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Nanoplasmonics driven by single-atom rearrangements

AuthorsKoval, P. CSIC ORCID; Marchesin, Federico CSIC; Barbry, Marc; Aizpurua, Javier CSIC ORCID ; Sánchez-Portal, Daniel CSIC ORCID
Issue Date2018
CitationCEN 2018
AbstractDimers of metal clusters can serve as model systems for tip-enhanced, surfaceenhanced or cavity-enhanced spectroscopies. Classical modeling is based on the electrodynamics of continuous media, using the framework of dielectric functions. Classical models correctly account for basic features of the near field in the vicinity of large clusters, measuring 10 nm and more. However, there are many effects that elude the classical modeling, especially when it comes to small clusters or to small distances between the clusters in the metallic dimers. Small clusters containing less than hundred atoms must be treated using quantum mechanics, while the atomistic nature of metal clusters may reveal itself in much larger clusters, containing thousands of atoms. In this work, we present a detailed ab initio atomistic calculation of induced near fields in the vicinity of a sodium cluster dimer 2×Na380 forming a plasmonic cavity. Our model consists of two almost icosahedral clusters. Each of the clusters exhibits facets, edges and tips made by atoms. Positions of atoms were optimized at the level of density functional theory (DFT), using the SIESTA software package. The induced near fields were determined at the level of time-dependent DFT, assuming an optical external perturbation. Induced near fields depend strongly on such geometry-dependent features as orientation of the external field, the distance between clusters and the mutual orientation of clusters. Besides these anticipated dependencies, there are more intriguing effects such as jump to contact due to atomic bridging and the formation of metal necks, taking place in the cavity during the initial approach and the consecutive retraction of the clusters, correspondingly. These effects involve the motion of atoms and have profound consequences on the geometrical and optical properties of the metallic cluster dimer. In particular, we show how structural reorganizations involving a few atoms, or even a single atom, lead to dramatic changes in the optical response of the whole structure. This effect is related to the conductance quantization in metal contacts of atomic cross-sections. Beyond the relatively simple setup of two metal clusters approaching and retracting, there are more complex scenarios such as ligand-protected noble-metal clusters, modeling of which may bring unexpected physics into play. Many aspects of these organo-metallic compounds may be captured by modeling based on ab initio molecular dynamics and time-dependent DFT allowed by our efficient numerical tools.
DescriptionResumen del trabajo presentado a la Spanish Conference on Nanophotonics (Conferencia Española de Nanofotónica-CEN), celebrada en Donostia-San Sebastián (España) del 3 al 5 de octubre de 2018.
Appears in Collections:(CFM) Comunicaciones congresos
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