Luminescence dynamics and enhancement of the UV and visible emissions of Tm3+ in LiYF4:Yb3+,Tm3+ upconverting nanoparticles

To maximize the intrinsic luminescence efficiency of the higher energy emissions of Tm3+ in LiYF4:Yb3+,Tm3+ upconverting nanoparticles, we investigated a specific range of Tm3+ dopant concentrations. Reported to be optimized at 25% Yb3+, 0.5% Tm3+, due to the multitude of Tm3+-to-Tm3+ interactions, the Tm3+ concentration commonly used may not be suitable for strong UV and visible emissions. Thus, we varied the concentration of Tm3+ in LiYF4 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% Tm3+ 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 Tm3+. 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.