Multi-wavelength pumped upconversion enhancement induced by Cu_2-xS plasmonic nanoparticles in NaYF₄@Cu_2-xS core-shell...

Cu_2-S nanoparticles (NPs) demonstrate unique tunable localized surface plasmon resonance (LSPR) and nonlinear optical properties, which are promising materials for photoelectric and display devices. In this work, we present highly improved upconversion luminescence (UCL) in the NaYF₄:Yb³⁺, Er³⁺@NaYF₄:Yb³⁺, Nd³⁺@Cu_2-S core-shell structure. The UCL enhancement is systemically studied under excitation of multi-wavelengths 808, 980, and 1540 nm, due to the broadband nature of Cu_2-S LSPR. Two different mechanisms synergistically contribute to the UCL enhancement, namely, the LSPR effect and two-photon effect, which lead to the extraordinary power dependence of UCL. UCL enhancement as high as 12-fold is achieved in the core-shell upconversion NPs (UCNPs). The core-shell NPs are printed on a paper substrate using a nano-printing technique, displaying different colors irradiated by different near-infrared light, and have potential applications in anti-counterfeiting, encryption, and display fields. These findings provide a method to design and optimize luminescent materials and demonstrate potential applications of plasmonic semiconductors and UCNPs.