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

We present here a systematic analysis of the influence of Tm³⁺ doping concentrations (x _Tm ) on the excitation power (P)-dependent upconversion luminescence and -performance of hexagonal-phase NaYF₄: 20% Yb³⁺, x _Tm % Tm³⁺ upconversion nanoparticles (UCNPs) for x _Tm of 0.2, 0.5, 0.8, 1.2, and 2.0, respectively. Our results reveal the influence of these different Tm³⁺ doping concentrations with respect to optimized upconversion quantum yield (Φ _UC ) values of the various Tm³⁺ upconversion emission bands, with the highest Φ _UC values of the Tm³⁺ emission bands above 700 nm resulting for different x _Tm values as the Tm³⁺ emission bands below 700 nm. This underlines the potential of Tm³⁺ 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 Tm³⁺ energy levels, like the P- and x _Tm -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 Tm³⁺ ions play a vital role in the (de)population of the excited energy levels of Yb³⁺, Tm³⁺ codoped nanocrystals.