Abstract
We study the effects of chemical bonding on Raman scattering from benzenethiol chemisorbed on silver clusters using time-dependent density functional theory (TDDFT). Raman scattering cross sections are computed using a formalism that employs analytical derivatives of frequency-dependent electronic polarizabilities, which treats both off-resonant and resonant enhancement within the same scheme. In the off-resonant regime, Raman scattering into molecular vibrational modes is enhanced by one order of magnitude and shows pronounced dependence on the orientation and the local symmetry of the molecule. Additional strong enhancement of the order of 10(2) arises from resonant transitions to mixed metal-molecular electronic states. The Raman enhancement is analyzed using Raman excitation profiles (REPs) for the range of excitation energies 1.6-3.0 eV, in which isolated benzenethiol does not have electronic transitions. The computed vibrational frequency shifts and relative Raman scattering cross sections of the metal-molecular complexes are in good agreement with experimental data on surface enhanced Raman scattering (SERS) from benzenethiol adsorbed on silver surfaces. Characterization and understanding of these effects, associated with chemical enhancement mechanism, may be used to improve the detection sensitivity in molecular Raman scattering.
Highlights
Raman scattering from molecules in proximity to a rough noble-metal surface or near a metal nanoparticle is strongly enhanced due to the interaction with surface plasmon modes and to the formation of metal–molecular complexes.1,2 This phenomenon, which allows for the measurement of Raman spectra of extremely low concentrations of molecules3 with a single-molecule detection as its ultimate limit,4,5 is very attractive for sensor applications
We study the effects of chemical bonding on Raman scattering from benzenethiol chemisorbed on silver clusters using time-dependent density functional theory (TDDFT)
The computed vibrational frequency shifts and relative Raman scattering cross sections of the metal–molecular complexes are in good agreement with experimental data on surface enhanced Raman scattering (SERS) for benzenethiol adsorbed on silver surfaces
Summary
Raman scattering from molecules in proximity to a rough noble-metal surface or near a metal nanoparticle is strongly enhanced due to the interaction with surface plasmon modes and to the formation of metal–molecular complexes. This phenomenon, which allows for the measurement of Raman spectra of extremely low concentrations of molecules with a single-molecule detection as its ultimate limit, is very attractive for sensor applications. To understand the chemical bonding effects in SERS, it is enough to simulate only the local environment of a molecule This is consistent with the “adatom model”,1 which assumes that the atomic-scale roughness features determine the hot spots on a metal surface. Schatz and co-workers reported a resonance Raman response calculation procedure using phenomenological lifetime parameters for electronic polarizability derivatives.23 This procedure has been applied to study SERS of pyridine adsorbed on silver clusters up to a few tens of atoms.. We identify the effects that contribute to the off-resonant Raman enhancement, such as the molecular orientational effect, the effect of the local symmetry of the adsorbate, and the effect of the proximity of the vibrational mode to a binding site Stochastic modulation of these effects by thermal motion is related to “blinking” events observed in recent SERS experiments on 4-aminobenzenethiol..
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