Rare-earth doped photon upconversion (UC) nanomaterials have numerous applications in different fields such as wavelength conversion layers in solar cells, security inks, and autofluorescence-free fluorescent labels for bioimaging. However, these materials suffer from inherent limitations; the small excitation cross section and the low quantum efficiency are the obstacles for the practical applications. A promising approach to overcome these problems is the formation of nanocomposites composed of UC nanomaterials and metal nanostructures and utilizing the enhanced electric fields accompanied by the localized surface plasmon (LSP) resonances. A variety of nanocomposite structures has been proposed and tested so far, and more than 100-fold enhancement of the UC intensity has been reported [T. Hinamoto et. al, JPCC, 121 (2017) 8077].
In order to maximize the UC enhancement by the LSP resonance, a metal nanostructure has to satisfy some criteria. First, it should have multiple resonances at largely separated wavelengths, because the upconverted photon energy is usually 1.5-3 times larger than the excitation one. A LSP mode at the excitation wavelength is preferably a dark mode with a large absorption cross-section and a small scattering cross-section, while that at the emission wavelength is vice versa. Furthermore, the electric field distribution has to be optimized to maximize the overlap between the field enhancement region and a volume of an UC material. Apparently, metal nanostructures satisfying all these criteria are not simple.
In this work, we develop a metal nanostructure composed of a metal core and a metal nanocap, and placed an UC material in between. In this structure, the LSP modes split into bonding and antibonding ones due to the plasmon hybridization, and the resonance wavelengths can be controlled in a wide wavelength range by the strength of the hybridization. Furthermore, a strong enhancement of the electric fields is expected in the gap, where an UC material exists.
We fabricated the nanocomposites as follows. First, a shell of an UC material (Er and Yb doped Y2O3) about 10 nm in thickness was formed around a Au nanoparticle core (64 nm in diameter) by a homogeneous precipitation method. The composite nanoparticles were placed on a fused silica substrate, and then Ag about 20 nm in thickness was deposited for the formation of a Ag nanocap. The structure of the nanocomposites was characterized by HAADF STEM observations and EDS element mappings. The scattering and UC of single nanocomposites were studied by a dark-field microscopy coupled with a monochromator and visible (CCD) and near-infrared (InGaAs diode array) detectors. The measured scattering spectra and electromagnetic field simulations based on a boundary element method revealed that the nanostructure has two scattering peaks due to the bonding and anti-bonding modes. In this work, an UC enhancement of 5-fold was achieved in not well-optimized structures.