• Poelman, Dirk
• Korthout, Katleen
• Claeys Bouuaert, Manuel
• Smet, Philippe

Abstract

White light emitting diodes (LEDs) are environmentally friendly lighting systems because of their low energy consumption and long lifetime. White LEDs are combinations of a blue LED and one or more phosphor materials. Rare earth doped nitridosilicates can be applied as phosphor in such white LEDs. Their advantages are a high thermal quenching temperature and a high stability [1,2]. Since the emission characteristics in these materials depend highly on the local environment of the dopant ions, a direct investigation of their neighborhood is necessary to optimize the phosphor.The photoluminescence of the rare earth doped nitridosilicates Ca2Si5N8:Ce and Ca2Si5N8:Eu was investigated in detail. Furthermore the incorporation of the dopant ions in the Ca2Si5N8 host lattice was investigated using x-ray absorption spectroscopy. Exafs data were collected at the BM01 (SNBL) and BM23 beamlines at the ESRF synchrotron facility in Grenoble (France).This study reports on the structure of the rare earth centers and the influence of a Li codopant on the emission spectrum, thermal quenching temperature and quantum efficiency of the phosphor. We found that the rare earth dopants, i.e. Ce and Eu, occupy both Ca2+ sites in the monoclinic host lattice, albeit with some preferential substitution (figure 1). From the x-ray absorption spectra it is found that Ca2Si5N8:Eu only contains Eu2+, and remarkably Ca2Si5N8:Ce contains not only Ce3+ but also an unexpected large fraction of Ce4+, which is known to be a luminescence killer [3]. It was proven both on the micro-scale (with SEM-CL and SEM-EDX mapping) and nano-scale (with Exafs) that this is not due to the presence of extra phases like CeO2. Moreover, because the fraction of Ce4+ shows the inverse trend as the quantum efficiency, it is believed that Ce4+ indeed builds into the lattice. It is thought that the most probable way for the Ce ions to do this is via two Ce3+ ions together with a nearby Ca2+ vacancy or via one Ce4+ ion with one nearby vacancy, since such clusters respect electrostatic equilibrium.In such cases of doping with Ce, Li+ is sometimes added as a codopant for charge compensation. Such codoping makes the previous cluster superfluous and the Ce ions rather build into the lattice with one or two Li+ ions nearby. This change in local Ce structure in codoped and non-codoped samples is investigated with Exafs.Photoluminescence measurements show that adding a codopant during synthesis does not affect the emission spectrum but influences the thermal quenching temperature and quantum efficiency of the phosphor – both of which can be explained by looking at the change in the local environment of the luminescent ion.

Topics

• impedance spectroscopy
• photoluminescence
• cluster
• phase
• scanning electron microscopy
• Energy-dispersive X-ray spectroscopy
• quenching
• vacancy
• extended X-ray absorption fine structure spectroscopy