솔-젤법으로 합성된 ZrO2:Eu3+ 나노 입자의 분광학 및 열형광 특성

Abstract

Optical spectroscopy and thermoluminescence properties of ZrO2:Eu3+ nano particles synthesized by sol-gel method

Jae Yong Je

Department of Physics, The Graduate School, Pukyong National University

Abstract ZrO2 and Eu3+-doped ZrO2 nanoparticles were synthesized by Sol-gel method and their luminescence and thermoluminescence properties are investigated by optical and laser-excitation spectroscopy. The doping concentration of Eu3+ ions in ZrO2 is 1.0 mol%. The structural characteristic were investigated by x-ray diffraction(XRD) and transmission electron microscopy(TEM). The XRD patterns are compared with JCPDS cards to identify the crystal phases of the ZrO2 nanoparticles sintered at different temperatures.

The XRD patterns confirm that ZrO2 and ZrO2:Eu3+ were synthesized with tetragonal and monoclinic phases sintered at temperatures of 700 and 1350 ℃, respectively. The samples sintered at 1000 ℃ show both of the tetragonal and monoclinic phases. The particle sizes are estimated to be 22.5 nm for tetragonal and 80.5 nm for monoclinic phases. The particle size of ZrO2 sintered at 1000 ℃ is estimated to be 47.5 nm. The results indicate that the particle size increases with increasing sintering temperature. The TEM images lead to be estimated the lattice constants of 0.312 nm (tetragonal phase) and 0.454 nm (monoclinic phase) which are in well agreement with the values in literatures. Both the tetragonal type (T-type) and monoclinic type (M-type) ZrO2:Eu3+ exhibit strong red emission peaking at 610 nm due to the 5D0 → 7F2 transition under the excitation at 266 and 355 nm. The relative emission intensity due to the 5D0 → 7F2 transition in comparison with that of the 5D0 → 7F1 transition becomes stronger for M-type ZrO2:Eu3+ which was synthesized at higher sintering temperature (1350 ℃). This is due to the lower crystal field symmetry of Eu3+ in the M-type ZrO2 than that of T-type ZrO2. It is interesting to note that the emission spectrum of T-type ZrO2:Eu3+ is much broader than that of M-type ZrO2:Eu3+. The results are attributed to the incomplete substitution Eu3+ ions in T-type ZrO2:Eu3+ indicating that part of the Eu3+ ions does not occupy the Zr4+ sites but forms Eu3+-clusters. At the sintering temperature of 700 ℃, the Eu3+ ions do not have enough thermal activation energy to occupy the Zr4+ sites in T-type ZrO2:Eu3+.

The luminescence decay times are estimated to be 0.659 and 0.573 ms for T-type and M-type ZrO2:Eu3+, respectively, at room temperature. Under excitation at 266 nm corresponding to the excitation at charge transfer band of Eu3+- O2- in ZrO2, the decay curves are nearly single exponential for M-type ZrO2:Eu3+, while slightly deviates from single exponential for T-type ZrO2:Eu3+. However, the decay curves deviate largely from single exponential for the luminescence obtained by exciting at 355 nm. The excitation energy of the 355 nm laser radiation corresponds to the energy located just below charge transfer band and does not match with Eu3+ energy levels in ZrO2. The band-edge-excitation by 355 nm results in the excitation of defect centers formed near Eu3+ ions and then transfer the energy from defect centers to Eu3+ ions with non-exponential decay. The excitation spectra obtained under the excitation at the 5D0 level also show different line broadening, i.e. broader for T-type and narrower for M-type. The thermoluminescent characteristics of the Eu3+-doped and undoped T-type ZrO2 and M-type ZrO₂are investigated by analyzing the glow curves after irradiation at 245 nm UV light and 6 MV X-rays. The glow curves of the samples are resolved into isolated glow peaks by Gaussian curve-fitting method in order to obtain peak shape parameters and activation energies. The glow curve of the M-type ZrO₂is resolved into three isolated glow peaks with the main peak locating at about 355 K. On the other hand the curve of T-type ZrO₂is composed of five isolated glow peaks. The results indicate that M-type ZrO₂has single dominant defect centers and T-type ZrO₂has several types of defect centers. The glow curves of M-type ZrO2:Eu3+ and T-type ZrO2:Eu3+ show narrower shape in comparison with Eu-undoped samples. The isolated glow peaks of Eu³+doped samples are located nearly at the same positions as the Eu-undoped samples.