| In the up-conversion(UC)nanomaterials,the presence of defects in the crystals is unavoidable due to the doping of rare earth ions and the high specific surface area.However,nowadays,the studies of defect state in nanomaterials focus on how to modify the crystal surface but neglect the application of the defects.In this paper,KLu2F7 and KLu3F10 upconversion nanoparticles have been prepared by using different surfactants,adjusting the reactants concentration and introducing other rare earth or metal ions.The effects of the crystal defect state,structure,morphology and optical properties of the samples have been investigated in detail.The research work of this paper is mainly divided into several parts as following:1.The improvement of the photon synergistic effect with the assistant of defect state was investigated.In this study,the photon synergistic effect of Yb3+/Er3+ doped KLu2F7 nanoparticles by using UV and NIR as the dual-excitation light source and the "photon avalanche" have been observed.Through adjusting the reaction conditions and the rare earth doped concentration,the photon synergistic effect has been effectively enhanced.In order to further verify the principle of enhanced photon synergistic effect,the(VK)were introduced by doping 10 mol%Ce3+ ions which can be used as shallow traps or bridges to transform deep traps into shallow traps to change the distribution of defect states.The increase of the shallow trap density effectively enhances the photon synergistic effect.According to the investigation in our work,we find that the photon synergistic effect derives from the three paths as follows:the electrons from DC could be re-excited to 4I11/2 level by 980 nm LD,the energy migration from DC process would be reused as the excitation resource and the defects could be acted as an energy level.2.The enhancement of the upconversion emission intensity by Li+ ions doping was studied.The introduction of the smaller effective ionic radius of Li+ ions could eliminate the defect states via the charge compensation leading to the promotion of the UC emission efficiency.It is found that the optimum doping concentration of Li+ ions is 10 mol%in which the UC emission intensity is enhanced about 12 times more than that of Li+-free sample.With the increasing of the 980 nm laser diode excitation power density,the luminescent color of 10 mol%Li+ doped KLu2F7:Yb3+,Er3+ samples changed from yellow to green.3.The effects of surfactants on the UC luminescence were studied.The surfactant CTAB can decrease the defect state density by modifying the nanocrystal surface,which is beneficial to improve the upconversion efficiency.The UC luminescence intensity and luminescent color of Yb3+/Er3+,Yb3+/Ho3+ and Yb3+/Tm3+ doped samples were controlled by different surfactants.The red,green and blue UC tricolor and NIR luminescence were obtained.It provides a wide range of ideas in the application of biological imaging.Thus,the study of defect states in UC nanomaterials is of great importance for its application research.By adjusting the defect states,the UC luminescence properties can be improved which provides a new idea for the application of UC nanomaterials in solar cells and bio-medical. |
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