Upconversion emission enhancement mechanisms of Nd3+-sensitized NaYF4:Yb3+,Er3+ nanoparticles using tunable plasmonic Au films: plasmonic-induced excitation, radiative decay rate and energy-transfer enhancement

Abstract

The upconversion luminescence (UCL) of rare earth ion-doped nanoparticles excited at 808 nm is more suitable for biological applications than those excited at 980 nm, as the former avoids overheating of biological tissues caused by 980 nm excitation light. However, one of the major challenges limiting the application of UCL of rare earth ion-doped nanoparticles excited at 808 nm is their low UCL efficiency. In this work, tunable plasmonic Au films were used to improve the UCL of Nd³⁺-sensitized NaYF₄:Yb³⁺,Er³⁺ nanoparticles. The results show that the enhancement factors and mechanisms of the UCL of Nd³⁺-sensitized nanoparticles are associated with the tunable plasmonic properties of Au films. The maximum enhancement factors of green and red UCL of NaYF₄:Nd³⁺,Yb³⁺,Er³⁺ nanoparticles excited at 808 nm are 6 and 5.8 on the Au film with ultra-broad plasmonic absorption band, respectively. A differentiation of UCL-enhanced mechanisms of NaYF₄:Nd³⁺,Yb³⁺,Er³⁺ nanoparticles on tunable plasmonic Au films was observed. The enhanced UCL of NaYF₄:Nd³⁺,Yb³⁺,Er³⁺ nanoparticles on the Au film with a narrow plasmonic absorption peak was due to the enhanced excitation field. The enhanced excitation field and energy-transfer enhancement was responsible for the UCL enhancement of NaYF₄:Nd³⁺,Yb³⁺,Er³⁺ nanoparticles on the Au film with ultra-broad plasmonic absorption.