Dual Europium Luminescence Centers in Colloidal Ga2O3Nanocrystals: Controlled in SituReduction of Eu(III) and Stabilization of Eu(II).

Introducing multiple luminescentcenters into colloidal nanocrystalsis an attractive way to impart new optical properties into this classof materials. Doping disparate ions into specific nanocrystals isoften challenging, due to the preferential incorporation of one typeof dopant. Here, we demonstrate the coexistence of europium dopantsas divalent and trivalent ions in colloidal Ga2O3nanocrystals, achieved by controlled in situreductionof Eu3+to Eu2+. The two dopant species exhibitdistinctly different steady-state and time-resolved photoluminescence,and their ratio can be modified via doping concentration, reactiontemperature, or thermal treatment of as-synthesized NCs. The Eu2+ions are proposed to be stabilized internally owing to theattractive interaction with oxygen vacancies, while Eu3+dopants partly reside in the nanocrystal surface region. The relationshipbetween the electronic structure of the native defects and the dopantcenters is discussed in the context of the overall emission properties.The exposure of these samples to X-ray radiation leads to the reductionof Eu3+to Eu2+, demonstrating an alternativeway of manipulating the oxidation state and suggesting the potentialapplication of this material as an X-ray storage phosphor. The coexistenceof Eu2+and Eu3+and the ability to controltheir relative fraction over the full oxidation state range in groupIII oxide nanocrystals allow for the design and preparation of newphotonic and light emitting materials. [ABSTRACT FROM AUTHOR]