Site-specific Raman spectroscopy and chemical dynamics of nanoscale interstitial systems

Site-specific spectroscopy is critical for a molecular-level understanding of the mechanisms and dynamics of the inhomogeneous chemical processes crucial for catalysis, surface and interfacial chemistry, and membrane protein dynamics in living cells. Surface-enhanced Raman scattering (SERS) spectroscopy and microscopy combined with atomic force microscopy (AFM) using metal-coated AFM tips have proven to be powerful in spectroscopic analysis of inhomogeneous processes, providing correlated topographic and spectroscopic information at the nanoscale from sites in highly heterogeneous environments. It has recently been observed that SERS spectral fluctuations are pertinent to site-specific spectroscopy and microscopy. Such fluctuations are important in that they hold promise for the study of molecular structure and dynamics at a single-molecule level. This article reviews our recent work on characterization and analysis of SERS spectral fluctuation dynamics at the nanoscale in metallic interstitial sites. Fluctuations were found to accompany nanoscale confined electromagnetic near-field enhancement. The result of such confinement is that only a few molecules dominate the far-field SERS spectral signal detected in microscopic measurements that probe one nanoscale 'hot' site at a time. The fluctuation amplitude significantly decreased with the number of molecules confined at the nanoscale-local field. A new AFM-coupled two-channel photon time-stamping system, enabling in situ correlation of the topographic and spectroscopic information for single nanoparticle clusters, was used to record the Raman intensity fluctuation trajectories at a submicrosecond resolution. Experimentally, we found that SERS fluctuation dynamics are highly inhomogeneous amongst nanocluster interstitial sites although molecular translational and rotational motions at the interstitial sites can account for the SERS spectral fluctuations. To further understand these fluctuations at the nanoscale interstitial sites and nanostructures, field enhancement and field distribution at different interstitial site topographies and rough fractal surfaces were studied using finite element method computational simulation in a classic electrodynamics approach.