Abstract
The electronic coupling between metal nanoparticles is responsible for intriguing new phenomena observed when the particles are near touching contact, which is exemplified by recent investigations of nanoparticle dimers. However, little is known about the role of the molecular orbitals of the nanoparticle dimers. The expectation is that the physics and chemistry of the system must be reflected in the orbitals that control the bonding at touching contact. This expectation is borne out in the present investigation in which we present a comprehensive theoretical study based on density-functional theory of the electronic coupling between two silver nanoparticles. We explain our findings by studying the molecular orbitals of the dimers as a function of the separation and relative orientation between the nanoparticles. We show that as the nanoparticles approach each other a bond-forming step takes place, and that the strength of the hybridization is a key element to determine various properties of the system. We find that the relative orientation between the nanoparticles plays an important role in determining the strength of the coupling which can be visualized by the spatial distribution of the highest occupied molecular orbitals. Moreover, the strength of the coupling will in turn determine the ease of their transition to the nonlinear dielectric-response regime. This effect allows for the tunability of the electronic coupling and magnetic moment of the dimer. Our findings are essential for understanding and tailoring desired physical and chemical properties of closely aggregated nanoparticles relevant for applications such as surface-enhanced Raman scattering and quantum transport in molecular devices.