Dynamic local structural symmetries and luminescence properties of the yellow emitting phosphor Ce3+-doped Y3Al5O12
Licentiatavhandling, 2016

This thesis centers on investigations of the local structure and vibrational properties of the oxide garnet Y3Al5O12 (YAG), which when substituted with a few percent of the activator ion Ce3+ to replace Y3+ (Y3−xCexAl5O12, YAG:Ce3+) is one of the most important phosphors for solid state lighting. The study builds on a comprehensive analysis of the nature of the phonons and localized vibrational modes in YAG:Ce3+ and how these depend on the Ce3+ concentration and temperature and how they affect key optical properties, such as the intensity and frequency of the emitted light. The investigations have been carried out by using a combination of Raman, infrared, luminescence, and neutron spectroscopies, together with mode-selective vibrational excitation, and are further supported by computer modeling. The results show that the static and dynamic structure of YAG:Ce3+ are dependent on both Ce3+ concentration and temperature. The substitution of the heavier Ce3+ ions compared to Y3+ is found to lower the vibrational frequencies of most of the phonons and localized vibrational modes around the Ce3+ ions, implying that they become more populated at a given temperature. As the temperature increases, vibrational modes of higher and higher frequency are activated, some of which induce significant dynamical tetragonal distortions around the Ce3+ ions. These distortions are shown to lead to a red-shift of the frequency of the emitted light. In addition, some of the high-frequency phonons are shown to be notably related to non-radiative relaxation of the excited-state electrons of the Ce3+ ions via electron-phonon coupling, which decreases the emission intensity when these phonons are activated. The reduction in emission intensity at elevated temperature is however a complicated process as it is found to relate also to thermal ionization of the excited-state electrons into the conduction band of the host crystal, which may be followed by charge trapping by defects.

free electron laser

neutron scattering

vibrational spectroscopy

phosphor

light emitting diode

luminescence

thermal quenching

garnet

PJ-salen
Opponent: Prof. Itai Panas, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Sweden

Författare

Yuan-Chih Lin

Chalmers, Fysik, Kondenserade materiens fysik

Drivkrafter

Hållbar utveckling

Styrkeområden

Energi

Materialvetenskap

Ämneskategorier

Annan materialteknik

Den kondenserade materiens fysik

PJ-salen

Opponent: Prof. Itai Panas, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Sweden