Boosting NIR emission through Yb concentration optimization in active-core/active shell upconversion nanocrystals

Official URL: https://dx.doi.org/10.1016/j.optmat.2024.115519

Here in, we report a new strategy to achieve a strong emission in the NIR region, especially in ∼802 nm, by introducing a core-shell structured UCNPs, denoted as NaYF4: 25%Yb3+, 0.5%Tm3+@NaYF4: x%Yb3+ (x = 0, 2, 5,15,100), offering a superior advantage for future biosensing-bioimaging application. Through the induction of specific Yb3+ ion concentrations (2 % and 5 %) into the shell, under 980 nm laser excitation, we noticed a selective reduction in emission peaks in the visible region with an intensified NIR (802 nm). To evaluate the results, the NIR/visible ratio was examined by increasing the Yb3+ dopant concentration in the shell, and the highest ratio was observed for NIR/red emission (NIR/646 nm) increasing from 38 (0%Yb3+). 62 (5%Yb3+) and 40.9 (15%Yb3+) indicate greater attenuation of the emission at 646 nm compared to 449 nm, and 473 nm. Decay time and power dependence analysis studied on the optimum nano crystals outcomes (NaYF4: 25%Yb3+,0.5%Tm3+@ NaYF4: x%Yb3+ (x = 0, 2, 5). The results highlighted a significant increase in decay time at the 3H4 → 3H6 state of Tm3+ ions in line with the enhancement of 802 nm emission. The power dependence analysis for the NIR (802 nm), the slope values obtained as 2.03 (core) to 1.68 (0%Yb), 2.01(2%Yb), 1.76 (5%Yb) and 1.97 (15%Yb), indicating the two-photon process is responsible upconversion process. These findings provide evidence that either decay time or power dependence studies align with the enhancement of 802 nm emission.