In 1997, two groups independently reported the startling observation of single molecule surface enhanced Raman spectroscopy (SMSERS). Over time, the evidence for the existence of SMSERS and the magnitude of the enhancement factor was questioned. This lecture will focus on our efforts to provide a robust existence proof for SMSERS. We'll attempt to answer fundamental questions such as: (1) what is the largest possible enhancement factor (EF) and (2) what nanostructure produces the largest EF will be given. Our approach to answering these questions involved the development of new tools using SMSERS, single nanoparticle SERS and single nanoparticle LSPR spectroscopy spatially correlated with high resolution transmission electron microscopy (HRTEM).            The existence of SMSERS has been proven employing a frequency-domain approach with two isotopologues of rhodamine 6G (R6G) that provide unique vibrational signatures. When an average of one molecule is adsorbed per Ag nanoparticle, only one isotopologue is typically observed under a dry N2 environment. The frequency-domain proof was subsequently used to study the structure of SMSERS active nanoparticle assemblies by HRTEM and electrodynamics theory as well as to perform excitation spectroscopy measurements. The isotopologue proof has recently been extended to the crystal violet (CV) system.            Understanding the relationship between single particle structure and SERS activity represents a significant challenge for plasmonics. To this end, the structural and optical properties of SERS nanoantennas comprised of aggregated Au nanoparticles that are coated with organic reporter molecules and encapsulated by a SiO2 shell have been determined using correlated transmission electron microscopy (TEM), SERS, LSPR, and theory. The results show that the distribution of SERS enhancement factors (EFs) for a structurally and optically diverse set of nanoantennas is remarkably narrow. The EFs are uncorrelated to aggregation state and localized surface plasmon resonance (LSPR), but are crucially dependent on the size of the interparticle gap.

Pushing the limits: Single molecule and single particle surface-enhanced Raman spectroscopy

242nd National Meeting (2011)

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