Upconversion Luminescence And Mechanisms Of Rare-earth-ions Dop Hydroxyapatite Materials

Hydroxyapatite (HA) has attracted widespread interest for bone substitute materials due to its excellent bioactivity and biocompatibility coming from the analogy to the mineral components and structure of natural bones and teeth. Rare earth doped upconversion (UP) material is a new kind of promising fluorescent label materials, and is especially attractive due to its desirable properties, such as easily excited by near-infrared (NIR) region, high tissues Penetration, achieving zero background detect. If rare earth ions can be doped into hydroxyapatite materials, not only hydroxyapatite can be applied in the biomedical field, but also we can conveniently detect the bone repair situation with the help of upconversion fluorescent label. Thus, in the thesis, samples of HA: E1³⁺, yb³⁺ were synthesized by the sol-gel method at the first time. Furthermore, the upconversion properties of HA: E1³⁺, yb³⁺codoped with F- and Li⁺ ions is studied under 980nm diode laser excitation.Well-proportioned samples of HA: E1³⁺, yb³⁺ were successfully synthesized by the sol-gel method at the first time and excited by 974nm diode lasers, Two distinct UC emission bands centered around 522 nm, 551 nm and 660 nm, assigned to (⁴S_3/2/²H_11/2)→⁴I_15/2 and 4F_9/2→⁴I_15/2 transitions were obtained. The green UC emissions state level are completely populated by two-photon process, while the population of the red UC emission arise from a combining effect of the one-photon and two-photon process. At high yb³⁺ ions concentrations, the strong coupling between E1³⁺ ions and yb³⁺ ions causes a large possibility of energy back transfer process, which will leads to the saturation of the 4I13/2 level. The upconversion mechanism was discussed and substantiated by steady-state rate equations theory.In HA:E1³⁺,yb³⁺ series,substitution of (OH)- by F- ions can results in the dramatically enhancement of upconversion intensity. After the replacement of (OH)- by F- ions, the vibration absorption peaks of hydroxyl disappear, which means the greatly reduction of the phonon energy of HA materials. As we know, the significantly decrease of the phonon energy in matrix materials will result in the distinct lengthening of the lifetimes of E1³⁺ ions'metastable states which greatly benefited E1³⁺ ions'upconversion radiations. As the green and red upconversion intensity is proportional or square to the lifetime of E1³⁺ ions'excited states, therefore, the greater of the level's lifetime, the stronger of the fluorescence intensity. In addition, theoretical investigations based on the steady-state rate equations demonstrate the correctness of the previous analysis. In FA:Er³⁺,Yb³⁺ series, it was observed that further doping with Li+ ions could obviously enhance Er³⁺ ions'green and red UC radiations. The changes of cell parameters of the same hexahedron HA lattice induced by Li+ ions doping were discovered, which indicate the change of Er³⁺ ions'local environment. Tailoring Er³⁺ ions'local environment will leads to the increment of the lifetimes of Er³⁺ ions'excited states, which can significantly favor the UC output. With more Li ions introduced, the lifetime of the Er ions'excited state become longer, so the green and red UC radiation become stronger. Additionally,the multiphoton processes of Er³⁺ ions'red upconversion radiations gradually changed from a combination of the one-photon and two-photon processes to a completely one-photon process after Li+ doping. These changes originated from that, the Li+ ions will leads to the dramatically increment of the Er³⁺ ions'excite state lifetime, which will greatly reduce the probability of non-radiative relaxation of the 4I11/2 level, so the two- phonon process for red UC emission is cut off.