Understanding the interactions between vibrational modes and excited state relaxation in garnet structured phosphors

Doktorsavhandling, 2018

This thesis concerns investigations of the local structural environments and
vibrational dynamics of the three garnet type oxide phosphors Ce3+-doped
Y3Al5O12 (YAG:Ce3+), Ca3Sc2Si3O12 (CSS:Ce3+), and Sr3Y2Ge3O12
(SYG:Ce3+), which show promising optical properties as luminescent materials
used in solid state white lighting technologies. The study focuses on a
comprehensive analysis of the nature of the long-range vibrations (phonons)
in terms of local atomic and molecular motions of the garnet structure, as well
as their dependence on the nature of the garnet chemical composition, Ce3+
concentration and temperature. The aim is to understand how these materials
properties aect key optical properties, such as the intensity and wavelength
(color) of the emitted light. The investigations have been conducted
by using a combination of Raman, infrared, luminescence, and neutron spectroscopies,
together with mode-selective vibrational excitation experiments,
and are further supported by theoretical and semi-empirical analyses and
computer modeling based on density functional theory.
The results show that increasing the Ce3+ concentration and/or temperature
cause(s) a red-shifting eect on the emission color due to an increased
crystal eld acting on the Ce3+ ions in YAG:Ce3+. This is primarily attributed
to the thermal excitation of certain high-frequency phonon modes
that induce dynamical tetragonal distortions of the local CeO8 moieties. A
reversal (blue-)shift of the emission color observed at higher temperatures is,
however, the result of counteracting thermal lattice expansion which turns
the local coordination of CeO8 into a more cubic symmetry. Specically, it is
found that the upward-shift of the frequencies of certain vibrational modes
in YAG:Ce3+ through decreasing the Ce3+ concentration or cosubstitution of
smaller and/or lighter atoms on the Y sites increases the thermal stability of
the emission intensity. This higher thermal stability of the emission intensity
is attributed to a less activation of modes that give rise to nonradiative relaxation
of electrons in the excited states via electron{phonon coupling and/or
energy migration processes. For SYG:Ce3+, the emission intensity is found
to decrease strongly with increasing temperature, as a result of thermal ionization
by promoting the electrons of Ce3+ ions into the conduction band of
the host, followed by charge trapping at defects. CSS:Ce3+ exhibits excellent
thermal stability up to very high temperatures, 860 K.