Plasmon Resonances in Nanohemisphere Monolayers

Plasmonic devices, consisting of nanoparticle monolayers, are conveniently fabricated by deposition techniques, wherein thermodynamics often favor the particle shape to be close to hemispherical. The present work investigates plasmon modes in Ag nanohemispheres (NHSs) using s- and p-polarized incident radiation at varying angles. The Ag NHSs, immobilized on nanoposts, resembling mushroom structures, allow for reduced substrate coupling and convenient resolution of the modes. Additionally, the modes are studied by in situ extinction acquisitions during nanoparticle synthesis and elucidated by numerical simulations. It is revealed that the broken symmetry by asymmetric particle shape leads to dipolar modes parallel and normal to the base, which are significantly different in terms of energy, excitation dependence on polarization, and particle–particle as well as particle–substrate couplings. In particular, the major parallel mode offers distinct advantages in plasmonics applications over nanospheres. For example, its strong substrate coupling may benefit thin film photovoltaics by efficient light coupling. Higher field concentrations are induced at the sharp edges of a NHS that may enhance hot electron injection in a photocatalyst. Unlike in a spherical dimer, where the field intensity peaks in the middle of the gap, the maximum field in a NHS dimer gap occurs on the metal surface (i.e., at the edges), overlaying with the chemical enhancement. Hence, a higher surface enhancement factor can be achieved in Raman scattering