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

Abstract

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.