Abstract
To maximize the intrinsic luminescence efficiency of the higher energy emissions of Tm(3+) in LiYF(4):Yb(3+),Tm(3+) upconverting nanoparticles, we investigated a specific range of Tm(3+) dopant concentrations. Reported to be optimized at 25% Yb(3+), 0.5% Tm(3+), due to the multitude of Tm(3+)-to-Tm(3+) interactions, the Tm(3+) concentration commonly used may not be suitable for strong UV and visible emissions. Thus, we varied the concentration of Tm(3+) in LiYF(4) nanoparticles between 0.08 and 0.55% to elucidate the effect of moderate changes of the dopant concentration on the UV, visible and NIR emissions. We determined a new optimized concentration of 0.24% Tm(3+) for maximal UV and visible emissions (nominally 0.2%). An extensive analysis of the luminescence spectra in the UV, visible and NIR regions and decay time measurements provides evidence for new luminescence mechanisms involving cross-relaxation pathways from the UV-emitting states of Tm(3+). Furthermore, we performed studies on an azobenzene derivative to demonstrate the substantial enhancement of the UV emissions by the newly optimized composition as evidenced by an increase in the degree of trans-cis photoisomerization.