Concentration Dependency of Eu2+ Doped La4-XCaxSi12O3+XN18-X Phosphors and the Energy Levels of the Rare-Earth Ions

Over the last decades, rare-earth doped silicon nitride and silicon oxynitride based materials have received considerable attention in luminescence research due to their wide range of possible structures, good luminescent properties, high thermal stability and chemical inertness. Examples hereof are Sr₂Si₅N₈:Eu²⁺, CaAlSiN₃:Eu²⁺ and α-sialon:Eu²⁺, which have shown to be good conversion phosphors for LEDs. The silicon nitride based luminescent materials are also considered for other applications like solar cells or as persistent luminescence materials.

In order to find new nitridosilicate phosphors, a single-particle-diagnosis approach [1] has been developed in our group, based on single-particle spectroscopy techniques. One of the phosphors that has been found via this method is Eu²⁺ doped La_4-xCa_xSi₁₂O_3+xN_18-x; a phosphor that was also found independently by Park et al. via a combi-chem approach [2]. The material has a strong 5d-4f emission band in the 450 to 650 nm range after 4f-5d excitation with UV or blue light, making it an interesting phosphor for LED applications.

For this study, La_4-xCa_xSi₁₂O_3+xN_4-x powders doped with rare-earths have been prepared via a solid-state reaction synthesis. The luminescence properties of the phosphors doped with Eu²⁺ have been studied as a function of the dopant concentration. An unusually large shift to longer wavelengths of the position of the emission band was observed with increasing Eu concentration, as is shown in Figure 1a. The shift can be attributed to a change in distribution of the Eu ions over the two different crystallographic sites, together with energy transfer between these sites. The effect of Eu concentration on the distribution and energy transfer, and with it the effects on the luminescence properties, have been investigated in detail by analyzing the luminescence decay time and temperature dependency of the luminescence as a function of the Eu concentration.

Doping with Ce also gave rise to 4f-5d absorption bands, that resulted in blue 5d-4f emission after excitation with UV light. The results with Ce doping were compared with the results with Eu doping, regarding the distribution of the lanthanides over the two crystallographic sites.

The information on the 4f-5d transitions of Eu and Ce, together with the thermal quenching data has been used to construct an energy level scheme (Figure 1b), showing the energy level positions of all lanthanides in La_4-xCa_xSi₁₂O_3+xN_18-x relative to the valence and conduction band. With such a scheme the optical properties of all other lanthanides in the same host can be predicted and understood. [3] The scheme could be used to explain the absorption and emission spectra of samples doped with samarium and ytterbium, with regards to the positions of the charge transfer bands, the valence of the dopants and the thermal quenching behavior. 

[1] N. Hirosaki, T. Takeda, S. Funahashi, R.J. Xie; Chem. Mater. 26 (2014) 4280-4288.

[2] W.B. Park, N. Shin, K.P. Hong, M. Pyo, K.S. Sohn; Adv. Funct. Mater. 22 (2012) 2258-2266.

[3] P. Dorenbos; J. Alloys Compd. 488 (2009) 568.