Optic Properties Of Yttrium Oxide Doped With Rare Earth Ions

Rare-earth ions possess a lot of merits such as rich energy levels, long lifetime of excited states, stable physicochemical properties and so forth. Recently, rare-earth luminescence materials have been extensively applied in the fields of laser, communication, color display, anticounterfeit, bio-medical and other high technological fields. It is well known that the emission of rare-earth ions is mainly attributed to the intra-4f electronic transitions, which are parity forbidden according to the quantum mechanical selection rules. Meanwhile, the phonon energy and the surface effect of host materials lead to the nonradiative process. So the low efficiency of these rare-earth luminescent materials has been a great difficulty to solve. In this thesis, as representative yttrium oxides which possess excellent optical and physicochemical properties, Y₂O₃ and Y₃Al₅O₁₂ were selected as the host materials. The Eu³⁺:Y₂O₃, Bi³⁺/Er³⁺:Y₂O₃, and Li⁺/Er³⁺:Y₃Al₅O₁₂ powders were prepared by precipitation, hydrothermal, microwave, and sol-gel methods. And the approaches of enhancing the emission efficiency of rare-earth materials were studied based on the mechanism influencing them.Eu³⁺:Y₂O₃ nanocystals were prepared by different chemical synthesis methods. The growth mechanism for the nanocrystals has been proposed meanwhile the correlation between the surface states and the optical properties of the Eu³⁺ doped Y₂O₃ nanocrystals is discussed. The Eu³⁺:Y₂O₃ nanocystals with high crystallinity is obtained through hydrothermal and microwave methods. The decrease in the surface defects of the nanocrystals results in the decline of nonradiative transition and the enhancement of emission intensity. These results indicate that the decrease in the surface states of the host materials can efficiently enhance the emission efficiency of rare earth ions. Furthermore, the reaction time is remarkably shortened by microwave method. For example, the luminescence intensity of Eu³⁺:Y₂O₃ prepared by microwave method under 200℃and 10min is comparable to that prepared by hydrothermal under 180℃and 2h.Y₂O₃ powders doped by Er³⁺, as activation ion, and Bi³⁺, as doping ion, with different concentrations were prepared by sol-gel combustion method. The influences of Bi³⁺ doping, under ultraviolet excitation and infrared excitation, on the emission properties and quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺ are discussed. The results show that under ultraviolet excitation the remarkably enhanced luminescent intensity of Er³⁺ originates from the efficient energy transfer from Bi³⁺ to Er³⁺ and the maximal magnification is 42 times. While under infrared excitation the enhancement is attributed to the modification of the local crystal field symmetry around the Er³⁺ and therefore the transition increases. Moreover, the doping of Bi³⁺ dissociates the Er³⁺ clusters and decreases the interaction between Er³⁺. Then the quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺ is enhanced by 2mol%. Doping of Bi³⁺ can not only sensitize Er³⁺ but also enhance its radiative transition rate and quenching concentration of ²H_11/2/⁴S_3/2 level.YAG powders codoped with Er³⁺and Li⁺ were prepared by sol-gel method. The influence and the mechanism of the influence of the site occupancy of Li⁺ on the upconversion emissions of Er³⁺ are investigated. We have found that the intensity of green emission increases slightly when the Li⁺ occupies substitutional site. In contrast, when the Li⁺ enters into interstitial site it enhances drastically, a maximal magnification of 36 times, accompanied with a change in the emission spectra. When Li⁺ occupies substitutional site, compared with the increase in the radiative transition rate, the decrease in nonradiative transition rate plays an important role in the enhancement of the emissions intensity. While the drastical enhancement of green emission and spectra change are attributed to the increase in the lifetime of ⁴I_11/2 level, the ratio of radiation rate in green emission and the absorptivity at 980nm, and the last one is the most important one. In a word, the doping of Li⁺ can modify the crystal field symmetry around Er³⁺, break the forbidden 4f-4f transition, enchance the radiative transition rate and the energy level life time, and as a result, the emission intensity of Er³⁺ is enhanced.In summary, we successfully enhanced the emission efficiency of rare-earth ions based on the mechanism influencing the efficiency of rare-earth luminescence materials. The results are in favor of the improvement of the luminescence intensity of rare-earth in practical applications.