A hybrid atomistic electrodynamics-quantum mechanical approach for simulating surface-enhanced Raman scattering.

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.

Direct test of the equivalency of dynamic IR and dynamic raman spectroscopies as techniques for observing ultrafast molecular dynamics.

We report the temperature-dependent infrared (IR) and Raman spectra of Fe(CO)3(η(4)-norbornadiene). This molecule undergoes carbonyl ligand site exchange on the vibrational time scale, and the effect of this exchange is observable as coalescence of the carbonyl bands in both the IR and Raman spectra. We outline a theory that we used to account for these effects and report simulations of the experimental spectra.

Tuning surface-enhanced Raman scattering from graphene substrates using the electric field effect and chemical doping.

Graphene recently has been demonstrated to support surface-enhanced Raman scattering. Here, we show that the enhancement of the Raman signal of methylene blue on graphene can be tuned by using either the electric field effect or chemical doping. Both doping experiments show that hole-doped graphene yields a larger enhancement than one which is electron-doped; however, chemical doping leads to a significantly larger modulation of the enhancements.

Photoreaction of matrix-isolated dihydroazulene-functionalized molecules on Au{111}.

Dihydroazulenes are photochromic molecules that reversibly switch between two distinct geometric and conductivity states. Molecular design, surface attachment, and precise control over the assembly of such molecular machines are critical in order to understand molecular function and motion at the nanoscale. Here, we use surface-enhanced Raman spectroscopy on special atomically flat, plasmonically enhanced substrates to measure the photoreaction kinetics of isolated dihydroazulene-functionalized molecules assembled on Au{111}, which undergo a ring-opening reaction upon illumination with UV light and switch back to the initial isomer via thermal relaxation.

Importance of Correctly Describing Charge-Transfer Excitations for Understanding the Chemical Effect in SERS.

The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) 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. Here, we investigate the importance of correctly describing charge-transfer excitations, using time-dependent density functional theory (TDDFT), when calculating the chemical coupling in SERS. It is well-known that TDDFT, using traditional functionals, underestimates the position of charge-transfer excitations.

A discrete interaction model/quantum mechanical method for simulating surface-enhanced Raman spectroscopy.

We have derived and implemented analytical gradients for the discrete interaction model/quantum mechanics (DIM/QM) method. DIM/QM combines an atomistic electrodynamics model with time-dependent density functional theory and thus enables modeling of the optical properties for a molecule while taking into account the local environment of a nanoparticle's surface. The DIM/QM analytical gradients allow for geometry optimizations, vibrational frequencies, and Raman spectra to be simulated for molecules interacting with metal nanoparticles.

A discrete interaction model/quantum mechanical method to describe the interaction of metal nanoparticles and molecular absorption.

A frequency-dependent quantum mechanics/molecular mechanics method for the calculation of response properties of molecules adsorbed on metal nanoparticles is presented. This discrete interaction model/quantum mechanics (DIM/QM) method represents the nanoparticle atomistically, thus accounting for the local environment of the nanoparticle surface on the optical properties of the adsorbed molecule. Using the DIM/QM method, we investigate the coupling between the absorption of a silver nanoparticle and of a substituted naphthoquinone.

A discrete interaction model/quantum mechanical method for describing response properties of molecules adsorbed on metal nanoparticles.

A new polarizable quantum mechanics/molecular mechanics method for the calculation of response properties of molecules adsorbed on metal nanoparticles is presented. This method, which we denote the discrete interaction model/quantum mechanics (DIM/QM) method, represents the nanoparticle atomistically which enables the modeling of the influence of the local environment of a nanoparticle surface on the optical properties of a molecule. Using DIM/QM, we investigate the excitation energies of rhodamine-6G (R6G) and crystal violet (CV) adsorbed on silver and gold nanoparticles of different quasispherical shapes and sizes.

Controlling the non-resonant chemical mechanism of SERS using a molecular photoswitch.

In this work we present a detailed investigation of the Raman properties of a dithienylethene photoswitch interacting with a small gold cluster (Au(19)(+)) using time-dependent density functional theory (TD-DFT). The enhancement mechanism (CHEM) due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized for this system. We demonstrate that it is possible to control the CHEM enhancement by switching the photoswitch from its closed form to its open form.

Understanding the molecule-surface chemical coupling in SERS.

The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized using time-dependent density functional theory. This has been achieved with a systematical study of the chemical enhancement of meta- and para-substituted pyridines interacting with a small silver cluster (Ag(20)). Changing the functional groups on pyridine enabled us to modulate the direct chemical interactions between the pyridine ring and the metal cluster.