Ion diffusion in lanthanide-doped nanocrystals

Abstract

The unique luminescence of lanthanide-doped nanocrystals has led to numerous scientific and technological applications. Over the last decades, synthesis methods have been developed to produce core/shell nanostructures, which enable spatial separation of dopant ions and thereby create new functionalities. Ionic diffusion is unwanted in these specific structures as it eliminates this spatial separation. On the other hand, the temperature dependence of ionic diffusion could also be useful, for instance to study migration of lanthanide ions under different conditions or to use in applications such as thermal history sensing. However, we currently lack the necessary practical, qualitative methods for measuring ion distribution. In this thesis, we study ion diffusion through simulations and experimental measurements. We calculated model decay curves from simulated concentration profiles to fit experimental decay curves from heated core/shell nanocrystals with different lanthanide dopants and shell lattices to study the effect of materials and temperature on diffusion speed. We found an increase by two orders of magnitude in the diffusion coefficient of NaYF4:Ho3+/NaYF4 from 1.31·10−24 to 1.11·10−22 m2/s when the temperature increased from 350 to 400 ◦C. Similar diffusion coefficients were found for NaYF4:Ho3+/NaGdF4. Thus, we have successfully developed a modelling technique to enable facile and quantitative tracking of ion diffusion. This technique can help to gain insight to improve core/shell structures and develop thermal history sensors for independently measuring the temperature and duration of a heating event.