As a promising plasmonic photocatalyst, an Ag-Pt core-shell nanostructure is able to convert sunlight into chemical energy. To fully exploit each function of the constituent materials, a numerical study on the optimal design of the hybrid nanostructures is presented in this work. Ag-Pt is demonstrated to be a good material configuration for the core-shell nanostructure because Ag has strong intrinsic plasmonic responses and a low imaginary dielectric function in the visible region, while Pt is catalytically active and has a large imaginary dielectric function. Considering the hot carrier generation and transfer processes in both plasmonic photocatalysis and photothermal catalysis, the catalytically active sites at the Pt shell can be revealed by high local heating power densities. For the dipole resonance, these sites distribute alternately with the spots where local electric fields are greatly enhanced. The former are along the “equatorial belt” of the nanoparticle, while the latter are in the two polar regions. It is then found that the high-efficiency hot carrier generation is related to multiple factors, including at least an ultrathin shell and a core of high aspect ratio with sharp tips. The physics behind these factors is further addressed.

For stabilization and functionalization, plasmonic nanoparticles (NPs) are often coated with dielectric shells, yet suffer from the weakening of local electric field enhancement. Herein, we numerically demonstrate that, instead of being weakened, the local electric fields can be further enhanced using an appropriate high refractive index dielectric coating. Because the high refractive index dielectric shell owns strong Mie resonances that can participate in the bonding plasmon hybridization, it can avoid excessive diminishing of light absorption when the shell becomes thick. Meanwhile, it can sustain a great leap of the local electric fields reaching the surface, which follows the boundary conditions at the interface within electrodynamics. The two mechanisms can work synergistically to offset the decay of the local electric fields inside the shell. It is also worth noting that a dielectric shell with too high a refractive index may badly suppress the light scattering by the core-shell NPs, going against the light coupling effect. These findings are of significant importance in extended applications of surface enhanced spectroscopies, especially for shell-isolated NP-enhanced Raman spectroscopy and surface enhanced fluorescence.

Plasmonic nanoparticle (NP) dimers, generating highly intense areas of electric field enhancement named hot spots, have been playing a vital role in various applications like surface enhanced Raman scattering. For stabilization and functionalization, such metallic NPs are often coated with dielectric shells, yet suffer from a rapid degeneration of the hot spot with the increase of the shell thickness. Herein, it is demonstrated that the use of appropriately high refractive dielectric coatings can greatly reduce the loss of local electric field enhancement, maintaining usable hot spots. Two mechianisms work synergistically. Firstly, the high refractive index dielectric coating enables a great leap of the local electric fields reaching the gap, which follows the boundary conditions at the interface within electrodynamics. Secondly, owing to its strong Mie resonances that can participate in the plasmon hybridization, the high refractive index dielectric coating contributes to a strong light coupling effect in terms of improving the light absorption. Taking advantage of the proposed physical process decomposition, both the resonance shift and local electric field enhancement can be elaborated. These findings should be of significant importance in extended applications of surface enhanced spectroscopies and related plasmonic devices based on hot spots.

Due to the generation of electromagnetic hot spots, metallic nanogaps have attracted great attention in various subjects, especially in plasmonics. For fixed nanogaps, using dielectric coating is an efficient way to narrow the gaps while improving their thermal and chemical stability. Herein, it is numerically demonstrated that the utilization of high refractive index dielectric coatings with zero or little loss, like TiO2, can lead to the generation of remarkable hot spots at the nanogaps. Two mechanisms work synergistically including the improved light coupling effect and a great leap of local electric field intensities at the dielectric-air interface. For higher refractive index dielectric coatings in practice, like graphene, despite a greater electric field leap at the interface, the weakening light coupling effect turns to play a dominant and negative role owing to the lossy dielectric properties. Our results provide a new perspective for the stabilization and functionalization of metallic nanogaps, which should be of significant importance in extended applications of surface enhanced spectroscopies as well as the fabrication, integration and packaging of plasmonic nanogap devices in the near future.

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