Colour-Optimized Quantum Yields of Yb, Tm Co-doped Upconversion Nanocrystals.

We present here a systematic analysis of the influence of Tm<sup>3+</sup> doping concentrations (<i>x<sub>Tm</sub></i>) on the excitation power (<i>P</i>)-dependent upconversion luminescence and -performance of hexagonal-phase NaYF<sub>4</sub>: 20 % Yb<sup>3+</sup>, <i>x<sub>Tm</sub></i> % Tm<sup>3+</sup> upconversion nanoparticles (UCNPs) for <i>x<sub>Tm</sub></i> of 0.2, 0.5, 0.8, 1.2, and 2.0, respectively. Our results reveal the influence of these different Tm<sup>3+</sup> doping concentrations with respect to optimized upconversion quantum yield (<i>Φ<sub>UC</sub></i>) values of the various Tm<sup>3+</sup> upconversion emission bands, with the highest <i>Φ<sub>UC</sub></i> values of the Tm<sup>3+</sup> emission bands above 700 nm resulting for different <i>x<sub>Tm</sub></i> values as the Tm<sup>3+</sup> emission bands below 700 nm. This underlines the potential of Tm<sup>3+</sup> 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<sup>3+</sup> energy levels, like the <i>P</i>- and <i>x<sub>Tm</sub></i>-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<sup>3+</sup> ions play a vital role in the (de)population of the excited energy levels of Yb<sup>3+</sup>, Tm<sup>3+</sup> codoped nanocrystals.