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. Figure 1a shows the structure of the alkaline earth sulfides, which become luminescent when doped with lanthanide ions such as europium or cerium. The corresponding energy diagram for the Eu²⁺ dopant (Figure 1b) indicates the involved transitions, which can be examined via theory. An example of an experimental photoluminescence emission spectrum is shown in Figure 1c. Figure 1 contains the various aspects of solid-state phosphors that can be investigated to unravel the luminescent behavior of the material. From the emission spectra, it is clear that a different mechanism is playing a role for CaS:Eu and SrS:Eu on the one hand and for BaS:Eu²⁺ on the other hand. In the former case, the luminescence is governed by 4f⁷-4f⁶5d¹ transitions in the Eu²⁺ ion. In the latter case, which is often called anomalous emission, the exact mechanism is still under debate. Possible explanations are the involvement of impurity trapped excitons, the 4f⁶6s¹ manifold of Eu²⁺ or states related to the conduction band. The case of MgS:Eu²⁺ is not depicted in the figure, but is also of interest because mechanical stress is thought to alter its luminescence properties.

Figure 1. Structure of the Eu²⁺ cluster in MS (M = Mg, Ca, Sr, Ba) (a), corresponding energy diagram (b) and photoluminescence emission spectrum (c).

In this thesis project, we will combine theory and experiment to investigate this interesting class of phosphors, based on the luminescence of broadband emitting lanthanide ions (such as Yb²⁺, Eu²⁺ or Ce³⁺, …) inside solid host crystals such as MgS, CaS, SrS and BaS.

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 in 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.

Optical spectroscopy allows distilling information on the electronic structure of the luminescent ion inside the host crystal directly from experiment. 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.

Aim 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.