Understanding the interactions between vibrational modes and excited state relaxation in garnet structured phosphors
Doctoral thesis, 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 a ect 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 e ect 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.
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 a ect 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 e ect 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.
free electron laser
thermal quenching.
luminescence
garnet
vibrational spectroscopy
neutron scattering
phosphor
light emitting diode
KC, Kemihuset, Kemivägen 4
Opponent: Professor Eugeniusz Zych, Faculty of Chemistry, University of Wroclaw, Poland
Author
Yuan-Chih Lin
Chalmers, Chemistry and Chemical Engineering, Energy and Material
Inorganic Phosphor Materials for Lighting
Topics in Current Chemistry,; Vol. 374(2016)p. 374-421
Review article
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,; Vol. 30(2018)p. 1865-1877
Journal article
Weak thermal quenching of the luminescence in the Ca3Sc2Si3O12:Ce3+ garnet phosphor
Journal of Materials Chemistry C,; Vol. 6(2018)p. 8923-8933
Journal article
Research on materials for energy applications represents one of the most fast-growing and important topics in materials science. This thesis focuses on investigations of, so called, phosphor materials for use in white-light Light Emitting Diodes (LEDs). These devices aim to replace the old and inefficient white lighting technologies that have been phased out (incandescent lamps) or are problematic from an environmental point of view (compact fluorescent lamps). However, a poor correlated color temperature and a poor color rendering index, which are measures of how warm and natural the light is perceived by the human eye, and a too low thermal stability of present-day white-light LED devices, hamper their wider usage and market breakthrough. The development of new devices depends on a better understanding of the fundamental properties of the light-matter interactions, but such understanding is, at present, lacking.
In order to take on this challenge, I have investigated in detail key fundamental properties, such as the crystal structure and dynamics of atoms and their correlation to light-matter interactions in a family of promising phosphors. In particular, I have used a combination of advanced experimental techniques and theoretical methodologies, available at Chalmers as well as at large-scale research facilities in the Netherlands, United Kingdom, and the United States, that allowed to obtain unique insights into the effects of atomic structure and dynamics on both the color and intensity of the emitted light generated in white-light LED devices. Generally, the results show that the color and intensity of the emitted light of the studied phosphors are very sensitive to even minute changes of the composition of the material as well as on temperature. More specifically, it is shown that an increase in the "local structural and dynamics disorder", as reflected by, e.g., a softening of the chemical bonds of the crystal lattice and a decrease of the vibrational frequency of certain vibrational modes of the material, is correlated with a general decrease of the emission intensity. Therefore, the new understanding of the relationships between the light-matter interactions and optical properties established in this thesis provides effective design principles for the future developments of new, more efficient, LED materials.
In order to take on this challenge, I have investigated in detail key fundamental properties, such as the crystal structure and dynamics of atoms and their correlation to light-matter interactions in a family of promising phosphors. In particular, I have used a combination of advanced experimental techniques and theoretical methodologies, available at Chalmers as well as at large-scale research facilities in the Netherlands, United Kingdom, and the United States, that allowed to obtain unique insights into the effects of atomic structure and dynamics on both the color and intensity of the emitted light generated in white-light LED devices. Generally, the results show that the color and intensity of the emitted light of the studied phosphors are very sensitive to even minute changes of the composition of the material as well as on temperature. More specifically, it is shown that an increase in the "local structural and dynamics disorder", as reflected by, e.g., a softening of the chemical bonds of the crystal lattice and a decrease of the vibrational frequency of certain vibrational modes of the material, is correlated with a general decrease of the emission intensity. Therefore, the new understanding of the relationships between the light-matter interactions and optical properties established in this thesis provides effective design principles for the future developments of new, more efficient, LED materials.
Subject Categories
Inorganic Chemistry
Atom and Molecular Physics and Optics
Condensed Matter Physics
ISBN
978-91-7597-798-0
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4479
Publisher
Chalmers
KC, Kemihuset, Kemivägen 4
Opponent: Professor Eugeniusz Zych, Faculty of Chemistry, University of Wroclaw, Poland