Surface-enhanced Raman spectroscopy at the single molecule level

This thesis presents the results of investigations aimed at elucidating the enhancement mechanism that enables the detection of single molecules with surface-enhanced Raman scattering (SERS). For a single molecule of Rhodamine 6G (R6G) adsorbed on a colloidal Ag nanocrystal, we have measured an average SERS scattering cross section of ∼2 × 10−14 cm2. To determine the electromagnetic contribution to the observed signal, resonant Rayleigh scattering spectra were measured for the individual Ag nanocrystals using dark-field microscopy. The scattering spectra indicate that no two particles have identical plasmon resonances. While all SERS-active particles do show some scattering intensity at the laser excitation wavelength, there is no correlation between the scattering resonance and the occurrence or intensity of SERS. Based on these results, we discuss a model in which huge SERS intensities result from single chemisorbed molecules interacting with ballistic electrons in optically excited large Ag particles.; Atomic force microscopy (AFM) measurements show that the Ag nanoparticles that yield surface-enhanced Raman scattering (SERS) of single molecules of Rhodamine (R6G) are all compact aggregates consisting of a minimum of two individual particles. Comparison of 514.5 nm and 632.8 nm excitation shows that the single molecule R6G signal is significantly higher when the excitation wavelength is resonant with the absorption band of R6G and suggests that the Raman excitation spectrum follows the absorption profile for R6G. We have also observed a superlinear power dependence of the SERS signal. We discuss these results in terms of model where the R6G molecule that yields single molecule SERS signals is located at the junction of two Ag nanoparticles. We have also modeled the system using molecular resonance Raman theory to provide further insight into the enhancement mechanism. Electric force microscopy (EFM) measurements have also been conducted to study the electrostatic properties of the colloidal Ag particles. While the majority of colloidal particles are negatively charged, EFM measurements indicate that positive and neutral particles exist as well. However, preliminary results suggest that the SERS-active aggregates are not characterized by unique electrostatic properties.