Plasmon Couplings from Subsystem Time-Dependent Density Functional Theory

Abstract

Many applications in plasmonics are related to the coupling between metallic nanoparticles (MNPs) or between an emitter and a MNP. The theoretical analysis of such a coupling is thus of fundamental importance to analyze the plasmonic behavior and to design new systems. While classical methods neglect quantum and spill-out effects, time-dependent density functional theory (TD-DFT) considers all of them and with Kohn–Sham orbitals delocalized over the whole system. Thus, within TD-DFT, no definite separation of the subsystems (the single MNP or the emitter) and their couplings is directly available. This important feature is obtained here using the subsystem formulation of TD-DFT, which has been originally developed in the context of weakly interacting organic molecules. In subsystem TD-DFT, interacting MNPs are treated independently, thus allowing us to compute the plasmon couplings directly from the subsystem TD-DFT transition densities. We show that subsystem TD-DFT, as well as a simplified version of it in which kinetic contributions are neglected, can reproduce the reference TD-DFT calculations for gap distances greater than about 6 Å or even smaller in the case of hybrid plasmonic systems (i.e., molecules interacting with MNPs). We also show that the subsystem TD-DFT can be also used as a tool to analyze the impact of charge-transfer effects.

The Seamless Connection of Local and Collective Excited States in Subsystem Time-Dependent Density Functional Theory

“…Subsystem TDDFT has led to numerous follow-up developments such as the description of plasmonic interactions of Ag 20 clusters . Furthermore, a subsystem version of the adiabatic-connection fluctuation–dissipation-theorem has been developed by Pavanello and co-workers based on the response function structure in FDEc-TDDFT.…”

“…82,127 Subsystem TDDFT has led to numerous follow-up developments such as the description of plasmonic interactions of Ag 20 clusters. 129 Furthermore, a subsystem version of the adiabaticconnection fluctuation−dissipation-theorem has been developed by Pavanello and co-workers based on the response function structure in FDEc-TDDFT. This methodology has led to an accurate description of van der Waals interactions between weakly interacting subsystems 130,131 and the determination of C 6 coefficients for molecules adsorbed on surfaces.…”

Revealing the Interplay between Hybrid and Charge-Transfer States in Polariton Chemistry

“…The theoretical analysis, performed within the TD-DFT framework, provides a deep understanding of the coupling mechanisms between the azobenzene photoswitch and a prototype plasmonic nanocluster, showing the effects of the localized surface plasmon on the onset of strong-coupling and CT phenomena. While polaritonic states appear only in the barrier region, charge-transfer excitations seem to be present along the selected isomerization pathway from trans to cis conformation in between S 1 and S 2 , this possibly provides additional channels whose relevance requires further investigation to be gauged and is likely dependent on the relative molecule-metal distance and orientation . We show that the photochemistry of azobenzene, in principle, could be manipulated or engineered by the formation of polaritons.…”

“…Recent works have shown that the strong coupling between surface plasmons and excitons could lead to the formation of hybrid states, namely, plexcitons. , These are analogous to polaritonic states with plasmonic modes , in the place of cavity modes. Here, we show the onset of these new states (indicated as polaritonic) for particular conditions and how these can modify the potential energy landscape of the molecule .…”

Coupling between Plasmonic and Molecular Excitations: TDDFT Investigation of an Ag-Nanorod/BODIPY–Dye Interaction

“…As is well-known, this is due to the presence in gold of the quenching effect of the 5d band, whereas in silver the 4d band is deeper and does not suppress the plasmon. The size and shape of Ag 50 has been chosen because for this nanocluster the plasmon resonance falls at the same energy as that of the lowest transition of the BODIPY, and this is expected to maximize their mutual interaction: we note, however, that, in general, it is easy to tune Ag nanorods to give the desired plasmon frequency to maximally interact with other transitions. − We start by briefly describing the excitation spectra of the isolated systems, then we couple them in a system which we shall designate as the adduct, and analyze and try to rationalize the changes occurred upon interaction. In the interaction system, the two fragments have been positioned in different mutual orientations, in order to have the dipole transition density of the BODIPY pointing toward or away from the nanorod, and at different distances.…”