Concentration dependent optical transition probabilities in ultra-small upconversion nanocrystals.

ABSTRACT

Transition probability is of vital importance for luminescence process, whereas the effects of doping concentration have not been explored in the Er3+NaGdF4. In this work, we investigate the radiative transition probabilities of Er3+ highly doped NaGdF4 sub 10 nm nanocrystals using J-O theory. It is found that the transition probabilities vary with changing Er3+ concentration, especially altering the ratio of Er3+ 2H11/2 to 4S3/2 level, which is highly useful for optical thermometers as they are thermally coupled. To validate the concentration dependent transition probabilities, significant enhancements of upconversion luminescence are achieved by epitaxial growth of the inert shell, and thermal sensing behaviors are investigated using the improved samples.