Surface Enhanced Spectroscopy by Second-Principles Calculations

Vibrational spectroscopies of molecules, materials and nanostructures, with light in the infra-red (IR) and optical (Raman) frequency range, are used in many scientific domains, from basic physics to biomedical applications, because these nondestructive methods have unmatched energy resolution and unique identification capabilities. However their limited spatial resolution and their low cross section are major drawbacks. In the last decades, major progress stemmed from the enhancement of spectroscopic responses when the applied electromagnetic radiation is resonant with a nanostructured plasmonic system, usually a surface or a tip, close to which the vibrating molecule is placed, leading to Surface Enhanced Vibrational Spectroscopies (SEVS). The most popular SEVS are SERS (Surface Enhanced Raman Spectroscopies) and SEIRA (Surface Enhanced IR Absorption).
The simulation of SEVS is a major scientific challenge. While the vibrational response of a system can now be computed with high accuracy by first-principles (1stP) quantum atomistic numerical methods, the calculation of the plasmonic response of a nanostructured system relies mainly on classical parameterized approaches. Indeed, 1stP calculations are very demanding in terms of computational time, which proscribes the accurate simulation of the plasmon resonant response of nanostructures, with typically more than one million active electrons. A unified method is however necessary to tackle unanswered questions related to the interpretation of the experimental data.
Various efforts have been devoted recently on second-principles (2ndP) methods, targeting mesoscale systems while keeping 1stP accuracy predictive power. 2ndP approaches aim at finding an effective way to reproduce 1stP data while avoiding the full quantum treatment of the electronic system. For the atomic vibrations, effective atomic potentials integrating out electronic degrees of freedom and accurately describing the 1stP Born-Oppenheimer potential energy surface can be constructed. Low-lying electronic excitations have also been tackled recently. However, the current status of the formalism does not deliver the crucial matching of the dielectric response of the material between the 1stP data and the corresponding 2ndP simulations.
In SURFASCOPE, we will design and implement a 2ndP numerical approach to interpret and guide SEVS. In order to remove the limitations of the existing formalism, the missing density response will be cast in terms of simplified treatments, either non-quantum (e.g. hydrodynamic or atomic polarizability), or quantum (e.g. simplified pseudo-orbitals or localized plasmon-poles). We will interface this 2ndP approach with at least two different state-of-the-art 1stP software packages based on quantum chemistry and material science methods. The effect of the plasmonic local field on the vibrational properties (frequency, activity,...) will then be fully described for systems of millions of electrons with a precision that will be derived from 1stP. This approach will allow us to deal with realistic nanostructured systems. We will demonstrate the capabilities of the new methodology by studying paradigmatic systems, recently investigated experimentally, based on graphene, gold resonant picocavities and metallic
nanoantennas.