Liren Lou

Optical properties of nanocrystalline Y2O3:Eu depending on its odd structure

The structure of nanocrystalline Y2O3:Eu prepared by a combustion reaction was analyzed by XRD and high-resolution electron microscopy. Compared with a large-scale particles, 5-nm Y2O3:Eu particles presented as distorted crystallite and rough surfaces. Luminescent and absorption properties of nano-Y2O3:Eu showed remarkably particle size effects. At Y2O3:Eu particle sizes smaller than 10 nm some new results were observed: (a) a red shift of the charge-transfer-state absorption; (b) new emission bands of Eu3+ in the 5D0 --> 7F2 region; (c) luminescent decay of energy level 5D0 of Eu3+ turning to a two-step exponential; and (d) a pronounced increase in quenching concentration and much lower phonon density compared with those of the bulk material. All these phenomena can be attributed to the effect of the softened lattice and surface state of the nanomaterial. The latter was confirmed by stronger excitation by the host absorption after the surface modification.

Preparation and size effect on concentration quenching of nanocrystalline Y2SiO5:Eu

Abstract Nanocrystalline Y2SiO5:Eu was prepared by the sol-gel technique. The photoluminescence spectra and quenching concentration of nanocrystalline Y2SiO5:Eu and normal Y2SiO5:Eu synthesized by the high-sintering method were measured and compared. Higher quenching concentration and stronger luminescent intensity in nanocrystalline Y2SiO5:Eu than in normal Y2SiO5:Eu was observed. This is ascribed to the influence of a confinement effect on resonant energy transfer in nanosized particles. These properties show that nanocrystalline Y2SiO5:Eu has important potential applications.

Luminescence concentration quenching of 1D2 state in YPO4:Pr3+

Under selective excitation of the 3P0 level of the Pr3+ ion in YPO4, the emission spectra and fluorescence decay curves were measured at different concentrations and temperatures. The origin of the spectral line located at 16 318 cm-1 (612.8 nm) was discussed and attributed to the 1D2→ 3H4 transition. The process of concentration quenching for the 1D2 state was also studied. Using the Inokuti-Hirayama model, the non-exponential fluorescence decay curves of the 1D2 level were fitted. The result shows that dipole-quadrupole interaction between Pr3+ ions, which causes 1D2→ 1G4 and 3H4→ 3F4 cross-relaxation, results in the quenching of 1D2 emissions.

Spectroscopic properties of Eu3+ ions in X1-Y2SiO5 at nanometric scale

Abstract The influence of concentration and temperature on the luminescence properties of the Eu 3+ activated X 1 –Y 2 SiO 5 are investigated for the first time. For high Eu 3+ concentration, a cross relaxation process between Eu 3+ ions results in the quenching of 5 D 1 emission. At low temperature and high concentration, the excitation transfer between Eu 3+ ions in the two sites enables the fluorescence of site 2 disappears entirely. In addition, the samples showed some interesting phenomenon related to size effect.

Planar TiO2 waveguides by the sol-gel process: the relationship of structure to properties

Recent works in our laboratories investigated the microstructure of titania films prepared from a colloidal solution and used as planar waveguides. The transmission electron microscopy including high resolution observations together with waveguide Raman spectroscopy, especially in the very low frequency range, showed a strong influence of the heat-treatment procedure on the films morphology. In addition, atomic force microscopy provided valuable information on their surface roughness. At lower temperatures (450 degree(s)C), the films structure consists of a mixture of amorphous TiO2 and anatase nanocrystals (4.5 - 5 nm) and its surface is smooth. On the contrary, when increasing the annealing temperature, the nanocrystals grow (50 nm at 1000 degree(s)C), pores (5 - 20 nm) are clearly distinguished and the films waveguiding properties disappear. Here, we discuss the optical behavior of these films and its relationship to microstructure. It is clearly shown that a high degree of crystallization induces an important porosity, a rough surface and thus dramatically affects the films waveguiding properties. Finally, we report a relatively simple strategy to stabilize the amorphous phase and control the crystallization in order to improve the optical quality of the waveguides.

Absorption and fluorescence spectra of Pr3+ in YPO4 ☆

Abstract Results of spectroscopic measurements of YPO 4 :Pr 3+ are reported. Based on these results, the fluorescent lines were identified. It is confirmed that the 612.8 nm emission line is due to 1 D 2 (Γ 3 )→ 3 H 4 (Γ 5 ) transition of Pr 3+ . It is found that the fluorescence lines from 1 D 2 are dominant for samples of low Pr 3+ concentration when 3 P 0 is selectively excited, which indicated very efficient relaxation from 3 P 0 to 1 D 2 .

Polarized waveguide Raman spectroscopy of the thorium phosphate xerogel thin films

Due to the difficulties in geometrical arrangement, up to now the polarization arrangement for waveguide Raman spectroscopy (WRS) was limited to only a few simple configurations. In this report, we present the experiments in extending polarization configurations for WRS. The resulted polarized WRS spectra of thorium phosphate xerogel thin films will be presented. In the region of Raman shift from 850 to 1200 cm-1 ,two components with different polarization behavior exist. Based on the symmetry arguments, these components are then identified to the different vibration modes of P043 ions. From the spectra it is also concluded that the film is amorphous and isotropic and has chain-like structure.