Structure, luminescence, and dynamics of Eu₂O₃ nanoparticles in MCM-41

The structure, luminescence spectroscopy, and lifetime decay dynamics of Eu₂O₃ nanoparticles formed in MCM-41 have been investigated. Both X-ray diffraction and high-resolution transmission electron microscopic observations indicate that Eu₂O₃ nanoparticles of monoclinic structure are formed inside channels of MCM-41 by heating at 140 °C. However, heat treatment at 600 or 700 °C causes migration of Eu₂O₃ from the MCM-41 channels, forming nanoparticles of cubic structure outside the MCM-41 channels. After heating to 900 °C, some of the cubic Eu₂O₃ particles change to monoclinic Eu₂O₃, and the MCM-41 structure breaks down and a different or disordered phase is formed. The feature of the hypersensitive ⁵D₀ f⁷F₂ emission profile of Eu³⁺ is used to follow the structural changes. In the luminescence spectrum of the sample prepared at 140 °C, the emission spectrum is dominated by peaks at 615 and 623 nm, while in the other samples a peak at 612 nm is prevalent. Photoluminescence lifetimes show the existence of short (<1 µs) and long (microsecond to millisecond) components for each sample. The fast decay is attributed to quenching by surface states of the nanoparticles or energy transfer to the MCM-41, while the longer time decays show the effects of concentration quenching. The monoclinic sample prepared at 140 °C shows a higher luminescence intensity than the cubic samples or the bulk powder. These observations indicate that MCM-41 as a template can be used for making and stabilizing monoclinic rare earth oxides, which normally are stable only at high temperatures and high pressures. More importantly, the nanophase Eu₂O₃/MCM-41 composite materials formed at low temperatures might represent a new type of efficient luminescence material with fast response, with potential applications in lighting and displays.