Plasmon-enhanced upconversion luminescence of the composite films through tunable ZnO spacer

Surface plasmon effects of Ag nanoparticles (NPs) have been extensively applied to enhance upconversion luminescence (UCL) properties of the nanoparticles doped with rare earth ions. However, it is important to control the effective coupling distance between the Ag NPs and the upconversion nanoparticles (UCNPs). In this work, the Ag/ZnO/UC composite films were fabricated by vacuum methods, and the influence of the ZnO spacer layer thickness on spectral properties was investigated. These studies reveal that the ZnO spacer thickness has a strong effect on the Ag NPs plasma resonance and UCL intensity. The highest enhancement factors of the red and green emissions were found to be 18.6 and 5.3 under 980 nm excitation, respectively. That is attributed to the enhancement of the absorption and excitation rate for UCNPs under the equilibrium between non-radiative energy and local field effect.