Electrodynamic and electronic structure methods to model surface enhanced spectroscopy and molecule-metal nanoparticle optical coupling

When metal materials decrease in size they change from colors of which we are familiar to various colors of the rainbow. These color changes arise from the oscillations of electrons on the surface of the metal, or surface plasmons. The interactions between plasmonic nanoparticles and molecules are of great interest because they are responsible for many useful phenomena, such as plasmon-exciton hybridization and surface enhanced spectroscopy. Plasmon-molecule interactions are very detailed in nature, and often result from interference between the electric fields of both systems and from overlap between the wavefunction of the metal and of the molecule. Because of these complex interactions, it is important to develop theoretical tools to help to simulate and understand these systems. One particularly exciting area of research in plasmon-molecule interactions is surface enhanced Raman spectroscopy (SERS) because it is both sensitive and selective, making it applicable for use in detection devices and biological monitoring. In SERS, the enhancement that arises from the strong local electric field of the plasmon is called the electromagnetic mechanism, and the enhancement that arises from wavefunction overlap is called the chemical mechanism. We have developed a model that allows us to understand the origin of non-resonant chemical mechanism of SERS. We have found that the magnitude of this enhancement mechanism is governed to a large extent by the energy difference between the highest occupied molecular orbital (HOMO) of the metal and the lowest unoccupied molecular orbital (LUMO) of the molecule. This implies that molecules that readily accept π-backbonding are likely to have a strong chemical enhancement. This model provides a framework for designing new molecules which exhibit high chemical enhancements. We have also developed a new method that aids in simulating and understanding metal nanoparticle-molecule interactions. This method, which we have called the discrete interaction model/quantum mechanics (DIM/QM) method, simulates the nanoparticle classically as a collection of polarizable atoms while retaining an electronic structure description of the adsorbed molecule. This allows us to model how a plasmon interacts with a molecule, and how the electronic structure of the molecule responds to the nanoparticle. With the DIM/QM method, we have found that the response of molecules is very dependent on the local environment on the surface of the nanoparticle. Further, we have found that the magnitude and very nature of the response is dependent on the relative orientation of the molecule. This has important ramifications in designing surface enhanced spectroscopy substrates. We demonstrate that DIM is capable of modeling very large nanoparticles (>1,000,000 atoms or 40 nm diameter). It is now possible to simulate and understand complex phenomena such as plasmon-exciton hybridization and single-molecule SERS.