Energy storage in lanthanide/s² ion activated phosphates

Promotor: Prof. dr. Philippe Smet en dr. Jonas Joos

Supervision: David Van der Heggen

Problem statement:

Photoluminescent (PL) materials or phosphors are ubiquitous in many technologies such as LEDs for lighting and information displays, medical imaging and scintillators to detect high energy radiation. These materials are typically composed of an inorganic host crystal with a large band gap that is deliberately doped with one or more impurities that activate the luminescence. These impurities can be a transition metal (e.g. Cr3+, Mn2+, Fe3+, …), a lanthanide (Ce3+, Eu2+, Tb3+, …) or heavy p-block ions that are also known as s2 ions (e.g. Pb2+, Bi3+, Tl+). Depending on the type of ion, different electronic transitions give rise to the luminescence. Typically, the luminescence process can be described by two steps, i.e. the absorption of a photon and the subsequent emission of a photon with slightly lower energy, the energy difference being dissipated as heat.

A special class of luminescent materials are those that can store the absorbed energy, typically by trapping photo-ionized charge carriers at intrinsic or extrinsic defects. It can be released after the application of external stresses such as pressure (mechanoluminescence, ML), temperature (thermoluminescence, TL) or the absorption of another photon (optically stimulated luminescence, OSL). Glow-in-the-dark materials are an example where the ambient temperature triggers the slow release of the stored energy. In this thesis, particular systems are investigated that are expected to show energy storage. Phosphate host crystals are selected due to their chemical tunability, offering a means to manipulate physical properties such as band gap energies, dielectric screening etc. Upon doping by lanthanide ions, in particular Ce3+, Eu2+ and Yb2+, efficient 4fN – 4fN-15d luminescence is activated. It has been shown that energy storage occurs in particular phosphate crystals without co-doping. Here s2 ions, in particular Bi3+ and Sb3+, are introduced as a co-dopant as they are believed to behave as traps for charge carriers. Up to now, their trapping mechanism is however not established. Furthermore, there is still disagreement on which charge states these s2 ions can take and how they behave electronically as impurity in inorganic hosts. By studying the luminescence of the lanthanide ions, it is the goal to learn about the behavior of the s2 ions in an indirect way.

Goal:

Luminescent phosphates will be prepared by a solid state reaction. The nature of the materials (orthophosphates, metaphosphates, pyrophosphates, …) will be varied by tuning the reaction parameters and precursor materials. Subsequently, lanthanide (Ce3+, Eu2+ and Yb2+), s2 (Bi3+ and Sb3+) co-doped materials will be selected for a profound study of their charge trapping dynamics, including the measurement of charging behavior, luminescent decay, TL and OSL. For this, the student will be educated to operate the state-of-the-art spectroscopic tools that are available within LumiLab. The outcome of this experimental investigation will serve as the input to devise energy level schemes of the impurity ion in the phosphate hosts. These schemes should help to explain the trapping/detrapping behavior in the materials and validate various claims on the luminescence mechanisms involving s2 ions. Finally, the most promising phosphors will be optimized with regard to the envisioned applications such as afterglow or storage phosphor.

Opmerking:

Dit thesisonderwerp kan ook opgenomen worden in het kader van de Educatieve Master Fysica en Sterrenkunde.