Infrared LEDs for spectroscopy applications realized by cooperating lanthanide ions
Radiative decay of a Eu-to-Tb charge transfer state in calcium sulfide gives rise to a broad emission band, ranging from the deep red to the near-infrared.
During the last decade, the lighting market has witnessed a disruption. Energy-consuming incandescent and potentially harmful fluorescent bulbs have made place for more sustainable and economic LED bulbs, based on light-emitting diodes. Next to the impressive reduction in global power consumption, new functionalities are possible due to the flexible design of LEDs.
The desired color can be finetuned by carefully designing the components of the LED, either the blue-emitting nitride light-emitting diode, or the blend of luminescent materials (a.k.a. phosphors) that partially convert this blue light to longer wavelengths, as is the case in common LED lamps. Next to the success of those white phosphor-converted LEDs, this technology also allows for more specialized and smarter applications. In particular, a recent interest has emerged for broadband infrared (IR) LEDs that find their way towards numerous spectroscopic applications. On the high-end side, medicine comes into the picture where IR light is proposed for imaging, but also for tissue analysis during medical interventions and examinations. More daily applications will be found in future smartphones that are able to analyze food, can remotely assess freshness, or exclude the presence of specific allergens. These emerging applications require new phosphors that convert blue light into a broad spectrum of IR wavelengths.
Infrared emitting phosphors based on single emitting ions, like a transition metal or a lanthanide ion, are already available. Unfortunately these materials fail at simultaneously being efficiently excitable by the blue pumping LEDs, as well as featuring a broad emission band that covers a sufficiently wide emission range required for the spectroscopic applications.
In our article, a new luminescence mechanism, yielding broadband IR emission, is described. We discovered this new IR emission by adding two different lanthanide ions, Eu2+ and Tb3+, to a specific crystal, namely calcium sulfide (CaS). A range of analytical and spectroscopic techniques were applied to show that the IR emission emerges as a cooperative effect between the Eu2+ and Tb3+ dopants, as the single ions only give green and red emission, respectively. Quantum mechanical calculations of the wavefunctions and excited state energies of the dopants and their clusters revealed that the IR emission is associated with an electron transfer between Eu2+ and Tb3+, forming a transient Eu3+-Tb2+ pair. To date, it was supposed that such charge-transfer states could only quench the common luminescence pathways. Here it is shown that they can also induce new types of luminescent transitions, provided that several microscopic conditions are fulfilled as explained by the structure-property relations following from our calculations.
Finally, the prepared CaS:Eu2+,Tb3+ phosphor was applied into a broadband IR emitting LED. An eye-catching bandwidth of 430 nm is achieved at a radiant power of 38 mW. These figures surpass the current state-of-the-art, yet there is still room for improvement in terms of spectrum and efficiency. Nevertheless, dopant-to-dopant-charge-transfer IR luminescence is a promising newcomer that should be on everyone’s watch list.
This work was recently published in Nature Communications:
“Broadband infrared LEDs based on europium-to-terbium charge transfer luminescence”, Nature Communications (2020). https://www.nature.com/articles/s41467-020-17469-x