Integrated Seamless Non-Noble Plasmonic Ni-Upconversion Nanofilm for Stable and Enhanced Fluorescence Performance

Thickness-controlled, easily patterned upconversion (UC) nanofilms are essential for high-precision optoelectronic devices, but challenges such as imprecise thickness control and low fluorescence intensity hinder their application. High-performance lanthanide-doped sodium yttrium fluoride UC materials are typically available in powder form, making direct integration into advanced devices difficult. Although physical vapor deposition (PVD) enables precise film formation, it often produces poor crystalline structures and weak fluorescence. To overcome these limitations, integrating non-noble plasmonic Ni with surface plasmon resonance to enhance fluorescence intensity is a promising yet understudied strategy, likely due to Ni's ultraviolet resonant wavelength and oxidation susceptibility. This study introduces an integrated Ni-UC nanofilm design, combining an ultrathin Ni layer with a NaYF₄:Tm, Yb UC layer via PVD, followed by post-annealing. Annealing at 500 °C transforms the UC layer into a hexagonal-phase crystal structure while protecting the Ni layer from oxidation. The unannealed UC nanofilm showed no fluorescence, whereas the annealed UC nanofilm displayed clear peaks at 476, 648, and 699 nm. Notably, the integrated Ni-UC nanofilm exhibited fluorescence intensities 5.29, 4.43, and 4.29 times higher at these wavelengths, respectively. Additionally, the integrated design exhibited high transparency and stability, highlighting its protective benefits. These results underscore the potential of the integrated Ni-UC nanofilm for advanced optoelectronics and sensing technologies, offering enhanced fluorescence, micro-processing compatibility, and robust performance in a cost-effective, non-noble plasmonic system.