were realized in nanocrystals NaYF4:Yb3+/Ho3+/Ce3+ and NaYF4:Yb3+/Er3+ by adjusting Ce3+ and Yb3+ contents, respectively. A novel core/shell structure was developed in fluoride nanocrystals, which can greatly increase the intensity of upconversion emission. The prepared fluoride nanocrystals were applied to optical tomorgraph in tissue, and it was found that upconversion fluoride nanocrystals can produce higher space resolution images than conventional fluorescent labels. Through animal experimentals, it is demonstrated that our designed fluoride nanocrystals can be successfully applied to autofluorescence-free in vivo multicolor imaging.