Ab initionanoplasmonics: The impact of atomic structure

Abstract

We present an ab initio study of the hybridization of localized surface plasmons in a metal nanoparticle dimer. The atomic structure, which is often neglected in theoretical studies of quantum nanoplasmonics, has a strong impact on the optical absorption properties when subnanometric gaps between the nanoparticles are considered. We demonstrate that this influences the hybridization of optical resonances of the dimer, and leads to significantly smaller electric field enhancements as compared to the standard jellium model. In addition, we show that the corrugation of the metal surface at a microscopic scale becomes as important as other well-known quantum corrections to the plasmonic response, implying that the atomic structure has to be taken into account to obtain quantitative predictions for realistic nanoplasmonic devices. There is a growing interest in the development and implementation of nanoplasmonic devices such as nanosensors [1,2], nanophotonic lasers [3][4][5], optoelectronic [6,7] and light-harvesting [8,9] structures, and nanoantennas [10,11]. Therefore, it is essential to have theoretical techniques with a sufficient predictive value to understand the physical processes of light-matter interactions at the nanoscale. In this regime, the standard analysis of the plasmonic response to external electromagnetic (EM) fields using the classical macroscopic Maxwell equations must be undertaken with caution. Indeed, genuine quantum effects such as the nonlocal nature of the electron-density response, the inhomogeneity of the conduction-electron density, or the possibility of charge transfer by tunneling have to be considered [12]. These effects can be incorporated into Maxwell equations in an approximate manner using, e.g., nonlocal dielectric functions [13][14][15][16][17][18][19] or the ad hoc inclusion of "virtual" dielectric materials [20][21][22]. While these semiclassical approximations have been successfully applied in many cases, they do not achieve the precision provided by first-principle calculations.A number of recent publications [20,[23][24][25][26][27] have treated the electronic response of plasmonic structures using stateof-the-art time-dependent density functional theory (TDDFT) [28,29]. However, the ionic structure is typically neglected and replaced by a homogeneous jellium background or by an unstructured effective potential. Although this approximation is sometimes justified by the collective nature of plasmon excitations [30,31], the charge oscillations associated with a localized surface plasmon (LSP) are mainly concentrated on the metal-vacuum interface. One may thus expect that the ionic structure in this region will have a quantitative and even qualitative impact. Therefore, there is a need to address the influence of the atomic configuration in the plasmonic response at the nanoscale. In this Rapid Communication we present ab initio calculations including the atomic structure, in accordance with the current paradigm in computational condensed matter physics [32] and physical chem...

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“…The conclusions derived from the JM calculations have been later confirmed by the experimental data 40,[66][67][68][69] , and ab-initio calculations at a full atomistic level [70][71][72] .…”

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“…Nonlocality arises as a consequence of the finite extent of the electronic wave function. It can also be important to explicitly include the underlying atomic structure rather than relying on simplifications such as the jellium model [17]. In this picture, MNPs behave like large molecules, and the plasmon, which * jf1914@ic.ac.uk † http://www.gianninilab.com can capture much of the spectral weight in the extinction cross section if optically active, is built up out of transitions between discrete energy levels mediated by the Coulomb interaction.…”

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