Plasmonic nanostructures can confine light at their interface with the dielectric surrounding through surface charge density oscillations called surface plasmon polaritons (SPP). By assembling such nanostructures into dimers, we can create nano-cavities that could confine optical fields to volumes much lower than the dffraction limit. The hybridized plasmon resonances generated due to the formation of such nanogaps critically depend on the morphology of the gap, the size, shape and material of the constituent monomers, and most importantly, on the inter-particle gap distance.

In this work, molecular linkers are used to assemble the gold nanospheres (AuNS) and palladium nanocubes (PdNC) to create dimers resulting in the formation of nano-cavities. The plasmon coupling behaviour of the coupled cavity mode is explored using single-particle dark-field scattering spectroscopy and the enhanced optical fields associated with the cavity modes are utilized to enhance the Raman scattering from molecules positioned in the gap. The inter-particle gap distances between the AuNS dimers with three different monomer sizes are precisely varied by using different linker molecules. Highly uniform structural characteristics
of monomer spheres, along with precise control over inter-particle gap distances through molecular linkers, result in highly uniform plasmonic properties at the single-particle level (precision plasmonics). The behaviour of gap plasmon modes at shorter gap distances points to a deviation from classical predictions due to the onset of quantum tunneling across the junction. A nanoparticle size-dependent onset of quantum tunneling is observed, with larger dimers exhibiting an earlier onset of quantum tunneling. 2D quantum-corrected simulations indicate that the curvature of the nano-cavity walls affects the rate of quantum tunneling. A
greater effective conductivity volume is available with larger dimers with lower curvature radius, which enables more electrons to tunnel across the junction than smaller dimers for the same gap distance. Subsequently, the field enhancement properties of AuNS dimer cavities are studied using polarization-resolved single-particle surface-enhanced Raman scattering (SP-SERS) spectroscopy and finite-difference time-domain (FDTD) simulations. Similarly, plasmon coupling and field enhancement properties in PdNC dimer cavities are also characterized by means of single-particle experiments and simulations. The simulations predict a
significant field enhancement in the dimer cavity, which is validated by polarization-resolved SP-SERS. Such PdNC dimers can therefore be employed for label-free SERS monitoring of Pd-catalyzed reactions.