Light emitting material, or phosphors, are used for obvious applications like lighting (LED phosphors) and displays (screens) but also for less obvious ones like in medical diagnostics, phosphors for security features as in bank notes, energy conversion phosphors for harvesting solar energy etc. One may distinguish direct conversion phosphors, afterglow phosphors, and storage phosphors. In afterglow and storage phosphors free electrons and/or holes created during the excitation phase or stored in charge carrier trapping centres. In the case of shallow trapping, the carriers are slowly liberated at room temperature leading to visible afterglow (glow in the dark phosphors). In the case of deep trapping, carriers needs to be stimulated by photons (Optically Stimulated Luminescence) or by increasing the temperature (Thermo Luminescence).
The Luminescence Materials group worked on the development of ZnGa2O4:Cr3+ afterglow phosphors. Figure 1 shows how nano-particles injected in the blood stream of a tumour bearing mouse are distributed. The distribution is monitored by detecting the red afterglow photons from Cr3+ that are able to escape the mouse.
Projects on phosphors
Afterglow and storage phosphors
We aim to engineer afterglow and storage phosphors by studying appropriate combinations of host material with luminescence centre and trapping centre. Combinations are limitless and to guide our research we wish to deliberate design materials. Our models on the electronic structure of lanthanide doped materials are used as design tool.
Solar conversion phosphors
To enhance the efficiency of solar cells we seek for materials that convert the entire solar spectrum to wavelengths where the solar cell has maximum efficiency. We particularly aim for efficient Tm2+ doped thin film materials.
Figure 1. The bio-distribution of ZnGa2O4:Cr3+ nano-particles in a tumour bearing mouse. From Nature Materials 13 (2014) p.418.