Abstract
Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine. Here we report broadband IR luminescence, cooperatively originating from Eu2+ and Tb3+ dopants in CaS. This peculiar emission overlaps with the red Eu2+ emission, ranges up to 1200 nm (full-width-at-half-maximum of 195 nm) and is 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 attribute the IR emission 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. Finally, a near-IR LED for versatile spectroscopic use is manufactured based on the MMCT emission.
Highlights
Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine
Broadband near-IR LEDs are a promising energy-efficient alternative for incandescent lamps for applications that rely on near-IR spectroscopy[1,2]
No trace of Eu3+ line emission is found. This does not exclude its presence, because intervalence CT (IVCT) states are known to quench the Eu3+ emission in case it pairs with Eu2+ ions[28]
Summary
In addition to this band, some excitation intensity can be found in the region around 370 nm where no allowed transitions for Eu2+ are found[31], suggesting the presence of additional excited states when Eu and Tb are codoped These features are visible at low temperature, as well as in the diffuse reflectance spectra Using this phenomenological model to fit the data yields a barrier height of ΔET,red = 1484 cm−1 (A = 1.07 × 103) This TQ performance is comparable to the one of singly doped CaS:Eu0.01 phosphors, which contain sufficient Eu for concentration quenching to be noticeable[29,58,59]. The IR TQ curve can be modeled by combining the ISC rate constant with a singlebarrier model for the TQ behavior,
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