Impact of the Nanoscale Gap Morphology on the Plasmon Coupling in Asymmetric Nanoparticle Dimer Antennas

Abstract

Coupling of plasmon resonances in metallic gap antennas is of interest for a wide range of applications due to the highly localized strong electric fields supported by these structures, and their high sensitivity to alterations of their structure, geometry, and environment. Morphological alterations of asymmetric nanoparticle dimer antennas with (sub)-nanometer size gaps are assigned to changes of their optical response in correlative dark-field spectroscopy and high-resolution transmission electron microscopy (HR-TEM) investigations. This multimodal approach to investigate individual dimer structures clearly demonstrates that the coupling of the plasmon modes, in addition to well-known parameters such as the particle geometry and the gap size, is also affected by the relative alignment of both nanoparticles. The investigations corroborate that the alignment of the gap forming facets, and with that the gap area, is crucial for their scattering properties. The impact of a flat versus a rounded gap structure on the optical properties of equivalent dimers becomes stronger with decreasing gap size. These results hint at a higher confinement of the electric field in the gap and possibly a different onset of quantum transport effects for flat and rounded gap antennas in corresponding structures for very narrow gaps.

Ideal Dimers of Gold Nanospheres for Precision Plasmonics: Synthesis and Characterization at the Single‐Particle Level for Identification of Higher Order Modes

“…For predicting the properties of AuNP dimers, computer simulations usually employ a pair of gold spheres separated by a constant gap distance . In contrast to theory, the experimental realization of such an ideal dimer of spheres is challenging because of structural inhomogeneities: (i) uncontrolled gap distance at the scale of Ångströms; (ii) AuNPs prepared by conventional reduction chemistry are faceted, nonspherical nanocrystals; and (iii) multiple gap morphologies, i.e., different configurations of the opposing crystal facets in the gap region . The combination of all these structural inhomogeneities leads to plasmonic inhomogeneities of the corresponding dimers.…”

“…The second issue, the nonperfect sphericity at the monomer level, has been solved in pioneering colloidal chemistry work using chemical etching yielding a smooth metal surface (Figure S2, Supporting Information) . The third issue, gap morphology, has recently been recognized as a decisive factor influencing the plasmonic properties of dimers or model system of dimers such as a particle on a mirror . However, it is currently not possible to experimentally control either gap morphology or to eliminate the influence of gap morphology in metal nanoparticle dimers.…”

Dumbbell gold nanoparticle dimer antennas with advanced optical properties

“…In principle, these observed variations in the far-field and near-field optical properties may have different origins. Particularly, the particle size, particle geometry and gap capacitor properties, such as, the gap size, the gap conductivity and the gap morphology are the most intuitive parameters, which have been shown to affect the LSPR position, width and scattering cross-section [18,25,28,36,59–60]. As a first parameter, variations in the gap size have to be considered.…”

“…Therefore, these NPs do not match with the shape of a perfect sphere used frequently for the simulations. Recently, it has been shown that the uniformity and size of the facets has strong influence on the homogeneity of their LSPR spectra [28,30,60]. From the high-magnification TEM images of the gap region of the dimers displayed in Fig.…”

“…In particular, DNA origami-structures provide a high versatility of the formed structures, however, the gap sizes on the sub-nanometer scale are difficult to control. Recently, we succeeded in the formation of dumbbell dimer antennas by means of the electrostatic interaction of positively charged 40 nm AuNPs and negatively charged 80 nm AuNPs [28]. Positively charged AuNPs were formed through a ligand exchange reaction with cysteamin.…”