Broadband Infrared LEDs based on Europium-To-Terbium Charge Transfer Luminescence

A broadband infrared (IR) light-emitting diode (LED) for versatile spectroscopic use is constructed, based on a novel type of broadband IR luminescence. This peculiar emission is identified in europium and terbium codoped CaS for which the spectrum partially overlaps with the red Eu2+ emission and ranges up to 1200 nm. It can be efficiently excited with visible light. Experimental evidence for metal-to-metal charge transfer (MMCT) luminescence is collected, comprising data from luminescence spectroscopy, microscopy and X-ray spectroscopy. State-of-the-art multiconfigurational ab initio calculations allow to attribute the infrared band to the radiative decay of a metastable MMCT state of a Eu2+-Tb3+ pair. The calculations explain why no MMCT emission is found in the similar compound SrS:Eu,Tb and are used to anticipate how to fine-tune the characteristics of the MMCT luminescence.