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

Yuan-Chih Lin (Institutionen för fysik, Kondenserade materiens fysik (Chalmers))
Göteborg : Chalmers University of Technology, 2016. - 46 s.
[Licentiatavhandling]

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.

Nyckelord: phosphor, garnet, luminescence, vibrational spectroscopy, neutron scattering, free electron laser, light emitting diode, thermal quenching



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Denna post skapades 2016-10-06. Senast ändrad 2016-10-17.
CPL Pubid: 243020

 

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Institutioner (Chalmers)

Institutionen för fysik, Kondenserade materiens fysik (Chalmers)

Ämnesområden

Energi
Materialvetenskap
Hållbar utveckling
Struktur- och vibrationsfysik
Övrig teknisk materialvetenskap

Chalmers infrastruktur

Examination

Datum: 2016-11-03
Tid: 14:15
Lokal: PJ-salen
Opponent: Prof. Itai Panas, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Sweden