Luminescent materials, also known as phosphors, have very interesting properties for a variety of applications. Traditional application domains are medical imaging and LED-based lighting. In the latter, 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 one or several well-chosen metal impurity ions that activate the luminescence. These impurities are mostly chosen from the first row of transition metals (3d series), such as Mn4+ or Cr3+ or from the lanthanides (4f series), such as Ce3+ or Eu2+. The particular choice depends on the intended application. Often, the incorporation of these luminescent ions induces the formation of so-called charge compensating defects, which are required to restore the charge balance in the crystal. Next to their stabilizing effect, these intrinsic defects are thought to have a strong influence on the performance of the luminescent ion itself, although this interaction is ignored in most studies. These effects can be detrimental for the phosphors’ properties as non-radiative decay channels might be generated. However, in some exceptional cases, the additional defects can generate metastable electronic states, allowing energy to be stored in the material. These metastable states can add functionality to the phosphor, turning it into a radiation dosimeter or a glow-in-the-dark phosphor, depending on the details of the potential energy landscape and the local charge carrier dynamics.

In this thesis, a model system will be chosen where compensating defects are known to play an important role in the phosphor performance. Thermodynamic stabilities of possible compensators will be computed and the interaction with luminescence activators studied.

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 their interaction with other defects.

Although the challenge of this thesis topic is clearly a computational one, depending on the preference of the student, experimental facilities are available to synthesize and characterize the optical and structural properties of luminescent materials. This offers the unique possibility to test whether the developed theoretical models comply with reality.

Goal

The goal of this master thesis is to study the role of charge compensating defects when trivalent lanthanide ions are incorporated on a divalent lattice site. Alkaline earth sulfides such as CaS or SrS, doped with Ce3+ form a perfect model system thanks to their high symmetry. These systems are known to feature interesting functional behavior such as energy storage, photochromism and afterglow. It is yet unknown which intrinsic defects (vacancies, interstitials, Frenkel pairs, …) are responsible for these phenomena. 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. In a second stage, defects are added to the unit cell and their stability and interaction with the luminescent center examined.

Interesting elective courses

- Simulations and Modelling for the Nanoscale
- Computational Materials Physics
- Luminescence

Aspects
Master of Science in Engineering Physics: This thesis subject is closely related to the following clusters of elective courses: MODELLING, PHOTONICS, MATERIALS and NANO. Physics aspects: use of quantum mechanical models and insight in quantum mechanical phenomena; engineering aspects: application to materials and optical properties