Nanogap plasmonic field enhancement on hydrogen-absorbing transition metals

Abstract

The electromagnetic field enhancement factors by gap plasmons between two spherical metal particles are calculated for hydrogen-absorbing transition metals Pd, Ti, and Ni, and reference noble metals Au, Ag, and Cu, in air, H2, or vacuum, and H2O. The dependence of the field enhancement factors on the metal species, the field wavelength, the electric field polarization, the separation of the two metal particles, and the observing location is systematically investigated. Field enhancement is observed significantly large in the gap of two metal particles and sensitive to the particle separation, but insensitive to the position in the gap, indicating a geometric flexibility for applications. The spectral peak field enhancement factors for Pd, Ti, and Ni do not compete with those for Au, Ag, and Cu, but do in the microwave regime. For the electric field parallel to the bipartite alignment, the field enhancement factors in the gap for Pd, Ti, and Ni are observed as large as several hundred and ten thousand for the separation-to-radius ratios of 0.1 and 0.01, respectively, for a wide wavelength region spanning from the visible to the infrared. The large field enhancements in the nanogaps of hydrogen-absorbing transition metals observed in this study can potentially be utilized for various energy applications, such as hydrogen storage, sensing, and nuclear fusion. In practical metallic material systems, it is important to account for such a gap-plasmon effect because nanoscale gaps commonly exist, for instance, on rough metal surfaces and in metal particle aggregates.