A Hybrid Atomistic Electrodynamics–Quantum Mechanical Approach for Simulating Surface-Enhanced Raman Scattering

Abstract

Surface-enhanced Raman scattering (SERS) is a technique that has broad implications for biological and chemical sensing applications by providing the ability to simultaneously detect and identify a single molecule. The Raman scattering of molecules adsorbed on metal nanoparticles can be enhanced by many orders of magnitude. These enhancements stem from a twofold mechanism: an electromagnetic mechanism (EM), which is due to the enhanced local field near the metal surface, and a chemical mechanism (CM), which is due to the adsorbate specific interactions between the metal surface and the molecules. The local field near the metal surface can be significantly enhanced due to the plasmon excitation, and therefore chemists generally accept that the EM provides the majority of the enhancements. While classical electrodynamics simulations can accurately simulate the local electric field around metal nanoparticles, they offer few insights into the spectral changes that occur in SERS. First-principles simulations can directly predict the Raman spectrum but are limited to small metal clusters and therefore are often used for understanding the CM. Thus, there is a need for developing new methods that bridge the electrodynamics simulations of the metal nanoparticle and the first-principles simulations of the molecule to facilitate direct simulations of SERS spectra. In this Account, we discuss our recent work on developing a hybrid atomistic electrodynamics-quantum mechanical approach to simulate SERS. This hybrid method is called the discrete interaction model/quantum mechanics (DIM/QM) method and consists of an atomistic electrodynamics model of the metal nanoparticle and a time-dependent density functional theory (TDDFT) description of the molecule. In contrast to most previous work, the DIM/QM method enables us to retain a detailed atomistic structure of the nanoparticle and provides a natural bridge between the electronic structure methods and the macroscopic electrodynamics description. Using the DIM/QM method, we have examined in detail the importance of the local environment on molecular excitation energies, enhanced molecular absorption, and SERS. Our results show that the molecular properties are strongly dependent not only on the distance of the molecule from the metal nanoparticle but also on its orientation relative to the nanoparticle and the specific local environment. Using DIM/QM to simulate SERS, we show that there is a significant dependence on the adsorption site. Furthermore, we present a detailed comparison between enhancements obtained from DIM/QM simulations and those from classical electrodynamics simulations of the local field. While we find qualitative agreement, there are significant differences due to the neglect of specific molecule-metal interactions in the classical electrodynamics simulations. Our results highlight the importance of explicitly considering the specific local environment in simulations of molecule-plasmon coupling.

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Nanoarchitecture Based SERS for Biomolecular Fingerprinting and Label-Free Disease Markers Diagnosis

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Spatial resolution of tip-enhanced Raman spectroscopy – DFT assessment of the chemical effect

“…53 Quantum mechanical investigations studying the electromagnetic effects of plasmons on the Raman scattering process of a molecule in the vicinity of a metal nanoparticle have been performed by Jensen et al who introduced a computational methodology to study plasmonic field effects on a substrate in the scope of SERS by means of a hybrid method combining atomistic electrodynamics and quantum mechanics. 48,54 Another approach by Dong et al presents a quantum mechanical description of the interaction between the molecule and the metal tip, treated as a highly confined plasmonic field, using an electric field function. 55 In contrast to the work of Aizpurua, Jensen and Dong, we will focus exclusively on the non-resonant chemical contributions to the signal enhancement/changes in TERS.…”