Highly thermally stable Dy3+/Sm3+ co-doped Na5Y9F32 single crystals for warm white LED

The optical behavior and thermal stability of Na5Y9F32 single crystals doped with fixed 1 mol% Dy3+ and various Sm3+ content grown by Bridgman technique were investigated by means of the fluorescence spectra, decay curves and temperature-dependent emission spectra. Upon 365 nm excitation, the emission color shifted from cool white emission to warm white emission as the Sm3+ concentration rise from 0 mol% to 1.6 mol%, in which the CIE coordinates experienced from (0.3445, 0.3888) to (0.3727, 0.3695) and the values of correlated color temperature (CCT) descended from 5114 K to 4174 K. The energy transfer (ET) from Dy3+ to Sm3+ identified by decay curves with emission spectra played significant effect on the change of emission color and the maximum energy transfer efficiency (ETE) was calculated to be 44.5% from the measured decay curves. Based on the Dexter's energy transfer formula, the mechanism of ET from Dy3+ to Sm3+ was determined as a dipole-dipole interaction. The down-conversion internal quantum yield (QY) of 1 mol% Dy3+/0.8 mol% Sm3+ co-doped Na5Y9F32 single crystal was estimated to be similar to 15.9%. When excited by 365 nm, Dy3+/Sm3+ co-doped Na5Y9F32 single crystals displayed excellent thermal stability deducing from the temperature-dependent emission spectra, in which the normalized emission intensity can still maintain 83% at 423 K. These results indicated that Dy3+/Sm3+ co-doped Na5Y9F32 single crystal held potential application prospect in warm white light-emitting diodes (w-LEDs.)