Large Upconversion Enhancement in the "Islands" Au-Ag Alloy/NaYF4: Yb3+, Tm3+/Er3+ Composite Films, and Fingerprint Identification

The surface plasmon (SP) modulation is a promised way to highly improve the strength of upconversion luminescence (UCL) and expand its applications. In this work, the islands Au-Ag alloy film is prepared by an organic removal template method and explored to improve the UCL of NaYF4: Yb3+, Tm3+/Er3+. After the optimization of Au-Ag molar ratio (Au-1.25-Ag-0.625) and the size of NaYF4 nanoparticles (NPs, approximate to 7 nm), an optimum enhancement as high as 180 folds is obtained (by reflection measurement) for the overall UCL intensity of Tm3+. Systematic studies indicate that the UCL enhancement factor (EF) increases with the increased size of metal NPs and the increase of diffuse reflection, with the decreased size of NaYF4 NPs, with the decreased power density of excitation light and with improving order of multiphoton populating. The total decay rate varies only ranging of about 20% while EF changes significantly. All the facts above indicate that the UCL enhancement mainly originates from coupling of SP with the excitation electromagnetic field. Furthermore, the fingerprint identification based on SP-enhanced UCL is realized in the metal/UC system, which provides a novel insight for the application of the metal/UC device.