Quantifying the plasmonic character of optical excitations in nanostructures from first principles

Localized surface plasmon resonances in nanostructures provide powerful tools to manipulate light at the nanoscale, below the diffraction limit. Metal nanoparticles are the most traditional plasmonic materials, but recently graphene and its nanostructures, whose molecular limit is represented by polycyclic aromatic hydrocarbons (PAHs), have been described as plasmonic as well. On the theoretical side, many efforts have been devoted to find a correct microscopic description of plasmons in nanostructures and it is still an open and controversial field [1]. While in large nanosystems optical and plasmonic properties are generally well described by semiclassical models of the frequency-dependent dielectric function, when the system size reaches a few nanometers, quantum finite-size effects become important and simplified descriptions fail. Thus atomistic first principles approaches are required to correctly describe plasmonic properties of nanosystems below ∼ 10 nm in size. Within the framework of solid-state physics, the electronic excitations of a system are generally classified as single-particle ones or plasmons. At the nanoscale this simple classification is usually no more applicable. A few numbers of approaches have been proposed recently in order to classify the excitations of nanosystems [2-4]. Those works provide inspiring insights into the microscopic origin of plasmonic resonances in small nanostructures, but they lack a simple quantification of the relative plasmonic character of the electronic excitations. Here we define an index that quantifies the plasmonicity of a given excitation and thus we present a classification of the excitations based on the quantitative assessment of their plasmonic character [5]. This approach results to be especially powerful for those systems where it is not always possible to clearly distinguish plasmons from single-particle excitations. In particular, we consider four different paradigmatic molecular systems that have been previously described as plasmonic. We take into account a linear sodium chain consisting of 20 atoms and a tetrahedral Ag 20 cluster as model plasmonic metallic nanosystems, a naphthalene molecule as an example of molecular system that hosts “molecular plasmons”, and a coupled system composed of a tetrahedral Ag 20 cluster and a pyridine molecule, which represents a prototypical hybrid system. Their electronic and optical absorption properties are investigated through DFT and TDDFT-based state-of-the-art first-principles codes [6]. The resulting description is consistent with the existing literature and provides the desired insight, thus paving the way for the application of our index to study complex systems whose plasmonic properties are unknown a priori . References [1] F.J.G. de Abajo, et al., Faraday Discuss. 178, 123 (2015). [2] S. Bernadotte, et al., J. Phys. Chem. C 117, 1863 (2013). [3] E. B. Guidez, et al., Nanoscale 6, 11512 (2014). [4] L. Bursi et al., ACS Photonics 1, 1049 (2014). [5] L. Bursi, et al., preprint (2015). [6] P. Giannozzi, et al., J. Phys.: Condens. Matter 21, 395502 (2009); X. Ge, et al., Comput. Phys. Commun. 185, 2080 (2014).