Luminescence dynamics and enhancement of the UV and visible emissions of Tm 3+ in LiYF 4 :Yb 3+ ,Tm 3+ upconverting nanoparticles.

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.