Michael R. Krames’s scientific contributions

Synthesis of high quality colloidal Cerium(III) doped yttrium aluminum garnet (Y3Al5O12:Ce³⁺, “YAG:Ce”) nanoparticles (NPs) meeting simultaneously both ultra‐small size and high photoluminescence (PL) performance is challenging, as generally a particle size/PL trade‐off has been observed for this type of nanomaterials. The glycothermal route is capable to yield ultra‐fine crystalline colloidal YAG:Ce nanoparticles with a particle size as small as 10 nm but with quantum yield (QY) no more than 20%. In this paper, the first ultra‐small YPO4‐YAG:Ce nanocomposite phosphor particles having an exceptional QY‐to‐size performance with an QY up to 53% while maintaining the particle size ≈10 nm is reported. The NPs are produced via a phosphoric acid‐ and extra yttrium acetate‐assisted glycothermal synthesis route. Localization of phosphate and extra yttrium entities with respect to cerium centers in the YAG host has been determined by fine structural analysis techniques such as X‐ray diffration (XRD), solid state nuclear magnetic resonance (NMR), and high resolution scanning transmission electron microscopy (HR‐STEM), and shows distinct YPO4 and YAG phases. Finally, a correlation between the additive‐induced physico‐chemical environment change around cerium centers and the increasing PL performance has been suggested based on electron paramagnetic resonance (EPR), X‐ray photoelectron spectrometry (XPS) data, and crystallographic simulation studies.

Modern urban human activities are largely restricted to the indoors, deprived of direct sunlight containing visible and near-infrared (NIR) wavelengths at high irradiance levels. Therapeutic exposure to doses of red and NIR, known as photobiomodulation (PBM), has been effective for a broad range of conditions. In a double-blind, randomized, placebo-controlled study, we aimed to assess the effects of a PBM home set-up on various aspects of well-being, health, sleep, and circadian rhythms in healthy human subjects with mild sleep complaints. The effects of three NIR light (850 nm) doses (1, 4, or 6.5 J·cm−2) were examined against the placebo. Exposure was presented five days per week between 9:30 am and 12:30 pm for four consecutive weeks. The study was conducted in both summer and winter to include seasonal variation. The results showed PBM treatment only at 6.5 J·cm−2 to have consistent positive benefits on well-being and health, specifically improving mood, reducing drowsiness, reducing IFN-γ, and resting heart rate. This was only observed in winter. No significant effects on sleep or circadian rhythms were noted. This study provides further evidence that adequate exposure to NIR, especially during low sunlight conditions, such as in the winter, can be beneficial for human health and wellness.

Commercial lighting for ambient and display applications is mostly based on blue light-emitting diodes (LEDs) combined with phosphor materials that convert some of the blue light into green, yellow, orange, and red. Not many phosphor materials can offer stable output under high incident light intensities for thousands of operating hours. Even the most promising LED phosphors saturate in high-power applications, that is, they show decreased light output. The saturation behavior is often poorly understood. Here, we review three popular commercial LED phosphor materials, Y3Al5O12 doped with Ce³⁺, CaAlSiN3 doped with Eu²⁺, and K2SiF6 doped with Mn⁴⁺, and unravel their saturation mechanisms. Experiments with square-wave-modulated laser excitation reveal the dynamics of absorption and decay of the luminescent centers. By modeling these dynamics and linking them to the saturation of the phosphor output intensity, we distinguish saturation by ground-state depletion, thermal quenching, and ionization of the centers. We discuss the implications of each of these processes for LED applications. Understanding the saturation mechanisms of popular LED phosphors could lead to strategies to improve their performance and efficiency or guide the development of new materials.

The renowned yellow phosphor yttrium aluminum garnet (YAG) doped with trivalent cerium has found its way into applications in many forms: as powder of micron sized crystals, as a ceramic, and even as a single crystal. However, additional technological advancement requires providing this material in new form factors, especially in terms of particle size. Where many materials have been developed on the nanoscale with excellent optical properties (e.g., semiconductor quantum dots, perovskite nanocrystals, and rare earth doped phosphors), it is surprising that the development of nanocrystalline YAG:Ce is not as mature as for these other materials. Control over size and shape is still in its infancy, and optical properties are not yet at the same level as other materials on the nanoscale, even though YAG:Ce microcrystalline materials exceed the performance of most other materials. This review highlights developments in synthesis methods and mechanisms and gives an overview of the state of the art morphologies, particle sizes, and optical properties of YAG:Ce on the nanoscale.

Phosphors have been used successfully for both research and commercial applications for decades. Eu³⁺-doped materials are especially promising, because of their extremely stable, efficient, and narrow red emission lines. Although these emission properties are ideal for lighting applications, weak absorption in the blue spectral range has until now prevented the use of Eu³⁺-based phosphors in applications based on blue light-emitting diodes (LEDs). Here, we demonstrate a sensitization mechanism of Eu³⁺ based on inter-particle Förster resonance energy transfer (IFRET) between lanthanide-doped inorganic nanocrystals (NCs). Compared to co-doping different lanthanides in the same host crystal, IFRET allows independent choice of host lattices for Eu³⁺ and its sensitizer while potentially greatly reducing metal-to-metal charge-transfer quenching. We demonstrate IFRET between NCs, resulting in red Eu³⁺ emission upon blue excitation at 485 nm using LaPO4:Tb/LaPO4:Eu, and LaPO4:Tb/YVPO4:Eu NC mixtures. These findings pave the road towards engineering blue-sensitized line emitters for solid-state lighting applications.

White light emitting diodes (LEDs) composed of a blue LED and a green/yellow downconverter material (phosphor) can be very efficient, but the color is often not considered very pleasant. Although the color rendering can be improved by adding a second, red-emitting phosphor, this generally results in significantly reduced efficacy of the device due to the broad emission of available conventional red-emitting phosphors. Trivalent europium is well-known for its characteristic narrow-band emission in the red region, with little radiation outside the eye sensitivity area, making it an ideal candidate for enabling high color quality as well as a high lumen equivalent of radiation from a spectrum point of view. However, a thorough study of the practical potential and challenges of Eu³⁺ as a red emitter for white LEDs has remained elusive so far due to the low excitation probability in the blue spectral range which is often even considered a fundamental limitation. Here, we show that the absorption in the blue region can be brought into an interesting regime for white LEDs and show that it is possible to increase both the color rendering and efficacy simultaneously using Eu³⁺ as a red emitter, compared to warm white LEDs comprising conventional materials.