  In this work, the plasmonic properties of nanoparticle dimers with optical
responses over a wide spectral range have been investigated by varying the
inter-particle gap, dimer geometry, gap morphology, nanoparticle composition,
and refractive index of the surrounding medium. In particular, we have
theoretically investigated the plasmonic properties of spatially overlapping
symmetric gold nanodisks, shape-asymmetric gold nanodisk nanoplates, and
compositionally asymmetric gold-silver nanodisk dimers by varying the gap
separation from touching to overlapping regime. In such a configuration, we
have observed the appearance of a dominant bonding dimer plasmon (BDP) mode
that blue-shifts as gap separation turns from touching to overlapping. In
addition, it is found that asymmetric dimer produces a broader resonance shift
compared to symmetric dimer because of the hybridization of bright and dark
plasmon modes, making it a viable option for sensing applications. It is also
found that blue shifting of the plasmon mode occurred by changing the gap
morphology of the contacting region of the dimer for fixed nanoparticle size
and dimer overlapping. Moreover, we explored the influence of overlapping
nanoparticle dimer thickness and observed a notable resonance shift by varying
the thickness of the nanoparticle dimer. Finally, based on this tunable
resonance shift, we explored the sensing applications of bonding dimer plasmon
mode with optimized geometries. Thus, the computed figure of merit (FOM) of the
overlapping symmetric, shape-asymmetric, and compositionally asymmetric
nanoparticle dimers were found to be 1.55, 2.08, and 3.04, respectively, and
comparative advantages among the three configurations with implications for
surface-based sensing have been thoroughly discussed.
