Dual Europium Luminescence Centers in Colloidal Ga₂O₃ Nanocrystals: Controlled in Situ Reduction of Eu(III) and Stabilization of Eu(II)

Introducing multiple luminescent centers into colloidal nanocrystals is an attractive way to impart new optical properties into this class of materials. Doping disparate ions into specific nanocrystals is often challenging, due to the preferential incorporation of one type of dopant. Here, we demonstrate the coexistence of europium dopants as divalent and trivalent ions in colloidal Ga 2 O 3 nanocrystals, achieved by controlled in situ reduction of Eu 3+ to Eu 2+ . The two dopant species exhibit distinctly different steady-state and time-resolved photoluminescence, and their ratio can be modified via doping concentration, reaction temperature, or thermal treatment of as-synthesized NCs. The Eu 2+ ions are proposed to be stabilized internally owing to the attractive interaction with oxygen vacancies, while Eu 3+ dopants partly reside in the nanocrystal surface region. The relationship between the electronic structure of the native defects and the dopant centers is discussed in the context of the overall emission properties. The exposure of these samples to X-ray radiation leads to the reduction of Eu 3+ to Eu 2+, demonstrating an alternative way of manipulating the oxidation state and suggesting the potential application of this material as an X-ray storage phosphor. The coexistence of Eu 2+ and Eu 3+ and the ability to control their relative fraction over the full oxidation state range in group III oxide nanocrystals allow for the design and preparation of new photonic and light emitting materials.