Luminescent materials, also known as phosphors, have very interesting properties for a variety of applications. Traditional application domains are LED-based lighting and imaging, where the phosphors convert blue light into green, yellow or red light in order to achieve a white emission spectrum from one (blue) pumping LED. To date, many questions remain on the precise physical processes responsible for their particular luminescent behavior, and a combination of theory and experiment is often needed to gain a deeper understanding.

Phosphors are typically built from an inorganic host compound which is activated by the addition of a well-chosen metal impurity that activates the luminescence. In this thesis, lanthanide ions such as Ce3+, Eu2+ and Yb2+ will be studied where the luminescence originates from localized 4fN-4fN-15d1 transitions. The color of the luminescence of these ions can be tuned over the full visible range, depending on the details of how the lanthanide ion forms chemical bonds with the atoms of the host compound (see Fig. 1). In this thesis, a particular host compound will be selected for an in-depth study of how the lanthanides get incorporated, their possible charge states and the mechanism of their luminescence. Systematic behavior across the lanthanide series will be looked for. This study will involve not only the 5d orbital, but we will also investigate the role of other, often overlooked excited states such as the 4fN-16s1 configuration or excitons that can be mobile or trapped at the impurity. Another interesting possibility is to investigate how the lanthanide impurities interact with intrinsic defects in the host compound. These defects might be responsible for competitive non-radiative decay channels which are detrimental for technological applications. This is however not well understood.

In this thesis project, we will combine theory and experiment to investigate luminescent materials, based on the luminescence of broadband emitting lanthanide ions.

Ab initio modeling of periodic systems has progressed enormously and can be used to rationalize and elucidate the structure-activity relationship of complex materials. In this respect, Density Functional Theory (DFT) is one of the most interesting electronic structure methods due to its excellent accuracy/computational cost ratio. It is in principle exact, although an approximated exchange-correlation functional has to be used. Modeling of luminescent solid state materials is based on accurate structure optimizations involving periodic codes. The simulations can reveal insight into the position of the lanthanide ion and stability of defects that are sometimes needed to obtain luminescence. Computation of the luminescent behavior itself requires optimization of the excited state and is no routine activity yet. Currently, there are many developments in this field and new implementations are written in existing software packages.

From an experimental point of view, optical spectroscopy allows directly distilling information on the electronic structure of the luminescent ion inside the host crystal. Furthermore, measurements of the decay dynamics of the luminescent material upon pulsed excitation yield information on the lifetime of the excited electronic state. Temperature dependent measurements allow probing the impurity levels of the dopant inside the band gap of the host crystal and can reveal energy transfer processes.

Goal
The goal of this master thesis is to unravel the luminescent behavior of lanthanide-doped solid state materials using a combined theoretical-experimental approach. Theoretical data will be computed using periodic unit cells. A first step will be the geometry optimization of the parent and lanthanide-doped material. The band structure and density of states can then be examined. Optimization of the excited state is more challenging, and newly developed methods will be tested for the computation of excited-state properties. Overall, the theoretical information will be compared with experimental data obtained by temperature and time dependent optical and luminescence spectroscopy. Phosphors with different concentrations will be investigated, to determine not only the properties of single, unperturbed luminescent ions, but to evaluate also relevant energy transfer processes.

The main challenge of this master thesis is the combination of theory and experiment. This combined approach ensures that the interested student gains a profound insight in the luminescent materials and the precise physical processes responsible for it. Due to the variety of aspects available in this master topic, the focus can be shifted to particular items depending on the interest of the student.

Physics / Engineering aspects

Physics: use of quantum mechanical models
Engineering: application to materials and optical properties