Design principles for achieving red emission in Eu2+/Eu3+ doped inorganic solids

Rare earth (RE) ions activated inorganic phosphors with multi-color emissions have received considerable attention because of their emerging applications in solid-state lighting, backlight displays, lasers, biomedical imaging, and so on. This tutorial review concerns the design principles for Eu2+ and Eu3+ activated red-emitting inorganic solids and highlights the influencing factors on the luminescence performance. Based on the recent advances in structural design of inorganic RE phosphors, we proposed several design principles for achieving red emission in Eu2+/Eu3+ ions doped solid-state materials. On the one hand, for the realization of red emission from Eu2+ ion, the used strategies include the following: (1) designed synthesis of new Eu2+-doped nitride or sulfide phosphors, (2) anionic substitution achieves large centroid shift, (3) Eu2+ occupies polyhedrons with small coordination numbers to obtain large ɛcfs, (4) doping concentration controls the distribution of Eu2+ ions, (5) mixed ligands induce large ΔS, and (6) doping Eu2+ in nitrides with UCr4C4-type structure to achieve narrow-band red emission. On the other hand, for the red emission originating from a Eu3+ ion, the design principles are listed as follows: (i) designed synthesis of Eu3+-doped phosphors with small CT energy, (ii) realization of low excitation energy by doping sensitizer ions, (iii) Eu3+ luminescence enhancement by charge compensation, and (iv) occupation of unsymmetrical sites to maintain high color purity of Eu3+. Finally, we discuss and look at the future opportunities for Eu2+/Eu3+ activated red phosphors.