Structural and Optical Characterization of Silica Nanospheres Embedded with Monodisperse CeO₂-Eu³⁺ Nanocrystals

Abstract

Luminescent nanocrystals embedded into silica microspheres were shown to be useful for silica labeling for biological applications, ensuring mechanical and chemical stability, nontoxicity, biocompatibility and optical properties. We used sol–gel technology to prepare silica nanospheres embedded with fluorescent and magnetic Eu3+(1 mol%)-doped CeO2 nanocrystals. The X-ray diffraction pattern analysis and transmission electron microscopy investigations showed CeO2:Eu3+(1 mol%) nanocrystals of about 9 nm size and Ce3+ ions substitution by the Eu3+ ions; the nanocrystals dispersed inside the nanosized silica spheres of about 400 nm diameters. The photoluminescence spectra recorded under UV-light excitation showed Eu3+ ions luminescence peaks (5D0-7FJ, J = 0–4) accompanied by a weaker 425 nm luminescence due to the silica matrix; the quantum yield was 0.14. The weak hysteresis loop and magnetization curves recorded up to 20,000 Oe showed dominantly paramagnetic behavior associated with the silica matrix; a slight opening of the hysteresis loop to a very small magnetic field (about 0.005 Oe) was due to the presence of the two rare earth ions. The photonic crystal properties of SiO2-CeO2:Eu3+(1 mol%) silica nanospheres deposited as films on quartz plates were revealed by the two weak attenuation peaks at 420 and 500 nm and were associated with the reflection from different planes. The SiO2-CeO2:Eu3+(1 mol%) nanospheres are attractive potential candidates for photonics-related applications or for multifunctional bio-labels by combining the luminescence and magnetic properties of the nanocrystals.

Keywords

• CeO₂
• europium
• silica spheres
• nanocrystals
• electron microscopy
• luminescence