Mn2+activated Ca-α-SiAlON-broadband deep-red luminescence and sensitization by Eu2+, Yb2+and Ce3+

Abstract

The covalent character of (oxy)nitrides has enabled tuning of emission color for new Eu2+- and Ce3+-doped phosphors and widespread implementation in white light LEDs. Mn2+-doped (oxy)nitrides form a relatively unexplored class of luminescent materials with great potential as orange to near infrared (NIR) phosphor in white light and NIR LEDs. Here we report deep red broad band emission around 700 nm for Mn2+ in Ca-α-SiAlON. The emission shows a complex and gradual temperature quenching behavior and an unexpectedly large blue shift of the band maximum from 740 nm at 75 K to 670 nm at 475 K. This peculiar luminescence behavior is explained by disorder in the local (N,O)7 polyhedral coordination of the Mn2+ ions resulting in a variation in emission maximum and quenching temperature for differently coordinated Mn2+ ions. Interestingly, the present results deviate from previous reports on narrow band orange emission from Mn2+ in Ca-α-SiAlON. Careful analysis reveals that the orange emission likely originates from an AlN:Mn2+ impurity phase. Sensitization of the Mn2+ emission by strongly absorbing Eu2+, Yb2+ and Ce3+ ions was observed but with low efficiency as energy transfer to Mn2+via exchange interaction is ineffective for the relatively large (>5 Å) nearest neighbor separation in Ca-α-SiAlON. The new insights on Mn2+ emission show that for efficient orange/red emission in (oxy)nitrides, a tetrahedral coordination is required for Mn2+ while deep red/NIR emission can be expected for higher coordination numbers but is prone to thermal quenching. Sensitization of the Mn2+ emission is efficient only for exchange interaction and requires short (