Yellow-to-violet upconversion in neodymium oxide nanocrystal/titania/ormosil composite sol-gel thin films derived at low temperature

Neodymium (III) oxide nanocrystals synthesized by the inverse microemulsion technique were dispersed in sol-gel titania/organically modified silane (ormosil) composite thin films for photonic applications. X-ray diffraction analysis and transmission electron microscopy observation show that the neodymium oxide nanoparticles have a nanocrystalline structure and that the size of the nanoparticles is in the range of 5-60 nm. An intense room-temperature yellow-to-violet upconversion emission at 402 nm (⁴D_3/2→⁴I_13/2) was measured from the composite thin film heated at 100 °C upon excitation with a yellow light at 587 nm. In addition to this violet emission, ultraviolet emission at 372 nm and blue emission at 468 nm were also observed. The lifetime of the violet emission was measured and the mechanism of the upconversion emission is proposed, and they indicate excited-state absorption originating from the long-lived ⁴F_3/2 excited state should be responsible for the yellow-to-violet upconversion.