April’s Paper of the Month

 The cover of the journal, Chemistry of Materials, highlights a unique phonon projection technique implemented on the yellow emitting phosphor, Y3−xCexAl5O12 (the phosphor applied in most commercial phosphor-converted white LEDs), which provides novel insights into local vibrational dynamics of the crystal and its effects on luminescence properties of the material.

The cover of the journal, Chemistry of Materials, highlights a unique phonon projection technique implemented on the yellow emitting phosphor, Y3−xCexAl5O12 (the phosphor applied in most commercial phosphor-converted white LEDs), which provides novel insights into local vibrational dynamics of the crystal and its effects on luminescence properties of the material.

Phosphor-converted white-light-emitting diodes (pc-WLEDs) are efficient light sources used in displays in electronic devices, lamps for indoor and outdoor lighting, and vehicle indicators, to name a few. The most common type of pc-WLEDs comprises an (In,Ga)N-based blue LED and a yellow phosphor, Y3−xCexAl5O12 (YAG:Ce3+), which is electronically excited by the blue LED and followed by yellow light emission. The admixture of the blue and yellow light appears as white light. Hereby, the luminescence properties of the device such as colour temperature, colour rendering index, efficiency, thermal stability, and so on, are strongly dependent on the luminescence performance of YAG:Ce3+.

In YAG:Ce3+, small amounts of the dopant Ce3+ ions serve as luminescent centers, whose electronic structure, which determines the energy transitions of excitation and emission, is predominantly controlled by the local static and dynamical structural environments of the host material, YAG. 

April’s Paper of the Month, from the Chalmers University of Technology, focus particularly on the vibrational dynamics around the Ce3+ ions using vibrational spectroscopy together with DFT-calculations and a unique phonon projection technique. The phonon projection technique is a novel means to interpret lattice vibrations, which allows the qualitative (symmetry) and quantitative (vibrational amplitude) determination of localized vibrations of individual YO8/CeO8, AlO6, and AlO4 moieties in the Y3−xCexAl5O12 crystal, in terms of symmetry coordinates.

They used the Linkam THMS600 in combination with a commercial Raman spectrometer, to measure temperature-dependent Raman spectra. The results reveal that the studied material, YAG/YAG:Ce3+, remains the same phase in the temperature range of 80 K (-193°C) and 870 K (597°C), however that the frequency of phonon modes changes as a function of temperature. The change in frequency of some specific vibrational modes have been shown to play an important role in the emission colour and luminescence efficiency, especially at high temperature.

The understanding of fundamental structural dynamical properties of one of the most important phosphors in this study, provides a promising design principle, through chemically tuning local static/dynamical structure around the luminescent centers, for developing new phosphors emitting at longer wavelengths, e.g. from greenish-yellow to reddish-yellow emission (to obtain warmer white light from pc-WLEDs), meanwhile exhibiting high luminescence efficiency at high temperature.

Y.-C. Lin, P. Erhart, M. Bettinelli, N. C. George, S. F. Parker, and M. Karlsson, Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y3–xCexAl5O12: Design Principles for Phosphors Based on 5d–4f Transitions. Chemistry of Materials 2018 30 (6), 1865-1877 DOI: 10.1021/acs.chemmater.7b04348