Molecular orbital view of the electronic coupling between two metal nanoparticles

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.

Theoretical Studies of Plasmonics using Electronic Structure Methods

“…There is tremendous interest in understanding the quantum nature of nanoparticle aggregrates, since aggregates provide a unique way of tuning the optical response by varying the distance between the nanoparticles. − Additional interest stems from the extremely large localized electric fields that can be found in the gap between nanoparticles (the so-called hot spots) when the plasmons are excited. As the gap between the particles decreases, the coupling increases, leading to the very high electric fields.…”

TDDFT and CIS Studies of Optical Properties of Dimers of Silver Tetrahedra

“…Dimers of metal nanoparticles, usually of Ag or Au, have been researched experimentally and theoretically. − A dimer of nanoparticles can play a key role in the understanding of hybridized plasmons in complex nanostructures and provide a simple system for systematic studies of plasmon interactions. These studies showed that there is red shift of the longitudinal plasmon resonance as the interparticle distance decreases in the nontouching regime, whereas it will start to blue shift when the two particles touch. ,,− In 2008, Lassiter et al investigated the plasmon interaction of individual pairs of nanoshells in the adjacent and touching regimes using environmental scanning electron microscopy (ESEM) experimentally and the boundary element method (BEM) simulation theoretically .…”

“…They determined how the energies and intensities of the dimer peaks change and new excitation peaks grow as the distance decreases. Recently, ab initio density functional theory has been used to study the electronic coupling between silver nanoparticles of different sizes and with different relative orientations. − …”

Plasmon Resonance of Isolated Gold Hollow Nanoparticles and Nanoparticle Pairs: Insights from Electronic Structure Calculations

“…In this case, two NPs are strongly coupled, and there is significant overlap between the wave functions of the neighboring atoms. A bond-forming step takes place . This is verified by the spatial distribution of LUMOs of Au17 – and Au32 NP pairs.…”

“…A bond-forming step takes place. 67 This is verified by the spatial distribution of LUMOs of Au17 À and Au32 NP pairs. (See Figures 7 and 8.)…”