Colour-optimized quantum yields of Yb, Tm Co-doped upconversion nanocrystals.

We present here a systematic analysis of the influence of Tm doping concentrations (x ) on the excitation power (P)-dependent upconversion luminescence and -performance of hexagonal-phase NaYF: 20% Yb, x % Tm upconversion nanoparticles (UCNPs) for x of 0.2, 0.5, 0.8, 1.2, and 2.0, respectively. Our results reveal the influence of these different Tm doping concentrations with respect to optimized upconversion quantum yield (Φ ) values of the various Tm upconversion emission bands, with the highest Φ values of the Tm emission bands above 700 nm resulting for different x values as the Tm emission bands below 700 nm. This underlines the potential of Tm dopant concentration for colour tuning. Special emphasis was dedicated to the spectroscopic parameters that can be linked to the (de)population pathways of the various Tm energy levels, like the P- and x -dependent slope factors and the intensity ratios of selected emission bands. The evaluation of all parameters indicates that not only energy transfer upconversion-, but also cross-relaxation processes between neighbouring Tm ions play a vital role in the (de)population of the excited energy levels of Yb, Tm codoped nanocrystals.