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

Transition probability is of vital importance for luminescence process, whereas the effects of doping concentration have not been explored in the Er:NaGdF. In this work, we investigate the radiative transition probabilities of Er highly doped NaGdF sub 10 nm nanocrystals using J-O theory. It is found that the transition probabilities vary with changing Er concentration, especially altering the ratio of ErH to S 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.