Page 143 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Rare Earth Doped Yttrium Oxide Nanophosphors Synthesis and Engineering- Controllable Photoluminescence Properties
diffraction peaks with calcination temperatures reflects an enhanced crystallinity at high temperatures. As a complementary characterization, Raman analysis is performed to provide a fingerprint of samples with/without Eu3+ doping (Figure 7 (c-d)). In both cases one can observe peaks at 130 cm-1, 329 cm-1, 378 cm-1, 469 cm-1, 592 cm-1 and 1085 cm-1, which are characteristic peaks of cubic yttria nanoparticles.20 However, several new peaks appear at 428 cm-1, 705 cm-1, 1262 cm-1, 1388 cm-1, 1659 cm-1, 1699 cm-1 and 1766 cm-1 in the spectrum of Eu3+ doped nanophosphors (marked by asterisks), revealing the chemical bonds and symmetry are changed due to the Eu3+ incorporation. This in conjunction with the EDX, XPS and XRD result indicates Eu3+ has been effectively and homogenously doped into the yttria lattice. In addition, the intensities of the peaks are found to increase significantly with the temperature, while the peaks width exhibits an inverse relationship. This is attributed to the improved crystallinity as well as the spatial-correlation effect. Particles annealed at higher temperatures have better crystallinity, leading to less surface defects and dislocations and eventually an increased Raman intensity. On the other hand, at lower temperatures particles have smaller crystalline size. The confinement of phonons in smaller volume can cause an increased uncertainty in the wave vector of the phonons and phonon momentum distribution, which in turn, resulting in the peak broadening phenomenon.20
Figure 7 (e-f) shows photoluminescence emission spectra of Eu3+ doped yttria nanophosphors prepared at different conditions. All spectra exhibit a sharp peak at 612 nm, which is originated from the 5D0→7F2 transition of Eu3+ in C2 symmetry.21 Less intense spectral features related to the 5D0→7F2 transition are also observed at 620-640 nm. There are several weak emission peaks in the range of 580-600 nm, which are assigned to the 5D0→7F1 transition of Eu3+ in S6 or C2 symmetries.22 The emission intensity of nanophosphors doped with same Eu3+ concentration is shown to increase drastically with the temperature. This is attributed to the crystalline effects. Bulk and surface defects widely exist in poorly-crystalline particles, which can act as non-radiative centers (quenching centers) and lower the luminescent efficiency. However, heat treatment can improve particle crystallinity and reduce the crystalline defects, allowing a better activation for the Eu3+ ions. By analyzing the highest peak (222) at 29.2◦ using the Scherrer formula, the crystallite size is estimated to increase drastically with the temperature, from 11.6 nm at 600 °C, 16.8 nm at 800 °C, 29.7 nm at 1000 °C and 53.5 nm at 1200 °C. The result verifies the XRD and Raman analysis relating to the improvement of the crystallinity at high temperatures.
131